III. SYNTHESIS GAS MANUFACTURE
The normal synthesis gas required for the Fischer-Tropsch process is a mixture of two volumes of hydrogen and one volume of carbon monoxide. This 2 to 1 ratio of hydrogen to carbon monoxide is subject to considerable variation, however.
Recent practice in the plants in the Ruhr, using the cobalt catalyst for synthesis of hydrocarbons, showed ratios from 1.8 to 2.0 for the low-pressure Fischer-Tropsch process and ratios as low as 1.5 for the medium-pressure process.*(1)
* Numerals in parentheses refer to bibliographical references at the end of the report.
In the Ruhchemie plant at Sterkrade, the medium-pressure process used different ratios for each of the three stages of synthesis, 1.4, 1.6, and 1.8, respectively, by introducing the requisite amount of "converted" water gas before each stage. (2)
For the iron-catalyst Fischer-Tropsch process a hydrogen-carbon monoxide ratio of 1.0 is suitable, but by changing conditions other ratios maybe used.(3)
In all cases the synthesis gas contains inerts, which seldom exceed 20% by volume.
A. Manufacture from Coke by Water-Gas Reaction
This was the process universally used by all plants in the Ruhr, sometimes supplemented by other processes such as cracking of coke-oven gas. The manufacture of blue water-gas from coke was carried out in standard water-gas sets and there were no new improvements in design or operation that would differentiate German operation from well-known American practices.
In order to obtain the higher ratio of hydrogen to carbon monoxide required for the standard Fischer-Tropsch operation, it was necessary to pass a portion of the water gas through convertors containing the so-called shift catalyst, whereby carbon monoxide was converted into hydrogen and carbon dioxide. Steam was introduced with the gas and the conversion was carried out at 450-500°C. Captured convertor catalyst analyzed 38.5% Fe2O3, 18.2% CaO, 5.4% Cr2O3, 5.2% MgO, with minor amounts of other constituents, including 18.0% water. In all respects this was normal shift catalyst.(4)
Since no new or usual practices in manufacture of water gas from coke or in conversion of the water gas were encountered, a minimum of plant operations data was secured. From the Rheinpreussen plant, however, drawings of the water-gas were encountered, a minimum of plant operations data was secured. From the Rheinpreussen plant, however, drawings of the water-gas plant and rather complete operating records were captured and some of the data are summarized and reference to original documents are given in the report on that plant. (4)
B. Manufacture by Cracking of Coke-Oven Gas
There were two plants in the Ruhr and possibly others located in Germany that were supplementing their production for synthesis gas. The high-hydrogen gas so produced was mixed with water gas thereby reducing the amount of water gas to be converted.
Records captured at the Rheinpreussen plant showed some of the operating data for the coke-oven-gas cracking installation there, (4) and some operating data were also obtained from the installation at the Castrop-Rauxel plant.(5)
C. Low-Temperature Carbonization followed by Gasification
At the Krupp Fischer-Tropsch plant at Wanne-Eickel was a large commercial installation of the Krupp-Lurgi process for m manufacture of low-temperature coke. Most of this coke was used in water-gas sets to make blue water-gas with a hydrogen-carbon monoxide ratio of 1.35, this ratio being nearer to the required ratio than that of water-gas from high-temperature coke. Summarized data on operation of this low-temperature coke plant were obtained by interrogation.(6)
This is a recent German development in the complete gasification of brown coal or other non-coking coals to make either synthesis gas or hydrogen. The necessary heat for the water-gas reaction is supplied by flowing a mixture of stream with previously formed water-gas through highly-heated regenerators which are alternately preheated by combustion of producer gas.
A rather large installation of this process was investigated at the coal hydrogenation plant at Wesseling on the Rhine River, and a summary of descriptive data has been included in a preliminary report on this plant. (7) Although this installation happens to have made hydrogen from brown coal, the same plant could have made synthesis gas with only minor modifications.
E. Schmalfeldt-Wintershall Process
This is a relatively new process for manufacture of synthesis gas from non-coking coal that was under large-scale development at the beginning of the war. Earlier information had described this process as the addition of very finely pulverized coal to a stream of hot recirculating gas which dried the coal in suspension with subsequent removal of the dried coal in a cyclone separator. The dried coal was then fed to a generator where it was completely gasified in a stream of gas highly preheated in large regenerators, this gas stream being constantly recirculated and supplied with enough water vapor to gasify the pulverized coal. A portion of the circulating gas stream was continuously with drawn as finished synthesis gas.
Interrogation has revealed that many difficulties were encountered in the practical, commercial operation of this process. A large installation of the process was installed by Wintershall, A. G. at Lutzkendorf in Central Germany. A document of I. G. captured at Leuna outlines some of the schemes to correct the various difficulties. (8)
One of the chief difficulties was lack of gas-producing capacity. the favored plan was to install a new type of rotating grate generator with oxygen added in a relatively small amount to increase the heat available for gasification, and thereby increase the make of synthesis gas per generator. For production of 75,000 metric tons per year of Fischer-Tropsch product, there was required 100,000 cubic meters per hour of synthesis gas (gas from this process contained only 76% hydrogen plus carbon monoxide). To make this four generators plus one spare would be required. Producer fuel gas for heating the large Cowper stoves or regenerators was required to the extent of 122,500 cubic per hour, and 1,400 cubic meters of oxygen per hour were necessary. Consumption of brow coal (dry basis) was at the rate of 0.8 kg per cubic meter of synthesis gas. The document suggests other means for increasing capacity but interrogation elsewhere indicates that the plant adopted the oxygen-addition scheme. (9)
A detailed account of other operating difficulties, plans for correction, material balances, heat balances and flow diagrams are given in the I. G. document (8) referred to above.
F. Purification of Synthesis Gas
It is well known that the Fischer-Tropsch process requires synthesis gas in which the total sulfur does not exceed 2.0 mg per cubic meter. This purification is always done in two stages, (1) removal of hydrogen sulfide, and (2) removal of organic sulfur.
The removal of hydrogen sulfide is almost universally carried out by the well-known iron-oxide process and practically nothing of a new nature concerning this process was discovered. At the Lutzkendorf plant the so-called "Alkazid" process had been installed and operating records captured (10) show results from this plant over certain typical periods. A solution of an alkaline organic compound absorbs the hydrogen sulfide, which is then continuously stripped from the solution by steam. At Lutzkendorf the evolved hydrogen sulfide was converted into elemental sulfur by a Claus oven.
The removal of organic sulfur was carried out in all Fischer-Tropsch plants examined by contacting the synthesis gas with a catalyst at temperatures varying from 180°C. with fresh catalyst to 280°C. with nearly exhausted catalyst. A captured sample of the catalyst from the Rheinpruessen plant showed 34.4% Fe2O3 and 23.8% Na2CO3. Other records showed a content of 30% of Na2CO3 (sodium carbonate), the remainder being essentially iron oxide and various inerts. Fully spent catalyst showed 33% Na2SO4, 0.3% Na2SO3, and 4% Na2CO3, showing that the reaction is essentially catalytic oxidation of the organic sulfur in the gas, and it is essential that a small percentage of oxygen be present in the synthesis gas to effect removal of the organic sulfur.(4)