Two reactor types predominate in plants built after 1970 (Cybulski 1994; Kirk — Othmer 1995). The ICI low-pressure process is an adiabatic reactor with cold unreacted gas injected between the catalyst beds (Figure 2.5, left). The subsequent heating and cooling leads to an inherent inefficiency, but the reactor is very reliable and therefore still predominant. The Lurgi system (Figure 2.5, right), with the catalyst loaded into tubes and a cooling medium circulating on the outside of the tubes, allows near-isothermal operation. Conversion to methanol is limited by equilibrium considerations and the high temperature sensitivity of the catalyst. Temperature moderation is achieved by recycling large amounts of hydrogen-rich gas, utilizing the higher heat capacity of H2 gas and the higher gas velocities to enhance the heat transfer. Typically a gas phase reactor is limited to about 16% CO gas in the inlet to the reactor, in order to limit the conversion per pass to avoid excess heating.

The methanol synthesis temperature is typically between 230 and 270°C. The pressure is between 50 and 150 bar. Higher pressures give an economic benefit,

FIGURE 2.5 Methanol reactor types: adiabatic quench (left) and isothermal steam raising (right).

since the equilibrium then favors methanol. Only a part of the CO in the feed gas is converted to methanol in one pass through the reactor, due to the low temperature at which the catalyst operates. The unreacted gas is recycled at a ratio typically between 2.3 and 6.

The copper catalyst is poisoned by both sulfur and chlorine, but the presence of free zinc oxides does help prevent poisoning.