The synthesis gas from the gasifier contains a considerable amount of CO2. After reforming or shifting, this amount increases. To get the ratio (H2-CO2)/(CO + CO2) to the value desired for methanol synthesis, part of the carbon dioxide could be removed. For this purpose, different physical and chemical processes are available. Chemical absorption using amines is the most conventional and com­mercially best-proven option. Physical absorption, using Selexol, has been devel­oped since the seventies and is an economically more attractive technology for gas streams containing higher concentrations of CO2. As a result of technological development, the choice for one technology or another could change in time, e. g., membrane technology, or still better amine combinations, could play an important role in future.

Chemical absorption using amines is especially suitable when CO2 partial pressures are low, around 0.1 bar. It is a technology that makes use of chemical equilibria, shifting with temperature rise or decline. Basically, CO2 binds chem­ically to the absorbent at lower temperatures and is later stripped off by hot steam. Commonly used absorbents are alkanolamines applied as solutions in water. Alkanolamines can be divided into three classes: primary, secondary, and tertiary amines. Most literature is focused on primary amines, especially monoethanola — mine (MEA), which is considered the most effective in recovering CO2 (Farla et al. 1995; Wilson et al. 1992), although it might well be that other agents are also suitable as absorbents (Hendriks 1994). The Union Carbide “Flue Guard” process and the Fluor Daniel Econamine FG process (formerly known as the

Dow Chemical Gas/Spec FT-1 process) use MEA, combined with inhibitors to reduce amine degradation and corrosion. The cost of amine-based capture are determined by the cost of the installation, the annual use of amines, the steam required for scrubbing and the electric power. There is influence of scale and a strong dependence on the CO2 concentration (Hendriks 1994). The investment costs are inversely proportional to the CO2 concentration in the feed gas when these range from 4% to 8%. MEA is partly entrained in the gas phase; this results in chemical consumption of 0.5-2 kg per tonne CO2 recovered (Farla et al. 1995; Suda et al. 1992). The presence of SO2 leads to an increased solvent consumption (Hendriks 1994).

When the CO2 content makes up an appreciable fraction of the total gas stream, the cost of removing it by heat regenerable reactive solvents may be out of proportion compared to the value of the CO2. To overcome the economic disadvantages of heat-regenerable processes, physical absorption processes have been developed that are based on the use of essentially anhydrous organic sol­vents, which dissolve the acid gases and can be stripped by reducing the acid-gas partial pressure without the application of heat. Physical absorption requires a high partial pressure of CO2 in the feed gas to be purified, 9.5 bar is given as an example by Hendriks (1994). Most physical absorption processes found in the literature are Selexol, which is licensed by Union Carbide, and Lurgi’s Rectisol (Hendriks 1994; Hydrocarbon Processing 1998; Riesenfeld et al. 1974). These processes are commercially available and frequently used in the chemical indus­try. In a countercurrent flow absorption column, the gas comes into contact with the solvent, a 95% solution of the dimethyl ether of polyethylene glycol in water. The CO2 rich solvent passes a recycle flash drum to recover co-absorbed CO and H2. The CO2 is recovered by reducing the pressure through expanders. This recovery is accomplished in serially connected drums. The CO2 is released partly at atmospheric pressure. After the desorption stages, the Selexol still contains 25-35% of the originally dissolved CO2. This CO2 is routed back to the absorber and is recovered in a later cycle. The CO2 recovery rate from the gas stream will be approximately 98% to 99% when all losses are taken into account. Half of the CO2 is released at 1 bar and half at elevated pressure: 4 bar. Minor gas impurities such as carbonyl sulfide, carbon disulfide and mercaptans are removed to a large extent, together with the acid gases. Also hydrocarbons above butane are largely removed. Complete acid-gas removal, i. e., to ppm level, is possible with physical absorption only, but is often achieved in combination with a chem­ical absorption process. Selexol can also remove H2S, if this were not done in the gas-cleaning step.

It has been suggested by De Lathouder (1982) to scrub CO2 using crude methanol from the synthesis reactor that has not yet been expanded. The pressure needed for the CO2 absorption into the methanol is similar to the methanol pressure directly after synthesis. This way only a limited amount of CO2 is removed, and the required CO2 partial pressure is high, but the desired R can be reached if conditions are well chosen. The advantage of this method is that no separate regeneration step is required and that it is not necessary to apply extra
cooling of the gas stream before the scrubbing operation. The CO2 loaded crude methanol can be expanded to about atmospheric pressure, so that the carbon dioxide is again released, after which the methanol is purified as would normally be the case.

Physical adsorption systems are based on the ability of porous materials (e. g., zeolites) to selectively adsorb specific molecules at high pressure and low tem­perature and desorb them at low pressure and high temperature. These processes are already commercially applied in hydrogen production, besides a highly pure hydrogen stream a pure carbon dioxide stream is coproduced. Physical adsorption technologies are not yet suitable for the separation of CO2 only, due to the high energy consumption (Ishibashi et al. 1998; Katofsky 1993).