as separation is distinguished from purification by
G the relative amounts of removable gas involved. In separation there usually an appreciable amount, while in purification there is but a small amount. Since bulk separation plants are usually preceded by gas purification systems, and since principh which in the past have been applied only to gas purification are now being applied to gas separation, no distinction is made between separation and purification in this review. kpcvoHon by Diffusion
Separation by diffusion through porous barriers is not widely used in industry. I t is mentioned mainly because it is the classical method used to determine the minimum theoretical work required for the separation of two gases using the concept of two semipermeable membranes each of which in permeable to one of the bases. Darling (1) has recently given data on the separation of
32
INDUSTRIAL A N D ENGINEERING CHEMISTRY
d
hydrogen from other gases using a membrane of palladium containing 23y0 silver. The data confirm Fick’s law of diffusion and indicate that the practical problem is to reduce to a minimum the thickness of the membrane so that the specific area available for diffusion is as high as possible. Darling describes a diffusion cell consisting of a nest of thin-walled, solid-drawn palladium alloy tubing l,/J8 in. inside diameter and 0.003 in. thick. The tubes, closed at one end, are brazed at the other end into a manifold, and the whole assembly is welded into a thick-walled stainless steel tube. Impure hydrogen circulates at pressures up to 300 p.s.i.g. and temperatures up to 500’ C. over the outside of the tubes through which the hydrogen diffuses. The excess hydrogen and other gases are bled off from the shell. As the proportion of bleed-off gas increases, the rate of diffusion increases but the efficiency of the cell decreases. Diffusion cells capable of an output of 500 cu. ft. per hour of pure hydrogen at a differential pressure of 2000 p.s.i. are commercially available. The process has been claimed suitable for specialized chemical, metallurgical, or electrical processes for which hydrogen having a dew point below -40’ C. is essential, the separation of hydrogen from cheap gas mixtures, the removal of unwanted hydrogen from process gas streams, and the separation of hydrogen isotopes. It is stated that hydrogen containing less than 1 p.p.m. of impurities may be obtained by the equipment. It has been stated ( 7 7) that another possible application of diffusion is the separation of helium from natural gas using silica glass. Separation by Gas Adsorption and Molecular Sieves
il
*
Gas adsorption has been used successfully for the removal of small amounts of impurities and a common application is the removal of water vapor from gas streams using silica gel as the adsorbent. Two types of adsorption should be recognized, one caused by the same type of intermolecular forces of attraction that produce normal condensation to the liquid state (van der Waals forces) and the other caused by specific chemical bonds between the atoms on the surface of the solid and the gas adsorbed (activated adsorption). Where the van der Waals forces of attraction are greater between a solid and gas than between the like molecules of the gas it is possible to condense the gas by adsorption on the solid surface at a temperature above the normal dew point. On a smooth surface, adsorption is restricted to a layer of one or a few molecules in thickness. However, on a solid possessing a minute capillary structure, surface adsorption is supplemented by capillary condensation. Capillary condensation occurs when the solid is wetted by the condensate, resulting in concave surfaces of the condensed liquid. The equilibrium vapor pressure of a liquid having a concave surface is less than the normal value by an amount depending upon the radius of curvature. For this reason vapors which AUTHOR D r . W . H . Granville is Principal Lecturer in the Bradford Institute of Technology, Bradford, England.
are at partial pressures much less than the normal saturation value are condensed, augmenting the adsorption normally taking place on flat surfaces. The adsorption power of a solid may vary over its surface and with its mode of preparation. Thus charcoal may be “activated” by heating it in the presence of steam or in a limited supply of air. In general the adsorption rate increases with the partial pressure of the adsorbate and decreases with increasing temperatures. Dunbar (5) has recently reported the properties and uses of certain Molecular Sieve products and one of these may be represented by a cell unit of composition Nalz(A102)1z(SiO)12.27H~O. In the activated state the water of crystallization is partially or completely driven off. The pore opening of such a cell is 4.2 Angstrom units, and it is useful to compare this pore size with the diameters of water and benzene molecules which are 2.3 and 6.8 Angstrom units, respectively. Dunbar describes two basic methods of separating molecules by the use of molecular sieves. Since the only entrance to the adsorptive area is through the small pore openings, the primary separation is between large and small molecules. Those molecules which have passed through the pores can then be separated on the basis of difference in polarity, unsaturation, or length of carbon chain. The activated adsorption sites which are left when the water of crystallization is driven off have a very strong attraction for water. Polar molecules such as ammonia, carbon dioxide, and carbon disulfide, as well as water, will be adsorbed. In the case of carbon bond saturation, compounds with double bonds will be more readily adsorbed than compounds with single bonds. Molecules with longer hydrocarbon chains are more readily adsorbed than molecules with shorter chains. Gas adsorption is a reversible process and thus the solids may be regenerated for further use. Regeneration may be achieved by using thermal stripping, pressure stripping, purge gas stripping or displacement stripping. Continuous operation may therefore be achieved by using two adsorbers, one for adsorption and one for stripping. If the stripping time is longer than the adsorption time the relative numbers of units may be suitably adjusted. The application of molecular sieves for bulk separation is illustrated by Dunbar by considering the removal of carbon dioxide from annealing gas containing 12% by volume of carbon dioxide. Gregory, Herbert, and Lane (7) have also reported the reduction of carbon dioxide concentrations from 12y0 to O . O l ~ ousing Molecular Sieve.
Separation by Heat Transfer
When a gas is passed through a partial condenser and the liquid and gas product streams are separated, some measure of gas separation takes place. Ideally the gas leaving the separator is in equilibrium with the liquid, and the gas stream is richer in the more volatile components than either the liquid or the feed streams. The VOL. 5 6
NO. 5
MAY 1964
33
degree of separation achieved is governed by the vaporliquid equilibrium relationships for the components present under the operating conditions of temperature and pressure. In general, the degree of separation is not sharp with one partial condenser, but better separation is achieved using a series of partial condensers working at progressively lower t e m p e r a m . Such a plant produces one gase-ous product richer in the more volatile components than the feed but also produce one separate liquid stream from each condenser. This type of prows is, therefore, used when the liquid streams may be either further processed or used as a fuel. Even so, this type of separation plant is usually d y a prelimiaary step used in conjunction with other reparationtechniques.
HOT 6 6 lNLET*---q
I
--I
TA6LE 1. ADSORPTION OF CO, FROM ANNEALING GAS
InlnGmditiDms Generator capacity cprbon dioxide urncentration Wata rrmocnfratioll Mdecular S i m requirement QFlctiu.eIaeacbbed: Msorpion
2wo &C.f.h. 15% wl. +80° F. dew point 570 Ib. p a bcd 1 hr. 1 hr.
Regmaation
u
1 hr.
Bed regatcratian temperature
5 0 0 0 P.
OutlaCollditan. WataamOcD~ cqrbon dioxideQmecntratiDll
Lastban - 100' F. dew point
When the gas stream contains only small quantities of impurities which are less volatile than the main gas s t w a n , the impurities may be removed by freezing them out on the cold surfaces of regenerators, reversing heat exchangers, or switched, dual-heat ucchangers. The impurities are then re-evaporated into the colder gas a t f e a m which is at a lower pressure. A typical regeneratm unit is shown in Figure 1, which illustrates the changeover values required. when these regeneram are applied to low-temperature distillation systems the purging gas is one of the recycled, pure gas streams. The principle of a reversing heat exchanger is similar to that of the regenerator in that it has two channels which may be used alternately for the two gas streams. When the purging cannot be carried out using pressure variations alone it is possible to use switched, dual-heat mcbangers. Impuritia are deposited in the feed channel only of the on-stream unit. Meanwhile the offstream unit is purged with the warmed pure gas from the o n e e a m unit. Provision is made for warming the d-stream unit to assist the purging operation and this is fobwed by a cooling operation before t h ~ unit is returned on stream. Thus in this case the purging effect relies on temperature rather than pressvre and can be costly, as a result of the drigeration required at lowa temperature levels. The latest development is the platelin heat exchanger in which each gas stream flows countacummtly to the other in alternate spaces between the plahs. This unit may be used as a reversing heat ex34
INDUSTllAL AND ENGINEERING CHEMISTRY
Figme 7. Rcgem&r for mohng andpunfying gam
changer or as a switched dual unit. Regenerators are used in air separation for the removal of carbon dioxide and water vapor and Denton (2,3)has investigated the use of reversible and switched dual units for the removal of carbon monoxide and nitrogen from hydrogen before its distillation at 20" K. kpclnHon by Gas Abrorp(ion
The separation of soluble gases from insoluble gases
by absorption is a common problem in the chemical industry. Usually it is soked by absorbing the soluble gases in a suitable solvent and then stripping the soluble
gas firm the solvent in a second column. Solvent from the bare of the stripping column is then recycled to the top of the absorption column. The solubility of a gas in a solvent incruuvs with its partial pressure in the gas s t r e a m and deaeascs with rising tCnIpaaNre. Such a plant, therefore, incorporates a number of heat exc h a w lor preheating the feed to the stripping column and for cooling the solvent feed to the absorption column. The heating and cooling cos61 increase with the solvent flow rate and it is desirable to maintain an economic flow rate. The author (6) has shown that in certain m u l b p e n t absorption systems the liquid rate may be considerably reduced using two or more absorption columns through which the gas flows in series and liquid in parallel. In general, the method applies to the recovery of two or more soluble gases from an inert gas stream when the soluble gases are present in appreciable concentrations and have different solubilities. Such a process is shown in Figure 2 where the most soluble gas, vinyl chloride, is absorbed in
P
I
,..,... ~.
. . ~_
..
..
,
,
.
-
. .
~,I, :. . . : . .I_ ,...
. .
the &it & I L ~with a soluble gas, - acetylene, is then more easily absorbed h the second column because its partial pressure is increasrd as a result of the elimination of vinyl chloride. This technique, usefd for absorption at near ambient temperature, may prove very desirable for absorption at very low temperatures. Palazzo,Schreiner, and Skaperdas (70) have described a low-temperatun condensation and absorption process 'Eor the recovery 8f hydrogen from refinery gases. The
concentrations it is more difficult to-satisfy the refrigeration requirements. This problem may be overcome by compressing the feed to a bigher pressure, by installing a recirculating hydrogen system, or by using cascade or n i t r o p refrigeration. It may be that when the concentrations of the impurities are appreciable then an additional absorber would be helpful. Jester (9) has described a low-temperature process for producing ammonia syntheais gas from a hydrogen95% of hydrogen. hydmcarbon mixture containing 7 plant in that the This plant is similar to the prev feed gas is cooled in stages at 300 to 400 p.s.i.g. pressure but in this ca8e the final purification for the removal of carbon monoxide and methane is carried out by absorption in'liquid nitrogen. The hydrogen leaving the a b s o r k is warmed in heat exchangers and mixed with sufficient nitrogen from an air-separation plant to form a 75y0 hydrogen and 25% nitrogen mixture for ammonia synthesis. As in the previous plant an elaborate heat exchange system is required. Sopamiion by Low-Tempwdun DidlMion
m I-m
IIpID(BIIB
nmmw
W
mum
-p1
-ml
mwm-mncun
ma MI m
(OYmBr
F i p n 3. Plant fa tlw lowrrnrpaoha ahwplion of nuthane in
PIOQmu a
t
feed contains 85.2% hydrogen, 8.5% methane, 4.4% ethane, 1.8% propane, and 0.1% butane, and the hydmgen pmduct is 99.9% pure. The dry gas at 320 ps.i.a pasm through a suiea of partial condensers aperating at -120' F., -160' F., -200' F., and -270' F. T h e oondenaera remove the bulk of the hydmcarhns and the 6nal purification is achieved by absorption of the remaining Bydrocarbons in liquid propane at 320 p.s.i.a. and -227' F. High pnssure hydragen leaving the absorber passes through tww expanSIOn engim~to produce the refrigeration required. The rich pmpane leaving the absorber is then depressurized, heated, and stripped of the hydrocarbon impuritie~bya portion of the product hydrogen. F3 is a simpliied l b w diagram of the absorption system.
Separation by distillation relies oh the differences i the volatilities of the components present in the feed stream. Fundamentally t h e is no difference between distillation at high tempemand dirtillation at low temperatures. In each case heat is provided in a reboiler to produce the necessary vapor stream and cold is applied at a condenser to pmduce the necessary d u x . In both cases a feed consisting of n components r e q u h a minimum of (n-7) columns in order to separate n p u n products, provided heat input is available at temperagreater than the bottom operating temperatures and heat removal is pwible at temperatures lower than the top operating temperaturea of the various columns. The main diffmnces bmKeen actual hightemperature and low-temperatureplants are : -high-fempaaturr
plants use &mal
heat muwa and sinks
while low-temperature plants utilize internal heat wurcea and sinks.
-high.tcmperature plants use the theoretical minimUm numkr of Columns whereas low-temperature plants may work above the minimum. -low-temperature pants work nearer the minimum reflux ratio than do high-temperature plants. -high-tanpture plants are not w carcfulIy designed for heat orchange between the various process streams and with the sumundings. -there is more freedom in the design and operation of bightemperature plants. -lawtemperature plants generally work at higher presaum. he ckssical example of low-temperature distillation
w
all the points stated above is that of Tsw dustrates .
air stparation. Assuming that air is a mixture of oxygen and nitrogen only, it would follow that one distillation column would be sufficient to produce oxygen and nitrogen at hiih degrees of purity. This column would, VOL 56
NO. 5 M A Y 1 9 6 4
35
n/r'
h e w , require a heat sink at a t e m p a t & lower diaa the boiling point of p& nitrogen. It is this proa heat SinL which complicates any practical for ah separation, and this has led to the d a of the Linde doublecolukn. Figurt 4 is a"simpIi6ed acrangcjnent of tbe 'doublecolumn'for ilrustration purposes. purified high pressure. air,'which has been mobxi by heat exchange with the '&duct gases, transfers heat, Ql,"in the reboiler of the bot& wbam before being .~~panded to the' prof the bottom whnmi whieh is often about 5.5 atmo&ph&e+. Thic d u m n pmduces reasonably pure liquid- nilrcgen, C', dnd an oxygen e&ed liquid air, D . Thrsa hvo a t k d are then expkdedtwthe pressurn, in the cop c+m which would bc about 1.5 atmospheres. StrUrm Bh'n&thefeedandCproyid&'the coldreiluxfor the top of the column. Pure 'nitrogen and oxygen are sejmarrd & g& at E and F. The heat f a the top d u m n , Qa is provided by the condensing nitrogen at the top of @e.bo~omd u m n , the necessary tempeyaaUr diffamees being provided by the diffacnecin working,pessura. A PonchonSavarit ~d+gram fw the doubkeolumi ilIustrated h &Own in F+ 5. The ?#iagmmis n0t;draWnto s i d e and.the enthalpy CuFVes anQ:+apor-liquidequilibrium tie.lines have^ b&if purp&y distce& to ilIustrawtkC basic the, @ an&. tn p a r t i a h , to demonamate the '&rdqiendcaca ofthe various h v x , cornpodtiom, and heat qxsaatih. Drawn accurately, the number of cialincs '
~
U
t I I D U S T R I A L ' A N D ENGINEERING C H E M I S T R Y
represents the number of themetical stagar required for hproeess. S i air comains about 0.93% argon, and since its bdling e t k between that of orygen andnitrogen then it will appear on all plates of the column and in the gasebus Iffoducts. Sinoe~themole fractions of argon an relatively small on the variolrs platen, and since its mold leteat heat is tairly clcse to that of o x w n , the PolMhon-Savarit diagram in Figurc 5 lnay still be used, reading the composition ax& as the mok fractions
lines then refer similar Ponctrom argon component. Jhthalpy EUMCS for ti& lutter diagram are then horizontal parametem f a v q i n g
nitrogen compositions. A tie-lie from point E on the nitrogen diagram gives the percentage nitrogen and the enthalpy of the liquid leaving the top plate of the upper column. The tie-line from point E on the argon diagram may then be drawn using the correct enthalpy. O p t i n g l i e s are then drawn on the nitrogen and argon diagrams. The nitrogen diagram gives the percentage nitrogen and enthalpy of the vapor stream leaving the second plate. This enthalpy value together with the operating line determines the percentage argon in the vapor stream leaving the second plate. In this way the two Ponchon-Savarit diagrams and the ternary vapor-liquid equilibrium data enable the n u m k of theoretical plates for the ternary system to be determined. There have been many developments in the design Os air separation plants and these have been summarized by Ruhemann (72, 73), Hazelden (S), and others.‘ To mention only two examples, the Linde-Frankl cycle u m a gaseous fad to the lower column and additional reflux for the upper column is provided by passing part F&4 7. O@&ing lincr (brokar) fadistil&bn c a I m @ded wdh of the initial air s h e a m through an expansion turbine continuow addition and removal of haat before its entry direct to the upper column, whereas the Oxyt0a cycle u s ~ la third distillation column which makes the operation-of the lower column more reversirectification sections of the column would reduce the ble. amount of heating and cooling required at the ends of One of the factors which contributes to the irreversithe column where these heating and cooling costs are bility of a distillation column is the fact that generally higher. The broken line in Figwe 7 indicates a more heat is only added and removed at the ends of the cotreversible actual operating l i e for the column using conumn. Figure 6 is a McCabe-Thide dmgram’showing tinuous addition and removal of heat. Savings to be the minimum reflux ratios required for various prtcr. of achieved by this type of operation depend on the tema binary distillation column with a saturated vapor ked. oerature distribution in the column. the shane of the The diagram demonstrates that the minimum reflux for > vapor-liquid equilibrium curve and the complexity of a fairly ideal system with a regular vapor-liquid equilibthe equipment r e q u i d . When the bottom product is rim w e is a maximum at the feed point. Addition required as u gas it may be more cemrenient to remove and removal of heat at stagea up the stripping and the product from the base of the column as a liquid and v a p d i e it later. ~h efficLncy d a low-&ahlre distillation plant depends on the application of the above factors together 4th good heat erehqege W t k n the various procesi SlMrns ’ . andei%cctiveinat#atien. Other applications of low-temperature dirtillation include the ~epar+tionof the rare pses, cracker-gas hydrocarbons, hydrogen isotopes, and coke-oven gas. These processes arc dkmsed in the texts by Ruhemann (72),Scott (74),andDinandCoclrett(4):
*
LITERATURE CITED (1) D a h
,A
8 , Sym
C b .&.,.&on,
ium
K3.
(ID “ h i
&-on
Me-
of ¶tion:. l n v ~
(2) Denton, W. H.,S h w , 8.. W a d , D.E., Tmm. Inrr. h. Ew. 36,179 (1958). (3) W , o 37,217 (1959). (4) nis F., Coebtt, A. € ‘I T. m, v Temp=mcum Tshniqua,” oemgc N &, laadon, 1960. (5) Ihmhr, C. L.. Sym ‘um on “ h i Cornman M u n of gpara+im,’’ Imf.
A%.
chqn.En#., London, (6) GRnsillc,W. H.,Bel. Chm.Eq. I, 629 (1963). ‘‘S. A. H a k t P. B. Lane I. I. S ( 7 ~ ~ & a t k n , ” I % w chm;. t~ Eng.,’M&. (8) Huddslq 0. G., T-. I W . Qm. Eeg. q6.21 (1948). (9) Il.ts,M.R.W.%,133(1958). (10) Pllano,D.P..&him, W. C., Skaprdu, 0. T., Im. ENS. O m . 49.685
pp”
F@ue 6. Minimton r.p.ratias for b h n y system
(1958). (11) Repmt, B”t.Chm. Eq. 4,101 (1959). M., “Ssplratioo ofGasa,” CLrmdoa R a s , London, 1945. (12) R-, (13) Ruhcnuno, M.,T-_ Imt. C h . Ew. 1 5 2 1 (1948).
V O L 5 6 NO. 5 M A Y 1 9 6 4
97
