Synthesis gas system

The mathematical properties of the set of equations describing chemical equilibrium in the synthesis gas system indicate that the carbon-producing regions are defined solely by pressure, temperature, and elemental analysis. Once a safe blend of reactants is determined from the ternary, the same set of equations which was used to derive the ternary may be used to determine the gas composition. 

Sulfur is a potential problem even at low levels for synthesis gas systems using certain types of catalysts. The production of methanol from synthesis gas, for example, uses catalysts that are poisoned by sulfur. Some tar cracking catalysts are also sulfur sensitive. In those systems, thorough removal of sulfur will be required. Fuel cell systems are also sulfur sensitive. 

Particulate removal requirements vary depending on the use of the product gas. For example, particulate levels must be reduced to below 50 mg/Nm3 for gas engines and below about 15 mg/Nm3 (>5 im) for turbines, and to perhaps 0.02 mg/Nm3 for synthesis gas systems. The primary types of systems include cyclonic filters, barrier filters, electrostatic filters, and wet scrubbers. 

An example of such recychng in a parallel reaction system is in the Oxo process for the production of C4 alcohols. Propylene and synthesis gas (a mixture of carbon monoxide and hydrogen) are first reacted to ra- and isobutyraldehydes using a cobalt-based catalyst. Two parallel reactions occur ... 

Sasol produces synthetic fuels and chemicals from coal-derived synthesis gas. Two significant variations of this technology have been commercialized, and new process variations are continually under development. Sasol One used both the fixed-bed (Arge) process, operated at about 240°C, as weU as a circulating fluidized-bed (Synthol) system operating at 340°C. Each ET reactor type has a characteristic product distribution that includes coproducts isolated for use in the chemical industry. Paraffin wax is one of the principal coproducts of the low temperature Arge process. Alcohols, ketones, and lower paraffins are among the valuable coproducts obtained from the Synthol process. 

Historically, formaldehyde has been and continues to be manufactured from methanol. EoUowing World War II, however, as much as 20% of the formaldehyde produced in the United States was made by the vapor-phase, noncatalytic oxidation of propane and butanes (72). This nonselective oxidation process produces a broad spectmm of coproducts (73) which requites a complex cosdy separation system (74). Hence, the methanol process is preferred. The methanol raw material is normally produced from synthesis gas that is produced from methane. 

A wide range and a number of purification steps are required to make available hydrogen/synthesis gas having the desired purity that depends on use. Technology is available in many forms and combinations for specific hydrogen purification requirements. Methods include physical and chemical treatments (solvent scmbbing) low temperature (cryogenic) systems adsorption on soHds, such as active carbon, metal oxides, and molecular sieves, and various membrane systems. Composition of the raw gas and the amount of impurities that can be tolerated in the product determine the selection of the most suitable process. 

A fundamentally different reaction system is under development by Air Products and Chem Systems (23). In this system, synthesis gas is bubbled through a slurry consisting of micrometer-sized methanol catalyst particles suspended in a paraffinic mineral oil. The Hquid phase serves as the heat sink to remove the heat of reaction. 

Direct conversion of methane [74-82-8] to methanol has been the subject of academic research for over a century. The various catalytic and noncatalytic systems investigated have been summarized (24,25). These methods have yet to demonstrate sufficient advantage over the conventional synthesis gas route to methanol to merit a potential for broad use. 

Other Specialty Chemicals. In fuel-ceU technology, nickel oxide cathodes have been demonstrated for the conversion of synthesis gas and the generation of electricity (199) (see Fuel cells). Nickel salts have been proposed as additions to water-flood tertiary cmde-oil recovery systems (see Petroleum, ENHANCED oil recovery). The salt forms nickel sulfide, which is an oxidation catalyst for H2S, and provides corrosion protection for downweU equipment. Sulfur-containing nickel complexes have been used to limit the oxidative deterioration of solvent-refined mineral oils (200). 

Ammonia production by partial oxidation of hydrocarbon feeds depends to some degree on the gasification step. The clean raw synthesis gas from a Shell partial oxidation system is first treated for sulfur removal, then passed through shift conversion. A Hquid nitrogen scmbbiag step follows. 

The saturated, cleaned raw synthesis gas from a Texaco partial oxidation system is first shifted by use of a sulfur resistant catalyst. Steam required for shifting is already present ia the gas by way of the quench operation ia the generator. The shifted gas is then processed for hydrogen sulfide and carbon dioxide removal followed by Hquid nitrogen scmbbiag. 

The choice of a specific CO2 removal system depends on the overall ammonia plant design and process integration. Important considerations include CO2 sHp required, CO2 partial pressure in the synthesis gas, presence or lack of sulfur, process energy demands, investment cost, availabiUty of solvent, and CO2 recovery requirements. Carbon dioxide is normally recovered for use in the manufacture of urea, in the carbonated beverage industry, or for enhanced oil recovery by miscible flooding. 

The Rectisol process is more readily appHcable for acid gas removal from synthesis gas made by partial oxidation of heavy feedstocks. The solvents used in Purisol, Fluor Solvent, and Selexol processes have low vapor pressures and hence solution losses are minimal. Absorption systems are generally corrosion-free. 

Chem Systems Inc. proposed a process in which ben2yl alcohol obtained by an undisclosed direct oxidation of toluene is homologated with synthesis gas to yield 2-phen5iethyl alcohol, which is then readily dehydrated to styrene (57). This process eliminates the intermediate formation of methanol from synthesis gas but does require the independent production of ben2yl alcohol. 

The nitrogen line pressure is set to a level much less than the experimental pressure. With the nitrogen cylinder open, a check valve prevents the feed gas from getting into the nitrogen cylinder. When the surge check valve cuts off the synthesis gas feed, the unit becomes depressurized. Then pressure drops below the nitrogen pressure level, nitrogen flow will start and flush out the system. 

For SNG manufacture, it is necessary to reduce the residual hydrogen to a minimum in order to achieve a high calorific value. This is best realized if the synthesis gas, instead of having a stoichiometric composition, contains a surplus of C02 which can be utilized to reduce the H2 content by the C02 methanation reaction to less than 1% according to equilibrium conditions. The surplus C02 must be removed at the end of the process sequence. It is, of course, also possible to operate a methanation plant with synthesis gas of stoichiometric composition then there is no need for a final C02 removal system. The residual H2 content will be higher, and therefore the heating value will be lower (cf. the two long term runs in Table II). 

The SASOL plant was operated with a surplus of C02 during a long term test of 4000 hrs. Of the C02 in the synthesis gas, 33.4% was metha-nated while the remaining 66.6% left the reaction system unconverted. Product gas from final methanation yielded specification grade SNG containing residual hydrogen of 0.7 vol % and residual CO of less than 0.1 vol %. The heating value was 973 Btu/standard cubic foot (scf) after C02 removal to 0.5 vol % (calc.). 

These tests were performed to establish the limits in flexibility and operability of a methanation scheme. The two demonstration plants have been operated in order to determine the optimum design parameters as well as the possible variation range which can be tolerated without an effect on catalyst life and SNG specification. Using a recycle methanation system, the requirements for the synthesis gas concerning H2/CO ratio, C02 content, and higher hydrocarbon content are not fixed to a small range only the content of poisons should be kept to a minimum. The catalyst has proved thermostability and resistance to high steam content with a resultant expected life of more than 16,000 hrs. 

On the basis of this past work and ongoing experiments, we feel that the liquid-phase methanation process promises to become an economic, reliable, and versatile means of converting synthesis gas mixtures to high Btu gas. Chem Systems believes that this technology is a key step in the transformation of fossil feeds into pipeline gas, and we look forward to its successful application in commercial coal gasification plants. 

Biomass gasification offers the potential for producing a fuel gas that can be used for power generation system or synthesis gas applications. The volatile matter contains a considerable amount of tar which is a complex mixture of aromatics. Despite extensive research efforts tar formation which causes the pipe plugging and the reduction of conversion efficiency is still a major problem in biomass gasification systems [1-6]. 

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