Method for the recovery of low purity carbon dioxide

A process for the economical recovery of carbon dioxide from a gas stream containing less than 85% carbon dioxide, by cooling the contaminating gas to remove water vapor, compressing the cooled gas to an elevated temperature and pressure, and drying the gas to a dewpoint of not more than about -85.degree. F.; condensing and removing the carbon dioxide from the dried compressed gas; and heating the remaining noncondensed gas mixture and expanding it to produce and recover kinetic energy and a cooled gas mixture.

TECHNICAL FIELD 
This invention relates generally to the recovery and liquefaction of carbon 
dioxide and relates more particularly to a process and application for 
liquefying a flow of relatively low purity carbon dioxide gas in 
conjunction with providing an economical commercially resalable food grade 
carbon dioxide product. 
BACKGROUND OF THE INVENTION 
The United States of America has a policy of utilizing the present national 
coal reserves of approximately 3.5 trillion tons of coal in the 820 power 
plants that will burn coal over the next 100 years. This will produce 
about 10.29 trillion tons of carbon dioxide to be vented to the 
atmosphere. The world is now being made aware of the "greenhouse effect" 
which predicts that the accumulation of carbon dioxide and other gases in 
the atmosphere will raise the global temperature by about 2.degree. C. by 
2050 and by about 5.degree. C. by 2100. This is expected to be disastrous 
and accordingly, all nations are preparing immediately to reduce carbon 
dioxide emissions drastically. The United States Government has issued a 
report of a study of this problem "Can We Delay a Greenhouse Warning?" 
This study notes that control of CO.sub.2 emissions from plants is an 
important step to take, and that the only technically feasible process is 
that of absorbing CO.sub.2 in nonethanolamine (MEA process), but that 
process is too costly in that it seriously reduces the capacities of the 
plants so much that it would not be economically feasible. Applicant's 
process is intended to meet that problem head-on and provide an 
economically feasible process for removing CO.sub.2 from stack gases or 
other gas streams low in CO.sub.2 content, i.e., less than about 85% 
CO.sub.2. 
Another advantage of the present process is to provide carbon dioxide and 
nitrogen for use in programs of enhanced oil recovery (EOR). These 
programs are designed to go beyond the present art of primary and 
secondary methods of recovering petroleum from underground reservoirs. 
Only about 25-30% of the petroleum is recovered by the conventional 
primary and secondary methods. EOR programs increase that recovery to 
about 45-50% using carbon dioxide and nitrogen. Approximately 900-5400 
cubic meters of CO.sub.2 are required per cubic meter of petroleum 
recovered. The applicant's process will provide an economical source of 
CO.sub.2 for such a process. 
The process of this invention was developed specifically, over a period of 
about 6 years to recover carbon dioxide economically from gas streams 
which all other processes, e.g., MEA process, fail to do at all or fail to 
do in an economically feasible fashion. Preferred procedures in the 
process of this invention are accomplished by using any of three types of 
patented separators for the separation and liquefaction of carbon dioxide 
from the treated gas stream contaminated with carbon dioxide. The patented 
separators are described in U.S. Pat. Nos. 4,498,303; 4,572,728; or 
4,639,262; and they describe gas-to-gas separation and gas-to-liquid 
separation. The step of separation is the key to an economically feasible 
process that does not rely on any expensive solution step such as in the 
MEA process. Other features of the present process are to utilize for 
heating or cooling any of the various gas and liquid streams in the 
process for heat exchange with other streams in the process. Furthermore, 
heat energy is converted to kinetic energy by expanding pressurized gas in 
turbines that may drive electric generators to produce electric power for 
use in the plant. The treated gas streams contain substantial amounts of 
nitrogen and oxygen and these gases are separated, purified and liquefied 
to produce valuable products that may be sold commercially and thereby 
reduce overall costs of removing carbon dioxide from flue gases and other 
gas streams vented to the atmosphere. There is no teaching in the prior 
art to expect that a process of this type would be successful in an 
economic sense. As a matter of fact, the consensus of the industry was 
that it would be impossible to accomplish. Hence, the MEA process was 
considered by the U.S. Government to be the only way to separate carbon 
dioxide from low purity streams, i.e., less than about 85% CO.sub.2. 
The present process also purifies the flue gas of oxides of sulfur and the 
oxides of nitrogen so that the purified gas stream that is vented to the 
atmosphere will meet more stringent specifications than the Government's 
Environmental Protection Agency standards. The contaminating flue gas 
stream will be purified of sulfur dioxide to less than 0.3 PPM, carbon 
monoxide less than 10 PPM, and oxides of nitrogen to less than 10 PPM by 
volume. These food grade carbon dioxide specifications are current 
industry standards. The purification of the contaminating gas of vaporous 
odors and particulates are not part of this invention and therefore, are 
not discussed for both simplicity and proprietary reasons. 
Various present methods of liquefying high purity 90% or better carbon 
dixoide gas are well known. Typically, the liquefaction process of a 
relatively pure carbon dioxide comprises of compressing the gaseous carbon 
dioxide to a pressure of approximately 233.85 psig to 312.1 psig and then 
removing the latent heat of condensation with a secondary refrigerant at 
an evaporating temperature below the saturation temperature of the carbon 
dioxide pressure or -12.degree. F. or -4.degree. F. respectively. The 
theoretical range of pressures over which vaporous carbon dioxide can be 
condensed to a liquid is approximately 60.43 psig to 1057.4 psig. 
Low purity carbon dioxide also contains contaminating gases with a lower 
temperature of condensation than carbon dioxide and these contaminating 
gases require a lower temperature refrigerant to condense than the carbon 
dioxide vapors. Therefore, the carbon dioxide may be separated from a 
contaminating gas source by fractional condensation. This invention 
specifically removes the carbon dioxide vapors from a gas stream between 
any compressor created saturation point down to the triple point of carbon 
dioxide. Any carbon dioxide below the triple point is unrecoverable. 
This invention relates to a process for recovering carbon dioxide vapors 
from a gas stream such as flue gas, industrial waste gas streams or any 
other low purity carbon dioxide gas stream, particularly to a process for 
recovering carbon dioxide at purities of less than about 85% that are too 
low to recover economically by a conventional carbon dioxide liquefaction 
system. It specifically replaces the MEA chemical absorption process. This 
invention produces carbon dioxide liquid or vapor at a substantial utility 
cost reduction below all existing MEA technology. 
It has proved to be most difficult and costly to recover, purify and 
liquefy the carbon dioxide vapors when they are present in low 
concentrations in a gas stream. Thus, all known processes which recover 
carbon dioxide vapors present in a gas at low concentrations involve high 
investment and/or production utility costs. In particular, in all MEA type 
absorption processes, the excessive amounts of steam required to 
regenerate the absorbent prohibits economic recovery of carbon dioxide 
from low purity gas sources, such as a steam boiler flue stack gases which 
are in the magnitude of 8 to 15% volume carbon dioxide purity. 
There are basically three types of carbon dioxide vapor and gas stream 
recovery combinations: (1) 85-100% pure carbon dioxide vapor-laden 
streams, (2) less than 85% and greater than 50% carbon dioxide vapor-laden 
gas streams, (3) 50% and less carbon dioxide vapor-laden gas streams. In 
Item (2) above, we are removing the non-condensable gases from the 
condensable carbon dioxide vapors. In Item (3) above, we are removing the 
condensable carbon dioxide vapor from the non-condensable gases. The above 
is determined mathematically by the ratio of the carbon dioxide vapor 
pressure to the partial pressure of the non-condensable gas stream. When 
this ratio is greater than one, we are removing the non-condensable gas 
from the carbon dioxide vapor. When this ratio is equal to one or less, we 
are removing the carbon dioxide vapor from the non-condensable gas. When 
we are removing a non-condensable gas from a carbon dioxide vapor we reach 
the point in fractional condensation where this ratio becomes one and then 
the carbon dioxide vapor must be removed from the non-condensable gas. 
The removal of carbon dioxide from the non-condensable gas can occur only 
when the carbon dioxide vapor pressure is above the carbon dioxide triple 
point of -69.9.degree. F. The removal of carbon dioxide vapor pressure 
below the triple point will cause freezing of the carbon dioxide. 
Therefore, the carbon dioxide vapors contained in the non-condensable gas 
who's dewpoint is below the triple point is non-recoverable vapors and are 
vented. 
The invention has two types of non-condensable vent procedures; a 
continuous vent process and a batch vent process. The batch vent process 
is applicable for approximately 50% or greater carbon dioxide purity gas 
stream. It's primary advantage is that it minimizes the amount of 
non-recoverable carbon dioxide vapor vented. It operates on the basic 
principle that the higher the non-condensable gas pressure, the less 
carbon dioxide vapor at saturation conditions it will hold. The carbon 
dioxide vapor pressure maintained equilibrium conditions and any increase 
in carbon dioxide vapor pressure will condense to a liquid. The continuous 
vent process will vent all the carbon dioxide vapors in the 
non-condensable gas stream. Example: a 95% carbon dioxide vapor stream at 
-12.degree. F. will vent 5.3% of the carbon dioxide vapor on a continuous 
vent process. The same 95% carbon dioxide vapor stream will vent 1.0% of 
the carbon dioxide vapor on a batch vent process. 
BRIEF SUMMARY OF THE INVENTION 
This invention relates to a process for the recovery of carbon dioxide from 
a gaseous mixture containing water and less than about 85% carbon dioxide, 
the process which comprises: 
a. cooling the gaseous mixture to remove substantially all water; 
b. compressing the cooled gaseous mixture to an elevated temperature and 
pressure and drying the compressed gaseous mixture to a dewpoint of not 
higher than about -85.degree. F.; 
c. cooling the compressed dried gas to liquefy said carbon dioxide therein 
and to separate the liquid carbon dioxide from the remaining noncondensed 
gas mixture; and 
d. heating the noncondensed gas mixture and expanding said gas to produce 
kinetic energy and a cooled gas mixture. 
In one specifically preferred embodiment the invention includes the use of 
any available hot gas or liquid stream to heat the noncondensed gas 
mixture in step d. and the conversion of the heat energy in that hot 
noncondensed gas mixture into kinetic energy by expansion in a turbine. In 
another embodiment the gaseous mixture treated in step a. is a flue gas 
containing less than 50% carbon dioxide. In still another embodiment the 
compressed dried gas of step c is introduced into a mass of liquid carbon 
dioxide to cause condensation of the carbon dioxide in that compressed 
dried gas. 
In one preferred embodiment the original gas mixture is cooled to condense 
and remove nearly all of the contaminating water vapor and at the same 
time to reduce the specific volume and density of the gas stream, thereby, 
reducing the horsepower requirements of compression. The gas stream is 
then compressed to an elevated pressure, so that the partial pressure of 
the carbon dioxide is equal to a saturation temperature of approximately 
-12.degree. F. or some other preferred saturation temperature. 
All water vapor is removed from the contaminating gas stream at either an 
intermediate pressure or the discharge pressure of the gas compressor by 
desiccant drying to produce a water vapor dewpoint at pressure (DPP) of 
-85.degree. F. This low dewpoint eliminates the freezing of water vapor 
during the separation and liquefaction of the vaporous carbon dioxide. The 
frost and ice formation in the carbon dioxide liquefier/separator would 
cause reduced capacity and eventual blockage of the liquefier/ separator 
with the results of no liquid carbon dioxide output to the storage tank. 
The compressed and dried low purity carbon dioxide gas then passes through 
a gas to gas regenerative type heat exchanger. It's primary function is to 
recover the mechanical refrigeration energy expended to cool the separated 
high pressure contaminating gases. The gas to gas cooler accomplishes this 
energy savings by cooling the compressed and dried low purity carbon 
dioxide gas stream while in count-current flow it warms the refrigerated 
or cooled contaminating gases. 
The compressed and dried low purity carbon dioxide gas then enters the gas 
to liquid separator (U.S. Pat. No. 4,498,303), or the gas to gas separator 
(U.S. Pat. No. 4,572,728 or U.S. Pat. No. 4,639,262) for liquefaction and 
separation of the carbon dioxide vapors from the contaminating gases. The 
gas to liquid or gas to gas separator is basically a vertical carbon 
dioxide absorber tower. The compressed carbon dixoide vapors are absorbed 
in the absorbent liquid carbon dioxide and the non-condensable gases pass 
through the absorbent and are vented. 
The cooled separated non-condensable gas, further, is used as the coolant 
to cool the low purity carbon dioxide gas stream prior to compression. 
This step of the process has a dual advantage in that it recovers the 
mechanical refrigeration energy required to cool the separated high 
pressure contaminating gasses and at the same time recovers waste heat 
energy from the low purity carbon dioxide gas stream for recovery in an 
expansion turbine as mechanical work. 
The recovery of the compression horsepower energy and waste heat is 
accomplished by an expansion turbine and converted by a generator to 
electrical power for the various compressor motors. In an especially 
advantageous mode the expansion turbine consists of multiple stages of 
expansion. Each stage is pre-heated by alternate sources of heat recovery. 
It is another object of the expansion turbine to use the expanded low 
pressure contaminating gas stream as a refrigerant for use in the 
liquefier/separator inplace of a conventional mechanical refrigeration 
system or for other process coolant requirements, such as gas coolers, 
compressors intercoolers and aftercoolers or precoolers. In the especially 
advantageous mode, by balancing the work generated into electrical power 
by the expansion turbine versus the refrigeration gas produced by the 
expansion process, will allow the lowest overall kilowatt reduction in the 
production of the food grade carbon dioxide. This has the results of 
minimum utility costs per pound of carbon dioxide produced. 
These and other objects, features and advantages of the present invention 
will become apparent after a review of the following detailed description 
of the disclosed embodiment and the appended drawings and claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the drawing there is shown a preferred embodiment 100 of the invention 
wherein a flow of relatively low purity carbon dioxide gas is purified, 
compressed, dried, separated and liquefied in conjunction with providing 
pure food grade liquid or gaseous carbon dioxide or an industrial grade 
liquid or gaseous carbon dioxide. The typical source of low purity carbon 
dioxide gas is from an industrial high sulfur 3 to 4% coal fired 
electrical power generation plant and commonly called flue gas. This flue 
gas contains relatively large amounts of contaminating gases such as 
nitrogen, water vapor, sulfur dioxide and oxygen. The major contaminate 
nitrogen has a substantially lower condensation temperature than that of 
carbon dioxide. The embodiment 100 will typically be used to advantage in 
a flue gas separation (FGS) plant for the commercial production of 
foodgrade liquid carbon dioxide and or nitrogen. 
The impure and lean carbon dioxide gas stream will be flue gas from the 
combustion of a fossil fuel. The flue gas is removed downstream of the 
electrostatic precipitator relatively free of solid particulates (fly ash, 
coal dust and mineral matter) and at a temperature of at least 350.degree. 
F. and perhaps as high as 1200.degree. F. or more. The sulfur content of 
the fuel has been reduced from 3.7% to less than 0.3PPM by volume of 
sulfur dioxide. The constituents of the cooled flue gas at 60.degree. F. 
and 14.7 psia or at the inlet to the flue gas compressor is approximately 
nitrogen, 77%, carbon dioxide 14%, oxygen 4%, sulfur about 0.3PPM, and the 
remainder water vapor. 
The flue gas is conducted via conduit 1 to the inlet of the flue gas 
cooler, heat exchanger, 2 the heat exchanger is either a conventional 
shell and tube or the finned coil type. The coolant in the shell is 
refrigerated nitrogen gas from the carbon dioxide separation process. The 
cooled flue gas and any condensed water is carried by conduit, 3 to water 
separator (knock-out drum) Item 4. All condensed water vapor is separated 
from the flue gas stream and the condensed water is sent to drain by a 
water trap or water-leg seal. The flue gas with a reduced water dewpoint 
is conducted by conduit 5 to the second stage flue gas cooler, heat 
exchanger Item 6. The flue gas cooler is either a conventional shell and 
tube or finned coil type. The coolant in the shell is evaporated ammonia 
from the mechanical refrigeration system. The cooled flue gas and any 
condensed water is carried by conduit 7 to water separator (knock-out 
drum) 8. All condensed water vapor is separated from the flue gas stream 
and the water is sent to drain by a water trap or water-leg seal. 
The flue gas with reduced water dewpoint is conducted by conduit 9 to the 
inlet of the gas turbine flue gas compressor set, 10. The gas turbine flue 
gas compressor set consists of the following items: 
10A - Flue Gas Compressor-Centrifugal Type, 
10B - Power Turbine, 
10C - Air Turbine, 
10D - Fuel Combustor, and 
10E - Air Compressor 
The centrifugal flue gas compressor using a gas turbine driver serves as 
the first stage or first two stages of gas compression. The flue gas is 
discharged at an elevated pressure and cooled by a conventional 
aftercooler (not shown) to 95.degree. F. 
This compressed and cooled gas is conducted via conduit 11 to the inlet of 
a direct contact flue gas cooler or water wash 12. This is a packed bed 
counter current flow vertical scrubber. The once-thru water coolant flow 
rate 46 is adjusted for 1 to 2.degree. F. temperature rise of the effluent 
discharge water to drain 47. 
The cooled and washed gas is conducted from the top outlet of the water 
wash 12 via conduit 13 to the inlet of the mulitple stage positive 
displacement flue gas compressor with electric motor driver, 14. All 
intercoolers and aftercoolers for simplicity are not shown. At an 
intermediate stage of gas compression of approximately 300 psig and 
95.degree. F. the flue gas is conducted via conduit 15 to a dessiccant 
type dryer 16 where all the water vapor is removed to a -85.degree. F. 
dewpoint at pressure (DPP). The dryed flue gas is then conducted via 
conduit 17 to the next stage of compression. The compressed flue gas is at 
an elevated pressure of 1200 to 2,000 psia and is discharged from the flue 
gas compressor at approximately 95.degree. F. downstream of the 
aftercooler. A trap dryer of a molecular sieve or a dessicant may be 
installed at the condensing pressure to guarantee a low dewpoint of the 
gas stream. 
This cooled and compressed gas is conducted via conduit 18 to and thru the 
gas to gas regenerative type precooler, 19. All sensible heat is removed 
from the flue gas stream and a small amount of latent heat of condensation 
of the vaporous carbon dioxide may occur. The coolant for the gas to gas 
precooler, 19, is refrigerated nitrogen gas from the carbon dioxide 
separation process in 21. 
The cooled flue gas is conducted by conduit 20 into the inlet of liquid 
carbon dioxide separator 21 (as explained in U.S. Pat. No. 4,498,303, 
dated Feb. 12, 1985). This is a fractional condensation 
liquefier/separator which liquefies the vaporous carbon dioxide and 
separates the non-condensable flue gases (N2, 02, etc.). The liquefier is 
basically a vertical carbon dioxide absorber tower. The compressed carbon 
dioxide vapors are absorbed in the liquid carbon dioxide (the absorbent) 
and the non-condensable gases pass through the absorbent and are vented 
via conduit 23. The liquid carbon dioxide is conducted by conduit 22 to a 
carbon dioxide liquid storage tank for use. The secondary refrigerant 
enters the liquefier/separator, 21, by conduit 50 and exits the 
liquefier/separator, 21, by conduit 49. This refrigerant may be supplied 
by either a conventional mechanical refrigeration system (two stage), 
cascade system, Joule-Thomson Valve or expander. Flow control valve 48 
maintains a back pressure on the carbon dioxide liquefier/ separator 21, 
so that the carbon dioxide condensing pressure is 75.1 psia at all times. 
The vented nitrogen gas is then conducted from valve 48 via conduit 51 to 
the gas-to-gas regenerative heat exchanger 19, and is heated from 
-69.degree. F. to +94.degree. F. The heat source is compressed dry flue 
gas which is being cooled down in temperature and then heated. Nitrogen 
vent gas is conducted by conduit 24 to the inlet of the flue gas cooler 2, 
where the gas is heated to within 6.degree. F. of the flue gas 
temperature. 
The heated nitrogen vent gas is then conducted via conduit 25, to the heat 
recovery heat exchanger 26, where the nitrogen vent gas is further heated. 
Heat is applied to the heat recovery heat exchanger via conduit 43 which 
conducts the exhaust gas at a temperature of at least 850.degree. F., 
e.g., 850.degree.-1200.degree. F. from the gas turbine engine. The heated 
nitrogen vent gas is then conducted by conduit 27 to the inlet of the 
first stage of expansion in the turbo-expander 28. The gas is then 
expanded down to the first stage discharge pressure. The work produced by 
the expansion process drives the electrical generator 33 and produces 
electricity to drive all electric motors on the multi-stage flue gas 
compressor and mechanical refrigeration compressor. The cooled and reduced 
pressure nitrogen vent gas is then conducted by conduit 29 to the heat 
recovery heat exchanger 26, where the nitrogen vent gas is once more 
heated. Additional stages of expansion and heat recovery are dependent on 
the waste heat available and the gas pressure available. The work produced 
by the expansion process drives the electrical generator 33, and produces 
electricity. 
The cooled and low pressure nitrogen vent gas is then conducted by conduit 
37 to the conduit 39 and returned to the chimney and a slip stream is 
separated by valves from the main gas stream. This slip stream is 
conducted by conduit 40 to the preheaters 41, which heat the dryer purge 
gas. The heated dryer purge gas is conducted by conduit 42 to the 
desiccant dryer where it is used to reactivate the dryers desiccant beds. 
The heat source conducted by conduit 44 for the dryer purge gas preheater 
is the gas turbine engines exhaust gas from the discharge of the heated 
recovery heat exchanger 26. 
The amount of heat recovery is dependent upon the total heat available from 
the flue gas stream which is recovered in the 1st stage flue gas cooler, 
and from heat available from other sources such as high temperature 
combustion gas, flue stack gas, and other waste heat streams. This will 
determine the number of turbo expander 28 stages. 
Further heat recovery is accomplished in a steam turbine 34. Any onsite 
waste steam available is conducted via conduit 35 to the inlet of steam 
turbine 34, which converts the steam heat energy into mechanical energy 
which drives the generator 33 and produces electricity and reduces the 
electrical KW costs for carbon dioxide production. The back pressure steam 
and condensate is conducted via conduit 36 for inplant process application 
or returned to the boiler as condensate. 
It is further part of this invention that both liquid carbon dioxide and 
liquid nitrogen may be produced simultaneously from the flue gas stream 
for commercial resale or use. The flow schematic would remain the same as 
the preferred mode of the embodiment as depicted in FIG. 1 with following 
process modifications. 
The vented non-condensables nitrogen gas in conduit 23 in the outlet of 
separator 21, a gas-to-liquid carbon dioxide separator, passes through 
flow control valve 48, which functions as a back pressure regulator. The 
vented nitrogen is conducted in conduit 51 where through separating valves 
a slip stream of nitrogen for recovery and liquefaction from a range of 1% 
to 100% is conducted into conduit 52. Conduit 52 conducts the nitrogen 
slip stream into a typical nitrogen purification system to reclaim and 
remove the by-product waste CO.sub.2. This CO.sub.2 must be removed prior 
to liquefaction of the nitrogen or it will cause freezing of heat 
exchangers and orifices. A conventional MEA or other chemical solvent 
process will be used. Conduit 52 conducts the nitrogen slip stream into a 
conventional nitrogen refrigeration system, a conventional liquid nitrogen 
generator or a typical Joule-Thomson Refrigerator. These three 
conventional nitrogen systems are depicted and explained in detail in the 
1968 ASHRAE, Guide And Data Book, entitled "Application in Chapter 49, 
Page 576, FIG. 3, Typical Joule-Thomson Refrigerator, Page 585, FIG. 19, 
Nitrogen Refrigeration System and Page 585, FIG. 20, Simplified Flow 
Diagram of Liquid Oxygen Generator. 
It is a further part of the invention that in place of 21 of the preferred 
embodiment of FIG. 1, there may be used a gas-to-liquid carbon dioxide 
separator/liquefier (U.S. Pat. No. 4,498,303 dated Feb. 12, 1985), a 
conventional horizontal or vertical carbon dioxide liquefier, having a 
shell-and-tube type heat exchanger, or a conventional liquid-to-gas 
separator. 
Further, it is part of this invention that the preferred mode of the 
embodiment of the combination expansion work and refrigeration process as 
depicted in FIG. 2 may be used to produce economical food grade carbon 
dioxide for commercial resale. The fundamental difference of the design is 
that the centrifugal flue gas compressor is not needed with a gas turbine 
driver 10 (FIG. 1), and the reheat cycle for the multiple stage turbine 
expander 28, in conjunction is not needed with the heat recovery heat 
exchanger 26. 
In the preferred mode of operation as depicted in FIG. 2, the discharge 
temperature of the nitrogen noncondensable vent gas at the outlet of the 
turbine expander will be approximately -130.degree. F. This cooled 
nitrogen gas can be used as a refrigerant precooler 23 and in after cooler 
19 (FIG. 2). The effluent-warmed nitrogen gas stream will be returned to 
the chimney at approximately +224.6.degree. F. via conduit 44. 
It is also part of this invention that any combination of the preferred 
mode of the embodiment of the expansion work process as depicted in FIG. 1 
and the advantageous mode of the embodiment of the combination expansion 
work and refrigeration process as depicted in FIG. 2 may be used in 
conjunction for the most efficient energy system for the specific Carbon 
Dioxide Recovery Plant installation. Typically, this would permit heat 
recovery from the flue gas chimney, boiler or other waste heat sources to 
be used, so that, all intermediate stages of the multiple stage turbo 
expander may be heated to 600.degree. to 650.degree. F. or other 
temperature in lieu of using the gas turbine engine exhaust gas. Further, 
a conventional electric motor driver may be used on the flue gas 
centrifugal compressor 10A of FIG. 1, in pace of the depicted gas turbine 
driver. 
It is also part of this invention that a conventional gas membrane 
separator may be used for the first and/or second stages of bulk gas 
separation. The membrane separator would be used to enrich the carbon 
dioxide volume percentage in the flue gas stream initially at about 8 to 
20% to approximately 60 to 80% carbon dioxide by volume or greater using 
multiple stages of membrane separators. The membrane separator would be 
installed after compression of the flue gases to an intermediate pressure 
of 250 to 600 psig. 
Although the present invention has been described in conjunction with the 
preferred embodiments, it is to be understood that modifications and 
variations may be utilized without departing from the principles and scope 
of the invention as defined by the following claims.