Adsorcion Gas

GAS-ADSORPTION PROCESSES - AN UPDATE

George E. Keller II Union Carbide Corporation

South Charleston. West Virginia

ABSTRACT

Gas-adsorption processes are commonly used in the petroleum. natural-gas, petrochemical and other industries. It is important to be aware of those process situations which favor the use of adsorption, and although no hard and fast rules can be given. some generalizations can be. and the first part of the paper is devoted to a discussion of these generalizations. Adsorption processes come in a wide range of physical embodiments. These embodiments are first discussed as idealizations, and their strong points and weak points are delineated. Next, several specific process flowsheets which have been commercialized rather recently and which may be extrapolable to other separations are discussed. Finally, the issue of where gas-adsorption technology is headed is confronted. Suggestions are made as to possible new and expanded applications.

INTRODUCTION

Distillation and related vapor-liquid-based separations (absorption and azeotropic and extractive distillation) are by far the most widely used separation processes in the natural-gas, petroleum, petrochemical and related industries. In addition, vapor-liquidbased separations have been practiced for several hundred years. and they are probably approaching a technological asymptote. By contrast, adsorption has been practiced, except for a few instances such as solutionclarification and for air purification in hospitals and on battlefields, for only about 60 years. But in this relatively short time, and having approached much less closely its technological asymptote, adsorption now ranks second to vapor-liquid-based separations in frequency of use in the above-named industries.

In this paper we will first describe those process conditions which favor the use of gas adsorption. We will then describe several archetype processes and delineate their strong and weak points. Next some specific variations

which have been commercialized will be discussed. and. finally. some suggestions will be made as to directions in which the technology is moving and opportunities for expanded applications for gas adsorption.

WHEN CAN ADSORPTION COMPETE?

Gas adsorption's chief competitors are obviously the vapor-liquid separations and. more recently, membrane-based processes. Vapor-liquid processes are formidable competitors because of their simple flowsheets; this translates into relatively low capital costs per unit of feed processed. Given the importance of capital costs in overall process economics, vapor-liquid separations will usually be a first choice if the energy costs are tolerable. And in fact, systems of distillation columns can often be heat-integrated to reduce energy costs, further increasing their economic viability in competition with other separations.

Nevertheless, there clearly are situations for which adsorption is the proper choice. Although we cannot cite precise criteria, we can enumerate several rough criteria. In the following list, distillation is assumed to be the chief competitor, and it is also assumed that an adsorbent with proper selectivity (greater than two for the adsorbate over less-adsorbing components) is available.

1. The relative volatility for distillation is low - 1.2-1.5 or less.

2. Several components must be separated from other components boiling in among the components to be separated.

3. Pressures greater than about 40 to 60 atmospheres (and especially if the feed gas is substantially lower in pressure than the column pressure) and/or cryogenic temperatures must be used.

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Proceedings from the Sixth Annual Industrial Energy Technology Conference Volume II, Houston, TX, April 15-18, 1984

4. Thermal damage to the products or rapid column fouling occurs at practical distillation conditions.

Criteria for selecting between adsorption and membrane-based processes are not yet available, and the proper choice must be made on a case-by-case analysis.

ARCHETYPE PROCESSES

Four archetype processes will be briefly discussed here. These processes differ primarily in the means by which desorption is effected. In a following section we will show some recent process modifications.

Temperature-Swing Adsorption - A schematic diagram is shown in Figure 1. Once a bed is loaded with adsorbate, it is taken off-line and regenerated by heating and (almost always) purging with an inert gas. Increasing the adsorbent temperature reduces the tenacity of the adsorbate-adsorbent bonds, and the inert gas reduces the partial pressure of adsorbate; both effects serve to facilitate desorption.

The advantages of this process are that high degrees of separation are possible and that investments are usually reasonable. The disadvantages are that the energy usage is very high per unit of adsorbate and that regeneration times are long - several hours to a day or more. As a result, this cycle is economical only for removing small amounts (generally a few weight percent or less) of materials from feed streams.

Less-Adsorbed Purge Gas Product

Heater

c o

Vent

Cooler Possible Recycle Adsorbate

Feed

Figure 1. Temperature-Swing Cycle

Pressure-Swing Adsorption (PSA~ - In this cy¢le, shown in its simplest form in igure 2, the! adsorbate is desorbed by lowering its partial pressure, partly by reducing total pressure and partly by using some of the non-adsorbate as a partial-pressure-reducing purge gas. Usually PSA processes employ at least three beds in ' parallel, with several purging and . blowdown/repressurization steps to minimize feed compression costs and maximize less-adsorbed product recovery.

Less-Adsorbed Product

rn rn c c

D D ~

0 ~

0 ~ ~

u W ~ 0

Adsorba1e Plus Le~sPressurized AdsorbedFeed Product Purge Gas

Figure 2. PSA Cycle

The advantages of PSA are that it can perform bulk separations (those for which t~e adsorbate concentration is greater than about 10 weight percent) and that investments are competitive with alternative processes. The chief disadvantage is that only the less-adsorbed product can be recovered in h~gh purity; the adsorbed product inevitably con~ains some of the less-adsorbed product used as a I purge. Desorption under vacuum can reduce adsorbed-product contamination, however.

Inert-Purge Adsorption - This cycle, shown in Figure 3, operates adiabatically and uses an inert gas to lower the partial pressure of ~he adsorbate and desorption. Occasionally total pressure will be reduced.

The chief advantages are process simplicity, leading to low investment, and the ability to perform bulk separations in some cases. Th~ disadvantages are the large exotherms, which limit the adsorbate loading, the fact that very large amounts of inert gas must be used if the adsorbate is tightly held, and the problem of recovery of the adsorbate from the inert gas.

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Less-Adsorbed Purgerroduct Gas

~ ~ ~ ~

~ ~

a ~ ~

a ~ ~ ~ ~ ~ Q

Adsorbate Plus

Feed Purge Gas

Figure 3. Inert-Purge Cycle

~lacement-Purge Adsorption - This final archetype cycle, shown in Figure 4, is similar in concept to inert~purge adsorption except that the displacement medium competitively adsorbs with adsorbate and effects desorption both by partial-pressure reduction and by mass action. The displacement medium boils outside the range of the products and is recovered from the products by distillation.

A = Adsorbates B = Less-adsorbed components C = Displacement agent

B

D

B+D

~

~

A ~ ~

a ~ ~ ~ ~D Q ~ ~

a ~

~

~ ~ ~

A+D Feed A+B

Q

D

The chief advantages are elimination of the exotherm, which makes higher loadings possible, and the ability to perform bulk separations. The disadvantages are process complexity, which leads to high investment, and the energy requirement to operate the stills.

Conclusion - Compared to distillation, gas-adsorption technology is substantially more varied, and clearly one flowsheet cannot be used for all separations. One must therefore be prepared to deal with both adsorbent development and flowsheet selection and development to effect a successful process development.

NEW DEVELOPMENTS

In this section we will highlight some recent developments - variations on the archetype processes - which either increase the number of separations amenable to adsorption or improve the economics of the archetype processes.

Continuous Temperature-Swing Adsorption - All of the archetype processes are discontinuous: adsorbate is first loaded and then desorbed in a fixed bed. The idea of performing continuous adsorptions by moving the adsorbent between adsorption and desorption zones is far from new and extends at least back to the 1930s. In 1950 the Hypersorption process was developed by Union Oil Company [1,2]. This process foundered because of excessive attrition of the activated-carbon adsorbent. Recently Kureha Chemical Company, Ltd., of Japan has developed a hard, microspherical activated carbon called bead activated carbon (SAC). This development has made possible a fluidized-bed/moving-bed process, shown in Figure 5, for removin9 small amounts (less than a few weight percent) of adsorbates from feed streams. The process,called GASTAK in Japan [3] is marketed in the United States under the name PURASIV HR [4,5] by Union Carbide. There are presently 12 operating units in the United States and over 40 in Japan. The units in the United States treat from 300 thousand to 5.5 million cubic feet per hour of feed, removing such materials as hydrocarbons and oxygenated and chlorinated hydrocarbons.

Nitrogen Recovery - Bergbau Forschung GmbH has developed a process for recovering nitrogen from air in up to 99.9 percent purity using a PSA-type cycle with vacuum desorption [6,7]. The adsorbent is a unique carbon molecular sieve which separates on the basis of differingdiffusion rates in the adsorbent pores rather than on inherent equilibrium selectivity; the carbon is virtually non-selective at equilibrium. Under diffusion control, however, oxygen penetrates into the particles much faster than nitrogen, leaving a nitrogen-enriched, gas-phase product. Nitrogen production using carbon molecular sieves is the only known commercial process using diffusion-selectivity as the basis for separation.

Figure 4. Displacement-Purge Cycle

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Less-Adsorbed Gas

Recovered Solvents

.~ ~. 1---

Feed

Steam

Condensa te

...

Li ft Gas

Figure 5. PURASTV HR Process

Other methods of using PSA for obtaining nitrogen from air have been revealed by Toray Industries, Inc. [8J and Air Products Corp. [9J. These processes, by adjusting feed and purge flows, produce a relatively pure adsorbate (nitrogen) stream and a relatively impure lessadsorbed (oxygen) stream, using more traditional moleculat" sieve adsorbents. A diagram of the Toray process is shown in Figure 6. Dried air is passed through the absorber at superatmospheric pr ssure, and nitrogen is preferentially adsorbed. Part of the nitrogen product from previous cycles is then passed into the bed to desorb small amounts of oxygen, after which the bed is reduced to atmospheric pressure to desorb nitrogen. Finally more nitrogen is recovered by vacuum desorption. The adsorbent used in this process is apparently the same zeolite molecular sieves - as that used in oxygen PSA processes.

This process has been demonstrated in pilot-scale. Its commercial status is unknown. The Air Products process has been commercialized.

"POLYBEO" PSA - Usually in the design of a process, the simpler the flowsheet, the better the economics. An exception to this generality is the POLYBED PSA process for hydrogen recovery, commercialized by Union Carbide [10-12J. Normally P5A processes will consist of four or fewer beds in parallel; POLYBEO PSA USes five and in some cases even over 10 beds in parallel. In addition, the process involves extensive gas interchanges and pressure eQuiliz tions between the beds. The net result is a process which can treat very large feed streams (up to about two million standard cubic feet per hour in a single train), recover higher percentages of hydrogen in the feed (about 86 percent vs. 70 to 75 percent for more conventional PSAs), and produce astoniShingly pure hydrogen (e.g., 99.999 percent) .

Azeotrope Breaking - Some of the more cos ly mixtures for separation by vapor-liquid means are azeotrope-forming solutions. Extractive or azeotropic distillation must be used, and several columns must be used. Quite recently a new adsorption-based process has surfaced which is especially suited for separating aQueousorganic mixtures containing up to 20 or more weight percent water [13J. An inert-purge cycle is used, and in the example discussed, carbo~ dioxide or nitrogen is used as the purge gas for separating the ethanol-water azeotrope using molecular sieves. The important feature of the process is that most of the heat of adsorption is stored in the bed during the adsorption p~rt

of the cycle, and that heat is then available for the subsequent desorption of the water. The net result is tha , along with an investment lower than that of azeotropic distillation, ohe energy requirement is less than 2000 Btu/gal. The process is being commercialized.

H20/C02 Separation Section ~ ,.-- --'A.~ ______

Removal ,------.---...,. - - - ,- - .,

I I I I I I I I

rn c

Ol C .D

Ol >N C .n 0

>-C1)

0

..A

Vl Ol 0 >- Vl ::;) OJ

0.. 0

I 1--II----__..... I L..... - ------~R-e~fl:-u-x---..6

Pressurized Ai r

Figure 6. Toray Nitrogen Process

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"TIP" Total Isomerization Process - Isomerization reactions often have equilibrium limitations which prevent attainment of one pure isomer. This problem can be obviated if a separation process can be coupled with the isomerization step to remove selectively the isomer(s) of interest. This has been done for paraffin isomerization by combining the Hysomer paraffinisomerization process. developed by Shell Research BV, with Union Carbide's IsoSiv normal/ isoparaffin separation process [14J. The overall process, known as the TIP Total Isomerization Process, is shown in Figure 7. The Hysomer reactor reacts the feed to a near-equilibrium mixture of isomers, which is fed to the adsorption unit. Normal paraffins are preferentially adsorbed, and the isoparaffins are recovered as a high-octane stream while the adsorbed normal paraffins are desorbed by a hydrogen purge and returned to the Hysomer unit. Product from a TIP unit has a Research Octane Number of 88 to 92, compared to a RON of 79 to 82 for the product from a Hysomer unit alone.

Light Pa ra Hi ns

H2 + Recycle Feed Normal Paraffins

Octane Isomer Produc t

Heater

C7\ C

.D <.. o Vl -0 «

H

Figure 7. TIP Total Isomerizc.tion Process

NEW DIRECTIONS FOR GAS ADSORPTION

It is unlikely that gas adsorption will ever rival the vapor-liquid separations in frequency of use in the natural-gas, petroleum, petrochemical and related industries. Nevertheless, gas adsorption will continue to make inroads as its technological limits are more closely approached. Gas adsorption's serious competition for the separations where it is now used will come chiefly from fixed-membrane processes. The extent of the inroad cannot yet be discerned. Below we will point out the likely areas of growth for gas

adsorption and suggest some technological innovations which could stimulate this growth.

Bulk Gas Separations - Adsorption for oxygen and nitrogen purification has been limited until recently primarily to applications requiring a few pounds to a few tons per day. Oxygen is now produced via PSA for small steel mills, waste-treatment plants, welding shops, and for in-house care of patients with various types of lung disease. On-board oxygen generation for aircraft will soon be a reality. But as process improvements continue, PSA oxygen will be able to compete economically with cryogenicallyproduced oxygen in much larger plants - 100 tons per day or more - in situations in which the higher argon level in the PSA oxygen is not a problem.

Nitrogen production via PSA is growing rapidly, for such uses as inerting of storage tanks and other vessels and areas, purging of process lines and vessels, creating modified atmospheres to prolong storage life in foodstorage areas, etc. And if grain-storage fumigants uch as ethylene dibromide come under increasing environmental pressure, nitrogen inerting may prove to be an ideal replacement for use in enclosed areas.

PSA hydrogen recovery will also grow in popularity in comparison to cryogenic methods. It will be possible to process feed streams of 1.5 to three million standard cubic feet per hour at pressures up to about 1000 psi. P A will also be favored in cases in which the virtual absence of inerts in the hydrogen product would minimize purge stream losses in other parts of the process.

The major challenge for PSA is to produce two nearly pure products at once in a simple fashion; such a capability would allow PSA to compete head-on with distillation for an increasing number of separations. Increasing the feed-to-purge pressure ratio is the primary means for improving the purity of the adsorbed product. Desorption under vacuum is one means though not without added costs - for increasing this ratio. Other PSA cycles not involving the need for high ratios would constitute a true breakthrough.

Inert-purge cycles, given the recently-demonstrated success in drying of azeotropes, would seem to be poised for use in several new separations. Prime candidates include those systems now separated by azeotropic and extractive distillation, many of which contain water as one constituent. The use of inert-purge cycles for isomer and other close-boiler separations should also grow for those systems whose components can be easily separated from the purge gas.

Displacement-purge cycles will not find many new uses, primarily because of the inherent complexity of these cycles.

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Fixed-bed, temperature-swing processes rarely turn out to be economical for bulk separations. Moving-bed and fluidized-bed processes based on thermal regeneration may prove to be much more economical because of lower heat requirements per unit of feed. The key to the success of these processes lies in the development of highly-attrition-resistant adsorbent particles.

Some other areas of new or expanded use for gas adsorption include synthesis gas ratio adjustment, carbon dioxide recovery from oil and gas-field streams. ammonia process vent-recovery, and purging of inerts from chemical-process streams.

Gas Purification - It is not likely that many process innovations will be needed for these separations. Fixed-bed, temperature-swing processes will continue to predominate, although moving-bed and fluidized-bed processes such as PURASIV HR should become more popular. Airpollution concerns should bring about increased applications for recovery of organics from various process vent streams, storage-tank vents. and air streams from solvent-pointing and other operations involving vaporization of organics. Major-use areas such as gas dehydration, removal of sulfur compounds and carbon dioxide and various specialty separations will grow about in proportion to the growth of those industries in which these separations are found.

References

[1] Berg, C., Petroleum Refiner 30, No.9, 241 (September,1951).

[2] Treybal, R. E., "Mass Transfer Operations", McGraw-Hill (1955).

[3] Sak aguch i, Y., Chemica1 Economy and Engineering Review ~, No. 12, 36 (December, 1976).

[4] Anon., Chemical Engineering, 39 (August 29, 1977).

[5] "PURASIV HR for Hydrocarbon Recovery", Union Carbide Corporation, Danbury, CT.

[6J Knoblauch, K., Chemical Engineering, 87 (November 6, 1978).

[7] "Pure Nitrogen Generator", Gas Services International, Ltd., Enfield, Middlesex, England.

[ 8J Miwa, K., and T. Inoue, Chemical Economy and Engineering Review 12, No. 11, 40 (November, 1980).

I [9J Sircar, S., and J. W. Zondlo, U. S. Patent

4.013,429, March 22, 1977. I

[ lOJ Heck, J. L., and T. Johansen, Hydrocarbon Processing. 175 (January, 1978).

[11 ] Corr, F., Dropp, F., and E. Rudelstorferr, Hydrocarbon Processing. 119 (March, 197~).

[12J Cassidy, R. 1., "POLYBED Pressure-Swing I Adsorption Hydrogen Processing", in Flank, W. H. (ed.), Adsorption and Ion Exchange with Synthetic Zeolites, Ameridan Chemical Society Symposium Series 135 (1980) .

[13J Garg, D. R., and J. P. Ausikaitis, Chem4 Eng. Progress, 60 (Apri 1, 1983).

[14J Symoniak, Hydrocarbon Processing, 110 n-lay, 1980) .

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Adsorcion Gas

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