In liquid-phase processes (Cybulski 1994; USDOE 1999), the heat transfer between the solid catalyst and the liquid phase is highly efficient, and therefore the process temperature is very uniform and steady. A gas phase delivers reactants to the finely divided catalyst and removes the products swiftly. This allows high conversions to be obtained without loss of catalyst activity. The higher conversion per pass (compared to fixed-bed technology) eliminates the need for a recycle loop, which implies less auxiliary equipment, fewer energy requirements, smaller volumetric flow through the reactor (Katofsky 1993). An additional advantage is the ability to withdraw a spent catalyst and add a fresh catalyst without interrupt­ing the process.

Different reactor types are possible for liquid-phase methanol production, such as fluidized beds and monolithic reactors. Air Products and Chemicals, Inc. invented a slurry bubble column reactor in the late 1970s, which was further developed and demonstrated in the 1980s and 1990s. From 1997 to 2003, a 300- tonne/day demonstration facility at Eastman Chemical Company in Kingsport, TN produced about 400 million liters methanol from coal via gasification (Hey — dorn et al. 2003).

In the slurry bubble column reactor (Figure 2.6), reactants from the gas bubbles dissolve in the liquid and diffuse to the catalyst surface, where they react. Products then diffuse through the liquid back to the gas phase. Heat is removed by generating steam in an internal tubular heat exchanger.

Commercial Cu/Zn/Al catalysts developed for the two-phase process are used for the three-phase process. The powdered catalyst particles typically measure 1 to 10 pm and are densely suspended in a thermostable oil, chemically resistant to components of the reaction mixture at process conditions, usually paraffin. Catalyst deactivation due to exposure to trace contaminants is a point of concern (Cybulski 1994).

Conversion per pass depends on reaction conditions, catalyst, solvent, and space velocity. Experimental results show 15-40% conversion for CO rich gases and 40-70% CO for balanced and H2 rich gases. Computation models predict future CO conversions of over 90%, up to 97% respectively (Cybulski 1994; Hagihara et al. 1995). Researchers at the Brookhaven National Laboratory have developed a low temperature (active as low as 100°C) liquid phase catalyst that can convert 90% of the CO in one pass (Katofsky 1993). With steam addition the reaction mixture becomes balanced through the water gas shift reaction. USDOE claims that the initial hydrogen to carbon monoxide ratio is allowed to vary from 0.4 to 5.6 without a negative effect on performance (USDOE 1999).

The investment costs for the liquid-phase methanol process are expected to be 5-23% less than for a gas-phase process of the same methanol capacity. Operating costs are 2-3% lower; this is mainly due to a four times lower electricity consumption (USDOE 1999).