WO2004018355A1

WO2004018355A1 - Process for the energy efficient removal of bromine from chlorine

Info

Publication number: WO2004018355A1
Application number: PCT/CA2003/001308
Authority: WO
Inventors: Clive M. H. Brereton
Original assignee: Noram Engineering And Constructors Ltd.
Priority date: 2002-08-23
Filing date: 2003-08-25
Publication date: 2004-03-04
Also published as: AU2003266037A1; KR20050058453A

Abstract

The invention provides a method for purification of chlorine comprising passing chlorine through a distillation column (12) to produce a chlorine gas, passing the chlorine gas from the distillation column through a compressor (28) to produce a compressed chlorine gas, and exchanging heat between the compressed chlorine gas and a liquid chlorine mixture in the reboiler. The invention also provides apparatus for purifying chlorine comprising a distillation column having a reboiler (20) to produce a chlorine gas, a compressor downstream of the distillation column for compressing the chlorine gas to produce a compressed chlorine gas, and a heat transfer apparatus connected downstream of the compressor for exchanging heat between the compressed chlorine gas and a liquid chlorine mixture in the reboiler.

Description

PROCESS FOR THE ENERGY EFFICIENT REMOVAL OF BROMINE FROM CHLORINE

Reference to Related Application [0001] This application claims priority from United States

Provisional Patent Application No. 60/405,307, filed 23 August 2002.

Technical Field

[0002] The invention relates to a process for removing bromine or other heavy materials from chlorine.

Background

[0003] Distillation is an established technique for separation of fluids such as chlorine and bromine which have different boiling points. Distillation has been used in the past in the chlor-alkali industry for production of high purity chlorine, free of bromine and organics, for specialty applications. Bromine is generally undesirable in purified chlorine, since the chlorine may be used in the production of plastics, and the presence of bromine causes unwanted coloration. Also, for chlorine which is to be used as a drinking water additive, it is desirable to eliminate as much bromine as possible since bromine poses health hazards.

[0004] Although it is effective, chlorine distillation is generally energy intensive. This is because chlorine is more volatile than bromine. The entire chlorine stream must be evaporated in order to distill it from the less volatile bromine. If distillation is performed on a condensed product, almost the entire product must be vaporized and re- condensed. This is expensive because: • It takes energy to vaporize chlorine/bromine in a reboiler and, • If the distillation takes place at low or modest pressure, energy is required to run chillers to produce a cold fluid to condense the chlorine gas produced by the distillation. [0005] Many chlor-alkali plants produce liquid chlorine. In some such plants chlorine gas is produced in a cellhouse. Chlorine from the cellhouse is generally passed on to a drying system. Dried chlorine is then compressed and liquefied typically in liquefiers where the refrigerant compressors may be maintenance intensive. Large chlor- alkali plants will usually use single or multistage centrifugal compressors to compress the chlorine; smaller plants may use less efficient but simpler acid ring machines. In plants employing centrifugal compression it is common to place a small column with a reboiler upstream of the compressor. An example of such a plant is shown in Figure 1A and is described further below.

[0006] The column may be termed either a "pre-cooler" or a "purification column" and serves two purposes:

• A small reflux of liquid chlorine is added to the column, in an amount just sufficient to chill the dry chlorine gas entering the centrifugal compressor; this increases the efficiency of the compression and decreases the size of the compression machine required.

• In addition, the column is a crude distillation column in which the reboiler may be batched with a heavy solvent, or the contents of the reboiler may be drained into a heavy solvent. Often in the past this has been carbon tetrachloride; it is now more commonly chloroform. The chloroform acts as a solvent and sink for heavy organics, and in particular nitrogen trichloride which otherwise can accumulate in the column and build up to dangerous levels. The reboiler is periodically tested to determine the levels of nitrogen trichloride and other impurities and, when these levels have reached certain thresholds, a batch of chloroform is removed and replaced with fresh material. Typically the contaminated chloroform, containing heavy organics, is drummed and disposed of by incineration off-site. The purification column, while it removes organics, may also contribute to organics contamination of the product if process upsets occur and organics are reboiled.

[0007] Figure 1A illustrates a prior art system 1 incorporating a typical pre-cooler/purification column 2. A small reflux 3 of liquid chlorine is added to column 2 above a feed 4 of chlorine gas containing impurities. The input rate of reflux 3 is generally less than 10% of that of feed 4. The amount of liquid chlorine used in reflux 3 is just enough so that when it evaporates it will chill the gas from feed 4 before it is sent to compression stage 28, with only a small amount of liquid draining down column 2 into reboiler 5. After compression stage 28, the chlorine gas passes through aftercooler 30 and is then liquefied by liquefaction train 32. The liquid chlorine for reflux 3 may be taken from liquefaction train 32, or from a storage system (not shown in Figure 1A) either on or off site. The liquid chlorine in column 2 drains into reboiler 5, bringing some impurities along with it. Reboiler 5, located below column 2, is partially filled with a heavy solvent to trap the impurities as the chlorine evaporates and is passed to compression stage 28 through column 2 and flowpath 26. Solvent contaminated with impurities is periodically removed from the bottom of reboiler 5 through flowpath 6.

[0008] A typical pre-cooler/purification column as shown in Figure

1A is not suitable for operation as a bromine/chlorine distillation column, because it cannot accomplish the requisite separation due to it's relatively small size and the low input rate of liquid chlorine in the reflux. [0009] Figure IB illustrates another prior art system 10 for removal of bromine and other impurities from chlorine at low pressure. The pre-cooler/purification column of Figure 1 A is replaced by a true distillation column 12. In column 12 of Figure IB, a reflux 14 of purified liquid chlorine is introduced at a higher flow rate than is reflux 3 in column 2 of Figure 1A. A bromine chlorine mixture of some desired strength is removed from the bottom of column 12 through flowpath 18. The liquid/gas mixture of bromine and chlorine is heated in reboiler 20, so that most of the chlorine evaporates and the chlorine gas returns to column 12 through flowpath 22. Substantially all of the bromine is removed from system 10 through purge 24 in liquid form.

[0010] Purified chlorine gas exits the top of column 12 through flowpath 26 and then enters compression stage 28. After compression stage 28, the compressed gas is typically cooled by aftercooler 30 before being passed on to the liquefaction train 32. Liquefaction train 32 typically comprises a plurality of liquefiers operated in parallel.

[0011] To properly purify the chlorine, the input rate of reflux 14 of Figure IB must generally be significantly higher than the input rate of reflux 3 in Figure 1A. Reflux 14 is generally provided from a storage system (not shown in Figures 1A and IB) downstream of liquefaction train 32. This liquid chlorine used for reflux 14 must be reboiled and liquefied again once it has been introduced into column 12 (i.e. a much larger volume of liquid chlorine must be vaporized and recondensed in comparison to the purification column of Figure 1A). This adds significantly to the cost and size of the liquefaction train and adds a large energy load, as compared to the energy load of system 1 in Figure 1A. [0012] There exists a need for a process for the distillation of chlorine to remove small concentrations of heavies such as bromine which also serves to reduce the capital and operating costs associated with the distillation.

Summary of Invention

[0013] The invention provides a method for purification of chlorine comprising passing chlorine through a distillation column to produce a purified chlorine gas, passing the purified chlorine gas from the distillation column through a compressor to produce a compressed chlorine gas, and exchanging heat between the compressed chlorine gas and a liquid chlorine mixture in the reboiler.

[0014] The step of exchanging heat may be achieved by passing the compressed chlorine gas relative to heat transfer surfaces in the reboiler. Alternatively, the heat exchange may be achieved by passing the compressed chlorine gas through a first heat exchanger carrying a heat transfer fluid and circulating the heat transfer fluid through a second heat exchanger associated with the reboiler.

[0015] The reflux may be maintained at a rate of at least 25 % of a rate of total chlorine feed to the distillation column to achieve a desired purity in the purified chlorine gas. The compressed chlorine gas may be liquefied to produce a purified liquid chlorine and the reflux may comprise the purified liquid chlorine.

[0016] Bromine may be passed into the distillation column along with the chlorine, and the method may comprise removing a liquid mixture of chlorine and bromine from the reboiler. The liquid mixture of chlorine and bromine may be passed through a secondary column having a secondary reboiler and purified bromine may be removed from the secondary reboiler.

[0017] The invention also provides an apparatus for purifying chlorine comprising a distillation column having a reboiler to produce a chlorine gas, a compressor downstream of the distillation column for compressing the chlorine gas to produce a compressed chlorine gas, and, a heat transfer apparatus connected downstream of the compressor to exchange heat between the compressed chlorine gas and a liquid chlorine mixture in the reboiler.

[0018] The heat transfer apparatus may comprise a flowpath for passing the compressed chlorine gas relative to heat transfer surfaces in the reboiler. Alternatively, the heat transfer apparatus may comprise a first heat exchanger thermally coupled to the compressed chlorine gas and a second heat exchanger thermally coupled to the liquid chlorine mixture in the reboiler. The first and second heat exchangers may be connected by a heat transfer loop carrying a heat transfer fluid.

[0019] The apparatus may further comprise a secondary column configured to receive a liquid mixture of chlorine and bromine from the reboiler of the distillation column. A vaporizer may be coupled between the reboiler of the distillation column and the secondary column for vaporizing the liquid mixture of chlorine and bromine. A secondary reboiler may be associated with the secondary column. The secondary reboiler may have an outlet for producing purified bromine.

Brief Description of Drawings

[0020] In drawings which illustrate non-limiting embodiments of the invention: Figure 1A is a schematic view of a prior art chlorine purification system;

Figure IB is a schematic view of another prior art system similar to that of Figure 1 A wherein the pre-cooler purification column has been replaced with a true distillation column;

Figure 2 is a schematic view of a chlorine purification system according to one embodiment of the invention;

Figure 3 is a schematic view of a chlorine purification system according to another embodiment of the invention; Figure 4 is a schematic view of a chlorine purification system according to another embodiment of the invention; and,

Figure 5 is a schematic view of a chlorine purification system according to another embodiment of the invention.

Description

[0021] Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.

[0022] The inventor has discovered that it is possible to take advantage of the need to compress and liquefy chlorine; this provides an opportunity to locate a distillation column upstream of compression and to operate this column with a heat integration loop. Heat from cooling and partial liquefaction of the compressed chlorine may be used to operate the reboiler of the distillation column, which contains a liquid chlorine mixture at a lower pressure than that of the compressed chlorine. With this approach it is possible to reduce the amount of liquef action which must be accomplished by liquefiers. Use of the invention will result in:

• reduced operating costs;

• substantially reduced capital costs associated with chlorine distillation; and/or

• reduced maintenance of large rotating equipment in the liquefiers.

[0023] Figure 2 is a schematic of a system 40 according to one embodiment of the invention for accomplishing separation of chlorine from bromine and other impurities. In system 40 of Figure 2, a pre- liquefier 42 is located downstream of compression stage 28 and aftercooler 30, and upstream of liquefaction train 32. Pre-liquifier 42 liquefies a portion of the chlorine and passes it to liquid chlorine storage 44, and passes the gaseous chlorine on to liquefaction train 32. Once liquefied, chlorine from liquefaction train 32 is sent to storage 44 through flowpath 33. Pre-liquefier 42 comprises a heat exchanger 46, which is thermally coupled to another heat exchanger 48 in reboiler 20 by means of heat transfer loop 50 which conveys a heat transfer fluid between heat exchangers 46, 48. Heat exchangers 46, 48 exchange heat between the compressed chlorine entering pre-liquifier 42 and the liquid chlorine mixture in reboiler 20. This eliminates the energy demand of reboiler 20 and reduces the liquefaction load, as a portion of the compressed chlorine is liquefied in pre-liquefier 42 as its heat is taken for reboiler 20.

[0024] The driving force for heat exchange is provided by the increase in pressure generated by compression stage 28, which causes the compressed chlorine downstream therefrom to have a higher pressure and thus a higher boiling point than the chlorine in reboiler 20 and column 12. Since the compression is a necessary part of the chlorine train, the increased cost is only the cost of the extra compression needed for the additional recycled chlorine which is added as reflux 14 to column 12, as compared to the compression required in the system of Figure 1A. Preferably the flow rate of chlorine in reflux 14 is at least 25 % of the flow rate of chlorine in feed 16 (whereas the flow rate of reflux 3 in Figure 1A is typically less than 10% of the flow rate of chlorine in feed 4). The cost of compressing this extra chlorine is modest compared to the energy costs of reboiling and liquefaction which are avoided by this scheme. A circulation pump 52 may optionally be provided in heat transfer loop 50 to circulate the heat transfer fluid.

[0025] The flow sheet can be configured in many ways depending upon the specific requirements of the plant.

[0026] Figure 2 is an installation where the liquid chlorine purge

24, typically containing only a small amount of the total chlorine feed to column 12 but containing up to 99% of the bromine, is vaporized and purged to a HC1 plant (not shown). Purge 24 also preferably purges nitrogen trichloride.

[0027] Figure 3 is a system 60 according to an alternative embodiment of the invention in which the heat exchange is accomplished directly by passing compressed chlorine from aftercooler 30 to reboiler 20 by means of flowpath 62. The compressed chlorine passes relative to heat transfer surfaces 64 in reboiler 20, which also acts as a pre-liquefier. The compressed chlorine liquefied in reboiler 20 is sent to storage 44 through flowpath 66, and the remaining compressed chlorine gas is sent to liquefaction train 32 through flowpath 68. This embodiment lacks a separate circulating fluid to exchange heat between the compressed chlorine gas and the liquid chlorine mixture in reboiler 20. System 60 of Figure 3 has the advantage of simplicity. Whether this is preferred will depend upon the relative elevations of the various system components in the plant.

[0028] Figures 4 and 5 show systems 70, 90 according to two further alternative embodiments of the invention. The embodiments shown in these Figures are adapted to produce a more concentrated bromine product. In both variants a secondary column 72 is employed in order to further purify the bromine from the relatively dilute stream taken off the bottom of the main column 12.

[0029] In system 70 of Figure 4, a liquid mixture of chlorine and bromine is passed to secondary column 72 by means of flowpath 74. The liquid mixture is then passed to secondary reboiler 76 by means of flowpath 78, where it is heated to evaporate most of the chlorine. The remaining liquid is mostly bromine, and is passed on to bromine outlet 80. The product removed from bromine outlet 80 may be greater than 90% bromine. The chlorine evaporated in secondary reboiler 76 returns to secondary column 72 by means of flowpath 82, and then returns to main column 12 by means of flowpath 84 and flowpath 22.

[0030] In system 90 of Figure 5, the liquid mixture of chlorine and bromine is passed through a vaporizer 92 on its way from reboiler 20 to secondary column 72. A secondary reflux 94 of purified liquid chlorine is added to secondary column 72 at a higher level than the vaporized mixture of chlorine and bromine. The liquid chlorine cools and condenses the bromine and a portion of the chlorine, and the resultant liquid mixture is then passed to secondary reboiler 76 by means of flowpath 78, where it is heated to evaporate most of the chlorine. The remaining liquid is mostly bromine, and is passed on to bromine outlet 80. The product removed from bromine outlet 80 may be greater than 90% bromine. The chlorine evaporated in secondary reboiler 76 returns to secondary column 72 by means of flowpath 82, and then returns to reboiler 20 by means of flowpath 96 and flowpath 18.

[0031] In the embodiments of Figures 4 and 5 secondary column 72 may be small compared to main column 12 and will typically have a low energy demand because the bulk of bromine concentration has been performed in the main energy-coupled column 12. Again the preferred choice of flowsheet will largely be dictated by plant elevations and whether the plant is new or retrofit.

[0032] Control of an energy integrated main column may be performed simply. Systems according to preferred embodiments of the invention are operated with a relatively constant gaseous feed 16 flow dictated by plant production. Reflux 14, typically from storage 44, is preferably also a relatively constant flow. It is desirable to maintain a boil-up rate in the reboiler 20 which matches the rates of feed 16 and reflux 14, while maintaining a fixed withdrawal rate from the reboiler sump, either by purge 24 or through secondary column 72. This may be accomplished in various ways including: • varying the circulation rate of the indirect heat transfer fluid circulating between reboiler 20 and pre-liquefier 32 in embodiments having a separate heat transfer loop 50, such as those shown in Figures 2, 4 and 5. • allowing the level of liquid in reboiler 20 to vary (between limits), so that as the liquid level moves up and down it covers and uncovers heat transfer surface and so adjusts the boil-up rate.

[0033] Systems according to the invention are self compensating.

If the operator sets a low rate for the withdrawal of liquid from the bottom of reboiler 20, the boil-up rate increases because the level of liquid in reboiler 20 rises until enough heat transfer surface of heat exchanger 48 or 64 is covered to provide for appropriate boil-up. Conversely, if the liquid withdrawal rate from the bottom of reboiler 20 is increased, the level of liquid in reboiler 20 drops until less heat transfer surface is exposed and the boil-up rate decreases.

[0034] As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method for purification of chlorine comprising:

(a) passing chlorine through a distillation column having a reboiler to produce a purified chlorine gas; (b) passing the purified chlorine gas from the distillation column through a compressor to produce a compressed chlorine gas; and, (c) exchanging heat between the compressed chlorine gas and a liquid chlorine mixture in the reboiler.

2. A method according to claim 1 wherein the step of exchanging heat comprises passing the compressed chlorine gas relative to heat transfer surfaces in the reboiler.

3. A method according to claim 1 wherein the step of exchanging heat comprises passing the compressed chlorine gas through a first heat exchanger carrying a heat transfer fluid and circulating the heat transfer fluid through a second heat exchanger associated with the reboiler.

4. A method according to claim 1 wherein a liquid chlorine reflux with a rate of at least 25 % of a rate of a total chlorine feed to the distillation column is maintained in the distillation column to achieve a desired purity in the purified chlorine gas.

5. A method according to claim 4 comprising liquefying the compressed chlorine gas at a location downstream from the compressor to produce a purified liquid chlorine and wherein the reflux comprises the purified liquid chlorine.

6. A method according to claim 1 wherein bromine is passed into the distillation column along with the chlorine, further comprising removing a liquid mixture of chlorine and bromine from the reboiler.

7. A method according to claim 6 further comprising passing the liquid mixture of chlorine and bromine through a secondary column having a secondary reboiler and removing purified bromine from the secondary reboiler.

8. Apparatus for purifying chlorine comprising:

(a) a distillation column having a reboiler for producing a chlorine gas;

(b) a compressor downstream of the distillation column for compressing the chlorine gas to produce a compressed chlorine gas; and,

(c) a heat transfer apparatus connected downstream of the compressor for exchanging heat between the compressed chlorine gas and a liquid chlorine mixture in the reboiler.

9. Apparatus according to claim 8 wherein the heat transfer apparatus comprises a flowpath for passing the compressed chlorine gas relative to heat transfer surfaces in the reboiler.

10. Apparatus according to claim 8 wherein the heat transfer apparatus comprises a first heat exchanger thermally coupled to the compressed chlorine gas and a second heat exchanger thermally coupled to the liquid chlorine mixture in the reboiler, wherein the first and second heat exchangers are connected by a heat transfer loop carrying a heat transfer fluid.

11. Apparatus according to claim 8 further comprising a secondary column configured to receive a liquid mixture of chlorine and bromine from the reboiler of the distillation column.

12. Apparatus according to claim 11 further comprising a vaporizer coupled between the reboiler of the distillation column and the secondary column for vaporizing the liquid mixture of chlorine and bromine.

13. Apparatus according to claim 11 further comprising a secondary reboiler associated with the secondary column, wherein the secondary reboiler has an outlet for producing purified bromine.

14. A method for purification of chlorine comprising: (a) contacting a reflux stream comprising liquified chlorine and a feed stream comprising gaseous chlorine and impurities in a distillation column; (b) vaporizing said liquified chlorine in a reboiler in communication with said distillation column; (c) withdrawing a purified stream of chlorine gas from an upper portion of said distillation column; (d) compressing said purified stream of chlorine gas in a compressor located downstream from said distillation column; and (e) circulating a heat transfer fluid between said compressor and said reboiler.

15. The method as defined in claim 14, wherein said heat transfer fluid comprises chlorine gas discharged from said compressor.

16. The method as defined in claim 14, wherein said heat transfer fluid is circulated between a first heat exchanger thermally coupled to chlorine gas discharged from said compressor and a second heat exchanger thermally coupled to liquid chlorine within said reboiler.

17. The method as defined in claim 14, wherein said feed stream comprises bromine.

18. The method as defined in claim 14, wherein the input rate of said reflux stream into said distillation column is at least 25 % of the input rate of said feed stream into said distillation column.