Design Considerations
The main considerations to be applied in developing a synthesis gas production scheme for methanol manufacture are the same as for ammonia, namely, selection of gasification pressure, syngas cooling arrangement, and acid-gas removal system. Contrary to the ammonia case, the optimization of oxidant quality is not a consideration, since any inerts in the syngas lower the conversion in the synthesis. The oxygen should simply be as pure as reasonably possible, which in effect means 99.5% purity.
• The selection of the exact pressure to run the methanol synthesis loop will depend on an OPEX/CAPEX optimization. For medium-size units of, for instance, 600 t/d the loop would operate at about 50 bar, that is, without any intermediate syngas compression. For a large unit of say 2,000 t/d, the pressure would be somewhat higher. The principles are shown in Figure 7-4. For the smaller unit, the clean gas from the C02 removal unit is fed to the suction side of the loop gas circulator,
while the CO shift bypass gas has sufficient pressure to enter the loop on the discharge side. For the larger plant, the gases are mixed together at the suction of the booster compressor.
• For the production of СО-rich gases, the use of a syngas cooler is usually the better selection. It provides an efficient use of the sensible heat in the raw gas leaving the gasifier, where the production of steam required for CO shift in a quench is of no advantage. The methanol case lies halfway between the extremes of all hydrogen and all CO production, in that about 50% of the gas must be shifted. In most actual plants, the syngas cooler option is used for the whole raw-gas stream, and a portion of the desulfurized gas is passed over a shift catalyst. It is, however, possible to divert some of the gas from a quench reactor into a syngas cooler, employing a so-called combi-reactor, and to use a raw-gas shift (Jungfer 1985). For our example, we will cool all the gas as described above.
• As with ammonia, Rectisol is the most advantageous acid-gas removal system. It is the only wash that will achieve the desired degree of desulfurization. Alternative systems would require an additional stage of COS hydrogenation and a subsequent zinc-oxide bed for final clean up. In the case of a methanol plant, Rectisol has the added advantage that the wash liquor is the plant product itself, thus enabling some saving of infrastructure (although it should not be overlooked that an inventory tank for H2S or C02 contaminated methanol is still required).
• It is worth pointing out that the use of an optimized synthesis gas quality also influences the choice of synthesis technology. Methanol formation from CO has a significantly higher heat of reaction than that from C02. The higher proportion of methanol produced from CO when using syngas from coal or oil gasification means that in such a plant more attention must be paid to the issue of heat removal than, for instance, in a natural gas-fed steam reformer-based plant. It is necessary not only to remove the larger quantity of heat. It is also necessary to perform this
in a manner that prevents the slightest local overheating in order to avoid by-product formation, since the production of impurities from side reactions increases with increasing temperature. The intense and intimate cooling provided by the boiling water in an isothermal reactor has therefore made it the preferred reactor system for gasifier-based methanol plants. Over 90% of gasification-based methanol production operates with isothermal reactors. This includes all the large capacity ones.
The above applies for gas-phase syntheses. A demonstration liquid phase synthesis has been built at the Eastman plant in Tennessee, which is also reported to be giving good service (Benedict, Lilly, Kornosky 2001).
The block flow diagram for a lOOOt/d methanol plant based on gasification of 29.5 t/h visbreaker residue in Figure 7-4 shows the result of these deliberations. The mass balance is given in Table 7-3. The residue with quality, as in Table 4-10, is gasified with 99.5% purity oxygen at a pressure of 60 bar. The raw gas is then desulfurized in a Rectisol wash to a residual total sulfur level of less than 0.1 ppmv. Approximately 50% of the desulfurized gas is subjected to a CO shift and C02 removal in a second Rectisol stage, providing a raw hydrogen with sufficient C02 slip to meet the overall synthesis gas specification. This gas is fed to the circulator of the methanol synthesis loop at a pressure of 52.5 bar. The remainder of the desulfurized gas contains about 48% CO and has a pressure of 48.8 bar. This gas is then added to the loop on the discharge side of the circulator. The exact ratio of the two streams of desulfurized gas is controlled to maintain the correct stoichiometric ratio.
The flowsheet shows with dotted lines an alternative means of adjusting the H2, CO, and C02 flows to maintain the synthesis gas specification, which is particularly appropriate in an environment where the methanol is utilized for acetyls production. Instead of converting CO to hydrogen in the shift unit, CO is removed from part of the desulfurized gas in a cryogenic unit and recovered as pure CO for acetic acid production. In this case, all the residual C02 in the desulfurized gas drawoff must be removed in an additional stage of the Rectisol unit prior to cryogenic treatment.