H2

Liquefied

Diethylamine petroleum gas Unit

Hydrotreating

Heavy atmospheric gas oil (Recycle) ^ Stabilized Product ^■Heavy gas oil

->Deasphalt oil (Recycle)

Solvent De-Ashing

FIGURE 5-36. HTI's direct coal liquefaction process scheme. (From HTI, Coal Liquefaction Technology (Hydrocarbon Technologies, Inc., Lawrenceville, NJ), www.htinj. com/products/liquefaction.html (accessed April 2004). With permission.)

Coal Conversion Reactors

Deasphalt oil Heavy atmospheric Make-up gas oil

Liquefied

Diethylamine petroleum gas Unit

Hydrotreating

Heavy atmospheric gas oil (Recycle) ^ Stabilized Product ^■Heavy gas oil

->Deasphalt oil (Recycle)

Solvent De-Ashing

FIGURE 5-36. HTI's direct coal liquefaction process scheme. (From HTI, Coal Liquefaction Technology (Hydrocarbon Technologies, Inc., Lawrenceville, NJ), www.htinj. com/products/liquefaction.html (accessed April 2004). With permission.)

expected to be operational in 2005. Shenhua Group intends to construct three additional plants in the Shengdong Coalfield of China, which spans Shaanxi Province and Inner Mongolia. These would be the only commercial, large-scale direct coal liquefaction plants operational in the world since the World War II-vintage plants constructed by the Germans.

Donor Solvent Processes

The main representative of this process is the Exxon donor solvent (EDS) process. In a donor solvent process, the solubilization of the coal is done by the hydrogen donor liquid, and hydrogen molecules come from the donor liquid and not gaseous hydrogen. The donor liquid is then recycled and hydrotreated to add hydrogen back into the donor liquid.

Exxon Donor Solvent Process In the EDS process, finely ground coal is mixed with the donor solvent, and the coal is liquefied in a noncatalytic tubular plug-flow reactor in the presence of molecular hydrogen and a hydrogen-rich donor solvent [2]. The liquefaction reactor operates at 800 to 880°F and 1100 to 2300 psig. The products from the liquefaction reactor are separated by distillation into light hydrocarbon gases, ranging from methane to propane and methylpropane, a naphtha fraction, a heavy distillate, and a bottoms fraction [22]. The naphtha and heavy distillate fractions are treated by conventional petroleum-refining technology. About 85% of the naphtha is recovered as gasoline, and about 50% of the heavy distillate is recovered as a mixture of benzene, toluene, and xylenes. Further processing of the heavy distillate produces fractions comparable to jet fuel and heating fuel [22]. A portion of the heavy distillate is hydrotreated and recycled to slurry the coal. In addition, the bottoms are either coked in a fluid coking plant or recycled to the liquefaction reactor. Recycling to the liquefaction unit results in a dramatic increase in the conversion of the coal to liquid products. In addition to small-scale test reactors, Exxon, the DOE, the EPRI, and an international group of industrial sponsors installed a pilot plant in Baytown, Texas, that produces 250 short tons coal per day. This was the minimum size needed for confident scale-up of the critical process and mechanical features of the EDS process.

Concluding Statements

Several direct liquefaction processes were introduced in this chapter. Although these have been some of the most important and, in most cases, the most successful processes, it must be noted that many processes have been conceived and researched. Due to economics, however, none has been able to compete with available liquid and gaseous fuels, especially in industrialized countries. The work of the Germans in the 1930s and 1940s and the facilities planned for China illustrate that large-scale, direct coal liquefaction commercial plants are technically feasible. They can be constructed and operated when necessary if dictated by political mandates, whether during a period of isolation in wartime or due to a decision to be self-sufficient and use indigenous resources.

0 0

Post a comment