CA2858057C

CA2858057C - Simplified high efficiency sulphur recovery

Info

Publication number: CA2858057C
Application number: CA2858057A
Authority: CA
Inventors: Gavin Craig Proudfoot
Original assignee: Suncor Energy Inc
Current assignee: Suncor Energy Inc
Priority date: 2014-07-30
Filing date: 2014-07-30
Publication date: 2017-01-03
Anticipated expiration: 2034-07-30
Also published as: CA2858057A1

Abstract

A modified Claus process and system for treating an acid gas stream may involve, in one illustrative embodiment, controlling gas stream temperatures to operate a second Claus stage reactor in above-dewpoint mode while operating a third Claus stage reactor in sub-dewpoint mode, and then, controlling gas stream temperatures to operate the second stage reactor in sub-dewpoint mode while operating the third stage reactor in above-dewpoint mode to regenerate its catalyst bed, without the need to use switching valves to divert the gas flow to effectively reorder the second and third stage reactors for such regeneration.

Description

SIMPLIFIED HIGH EFFICIENCY SULPHUR RECOVERY
BACKGROUND OF THE INVENTION
1. Field of Invention This invention relates generally to the field of sulphur recovery and to a modified Claus process and system for treating an acid gas stream.

2. Description of Related Art Sour gas or acid gas may be treated by removing its hydrogen sulphide (H2S).
For example, a Claus Plant uses a series of catalytic reactor stages to heat and cool a gas stream carrying H2S so that the H2S reacts with sulphur dioxide (SO2) to produce water (H20) and elemental sulphur (Se). Typically, each process stage operates at a slightly lower temperature. The main objective is to recover as much sulphur as possible, thereby minimizing SO2 emissions.
It is desirable to remove sulphur from H2S-containing gases efficiently.
Increased conversion efficiencies are gained at reduced temperatures. At sufficiently low temperatures, sulphur is deposited on the catalyst bed. This sub-dewpoint sulphur condensing process gradually deactivates the catalyst bed. Accordingly, Claus processes that operate below the sulphur dew point must provide for the regeneration of the catalyst bed and perform this function in a manner that does not significantly undermine the sulphur recovery process.
The regeneration of a catalyst bed containing liquid sulphur may be carried out by using switching valves in conjunction with existing pipe infrastructure. A
switching valve sequence is invoked to effectively change the order of two catalyst beds. For example, a catalyst bed that has been loaded with sulphur due to sub-dewpoint operation is effectively repositioned upstream of the other catalyst bed using the switching valves and pipe infrastructure, and is then regenerated by increasing its inlet gas stream temperature. Once the sulphur has been removed from this bed, it is ready to be operated again in sub-dewpoint mode after using the switching valves to effectively change the order of the two catalyst beds once again.
However, a standard Claus plant typically does not include gas diversion infrastructure, and retrofitting the plant to add the infrastructure would be time-consuming and costly. Secondly, adding such infrastructure increases system maintenance requirements and creates additional points of potential failure.
Thirdly, diversion of the feed gas stream to a new reactor using switching valves may cause undesirable emission spikes or flow changes in some systems.
SUMMARY OF THE INVENTION
A method of sulphur recovery and a control system is hereby provided in accordance with a variety of embodiments.
Certain embodiments of the present invention may provide one of the following advantages. Embodiments of the invention may be relatively easily implemented in a standard Claus plant without substantial physical modifications thereto. It can be unnecessary to switch the flow order of the reactors: no diversion of the feed gas stream is needed (e.g., for regeneration) and therefore:
(a) there is no need to build infrastructure for diverting the stream between reactors (e.g., switching valves and/or extra piping);
(b) emission spikes and flow changes due to switching are avoided;
(c) maintenance costs (e.g., stuck or leaky valves) are reduced;
(d) due to greater system simplicity, troubleshooting is easier; and

-3-(e) the sulphur recovery efficiency is improved compared to the standard Claus configuration that does not utilize the sub-dewpoint process.
In accordance with one aspect of the invention, there is provided a method of removing sulphur from a gas stream. The method involves:
(a) serially feeding a gas stream through first, second, and third Claus stages connected in series, each including a respective reactor with a catalyst bed, to remove sulphur from the gas stream;
(b) heating the gas stream entering the first reactor in the first Claus stage and heating the gas stream entering the second reactor in the second Claus stage;
(c) controlling the temperature of the gas stream entering the third reactor in the third Claus stage from the second Claus stage to operate the third reactor in sub-dewpoint mode;
(d) as the third reactor approaches an upper sulphur deposition limit, transitioning the second reactor to sub-dewpoint mode, and heating the treated gas stream entering the third reactor from the second Claus stage to vaporize sulphur deposited in the third reactor; and then, (e) heating the gas stream entering the second reactor from the first Claus stage to vaporize sulphur deposited in the second reactor, and returning the third reactor to sub-dewpoint mode.
The method may further involve repeating steps (d) to (e).

-4-Step (d) may be carried out while the first, second and third Claus stages continue to be connected in series in the manner described in step (a).
Transitioning the second reactor to sub-dewpoint mode may involve not heating the gas stream entering the second reactor.
Transitioning the second reactor to sub-dewpoint mode may involve heating the gas stream entering the second reactor to a temperature of 140 to 150 C.
The method may further involve, following step (d), transitioning the second reactor to heat soak mode.
Step (f) may further involve transitioning the second reactor to normal Claus mode operation.
Step (c) may involve allowing substantially the entire third reactor to enter sub-dewpoint mode.
The third reactor may include a catalyst bed, and heating the treated gas stream entering the third reactor in step (e) may involve regenerating substantially the entire catalyst bed of the third reactor before the second reactor is saturated with sulphur due to the second reactor operating in sub-dewpoint mode.
The method may further involve heating the third reactor in step (d) to regenerate the catalyst bed of the third reactor only until the second reactor begins to approach capacity in sub-dewpoint mode.
Beginning to approach capacity may involve approaching only within a predefined safety margin of actual capacity of the second reactor before entering step (e).

-5-The method may further involve operating each of the reactors in the first, second and third Claus stages in normal Claus mode.
The method may further involve controlling a reheater to heat the gas stream to the third reactor in step (d) to a temperature of between about 250 C and about 300 C.
Heating the gas stream entering the second reactor may involve controlling the gas stream's temperature to be sufficient to vaporize substantially all the sulphur condensed in the second reactor within a predetermined amount of time.
In accordance with another aspect of the invention, there is provided a method of processing sour gas in a Claus sulphur recovery system having at least an upstream Claus stage and a downstream Claus stage, the upstream Claus stage including a first catalyst bed and first condenser connected in series and the downstream Claus stage including a second catalyst bed and a second condenser connected in series. The method involves:
(a) receiving a first input gas stream from an upstream source;
(b) passing the first input gas stream through the upstream Claus stage, including the first catalyst bed and the first condenser, to produce a second input gas stream; and (c) passing the second input gas stream through the downstream Claus stage, including the second catalyst bed and the second condenser, to produce a processed output gas stream;
wherein steps (a)-(c) further involve:

-6-(d) during a first phase of processing the sour gas, operating the first catalyst bed in above-dewpoint mode to recover at least some sulphur from the first input gas stream to produce the second input gas stream, and operating the second catalyst bed in sub-dewpoint mode to recover additional sulphur from the second input gas stream to produce the output gas stream; and (e) during a second phase of processing, following the first phase, transitioning the first catalyst bed to sub-dewpoint mode operation while continuing to operate the second catalyst bed in sub-dewpoint mode to produce the output gas stream.
The method may further include one or more of the following:
(f) during a third phase of processing, following the second phase, operating the first catalyst bed in sub-dewpoint mode to recover additional sulphur from the first input gas stream, and transitioning the second catalyst bed to catalyst regeneration mode to vaporize condensed sulphur from the second catalyst bed;
(g) during a fourth phase of processing, following the third phase, transitioning the first catalyst bed to heat soak mode or catalyst regeneration mode, while continuing to operate the second catalyst bed in catalyst regeneration mode; and (h) during a fifth phase of processing, following the fourth phase, operating the first catalyst bed in above-dewpoint mode, while transitioning the second catalyst bed to sub-dewpoint mode.
The fifth phase may involve operating the first catalyst bed in normal Claus mode.

-7-The fifth phase may involve operating the first catalyst bed in heat soak mode.
The method may further involve:
(i) during a sixth phase, following the fifth phase, (A) processing the first gas stream received from the upstream source using the first catalyst bed and first condenser, with the first catalyst bed operating in normal Claus recovery mode, to produce the first processed gas stream; and (B) processing the first processed gas stream using the second catalyst bed and second condenser, with the second catalyst bed operating in sub-dewpoint mode, to produce the second processed gas stream.
The method may further involve, during the first phase, processing the first processed gas stream with the second catalyst bed operating in sub-dewpoint mode until the second catalyst bed is substantially saturated with sulphur, and then commencing the second phase of processing.
The method may further involve regenerating the second catalyst bed during the third phase by heating the first gas stream with a reheater located upstream of the second catalyst bed and downstream of the first condenser in the upstream Claus stage.
The third phase may be much shorter than the first phase.

-8-The upstream source may be an upstream condenser associated with a preceding upstream Claus catalyst bed.
In accordance with another aspect of the invention, there is provided a method of recovering sulphur from a gas stream by operating a Claus sulphur recovery system having first, second and third stages which include first, second and third reactors, respectively. The method involves:
(a) operating substantially the entire second reactor in above-dewpoint mode while operating substantially the entire third reactor in sub-dewpoint mode, to recover at least some sulphur from the gas stream in each of the second and third stages; and then, (b) operating the substantially the entire second reactor in sub-dewpoint mode to recover at least some sulphur from the gas stream in the second stage while operating the third reactor in above-dewpoint mode to regenerate substantially all the catalyst in the third reactor;
wherein during steps (a) and (b), the gas stream entering the first stage of the Claus sulphur recovery system is passed through to the second stage and then passed through the second stage to the third stage.
The method may further involve transitioning the second reactor to sub-dewpoint mode before the third reactor is placed in above-dewpoint mode such that at least a portion of each of the second and third reactors is in sub-dewpoint mode simultaneously.
The method may further involve operating the second reactor in above-dewpoint mode to regenerate a catalyst in the second reactor while operating the third reactor in sub-dewpoint mode.

-9-In accordance with another aspect of the invention, there is provided a control system for a Claus sulphur recovery system which is operable to implement one or more of the above-mentioned methods.
In accordance with another aspect of the invention, there is provided a non-transitory computer-readable medium storing codes for instructing a processor circuit to execute one or more of the above methods.
In accordance with one aspect of the invention, there is provided a control system operable to control the operation of upstream and downstream Claus converter stages that are interconnected to recover sulphur from a gas stream, wherein (i) the upstream Claus converter stage includes a first reheater, a first reactor and a first condenser connected in series, and (ii) the downstream Claus converter stage includes a second reheater, a second reactor and a second condenser connected in series, the control system including at least one processor circuit operably configured to control the first and second reheaters to selectively reheat the respective portions of the gas stream about to enter the first and second reactors to have a first and second temperature, respectively, by:
(a) in a first time period, controlling the first reheater to maintain the first temperature sufficiently high to prevent the first reactor from operating in sub-dewpoint mode, and controlling the second reheater to permit the second temperature to fall sufficiently to cause the second reactor to operate in sub-dewpoint mode; and (b) in a second time period, controlling the first reheater to permit the first temperature to fall sufficiently to permit the first reactor to operate in sub-dewpoint mode, and controlling the second reheater to raise the second

-10-temperature sufficiently high to cause a catalyst in the second reactor to be regenerated.
The processor circuit may be in communication with a first temperature sensor measuring the first temperature of gas entering the first reactor and a second temperature sensor measuring the second temperature of gas entering the second reactor.
During the second time period, the first reheater may be controlled to allow the first temperature to fall sufficiently to cause the first reactor to enter sub-dewpoint mode before the second reheater is controlled to raise the second temperature to regenerate the catalyst in the second reactor.
The system may further include:
(a) at least one input line for providing temperature data from the first and second temperature sensors to the processor circuit; and (b) at least one output control line allowing the processor circuit to control the operation of the first and second reheaters.
During the first time period, the second reheater may be turned off by the processor circuit long enough to allow substantially the entire second reactor to enter sub-dewpoint mode.
During the second time period, the first reheater may be turned off by the processor circuit long enough to allow substantially the entire first reactor to enter sub-dewpoint mode.

-11-It should be appreciated that the invention is not limited to the illustrative embodiments described in this summary section, but may also be implemented in other different embodiments, including embodiments which combine some of the features described above.
Moreover, this summary section does not purport to exhaustively summarize all advantages of every possible embodiment of the invention.
It should be appreciated that different embodiments of the invention may provide different advantages relative to each other and relative to the prior art and may address different disadvantages present in the prior art.
Other aspects, advantages, benefits and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

-12-BRIEF DESCRIPTION OF THE DRAWINGS
In drawings which illustrate embodiments of the invention, Figure IA is a schematic view of a three stage Claus reactor system for carrying out the process of the invention in certain embodiments;
Figure 1B is a schematic view of a four stage Claus reactor system for carrying out the process of the invention in certain embodiments;
Figure 2 is an embodiment of a control system for controlling the process in a reactor system such as the ones shown in Figures 1A and 1B;
Figure 3 is a set of process parameter plots illustrating operation of one embodiment of the invention under a particular set of conditions; and Figure 4 is a flowchart providing an overview of a process of the invention in accordance with certain illustrative embodiments.
A detailed description of various illustrative embodiments will now be provided.

-13-DETAILED DESCRIPTION
Three Stage System Referring to Figure 1A, one illustrative Claus sulphur recovery system for carrying out the invention in various embodiments is shown generally at 100.
The system 100 shown has three stages (120, 130, 140) connected in series.
Typically, hot acid gas from an upstream reaction furnace is provided to the input 121 of the first stage 120 for treatment, is passed through a first stage reheater 122, and then via line 123 is passed into a first stage reactor 124 containing a first catalyst bed for facilitating the conversion of H2S into elemental sulphur. The output of the first stage reactor 124 is provided via line 125 to a first stage condenser 126, which cools the gas stream, thereby causing at least some of the elemental sulphur in the gas stream to condense into liquid form for discharge via line 127 into a sulphur collection system such as a sulphur pit 150. The remainder of the cooled gas stream from the condenser 126 is fed via line 128 to an input 131 of the second stage 130.
The partially treated gas stream passes in turn from the input 131 through a second stage reheater 132 via line 133 to a second stage reactor 134, containing a second catalyst bed to facilitate the conversion of H2S into elemental sulphur.
The gas stream then passes via line 135 into a second stage condenser 136 to cause at least some sulphur to condense into liquid form for disposal via line into sulphur pit 150, whereas the partially processed gas stream is fed via line 138 to the input 141 of the third stage 140. The third stage 140 similarly passes the gas stream in turn through a reheater 142, reactor 144 (via line 143) and condenser 146 (via line 145), and disposes of any condensed sulphur into the sulphur pit 150 (via line 147), to provide a final treated gas stream 148. The composition of the final gas stream 148, in particular, H2S and SO2 content, may be monitored, for example, via line 149 by an air demand analyzer (ADA) 152 and/or a Continuous Stack Emission Monitor (CSEM).

-14-Four Stage System Referring now to Figure 1B, an illustrative Claus sulphur recovery system for carrying out the invention in further embodiments is shown generally at 102.
In comparison to the system shown in Figure 1A, the system 102 includes an extra Claus stage such that it has a series of four Claus stages (110, 120, 130, 140), each including a respective reheater (112, 122, 132 and 142), reactor (114, 124, 134 and 144), and condenser (116, 126, 136 and 146), connected in series.
Sulphur vapour enters the respective gas inlet (115, 125, 135, and 145) of each condenser, is cooled and thus condensed into liquid form within the condenser, and then is discharged into a sulphur pit 150 (e.g., via lines 117, 127, 137, 147), while a remainder of the gas stream exits from the respective gas outlet of each condenser (118, 128, 138, 148). The gas stream entering at input 111 typically comes from an upstream combustion chamber that thermally facilitates conversion of H2S into elemental sulphur.
In such embodiments, a raw sour gas stream that enters the system at input 111 is processed in each of the stages in turn (110, 120, 130, 140) to incrementally remove at least some sulphur content in each stage, to produce a final treated gas stream at output 148. In each Claus stage (110, 120, 130, 140), an input line (111, 121, 131 and 141) allows a feedstock, namely, a respective gas stream, to enter the stage, and be suitably heated by the respective reheater (112, 122, and 142) to provide a respective reheated gas stream (113, 123, 133, 143) to the respective reactor (114, 124, 134 and 144). Each reactor has a gas inlet for receiving its respective gas stream and a gas outlet for discharging the gas stream after processing, and includes a respective catalyst bed operable to facilitate the conversion of at least some hydrogen sulphide (H2S) in the gas stream into elemental sulphur. The final treated gas stream 148 may be conveyed via line 149 to a tail gas analyser or air demand analyser (ADA) 152 monitoring the composition of the final gas.

-.15-While different numbers of stages could be used in various embodiments of a Claus system constructed and operated in accordance with the invention, a 3-stage Claus reactor system (e.g., Fig. 1A) will be assumed for purposes of the following description, such that the stages 120, 130 and 140 respectively constitute the first, second and third stages of the overall Claus system 100.
Catalytic conversion of H2S
The catalytic chemical reaction at the heart of the Claus sulphur recovery process is limited as to its completion by high temperatures and by the presence of elemental sulphur in reaction equilibrium with other molecules.
Consequently, catalytic conversion of H2S and SO2 into elemental sulphur in a reactor is maximized at lower temperatures and upon the removal of elemental sulphur.
If a given reactor is operated at a temperature above a sulphur dewpoint temperature, any elemental sulphur that is formed will remain in gaseous form.
If the reactor temperature falls below the sulphur dewpoint temperature, more of the H2S and SO2 in the gas stream will be converted into elemental sulphur, however, a portion of the elemental sulphur will be condensed and deposited onto the catalyst bed as various allotropes in liquid form, thereby gradually saturating the catalyst bed and deactivating the operation of the catalyst.
For example, liquid sulphur may be adsorbed at the actives sites of the catalyst, thus preventing them from facilitating the Claus reaction. Each catalyst bed has a limited capacity for deposited sulphur and thus will eventually require regeneration (or replacement) in order to remain capable of converting H2S and SO2 to elemental sulphur. Regeneration may be done by heating a reactor's gas feed. In general, the hotter the gas feed, the better it is able to vaporize and carry away sulphur deposited on the reactor's catalyst bed, thereby regenerating it.

A line (e.g., 125, 135, 145) connects the output of each reactor (e.g., 124, and 144) to the respective condenser (e.g., 126, 136, 146), which cools the respective gas stream to between about 130 C to 150 C, to condense sulphur vapour to liquid form for discharge into the sulphur pit 150. As mentioned above, such removal of sulphur helps the Claus reaction to proceed.
Catalyst Beds Each reactor contains a bed of catalyst which facilitates the reaction between two molecules of H2S and one of SO2 to produce sulphur and water (commonly called the Claus reaction). As described above, if a catalyst bed is operating in sub-dewpoint mode, sulphur will tend to condense on the catalyst as a liquid.
In some embodiments, the catalyst bed will be able to condense up to about 50% of its weight in sulphur before it becomes deactivated such that regeneration is needed. The deposition of sulphur onto the catalyst bed is reversible by applying heat sufficient to operate the catalyst bed at a temperature sufficiently above the sulphur dewpoint (i.e., in "above-dewpoint mode"). Catalyst beds for the Claus reaction may be approximately 1 meter in depth (in typical embodiments) and subject to heat released by the Claus reaction. Catalyst beds may be heated and cooled unevenly.
Accordingly, such catalyst beds may have a plurality of thermocouples for measuring respective local temperatures in particular portions of the catalyst bed to provide operational feedback. For example, a catalyst bed may have top, middle and bottom thermocouples. In one exemplary embodiment, the first reactor 124 is fed an acid gas via line 123 that has a temperature of 238 C, which may cause the top, middle and bottom thermocouples to register temperatures of 278 C, 315 C and 322 C, respectively. In this example, the measured temperatures are up to about 90 C higher than the input gas feed because the catalytic reaction is strongly exothermic and generates heat. Clearly, the first reactor is converting a substantial amount of H2S and SO2 into sulphur. The gas at the output of the second reheater 132 at line 133 may have a temperature of 213 C, however, the top, middle and bottom of the catalyst bed in the second reactor 134 may be at temperatures of 220 C, 224 C and 224 C. The second stage reactor only creates a temperature rise of up to about 11 C. In the third reactor bed 144, the gas may be at 195 C going in, but its thermocouples will show the top at 197 C, the middle at 198 C and the bottom at 197 C, for example. The temperature profiles of each of the catalyst beds illustrate that the rate of sulphur extraction in the second and third stages (130, 140) is considerably lower than that in the first stage 120. It will be appreciated that the above example is for illustration only, and that the precise temperatures in a given system will depend on a number of factors including the nature of the gas feed, the operation of the reheaters, the amount and condition of the catalyst beds, and other operating conditions.
In regular Claus operation, feedback from the thermocouples can be used to assess the health of the catalyst bed.
However, feedback from the thermocouples can be used for purposes of this invention to monitor the transition of each catalyst bed between different operating modes. Given the depth and mass of a given catalyst bed, temperature changes within it are not instantaneous. The profile of temperature transitions depends on factors including the mass and condition of the catalyst bed and temperature differentials between various portions of the bed and the gas stream. For example, turning off (or sufficiently turning down) a reheater will initiate a transition to sub-dewpoint mode operation, but it will take time for the top portions of the associated catalyst bed to cool off, the middle portions will cool off later, and the bottom portions will cool off last. Similarly, when a heating mode is initiated (e.g., when transitioning from sub-dewpoint mode to regular Claus mode, heat soak mode, or regeneration mode), it will take least a short period of time for the top thermocouple to see the temperature increase, longer for the middle one, and longer yet for the bottom one.

As an illustration, consider one embodiment in which the second reactor is being transitioned to regeneration mode from a stint in sub-dewpoint mode. Assume further that a bottom portion of the catalyst bed is still relatively "hot", although the top portion is "cold". For example, the top portion of second catalyst bed may be at 140 C, the middle portion at 150 C and the bottom portion at 180 C. If heat is added to the top of the bed, the "cold front" from the top portion (initially 140 C) will work its way down into the bed. In other words, when the transition to regeneration mode is initiated, the top portion of the bed (initially at 140 C) begins to heat up, but it also causes cooling of the middle portion, which in turn causes some cooling of the bottom portion. Thus, the "cold front" gets pushed down the bed such that the bottom portion of the bed may cool down to about 150 C or 160 C before it begins to heat up again due to the heating of the top portion. In this example, it may take up to about 6 hours for the entire bed to reach steady state temperatures. The actual time and temperature profile of the 2nd catalyst bed during its time in sub-dewpoint mode and its transition to above-dewpoint mode will depend on many factors such as flow rate, catalyst bed size, time to regenerate the final bed and other factors. Such considerations will allow for the optimization of embodiments and may vary over the life cycle of a sulphur recovery facility.
One advantage of most embodiments of the present invention is that they are capable of using standard Claus process catalysts, in contrast to some modified Claus systems, which require exotic catalysts for their operation.
Reheaters The system 100 includes a plurality of reheaters (e.g., 122, 132, 142) for heating the gas feed within each stage (120, 130, 140) to a suitable temperature.
Reheaters may use indirect heating (e.g., electrical coils or a steam-based heat exchanger) or direct heating (e.g., burning acid gas or fuel gas within the gas stream that is to be heated). Preferably, the reheaters have turndown capability to generate a low-end gas temperature of between about 140 C to about 150 C, if not lower (e.g., 130 C). In extreme winter conditions, the low-end temperatures may be set a bit higher. Preferably, the reheaters provide infinite levels of heating control, however, the method can still be implemented even if the reheater is limited to discrete levels of heating.
Direct reheaters can be operably configured to burn a certain volume of acid gas with a predetermined proportion of air to achieve a desired temperature. In general, a higher temperature is created by burning more fuel mixed with air using a proper fuel-air ratio. Some direct reheaters have turndown limitations: if the flame is set too low, it may flicker and blow out. This can be problematic if the burner needs to be manually relit. Consequently, some direct reheaters may have an idle mode which represents a minimum flame (and thus, a minimum reheater temperature) that can safely be used without blowing out. In such embodiments, a reactor would be put into sub-dewpoint mode by turning down the reheater to be at or near the minimum temperature. To provide turndown flexibility in an acid gas based reheater, it may be desirable to use a strong pilot flame or else to use two burners.
In contrast, turning off electric reheaters is easily achievable. However, if the heating coils become colder than the incoming gas stream, this may lead to sulphur condensing on the coils.
If this is desirable to prevent such condensation, rather than turning off the electrical coils completely during sub-dewpoint operation, the electrical reheater may be set to generate a low level of heat to keep its components a bit warmer than the temperature of the passing gas feed, which may be considered an "idle mode" of operation.
Reheaters may be operably configured by setpoints to maintain a particular temperature output. If the gas feed falls below the setpoint (e.g., by at least a minimum amount), the reheater is turned on to maintain the temperature as set.

Setpoints may be programmed either from a central control system, a distributed control system, or locally, depending on the embodiment.
Sulphur Dewpoint Before discussing the various sub-dewpoint and above-dewpoint reactor modes which are utilized in embodiments of the invention, it is worth briefly explaining sulphur dewpoint. Sulphur dewpoint is the temperature at which sulphur vapour condenses into liquid form. Sulphur dewpoint is not static but depends on a number of factors including humidity, i.e., the amount of sulphur in the vapour phase. As sulphur is removed in each stage of the system, the dewpoint temperature drops. Thus, the sulphur dewpoint will vary somewhat as between the first, second and third reactors. Typically, the first reactor runs hot enough that it never goes below its sulphur dewpoint. The second reactor may have a dewpoint of about 200 C (e.g., as shown graph 302 in Fig. 3), thus it may be run at about 210 C for normal Claus operation. The third reactor might have a sulphur dewpoint temperature of about 180 C (e.g., graph 308 in Fig. 3), thus it may be run at about 190 C to 195 C in regular Claus operation.
Regular Claus Mode An example of normal Claus mode operation is now provided to provide a point of comparison to other reactor temperature modes of the invention. Normal Claus operation is usually at a temperature of about 5 to 15 C above the sulphur dewpoint. In one embodiment which uses normal Claus mode operation, the operation of the reheaters may be as follows. The first reheater takes gas stream at about 130 C as an input, and heats it to about 238 C to 240 C, which elevates temperatures inside the first reactor as high as about 315 C to about 330 C (due to the exothermic nature of the catalytic reaction). The second stage reheater may have gas coming in at between about 137 C to 142 C and coming out at a set point of about 213 C, which causes a second reactor temperature reach internal temperatures of up to about 220 C to 225 C. The third stage reheater may heat the gas stream from about 135 C to about 195 C, causing the third reactor to reach temperatures of about 198 C. It will be appreciated that this example of regular Claus operation is not limiting and that other combinations of temperatures are possible. Some embodiments of the invention may be able to transition between regular Claus operation and other reactor temperature modes, as discussed below.
Other Reactor Temperature Modes or Regimes While normal Claus mode typically uses a reactor temperature of about 5 C to about 15 C above the sulphur dewpoint, embodiments of the invention use a variety of other reactor temperatures to achieve superior sulphur recovery. As mentioned earlier, the catalytic reaction achieves higher recovery at low temperatures. Accordingly, having a final catalyst reactor operate at a temperature below the sulphur dewpoint (i.e., "sub-dewpoint mode" operation) increases the amount of sulphur recovered and improves the sulphur recovery efficiency. However, as discussed earlier, sub-dewpoint mode operation also causes sulphur to be deposited on the catalyst bed, which eventually deactivates it. Consequently, it is necessary to use one or more above-dewpoint temperature regimes in order to vaporize the deposited sulphur.
While regular Claus operation mode is an above-dewpoint mode of operation, it is not very useful in vaporizing sulphur. While some elemental sulphur will be carried away, this may occur too slowly to regenerate a catalyst bed within the time constraints required in the processes described below. Thus, higher temperature modes of operation are used to facilitate catalyst regeneration.

In some embodiments, a "heat soak" mode of operation involves running a reactor at about 25 C above the normal Claus operating temperature, that is, about 30 to 50 C higher than dewpoint temperature. In some embodiments, an even higher temperature range for is used for reactor operation, namely, a "regeneration mode" of operation may be about 250 C to 330 C or a sub-range thereof, such as about 270 C to 300 C, which may be about 50 to 130 C higher than the sulphur dewpoint temperature. Temperatures above 330 C may lead to metallurgical problems (e.g., oxidation or corrosion in the pipes), and in any event, provide diminishing returns. It will be appreciated that the difference between the "heat soak" and "regeneration" modes of operation is a matter of degree, not kind. Also, the temperature dividing line between the latter two modes is somewhat arbitrary and could be drawn elsewhere.
In summary, in addition to the possibility of operation in regular Claus mode, embodiments of the invention may use three different ranges of temperature in at least two reactor beds:
(a) a sub-dewpoint mode of operation;
(b) a low elevated temperature range mode ("heat soak mode"); and (c) a high elevated temperature range mode ("regeneration mode").
If a milder form of regeneration is sufficient or required, "heat soak mode"
(or a similar low elevated temperature range) may be used. Advantageously, heat soak mode not only is capable of vaporizing sulphur (at an intermediate rate), but it can allow the Claus reaction to continue to proceed (at a diminished rate).
In other words, operating a reactor in heat soak mode increases the sulphur removal capacity of the incoming gas stream by a moderate amount without decreasing the efficiency of the Claus reaction too much.
If an aggressive form of regeneration is required, "regeneration mode" (or a similar high elevated temperature range) may be used. Advantageously, the regeneration mode will vaporize and remove the condensed sulphur more quickly than the heat soak mode, however, it also greatly decreases the efficiency of the Claus reaction. In some situations, this trade-off of speed/efficiency is worthwhile, e.g., when trying to regenerate the third or final stage reactor catalyst in a Claus system, it is advantageous to do this quickly as the preceding catalyst bed may fill up quickly in sub-dewpoint mode.
Operation In one embodiment, a method of recovering sulphur from a gas stream involves passing it through the Claus sulphur recovery unit 100 which has first, second and third stages (120, 130, 140) comprising first, second and third reactors (124, 134, 144). The raw gas stream input 121 to be processed typically originates from an upstream reaction furnace operating at extremely high temperatures in order to destroy certain kinds of molecules (e.g., ammonia, methane, etc.) and to produce elemental sulphur in vapour form. After passing through an upstream condenser which cools the gas stream to extract some of the elemental sulphur, the gas stream passes through the first reheater. The method involves operating the first reactor 124 to recover at least some sulphur in the first stage 120, wherein the gas stream entering (via 123) the first reactor is at a first temperature (e.g., about 220 C to about 240 degrees Celsius). Typically, the first temperature is above the sulphur dewpoint such that the first reactor 124 is operated in above-dewpoint mode. In addition to the heat input from the first reheater, the reaction within the first reactor itself will add up to about 90 C of temperature, raising the overall temperature to about 300 to about 330 C. This high temperature permits certain difficult chemical reactions to take place in the first reactor. The first condenser then removes the majority of the vaporous sulphur created in the first reactor.
The second reactor 134 is operated in above-dewpoint mode to recover sulphur in the second stage 130, for example, the gas stream entering the second reactor (via 133) may be at a temperature of 5 to 15 C above the sulphur dew point (approximately 200 to 220 C). Simultaneously, the third reactor 144 is operated in sub-dewpoint mode to increase the recovery of sulphur and increase the sulphur recovery efficiency. The gas stream entering the third reactor is at a temperature below the sulphur dew point (e.g., 140 C, or from about 130 C to about 160 C). These lower temperature are needed to operate the third reactor 144 in sub-dewpoint mode and may be attained by turning off or reducing the setpoint on the third reheater 142.
The respective gas streams (125, 135, 145) exiting the reactors are cooled by the first, second and third condensers (126, 136, 146) to condense some of the elemental sulphur existing in a vapour form into a liquid form. Liquid sulphur is discharged via one of the lines 127, 137, and 147 into the sulphur pit 150.
(Notably, very little sulphur is recovered in the condenser servicing the catalyst bed operating in sub-dewpoint mode. Sulphur is condensed in the catalyst bed and removed in batch operation during the regeneration cycle.) After the first, second, and third reactors (124, 134, 144) have operated for some time, due to the sub-dewpoint operation of the third reactor, its catalyst bed will approach capacity (i.e., it will become saturated with condensed sulphur to the extent that its catalytic conversion efficiency will be substantially impaired). At this juncture, the second reactor 134 is transitioned into operating in sub-dewpoint mode to increase its conversion efficiency, for example, by turning down or turning off the second reheater 132. Subsequently, the second reactor 134 is operated in sub-dewpoint mode while the third reactor 144 is operated above the sulphur dewpoint temperature in regeneration mode (e.g., at temperatures of about 250 to 300 C) in order to regenerate the catalyst in the third reactor 144. In some embodiments, at least a portion of the second reactor 134 is placed in sub-dewpoint mode before the third reactor 144 is taken out of sub-dewpoint mode to help avoid emission spikes. Because the input gas stream 141 into the third stage 140 has been cooled by the second condenser 136, the third reheater 142 is controlled to generate the higher temperatures necessary for the third reactor 144 to operate in regeneration mode. When the catalyst bed in the third reactor 144 has been suitably regenerated, the third reactor may be returned to operating in sub-dewpoint mode.
It will be appreciated that running the second reactor 134 in sub-dewpoint mode for even a short period of time will cause a degree of sulphur deposition on its catalyst bed. As a further step, the second reactor 134 may be transitioned to operate at a temperature above the sulphur dewpoint to remove deposited sulphur from the catalyst in the second reactor. The transition of the second reactor 134 may take place while (or, in some embodiments, before) the third reactor 144 is transitioning from regeneration mode to sub-dewpoint mode, provided that at least a portion of the second reactor 134 catalyst bed remains in sub-dewpoint mode until at least a portion of the third reactor 144 completes its transition to sub-dewpoint mode. Once the catalyst in the second reactor 124 has been adequately regenerated (or the sulphur is removed from it by heat soaking), the second reactor 134 may be configured to operate at a lower temperature in above-dewpoint mode (e.g., regular Claus mode), by suitably adjusting the second reheater 132. As discussed, at an appropriate point in this process, the third reactor will be transitioned to sub-dewpoint mode operation once again.
While operating the first, second and third reactors (124, 134, 144), as described above, the raw gas stream provided at the input 121 is serially fed through and processed in the first, second, and third Claus stages (120, 130, 140) in that order, to form the final processed gas stream 148. In particular, the gas stream 128 exiting the first stage 120 is fed (at 131) into the second stage 130 and the gas stream 128 exiting the second stage is fed (at 141) into the third stage 140, and no switching valves are used to modify the serial gas stream flow pattern during operation. As described above, the temporary operation of certain reactors in sub-dewpoint mode or above-dewpoint mode (e.g., regeneration mode) is attained by controlling the heat output of the respective reactor reheaters, and does not require diversion of gas flow.
Still further aspects of system operation will be discussed below with respect to a control system and a detailed simulation example.
Control System Figure 2 illustrates a control system, shown generally at 200, for controlling the modified Claus process in certain embodiments of the invention. The control system 200 is operable to control heating cycles in the overall Claus system 100, for example, to cause specific reactors to enter or exit sub-dewpoint mode and above-dewpoint mode at suitable points. The control system 200 may facilitate automatic and/or manual operation of the reheaters (e.g., 122, 132, 142) and setting temperature set points for the reheaters, in some cases in response to process parameters or sensor signals.
Referring to Figure 2, the control system 200 includes a processor circuit 210 which may be in communication with a memory 212 and other devices through an I/O bus or input/output circuit 214. In various embodiments, the processor circuit 210 may include, for example, a programmable logic controller or PLC, a microprocessor or microcontroller, an Application Specific Integrated Circuit or ASIC, a programmable circuit such as an FPGA, a multiprocessor network, or the like. The memory 212 may include, for example, a random access memory (RAM), a read only memory (ROM), a flash memory, a cache, and/or a set of control registers. The processor circuit 210 of the control system 200 also may be in I/O communication with a storage device 220 (e.g., a hard disk drive or remote memory), a network 218 via a network interface (e.g., Ethernet), a removable media device 222 (e.g., an optical drive). At least one of the memory 212, storage 220 and removable media, may store processor- or computer-readable instructions for directing the processor circuit 210 to carry out any of the methods described herein.
The control system 200 may include a human-machine interface (HMI) 216 operable to allow a user to interact with the control system 200, for example, to configure it for operation and/or to obtain process parameter information. The HMI 216 may be implemented by control software hosted on the processor circuit 210 and operably configured to display information on a display device to the user, as well as to receive user input, for example, via a keyboard or mouse.
Alternatively, the HMI 216 may be implemented by a standalone computer system interconnected with the control system 200. The HMI 216 may provide a graphical user interface (GUI). The GUI may provide a status display screen for the plant operator, providing a schematic of the entire sulphur plant and indicating the operational status thereof. The GUI may provide a clock or state diagram indicating where the process is in the cycle of operational states described herein and how far the process has to go to enter the next step or state (e.g., "step S4 will be initiated in 1 hour"). The GUI may provide a visual display of measured operational parameters (e.g., temperatures, emission rates) as compared to expected results. The GUI may also provide an interface for the operator to enter parameters and variables and otherwise configure the overall process. For example, the operator could use a keyboard or mouse to indicate that, instead of a 72 hour cycle, he or she wants a 60 hour or 80 hour cycle.
The operator may also review or edit sets of parameters associated with a particular kind of operation, e.g., fine-tuning the precise reheater heat output in regeneration mode. The HMI 216 may also annunciate alarm conditions and warnings.

Optionally, the control system 200 may be configured for operation from a remote location and/or configured to display process parameter information at the remote location via a connection to a network 218 such as the Internet.
The control system 200 may receive a plurality of inputs from the external world, including at least one temperature sensor input 230, at least one gas composition sensor input 232, and other parameter inputs 234 which are useful for controlling the system 100. The processor circuit 210 may be configured to control at least one reheater control line 240 and/or other outputs 244 to the external world in response to the plurality of inputs. It will be appreciated that multiple input or output signals can be multiplexed onto one electrical, optical or wireless connection. Alternatively, inputs/outputs may have their own dedicated electrical, optical or RF connections to the world.
The at least one temperature sensor input 230 may be electrically, optically or otherwise in communication with temperature sensors disposed in the piping going into each reactor (e.g., 123 goes to 124). Temperature measurements received from such temperature sensors may be used by the control system 200 as at least a partial basis for controlling the heat output of the reheaters (e.g., 122, 132, 142) to maintain the temperature of gas entering each respective reactor temperature within a suitable range in accordance with that reactor's current mode of operation. For example, the control system 200 may provide control signals 244 to control the amount of fuel gas being burned or the amount of electrical current consumed by each reheater, in response to the temperature of the gas entering each reactor (e.g., 124, 134, 144).
Alternatively or in addition, the at least one temperature sensor input 230 may be connected to temperature sensors such as thermocouples in the reactor bed. In one embodiment, one thermocouple is disposed near the top of the reactor bed, one thermocouple is near the middle, and one thermocouple is near the bottom of the bed. In other embodiments, catalyst beds may have more or fewer thermocouples. The instantaneous temperature at each level of the catalyst bed and rate of temperature change can be read by the control system 200 to track the bed operation and indicate the health of the catalyst.
The at least one gas composition sensor input 232 may be connected to at least one gas composition sensor, for example, the air demand analyzer (ADA) 152 disposed at an output end of system 100 to analyse the ratio of H2S and SO2 in the treated gas stream 148. In response to feedback from the air demand analyzer 152, the operating system 200 may adjust or control the amount of air that is mixed with the H2S at a front end of the system. In addition, the system 100 may include a Continuous Stack Emission Monitor (CSEM) downstream of the ADA 152 to continuously monitor and record the amount of SO2 that is leaving the facility. A CSEM output signal could be provided to the control system 200 as an input. Normally, the CSEM will provide immediate feedback relating to the sulphur recovery efficiency of the different modes of operation.
This information will allow for optimizing or simplifying the process temperatures and/or times for each of the sub-dewpoint and above-dewpoint modes of operation.
In certain embodiments, the control system 200 is operable to control the operation of at least two Claus converter stages (e.g., 130 and 140) connected in series to recover sulphur from a gas stream, for example: (i) the Claus converter stage 130 comprises a reheater 132, a reactor 134 and a condenser 134 connected in series, and (ii) the Claus converter stage 140 comprises a reheater 142, a reactor 144 and a condenser 146 connected in series. The control system includes the processor circuit 210, which is operably configured to control the two reheaters 132, 142 to selectively reheat gas entering the reactors 134, 144 to control a temperature of gas entering the reactor 134 and a temperature of gas entering the reactor 144. The control of the reheaters 132, 142 in such embodiments may involve the processor circuit 210 directing the control system 200 to do the following:
(a) during a first time period, controlling the reheater 132 to maintain the temperature of gas entering the reactor 134 above a sulphur dewpoint temperature to prevent the reactor 134 from operating in sub-dewpoint mode, and controlling the reheater 142 to permit the temperature of gas entering the reactor 144 to fall below the sulphur dewpoint temperature to permit the reactor 144 to operate in sub-dewpoint mode; and (b) during a second time period, controlling the reheater 132 to permit the temperature of gas entering the reactor 134 to fall below the sulphur dewpoint temperature to permit the reactor 134 to operate in sub-dewpoint mode, and controlling the reheater 142 to sufficiently raise the temperature of gas entering the reactor 144 above the sulphur dewpoint temperature to cause a catalyst in the reactor 144 to be regenerated within an operational time budget.
In general, the sulphur recovery efficiency is highest when the final reactor catalyst bed is operating in sub-dewpoint mode and the second reactor 134 is in regular Claus mode. In some embodiments, this may be accomplished by lowering the third reheater 142 set point temperature to between about 130 C
to about 160 C. Depending on the nature of the third reheater and its turndown ability, it may be possible to set the temperature even lower in idle mode or to turn off the third reheater altogether. In the meantime, the second reactor may be operated from about 5 to about 15 C above the sulphur dewpoint temperature, until the third reactor approaches its sulphur deposition capacity or some lower user specified limit or threshold.

Other embodiments of the control strategy are disclosed below, especially in respect of the detailed example provided below with reference to Figure 3.
The control strategy may use time interval set points to control the length of time spent in a particular operational regime (e.g., "operate the third reactor in sub-dewpoint mode at 140 C for 48 hours while the second reactor operates in regular Claus mode at 210 C"). The time interval setpoints may be pre-programmed into a memory 212 or storage device 220, 222 associated with the control system, or they may be calculated dynamically based on the current operational conditions. For example, the sulphur capacity of the second reactor 134 and the rate at which sulphur is deposited thereon in sub-dewpoint mode, may be used by the control system 200 to calculate (possibly subject to pre-configured safety margins) a maximum amount of time that can be spent by the second reactor 134 in sub-dewpoint mode, which in turn, limits the amount of time that can be spent by the third reactor 144 in regeneration mode. The latter time limit establishes the length of time that the third reactor 144 can be safely operated in sub-dewpoint mode in view of the rate of sulphur deposition in that mode. Once again, preconfigured safety margins may be part of the system's calculations.
The control system 200 may include Claus process modelling algorithms, or may be configured to accept input from simulation software such as Sulfsim. The control system 200 may also be operably configured to receive data from other systems, for example, data inputs indicating the gas composition, and use this information, in conjunction with operator input, to carry out calculations or perform control functions. In some embodiments, the system 200 may include user programmable features such as PLC ladder logic.
The control system 200 may take a clock input 236 (or it may generate a clock internally and provide it as an output 244), for controlling part or all of the system 100 (or for controlling a distributed control system that is configured to control the system 100). A set of operational parameters may be stored in memory to instruct the control system 200 take particular actions at particular times of the overall processing cycle. For example, the system 200 could be operably configured at time T = N*C + 48 hours, to transition the third reheater to regeneration mode and to turn off the second reheater, where N is the total number of completed cycles, C is the cycle length, and 48 hours is an offset.
Thus, in the case of a 100-hour overall cycle, the same set of actions would be initiated by the system at the following times: T=48, T=148, T=248, etc.
The control system 200 or a distributed control system could dynamically calculate control parameters with the processor circuit 210 by executing a control philosophy implemented by processor readable instructions in memory 212 based on a combination of inputs including: (1) operator inputs via the HMI
216, (2) measurements from temperature sensors 230, (3) measurements from gas composition sensors 232, and (4) other inputs received 234 (e.g., alarm or fault conditions), as in the following examples:
1.
The control system 200 could perform calculations to ensure that the third stage reactor is regenerated in a sufficiently short time that the second stage reactor does not get overloaded, bearing in mind that the sulphur content of the gas feed to which the second stage reactor is exposed is substantially higher than that of the third stage reactor and thus the second reactor will fill up with sulphur much faster in sub-dewpoint mode.
The amount of sulphur that would be being laid down in a reactor would be estimated by the processor circuit 210 by taking into account operational parameters such as gas composition and flow rates, operating temperatures, and the nature and condition of the reactor catalyst bed.
Thus, the control system 200 could control the reheater control outputs 240 such that the second stage reactor is kept in sub-dewpoint mode long enough to sufficiently regenerate the third stage reactor but not so long that the second reactor is overloaded.
2. The control system 200 could be configured to recognize that a high-temperature regeneration mode is preferred for regenerating the third stage reactor bed. On the other hand, while regenerating the second stage reactor, it may be undesirable in some embodiments to drive too much sulphur into the third stage reactor (which may go to a higher temperature and start to operate at lower efficiency). Thus, while it is desirable to regenerate the second stage reactor sufficiently quickly, if the amount of sulphur being sent to the third stage reactor would be too high in a high-temperature regeneration mode (e.g., about 270-290 C), the control system 200 could be configured to instead regenerate the second stage reactor more gently over a longer period (e.g., 48 hours) by using a lower-temperature heat soak mode (e.g., about 230 C).
3. In some embodiments, the ADA 152 may see a skewed H2S / SO2 ratio while a catalyst bed is being regenerated, especially at the beginning of regeneration. Given that the output of the ADA 152 is typically used to regulate the air to acid gas ratio, the program logic of the control system 200 may need to recognize the reason for the skewed H2S / SO2 ratio, to avoid making unnecessary drastic changes to the air/acid gas ratio in response. Another option may be to relocate (or add) an ADA device to a location downstream of the first converter where its H2S and SO2 readings will remain unaffected by catalyst regeneration.
4. In some embodiments, the system may have preconfigured safety margins for switching between reactor operating modes. Rather than waiting until a catalyst bed is fully loaded, mode transitions may be timed to occur earlier by a preset amount to avoid the risk of causing an H2S or SO2 emission spike. The safety margin may be configurable by a plant operator through the HMI 216 of the control system 200. In addition, the control system 200 may be operably configured to estimate the sulphur load capacity of a particular catalyst bed and to calculate the amount of time that that catalyst bed can remain in sub-dewpoint operation given a particular set of operating conditions (e.g., a particular acid gas mix being processed at a particular sub-dewpoint temperature with a particular mass of catalyst) before being saturated. The system 200 may further be operably configured to transition away from sub-dewpoint operation of the reactor bed before the point of saturation (minus the applicable safety margins, if any), in order to initiate an above-dewpoint mode of operation in which the catalyst bed is regenerated and restored as described herein.
In some embodiments, the system may rely on an upper sulphur deposition limit of the third catalyst bed to trigger a transition of the second reactor to sub-dewpoint operation, in order to prepare to regenerate the third bed. The upper sulphur deposition limit of a catalyst bed may represent an amount of deposited sulphur which will not degrade operation of the catalyst bed. It may be equal to the maximum capacity of the catalyst bed minus a system-calculated or user-defined safety margin.
5. If the inputs to the control system 200 indicate a system upset or fault condition, the control system 200 may be operably configured to cause the entire system 100 to gracefully transition to a default mode of operation, for example, to transition to regular Claus operation.
The above examples of control philosophy are not intended to be exhaustive or limiting. Furthermore, the optimal temperatures and times for each stage of the process for a given system operating under specific operational conditions can be estimated by the control system 200 through calculation and/or simulation.

Computer generated estimates and simulations can be verified by experimentation (e.g., field data) and improved by optimization.
Alternatively, a distributed control system could be implemented. For example, multiple control systems akin to the control system 200 in Figure 2 could be present at multiple points of the system and internetworked. Of course, a distributed control system could be much simpler than the one in Figure 2. For example, each reheater unit could simply take a clock signal as an input and determine based on the clock (or a "state" signal) which step of the process it should be carrying out at any given moment. Thus, each reheater could read or receive parameters which indicate whether it should be heating the gas stream, and if so, by how much, depending on the cycle time. That is, the control system would know what to do in each part of the cycle.
In some embodiments lacking a central or distributed control system, the method described could be implemented by manually switching the at least two reheater units (e.g., 132, 142) to the right temperatures at the right times.
EXAMPLE
An illustrative example is now given based on simulation results of a particular operational scenario in Sulfsim. In this example, a 3-bed Claus plant operating according to an embodiment of the invention was compared to a conventional 3-bed Claus plant operating in a conventional way (i.e., "the base case").
A medium quality acid gas having an H2S mole fraction of 0.528 was processed, the inlet gas feed containing 10 tons/day of sulphur equivalence. The base case in normal operation achieved a sulphur recovery efficiency of 98.15%. In comparison, the modified operating process (which included periodic sub-dewpoint mode and regeneration mode operation in the second and third catalyst beds as described herein) allowed the 3-bed Claus plant to achieve an average recovery efficiency of 98.9% over a 3-day cycle. Thus, the new method improved sulphur plant recovery efficiency by about 40% (under ideal conditions and assuming a steady gas feed in the plant).
Referring now to Figure 3, a set of graphs illustrating certain process parameters and conditions plotted against time (in hours) is shown generally at 300.
Graphs 306 and 312 show the reheater level controls for the second and third stage heaters, respectively. Graphs 302 and 308 show the average temperatures of the beds in the second and third reactors, respectively, as controlled or set by operation of the reheaters in response to the reheater level controls. Graphs and 310 illustrate the level of sulphur saturation or deactivation of the catalyst beds in the second and third reactors, respectively, where saturation gradually increases as reactors operates in sub-dewpoint mode and decreases as reactors operate in regeneration mode.
Graphs 306 and 312 show that the reheater level controls can assume one of a plurality of levels (in this embodiment: "off', "intermediate", and "high"):
1. As an example of a reheater being "off', graph 312 shows reheater #3 in this state at time = 4. The corresponding reactor temperature graph 308 at this time shows the third reactor in sub-dewpoint mode.
2. As an example of a reheater being controlled to reheat gas with an intermediate level of heat, graph 306 shows reheater #2 in this state at time = 4. The corresponding reactor temperature graph 302 at this time shows operation of the second reactor in an above-dewpoint mode.
3. As an example of a reheater being set to a high level of heating, graph 306 shows reheater #2 at a high setting at time = 64, with the corresponding reactor temperature graph 302 at time = 64 showing operation of the second reactor in regeneration mode at a temperature higher than during regular above-dewpoint operation (e.g., at time = 4).
In some embodiments, a reheater may include a bank of heaters, wherein turning on individual heaters in the bank allows step-wise increments in the amount of heat generated and applied to the gas stream. In other embodiments, there may be infinite levels of adjustment within the operational constraints of the heater.
For example, an electrical heater may be infinitely adjustable from "off' to its maximum heat output. Some acid gas burning heaters may be infinitely adjustable between their maximum turndown level (i.e., a minimal flame just short of being blow out) to its maximum output level.
Still referring to Figure 3, an exemplary embodiment of the process of the invention will now be described in detail. Step one (designated as "S1" in Figure 3) involves operating the second reactor above-dewpoint at a normal Claus temperature of about 200 to 220 C (e.g., about 210 C) while running the third reactor in sub-dewpoint mode at a temperature in the range of about 130 to 160 (e.g., about 140 C at the third reactor inlet). The equilibrium temperature inside the third stage reactor depends on the outlet temperature of the second stage condenser, the turndown ability of the third reheater, the amount of H2S and in the acid gas feed at that point, and other factors. This phase of operation may last about 24 to 48 hours (or even longer, if the third catalyst bed has sufficient capacity). The duration of this phase depends on how quickly the final reactor can be regenerated. Preferably, the final bed would spend a long time in sub-dewpoint mode, and a relatively short time in regeneration mode.
Step two ("S2" in Figure 3) is a transition step in which the second reactor is placed into sub-dewpoint mode by reducing the outlet temperature of the second reheater to about 140 C to 160 C. This phase could take about 1 to 6 hours, before step three ("S3") is undertaken to change the operation of the third reactor. During this period of time, no change is made to the operation of the third reactor, which continues to operate in sub-dewpoint mode.
Step three ("S3" in Figure 3) provides for the regeneration of the catalyst bed in the third reactor. The regeneration of the final catalyst bed removes the substantial amount of liquid sulphur which has condensed in the bed during steps S1 and S2. To continue to operate the third reactor in sub-dewpoint mode at this point would cause the bed to exceed its sulphur bearing capacity, i.e., it would eventually lead to saturation and deactivation of its catalyst bed, thereby causing a gradual increase in SO2 emissions. To avoid this, the third reactor is put into regeneration mode by setting the outlet temperature of the third reheater in the range of about 250 to 300 C. The time for regeneration of the third (final) reactor depends on variables such as gas flow rate, outlet temperature of the third reheater, converter size, and the amount of sulphur condensed in the third reactor that needs to be removed.
In a fourth step ("S4" in Figure 3), the catalyst bed in the second reactor is transitioned into a heat soak or regeneration mode. Accordingly, the temperature of the second reheater is increased above the sulphur dewpoint and may vary from about 225 to 300 C, for example. In some embodiments, this step may last between 6 to 18 hours, but the duration will depend on many operational factors such as flow rate, gas temperature, reactor size, etc.
Notably, there is no immediate urgency to remove the condensed sulphur in the upstream catalyst bed. The upstream reheater temperature setpoint can be restored to normal operation (or normal plus about 10 C to 30 C) and the condensed sulphur in the catalyst bed will be removed over a longer period of time. The function of the upstream bed is to provide a gas that is relatively lean in H2S and SO2 (resulting from the bed being in sub-dewpoint mode), and this lean H2S and SO2 gas is used to regenerate the final bed. The sulphur recovery efficiency may be the lowest at step S3 and part of step S4.
Notably, the process can take advantage of the fact that temperature transitions within the catalyst bed are gradual and layered. Having previously put the second reactor in sub-dewpoint mode, there is at least a portion of the second catalyst bed at a "cold" temperature (e.g., about 140 C). It is possible to begin adding heat to the second catalyst bed before the regeneration of the third catalyst bed is complete because the "cold" layer will work its way down the second bed for a few hours while continuing to achieve higher sulphur recovery, even as heat is added to the top portion of the second catalyst bed.
A fifth step ("S5" in Figure 3) involves returning the operation back to the main operating mode, as described with reference to step one (S1). In this step, the temperature of the third reheater is decreased to about 130 to 160 C, such that the third reactor operates in sub-dewpoint mode. The fifth step may be initiated while the transition in step four (S4) is in progress, which, it will be recalled, involves initiating the regeneration or heat soak of the second reactor. The timing of the fifth step is dependent on a number of operational factors and may commence, e.g., about 2 to 6 hours after Step 4 (S4) starts.
Step S5 can be initiated before the full completion of step S4 without causing an emission spike. While most of the third reactor bed may be at a high temperature (e.g., at time 1=60 to 62 hrs) and the second reheater has been suddenly turned up, at least a portion of the second bed will remain in sub-dewpoint operation until the "hot front" percolates its way down from the top of the second bed towards the bottom. By the time all the catalyst in the second bed heats up sufficiently to leave sub-dewpoint mode operation, at least a portion of the third bed will have cooled down enough to enter sub-dewpoint operation (as its "cold front" percolates its way down the catalyst bed).

In a sixth and final step ("S6" in Figure 3), in order to return to step one (S1), the second reheater temperature is lowered to about 200 to 220 C (i.e., normal service) after the second reactor has had a sufficient amount of deposited sulphur removed (e.g., by previously undergoing an elevated temperature regime such as heat soak mode or regeneration mode). Once the temperature transition inside the second reactor completes (i.e., once steady state or temperature equilibrium is achieved), in effect, the second rector will have been returned to regular Claus mode operation (S1).
Table 1 shows the states of the last two reactors over one complete cycle.
Table I. Summary of Reactor States During One Cycle Step Penultimate Reactor Ultimate Reactor Notes S1 above-dewpoint (normal) sub-dewpoint (longest step) S2 transition to sub-dewpoint sub-dewpoint S3 sub-dewpoint transition to regeneration S4 transition to heat soak or regeneration regeneration mode S5 heat soak or regeneration transition to (may overlap S4) sub-dewpoint S6 transition to normal Claus sub-dewpoint (go to S1) above-dewpoint operation Table 2 shows the states of the reheaters over one complete cycle (Fig. 3).
Table 2. Summary of Reheater States During One Cycle in Fig. 3 Step Penultimate Reheater Ultimate Reheater Time (hrs) S1 medium off or low 1=0 to 48 S2 TURN off or low off or low T=48 to 54 S3 off or low TURN high T=54 to 60 S4 TURN med.-high* or high high T=60 to 66 S5 med.-high* or high TURN off or low T=62 overlaps S4 S6 TURN medium off or low T=68 (ao to S1) * "med.-high" means "medium high", which is higher than "medium" but lower than "high" (each of which are above-dewpoint temp. ranges) Step six (S6) thus brings the process full circle to the first step (S1).
Steps S1 to S6 may be repeated many times. In the embodiment illustrated in Figure 3, the cycle time for proceeding through all these steps is about 3 days overall.
Several observations made based on the above example are as follows:
I. During sub-dewpoint mode of the 3rd reactor (inlet temperature of 140 C), the sulphur recovery efficiency was calculated to be 99.15%;
2. In this sub-dewpoint mode, the third reactor was condensing and holding about 0.2 ton/day of sulphur, accumulating 0.5 tons in 2.5 days;
3. Then, the second reactor was placed in sub-dewpoint mode for only about 12 hours, to allow the third reactor to be quickly regenerated with partially processed sour gas that had smaller amounts of H25 and SO2;
4. Despite the use of regeneration mode, the sulphur recovery efficiency of this embodiment (98.4%) was still higher than the base case scenario;
Other Embodiments In some embodiments, a modified 3-bed Claus sulphur recovery system is operated during a first time period with the first and second beds operating in Claus sulphur recovery mode and the third bed operating in sub-dewpoint mode, wherein the third bed operates in a regeneration mode during a second time period during which the preceding bed is operated in sub-dewpoint mode at least until the third bed has been sufficiently regenerated to resume operation in sub-dewpoint mode.
It will be appreciated that no special switching valves are needed to cause the third bed to be regenerated with hot gas, nor is there any need to take the third bed offline during regeneration thereof. Thus, as in many previously described embodiments, specific problems can be avoided, including: valves switching out of sequence, leaking, becoming stuck, temperature spikes, emission spikes, or troubleshooting difficulties (given that two or more valves may switch at about the same time in some competing sub dew point systems).
It will be appreciated that reheaters will be designed for sufficient turndown and adequate duty for regeneration. In addition, the condenser duty needs to be adequate to condense sulphur-containing gas from the reactor outlet that has a high loading of sulphur and is at a higher inlet temperature.
Plants that process more than 10 tonnes/d normally have three Claus stages. In some embodiments, a Claus system may be provided that operates in the manner of any of the aforesaid 3-bed Claus systems, however, a fourth Claus stage, added at the downstream end, is not operated in sub-dewpoint mode but is used simply for "polishing" the gas stream in regular Claus mode.
In some embodiments, the amount of catalyst that is put into each reactor bed may be optimized for the improved sulphur removal process described herein. In a normal Claus plant, the same amount of catalyst is typically placed in each bed. For example, in some embodiments of this invention, it might be advantageous to put more catalyst into the second reactor, to potentially allow the second reactor to stay longer in sub-dewpoint mode, which opens the door to allowing the third reactor to spend more time in regeneration mode, which in turn, allows the third reactor to operate longer in sub-dewpoint mode.

In different embodiments, there would be variations in the time and temperature of the regeneration and sub-dewpoint modes depending on the acid gas composition leaving the first reactor, the size of the second and third reactors (or the volume of catalyst present), and the amount of flow of acid gas through them.
Regeneration time will depend on the amount of condensed sulphur in a reactor bed needing removal, the mass flow rate and composition of sour gas through the bed, and the temperature of the regeneration gas (as may be controlled by a reheater), for example. Regeneration time may be also affected by channelling or by-passing in the bed. Generally, the hotter the regeneration temperature, the faster the sulphur is removed; higher temperature regeneration gas can evaporate more sulphur.
Rapid catalyst regeneration will typically occur at temperatures ranging from about 250 to 300 C. It will be appreciated that the condenser will be adequately sized to handle the increased heat load during regeneration. The operational sweet spot for maximum sulphur recovery for a set of conditions can be found by optimization including preliminary calculations and follow-up simulations and/or testing.
While specific embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims. It should also be appreciated that the embodiments disclosed herein are not necessarily mutually exclusive such that features of one embodiment may be combined with those of another embodiment to form further embodiments falling within the scope of the claims. Similarly, it will be appreciated that various other combinations and permutations of the components and steps described herein may form further embodiments falling within the scope of the invention described in the claims.

Claims

What is claimed is:

1. A method of removing sulphur from a gas stream, comprising:
(a) serially feeding a gas stream through first, second, and third Claus stages connected in series, each comprising a respective reactor with a catalyst bed, to remove sulphur from the gas stream; and the method further comprising:
(b) heating the gas stream entering the first reactor in the first Claus stage and heating the gas stream originating from the first reactor and entering the second reactor in the second Claus stage;
(c) operating the third reactor in the third Claus stage in sub-dewpoint mode in which the temperatures across the entire third reactor catalyst bed are at a steady state, by maintaining the temperature of the gas stream originating from the second Claus stage and entering the third reactor in the third Claus stage at a first temperature based on a first temperature setpoint;
(d) as the third reactor approaches an upper sulphur deposition limit, transitioning the second reactor to operate in sub-dewpoint mode in which the temperatures across the entire second reactor catalyst bed are at a steady state, by reducing the temperature of the gas stream originating from the first Claus stage and entering the second reactor, and heating the treated gas stream originating from the second Claus stage and entering the third reactor to a second temperature based on a second temperature setpoint to regenerate the catalyst bed of the third reactor by vaporizing sulphur deposited in the third reactor; and then, (e) heating the gas stream originating from the first Claus stage and entering the second reactor, to vaporize sulphur deposited in the second reactor, and returning the third reactor to sub-dewpoint mode in which the temperatures across the third reactor catalyst bed are at a steady state, wherein the gas stream flows through the first Claus stage, the second Claus stage, and the third Claus stage sequentially during steps (b) through (e).

2. The method of claim 1 wherein in step (c), the third reactor is operated for a time period that is at least twice as long as the time for regenerating the entire catalyst bed in step (d).

3. The method of claim 1 wherein, in transitioning step (d), the gas stream entering the second reactor is not heated.

4. The method of claim 1 wherein transitioning the second reactor to sub-dewpoint mode comprises heating the gas stream entering the second reactor to a temperature of 140°C to 150°C.

5. The method of claim 1 further comprising, following step (d), transitioning the second reactor to heat soak mode in which the second reactor is operated at 30 C to 50 C higher than the sulphur dewpoint temperature.

6. The method of claim 1 wherein step (e) further comprises transitioning the second reactor to normal Claus mode operation.

7. The method of claim 1 wherein the third reactor is operated in sub-dewpoint mode for at least 12 hours in step (c).

8. The method of claim 1 wherein the entire catalyst bed of the third reactor is regenerated before the second reactor is saturated with sulphur due to the second reactor operating in sub-dewpoint mode.

9. The method of claim 1 further comprising heating the third reactor in step (d) to regenerate the catalyst bed of the third reactor only until the second reactor begins to approach capacity in sub-dewpoint mode.

10. The method of claim 9 wherein beginning to approach capacity comprises approaching only within a predefined safety margin of actual capacity of the second reactor before entering step (e).

11. The method of claim 1 further comprising operating each of the reactors in the first and second Claus stages in normal Claus mode prior to step (d).

12. The method of claim 1 further comprising controlling a reheater to heat the gas stream to the third reactor in step (d) to a temperature of between about 250°C
and about 300°C.

13. The method of claim 1 wherein heating the gas stream entering the second reactor comprises controlling the gas stream's temperature to be sufficient to vaporize all of the sulphur condensed in the second reactor within a predetermined amount of time.

14. A method of processing sour gas in a Claus sulphur recovery system having at least an upstream Claus stage and a downstream Claus stage, the upstream Claus stage comprising a first catalyst bed and first condenser connected in series and the downstream Claus stage comprising a second catalyst bed and a second condenser connected in series, the method comprising:
(a) receiving a first input gas stream from an upstream source;
(b) passing the first input gas stream through the upstream Claus stage, including the first catalyst bed and the first condenser, to produce a second input gas stream;
and (c) passing the second input gas stream through the downstream Claus stage, including the second catalyst bed and the second condenser, to produce a processed output gas stream;
wherein steps (a)-(c) further comprise:
(d) during a first phase of processing the sour gas, operating the first catalyst bed in above-dewpoint mode to recover at least some sulphur from the first input gas stream to produce the second input gas stream, and operating the second catalyst bed in sub-dewpoint mode in which the temperatures across the entire second catalyst bed are at a steady state, by controlling reduction of the inlet temperature of the second input gas stream based on a first setpoint, to recover additional sulphur from the second input gas stream to produce the output gas stream;

(e) during a second phase of processing, following the first phase, transitioning the first catalyst bed to operate in sub-dewpoint mode in which the temperatures across the entire first catalyst bed are at a steady state by controlling reduction of the inlet temperature of the first input gas stream based on a second setpoint, while continuing to operate the second catalyst bed in sub-dewpoint mode in which the temperatures across the entire second catalyst bed are at a steady state to produce the output gas stream; and (f) during a third phase of processing, following the second phase, continuing to operate the first catalyst bed in sub-dewpoint mode in which the temperatures across the entire first catalyst bed are at a steady state to recover additional sulphur from the first input gas stream, and transitioning the second catalyst bed to operate in catalyst regeneration mode by controlling increase of the inlet temperature of the second input gas stream based on a third setpoint, to regenerate the second catalyst bed by vaporizing condensed sulphur deposited on the second catalyst bed.

15. The method of claim 14 further comprising:
(g) during a fourth phase of processing, following the third phase, transitioning the first catalyst bed to operate in heat soak mode or catalyst regeneration mode, while continuing to operate the second catalyst bed in catalyst regeneration mode.

16. The method of claim 15 further comprising:
(h) during a fifth phase of processing, following the fourth phase, operating the first catalyst bed in above-dewpoint mode, while transitioning the second catalyst bed to operate in sub-dewpoint mode in which the temperatures across the entire second reactor catalyst bed are at a steady state.

17. The method of claim 16 wherein the fifth phase comprises operating the first catalyst bed in normal Claus mode.

18. The method of claim 16 wherein the fifth phase comprises operating the first catalyst bed in heat soak mode.

19. The method of claim 16 further comprising:
(i) during a sixth phase, following the fifth phase, (A) processing the first input gas stream received from the upstream source using the first catalyst bed and first condenser, with the first catalyst bed operating in normal Claus recovery mode, to produce the second input gas stream; and (B) processing the second input gas stream using the second catalyst bed and second condenser, by operating the second catalyst bed in sub-dewpoint mode in which the temperatures across the entire second catalyst bed are at a steady state, to produce the processed output gas stream.

20. The method of claim 14 further comprising, during the first phase, processing the second input gas stream with the second catalyst bed operating in sub-dewpoint mode in which the temperatures across the entire second catalyst bed are at a steady state until the second catalyst bed is substantially saturated with sulphur, and then commencing the second phase of processing.

21. The method of claim 14 further comprising regenerating the second catalyst bed during the third phase by heating the second input gas stream with a reheater located upstream of the second catalyst bed and downstream of the first condenser in the upstream Claus stage.

22. The method of claim 14 wherein the third phase is shorter than the first phase.

23. The method of claim 14 wherein the upstream source is an upstream condenser associated with a preceding upstream Claus catalyst bed.

24. A method of recovering sulphur from a gas stream by operating a Claus sulphur recovery system having first, second, and third stages comprising first, second, and third reactors with a respective catalyst bed, the method comprising:
(a) operating the second reactor in above-dewpoint mode by controlling reduction of the inlet temperature of the gas stream entering the second reactor while operating the third reactor in sub-dewpoint mode in which the temperatures across the entire third reactor catalyst bed are at a steady state, by controlling increase of the inlet temperature of the gas stream entering the third reactor based on a first setpoint, to recover at least some sulphur from the gas stream in each of the second and third stages; and then, (b) operating the second reactor in sub-dewpoint mode in which the temperatures across the entire second reactor catalyst bed are at a steady state, by controlling reduction of the inlet temperature of the gas stream entering the second reactor based on a second setpoint, to recover at least some sulphur from the gas stream in the second stage while operating the third reactor in above-dewpoint mode by controlling increase of the inlet temperature of the gas stream entering the third reactor based on a third setpoint to regenerate all of the catalyst in the third reactor by vaporizing the sulfur deposited in the third reactor;
wherein during steps (a) and (b), the gas stream entering the first stage of the Claus sulphur recovery system is passed through to the second stage and then passed through the second stage to the third stage sequentially.

25. The method of claim 24 further comprising transitioning the second reactor to sub-dewpoint mode before the third reactor is placed in above-dewpoint mode such that at least a portion of each of the second and third reactors is in sub-dewpoint mode simultaneously.

26. The method of claim 24 further comprising:
operating the second reactor in above-dewpoint mode to regenerate the catalyst in the second reactor while operating the third reactor in sub-dewpoint mode in which the temperatures across the entire third reactor catalyst bed are at a steady state.

27. A control system for a Claus sulphur recovery system, the control system operable to implement the method of any one of claims 1-26.

28. A non-transitory computer-readable medium storing codes for instructing a processor circuit to execute the control steps of the method of any one of claims 1-26.

29. A control system operable to control the operation of upstream and downstream Claus converter stages interconnected to recover sulphur from a gas stream, wherein (i) the upstream Claus converter stage comprises a first reheater, a first reactor having a first catalyst bed, and a first condenser connected in series, and (ii) the downstream Claus converter stage comprises a second reheater, a second reactor having a second catalyst bed, and a second condenser connected in series, the control system comprising:
at least one processor circuit operably configured to control the first and second reheaters to selectively reheat the respective portions of the gas stream about to enter the first and second reactors to have a first and second temperature, respectively, by:
(a) in a first time period, controlling the first reheater to maintain the first temperature sufficiently high to prevent the first reactor from operating in sub-dewpoint mode, and controlling the second reheater to permit the second temperature to fall sufficiently to cause the second reactor to operate in sub-dewpoint mode in which the temperatures across the entire second catalyst bed are at a steady state; and (b) in a second time period, controlling the first reheater to permit the first temperature to fall sufficiently to permit the first reactor to operate in sub-dewpoint mode in which the temperatures across the entire first catalyst bed are at a steady state, and controlling the second reheater to raise the second temperature sufficiently high to cause a catalyst in the second reactor to be regenerated.

30. The system of claim 29 wherein the processor circuit is in communication with a first temperature sensor measuring the first temperature of gas entering the first reactor and a second temperature sensor measuring the second temperature of gas entering the second reactor.

31. The system of claim 29 wherein, during the second time period, the first reheater is controlled to allow the first temperature to fall sufficiently to cause the first reactor to enter and be operated in sub-dewpoint mode before the second reheater is controlled to raise the second temperature to regenerate the catalyst in the second reactor.

32. The system of claim 30 further comprising:
(a) at least one input line for providing temperature data from the first and second temperature sensors to the processor circuit;
(b) at least one output control line allowing the processor circuit to control the operation of the first and second reheaters.

33. The system of claim 29 wherein, during the first time period, the second reheater is turned off by the processor circuit long enough to allow the second reactor to enter and be operated in sub-dewpoint mode.

34. The system of claim 33 wherein, during the second time period, the first reheater is turned off by the processor circuit long enough to allow a substantial portion of the first reactor to enter and be operated in sub-dewpoint mode.

35. The system of claim 29 wherein the first and second reheaters are controlled using setpoints corresponding to operational states of the first reactor and the second reactor.