Argon Recovery From Ammonia Plant Purge Gas Utilizing a Combination of Cryogenic and Non-Cryogenic Separating Means

Economic production of argon via air separation plants is linked to the production of equivalent quantities of nitrogen or oxygen or both. In recent years, however, the demand for argon has been growing at a more rapid rate than the corresponding growth rate of either nitrogen or oxygen. Alternative sources for argon production have thus become attractive. One such alternative source is the purge gas from ammonia synthesis plants.

In an ammonia synthesis plant, it becomes necessary to purge a fraction of the gas stream in order to maintain the inert concentration below a specified level. Higher inert levels reduce the partial pressure of the reactants and cause an unfavorable shift of the ammonia synthesis reaction equilibrium. Methane and argon constitute the inert gases of concern. A typical composition of the ammonia purge gas available at approximately 1900 psig pressure is as follows: 60.5% H.sub.2, 20% N.sub.2, 4.5% Ar, 13% CH and 2% NH.sub.3. Depending on the ammonia plant design, the purge gas may be available at much higher pressures or at slightly different compositions.

Present technology for argon recovery from ammonia plant purge gas employs a cryogenic process that consists of a pre-treatment section for ammonia removal and three cryogenic distillation columns. The first two columns are for stripping hydrogen and nitrogen in the feed gas and the final column is for separating argon and methane to obtain pure liquid argon product and also pure methane for use as fuel.

The primary object of the invention was to develop an improved process for recovering argon from ammonia synthesis plant purge gas. A further object of the present invention was to develop a process employing an advantageous combination of non-cryogenic and cryogenic steps for argon recovery from an ammonia synthesis plant purge gas. Yet a further object of the present invention was to utilize a PSA system to accomplish removal of methane from the purge gas exiting an ammonia synthesis plant.

In the following description of the invention, the term "pressure swing adsorption" or its acronym "PSA" is used in reference to a type of process and apparatus that is now well known and widely used with respect to separating the components of a gaseous mixture. A PSA system basically comprises passing a feed gas mixture through one or more adsorption beds containing a sieve material which has greater selectivity for more strongly adsorbed components than more weakly adsorbed components of the gas mixture. In the normal operation of a typical 2-bed PSA system, the connecting conduits, valves, timers, and the like are coordinated and arranged so that when adsorption is occurring in a first bed, regeneration is occurring in a second bed. In the usual cycle, sequential steps with respect to each adsorption bed include bed pressurization, product release and bed regeneration. Basic PSA systems are described in U.S. Pat. No. 2,944,627, U.S. Pat. No. 3,801,513, and U.S. Pat. No. 3,960,522.

Various modifications and improvements to the basic PSA process and apparatus have been described in the literature, for example, in U.S. Pat. No. 4,415,340, issued on Nov. 15, 1983 and U.S. Pat. No. 4,340,398 issued on July 20, 1982. The present invention is not limited to the use of any particular PSA process or apparatus design. A design that results in high argon yield, however, is detailed below as an example.

A new and improved process has been developed for recovering argon from the purge gas flowing from an ammonia synthesis plant. This process employs a non-cryogenic means comprised of a pressure swing adsorption (PSA) unit for accomplishing a critical separation between argon and methane as well as removing most of the nitrogen.

The present invention has several important advantages over the three stage prior art cryogenic recovery of argon. A considerable reduction in capital cost and operating expense is achieved through the use of a gas phase methane separation. In fact, the high pressure of the purge gas exiting from the ammonia plant can be used to provide most or all of the energy requirements in the non-cryogenic separation. Furthermore, it is possible, as a further energy saving measure, to pass the purge gas stream through a turbine in order to provide cooling needed for the later cryogenic separation. Further advantages of the present process stem from the use of PSA units for ammonia separation and for methane separation ahead of a membrane for hydrogen separation. The ammonia purge stream is at a cold temperature (about -10.degree. F.), at which pressure swing adsorption is more effective. Because of the heats of adsorption, the product gas from the PSA units will be warmer. Membranes operate more effectively at higher temperatures (about 70.degree. F.). The temperatures noted here are for an ammonia plant operating at about 2000 psia and may be slighty different in the event the plant is designed to operate at other pressures.

Another advantage of the present invention is that it offers the option of separating ammonia simultaneously with methane and nitrogen in a single PSA system. This option is advantageous when ammonia in the feed is at such low concentrations that the recovery or recycle of ammonia in the purge gas is not critical. Expensive ammonia separation equipment are then eliminated, making the process even more cost effective. Finally, the compact units employed in the present process are more portable and, as a result, the purge gas available at numerous ammonia plant sites over a wide geographical range can be more expeditiously tapped for argon in order to meet the growing demand for this industrial gas.

A first embodiment of the process of the present invention involves the following stages:

(1) Ammonia from the ammonia synthesis plant purge gas is removed by controlled adsorption at high pressures, on the order of 1100 psig, using zeolite molecular sieve material. Regeneration of this material is carried out by purge with a hydrogen rich stream produced in the overall process. The hydrogen rich stream is optionally heated. Purge is combined with pressure reduction to approximately 400 psig. The removed ammonia may be recycled to the ammonia synthesis plant.

(2) The product gas, following ammonia adsorption, is passed to a PSA unit where essentially all of the methane and most of the nitrogen are removed by using an adsorbent material possessing the necessary selectivity.

(3) The product gas from the PSA system is passed to a cryogenic distillation unit consisting of two cryogenic distillation columns. In the first column, hydrogen is separated; most of the hydrogen rich stream is directly recycled, although a portion thereof may be used as the purge gas for regeneration in the ammonia adsorption step and then recycled. In the second column, pure liquid argon is produced as a bottom product, whereas a nitrogen product with small amounts of hydrogen is the distillate.

In an alternative embodiment, the product gas from the PSA unit is sent to a membrane separator for hydrogen removal and recycle. Part of the hydrogen stream may be used for regeneration in the ammonia adsorption step and then recycled. The non-permeate gas consisting of nitrogen and argon is then cryogenically treated in a single column to produce pure liquid argon.

Although ammonia adsorption is preferred, a conventional method for ammonia removal may be employed, in which the purge gas is scrubbed with water and the ammonia-water mixture is then separated by fractionation. Residual moisture in the gas leaving the scrubber can be removed by adsorption if the downstream processing cannot tolerate water content.

If ammonia in the feed is in small amounts and it is not critical to recover or recycle the same, then the ammonia adsorption of step (1) may be combined in a single PSA unit with the methane and nitrogen separation of step (2).

The invention will be more clearly understood by reference to the following description of exemplary embodiments thereof in conjunction with the following drawings in which:

FIG. 1 discloses a schemtic flow diagram, according to one embodiment of the present invention, of a process for recovering argon from an ammonia synthesis plant purge gas; and

FIG. 2 discloses a schematic flow diagram according to a second embodiment of the present invention;

FIG. 3 discloses a schematic of a PSA unit illustrating valve positions and auxillary equipment according to one specific design; and

FIG. 4 is a timing diagram illustrating a full cycle sequence of PSA operation.

Referring to FIG. 1, ammonia synthesis plant purge gas stream 1, at 1900 psig and comprising approximately 60.5% hydrogen, 20% nitrogen, 13% methane, 4.5% argon, and 2% ammonia, enters a molecular sieve ammonia adsorption unit 2 operating at an elevated pressure. The unit 2 is regenerated by purge with a gas consisting of optionally heated hydrogen coupled with pressure reduction to 400 psig. The purge gas is part of the distillate stream exiting the cryogenic column 9. The ammonia removed by unit 2 exits as stream 3 along with the gas used to purge the unit, and is recycled to the ammonia synthesis loop at 400 psig.

The product stream 5, following removal of ammonia in adsorption unit 2, and optional cooling by expansion means 15, such as a turbine, enters a pressure swing adsorption (PSA) unit 6 for removal of essentially all of the methane and most of the nitrogen. The PSA unit 6 contains a molecular sieve adsorbent material which has a greater selectivity for methane than argon. The PSA vent stream 7 containing methane and nitrogen is usable as fuel for the ammonia plant. The product stream 8, comprising the predominant amount of argon, next enters a high pressure column 9, where cryogenic distillation, at a typical pressure of 400 psig, produces a top stream 10 of hydrogen containing a small amount of nitrogen, which may be recycled to the ammonia synthesis plant after being partly used as purge stream 4 for regeneration of unit 2. The bottoms product stream 11, containing the concentrated argon, next enters a second cryogenic distillation column 12, wherein remaining quantities of nitrogen and hydrogen exit the top of column 12 as gas stream 13. This may either be vented as a waste gas stream, used to purge the methane PSA unit 6, or used as fuel. The final product, essentially pure liquified argon, exits as stream 14 from the bottom of column 12. Argon yields of about 70 percent are obtainable at product methane concentrations of under about 20 ppm. High purity argon product containing less than 0.5 ppm methane can also be achieved.

In the first embodiment of the present invention, described above in reference to FIG. 1, the pressure swing adsorption (PSA) unit 2 can be made more effective by coupling with pressure reduction down to 400 psig, which is the operating pressure of the high pressure hydrogen separation column 9. Whereas the methane PSA unit operates at approximately 400 psig or above for the cryogenic hydrogen separation embodiment, much higher pressures, for example, approximately 1100 psig, are required for the embodiment shown in FIG. 2.

Referring now to FIG. 2, an alternative embodiment is shown in which the two column cryogenic separation described above, is modified to a single column separation by use of a hydrogen membrane separator.

In FIG. 2, numbered items 1 to 8 are the same as described above in reference to FIG. 1. Upon exiting from the PSA unit 6, however, the product stream 8 enters a hydrogen membrane separator 9, which removes hydrogen as permeate stream 10. A small part of the permeate is used as purge gas for regeneration of the ammonia adsorbing material. The non-permeate product stream 11 from the membrane separator 9, after being cooled by expansion means 16, next enters a cryogenic distillation column 12 in which an essentially pure liquid argon final product is obtained as bottoms stream 14. The more volatile nitrogen and hydrogen components of the product stream 11 exit from the top of the cryogenic distillation column 12, to form stream 13 which may be vented, used as purge gas for regeneration of the methane PSA unit 6, or used as fuel.

The ammonia adsorption unit 2 in FIG. 1 and FIG. 2, is suitably a three bed adsorber system described by Ruhemann (Petrocarbon Ltd.) in the article entitled "New developments in the treatment of ammonia purge gas", Indian J. Cryogenics 1982, Vol. 7, No. 3, pp 111-116. The three beds operate at very high pressures (typically 1100 psig) and may be regenerated with a hot purge gas comprised of a hydrogen-nitrogen mixture generated later in the overall process. The regenerated ammonia along with the purge gas is recycled to the ammonia synthesis loop.

Alternatively, it is also possible that the ammonia and methane adsorbers may be combined, in a PSA unit, provided the ammonia is not of much value. For low ammonia concentrations in the feed, this is often the case. In this case, regeneration is carried out at 25 psia or lower combined with purge. The vent stream may be used as a low BTU fuel.

The adsorbent material for the PSA unit 6 shown in FIG. 1 and FIG. 2 must have a greater selectivity for methane than argon. Both calcium and sodium aluminosilicate zeolites and activated carbons are suitable materials. Carbon molecular sieves and silica molecular sieves are also functional. Suitable zeolites include, but are not limited to the following: 5A, 10X, 13X or mordenites. Preferred zeolites are 5A medical grade zeolite sold by Union Carbide, the 5A HC sieve sold by LaPorte Industries or molecular sieves with comparable pore size and molecular attraction. The 5A medical grade zeolite was found to provide excellent argon-methane selectivity and to exhibit the ability to remove methane completely so that the PSA product gas will contain as low as fractional ppm levels. Removal of the methane to low levels is an important criterion; methane in the product gas would otherwise concentrate in the pure argon produced in the cryogenic section. Hence, expensive downstream purification steps would be required if the PSA product gas contains undesirable levels of methane. A methane level of less than 20 ppm is required and a level of less than 1 ppm is required. A methane level of 0.5 ppm or below is preferred.

A suitable operating pressure in the PSA for methane separation is in a range of 50 psig to 1100 psig. A range of 400 to 1100 psig is preferred in order to maintain the pressure of the gases downstream and to avoid downstream recompression. A pressure of 400 to 600 psi is preferred for the first embodiment, whereas approximately 1100 psi is preferred for the second embodiment.

A typical PSA unit is shown in FIG. 3. In FIG. 3, items 1 to 16 represent conventional valves. Other items in the FIG. 3 are adsorption bed A, adsorption bed B, equalization tank C, backfill tank D, product reservoir E and backpressure valve 17. This PSA unit may be operated according to the full cycle sequence shown in Table I for a 2-bed PSA system with tank equalization, to reduce void gas loss, and with product backfill, to achieve desired purities. In FIG. 4, the timing sequence and valve position for each step of the sequence is shown.

By varying the product to feed ratio either by changing product flow or cycle time, the argon yield at various operating pressures that corresponded to zero methane concentration in the product can be determined by thermal conductivity analysis of the PSA product stream in a gas chromatograph. There is a moderate reduction in argon yield with increasing pressure. The variation of argon yield with pressure indicates that the separation of methane is regeneration controlled; the higher the amount of methane regenerated, the better will be the argon-methane selectivity.

The PSA unit must be regenerated periodically. Typical modes of regeneration include (i) regeneration at atmospheric or slightly elevated pressures coupled with product purge, (ii) regeneration at atmospheric or slightly elevated pressures coupled with purge using hydrogen or a hydrogen-nitrogen mixture, and (iii) vacuum regeneration.

When using product purge, it is preferred to restrict the purge to different portions of the half cycle. Typically, the product released immediately after pressurization of the bed contains mostly hydrogen. The product released at the tail end of the production half cycle has minimum purity. To take account of this fact, two purge steps may be employed: the first purge step, immediately following pressurization of the adsorbing bed, and the second purge step towards the end of the half cycle. By suitable choice of time for the two purge steps, the maximum argon yield with this mode of regeneration can be obtained.

A disadvantage of product purge is low argon yield due to loss of the product purge gas itself. For separating all of the methane in the feed gas, the purge gas requirement is considerable and accounts for a large percentage of the argon lost.

An alternative to purge with product is purge with a gas available from a source external to the PSA; the hydrogen rich stream or the hydrogen-nitrogen mixture which are the distillate products from the two downstream columns can be effectively used for purge.

A third alternative mode of regeneration is vacuum regeneration. The yield obtained using vacuum regeneration is generally superior to the yield using purge. Vacuum regeneration, however, increases the capital investment for the process slightly and increases the energy requirement appreciably. Since the vent stream is used as a fuel, recompression to about 25 psia is also necessary unless special low pressure burners are installed. In determining the best regeneration procedure, the increase in argon yield that results with vacuum regeneration must be weighed against the incremental capital cost and energy requirements.

The operating pressures for the ammonia adsorber and the methane PSA unit are selected based on a typical ammonia plant design where the purge gas is available at about 2000 psia and the hydrogen rich recycle gas is required to be at 350 psia for introduction to the second stage ammonia compressors of the ammonia plant. It must be recognized that the process detailed here is applicable to any ammonia plant design; a suitable choice will have to be made for the pressures at various separation stages so that the recycle can be sent to the ammonia plant at a desired pressure.

The following working example illustrates a design based on actual plant data, experimental results, or, where appropriate, theoretical calculations assuming well mixed streams.

An ammonia purge gas stream comprising approximately 60.5% hydrogen, 20% nitrogen, 13% methane, 4.5% argon, and 2% ammonia enters a three bed ammonia adsorption unit containing 4 A molecular sieve material and operating at 1200 psig. This adsorption unit removes the ammonia and is periodically regenerated at 400 psig with a hydrogen rich purge stream. Ammonia rich vent is recycled to the synthesis compressor of the ammonia plant. After being cooled by expansion, the ammonia depleted stream, comprising 61.7% hydrogen, 20.5% nitrogen, 4.5% argon, and 13.3% methane, enters a PSA unit comprising two beds containing 5A medical grade molecular sieve material. The operating pressure within the beds is approximately 450 psig during adsorption. This PSA unit removes essentially all of the methane and most of the nitrogen. The vent stream comprises 9.0% hydrogen, 48.1% nitrogen, 4.0% argon and 38.9% methane. The product stream, comprising 88.9% hydrogen, 4.9% argon, and 6.2% nitrogen, enters a cryogenic distillation column operating at 400 psig. This cryogenic distillation column produces a top product of hydrogen, capable of being recycled to the ammonia synthesis plant, and a bottoms product stream of concentrated argon. This product stream is cooled by expansion and next enters, at an operating pressure of about 30 psig, a second cryogenic distillation column which, after removing residual nitrogen and hydrogen as a distillate, produces a liquified argon final product. An argon yield of 68% with a methane concentration of 0.5 ppm is obtained.

Temperature, pressure, flow rate and composition of various streams are summarized in Table II.

The above described embodiments are illustrative, but not limitative, of the present invention for argon recovery from an ammonia purge gas stream. Modifications or variations, within the scope and spirit of the invention, may be apparent to those skilled in the art.