WO2021073917A1

WO2021073917A1 - Process for producing an ammonia synthesis gas

Info

Publication number: WO2021073917A1
Application number: PCT/EP2020/077801
Authority: WO
Inventors: Per Juul Dahl
Original assignee: Haldor Topsøe A/S
Priority date: 2019-10-15
Filing date: 2020-10-05
Publication date: 2021-04-22
Also published as: AR120216A1

Abstract

A process for producing an ammonia synthesis gas, said process comprising the steps of: - Reforming a hydrocarbon feed in a reforming step thereby obtaining a synthesis gas comprising CH₄, CO, CO₂, H₂and H₂O, - Shifting said synthesis gas in a high temperature shift step over a promoted zinc-aluminum oxide based high temperature shift catalyst, wherein the steam/carbon ratio in the reforming and HT shift step is less than 2.6.

Description

Title: Process for producing an ammonia synthesis gas

Following today’s demand and competitiveness in ammonia production, significant ef forts have been put into developing optimized production for ammonia plants, with the objective to improve overall energy efficiency and reduce capital cost. The need for more cost-efficient ammonia production has spurred the development of technology and catalysts for large-scale ammonia production units, in order to benefit from econ omy of scale.

Topsoe’s latest innovations within ammonia production technology and the develop ment of a new generation of state-of-the-art catalysts ensures highly cost efficient am monia production and high plant reliability also for single line capacities of 5000 MTPD ammonia or more where todays standard is up to only 3300 MTPD.

In a first aspect of the present invention is provided a process enabling a process scheme utilizing proven reforming technology operating with low steam/carbon.

In a second aspect of the present invention is provided a process enabling operation of the high temperature (HT) shift downstream the reforming section at the same low steam/carbon ratio as the reforming section.

In a third aspect of the present invention is provided a process scheme without the ne cessity of a methanation section to remove residual carbon components in the make up synthesis gas for the ammonia synthesis.

In a fourth aspect of the present invention is provided an overall process layout ena bling maximum single line capacity.

These and further advantages are achieved by a process for producing an ammonia synthesis gas, said process comprising the steps of:

Steam reforming of a hydrocarbon feed followed by oxygen reforming thereby obtaining a synthesis gas comprising CFU, CO, CO2 , H2 and H2O Shifting said synthesis gas in a high temperature shift step over a promoted zinc-aluminum oxide based high temperature shift catalyst, wherein The steam/carbon ratio in the reforming step is less than 2.6.

The wording “oxygen reforming” as used herein and in the following description and claims refers to oxygen blown reforming or autothermal reforming (ATR)

HT shift is defined as a process step where a synthesis gas containing CO, CO2, H2 and H2O undergoes the shift reaction in the temperature range from 300 °C to 600⁰

In a conventional ammonia plant the standard use of iron based HT shift catalyst re quires a steam/carbon ratio of around 3.0 to avoid iron carbide formation.

(1) 5Fe₃0₄ + 32CO <-» 3Fe₅C₂ + 26 C0₂

Formation of iron carbide will weaken the catalyst pellets and may result in catalyst dis integration and pressure drop increase.

Iron carbide will catalyse Fischer-Tropsch by-product formation

(2) nCO + (n+m/2)H <-» C_nH_m + nH0

The Fischer-Tropsch reactions consume hydrogen, whereby the efficiency of the shift section is reduced.

However, according to the present invention a non Fe-catalyst is used, such as a pro moted zinc-aluminum oxide based catalyst. For example the Topsoe SK-501 Flex™ HT shift catalyst which enables operation of the reforming section and HT shift section at a steam/carbon ratio down to 0.3.

Thus, the present process operating at a steam/carbon ratio down to 0.8 is in contrast to today’s traditional ammonia plants which are based on reforming and/or HT shift sections operating at a steam/carbon ratio of 2.6 or higher. In advantageous embodi ments of the process the zinc-aluminum oxide based catalyst in its active form com prises a mixture of zinc aluminum spinel and zinc oxide in combination with an alkali metal selected from the group consisting of Na, K, Rb, Cs and mixtures thereof, and optionally in combination with Cu. The catalyst may have a Zn/AI molar ratio in the range 0.5 to 1.0, a content of alkali metal in the range 0.4 to 8.0 wt % and a copper content in the range 0-10% based on the weight of oxidized catalyst.

The HT shift catalyst used according to the present process is not limited by strict re quirements to steam to carbon ratios, which makes it possible to reduce steam/carbon ratio in the shift section as well as the reforming section.

A steam/carbon ratio of less than 2.6 has several advantages. Reducing steam/carbon ratio on a general basis leads to reduced feed plus steam flow through the reforming section and the downstream cooling and synthesis gas preparation sections.

Low steam/carbon ratio on reforming section and shift section enables higher syngas throughput compared to high steam/carbon ratio. Nitrogen added via the Nitrogen wash enables higher syngas throughput compared to adding nitrogen in the reforming sec tion. No methanation section reduces pressure loss and using an inert free gas in the ammonia synthesis section enables higher throughput in the ammonia synthesis sec tion

Reduced mass flow through these sections means smaller equipment and piping sizes. The reduced mass flow also results in reduced production of low temperature calories, which can often not be utilised. This means that there is a potential for both lower CAPEX and OPEX.

The present process may further comprise one or more of the following steps:

Shifting the HT shift outlet gas in one or more medium temperature(MT)/low temperature(LT) shift step(s). The MT/LT shift step(s) can optionally be per formed at a higher steam/carbon ratio than the HT shift to limit byproduct for mation such as methanol.

Optionally remove methanol from the MT/LT shift outlet gas in a water wash Remove C0₂ from the MT/LT shift outlet gas/water wash outlet gas down to a level lower than 500 ppm preferably down to below 20 ppm.

Remove residual CO2 and H2O from the gas leaving the CO2 removal section in a molecular sieve dryer section.

Remove ChU, CO and inerts such as Ar and He from the gas leaving the molec ular sieve dryer section in a nitrogen wash section and adjust the N2/H2 ratio to approximate 3 as needed for the ammonia synthesis.

Convert the adjusted outlet gas from the nitrogen wash to ammonia in an inert free ammonia synthesis section.

- An alternative cleaning method of the gas leaving the CO2 absorber is to send it through a PSA. The pure hydrogen stream can be mixed with N2 from from the ASU to a preferred ratio H2/N2 ratio of approximate 3 in the make-up stream to the ammonia synthesis loop. The off-gas the PSA can be used as fuel in the steam reformer

In preferred embodiments the reforming step comprises at least an autothermal re former (ATR) being operated with oxygen an oxygen source containing above 90%, preferably above 98 % oxygen .

As the requirements to the steam/carbon ratio in the HT shift step by the present pro cess is significantly reduced compared to known technologies it is possible by the pre sent invention to reduce steam/carbon ratio through the front end to e.g. 0.8 or as low as possible dependent on the possible shift solutions. An advantage of a low steam/carbon ratio to the reforming step and in the overall process is that smaller equipment is required in the front-end due to the lower total mass flow through the plant.

The carbon feed for the ATR is mixed with an oxygen source and additional steam in the ATR, and a combination of at least two types of reactions take place. These two re actions are combustion and steam reforming.

Combustion zone:

(3) 2¾ + 0₂ 2¾0 + heat

(4) CH₄ + 3/20₂ CO + 2¾0 + heat Thermal and catalytic zone:

(5) CH₄ + H₂0 + heat <-» CO + 3¾ (6) CO + H₂0 <- C0₂ + H₂ + heat

The combustion of methane to carbon monoxide and water (4) is a highly exothermic process. Excess methane may be present at the combustion zone exit after all oxygen has been converted. The thermal zone is part of the combustion chamber where further conversion of the hydrocarbons proceed by homogenous gas phase reactions, mostly (5) and (6). The endothermic steam reforming of methane (5) consumes a large part of the heat devel oped in the combustion zone. Following the combustion chamber there may be a fixed catalyst bed, the catalytic zone, in which the final hydrocarbon conversion takes place through heterogeneous catalytic reactions. At the exit of the catalytic zone, the synthesis gas preferably is close to equilibrium with respect to reaction (5) and (6). The steam/carbon ratio in the reforming section may be 2.6 - 0.8, 2.4 - 1.0 or 2.0 - 1.2, such as 1.8 or 1.5.

The steam/carbon ratio is defined as the ratio of all steam added to the reforming sec tion and the HT shift section i.e. steam which may have been added to the reforming section via the feedgas, oxygen feed, by addition to burners, by addition to the HT shift reactors etc. and the hydrocarbons in the feedgas to the reforming section on molar ba sis.

In advantageous embodiments the space velocity in the ATR is low, such as less than 20.000 Nm³ C/m³/h, preferably less than 12.000 Nm³ C/m³/h and most preferably less 7000 Nm³ C/m³/h. The space velocity can be defined as the volumetric carbon flow per catalyst volume and is thus independent of the conversion in the catalyst zone. In preferred embodiments the temperature in the HT shift step is in the range 300 - 600°C, such as 360-470°C. This means that according to the present process it is pos sible to run a high temperature shift reaction on a feed with much lower steam/carbon ratio than possible by known processes. For example the high temperature shift inlet temperature may be 300 - 400°C, such as 350 - 380°C.

Preferably a prereformer is provided as part of the reforming section upstream the steam reforming . In the prereformer all higher hydrocarbons can be converted to car bon oxides and methane, but also for light hydrocarbons the prereformer is advanta geous. The prereformer may provide an efficient sulphur guard resulting in a practical sulphur free feed gas entering the steam reformer and the downstream system. The prereforming step may be carried out at temperatures between 300 - 650°C, preferably 390-480°C.

The low steam/carbon ratio may result in a lower than optimal shift conversion which means that it in some embodiments may be advantageous to provide one or more ad ditional shift steps. The one or more additional shift steps may include a MT shift and/or a LT shift and/or a HT shift. Generally speaking, the more converted CO in the shift steps the more gained H2 and the smaller front end required.

This is also seen from the exothermic shift reaction given below

(7) CO + H₂0 <-» C0₂ + H₂ + heat

Steam may optionally be added to the shift section such as before one or more of the HT, MT or LT shift steps in order to maximize performance

Having two or more HT shift steps in series (such as a HT shift step comprising two or more shift reactors in series e.g. with the possibility for cooling and/or steam addition in between) may be advantageous as it may provide increased shift conversion at high temperature which gives a possible reduction in required shift catalyst volume and therefore a possible reduction in capex. Furthermore, high temperature reduces the for mation of methanol, a typical shift step byproduct. Preferably the MT and LT shift steps may be carried out over promoted cop- per/zink/aluminia catalysts. For example the low temperature shift catalyst type may be LK-821-2, which is characterized by high activity, high strength, and high tolerance to wards sulphur poisoning. A top layer of a special catalyst may be installed to catch pos sible chlorine in the gas and to prevent liquid droplets from reaching the shift catalyst.

The MT shift step may be carried out at temperatures at 190 - 360°C.

The LT shift step may be carried out at temperatures at T_dew+15 - 290°C, such as, 200 - 280°C. For example the low temperature shift inlet temperature is from T_dew+15 - 250°C, such as 190 - 210°C.

Reducing the steam/carbon ratio leads to reduced dew point of the process gas, which means that the inlet temperature to the MT and/or LT shift steps can be lowered. A lower inlet temperature can mean lower CO slippage outlet the shift reactors.

It is well known that MT/LT shift catalysts are prone to produce methanol as byproduct. Such byproduct formation can be reduced by increasing steam/carbon. The CO2 wash following the MT/LT shifts requires heat for regeneration of the CO2 absorption solution. This heat is normally provided as sensible heat from the process gas but this is not al ways enough. Typically an additionally steam fired reboiler is providing the missing heat. Optionally adding steam to the process gas can replace this additionally steam fired reboiler and simultaneously ensures reduction of byproduct formation in the shift section.

The methanol formed by the MT/LT shift catalyst can optionally be removed from the synthesis gas in a water wash to be placed upstream the CO2 removal step or on the CO2 product stream.

In many advantageous embodiments a CO2 removal step may be carried out af ter/downstream the one or more shift steps. In standard design the CO2 content is 500 vppm in the treated gas. In preferred embodiments a CO₂ removal step may be used to bring the CO₂ content down to less than 400 vppm CO₂, such as below 100 vppm or in some preferred em bodiments down to 20 vppm or below.

The process may further comprise a washing step, preferably a N₂ wash. The N₂ wash may serve several purposes such as purification of the syngas as well as to add the stoichiometric required nitrogen for a downstream ammonia synthesis.

The nitrogen for the N₂ wash unit (NWU) may be supplied by an air separation unit (ASU) which separates atmospheric air into its primary components nitrogen and oxy gen. The oxygen is used in the ATR and the nitrogen in the NWU.

After the one or more shift sections and CO₂ removal unit the gas may contain residual CO and CO₂ together with small amounts of ChU, Ar, He and H₂O.

CO₂ and H₂O are preferably removed before the N₂ wash because they otherwise would freeze at the low operating temperature of the N₂ wash. This may for example be done by adsorption in a molecular sieve dryer consisting of at least two vessels one in operation while the other is being regenerated. Nitrogen may be used as dry gas for re generation.

In the NWU the syngas is washed by liquid nitrogen in a column where CH₄, Ar, He and CO are removed. The purified syngas preferably contains only ppm levels of Ar and

CH₄.

The waste gas containing the impurities together with some lost nitrogen may advanta geously be used as fuel in the fired heater.

After the NWU, nitrogen gas may be added to the process stream in order to adjust the N₂ content to a preferred ratio H₂/N₂ ratio of 3 in the make-up stream to the ammonia synthesis loop.

An alternative cleaning method of the gas leaving the CO₂ absorber is to send it through a PSA. The pure hydrogen stream can be mixed with N₂ from from the ASU to a preferred ratio H₂/N₂ ratio of approximate 3 in the make-up stream to the ammonia synthesis loop. The off-gas the PSA can be used as fuel in the steam reformer.

The selection between the two methods, NWU or PSA, will be based on cost, CO₂ and/or H₂ requirement connected to the ammonia production

Because the purified syngas now only contains H₂ and N₂ in the correct stoichiometric ratio for ammonia synthesis plus ppm levels of Ar and ChU the ammonia synthesis sec tion can be considered inert free.

An ammonia synthesis loop is defined as inert free when it is not required to purge gas from the loop because the build-up of inerts is negligible without such purge.

Claims

1. A process for producing an ammonia synthesis gas, said process comprising the steps of: - Steam reforming of a hydrocarbon feed followed by oxygen reforming thereby obtaining a synthesis gas comprising ChU, CO, CO2 , H2 and H2O Shifting said synthesis gas in a high temperature (HT) shift step over a pro moted zinc-aluminum oxide based high temperature shift catalyst, wherein The steam/carbon ratio in the reforming and HT shift step are less than 2.6.

2. Process according to claim 1, wherein the temperature in the high temperature shift step is 300 - 600 °C, such as 345 - 550 °C.

3. Process according to any of the preceding claims, wherein the promoted zinc-alumi- num oxide based HT shift catalyst comprises in its active form a Zn/AI molar ratio in the range 0.5 to 1.0 and a content of alkali metal in the range 0.4 to 8.0 wt % and a copper content in the range 0-10% based on the weight of oxidized catalyst.

4. Process according to any of the preceding claims, wherein the steam/carbon ratio in the reforming step is 2.6 - 0.8, 2.4 - 1.0 or 2.0 - 1.2, such as 1.8 or 1.5.

5. Process according to any of the preceding claims, wherein the reforming is carried out by tubular steam reforming followed by autothermal reforming with an oxygen source, wherein the oxygen source contains above 90%, preferably above 98 % oxy- gen.

6. Process according to any of the preceding claims, wherein the space velocity in the oxygen reforming is less than 20.000 Nm³ C/m³/h, preferably less than 12.000 Nm³ C/m³/h and most preferably less 7000 Nm³ C/m³/h.

7. Process according to any of the preceding claims further comprising a prereforming step.

8. Process according to any of the preceding claims, wherein the high tempera ture shift step is one or more high temperature shift steps in series, preferably with possibility for cooling and/or steam addition in between.

9. Process according to any of the preceding claims further comprising one or more ad ditional shift step downstream the high temperature shift step.

10. Process according to any of the preceding claims, wherein the one or more addi tional shift steps are one or more medium temperature shift steps and/or one or more low temperature shift steps.

11. Process according to any of the preceding claims, wherein steam is optionally added to the synthesis gas before the one or more additional shift steps downstream the high temperature shift step.

12. Process according to any of the preceding claims, wherein the synthesis gas leav ing the one or more additional shift step downstream the high temperature shift step is optionally washed with water to reduce the methanol content.

13. Process according to any of the preceding claims further comprising a CO2 re moval step removing CO2 from the synthesis gas down to a level less than 400 vpp CO2, such as below 100 vpp or in some preferred embodiments down to 20 vpp or below.

14. Process according to any of the preceding claims further comprising a N2 wash step.

15. Process according to claims 1 -13 further comprising a PSA.

16. Process for producing ammonia, wherein the ammonia synthesis gas is prepared by the process according to claim any one of claims 1 - 15.

17. Process for producing ammonia according to claim 16, wherein the ammonia pro cess loop is an inert free loop. Ċ

18. Ammonia plant arranged to carry out the processes according to any one of claims