Methanol dehydration

Methanol is converted from syngas at high temperatures and pressures. The gas stream leaving the converter, at 300.degree. to 500.degree. C., above atmospheric pressure--usually between 750 to 7000 pounds per square inch, has besides the unreacted permanent gases of the syngas and others, methanol and water as well as other liquid impurities. Based on the weight of methanol, water may be present in from 0.5 to 20%, also small amounts of higher alcohols, dimethyl ether, etc. Condensation gives an aqueous liquid, from which the water must be separated if the methanol is to be used as a motor fuel. When this amount of water is low e.g. 0.5 to 12% this may be separated by adsorption most economically from the gas before condensation of the methanol. The energy in this gas stream at a high temperature and pressure above the ambient may be used to dehydrate completely the methanol, by the use of a conventional desiccant.

The process of this invention separates a relatively small amount of water 
from a hot gas mixture containing, besides permanent gases, mostly 
methanol. Often it contains other vapors also as small amounts of some 
higher alcohols. 
This method is particularly useful in removing water from the gas mixture 
from a converter producing synthetic methanol, when the methanol is to be 
used in admixture with gasoline as a motor fuel, or when for other reasons 
it must be rigorously anhydrous. Water formed with the methanol as a 
by-product in its synthesis from hydrogen and one or both carbon oxides 
prevents the complete miscibility with gasoline which is necessary when 
used in automobiles. It may also be used in dehydrating other gas streams 
containing methanol and a relatively small amount of water as vapors. 
In making synthetic methanol, conventional practice has produced a hot gas 
stream having vapors of methanol and as much as 20% water based on the 
methanol content. This water is obtained in the product on the 
condensation of the vapors in the gas leaving the catalytic converter. 
Modern processing has reduced this to 10% in many cases, and in U.S. Pat. 
No. 4,235,799, the amount of water present may be less than 1%. This is 
based on methanol produced, as is the somewhat less than 2 or 3% of higher 
alcohols, principally ethyl. Also dimethyl ether is in the gas stream 
formed in the converter, and these organics may be condensed along with 
the methanol and water and the mixture used directly as a fuel. The water 
alone must be separated if the liquid is to be mixed with gasoline. 
Methanol has long been separated from water by distillation with 
rectification. All of the methanol (b.p. 65.degree. C.) is readily 
distilled--with reflux--away from the water; and this may be the most 
economical separation method when 15 to 20% water is present. However, 
when the water is present in smaller amounts, i.e. 0.5 to 10 or 12%, and 
especially when present in an amount of from 0.05% to 2%, the distillation 
of all of the methanol away from the small amount of water requires a 
substantial amount of heat and a large distillation system compared to the 
adsorption system of the present invention. 
Fractional adsorption of the small amount of water away from the methanol 
has been found to use much less energy. Then for fuel use, the methanol 
requires no distillation or other refining, as the other impurities the 
higher alcohols, and other organics are also good fuels. Also, in the 
dehydration of the methanol vapors present in the gas stream leaving the 
catalytic converter at a pressure and temperature above the ambient, it 
has been found that the use of some part of the heat contained in these 
vapors above the ambient temperature may be used for the regeneration of 
the adsorbent material, always an essential step in any continuous 
adsorption operation. 
The several processes for converting syngas to methanol discharge a hot gas 
stream from the converter containing from 3 to 25 mol % methanol and at a 
pressure of from 750 to 7000 pounds per square inch gage and a temperature 
of from 300.degree. C. to 500.degree. C. The thermal and mechanical energy 
in this gas stream at these high pressures and temperatures compared to 
the ambient have been found usually to be more than adequate to supply the 
energy needs of the separating process for water, the subject of this 
invention. 
In the operation of some converters, e.g. that of U.S. Pat. No. 4,235,799, 
the high temperature of the hot gas stream is reduced by transfer of some 
of its heat in a steam generator, and the steam so produced gives 
mechanical and or electrical power. It has been found that the energy so 
developed in cooling the hot gas stream may be at least sufficient to 
drive the pumps, compressors, etc used as essential accessories in this 
separation process. This energy is principally the heat at the high 
temperature of these gases--or even after being cooled somewhat in the 
steam generator. It has been found to be sufficient to accomplish the 
desorption operation either by heat transfer through the walls of the 
adsorber of by heating an inert gas which circulates in direct contact 
with the desiccant particles. More energy may be available than may be so 
required; and this energy coming from the steam generator may be used in 
other parts of the plant of which this process is a part. The hot gas 
stream may be cooled to as low a temperature as 25.degree. C. before being 
dehydrated, as an aid thereto, so as to improve the dehydration. Other 
cooling agents than boiling water would then be used in other heat 
exchangers. 
OBJECT OF THE INVENTION 
It is thus the object of this invention, which may be accomplished readily, 
to separate the water from methanol and other vapors or gases which come 
from methanol synthesis in a converter at a superatmospheric pressure and 
a temperature between about 300.degree. and 600.degree. C. and with or 
without some cooling in a heat exchanger. Selective adsorption of the 
water present is accomplished, using some part of the sensible heat of the 
gas stream for generating mechanical and/or electrical energy and for 
heating of the adsorbent bed for regeneration of the adsorbent or 
desiccant through evaporation of the adsorbed water so that no energy from 
outside the system is added or required for operating this dehydration. 
Adsorbent--Desiccants 
Numerous solid materials have the ability to adsorb water vapor from a gas 
stream at various temperatures and pressures; and some of these desiccants 
may have more or less advantages when used in the present process. 
In general, the particle size of a solid desiccant may be from that of a 
powder to that of a bead or pellet of any shape and having an average 
diameter of up to 1/8 to 1/4 inch or even larger. Desireable particle size 
for optimum usefulness depends largely on mechanical considerations, the 
design and configuration of the adsorber bed or container, the method of 
support of the desiccant particles therein, the ease of flow of gas 
through such a bed, etc. 
Such adsorbents or desiccants depend on a chemical and/or a physical 
attraction for the water molecule. When any amount of water is held by a 
solid desiccant, this water has a vapor pressure at every temperature 
which is very much reduced from the normal vapor pressure of water at that 
temperature. This vapor pressure increases with increasing amounts of 
water held, up to the maximum or saturation amount of the desiccant. For 
any constant amount of water or other fluid which has been adsorbed, the 
vapor pressure relations with temperature follow the same laws as for a 
pure substance (see for example, Othmer and Sawyer, Industrial & 
Engineering Chemistry Vol. 35, p. 1269, 1943). 
The vapor pressure of the adsorbed water comes to equilibrium with that in 
the moist gas by adsorption of water therefrom, until the partial pressure 
of water in the gas is equal to the vapor pressure of the water adsorbed 
in the desiccant. 
Regeneration, drying, or dewatering of the desiccant which has taken up 
water, is accomplished by (a) heating to a higher temperature (b) reducing 
the pressures (c) passing a stream of dry gas, or (d) passing a stream of 
gas containing a compound preferentially adsorbed by the solid. In the 
first three of these methods, regeneration of the desiccant is such as to 
give the water adsorbed a higher vapor pressure than that in the 
surroundings, so the water vaporizes. A combination of two or more of 
these methods of regeneration may be used. 
A consideration of this vapor pressure of water from the desiccant explains 
why it is desirable that the adsorption--in some sense a type of 
condensation--of water from any gas stream carrying a fixed percentage of 
water can be more complete if the adsorption is conducted at a lower 
temperature, and/or a higher pressure. Correspondingly the regeneration is 
better at a higher temperature and/or a lower pressure. 
In the condensation or adsorption of the water on the desiccant there is 
involved not only the usual latent heat of condensation of the water, but 
also the physical or chemical heat of the adsorption process. Both heats 
are involved likewise in the regeneration or evaporation of water from the 
desiccant. 
The effectiveness of any desiccant is measured by the amount of water it 
will take up at any given temperature when its vapor pressure is in 
equilibrium with the partial pressure of water in the gas stream at a 
desired operating temperature. Other considerations are also very 
important, e.g. stability in repeated cycles of use and regeneration, 
inertness to other gases in the stream, mechanical strength to minimize 
breakage and crumbling, cost, etc. 
Various solids have been found to have a greater or less extent the 
properties required in this process depending on the particular conditions 
involved, for example:--calcium sulfate, activated alumina, silica gel, 
and zeolites (aluminosilicates with alkali metal cations), among others. 
The first three may be less useful in some cases because of operating 
temperature ranges, or inefficiencies in separation in the presence of 
other components of the gas mixtures. 
The so called "soluble" form of calcium sulfate (trade name Drierite) is a 
low cost desiccant which has been found useful when the range of 
adsorption temperature desired is at about 30.degree. to 50.degree. C. and 
the desorption is to be accomplished at 190.degree. to 220.degree. C. This 
low temperature range of adsorption gives at 30.degree. C. an equilibrium 
moisture content in the gas stream of only about 0.005 milligrams of water 
per liter gas with a water adsorption of a little over 6%. However with a 
slightly less efficient water removal, this desiccant will remove up to 12 
or 14% water by weight before regeneration is necessary. 
Activated alumina has been found to give comparable drying to calcium 
sulfate for the gas at temperatures near the ambient but with a slightly 
higher capacity before regeneration at 150.degree. to 300.degree. C. 
The zeolites desiccants are usually called Molecular Sieves. They have a 
crystalline structure, synthesized so as to give pores of a specific, 
uniform size which will admit and hold molecules of a definite size, and 
reject molecules of other sizes. They have been found to be particularly 
efficient in the process of this invention because of their selectivity 
for water also because of other properties than simply accepting a 
molecule of the size, the water molecule. These include their capability 
of holding large amounts of water with very low vapor pressures out of the 
desiccant and their stability over many cycles of dehydration and 
regeneration. 
Different manufacturers provide standard grades of molecular sieves in 
powders and pellets or beads. All types have been found to be excellent 
desiccants in this use, with long service lives, particularly in the beads 
of screen mesh sizes of 4.times.8 or 8.times.12, or pellets of 1/16" or 
1/8" diameter, especially of type 3A and 13X. 
They have been used at higher temperatures than other desiccants in this 
service; and capacities have been found to be over 15% by weight of water 
adsorbed even at 100.degree. C.--somewhat less at 150.degree. C., and 
extremely low residual water in the gas stream. Regeneration has been at 
temperatures of 200.degree. to 350.degree. C.

While a diagram of a single adsorber operated batch wise could be drawn, 
this is elementary; and its operation will be obvious from the description 
below to those familiar with the adsorption operation. Also no figures are 
drawn to indicate the many well known variations of such, so called 
continuous adsorption operation as those of FIGS. 1 and 2, which also may 
be used to implement this invention. 
REGENERATION OF ADSORBENT BEDS BY DIRECT TRANSFER OF HEAT 
FIG. 1 shows one of the various flow sheets of vapors and liquids to 
accomplish the dehydration by adsorption of water away from methanol and 
indeed away from the other components of the gas stream leaving the 
converter of a methanol synthesis plant. Modifications of this adsorption 
flow sheet familiar to the art, may also be used by this process to 
separate the water present in hot methanol vapors and other constituents 
of other gas streams, and using the sensible heat of the gaseous stream to 
regenerate the adsorbent through evaporating therefrom the water which has 
been adsorbed. 
The hot gases from the methanol synthesis converter pass through line 101, 
and heat exchanger 102. This may be a steam generator to recover a large 
amount of the sensible heat in this gas stream, coming principally from 
the exothermic synthesis reactions which produce methanol. Steam for power 
and process use thus leaves the steam generator by line 103. Depending on 
the energy balance, some part of the gases flowing in 101 may by-pass the 
heat exchanger by a line, not shown, so as to give a higher temperature in 
the line 104 going to the adsorbers. 
FIG. 1 diagrams two identical adsorbers, A and B, in parallel, each with 
multiple beds charged with a suitable adsorbent-desiccant preferably in 
pellet form. Both resemble a standard shell and tube heat exchanger, with 
very large tubes held between standard tube sheets, although other 
standard arrangements may be used. These adsorbers are used alternately to 
each other, so that while one is adsorbing water to dehydrate the gas 
stream, the other adsorber is being regenerated by having the adsorbed 
water vaporized away from the adsorbent. A third adsorber may be 
interconnected to this parallel arrangement to allow a spare unit or to 
allow some flexibility in timing of the shifting from the adsorption to 
the regeneration sequences of the cycle, or back. Such usage is 
conventional in adsorption practice. 
Assuming that adsorbing beds 107 of A have become charged with water in the 
previous cycle, then the hot gases substantially at the pressure of the 
converter pass through lines 104 and 105 to enter the space around the 
tubes 108 inside the shell of A and to heat the adsorbent particles which 
are supported by suitable means at the level of the bottom tube sheet. 
These particles are packed inside several or more metal tubes of large 
diameter. Heat transferred through the walls of the tubes supplies the 
heat of evaporation plus the heat of adsorption--often about an equivalent 
amount--so that the water is desorbed, evaporated, and driven off. These 
large tubes containing the desiccant may be of any convenient diameter, 
however, if more than about 12 inches the essential heat transfer is 
undesirably slow. The hot gases mainly 10-15% methanol vapors as described 
in U.S. Pat. No. 4,235,799 in admixture with permanent gases and with the 
small amount of water vapor to be removed leave the shell side of A by 
line 109. 
This gas mixture, now somewhat cooler, passes by way of lines 111 through a 
heat exchanger 112, which recovers more of their heat, which has not been 
used in the regeneration of the adsorbent in the beds of A. This heat 
recovery may be by generation of steam by 112 or the heating of boiler 
feed water going to steam generator 102. However the most important 
function of this heat exchanging complex, designated simply as 112, will 
be the counter current preheating of recycle gas in line 126. 
Also the heat exchanging noted in 112 will usually include a final cooling 
of the gas stream in 111 by an additional flow of cooling water, not 
shown. It is desired to have as low as practical a temperature for the 
adsorption of water in the gas stream in 111 by the desiccant in 107 of B. 
The gas stream of 111, now further cooled and at a pressure somewhat less 
than that of the methanol converter passes through line 114 to the bed 
side of the other adsorber, B, and down through the particles of 
adsorbent-desiccant in the beds 107. Water is adsorbed from the gas 
stream, and the dehydrated methanol vapors--with a large amount of 
permanent gases principally CO, CO.sub.2, H.sub.2, and others--leaves B by 
line 116. Then this stream goes through line 117 to pass to the 
condenser-cooler 118. Here methanol is condensed together with any other 
condensables. These may be principally higher alcohols in from one to 
several percent, possibly some dimethyl ether in very small amounts, 
depending on the particular catalysts and flow sheet of the converter 
loop. 
The liquid and the gas phases leaving 118 pass through line 119 to a 
separating flash drum 120; and the liquid, principally methanol, 
discharges from the system through line 121. 
All of the process described so far, including this condensation of 
methanol, preferably has been under the pressure of the converter--or 
slightly less, due to the drop in pressure through piping and equipment. 
Any lower pressure down to atmospheric may be maintained--usually less 
advantageously. Now the liquid methanol and any non-condensable gases 
dissolved therein pass through a reducing valve in line 121 to discharge 
at atmospheric pressure. 
It may be desirable to discharge this methanol into a second flash drum, 
not shown, to separate the non-condensable gases, relatively small in 
amount compared to those separated in 120, which may be dissolved in the 
liquid condensate. If so, this gas stream may be wasted as part of the 
gases purged from the system, or it may be recompressed to the pressure of 
120 and passed back to line 117 leading to the condenser 118 to condense 
out the methanol in this gas stream. 
The residual syngas and inert permanent gases which are flashed out of the 
liquid methanol in 120 discharge through line 122, some small part is 
discharged through the purgeline 123 for other processing or waste as 
desired. The balance is passed by line 124 to be compressed by compressor, 
125 to the pressure of line 126, thence heat exchanger 112, and finally 
they are passed as recycle back to the methanol conversion loop. 
Usually the dehydration-adsorption may be done best in B at a high 
pressure, not much below that of the converter but at as low a temperature 
as may be economic to attain. On the other hand, the regeneration process 
taking place in adsorber A, while B is adsorbing, may be done at a high 
pressure, somewhat less than that of the converter or it may be done at or 
near atmospheric pressure. A low pressure in the regeneration, with a high 
pressure for the adsorption-dehydration allows rapid and complete 
desorption of water in a pressure-swing (a pressure-differential type of 
regeneration). This will be assumed for the present example, which depends 
on both a temperature swing and a pressure swing, although either would 
suffice. However, since the gas stream 101 has both a very high 
temperature and a very high pressure compared to the ambient, the pressure 
swing may also be used. This allows a lower temperature to be attained in 
102. 
The hot gases coming from the heat exchanger 102, following the converter 
and passing to the shell side 108, of adsorber A, passes through the wall 
of the tubes to the desiccant 107 in the adsorber tube-beds and evaporates 
the water therefrom. This desorption, when done at a low final pressure, 
e.g., below atmospheric, will allow a lower temperature drop of the hot 
gases entering at 105 and leaving at 109 because of the great aid to the 
desorption due to this pressure-swing. 
By operating the regeneration in A at as low a temperature drop as possible 
depending finally on a pressure-swing regeneration, it is possible to have 
the heat exchanger 102, cool the gas stream to a lower temperature than 
could be possible if desorption was done at a high final pressure. Thus 
more heat is recovered in 102; and the cost of the overall operation is 
reduced. 
This water vapor from the regeneration passes through lines 130 and 132 to 
a condenser-cooler 133, operated preferably at atmospheric pressure. Water 
previously adsorbed in A, and now being desorbed, is condensed in 133, and 
discharged through line 134 to a receiving tank 135, with a drainpipe 136; 
while non-condensable gases are discharged through vent line 137. When no 
more condensate, water, discharges from 136, suction may be applied to 
vent line 131 to complete the regeneration of the desiccant, and a small 
additional amount of water may be received in 135. 
If there is syngas of value in the exhaust gases discharging through 137, 
this line or the discharge of the unit supplying suction may be compressed 
and recycled back to the converter. This is usually not worthwhile, and 
this wasted gas may be part of the necessary purge of the system. 
Alternatively, before the dessicant side of adsorber A, is depressurized; 
and at the start of the regeneration of its beds 107, a compressor--not 
shown--may take a gas stream from line 130 and discharge it back into line 
111 or 114, thence to go with the gas stream to adsorber B. This will 
exhaust the syngas and other gases from the bed side of the adsorber A, 
and leave the water almost alone in being held in the adsorbent beds 107 
of A, since water has been found to be held on the desiccant-adsorbent 
most firmly of the several constituents of the gas stream coming from the 
converter. 
This same object can be obtained, also before depressurizing the adsorption 
side of A and the condenser and receiver assembly, by connecting a 
compressor--not shown on the figure--from the vent 137, to the line 111. 
Gas under the adsorption pressure is then passed from the adsorption side 
of A through the cold condenser 133, also 134, 135, and 137, then 
compressed over a comparatively narrow range of pressure, that due to pipe 
friction, etc. and passed through lines 111 and 114 to the adsorber, B. 
When the adsorber side of A is thus "degassed," the pressure there may be 
allowed to fall, with vent 137 open to the atmosphere. Meanwhile water 
vapor is desorbing from the beds 107, in the adsorber side of A and is 
being condensed in 135 and discharged from 136. The last of the water may 
be stripped out, with valve on line 136 closed, by use of this compressor 
as a vacuum pump discharging to atmospheric pressure; and the last of the 
water will be collected in 135 as a vacuum receiver. The 
adsorbent-desiccant in 107 of A, is thus completely stripped of water, 
while that in 107 of B has become completely charged with water. 
This marks the end of a cycle; and by proper manipulation of the valves 
(i.e. the change of all those full open to full closed and vice versa.) 
the operation with A becomes that of adsorption and that with B is that of 
desorption. For B to act as the desorber, hot gases from line 104 pass 
through the now open valve 106, and lose some heat and reduced in 
temperature in going through 108 of B, next line 110 with its now open 
valve, then through line 111, the now open valve 113, the desiccant in 107 
of A, and out--free of water--through now open valve 115 to pass through 
line 117 and successive steps as before. Cycle after cycle is accomplished 
through manual or automatic opening and closing of these valves to control 
the flow of the several gas and liquid streams. 
In pressure, the higher than atmospheric in the methanol catalytic 
converter loop may be from 750 psi to 7500 psi or even more, to which this 
adsorption-separation system may be attached as an integral part. A large 
part of the gas necessarily is recycled after the adsorption-separation, 
and after condensing out the methanol, particularly, also separately, the 
water. Thus it will be apparent that insofar as the recycle gas is 
concerned, if the adsorption is done under full pressure, the separation 
may be accomplished without a substantial loss of pressure between the gas 
taken from the methanol loop and the large part of it which is returned. 
Regeneration of Adsorbent Beds by Circulating a Hot Gas 
FIG. 2 shows another flow sheet of the water being separated by the process 
of the invention. Here a hot inert gas is used as the means of supplying 
heat to the adsorbent-desiccant for the regeneration or desorbing 
operation of removing water. Heat transfer is slow through the walls of 
the container for the adsorbent in FIG. 1 and particularly through the bed 
itself. Thus the container cannot have a large diameter, and the 
temperature difference from gas to an average particle in the bed must be 
large to accomplish the heating for regeneration in a reasonable time. 
Instead of heat transfer from outside the bed, a heated inert gas may be 
circulated directly through the bed to heat and simultaneously to purge 
water from the particles of adsorbent. This gas may be by-product nitrogen 
from the air separation plant making oxygen, if the synthesis gas to make 
methanol is produced by partial oxidation of a carbonaceous fuel. Or it 
may be carbon dioxide in those plants removing carbon dioxide from the 
syngas, 
Syngas itself has been found to be suitable for use; but it would lose a 
part of its water, and would then have to be recycled to the adsorber for 
removal of the balance. Indeed also the gas in the stream following the 
condensation out of the methanol may be used as heat carrier in this 
separating process. This may be called the "spent" gas and might be 
obtained from a draw-off connection to line 223 or line 225 or line 226 in 
FIG. 2, now to be explained. This would be connected to the inert gas 
inlet at 250. Use of syngas if anhydrous or of this spent gas adds no new 
component to the system of the circulating gases. An additional advantage 
will be that either may be at nearly the pressure of the system and thus 
does not require substantial compression in its use, if desorption is done 
under high pressure. 
The operation of much of the adsorption side of the flow sheet of FIG. 2 
duplicates that of FIG. 1. However, the beds for adsorbent may be much 
larger in diameter and the adsorbent is filled inside the walls of large 
cylindrical vessels A and B, which are provided with suitable conventional 
supports for the adsorbent. 
Here again the valves are indicated as being in the appropriate open or 
closed positions for operation of B as adsorbing and A being regenerated. 
Thus, the gas stream coming from the methanol converter, enters at 201, 
passes through the heat recovery exchanger, preferably a steam generator 
202, through a second exchanger 260 which preheats the regenerating gas 
and via line 204, heat exchanger 212, and lines 211 and 214 to enter the 
adsorbent bed 207, of B. 
Water is adsorbed in the bed 207 of B, the dehydrated gas stream passes 
through lines 216 and 217 to the condenser-cooler 218; the condensate, 
methanol drains to receiver 220, then through line 221 to storage. The 
dehydrated and demethanolized spent gas is separated in 220. Some part is 
purged from the system by line 223; and a part of this may be used as 
regenerating gas by a line, not shown, connecting to 250. The balance of 
the stream leaving 220 is compressed by 224 back to converter pressure (to 
compensate for the pressure drop of the adsorbing part of the cycle). This 
stream then passes through line 226 and is reheated by heat exchangers 212 
and 202 on its way back as recycle to the converter via line 225. 
This passage of the product gas from the converter is continued until the 
adsorbent 207 of B is fully charged with water, when a cutover of B to 
regeneration by desorption is made through proper adjustment of valves. 
Meanwhile adsorber A has been operating on its desorption half of the 
cycle. The desorption gas system has been charged with inert gas. This is 
preferably a part of the stream in line 223 or line 226 supplied by a line 
not shown. The inert gas enters through supply line 250. It then passes 
through line 252 and compressor 253 which essentially makes up for the 
pressure drop in circulating the desorbing gas through the system. Usually 
the absolute pressure will be at or near atmospheric during most of the 
desorption operation, but it may be at any desired pressure including a 
partial vacuum. 
Desorbing gas goes through line 254 and is heated in heat exchanger 255 by 
the desorbing gas charged with water--or bypassed 255 by line 256 
(dashed)--then through line 257 to be heated further in 260 by incoming 
gas from the converter or by-passed 260 through line 259. Now at an 
elevated temperature, the gas stream passes through line 251 to pass by 
line 262 to adsorber A and its bed 207 of adsorbent. Here the gas stream 
evaporates the water and picks it up as vapor, is cooled somewhat thereby 
leaves by line 230, and passes via line 232, heat exchanger 255, then to 
water-cooled condenser-cooler 233 where water is condensed. This and 
non-condensed gases pass to separator tank 235 from which water is 
discharged from 236. 
When the water thus is removed from bed 207 of A which has been well 
heated, the same stream cycle may be used to cool 207 of A, by bypassing 
the heat exchangers 255 and 250 through lines (dashed) 256 and 259. Thus 
the desorbing stream, cooled by condenser-cooler 233 passes the route 235, 
253, 254, 256, 257, 259, 251, 262, through bed 207 of A and circulates 
back through 230, 232, 233, 234, and 235 until the bed A is cooled. At 
that time, there may be connected, if desired, a vacuum pump to line 261 
to exhaust the desorbing gas and to reduce the pressure of bed 207 of A. 
When anhydrous syngas for the methanol production is used in this way, and 
it is desired to use syngas for desorption, this relatively small amount 
can be added to or taken from the stream used in methanol production 
without substantial effect. If the inert gas is taken from line 223, as is 
often preferable, the gas may be discarded from the system, or returned to 
the gas purification plant for recovery of its values. 
The cycle is complete, and the operation of A is now changed to adsorption 
and of B to desorption, with a continuing sequence of alternations of 
adsorption and desorption in the respective beds, by proper manipulation 
of the valves. When B is operated as the desorber, the hot gas for 
desorption in line 251 enters B and its bed of desiccant by line 263, 
while the incoming gas stream from 211 is dehydrated by the desiccant in 
bed 207 of A, and is passed, anhydrous, out through 215 and 217 to the 
subsequent steps as before. 
The use of a separate gas for cooling after desorption as well as heating 
for desorption has the advantage of excellent heat transfer to or from the 
gas, from or to the adsorbent particles; and the additional advantage, in 
this case, where water is being desorbed, is that if some of this 
adsorbate is lost by not being entirely removed from this gas stream, it 
has no value. This may be different from the usual recovery of solvents by 
adsorption.