yngas production means, in most cases, the manu- S facture of gases consisting mainly of hydrogen and carbon monoxide. The H*/CO ratio varies from about 1 : 1 for oxo alcohols through 2: 1 for methanol, to pure hydrogen for ammonia, as well as for various hydrogena- tion processes such as the hydrocracking of heavy crude or residual oils. The manufacture of these syngases starts nowadays normally with steam reforming or partial oxidation of hydrocarbons, thereby producing gas mix- tures that consist mainly of hydrogen and carbon mon- oxide. In most cases, CO shift conversion follows that should be as complete as possible, if pure hydrogen is re- quired as the final product.

The most common feedstock for syngas production in the U.S. is natural gas--i.e., methane, which has a carbon to hydrogen ratio of 1 :4. Naphtha is also used for steam reforming, and has an average C/H ratio of 1 : 2. The highest carbon content is reached in heavy crude or residual oil with an average ratio of about 1 : 1.5. For hydrogen manufacture, almost all of the carbon in the feedstock reappears after shift conversion in the form of COz. Its concentration then ranges from about 18% for shifted steam reformer gas from methane to 35% and more for shifted gas from partial oxidation of heavy crude or residual oil.

Since all COz produced has to be removed from the syngas, the respective gas treating process contributes a considerable part to the production cost. Therefore, the selection of a suitable COZ removal system is very important for the overall economics of gas production. The gas treating costs are of specific importance if heavy crude or residual oil is gasified. This follows not only from the high carbon content in the feedstock, which leads to a corresponding high amount of COZ after shift, but also from the fact that crude oils normally have a high sulfur content of up to 4% and more, which cannot be economically removed in the liquid state. Therefore, the oil is gasified by partial oxidation together with the sulfur, which is then removed from the raw gas mainly in the form of hydrogen sulfide and carbonyl sulfide. This desulfurization is an additional cost factor for syngas production.

I t is the intent of this paper to present some modern solutions for the purification of syngases-desulfurization and CO2 removal-that offer several advantages re- garding economics as well as safety of operation. Efficient Acid Gas Removal

Rectisol and

Purisol G. HOCHGESAND

General Aspects of Physical Absorption Lurgi has invented two gas treating processes for the

specific requirements of high pressure syngas production : Rectisol (3, 6) and Purisol (7). The first process (jointly developed with German Linde) was originally designed and applied in conjunction with Lurgi’s pressure gasifi- cation of coal, which involved more than sweetening sour gas (5) : gas naphtha, gumformers, ammonia, and cyanides have to be removed together with COz and sulfur compounds. Ten big Lurgi plants running suc- cessfully on coal gas have demonstrated the reliability of the Rectisol process for that difficult function. In a simplified modification, the same process can be applied

for High Pressure Hydrogen

and Syngas Production

VOL. 6 2 NO. 7 J U L Y 1 9 7 0 37

TREATED GAS TREATED GAS

FLAWIN2 SlRppNG HOT cou) REGENERATION REGENERATION

Figure 7 . Dzyerent types of regeneration

Figure 2. Equilibrium lines f o r chemical and physical absorption

to the less contaminated gas produced from liquid hydro- carbons.

The Purisol process-developed by Lurgi alone- was first applied for natural gas sweetening. But the new trends in syngas production have shown that it can serve with increased efficiency for the purification of high pressure hydrogen.

Both processes are based entirely on physical absorp- tion and desorption of the gas impurities. They differ, therefore, from most of the conventional gas treating processes, such as ethanolamine and hot carbonate sys- tems based on chemical absorption. It might therefore be useful to outline the major pros and cons of physical absorption by comparing it with chemical systems.

Figure 1 shows some basic schemes of gas absorption systems. The absorption step of all these systems is more or less similar: a trayed or packed absorber in which the raw gas is contacted countercurrently with the solvent.

There are three types of regeneration that differ in principle as well as in suitability for chemical and physical processes. The most economic method of regeneration is flash desorption, which is shown in the left-hand section of Figure 1. The rich solvent, under pressure at the base of the absorber, is let down in several stages. The operating pressure determines the amount of dissolved gases released in each flash. The acid gas content of the regenerated solvent is related to the pres- sure of the last stage, which is normally held somewhat above atmospheric pressure. For example, if one assumes that only COZ is dissolved and the final flash operated a t about 17 psia, then the solvent recycled to the top of the absorber will have a COZ equilibrium vapor pressure of 17 psia. The COZ partial pressure of the treated gas can therefore be lowered to about 25 to 30 psia, provided there is a reasonable number of trays in the absorber. At a total pressure of 1,000 psia, this partial pressure corresponds to a final COZ content of 2.5 to 3.0 vol yo, which is already sufficient for some syngas requirements. If further COZ removal from the treated gas is required, a vacuum flash can be added; however, this is only economical down to approximately 4 psia.

The solvent is again degassed first by pressure reduction, followed by reduction of partial pressure of the com- ponent to be desorbed by stripping the solvent with an inert gas. The partial pressure of that component is normally zero at the inlet. Any treated gas purity can theoretically be obtained by this method. However, there is one possible disadvantage in that the regenerator off-gas is diluted with stripping gas. This is acceptable for COz in most cases, but may be unacceptable if the HzS removed from the raw gas is to be used as feed for a Claw-type sulfur recovery unit.

The highest treated gas purity (and concentration of acid gases in the off-gas) can be attained by the use of hot regeneration. Here, the effect of decreasing solu- bility with increasing temperature is combined with heating the solution to its boiling point and stripping it with its own vapor. However, this method requires

Regeneration by stripping is basically similar.

38 I N D U S T R I A L A N D ENGINEERING CHEMISTRY

expensive heat exchangers and consumes a large amount of the heat that is necessary for production of stripping vapor, for supplying heat of desorption of the dissolved gases, and to makeup heat losses inherent in the use of heat exchange.

For any of these basic types of absorption and de- sorption, the economics mainly depend on the circula- tion rate. I t influences the size of all equipment and, therefore, the investment costs as well as the operating costs, because the pumping costs are proportional to the circulation rate and the regeneration cost is almost pro- portional to the circulation rate. A comparison of circulation rates required for physical and chemical absorbents shows that the inexpensive systems using flashing and stripping without application of heat, are practical only when applied to physical absorbents, and when treating raw gases with high acid gas concentra- tions. The chemical absorbents like MEA (mono- ethanolamine) normally have to be regenerated by re- boiling. This follows from the different types of equilib- rium, as shown in Figure 2.

Figure 2 presents typical equilibrium lines for chemical and physical absorption, in which the equilibrium vapor pressure of the dissolved gas component, for example, COZ or HZS, is related to its concentration in the liquid phase, expressed as volume sour gas per volume solvent. O n principle, physical absorption is favored a t high partial pressures, and chemical absorption at low partial pressures of the component to be removed. At low partial pressures, for example, p2 in Figure 2, the loading of the chemical solvent, C2,h is much higher than the corresponding loading CZph for physical absorption.

At high partial pressures above the intersection of the equilibrium lines, for example, 61 in Figure 2, physical absorption has the higher capacity. While chemical absorption is characterized by a saturation of the chemical agent, loading of the physical solvent increases steadily with the partial pressure (according to Henry's law, almost proportionally). Owing to this relation- ship, the circulation rate is substantially independent of acid gas concentration in the feed, and is determined by the feed gas rate only. On the other hand, the circula- tion rate decreases as the operating pressure of the ab- sorber increases.

For simple flash regeneration the strong dependency of the solute concentration on partial pressure for physi- cal absorption can be very efficiently utilized, if the solvent is saturated under high partial pressures. As- suming a high pressure gas with large amounts of sour

SOLVENT T E W

OC PHYSICAL

WATER *35

PURISOL (NMP) +35

RECTISOL 1 -10

AUTHOR G. Hochgesand, is Head of the Department for gas purijication at Lurgi Gesellschaft f u r Warme- und Chemotechnik m.b.H., Frankfurt, ( M a i n ) , West Germany. This paper was presented at the 758th ACS Meeting, New York, N . Y., Sep- tember 7-12, 1969, aspar t of the Symposium on Novel Pro- cesses and Technology of the European and Japanese Chemical Industry.

Vol SOUR GAS/Vol.SOLVENT SELECTIVITY AT EQUILIBRIUM . c

p=10 ata p = 1 ata p = 1 ata co2 H2s &H2S

aC02 -

5,5 0,6 1,s 3P 32 3 25 8,3

100 1 8 11 5,1

( M E W 1

MEA (2,5 Mol/l)

CHEMICAL

HOT CARB'TE (1,9 Molll)

-30 270 15 92 6J

4 0 50 39 54

26 39 +I10 40

Figure 3. Loading capacity of typical solvents

components, for instance, shifted gas with 30y0 COZ with the partial pressure in the feed being $1, and the last flash pressure being $2, the physical solvent can offer an effective capacity of Atph. The corresponding value for chemical absorption, Atch, is considerably lower ac- cording to the curvature of the equilibrium line. Physi- cal absorption, therefore, is particularly attractive for bulk removal of sour components from high pressure gases.

Several physical absorption processes have been de- veloped in the past; water wash is probably the most well-known. However, its economics are limited by the relatively poor solubility of sour components in water, resulting in large cirdulation rates and, therefore, high pumping costs. This is different with organic solvents, which possess a much higher solvent capacity than water.

After extensive research and development work in conjunction with the experience from a large number of commercial plants, Lurgi now has available two dif- ferent types of organic solvents. They are (1) low boiling solvents, primarily methanol, as used in the Rectisol process, with very low viscosity, which allow low operating temperatures that have the advantage of very high sour gas pick up even at moderate partial pressures of the components to be removed, and (2) high boiling solvents, primarily N-methyl-pyrrolidone (NMP or M-Pyrol), as used in the Purisol process, which offer special advantages for treating high pressure gases with high sour gas concentrations at ambient tempera- tures. NMP is also used by Lurgi as solvent for the re- covery and concentration of acetylene and butadiene (under license from BASF), and for the extractive re- covery of pure aromatics (Lurgi Arosolvan and Dis- tapex processes).

The solvent power of these solvents is illustrated in Figure 3 where equilibrium data are listed for water, NMP, and methanol in comparison with two chemical solvents, MEA and hot carbonate. For typical ab- sorber effluent temperatures listed in the second row,

VOL. 6 2 NO. 7 J U L Y 1 9 7 0 39

t 1 WASTE K A T RECOMRY 1

Figure 4. oil ( M P route)

Hydrogen or ammonia sjngas from heavy crude or residual

the third row shows liquid concentrations a t equilibrium for C 0 2 absorption a t a partial pressure of 10 atmo- spheres--i.e., about 140 psia, and the fourth row shows corresponding values for atmospheric pressure. At 10 atm the absorption capacity of organic solvents at low temperatures can be fifty times that of water at ambient temperatures. A comparison with the values in the fourth row shows that always 90Yc of the dissolved CO2 and more is released again from the solvent when iso- thermally flashed to atmospheric pressure.

In the fifth row data for HZS absorption, again a t atmospheric pressure, are given. Because the boiling point of HzS is considerabIy higher than that of COZ, the solubility for HzS is always higher. Therefore, physical solvents that can absorb H2S and COZ at the same time, but almost independently from each other, always offer a certain selectivity for HSS us. COZ. This is illustrated by the last row where the ratio of the ab- sorption coefficients for H2S and CO? is listed. For instance, the absorption of H2S at a partial pressure of 1 atm in methanol needs only '/e to ' / j of that circula- tion rate that would be necessary to absorb COZ under similar conditions.

For normal operation of high pressure Rectisol plants the actual selectivity is always somewhat lower, but in any case, it is possible to remove H2S completely from the raw gas whereas the major part of the COZ remains untouched so that an off-gas rich in H2S can be obtained even from gases very low in H2S.

For chemical systems, MEA and hot carbonate, the solvent power is already comparatively high at atmo- spheric pressure, while the excess capacity for increased partial pressures is relatively poor. A chemical solvent charged under 10 atm of C O Z releases only about l/3

and less of the dissolved COZ when flashed to atmospheric pressure, so that a high residual amount has to be stripped by expensive hot regeneration.

A selectivity related to different equilibrium ratios of H2S and C 0 2 cannot be defined for chemical solvents,

because H2S and COZ influence each other during chem- ical reaction. Normally, a chemical solvent can remove HZS completely, only if almost all of the COS is removed, too. There are, however, special chemical systems work- ing with short contact times, thereby offering a certain selectivity owing to different reaction rates for H2S and COS. But these processes normally provide a limited purity only and shall not be discussed in more detail here.

Apart from these solubility characteristics, the organic solvents for the Rectisol as well as for the Purisol process offer some other advantages, which can be summarized as follows:

Both methanol and NMP do not foam and are com- pletely miscible with water. They can work as dehydra- tion agents, and vice versa, they can easily be recovered from off-gas streams by backwashing with small amounts of water, if necessary, in which case the solvent/water mixture is separated by simple distillation.

Both solvents have a high thermal and chemical sta- bility and are not corrosive. There are no degradation problems, and carbon steel can be widely used for the equipment.

Under process conditions, both have low vapor pres- sures so that losses by vaporization are also low.

Finally, both solvents are produced in big quantities and readily available.

After these general remarks on the characteristics of physical absorption as applied in the Rectisol and Purisol processes, some typical examples will be given to illus- trate their application to syngas purification.

Application of Rectisol and Purisol for Syngas Manufacture

Two-stage Rectisol for medium pressure partial oxidation. Figure 4 shows a block scheme for the production of hydrogen or ammonia syngas based on medium pressure partial oxidation of heavy crude or residual oil. In the first step oil, oxygen, and steam are reacted at temperatures of about 2500°F and pressures normally between 500 and 800 psi. A waste heat boiler produces high pressure steam of up to 1500 psi, which can provide mechanical power by driving back pressure turbines before being introduced into the process. After that, the carbon produced during gasification is removed from the gas by quenching and scrubbing the gas with water, thereby cooling the gas down to ambient temperatures (70, 7 7 ) .

At that temperature level, it is very economical to desulfurize the raw gas before shift conversion. I t is a general advantage of medium process pressures that they allow the application of low temperature shift, provided the desulfurization process guarantees a very high purity regarding sulfur comp0und.s that might poison the highly sensitive copper catalysts. The small residual content of CO after shift, together with the low amount of methane formed at medium process pressure, give an optimum purity for the hydrogen product of about 9870 after final treatment.

40 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

For desulfurization, a first Rectisol stage is applied, which has to treat a feed gas with about 5 to 6% COz and 1% HZS and COS, if a feedstock with 4% sulfur is as- sumed. This might be surprising since the general discus- sion of physical absorption has led to the conclusion that for small amounts of sour impurities chemical absorption is preferable.

There are the following facts to be considered :

1. The Rectisol system can easily reduce the total sulfur, HZS + COS down to a guaranteed purity of less than 0.1 ppm, which cannot be accomplished easily with a once-through chemical system.

A chemical system would remove the COz com- pletely with corresponding cost of utilities, while the physical solvent can remove the sulfur components only, as will be explained later.

The physical Rectisol process, by selective absorp- tion of H2S and COS, gives an off-gas with a higher sulfur concentration that contributes toward economical sulfur recovery.

The Rectisol system also removes other gas im- purities without degradation of the solvent.

The economics of Rectisol stage I, when operated alone, is not very attractive, but it can be improved con- siderably by combining it with Rectisol stage I1 for COS removal after shift conversion. By this measure, some of the facilities for regeneration of the solvent can be used for both stages so that the overall costs of both Rectisol stages are very favorable compared with other systems serving the same functions.

2.

3.

4.

5.

Rectisol stage I1 recovers the C02 from the gas com- pletely free of sulfur so that it can be discharged into the atmosphere without violating air pollution regulations. A large portion of this COZ has a purity of more than 98.5y0 and can therefore be used for urea synthesis, which is often combined with the production of am- monia. In the latter case, the Rectisol system would de- liver the purified hydrogen gas with less than 5 ppm C 0 2 to allow final treatment with liquid nitrogen. In this system, all remaining impurities including methane, carbon monoxide, and argon are efficiently removed to give an optimum ammonia syngas. If hydrogen is the final product, for instance, in refineries for hydrocrack- ing, methanation is applied as the final step to convert the residual CO. In this case, a COS content of 0.1% would be sufficient after Rectisol stage 11.

In both cases the Rectisol system is an ideal process for C02 removal for the following reasons :

1. Process pressure and COZ concentration are rather high after shift conversion; for an average total pressure of 700 psi and a concentration of 35%, the COz partial pressure is nearly 250 psi. At this level, the Rectisol sol- vent offers a much higher absorption power than any other known system.

For the same reason, most of the COZ absorbed in the solvent can be released by simple flashing, the rest being removed by stripping at the same temperature. This can be done most economically with impure nitro-

2.

1

Figure 5. Rectisol two-stage desulfurization and COZ removal

gen that is available free of cost from the air separation unit providing oxygen for gasification.

Of course, the process steps ahead and downstream of each purification stage need some adjustments with respect to the specific operating conditions of the Rectisol process to provide for optimum integration. But this is not discussed in more detail.

With reference to Figure 5 a description is now given of the Rectisol purification process. Figure 5 includes both Rectisol stages, stage I for desulfurization of the raw gas on the left and stage I1 for C02 removal from shifted gas on the right. Absorption columns are shown in the top half and regenerators in the bottom half of Figure 5 .

The working temperature of both absorbers is below O°F and it is maintained by evaporating ammonia pro- vided by a refrigeration unit that normally operates on inexpensive waste heat from shifted gas. Ice formation in saturated feed gas coolers is prevented by methanol in- jection, the methanol/water condensate being separated in a distillation column not shown here. In both cases, the fat solvent is preflashed to set free small amounts of coabsorbed H2 and C O for recycling to the feed gas. Then, the regeneration follows different routes for Rectisol stages I and 11. Sulfur-containing solvent leaving stage I is regenerated by reboiling, and the solvent from stage I1 is regenerated by further flashing and stripping with nitrogen.

A special advantage of the integrated Rectisol system is that the two solvent circuits can be superimposed to allow two-fold use of the reboiled methanol. It serves first for COz removal and then, after flashing, for desul- furization without the remaining C02 dissolved in the methanol having an adverse effect on the solvent power for sulfur components.

A Rectisol unit as described above is included in a 1350-short-tons-per-day ammonia plant for which Lurgi was selected recently as the main contractor. Syngas production is arranged according to the last block scheme including liquid nitrogen wash as final treatment ahead

VOL. 6 2 NO. 7 J U L Y 1 9 7 0 41

23 PARTIAL OXIDATION

HzS+COS COP

t 1 Y*113 N? H2

Figure 6 . oil (HP route)

Hydrogen or ammonia syngas jrom heavy crude or residual

I I : I I

HOT REGENERATION

I

Figure 7. Rectisol combined desuyurization and COZ removal

of syngas compression. Similar Rectisol plants will be built for a number of coal-based fertilizer plants in India, which follow substantially the same process sequence in which coal gasification can be regarded as a special type of partial oxidation.

The production of syngas for methanol synthesis is very similar to the scheme described here. I t differs only inso- far as part of the desulfurized gas is shifted while the remainder is bypassed and admixed directly with the purified gas ex Rectisol stage 11, thus providing the re- quired Hz : CO ratio of about 2 : 1. Another difference is that a final CO2 content of about 1 to 2y0 is allowed and that no additional treatment such as methanation is necessary. In early 1969 a unit of this type was started up successfully as part of a methanol syngas production plant with a capacity of 50 million std cu ft/day.

Once-through Rectisol purification for high pres- sure partial oxidation. After the first example illus- trating two-stage Rectisol gas purification, the second example is devoted to a once-through process for syngas manufacture by high pressure partial oxidation.

Figure 6 shows a block scheme for this type of gasifica- tion. The first step is similar to the corresponding one at medium pressure, but it operates a t about 1200 psi. No waste heat boiler is applied normally, and carbon scrub- bing is operated at relatively high temperatures. This leaves enough steam in the gas to go directly--z.e., without desulfurization at lower temperatures, to high tempera- ture shift conversion. This procedure is possible because only high temperature shift is applied, the catalysts of which are not attacked by sulfur. This, of course, results in a higher residual CO content, the methanation of which is rather difficult because of the high temperature gradients resulting from the exothermic reaction of CO with hydrogen. The latter does not apply if copper liquor wash is provided as final treatment for hydrogen production, or if liquid nitrogen wash is used ahead of ammonia synthesis (2, 9 ) .

This process sequence includes only one gas purifica- tion step after shift conversion. In fact, it is a two-stage operation since there is again the twofold function, namely, selective gas desulfurization and recovery of sulfur-free COZ, unless one considers simultaneous re- moval of H2S and COz, and separate treating of the combined sour off-gases. The Rectisol process offers most economic solutions for both wa)-s; the first way shall be discussed here.

The following will characterize the process conditions that are much more difficult compared with the first example, because the H2S : COz ratio is less favorable. In the first example, it was about 1 : 5 for a feedstock with 4% sulfur. Here, the ratio is as low as 1 : 50 because the COZ content rises to about 35y0 during shift, while the H2S content drops from ITo to about 0.7Oj, owing to the increase in gas volume.

Figure 7 shows a flow scheme of a Rectisol unit for this more difficult function. Similar to Figure 5 the gas is purified in two stages working under low, temperatures : stage I for desulfurization and stage I1 for COZ removal, which in this case follows directly. The most important difference compared with Figure 5 is a rectifying column (lower left) to concentrate the H2S within the solvent. This is done by stripping most of the COz from the solvent with nitrogen, the H2S being reabsorbed and fed to hot regeneration from which H2S-rich gas is recovered.

A Rectisol system of this type offers about the same economics in terms of investment and utilities as that in the first example despite the fact that the hydrogen gas is cooled down only once, and elevated pressures generally favor ph)sical absorption systems. This has two main reasons :

1. The desulfurization stage has to handle a larger qas quantity corresponding to the increase in volume during shift.

2. All methanol, including that for COz removal, has to be regenerated by reboiling, which is much more cx- pensive than flashing and stripping at low temperature only.

Otherwise, this solution also allows an ideal combination with liquid nitrogen wash for the production of ammonia

(For the second, see 4, 9.)

42 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

syngas. In the process diagram, the purified gas therefore goes to the final treatment before being rewarmed to ambient temperature for syngas compression.

For hydrogen manufacture by high pressure partial oxidation, such a combination of low temperature pro- cesses is not applicable. Therefore, an alternative might be considered using only gas treating processes that work a t ambient temperature.

High pressure GO2 removal with Purisol. The third example presents a typical application of the Purisol process for treating high pressure hydrogen gas a t ambient temperatures. Referring back to the first and second example, the Rectisol system already offers optimum economics a t medium pressures. The appli- cation of higher pressure only results in a slight increase in economics for various reasons. One reason is that the higher the loading of the solvent, the higher the temperature gradients during absorption, since the heat of absorption is set free in a lesser quantity of solvent. These temperature gradients can be compensated only by additional heat exchange or elevated process tempera- tures, both steps limiting the further increase of the overall economics.

O n the other hand, the Purisol solvent, which has a relatively small absorption power at low to medium pres- sures, becomes very effective a t high pressures, especially for COz removal from shifted hydrogen gas. This can easily be derived from the solubility data in Figure 3, con- sidering that 35% COZ contained in a gas a t a total pres- sure of 1000 psi corresponds to a partial pressure of as high as 350 psi.

Figure 8 shows such a COz removal unit that may be applied to desulfurized and shifted hydrogen gas from high pressure partial oxidation, as well as to raw gas pro- duced by medium pressure steam reforming of natural gas that is compressed to the high pressure level after shift. The second application may be surprising, con- sidering the additional energy consumption needed for compression of the C02 contained in the hydrogen gas. But this disadvantage can be compensated by savings in the utility consumption for COZ removal. Another ad- vantage is offered by the possibility of replacing reciprocal compressors by centrifugals, because the volume and the density of the COz-containing gas are much higher com- pared with pure hydrogen ( I , 8 ) .

Figure 8 shows gas absorption on the left and regenera- tion of the Purisol solvent NMP in the middle. The main solvent circuit includes absorption and two-stage flashing and stripping with nitrogen or air. As in Figures 5 and 7, the first flash serves to set free coabsorbed hydrogen for recycling to the feed gas.

The complete miscibility of NMP with water allows a very practical arrangement for gas dehydration and solvent recovery: a few trays in the bottom of the ab- sorber and stripper are charged with small side-streams of NMP for feed and strip gas drying, while corresponding top trays serve for solvent recovery by water wash. All NMP/water mixtures are fed to the dryer (right) for distillation, which is combined with solvent recovery from C02 off-gas.

TREATED GAS

SOLVENT STRIPPER DRYER

I

i I I

Figure 8. Purisol COa removal

Two Purisol units of the type shown in Figure 8 with a raw gas capacity of about 100 million std cu ft/day each have been built for a major U.S. oil company under license from Lurgi. They were started up in early 1970.

Another Purisol process scheme is under development that serves for combined desulfurization and COz re- moval. I t is similar to the second example for the appli- cation of the Rectisol process.

Conclusion The general discussion over physical absorption,

together with the examples given for the application of Rectisol and Purisol may have demonstrated the abilities of these processes and their special advantages for the manufacture of synthesis gases a t medium to high process pressures. They allow optimum solutions for the overall scheme by providing extreme purities, wide use of inex- pensive regeneration by flashing and stripping, and the ability to process gas quantities of 150 million std cu ft/ day and more in a single train. I n addition, the opera- tion of these processes is simplified considerably by the absence of chemical reactions. There are no serious prob- lems regarding corrosion or degradation of the organic solvents, and process control is restricted mainly to pres- sures and temperatures.

Lurgi has built or designed 22 Rectisol units and 3 Purisol units, with the first plant operating successfully for 14 years. Both processes are covered by various patents, and are licensed world-wide by Lurgi.

REFER EN CES (1) Beavon, D. K., and Roszkowski, T. R . , 011 Gas J., 68 (15), 138 (1969). (2) Garvie, J. H., Chem. Process Eng., 48 ( l l ) , 55 (1967). (3) Herbert, W., Erdoel Kohle, 9, 77 (1956). (4) Hochgesand, G. , Chem.-Ing.-Tech., 40, 432 (1968). (5) Hoogendoorn, J. C . , and Salomon, J. M., Bri t . Chem. E n g . , 2, 238 (1957). (6) Kohrt, H. U., Kaeltetechnzk, 11, 130 (1959). (7) Kohrt, H. U., Thorrnann, K., and Bratzler, K., Erdoel Kohle, 16, 96 (1963). (8) McLeod, W. J., Smith, C. S., and Haritatos, N. J., Oil Gas J., 68 (23), 72

(9) Morrison, J., ibid. , (8), p 76. (10) Reinmuth, E., Erdoel Kohle; 22, 378 (1969). (11) Van den Berg, G . J., Rijnaard, P . , and Byme, D. J., Hydrocarbon Process., 45

(1969).

(5), 193 (1966).

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