Merichem-A Unique Syngas Cleanup Scheme
Merichem-A Unique Syngas Cleanup Scheme...
A Unique Syngas Cleanup Scheme By Gary J. Nagl Merichem Company
Abstract Syngas produced from the gasification of coal has become an acceptable means of producing feedstocks for Coal-to-Chemical plants in regions such as China where coal is abundant and inexpensive while petroleum and natural gas are relatively scarce and expensive. The production of chemicals such as methanol, acetic acid, ammonia, dimethyl ether, DME, or hydrogen from coal has become quite common place in China. A common characteristic of these plants is that they are all relatively small compared to the more familiar Integrated Gasification Combined Cycle (IGCC) facilities; consequently, the gas treatment train differs somewhat from the larger IGCC facilities. A Chinese client requested Merichem Company to develop a processing scheme for its planned acetic acid, Coal-to-Chemical facility. The processing scheme was required to produce syngas with a total sulfur content (COS, CS2, and H2S) and a hydrogen cyanide content of less than 0.1 ppm. An additional stipulation was the exclusion of a zinc oxide system from the design. This paper describes a unique processing scheme utilizing a combination of multi-stage hydrolysis and liquid redox to achieve the stated goals.
Introduction China has the third largest recoverable reserves of coal and is the leading producer and consumer of coal in the world; however, China has only limited reserves of expensive crude oil and natural gas. Consequently, China is actively pursuing the exploitation of this indigenous coal resource as a primary energy source and as a primary raw material for the production of petrochemicals. For this reason, China has constructed more than 20 coal gasification facilities over the past few years with the sole purpose of producing a synthesis gas (syngas) to be employed as a building block for myriad petrochemicals. Figure 1 illustrates the variety of products, which can be produced from a coal derived syngas. Because China is not hampered by stringent permitting or regulations and has an ample supply of inexpensive labor, gasification plants can be built and operated in approximately 30 months and at 2/3 to1/2 the cost of similar plants in the U.S. and Europe. Gasification is the conversion of a carbon-based material in an oxidant-lean environment for the purpose of creating a gaseous mixture rich in hydrogen and carbon monoxide. As previously noted this H2:CO mixture is referred to as “Syngas”. The oxidant employed is usually oxygen produced in an air separation unit. A simplified block flow diagram of a coal gasification facility is shown in Figure 2
Gasoline, Diesel, etc.
Vinyl Acetate Acetic Anhydride Terephthalic Acid
Hydrocarbon SYNGAS CO & H2
Figure 1 Coal Derived Chemicals
Air Separation Unit
Feedstock Preparation Steam
C XH Y
COOLING CLEAN ASH REMOVAL SYNGAS WATER GAS SHIFT SULFUR REMOVAL CO2 REMOVAL
POWER PLANT CHEMICALS
Figure 2 Block Flow Diagram of a Gasification Facility The overall gasification reaction can be represented as follows: CxHy + (x/2) O2
xCO +(y/2) H2
The above equation applies to the carbon and hydrogen constituents in a feed stock; however, a feedstock such as coal has many more constituents, which produce byproducts such as H2S, HCN, COS, CS2, CO2, Cl2, and ash or slag.
CASE STUDY Merichem Company was approached by a Chinese client who desired to produce acetic acid from a coal-derived syngas by first producing methanol followed by methanol conversion to acetic acid as illustrated by the following reactions: CO + 2H2
CH3 + CO
As can be seen from the above equations, both reactions are equilibrium limited and both require a catalyst. In addition, both of the catalysts are extremely sensitive to sulfur and hydrogen cyanide, which is the reason for some unique processing requirements from the client. In normal applications such as this, the syngas would be treated with an absorption type process such as amine, which would reduce the overall sulfur content to something less than 10 ppm. The gas would then be treated in a guard-bed system generally consisting zinc oxide, which would remove the remaining sulfur compounds. However, the Chinese have had very bad luck with this approach. Apparently, the absorption processes they employed upstream of the guard beds were either not capable of or were not operated properly to achieve low levels of sulfur compounds in the effluent gas resulting in unacceptable change out frequencies of the zinc oxide. Based on this experience, the client requested that Merichem design a system for treating the gas stream described in Table 1, which would produce an effluent gas having total sulfur and cyanide concentrations of less than 0.1 ppm without employing zinc oxide. Table 1 Raw Syngas Conditions Flowrate Pressure Temperature CO H2 H 2O CO2 N2 H 2S COS HCN Total Sulfur
37,800 Nm3/hr 37.0 Bar(g) o 170 C 46.8 % 19.4 % 20.1 % 5.4 % 7.8 % 3,600 ppm 500 ppm 160 ppm 5.1 MTPD
Merichem’s design approach consisted of first cooling the gas stream to remove a large portion of the water vapor thus reducing the amount of gas to be treated. This would then be followed by two stages of hydrolysis with intermittent H2S removal via liquid redox. The flow scheme is illustrated in Figure 3.
HEATER Sweet Syngas
SYNGAS HEATER H2O
Primary Hydrolysis Reactor
Primary LO-CAT Absorber
Polishing Hydrolysis Reactor
Polishing LO-CAT Absorber
Figure 3 Flow Scheme The presence of COS, HCN and possibly CS2 prompted the selection of catalytic hydrolysis, to accomplish the following reactions: COS + H2O
H2S + CO2
CS2 + H2O
2 H2S + CO2
HCN + H2O
NH3 + CO
As illustrated, all of the hydrolysis reactions are equilibrium limited, which resulted in the requirement for a two stage hydrolysis approach. After removal of the water vapor, the H2S concentration in the syngas feeding the first reactor increases to approximately 4400 ppm, which subsequently limits the equilibrium conversions of COS and CS2. Consequently, to achieve a very high overall total sulfur conversion an intermediate H2S removal step had to be incorporated into the processing scheme. This intermediate H2S removal was then followed by a final hydrolysis reactor and liquid redox absorber.
H2S REMOVAL The critical element to the success of the flow scheme illustrated in Figure 3 was the selection of the H2S removal process. The process had to be commercially proven to be able to achieve very high H2S removals (>99.9%) at elevated pressures and at favorable economics. In addition, due to the nature of the upstream gasifier, the process required turndowns approaching 100% for both flowrate and sulfur loading. To meet these requirements a liquid redox process was selected – ® the LOCAT process - due to its long, proven history in similar applications. The LOCAT process is an ambient temperature, aqueous-based, catalytic (‘Chelated Iron”), redox process that converts H2S to solid, elemental sulfur. The redox reactions are the oxidation of = o +3 sulfide ions (S ) to elemental sulfur (S ) and the corresponding reduction of ferric ions (Fe ) to
ferrous ions (Fe ). Although there are several intermediate steps, the overall process can be best represented by the direct oxidation of H2S with oxygen as follows: H 2S + ½ O 2
H 2O + S
Since there is a phase change – the formation of solid elemental sulfur – the reaction is not equilibrium limited; consequently, provided that there are sufficient mass transfer stages to absorb the required amount of H2S into the aqueous, catalytic solution, very high H2S removal efficiencies can be achieved. The source of the oxygen is generally air; however, enriched air or pure oxygen have also been employed. In this application it is imperative that the syngas not be contaminated with oxygen or nitrogen; consequently, the absorption of the H2S, the formation of sulfur and the reduction of the ferric ions are accomplished in a separate absorber vessel while the introduction of air and the oxidation of the ferrous ions back to the active ferric state are accomplished in a separate oxidizer vessel. Since the LOCAT process operates at ambient temperature, burners are not required and the heat content of the sour gas stream is immaterial; consequently, turndown ratios in regards to gas flow and H2S concentration approach 100%. In addition, since no heat-up is required, the process can be started up in a matter of hours. To prevent sulfur accumulation in the system, sulfur is removed on a continuous basis via filtration by means of gravity, vacuum or pressure filtration, If desired the sulfur can be further processed in a proprietary melter system to produce molten sulfur suitable for sulfuric acid plants. As illustrated in Figure 3 there are several wastewater streams produced in the process. The wastewater streams will contain both dissolved H2S and NH3, which must be removed prior to discharge. In this case a dual column, sour water stripper system was employed as illustrated in Figure 4.
H2S to LO-CAT Unit H2S Stripper
NH 3 to Flare
Vent to LO-CAT Unit Sour Water
Degassing Vessel Stripped Water
Figure 4 Sour Water Stripper System In this system the H2S and NH3 are separated into two separate streams. The H2S stream is sent back to the LOCAT unit for processing while the NH3 is directed to flare.
SUMMARY Based on a client’s specification for treating a coal-derived syngas to very low levels of total sulfur and hydrogen cyanide without employing zinc oxide, Merichem designed a unique processing scheme utilizing a combination of multi-stage hydrolysis and liquid redox to achieve the stated goals. The secret to success of the system was the liquid redox process – the LOCAT process – due to its ability to achieve, in an economical manner, H2S removal efficiencies exceeding 99.9%. The entire facility consisting of a coal gasification unit, syngas clean up facilities and an acetic acid plant is still the initial startup mode, all indications are that the stated goals will be met.