Hydrotreating Synthoil

The so-called cobalt molybdate catalyst has been used much in the petroleum industry for hydrotreating and hydrodesulfurization. More recently, these catalysts have been employed in coal liquefaction and synthoil upgrading. The latter probably accounts for the recent rash of publications on this very interesting catalyst system. Indeed, of the papers surveyed for this review, the majority have been published in the past 5 years with no letup in sight. 

In the Energy Research and Development Administration s SYNTHOIL process, slurries of coal in recycle oil are hydrotreated on Co-Mo/Si02 Al203 catalyst in turbulent flow, packed-bed reactors. The reaction is conducted at 2,000 to 4,000 psi and about 450° C under which conditions coal is converted to low-sulfur liquid hydrocarbons and sulfur is eliminated as E2S. 

Primary coal liquefaction products from three processes— solvent-refined coal, Synthoil, and H-Coal—were hydrotreated. Upgrading was measured in terms of the decrease in heptane and benzene insolubles, the decrease in sulfur, nitrogen, and oxygen, and the increase in hydrogen content. Hydrotreating substantially eliminated benzene insolubles and sulfur. An 85% conversion of heptane insolubles and an 80% conversion of nitrogen was obtained. Catalyst stability was affected by metals and particulates in the feedstocks. 

Table VI recapitulates the relative response of Synthoil and SRC to hydrotreating in terms of percentage conversion of heptane insolubles, as reported in Table II. Conversion of heptane insolubles was substantially greater in the case of SRC. While the Synthoil was processed at a higher space velocity, the throughput rate of Synthoil feed heptane insolubles was only half that of the SRC feed heptane insolubles. Temperature in the Synthoil run was 5°-10°C lower than in the SRC run.

This comparison is reinforced by a comparison of Synthoil with UOP-filtered SRC filter feed. Figure 4 illustrates the response of the two stocks to hydrotreating as a function of pressure and space velocity. The temperature range of the experiments was 9°C. Conversion of SRC heptane insolubles was first order up to 98%, and the effect of pressure was relatively small. Conversion of Synthoil heptane insolubles never exceeded 80% under comparable conditions, and it was strongly dependent on pressure. 

The asphaltenes in SRC stocks are more responsive to hydrotreating than those in Synthoil. Whether this is because of differences in the two processes, or because of differences in the coals used to produce the particular materials studied, is a matter of speculation. 

A recapitulation of the catalyst stability data reported indicates that within the time scale of the hydrotreating runs, the UOP-filtered SRC filter feed gave relatively stable performance. SRC itself caused substantial catalyst deactivation Synthoil gave stable performance after an initial deactivation. Since dissolved metals and particulate matter are known to have an adverse effect on catalysts, a correlation was sought based on an analysis for these components. 

Removal of asphaltenes and preasphaltenes was easier than heteroatoms. At an increase of a hydrogen-to-carbon atomic ratio by 0.16, they were 80% removed. A comparison can be made with Synthoil process product of hydrogen-to-carbon atomic ratio equal to 1.04 which was hydrotreated to hydrogen-to-carbon atomic ratio equal to 1.24. The asphaltenes and preasphaltenes were 77% and 99% removed, respectively (7). Squires (8) concluded that the preasphaltenes can be converted to asphaltenes and oils with very little consumption of hydrogen. Asphaltenes are the major consumers of hydrogen. Although Squires conclusions were based on donor-solvent coal liquefaction, similar results were reported... 

A hydrotreating run of 436 hr was completed on pretreated Synthoil. Analyses of the composite product are compared with those of the feed in Table II. 

---