Heavy paraffin synthesis

Fig. 3. The Shell middle distillate synthesis (SMDS) process. HPS = heavy paraffin synthesis. HPC = heavy paraffin conversion.

The first stage, Heavy Paraffin Synthesis (HPS), converts hydrogen and carbon monoxide into heavy paraffins by the Fischer-Tropsch process. The product distribution is in accordance with Schultz-Flory polymerization kinetics, which is characterized by, the probability of chain growth. 

Shell has been involved in syngas chemistry for many years, giving special attention to the options for the conversion of natural gas into more easily transportable liquid hydrocarbons. The first result of this effort has been the Shell Middle Distillate Synthesis (SMDS) plant commissioned in Malaysia in 1993. This plant makes use of cobalt FT catalyst and tubular reactors in the Heavy Paraffin Synthesis unit (HPS). A simplified flow scheme of this plant is presented in Figure 7. 

The second step is Heavy Paraffin Synthesis, in which Syngas is passed through proprietary multi-tubular fixed-bed synthesis reactors and converted into long-chained... 

The design approach for Pearl GTL is that essentially limited scale-up risks have been taken at equipment level. As a result the Air Separation Units (ASU s), The Shell Gasification Process reactors (SGP s) and Heavy Paraffin Synthesis reactors (HPS s) are all built in a modular fashion, i.e. with multiple parallel imits. 

HPS HEAVY PARAFFIN SYNTHESIS HPC HEAVY PARAFFIN CONVERSION... 

For maximum yield of liquid hydrocarbons and minimum yield of gases, FT synthesis is optimised to produce predominantly heavy products (heavy paraffins),7 i.e., producing hydrocarbon chains as long as possible at maximum hydrocarbon chain growth probability. 

Consecutively, the heavy paraffins are cracked into lighter hydrocarbon fractions by hydro-cracking. For example, for the Shell Middle Distillate Synthesis (SMDS) process, the liquid product stream is composed of 60% gasoil (diesel), 25% kerosene and 15% naphtha. The gaseous product mainly consists of LPG (a mixture of propane and butane) (Eilers et al., 1990). Figure 7.3 shows a simplified diagram comprising all process steps to produce synthetic hydrocarbons from biomass, natural gas and coal. 

Compared to SMDS, this simplified process employs a different catalyst in the synthesis stage and does not include a Heavy Paraffinic Conversion stage for producing the finished middle distillate fractions. The syncrude product is a broad boiling range of hydrocarbons (Table 4), and the relative amounts of individual products can be varied by adjusting the reaction conditions. Alternatively, syncrude products can be processed in existing refineries into finished transportation fuels. 

Figure 6. Reactors suitable for synthesis of heavy paraffins.

Another application of the hydrocracking process could become important during the next decade, when the transformation of natural gas into liquid fuels is considered to be economically feasible. In a route going via the manufacture of synthesis gas and the production of heavy paraffins by Fischer-Tropsch synthesis, a hydrocracking operation could be the final step in order to produce a high quality diesel engine fuel. 

One of the most important, and perhaps the best studied, applications of three-phase fluidization is for the hydrogenation of carbon monoxide by the Fischer-Tropsch (F-T) process in the liquid phase. In this process, synthesis gas of relatively low hydrogen to carbon monoxide ratio (0.6 0.7) is bubbled through a slurry of precipitated catalyst suspended in a heavy oil medium. The F-T synthesis forms saturated and unsaturated hydrocarbon compounds ranging from methane to high-melting paraffin waxes (MW > 20,000) via the following two-step reaction ... 

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