A new process for argon recovery from hydrogen depleted ammonia plant purge

R. Krishnamurthy, Steven L. Lerner and Donald L. MacLean

gas

The BOC Group, Inc., Group Technical Center *, 100 Mountain Avenue, Murray Hill, New Jersey 07974, USA

Received 2 7 April 198 7; revised 25 June 198 7

A number of ammonia plants employ membranes or cryogenic hydrogen recovery units to separate hydrogen contained in the purge gas for recycle to the ammonia synthesis loop. The resulting hydrogen depleted purge gas, which is usually used for fuel, is an attractive source of argon. This paper presents the novel features of a process which employs a combination of pressure swing adsorption (PSA) and cryogenic technology to separate the argon from this hydrogen depleted purge gas stream. This new proprietary Hybrid Argon Recovery Process ( HARPTM) plant+ is an effective alternative to a conventional all-cryogenic plant.

Keywords: HARPTM; argon recovery; ammonia plant, hydrogen depleted; purge gas; PSA; cryogenic technology

Air contains slightly less than one percent argon by volume and is a primary source for argon manufacture. The 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, argon has found increasing applications in ferrous and non-ferrous industries as well as electronic component manufacturing. This has resulted in a rapidly growing demand for argon that has exceeded the corresponding growth rates ofeither oxygen or nitrogen. Alternative sources of argon have thus become very attractive. One such alternative source is the purge gas from ammonia synthesis plants.

At an ammonia plant, a fraction of the gas stream is purged from the synthesis loop to maintain an inert concentration below a specified level. Undesirably high inert levels reduce the partial pressure of the reactants and cause an unfavourable shift in the ammonia synthesis reaction equilibrium. Methane and argon are the inert gases. Argon originates from the air supplied to the secondary reformer for providing the nitrogen required in ammonia synthesis. Methane results primarily from the methanation process employed to remove trace levels of carbon monoxide and carbon dioxide that poison the synthesis reaction catalyst. A typical composition of the ammonia purge gas, available at approximately 12.68 MPa (1900 psi), is as follows: 61% hydrogen, 20% nitrogen, 4% argon, 13% methane and 2% ammonia. The argon concentration varies from plant to plant and can range from 3-6%.

*Presented at AlChE Spring National Meeting Houston, Texas, 29 March-l April, 1987 Warious patents pending in the United States and abroad *The 8OC Group Technical Center performs research for group companies such as Airco, BOCL, Commonwealth Industrial gases and Osaka Sanso 0950-4214/87/01016-07$03.00

Q 1987 Bunerworth 8 Co (Publishers) Ltd.

Hydrogen production by steam reforming is a very energy-intensive portion of the overall ammonia synthesis process. Energy reduction in the ammonia plant is achieved by recovering hydrogen in the purge gas for recycle to the synthesis loop’. This maximizes the hydrogen available for conversion to ammonia and hence improves operating efficiency. A number of ammonia plants employ membranes2. 6, or cryogenic recovery units3*4 for this purpose. A hydrogen PSA (pressure swing adsorption) system has also been developed and some- times employed. All of the processes mentioned above separate > 80% of the hydrogen in the purge gas for recycle to the ammonia plant. The waste gas from the hydrogen recovery units is termed the hydrogen depleted purge gas and is usually burnt as fuel in the ammonia plant primary reformer. The stream contains between 6 and 12% argon admixed with hydrogen, nitrogen and methane and serves as a valuable source of argon. Removing the argon does not affect its final use as fuel.

Processes for argon recovery from ammonia purge gas that are currently offered are based solely on cryogenic technology. A three column cryogenic system3 combines argon recovery with the recovery and recycle of hydrogen to the ammonia synthesis loop. For ammonia plants where membranes are used to recover hydrogen, a two column cryogenic process is employed for argon recovery.

The object of this paper is to present the features of a new plant design developed for argon recovery from hydrogen depleted purge gas. This is an alternative to existing all-cryogenic process plants. The HARPTM plant effectively employs a combination of pressure swing adsorption (PSA) and cryogenic technology to separate argon.

A brief review of all-cryogenic processes is provided below. This is followed by a detailed description of the

16 Gas Separation and Purification 1987 Vol 1 September

A new process for argon recovery: R. Krishnamurthy et al.

plant and its essential features. Special emphasis is placed on the gas phase adsorptive removal of all of the methane and a major portion of nitrogen in the feed gas. Finally, those conditions that make the plant an attractive choice for argon recovery at ammonia plants are outlined.

Cryogenic argon recovery

A brief description of cryogenic argon recovery from ammonia plants is presented here to provide a basis for contrasting the HARPTM plant with current cryogenic technology. A three column cryogenic process is typically used for argon recovery from ammonia purge gas. A pretreatment section upstream of the columns removes ammonia by absorption in water prior to drying via adsorption. The dried gas passes through a train of three columns operated at cryogenic temperatures. The first column, which separates a hydrogen rich stream for recycle to the ammonia synthesis loop, operates at high pressure. The second column strips all the nitrogen in the feed gas. Finally, the third column separates the argon- methane mixture into pure products. The methane is returned to the ammonia plant fuel system. The plant also comprises a nitrogen recirculating refrigeration system which acts as a heat pump. Liquid nitrogen required in excess of that produced at the reboilers is made available by expanding a high pressure nitrogen stream.

For the many ammonia plants that are already fitted with a hydrogen recovery system, only two columns are needed for argon production. A two column cryogenic system can accept feed gas from either a membrane hydrogen recovery unit or an existing cryogenic unit. The pretreatment section is also not required since ammonia removal and purge gas drying are effected upstream of the hydrogen recovery unit. A small adsorption unit, however, is still typically provided to remove any traces of ammonia or moisture present in the feed gas. The hydrogen depleted purge gas is treated in a first column to strip all of the hydrogen and nitrogen. The hydrogen/nitrogen mixture obtained as a vapour distillate can be returned to the ammonia plant if desired. The second column separates the argon-methane stream into pure products. Apart from eliminating one column (the hydrogen stripper), the two column process utilizes a much simplified nitrogen refrigeration circuit compared to the three column cryogenic process. The power requirement for the two column process is also significantly lower than that corresponding to the three column process.

Overview of hybrid argon recovery process plant

The pressure and composition of the hydrogen depleted purge gas depends on the design and operation of the ammonia plant as well as the hydrogen recovery plant.

Table 1 Conditions of process streams identified in flow diagram

The pressure ranges from 0.333 MPa (50 psia) to 12.68 MPa (1900 psia); typically, high pressure streams are from membrane separators and low pressure streams are from cryogenic units. The volumetric composition of the hydrogen depleted purge gas is in the range: hydrogen 7-3096; nitrogen 35-60%; argon 6-12%; methane 25-38%.

The composition of a typical feedstream from a membrane hydrogen recovery unit is 9% hydrogen, 12% argon, 54% nitrogen and 25% methane. Feedstreams from cryogenic hydrogen recovery units generally contain higher amounts of hydrogen and methane per unit of argon. The methane to argon ratio in either feedstream varies from about 2 to over 5. The feed gas to the HARPTM plant is assumed to be essentially dry with no ammonia present. Again, as in a cryogenic plant, an upstream adsorber unit removes any traces of moisture and ammonia in the feed gas.

A simplified overall flow diagram is given in Figure I. Temperature, pressure, flow rate and composition of various streams identified in Figure 1 and in the text below, are summarized in Table 1. The feed gas (stream no. 1) enters a two bed pressure swing adsorption system. The PSA separates the feed into two product streams; an argon rich high pressure product (2) and a methane rich low pressure product (3). The PSA is operated such that the argon rich product contains very low, fractional ppm concentrations of methane. This ensures that the final argon product purity is within merchant grade specifica- tions.

The cryogenic section of the plant consists of a single column and a nitrogen refrigeration circuit. For the design conditions in Figure 1 and Table I, the column requires 40 theoretical trays and the nitrogen flow in the refrigeration circuit is 5.3 kmol s-l with a pressure ratio of 16. The argon rich product is fed to the column (4) after being cooled in the main exchanger using warming streams in the nitrogen refrigeration cycle. In the column, pure liquid argon is produced as a bottom product (5) and a hydrogen-nitrogen mixture is obtained as a vapour distillate product (6). A small quantity of nitrogen rich liquid distillate is also withdrawn and is mixed in the refrigeration cycle to make-up for losses from the compressors and expander. The hydrogen-nitrogen distillate (7) is warmed in the main heat exchanger to recover cooling energy. It is then sent at ambient temperature to the PSA unit for bed regeneration. During regeneration it mixes with the methane rich vent stream. The mixed vent and regeneration gas is sent back to the ammonia plant to recoup the fuel value.

A detailed schematic of the PSA unit is shown in Figure2 and a process cycle sequence is given in Table2. The PSA unit is a two bed adsorption system. For the design conditions shown in Figure 1 and Table 1, the total volume

Stream number (see Figure I)

1 2 3 4 5 6 7

Temperature

W)

298 303 293 116

89: 298

Pressure Flow rate

(MW (kmol s-l)

1.17 4.86 1.10 1.91 0.1 1 4.28 0.30 1.91 0.27 0.41 0.28 1.50 0.25 1.33

Hz

9.0 26.4 10.2 26.4

33.5 37.8

Composition (mole %)

Ar N2 CH4

12.0 54.0 25.0 21.4 52.2

4.1 57.3 28.4 21.4 52.2

100.0 0.2 66.3 0.2 62.0

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A new process for argon recovery: R. Krishnamurthy et al.

Nitrogen refrigeration cycle I 1

Make-up N2

PSA product (Hz, Ar, Nz 1

Hz /Nz purge gas 1

I t I

Main heat exchanger ?’ i

HZ Ar

N2

CH4

Vent (+ purge)

CH4

N2 H2 (purge) Ar (losses)

To NH3 plant fuel system

Hz/N2

Sub Argon cooler column

---_

<: ---A

Figure 1 Simplified flow diagram of hybrid argon recovery process

of sieve required in the two beds is approximately 17 m3. In addition to the beds, the PSA unit consists of an equalization tank(C), a backfill tank(D) and receivers for product (E), feed (F), vent (G) and regeneration gas (H). The equalization tank is used to reduce argon losses by recovering the additional argon rich void volume gas prior to bed regeneration. The backfill tank is used to transfer product gas to partially repressurize the beds as well as cleaning the product end flowlines of any residual methane at the start of the production cycle. The various receivers are used to supply uniform and continuous streams in spite of discontinuities in the PSA process steps. A brief description of the various process steps during one full cycle of the PSA unit are given below.

Feedpressurization. At the start of the cycle, bed A is at a pressure intermediate between the high adsorption and low desorption pressures. From this condition, bed A is pressurized with feed gas to the PSAoperating (adsorption) pressure. No product removal occurs during this step.

Production. Once bed A reaches the set point of the back pressure regulator on the product line, production commences. The initial production rate is slightly higher to make up for the lack of production during bed pressure equalization and pressurization. After this short initial period when the pressure in the product receiver increases, production continues over a half cycle at a more or less constant rate determined by the system demand. Produc- tion is stopped once the impurity front approaches the product end of the bed.

1, Liquid argon to storage

Bedpressure equalization. When bed A had completed its production cycle, bed B has completed regeneration and partial repressurization with gas from the equalization tank. The two beds are pressure equalized at this juncture to transfer argon rich void gas from bed A to bed B.

Fotward tank equalization. Bed A is further pressure equalized with the equalization tank through the product end. Argon rich void gas at the top end of the bed is transferred to the tank which is at low pressure at the commencement of this step.

Bed vent. Bed A is finally depressurized to a low desorption pressure (at or slightly above atmospheric) through the feed end into the vent gas receiver.

Table 2 HARPTM PSA cycle sequence

S. No: Bed A Bed B

1 Bed balance Bed balance

2 Product backfill Tank equalization

3 Feed pressurization Tank equalization

4 Feed + Production Vent 5 Feed + Production Vent + Hz/N,

regeneration

6 Feed + Production Tank equalization

7 Bed balance Bed balance

8 Tank equalization Product backfill

9 Tank equalization Feed pressurization

10 Vent Feed + Production

11 Vent + Hz/N, regeneration Feed + Production

12 Tank equalization Feed + Production

18 Gas.Separation and Purification 1987 Vol 1 September

ic

Y

Figure 2 Schematic representation of HARPTM PSA separation

Bed regeneration, While bed A is still open to the vent receiver through the feed end, the hydrogen-nitrogen column distillate mixture is admitted at the top end of the bed. This regeneration gas helps to desorb methane, which is strongly held on the adsorbent, by lowering the gas phase partial pressure of methane. At the end of this step, the bed is filled with hydrogen and nitrogen at desorption pressure. The vent gas together with the regeneration gas is compressed to desired fuel pressure for return to the ammonia plant.

Return tank equalization. Regenerated bed A is partially repressurized by a return pressure equalization with the tank. The tank is now initially at a higher pressure and hence the gas conserved in it during the forward tank equalization is returned to bed A.

Product backfill. Bed A is repressurized by pressure equalization with the backfill tank which contains product quality gas. During this process step, the lines at the product end are also cleared of methane as the residual gas is driven toward the feed end of the bed.

While bed A is in the pressurization and production cycle, bed B is depressurized and regenerated. The cycle then reverts with bed A undergoing depressurization and regeneration while bed B is pressurized and starts to produce.

PSA separation

Process performance hurdles

The key feature of the HARPTM plant is the pressure swing adsorptive removal of all of the methane and a major portion of the nitrogen in the feed gas. The most important performance requirement of the PSA separation is the reduction of methane from N 30% in the feed to fractional ppm levels. The reason for such stringent purity requirements is that any methane in the argon rich PSA product will separate in the distillation column with the pure argon product. For example, if the final argon

product specification is 0.5 ppm methane and the PSA product contains 20% argon, methane must be reduced in the PSA to 0.1 ppm. In devising a PSA process, the primary objective is to maximize argon yield while meeting the product purity with respect to methane. A secondary objective is to minimize the quantity of nitrogen that separates with the argon rich product down to a minimum acceptable level. It is desirable to reduce the nitrogen content since the quantity of nitrogen determines the power required for the downstream cryogenic separation. However, a certain minimum amount of nitrogen must be present in the PSA product so that: (1) the cryogenic separation can be performed without significant argon losses, and (2) the column can operate at a pressure lower than available feed pressure to avoid feed compression. For a given quantity of hydrogen in the feed, the pressure required for the cryogenic separation decreases with an increase in the quantity of nitrogen that separates with the PSA product.

Laboratory tests

Extensive laboratory testing and evaluation of the PSA separation, using various size units, has been performed. These tests have demonstrated both process feasibility and flexibility. It has been shown that methane in the feed gas can be effectively removed to extremely low concen- trations. An argon rich PSA product containing < 0.05 ppm (the instrument detection limit) has been achieved and sustained. Overall argon yields > 70% have consistently been achieved using a simple PSA process cycle for a number of feed streams. Process variations that further increase argon yield to > 85% have been identified and evaluation of these process options is currently being performed.

Experimental procedure

Experiments were carried out using a small scale, automated, two bed PSA unit. All processing steps recommended for the full scale process were included in the testing procedure. The four component mixture that simulates the hydrogen depleted purge gas was obtained by mixing pure gases using a multicomponent gas mixing system specifically designed and developed for this application. The mixing system supplies a continuous, uninterrupted source of the gas mixture; the desired composition is dialed in from a control panel. A programmable controller was used to regulate the opening and closing of solenoid valves as required by a specific cycle sequence. The compositions of the feed and product streams were analysed using a gas chromatograph. For the argon rich product stream, a dual level analysis was employed. The bulk composition was analysed using a thermal conductivity detector while the trace methane was analysed using a flame ionization detector. Test measure- ments also included determination of volumetric flow rate of products. Since the low pressure product was dis- continuous, flow and composition were determined as an average over a cycle. Mass balance closures were determined for all experiments to verify accuracy of the various measurements.

Adsorbent

The PSA separation employs a commercially available type 5A zeolite molecular sieve as the adsorbent material. This particular sieve was found to offer high argon to

A new process for argon recovery: R. Krishnamurthy et al.

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A new process for argon recovery: R. Krishnamutthy et al.

methane selectivity and consistently reduced methane to desired low levels. The equilibrium capacity for the sieve is in the following order: methane > nitrogen > argon > hydrogen. All of the hydrogen, therefore, separates with the argon rich stream and the nitrogen distributes between the argon rich and methane rich products. Various other sieves evaluated includes two grades of activated carbon, zeolites 4A, 10X 13X and sodium mordenite.

PSA operating pressure

Laboratory tests have confirmed PSA process feasibility between 0.333 MPa (50 psia) and 3.33 MPa (500 psia). The optimum argon yields, however, decreases at pressures above 1.33 MPa (200 psia). Therefore, a desired range of operating pressure for the PSA is 0.333 MPa (50 psia) to 1.33 MPa (200 psia). The PSA operating pressure selected for a particular plant must be preferably less than available feed pressure. The hydrogen purge gas from membrane separators is typically available at a high pressure. However, the pressure of feedstreams from cryogenic hydrogen recovery units may be as low as the ammonia plant fuel header pressure. In most ammonia plants, this pressure is above 0.333 MPa (50 psia).

Feed composition

PSA process flexibility has been demonstrated by confirming performance over a wide range of feed conditions; argon and methane concentrations between 3 and 12% and 13 and 38%, respectively. Laboratory tests were also conducted on a feed that corresponded to the hydrogen rich ammonia plant purge gas. Performance comparable to that obtained with the hydrogen depleted purge gas was achieved. Argon yield in the PSA has been determined to be almost independent of the methane to argon ratio or the absolute argon concentration in the feed.

Hydrogen in the feedstream has the following effects:

1 all of the hydrogen in the feed is sent to the cryogenic section and returns via the distillate stream as a regeneration gas source for the PSA. Increased regeneration gas availability improves methane removal and hence PSA performance; and

2 as the amount of hydrogen in the feed increases, the hydrogen concentration of the PSA product increases. This results in a higher pressure requirement for the cryogenic separation.

The composition of the hydrogen depleted purge gas is dependent on the design and operation of the ammonia plant as well as the hydrogen recovery unit and can vary over a wide range from plant to plant. Further, even for a specific ammonia plant, variations in feed conditions are expected when changes occur in the ammonia purge gas rate and other operating parameters. Ammonia plant loading is a primary cause for operational variations.

PSA bed regeneration

Optimum PSA performance and increased PSA-cryogenic integration in the HARPTM plant is achieved by using the hydrogen-nitrogen mitiure separated as a distillate vapour from the column for PSA bed regeneration. Complete regeneration of methane ensures product purity requirement, improves bed utilization and increases argon yield. Increased bed utilization also results in a

higher specific product (production rate per unit volume of adsorbent), thereby reducing required adsorbent volume. Bed regeneration is, therefore, a critical step in the PSA separation.

Three modes of bed regeneration evaluated for the HARPTM plant PSA separation include vacuum, product purge and hydrogen/nitrogen purge regeneration. Use of hydrogen/nitrogen purge was determined to be the most efficient for the following reasons:

a significant quantity of hydrogen/nitrogen purge gas is available. Although the exact amount available is dependent on the concentration of hydrogen and nitrogen in the feed gas, the ratio of regeneration gas to feed is typically in the range 30-45%; a negligible quantity of argon is present in the hydrogen/nitrogen mixture obtained as the distillate product. Argon losses through the regeneration gas is, therefore, insignificant. In contrast, using the product purge method results in a 20-30% argon loss solely through the regeneration gas; this method combines the pressure swing with both inert gas and displacement gas modes’ of bed regeneration. Hydrogen serves as an inert gas as it is not adsorbed on the sieve. Nitrogen, however, displaces methane during regeneration and results in complete methane removal; the nitrogen adsorbed on the sieve during the regeneration step results in an initial condition that improves argon to methane selectivity on the sieve during the production cycle. Advantage is taken of this higher selectivity by operating at a higher ratio of product rate to feed rate; and compared to vacuum regeneration, use of hydrogen/ nitrogen mixture is significantly less capital or energy intensive. Further, with vacuum regeneration, there exists a possibility of air leakage into the beds. This will introduce traces of oxygen into the system, thereby necessitating additional downstream purification steps. Finally, with vacuum regeneration, most of the nitrogen is removed with the methane product. The cryogenic separation becomes more difficult because of the high hydrogen concentration in the argon rich product.

Alternative processing options

The process described earlier represents an optimum selection of processing methods for a typical plant. A number of alternative process options were identified during the initial phase of process development. These alternatives may be attractive for specific cases and are summarized below.

For hydrogen rich feeds, the cryogenic separation of the PSA product gas may require high pressures. Removal of hydrogen prior to final cryogenic purification will be an energy saving alternative. Two methods were identified to remove hydrogen non-cryogenically.

If the PSA is operated at, or over 1.33 MPa (200 psi), a membrane separator can be provided to treat the PSA product. The hydrogen rich product from the membrane separator at low pressure can be used as PSA regeneration gas or returned to the ammonia plant. The argon rich product at high pressure has a very low hydrogen concentration. It can be cryogenically purified at a low pressure, resulting in energy savings.

20 Gas Separation and Purification 1987 Vol 1 September

A new process for argon recovery: R. Krishnamurthy et al.

For feedstreams available at less than 1.33 MPa (200 psia), the PSA bed may be tilled with a mixed adsorbent. In addition to zeolite molecular sieve, the bed would also contain a metal hydride, for example, Hystor alloy La Ni4.7Al0.3 (a commercial product from Ergenics Inc., Wycoff, New Jersey). The two adsorbents may be mixed throughout the bed or the hydride can be placed on top of the zeolite in a two layered arrangement. The zeolite adsorbs all the methane and a portion of nitrogen. The metal hydride adsorbs the hydrogen in the feed gas. During regeneration, the hydrogen desorbed from the top of the bed aids in the regeneration of methane and nitrogen from the zeolite. The PSA product contains significantly less hydrogen than if metal hydrides were not used. Consequently, the cryogenic separation can be carried out at a lower pressure.

For low argon capacity plants (e.g. < 10 tons/day), a significantly lower capital alternative would be to provide liquid nitrogen from storage tanks to meet the cooling energy required at the condenser. The vaporized nitrogen can be returned to the ammonia plant air compressor suction. The small amount of cooling provided by nitrogen vapour at the plant air compressor will result in a marginal increase in air intake and favour improved compressor operation. Use of liquid nitrogen eliminates the nitrogen refrigeration cycle, which is a capital intensive part of the process. This process option would be very attractive, if a liquid nitrogen plant exists close to the ammonia plant.

HARPTM plant and all-cryogenic plant comparison

A quantitative economic comparison of a HARPTM plant with an all-cryogenic plant is feedstream specific and such an exercise is beyond the scope of this paper. However, major cost factors for the two processes are highlighted below as well as possible guidelines for plant selection.

The HARPTM plant has been developed with the objective of offering a lower capital and lower power alternative. The capital cost reduction over an all- cryogenic plant is achieved by:

1 eliminating one column and auxiliary equipment by complete methane removal in the PSA;

2 reducing cryogenic equipment size by separation of > 50% of the feed gas in the PSA, and

3 simplifying the nitrogen refrigeration cycle.

It must be noted that the capital cost savings achieved in the cryogenic section must also support the cost of the PSA unit in the HARPTM plant. Unlike the cryogenic section, however, which requires several expensive auxiliary pieces of equipment such as a cold box for insulation, the PSA unit hardware is relatively simple and low cost. The cost of the PSA unit is between 10 and 20% of total plant cost.

A HARPTM plant achieves a significant energy reduc- tion compared to an all-cryogenic plant by:

1 performing the PSA separation using available feed pressure energy;

2 significantly reducing the column separation energy by accomplishing bulk removal of unwanted ponents in the feed gas using the PSA; and

3 reducing the quantity of gas to be condensed.

com-

For returning the fuel to the ammonia plant, compression may be required depending on the fuel header pressure. Returning this fuel gas directly to dedicated burners may minimize or totally eliminate the need for additional compression. Fuel gas compression energy, if required, will be higher for the HARPTM plant compared to an all-cryogenic plant since methane and hydrogen streams can be removed by the latter at a slightly elevated pressure. Net energy savings realized by a HARPTM plant, therefore, is the difference in compression energy required in the nitrogen refrigeration cycles less the difference in the fuel gas compression energy.

In the cryogenic process, an argon yield of > 95% can be achieved. A HARPTM plant operates at a lower argon production rate due to argon losses with the methane rich PSAvent gas. This factor must be taken into consideration in determining overall savings and project return. For a given feedstream, there exists a cut-off in argon production rate that must be exceeded by the HARPTM plant in order to have a lower unit production cost. Laboratory results and economic calculations indicate that this cut-off argon yield is exceeded for a number of design feedstreams. For a given amount of argon in the feed stream, a HARPSU plant becomes more attractive over an all-cryogenic plant as the quantity of methane and nitrogen increases. The single most important factor that determines the potential savings is the ratio of the amount of methane plus half of the nitrogen in the feed to the amount of argon in the feed. This factor expresses the quantity of gas removed at ambient temperature by the PSA. The simplifications introduced in the HARPTM plant design compared to an all-cryogenic process also make it an attractive choice for very low argon capacity plants.

Conclusions

A Hybrid Argon Recovery Process plant which employs a combination of PSA separation and cryogenic distillation, has been developed as an alternative to an all-cryogenic plant for argon recovery from hydrogen depleted ammonia plant purge gas. A novel aspect of the plant is the PSA removal of methane and nitrogen in the feedstream. This gas phase argon enrichment is carried out at ambient temperature.

Laboratory testing has confirmed the feasibility of the PSA bulk separation and the removal of methane down to fractional ppm levels. Process flexibility with respect to feed composition and pressure has also been demonstrated. The high argon yields that have been achieved using a simple 2-bed PSA process cycle make the plant a viable and attractive alternative. New variations in the PSA process step that can further increase argon yield have been identified and evaluation of these improvements are actively being pursued.

The HARPTM plant achieves significant capital and power savings over all-cryogenic plant for feedstreams with a high methane to argon ratio. These savings are a result of:

1 elimination of one column achieved through complete methane removal in the PSA;

2 reduction in the quantity of gas processed in the cryogenic section realized through gas phase removal of methane and nitrogen using feed pressure energy; and

3 simplifications to the cryogenic nitrogen refrigeration cycle.

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A new process for argon recovety: R. Krishnamorthy et al.

Acknowledgements 2 Chae, Y.C., Legendre, G.S. and van Gelder, J.M. Hydrogen Recovery from Ammonia Plant Purge Gas Using Hollow-Fibre

Preliminary process and engineering specifications for the PSA and cryogenic portions of a full size HARPTM plant were developed by Airco PSA and BOC Cryoplants, respectively. The contributions of Y. Shukla in process development and B.J. Mychajliw in laboratory testing is gratefully acknowledged.

References

Gas Separators 4th Fert. Nitrogen Proc., Br. Sulphur Corp. Int. Conf Ferr. Technol. (1985) 457-469

3 Gilmartin, B., Hwang, W., Kamrath, D. and Musicus, P. Production of Argon from Ammonia Plants The BOC Group Technology Magazine (1985) 2 19-24

4 Haslam, A., Brook, P. and Isalski, H. Recycle Hydrogen in Ammonia Purge Gas Hydrocurbon Proc. (1976) 103-106

5 Keller II, G.E. Gas-Adsorption Processes: State of the Art in ‘Industrial Gas Separations’ ACS Symp. Ser. (1983) 223 145-169

1 Brykowski, FJ. (Ed.) Ammonia and Synthesis Gas - Recent 6 MacLean, D.L., Prince, C.E. and Chae, Y.C. Energy-Saving and Energy Saving Processes Chem. Tech. Rev. (1981) 193; Modifications in Ammonia Plants, Chem. Eng. Prog. (1980) 3 Energy Tech. Rev. (1981) 68 98-104

22 Gas Separation and Purification 1987 Vol 1 September