Distillative separation of carbon dioxide from hydrogen sulfide

A distillative separation of carbon dioxide and hydrogen sulfide is improved by adding a C.sub.3 -C.sub.6 alkane, a mixture of C.sub.3 -C.sub.6 alkanes, SO.sub.2 or SO.sub.3 to a distillation column to increase the relative volatility of carbon dioxide to hydrogen sulfide. Increasing the relative volatility facilitates the separation.

TECHNICAL FIELD 
This invention is in the field of distillation. 
BACKGROUND ART 
Certain gas mixtures contain both carbon dioxide and hydrogen sulfide. For 
example, the overhead carbon dioxide stream produced in the distillative 
separation of carbon dioxide from ethane described in our copending 
application Ser. No. 131,416, filed Mar. 18, 1980, might contain hydrogen 
sulfide in addition to carbon dioxide for many sources of the gas stream 
processed. Other sources of gas mixtures which might contain high 
percentages of carbon dioxide together with hydrogen sulfide are: natural 
occurring gas and associated gas mixtures; coal gasification or 
liquefaction crude gas products; and sulfur processing "tail" gases. 
Recently, there has been an increased interest for certain industrial 
applications to resolve mixtures of carbon dioxide and hydrogen sulfide 
into relatively pure fractions. Many times, in enhanced oil recovery 
situations, for example, it is desirable to employ carbon dioxide having 
less than about 100 ppm hydrogen sulfide. It is also desirable to remove 
hydrogen sulfide from carbon dioxide streams which are vented. Relatively 
high purity hydrogen sulfide streams are also desired or necessary, for 
the production of elemental sulfur, by processes such as the Claus sulfur 
process. 
Previously, the separation of carbon dioxide and hydrogen sulfide into two 
product streams, one containing carbon dioxide without a significant 
amount of hydrogen sulfide and one containing virtually all of the 
hydrogen sulfide, was difficult and extremely costly by distillative 
techniques. This is due to the relatively close volatilities of carbon 
dioxide and hydrogen sulfide at high carbon dioxide concentrations. See 
Bierlein, J. A. and Kay, W. B., Industrial and Engineering Chemistry, Vol. 
45, No. 3, "Phase-Equilibrium Properties of System Carbon Dioxide-Hydrogen 
Sulfide", pages 618-624 (1953). Bierlein and Kay determined the 
phase-equilibrium properties of carbon dioxide/hydrogen sulfide systems 
and concluded that no azeotrope existed. Nevertheless, there was evidence 
that intermolecular forces of the kind causing azeotrope formation were 
strongly developed at the carbon dioxide-rich end of the system, together 
with a very flat terminal slope, which suggested a strong tendency towards 
formation of a minimum-boiling mixture. Based upon these data, Bierlein 
and Kay concluded that separation of carbon dioxide from hydrogen sulfide 
in the binary system became very difficult above 0.8 mole fraction of 
carbon dioxide, and would require a large number of theoretical stages for 
further separation. 
These predictions are born out and the difficulty of separating carbon 
dioxide from hydrogen sulfide by distillation is illustrated in U.S. Pat. 
No. 3,417,572 issued to Pryor. In the Pryor invention, a distillation 
column is employed to separate a mixture of hydrogen sulfide and carbon 
dioxide into a hydrogen sulfide bottoms product stream and a carbon 
dioxide overhead product stream. Although the overhead carbon dioxide 
stream obtained has high purity, the bottoms product stream of hydrogen 
sulfide has a hydrogen sulfide concentration minimally adequate as a feed 
to a Claus process, which may be as little as slightly over 10 mole 
percent hydrogen sulfide. Even to obtain this separation, it is indicated 
that 100 trays were used in the distillation column. 
In view of the difficult nature of complete distillative separations of 
carbon dioxide from hydrogen sulfide, such separations have been 
accomplished commercially primarily by solvent extraction techniques. 
Solvent extraction techniques are costly, albeit less so than prior 
distillative techniques. 
DISCLOSURE OF THE INVENTION 
This invention relates to distillative separation of carbon dioxide from 
hydrogen sulfide in mixtures containing both, particularly where such 
separations are normally difficult and/or impractical due to the low 
relative volatility of carbon dioxide to hydrogen sulfide at high carbon 
dioxide concentrations. The feed mixtures in such distillative separations 
can, of course, contain additional components. 
In this method, a distillation column is used to separate a feed mixture 
containing carbon dioxide and hydrogen sulfide into an overhead stream 
enriched in carbon dioxide and a bottoms stream enriched in hydrogen 
sulfide. The improvement of this invention comprises adding to the 
distillation column, at a point above where the feed is introduced, an 
agent for increasing the relative volatility of carbon dioxide to hydrogen 
sulfide. Surprisingly, it has been found that certain materials, such as 
C.sub.3 -C.sub.6 alkanes, produce this desired increase in relative 
volatility at high concentrations of carbon dioxide. 
Thus, the method disclosed herein results in the ability to obtain an 
overhead stream enriched in carbon dioxide without a significant amount of 
hydrogen sulfide and a bottoms stream enriched in hydrogen sulfide in 
distillative separation of a mixture containing carbon dioxide and 
hydrogen sulfide. Additionally, since certain materials capable of 
increasing the relative volatility of carbon dioxide to hydrogen sulfide 
are often present in feed mixtures to be distilled, these can be separated 
and fed back to the distillation column to serve the function of 
increasing the relative volatilities. However, the mere presence of such 
materials in the feed is not sufficient to produce the required increase 
in relative volatility at the upper portion of a column. Agents present in 
the feed may be separated and added back to the column at an appropriate 
point, such as above the feed point, to function in the manner described 
herein. When this is done, advantage is taken of materials which are 
already present in the feed and the feed thus serves as a convenient 
source of agents.

BEST MODE FOR CARRYING OUT THE INVENTION 
This invention will now be further described in further detail with regard 
to the Figures. 
Much of the data presented in the following description, as well as that 
shown in the Figures, was obtained using a calculation program to simulate 
conditions within a distillation column for certain given or desired 
operating conditions. Unless otherwise stated, the program employed was 
the PROCESS simulation program of Simulation Sciences, Inc., Fullerton, 
Calif., Dec., 1979-Apr., 1980. Vapor-liquid equilibria and thermodynamic 
data were calculated based upon the Soave-Redlich-Kwong equation of state. 
While the total accuracy of the data obtained cannot be assured, and in 
fact will change somewhat depending upon the constants chosen, the data is 
believed to be representative of actual data and is certainly appropriate 
for illustrating and substantiating the benefit gained by employing an 
agent to raise the relative volatility of carbon dioxide to hydrogen 
sulfide in distillative separation. For purposes of simplifying the plots, 
data from systems which were not binary were plotted on a pseudobinary 
basis in which mole fractions are calculated as if the components beyond 
those in the binary were not present. 
The practical difficulty of obtaining a substantially complete separation 
of carbon dioxide from hydrogen sulfide in a gas mixture containing both 
can be seen by referring to FIG. 1. Therein, it can be seen that the 
vapor-liquid equilibrium plot for the pure binary at 600 psia tends to 
pinch together at the high carbon dioxide concentrations. Once a 
composition containing about 80% carbon dioxide has been reached, further 
separation is very difficult. Thus, the overhead product from a column 
would normally be limited to about 80% carbon dioxide, unless a large 
number of theoretical stages is added to the column. The beneficial effect 
of adding an agent to increase the relative volatility of carbon dioxide 
to hydrogen sulfide can be seen in FIG. 1, also. When n-butane is added to 
a concentration of 20% in the liquid phase, the right hand portion of the 
vapor-liquid plot is opened considerably making further separation much 
easier. Further opening of the vapor-liquid equilibria plot is illustrated 
when n-butane is added to a concentration of 40% in the liquid phase. 
The beneficial effect of the agent is more dramatically demonstrated by the 
data regarding the relative voltatility of carbon dioxide to hydrogen 
sulfide presented in FIG. 2. As illustrated, with no agent present, the 
relative volatility approaches 1.45 at high concentrations of carbon 
dioxide. The addition of n-butane to a level of 20% in the liquid phase 
significantly raises the relative volatility, and addition of n-butane to 
a level of 40% raises it even further. 
FIG. 3 compares the effect of three potential agents on the relative 
volatility of carbon dioxide to hydrogen sulfide. It can be seen that 
sulfur dioxide and n-butane have a similar effect up to about 22% agent. 
Above this amount, n-butane has a slightly larger effect on relative 
volatility than sulfur dioxide. Sulfur trioxide exhibits a greater effect 
on relative volatility than both n-butane and sulfur dioxide at the 
concentrations shown. 
Of course, considerations beyond the effect of agents on relative 
volatility must be considered in selecting agents. For example, the 
potential for reactions between the agent and components in the mixture to 
be separated should be considered, as should the ease of separating the 
agent from hydrogen sulfide if such separations are desired or necessary 
in subsequent processing of the bottoms stream. 
In general, a wide variety of materials or mixtures of materials which 
cause the relative volatility of carbon dioxide to hydrogen sulfide to be 
significantly increased over the range of interest is satisfactory as an 
agent for this invention. Agents which are components in the feed mixture 
are preferred agents because they are easy to separate and recycle and 
often have a very beneficial effect in causing the carbon dioxide to be 
more volatile relative to hydrogen sulfide. Natural gas liquids (NGL) 
contain alkanes, such as C.sub.3 -C.sub.6 alkanes, which can often be 
separated from bottoms product in conventional separation equipment. Thus, 
NGL or components thereof can be conveniently recycled to provide a 
beneficial agent. It is also clear that materials satisfactory as an agent 
need not be pure materials. In general, the agent should be miscible in 
the liquid phase at all conditions in the distillation column. It is 
desirable, of course, to have agents which have volatilities lower than 
the components to be separated. Also, the agent should have a freezing 
point sufficiently low to avoid solids formation in the column. 
In addition to the preferred materials mentioned above, there are other 
classes of materials which meet these requirements. For example, other 
hydrocarbons such as higher alkanes and naphthenes, halogenated 
hydrocarbons such as fluoro-chloromethane and fluoro-chloroethane 
compounds, sulphur dioxide, sulfur trioxide, etc., are believed to be 
suitable. Those skilled in the art will know, or be able to ascertain 
using no more than routine experimentation, other suitable agents for use 
with the invention described herein. 
The amount of agent added will be dependent upon factors such as the 
composition of the feed, operating pressure, throughput of the column, 
recovery of overhead and bottoms product desired, etc. Such factors can be 
taken into account by those skilled in the art by determining the 
operative amounts for any given separation using no more than routine 
experimentation. 
Agent is added to the tower at a point above where feed is introduced since 
this is where relative volatility needs to be increased. Although some 
materials which are suitable agents are contained in the feed in some 
cases, this alone is not sufficient. This is because the agent is usually 
not sufficiently volatile to rise up the column to the problem area in 
sufficient concentration. Thus, even if present in the feed, the agent 
should be separated and added to a point above the feed. 
Although it is possible to add agent to the top of the column, including 
into the condenser, this is usually not desirable because agent cannot 
then be separated efficiently from overhead product. Thus, it is 
preferable to add agent in most cases at a point below the column top to 
thereby allow separation of additive from the desired overhead product. 
In some cases, it is desirable to add agent at more than one column 
location on a simultaneous basis. 
An apparatus for carrying out a separation of carbon dioxide from hydrogen 
sulfide according to this invention is schematically illustrated in FIG. 
4. Therein, feed mixture 10, containing a mixture of carbon dioxide and 
hydrogen sulfide, and usually other components such as hydrocarbons, 
nitrogen, etc., enters through feed line 12 into distillation column 14. 
Column 14 contains a number of vapor-liquid contact devices such as trays 
or packing, with the exact number of contact stages depending upon the 
required operating conditions. 
Overhead stream 20 is rich in carbon dioxide and passes to partial 
condenser 22 from which the remaining vapor stream 24 exits as carbon 
dioxide product. This product stream also contains, of course, components 
present in the feed which are more volatile than carbon dioxide, such as 
any hydrogen, carbon monoxide, nitrogen and light hydrocarbons present in 
the feed. Liquid from the partial condenser returns to column 14 in line 
26 where it serves as reflux for tower 14. Condenser 22 is cooled by 
external cooling source 28. 
The bottoms stream exits from the lower portion of column 14 in bottoms 
line 30 and contains hydrogen sulfide and other less volatile hydrocarbons 
or other components, and any agent added to increase carbon 
dioxide/hydrogen sulfide relative volatility. A portion of the bottoms 
product is passed through reboiler 32 and back to column 14 in line 34. 
Reboiler 32 is heated by an external heat source 36. 
The bottoms product passes in line 38 to further separation equipment 40, 
such as another distillation column. Separation equipment 40 is employed 
to separate out the agent which is recycled in line 42 back to the column. 
The amount of recycled agent can be controlled by valve 46. An hydrogen 
sulfide fraction is also separated in equipment 40 and is directed in line 
44 to suitable hydrogen sulfide product facilities. 
Agent for increasing the relative volatility of carbon dioxide may also be 
added to the system through line 50 and valve 52. Such externally added 
agent may be used in lieu of recycled agent or in conjunction with 
recycled agent. In either case, the agent is cooled in heat exchanger 54, 
cooled by cooling source 56, and directed through flow line 58 back 
towards the column 14. 
Agent can be added at a number of different location, either individually 
or at several locations simultaneously. As illustrated, agent can be 
directed in line 64 to valve 66 and flow line 68 and introduced directly 
onto a tray in the upper section of column 14. Similarly, agent can be 
added to a higher column tray, such as by passing it in line 70 through 
control valve 72 and line 74. Agent can also be introduced into condenser 
22 by directing agent through line 60, flow control valve 62 and line 63. 
Other suitable points of addition can be determined, of course, for each 
particular separation being performed. 
INDUSTRIAL APPLICABILITY 
This invention is useful in the distillative separation of carbon dioxide 
from hydrogen sulfide. 
EQUIVALENTS 
Those skilled in the art will recognize, or be able to determine using no 
more than routine experimentation, other equivalents to the specific 
embodiments described herein.