Process for drying a gas stream comprising 2,3,3,3 tetrafluoropropene

Abstract

A method of drying a fluid comprising a fluoropropene, which method comprises the step of contacting the fluid with a desiccant comprising a molecular sieve having openings which have a size across their largest dimension of from about 3 to about 5 . A heat transfer device comprising a heat transfer fluid comprising a fluoropropene, and a desiccant comprising a molecular sieve having openings which have a size across their largest dimension of from about 3 to about 5 . Preferably, the fluoropropene is R134yf or R-1225ye.

Claims

1. A method of drying a fluid comprising a fluoropropene and water, the method comprises the step of: contacting the fluid with a desiccant comprising a molecular sieve having openings which have a size across their largest dimension of from 3 to 5 , wherein the ratio by weight of desiccant to fluid is greater than 1:100.

2. The method according to claim 1 wherein the molecular sieve has openings which have a size across their largest dimension of from 3 to 4 .

3. The method according to claim 2 wherein the molecular sieve has openings which have a size across their largest dimension of about 4 .

4. The method according to claim 1 wherein the fluoropropene is R-1234yf or R-1225ye.

5. The method according to claim 4 wherein the fluid comprises at least one additional refrigerant component.

6. The method according to claim 5 wherein the at least one additional refrigerant component is selected from CF.sub.3I, R-134a and R152a.

7. The method according to claim 4 wherein the fluid further comprises a lubricant.

8. The method according to claim 7 wherein the lubricant is selected from the group consisting of esters, PAGs, PVEs, mineral oils and synthetic hydrocarbons.

9. The method according claim 1 wherein the fluid further comprises a stabilizer.

10. The method according to claim 9 wherein the stabiliser is selected from epoxides, dienes and phenols.

11. The method according to claim 1 wherein the fluid further comprises a dye.

12. The method according to claim 11 wherein the dye is a fluorescene.

13. The method according to claim 1 wherein the desiccant comprises at least one further desiccant or adsorbent other than the molecular sieve.

14. The method according to claim 13 wherein the at least one further desiccant or adsorbent is selected from alumina, silica and activated carbon.

15. The method according to claim 14 wherein the desiccant is contained in a cartridge.

16. The method according to claim 1 wherein the desiccant does not contain any further desiccant other than the molecular sieve.

17. The method according to claim 1 wherein the fluid is a heat transfer fluid.

18. A method of providing cooling, the method comprising: providing a heat transfer fluid comprising a fluoropropene and water; drying the fluid by contacting the heat transfer fluid with a desiccant, said desiccant comprising a molecular sieve having openings which have a size across their largest dimension of from 3 to 5 ; and using the fluid to provide cooling, wherein the ratio by weight of desiccant to fluid is greater than 1:100.

19. A heat transfer device comprising: a heat transfer fluid comprising a fluoropropene and; and a desiccant comprising a molecular sieve having openings which have a size across their largest dimension of from 3 to 5 , wherein said molecular sieve is designed to remove water from said heat transfer fluid, wherein the ratio weight of desiccant to fluid is greater than 1:100.

20. The heat transfer device according to claim 19 wherein the heat transfer device is a refrigeration system, and wherein said fluoropropene is R-1234yf.

21. The method of claim 1, wherein the ratio by weight of desiccant to fluid is greater than 1:87.5.

Description

DESCRIPTION OF THE DRAWING FIGURES

(1) The invention will now be described, by way of example, with reference to the accompanying figure, in which;

(2) FIG. 1 shows the levels of water in CF.sub.3I (TFIM), R-1234yf and R-134a, after drying with molecular sieves; and,

(3) FIG. 2 shows the capacity of desiccants in the drying of CF.sub.3I (TFIM), R-1234yf and R-134a.

DETAILED DESCRIPTION

(4) We have surprisingly found that zeolite molecular sieve desiccants having nominal pore dimensions of below about 5 may be used with refrigerants comprising at least one fluoropropene, optionally blended with at least one additional refrigerant component. In particular we have found that zeolite molecular sieve desiccants having nominal pore dimensions of below about 5 may be beneficially used to remove moisture from R-1234yf or R-1225ye and blends of R-1234yf and R-1225ye with other refrigerant components, particularly R-134a, R-152a and/or CF.sub.3I.

(5) FIG. 1 represent the equilibrium water content of refrigerants in contact with a number of zeolite molecular sieves with nominal pore dimensions ranging from 3 (M3 ) to 10 (MS13X). It is clear that as the nominal pore size of the molecular sieve increases, the water content of all of the refrigerants examined also increases, from approximately 0.003 mg H.sub.2O per mg of zeolite with the 3 zeolite to approximately 0.02 mg H.sub.2O per mg of zeolite with the 10 zeolite. Surprisingly, the 5 zeolite performs poorly with R-1234yf as the refrigerant, only achieving a water content comparable to that of the 10 zeolite. This is particularly surprising since the fluorinated propene would be expected to be larger than R-134a and so be expected to be less effectively absorbed by the 5 zeolite.

(6) Also, WO01/83411 discloses the removal of (hydro)haloalkene impurities from product streams using a solid adsorbent. In particular, WO01/83411 describes the removal of R-1234yf from product streams using an adsorbent having a pore size of from about 7 to about 10 .

(7) From the experimental results, we have found that in order to provide effective drying of refrigerant fluids comprising R-1234yf, a zeolite molecular sieve with a nominal pore dimension less than 5 is required. Because of the reduced water absorption rates associated with zeolite molecular sieves with nominal pore dimensions below about 3 , this value represents the lower end of the acceptable range of zeolite pore dimensions.

(8) The poor performance of 5 pore zeolites is reflected in the corresponding molecular sieve water content represented in FIG. 2. Here, the 5 zeolite has a water content similar to that of the 10 system, approximately 0.08 mg H.sub.2O per mg of zeolite. In contrast, both R-134a and CF.sub.3I result in capacities of approximately 0.09 mg H.sub.2O per mg of zeolite. Clearly, a desiccant having a lower water capacity may still be used to dry a refrigerant fluid but would require a greater quantity to be used in order to absorb a set quantity of moisture from the fluid.

(9) The desiccants of the invention are unlikely to act to remove additives such as stabilisers, dyes or lubricity enhancers from the circulating refrigeration fluid comprising a fluoropropene and a lubricant. In this way, the desiccants of the invention are unlikely to compromise the thermal stability of the refrigeration fluid or to compromise the performance of the compressor lubricant.

EXPERIMENTAL

Example 1

(10) Zeolite molecular sieves 3 , 5 and 13X were purchased from Aldrich Chemical Company. The 4 XE molecular sieve was obtained from National Refrigeration Suppliers contained within spun-copper cartridges. Karl-Fischer titration for determining water content of the refrigerants was conducted on a Cou-Lo Compact instrument supplied by GRScientific.

(11) A weighed quantity, approximately 0.4 g, of zeolite molecular sieve that had been previously dried in a stream of nitrogen gas at 150 C. for 12 hours, was placed in a stainless steel pressure vessel having an internal volume of 50 ml fitted with an access valve. The vessel valve was used to evacuate the vessel prior to introduction of a weighed quantity of liquid refrigerant, approximately 40 g, containing a known quantity, approximately 40 mg, of water. After initial mixing, the vessel was allowed to stand at approximately 23 C. for a period of 3 hours before a small sample of liquid refrigerant was removed from the vessel via the access valve and the water content determined by Karl-Fischer titration. The vessel was allowed to stand for a total of 24 hours to reach equilibrium before determining the water content of the refrigerant by removal of a further liquid sample.

(12) To determine refrigerant moisture content, a sample of liquid refrigerant was passed through the access valve and allowed to evaporate within a length of stainless steel tubing leading to the Karl-Fischer titration cell. The resulting vapour was metered into the Karl-Fischer titration cell. Moisture content of the desiccant sample was calculated by difference.

(13) Refrigerant moisture at. 24 hours (mg H.sub.2O/g refrigerant)

(14) TABLE-US-00001 MS3A MS4A MS5A MS13X TFIM 0.003 0.012 0.009 0.02 R-1234yf 0.003 0.01 0.021 0.019 R-134a 0.004 0.007 0.009 0.02
Sieve capacity at 24 hours (mg H.sub.2O/mg sieve)

(15) TABLE-US-00002 MS3A MS4A MS5A MS13X TFIM 0.097 0.088 0.091 0.08 R-1234yf 0.097 0.09 0.079 0.081 R-134a 0.096 0.093 0.091 0.08
MS13X molecular sieve has a 10 nominal pore diameter. Preferred zeolites are those having pore sizes in the range of from about 3 to below about 5 , in particular, those having pore dimensions in the range of about. 3 to about 4 . Particular examples of commercially available zeolite molecular sieves falling within this preferred range include XH-7 and XH-9 manufactured by UOP, and MS594 and MS592 manufactured by Grace.

Example 2

(16) In a variation of Example 1, wet R-1225ye was tested with a variety of molecular sieves and compared to R-134a.

(17) In this experiment, the refrigerant was wetted by adding 0.5 g distilled water to an evacuated cylinder, to which was then added 250 g refrigerant. This mixture was allowed to equilibrate at room temperature for several hours before being analysed by Karl Fischer for liquid moisture contact.

(18) The desiccant was prepared by drying it at 200 C. with dry nitrogen passing through it for at least 16 hours, 0.8 g desiccant was then added to a clear, dry cylinder which was then evacuated before adding 70 g of wet liquid refrigerant. This was then left at room temperature and analysed in duplicate providing an average value for liquid moisture after 3 and 24 hours.

(19) TABLE-US-00003 3A 4A 5A 13X Time/Hrs 0 3 24 0 3 24 0 3 24 0 3 24 R-134a 769 382 31 R-134a 656 79 45 R-134a 816 132 70 R-134a 833 202 163 R-1225ye 641 275 55 R- 574 306 78 R- 598 329 181 R- 641 404 357 1225ye 1225ye 1225ye 3A mg water per mg sieve mg water per g refrigerant R-1225ye 0.095 0.005 R-134a 0.096 0.004 5A mg water per mg sieve mg water per g refrigerant R-1225ye 0.084 0.016 R-134a 0.091 0.009 4A mg water per mg sieve mg water per g refrigerant R-1225ye 0.093 0.007 R-134a 0.093 0.007 13X mg water per mg sieve mg water per g refrigerant R-1225ye 0.069 0.031 R-134a 0.080 0.020

(20) The results indicate an optimum sieve size of 3-4 ; performance of sieves with a pore size greater than 4 is poor.

(21) In certain embodiments, it may be beneficial to include another desiccant with the molecular sieve of the invention. Thus, in certain applications the desiccant material could comprise the molecular sieve alone, whilst other applications may use a desiccant that contains the molecular sieve in addition to one or more auxiliary agent such as alumina, silica and/or activated carbon, for example for the removal of acids.

(22) The desiccant which is used in the process of the present invention comprises a molecular sieve containing pores having openings which have a size across their largest dimension in the range of from about 3 to about 5 , e. g. from greater than or equal to 3 to less than 5 . By opening we are referring to the mouth of the pore by which the molecule to be adsorbed enters the body of the pore where it becomes trapped. The openings to the pores may be elliptically shaped, essentially circular or even irregularly shaped, but will generally be elliptically shaped or essentially circular. When the pore openings are essentially circular, they should have a diameter in the range of from about 3 to about 5 , e.g. from greater than or equal to 3 to less than 5 .

(23) Preferred adsorbents are those comprising pores having openings which have a size across their largest dimension in the range of from about 3 to about 4 .

(24) The desiccant may contain more than one distribution of pore sizes, so that in addition to the pores of the required dimension in which the openings to the pores have a size across their largest dimension in the range of from 3 to 5 , the adsorbent, may also contain pores which are either larger or smaller. Thus, the adsorbent does not have to contain exclusively pores within the 3 to 5 range. However, any pores outside this range will not be as effective at selectively removing moisture from the fluoropropenes.

(25) The desiccant should be in particulate form and is conveniently in the form of pellets or beads. When used in the manufacture of fluoropropenes, the particulate adsorbent is typically arranged as a bed or layer in an adsorption tower or column and the product stream may be conveyed over or through the bed. The desiccant bed may be a fluidised or moving bed, but in a preferred embodiment is a fixed or static bed. When used to dry the circulating fluid in a refrigeration system, the desiccant is conveniently in the form of pellets or beads contained within a cartridge, through which liquid refrigerant, containing any circulating compressor lubricant, is passed. Alternatively, the desiccant may be in the form of a solid, porous core comprising a zeolite of the present invention, a binder and any auxiliary desiccants or adsorbents such as silica gel, alumina or activated carbon. In use, the core is contained within a cartridge and the circulating refrigeration fluid is caused to pass through the cartridge and into contact with the core.

(26) The desiccant typically has a surface area in the range of from 300 to 900 m.sup.2/g.

(27) The desiccant is normally ore-treated prior to use by heating in a dry gas stream, such as dry air or dry nitrogen. This process is known to those skilled in the art and has the effect of activating the desiccant.

(28) Typical temperatures for the pre-treatment are in the range of from 100 to 400 C.

(29) The process of the present invention can be conducted with the product stream in the liquid phase or the vapour phase. In a fluoropropene manufacturing process, the product stream exiting the reactor will typically be pre-treated before it is subjected to the process of the present invention in order to reduce the overall level of impurities in the product stream. This pre-treatment will typically include a distillation step. The product stream may also be re-circulated and conducted several times through the same adsorbent bed in order to achieve the desired low level of water.

(30) The process of the invention may be operated in a batch or continuous manner, although continuous operation is preferred.

(31) The present process is preferably operated at a temperature in the range of from 20 to 100 C., more preferably in the range of from 10 to 70 C. and particularly in the range of from 10 to 50 C.

(32) The preferred operating pressures are in the range of from 1 to 30 bar, more preferably in the range of from 5 to 20 bar and particularly in the range of from 6 to 12 bar.

(33) The preferred feed rate for the product stream to the desiccant bed is in the range of from 0.1 to 50 hour.sup.1, more preferably in the range of from. 1 to 30 hour.sup.1 for liquid phase product streams and in the range of from 10 to 10,000 hour.sup.1, more preferably in the range of from 100 to 5,000 hour.sup.1 for vapour phase product streams.

(34) During operation of the present process, the adsorption capability of the desiccant is gradually consumed as the pores become occupied with water. In the manufacture of fluoropropenes, the ability of the adsorbent to adsorb water will eventually be substantially impaired and at this stage the adsorbent must be regenerated. Regeneration is typically effected by heating the used adsorbent in a dry gas stream, such as dry air or dry nitrogen, at a temperature in the range of from. 100 to 300 C., e.g. 100 to 200 C., and a pressure in the range of from 1 to 30 bar, e, g. 5 to 15 bar. This process is known to those skilled in the art. When used in refrigeration systems, the desiccant cartridge or core is normally replaced on exhaustion of the water absorption capacity of the desiccant or as part of a routine service.