Process for the purification of crude gases with simultaneous production of synthesis gas and fuel gas

In a process for the purification of coal gasification gases, synthesis gas and fuel gas are simultaneously produced. In order to obtain a fuel gas rich in CO.sub.2, capable of handling fluctuations in demand, and to produce at the same time a highly concentrated H.sub.2 S fluid fraction, a portion of the crude gas, scrubbed to synthesis gas purity, is utilized for stripping out CO.sub.2 under pressure from scrubbing medium loaded exclusively with CO.sub.2. The partially stripped CO.sub.2 -loaded scrubbing medium is employed for the concentration of sulfur compounds in an H.sub.2 S/COS-loaded scrubbing medium.

BACKGROUND OF THE INVENTION 
The invention relates to a process and apparatus for the purification of 
crude gases from a carbonaceous feedstock gasification with simultaneous 
production of: (1) synthesis gas, (2) CO.sub.2 -enriched fuel gas suitable 
for the generation of electrical energy, e.g., by means of a gas turbine, 
and (3) H.sub.2 S/COS-rich gas suitable for producing sulfur in a Claus 
unit. After conversion to the desired H.sub.2 to carbon oxide ratio, the 
crude gas is cooled and, subsequently, H.sub.2 S and COS are removed 
therefrom in a lower section of a scrubbing column using a scrubbing 
medium previously loaded with CO.sub.2. In an upper section of the 
scrubbing column, CO.sub.2 is scrubbed from the crude gas. Purified 
synthesis gas is discharged from the scrubbing column. 
In the gasification of carbonaceous feedstock (e.g., coal, oil, refinery 
residues), a gaseous mixture is formed comprising preferably H.sub.2 and 
CO, but also containing other components, such as CO.sub.2, H.sub.2 S and 
inert gases. 
In correspondence with further usage of the gaseous mixture as synthesis 
gas (e.g., NH.sub.3 or methanol synthesis), as a starting compound for 
obtaining H.sub.2 or CO, or as a fuel gas, the hot crude gas, in 
conventional methods, is first converted in accordance with the marginal 
range of conditions yielding the required synthesis gas. After conversion, 
the gas is partially cooled--either in a waste heat boiler for steam 
generation or by water quenching--and then purified in a two-stage, sour 
gas scrubbing column. H.sub.2 S and COS are removed in a lower column 
section while CO.sub.2 is scrubbed out to the required purity in an upper 
section. Normally, physical scrubbing techniques are utilized for removal 
of CO.sub.2, H.sub.2 S and COS. 
A conventional process for the simultaneous production of synthesis gas and 
fuel gas, the latter being used for generating electrical energy by means 
of gas turbines, is disclosed in DE-OS 3,427,633. In this process, two 
crude gas streams are cooled separately. Only the crude gas stream 
intended for the production of synthesis gas is converted (i.e., reaction 
of CO with steam to produce H.sub.2 and CO.sub.2). The gas streams are 
subsequently scrubbed in two parallel scrubbing columns. During this 
process, the synthesis gas is first selectively desulfurized in an H.sub.2 
S scrubbing step and subsequently subjected to a CO.sub.2 scrubbing step. 
In the conventional process, the excess, CO.sub.2 -loaded scrubbing agent 
from the synthesis gas scrubbing stage is utilized for desulfurization of 
the fuel gas. By virtue of this combination, it is possible to strip out a 
portion of the CO.sub.2, that had been scrubbed out in the synthesis 
scrubbing stage, and to combine it with the fuel gas without any 
significant additional expenditure of energy. By this procedure, a savings 
in compression energy is realized in the subsequent admixture of air with 
the fuel gas inasmuch as less excess air is required to be mixed with the 
fuel gas in order to limit the maximum combustion temperature in the gas 
turbines. 
However, despite the advantage of CO.sub.2 enrichment in the fuel gas, the 
conventional process exhibits several substantial drawbacks. 
In the H.sub.2 S scrubbing step, in addition to H.sub.2 S and COS, a large 
amount of CO.sub.2 is scrubbed out as well (in correspondence with the 
CO.sub.2 partial pressure and CO.sub.2 solubility). In the usual selective 
sour gas scrubbing operations, H.sub.2 S and COS are concentrated in a 
downstream enrichment column to such an extent that the H.sub.2 S fraction 
obtained during hot regeneration of the scrubbing medium can be reacted to 
elemental sulfur in a Claus unit. 
However, in the enrichment column, a cold scrubbing medium already loaded 
with CO.sub.2 is needed for retaining H.sub.2 S and COS. The cold. 
CO.sub.2 -loaded scrubbing medium is used to rescrub the H.sub.2 S 
released together with the CO.sub.2 and so as to deliver a sulfur-free 
residual gas from the H.sub.2 S enrichment column. Normally, the excess 
scrubbing medium for the CO.sub.2 scrubbing operation is utilized for this 
rescrubbing step. However, in the combination disclosed in the 
above-described conventional method of DE-OS 427,633, there is no 
available CO.sub.2 -loaded scrubbing medium. Enrichment of H.sub.2 S and 
COS is therefore impossible. Due to this problem, the sulfur content of 
the sulfur fraction drops to such a great extent that a Claus unit cannot 
be employed. Although H.sub.2 S from such a gaseous mixture can be reacted 
to elemental sulfur by means of an oxidative scrubbing step, these 
scrubbing operations have the disadvantage that COS does not react and 
thus cannot be scrubbed out. Due to the high COS content of unconverted 
crude gas (about 4-5% of the sulfur is generally present in the form of 
COS), significant environmental pollution problems are encountered. 
Normally, electric current generation is adapted to demand. Therefore, 
fluctuations occur on a daily and seasonal basis. For this reason, the 
amount of fuel gas needed for the generation of electrical energy produced 
by coal gasification and the like is likewise variable. To avoid load 
fluctuations of gasification and of the air fractionator, it is normally 
desirable to operate gasification at a constant rate and to absorb 
quantitative fluctuations of the required fuel gas in the synthesis gas 
production. As a prerequisite of this, the quenching, conversion, and the 
two parallel scrubbing operations must operate under fluctuating marginal 
conditions, requiring a high expenditure for control means for the 
operation of the total installation. 
Both lines (gas cooling, conversion, and both sour gas scrubbing stages) 
must be designed for the respectively maximum amount of gas. Therefore, 
both lines are oversized and normally do not operate at optimum operating 
conditions. At the same time, over-dimensioning entails higher initial 
investment costs. 
SUMMARY OF THE INVENTION 
An object of one aspect of this invention is to structure a process for the 
purification of crude gases with simultaneous production of synthesis gas 
and fuel gas in such a way that, a CO.sub.2 -enriched fuel gas is obtained 
while simultaneously producing a highly concentrated H.sub.2 S fraction, 
and the response to fuel gas demand is facilitated. 
An object of another aspect of this invention is to provide apparatus for 
conducting the process. 
Upon further study of the specification and appended claims, further 
objects and advantages of this invention will become apparent to those 
skilled in the art. 
The process aspect of the invention is attained according to the invention 
by delivering a branched-off portion of synthesis gas, produced by a 
scrubbing step, to a stripping step wherein the portion of synthesis gas 
is used to strip CO.sub.2 from a scrubbing medium, loaded essentially with 
CO.sub.2, discharged from the scrubbing step, removing CO.sub.2 -enriched 
fuel gas from said stripping step and delivering partially stripped 
scrubbing medium, loaded essentially with CO.sub.2, to an H.sub.2 S 
enrichment column. 
According to another aspect, the process of the invention is performed by 
branching-off a portion of the obtained synthesis gas and utilizing this 
portion for stripping out CO.sub.2 under pressure from scrubbing medium 
removed from the upper column section of the scrubbing column. This 
scrubbing medium is loaded essentially with CO.sub.2. A CO.sub.2 -enriched 
fuel gas is obtained as a product of the CO.sub.2 stripping step. The 
partially stripped scrubbing medium, also loaded essentially with 
CO.sub.2, is subsequently used to concentrate sulfur compounds in an 
H.sub.2 S enrichment column. 
Suitable scrubbing media are methanol, ethanol, acetone, 
N-methylpyrrolidone (NMP), dimethylformamide (DMF), propylene carbonate, 
polyethylene glycol dialkyl ethers as well as mixtures thereof. 
Due to this procedure of the invention, the fuel gas is enriched with 
CO.sub.2, and, at the same time, the partially stripped scrubbing medium 
loaded solely with CO.sub.2 is made available for the H.sub.2 S enrichment 
column. The branched-off synthesis gas contains little if any sulfur 
compounds and the stripped-out CO.sub.2 withdraws the heat of solution 
from the scrubbing medium whereby the latter is cooled off. As seen from 
the viewpoint of a cold value balance, stripping off CO.sub.2 under 
pressure is equivalent to expansion of the loaded scrubbing medium and 
concomitant partial vaporization of CO.sub.2, as employed in the 
conventional methods. 
An additional advantage resides in that the crude gas production, 
conversion and sour gas scrubbing step can be performed under constant 
conditions while only the stripping column is charged with a variable 
amount of synthesis gas. 
According to the invention, conversion, though not required for obtaining 
fuel gas, is still needed for the production of, for example, H.sub.2, 
methanol or NH.sub.3 synthesis gas. For this reason, conversion is 
performed prior to cooling the crude gas. 
In a further development of the invention, a portion of the crude gas can 
be branched-off prior to conversion and subjected, unconverted, to sour 
gas scrubbing. This procedure is utilized with preference either if, for 
further processing, an unconverted gas is required in addition to the fuel 
gas, for example for obtaining pure CO, or if no great load fluctuations 
of the fuel gas are expected, for example if the electrical energy, 
obtained by means of gas turbines, is required only for synthesis gas 
production and further processing of the crude gas (i.e., a 
self-sufficient plant). 
In another modification of the process aspect of the invention, a portion 
of the crude gas is branched-off, prior to conversion, and desulfurized. 
The desulfurized, unconverted crude gas is then employed for stripping out 
CO.sub.2. 
Normally, stripping of CO.sub.2 is performed under the pressure of the 
scrubbing column. Another embodiment of the process according to this 
invention, however, provides that, in cases where the synthesis gas 
production, i.e., conversion and scrubbing, on account of the subsequent 
synthesis, is conducted under a pressure higher than the required pressure 
for the combustion chamber, the branched-off synthesis gas is 
engine-expanded in an expansion turbine to the pressure of the combustion 
chamber upstream of the CO.sub.2 stripping column. The CO.sub.2 stripping 
step is carried out, in this case, at a reduced pressure. 
In accordance with a special embodiment of the process of this invention, 
it is possible to use also other gases fulfilling certain marginal 
conditions, in addition to the fuel gas, during the stripping out of 
CO.sub.2 under pressure. For example, purge gases from methanol synthesis, 
residual gases from a nitrogen scrubbing stage, or N.sub.2 from an air 
fractionator can be employed, individually or combined, during stripping. 
According to the invention, the concentration of sulfur compounds takes 
place in an H.sub.2 S enrichment column by rescrubbing the liberated 
H.sub.2 S/COS with scrubbing medium loaded with CO.sub.2 obtained from the 
CO.sub.2 stripping column. The H.sub.2 S/COS is thereby enriched in the 
bottoms of the H.sub.2 S enrichment column. N.sub.2 or another suitable 
gas is utilized for stripping out CO.sub.2 from the scrubbing medium. 
With special advantage, the cold scrubbing medium is withdrawn, in this 
process, as a side stream from an upper section of the enrichment column 
and, after being heated against regenerated scrubbing medium, is 
reintroduced into the column in a lower section. 
Furthermore, the H.sub.2 S-loaded scrubbing medium is subjected to hot 
regeneration for obtaining a gaseous fraction, rich in H.sub.2 S/COS, and 
regenerated scrubbing medium for the scrubbing column. 
In a further development of this invention, the residual gas--preferably 
CO.sub.2 and N.sub.2, but essentially free of H.sub.2 S--obtained in the 
enrichment column is, after being heated against the crude feed gas, 
compressed together with the combustion air, and admixed with the fuel gas 
upstream of the combustion chamber. 
In this connection, a special advantage resides in that less gas needs to 
be compressed for lowering the combustion temperature in the combustion 
chamber, since CO.sub.2 has a higher specific heat than air. 
A further advantage lies in that no scrubbing medium vapor emission 
problems occur due to the recycling inasmuch as the residual gas is 
saturated with scrubbing medium vapor in correspondence with pressure and 
temperature. 
Normally, it is practical from an economic viewpoint and also on account of 
local antipollution requirements to expand the loaded scrubbing medium to 
an intermediate pressure and to return the resultant flash gases 
(preferably H.sub.2 and CO), after compression, into the crude gas. In the 
admixing of the residual gas with the fuel gas, according to a further 
embodiment of the idea of this invention, it is of advantage that the 
intermediate expansion tanks and the recycle compressor are eliminated 
inasmuch as the combustible components are likewise combusted. 
According to the apparatus aspect of the invention, the apparatus comprises 
a scrubbing column, a CO.sub.2 stripping column, and an H.sub.2 S 
enrichment column, wherein, from a conduit removing synthesis gas from the 
scrubbing column, a second conduit is branched-off and leads into the 
lower section of the CO.sub.2 stripping column. 
The scrubbing column is preferably connected to the CO.sub.2 stripping 
column in such a way that a conduit for CO.sub.2 -loaded scrubbing medium 
leads from the upper portion of the scrubbing column to the stripping 
column. The stripping column is preferably connected by a bottom conduit 
with the enrichment column whereby partially stripped scrubbing medium is 
delivered to the latter. Also, it is preferred that a conduit for 
scrubbing medium rich in H.sub.2 S/COS leads from the lower section of the 
scrubbing column to the enrichment column, and that a conduit leads from 
the bottom of the enrichment column to a hot scrubbing medium regenerator. 
A Claus unit is preferably in communication with the hot regenerator. 
Also, a heat exchanger is preferably connected to a conduit leading from 
the regenerator to the upper section of the scrubbing column. The heat 
exchanger is preferably further connected to a conduit extending laterally 
out of the enrichment column.

DETAILED DESCRIPTION OF THE DRAWINGS 
A crude gas containing H.sub.2 and CO, but also CO.sub.2, H.sub.2 S, COS 
and inert gases, is cooled off, after adding a small amount of methanol, 
in a heat exchanger 1 and introduced by way of a methanol/H.sub.2 O 
separator 2 into the lower section of a scrubbing column 3. In the 
scrubbing column, the crude gas is freed of CO.sub.2 and H.sub.2 S/COS by 
being sprayed with methanol fed via conduit 19 which absorbs the 
impurities. The thus-obtained synthesis gas is withdrawn from the 
scrubbing column via a conduit 4. A portion is branched-off and conducted 
via a conduit 5 into the lower section of a CO.sub.2 stripping column 6. 
Methanol loaded with CO.sub.2 is withdrawn from the side of the scrubbing 
column and conducted via a conduit 7 into the upper section of the 
CO.sub.2 stripping column 6 where the branched-off synthesis gas is 
utilized for stripping out CO.sub.2. The resultant CO.sub.2 -enriched fuel 
gas is conducted via a conduit 8 into a combustion chamber (not shown) 
wherein it is used as fuel for driving a gas turbine (not shown) to 
produce electrical energy. 
A portion of the CO.sub.2 -loaded methanol removed via conduit 7 is 
returned to a lower section of the scrubbing column for scrubbing out 
H.sub.2 S/COS from the crude gas. The cold, partially stripped, CO.sub.2 
-loaded methanol is discharged from the stripping column by way of a 
conduit 9. In an H.sub.2 S enrichment column 10, this scrubbing medium 
serves for rewashing the H.sub.2 S/COS released in this column from the 
scrubbing medium loaded with H.sub.2 S/COS which was introduced via 
conduit 11 from the scrubbing column; thereby, the H.sub.2 S/COS is 
concentrated in the bottoms of the H.sub.2 S enrichment column 10. 
Laterally of the enrichment column, the cold methanol is withdrawn and 
conducted via a conduit 12 to a heat exchanger 13 where it is heated in 
heat exchange with regenerated methanol. Thereafter, the withdrawn 
methanol is reintroduced to the enrichment column in the lower section 
thereof. By way of a conduit 14, N.sub.2 is introduced into the lower 
section of the enrichment column, where the N.sub.2 strips out CO.sub.2 
from the methanol, which is additionally loaded with H.sub.2 S/COS. The 
methanol is conducted from the bottom of the enrichment column via conduit 
15 into a hot regenerator 16. The methanol/H.sub.2 O condensate from 
separator 2 is furthermore introduced into the hot regenerator 16 via 
conduit 21. From the hot regenerator 16, a conduit 17 conducts the H.sub.2 
S/COS-rich gas into a Claus unit 18, and a conduit 19 conducts regenerated 
methanol back to the scrubbing column 3. Residual gas which contains 
CO.sub.2 and N.sub.2 is removed from the enrichment column 10 by way of 
conduit 20. 
The quantities and compositions of materials converted in an exemplary 
process, as well as several pressures and temperatures, are compiled in 
the product balance of the table below. 
TABLE 
__________________________________________________________________________ 
Product Balance 
4 8 14 17 20 
Reference Numeral 
1 Synthesis 
7 Fuel 
Stripping 
Sour 
19 Residual 
11 
Type of Material 
Crude Gas 
Gas CH.sub.3 OH 
Gas Gas Gas 
CH.sub.3 OH 
Gas CH.sub.3 OH 
__________________________________________________________________________ 
Components, vol % 
H.sub.2 56.6 96.9 82.0 0.2 
N.sub.2 + CO + CH.sub.4 
1.9 3.1 2.6 100.0 
2.0 10.1 
CO.sub.2 40.8 10 ppm 15.4 64.3 89.7 
H.sub.2 S/COS 
0.7 0.1 ppm 1 ppm 33.7 10 ppm 
Amount, Nm.sup.3 /h 
100,000 
24,620 
62 t/h 
39,860 
3,750 
2,180 
125 t/h 
37,090 
63 t/h 
plus plus plus 
dissolved dissolved diss. 
compon. compon. comp. 
Pressure, bar 
58 56 55 4 2 1.1 
Temperature, .degree.C. 
35 25 25 20 30 25 
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The gas flow and composition of the crude gas depend mainly on the further 
usage of the synthesis gas. The most important syntheses, which are 
combined with partial oxidation of coal or heavy hydrocarbons (oil, 
residuals, tar) are ammonia production, normally combined with the 
production of urea, methanol production sometimes combined with the 
production of acetic acid (i.e additional production of CO), oxoalcohol 
production, synthetic natural gas (SNG) production and hydrogen production 
for refineries. The composition of the crude gas depends on the further 
processing and varies mainly in the degree of CO-conversion, e.g. for the 
production of ammonia and hydrogen, a nearly complete conversion of CO to 
hydrogen and CO.sub.2 is sought to be attained, whereas for alcohol 
synthesis a certain ratio of H.sub.2 : CO is required. 
The amount of crude gas depends on the expected amount of product and can 
vary between 10.000 Nm.sup.3 /h (10 MMSCFD) for oxoalcohol production and 
350.000 Nm.sup.3 /h (313 MMSCFD) for ammonia or SNG production. To this 
the required amount of fuel gas has to be added. 
The operation pressure of the scrubbing column and the stripping column 
depends on the type of gasification and the pressure of the synthesis. A 
usual pressure range will be 10 to 150 bar, especially 20 to 100 bar. The 
H.sub.2 S enrichment column is usually operated at 1,5 to 5 bar. 
The operating temperature depends mainly on the feed gas pressure and 
composition and can vary between ambient temperature and -90.degree. C. in 
the scrubbing column, the stripping column and the H.sub.2 S enrichment 
column being operated at colder temperatures. In order to compensate for 
the loss of cold within the process, a refrigerant unit is used working in 
the temperature range between 20 and -50.degree. C. 
Normally it is desired to process the acid gas obtained from the 
regenerator in a Claus unit to produce elemental sulfur. This means, that 
the acid gas should have a H.sub.2 S concentration of at least 20 mol%. In 
principle, it is also possible to process a H.sub.2 S containing gas 
stream in an oxidative sulfur wash system. Such a process can be of 
interest if a very small crude gas flow has to be processed, if the crude 
gas contains only a very low H.sub.2 S content or if the crude gas 
contains heavier hydrocarbons, as is usual in low temperature coal 
gasification. The amount of CO.sub.2 in the fuel gas depends on the amount 
in feed gas. 
In general the acid gas removal unit has to be adapted to the conditions of 
the overall plant and especially to the changes on the fuel gas demand. 
The proposed process is very flexible and can be adapted to the different 
demands and conditions. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention, and without departing 
from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions.