2. Literature Review
2.1 Sasol’s Coal to Synthetic Polymer Processes
Sasol leads the world in the conversion of low grade coal to motor fuels and synthetic polymers. In addition to producing more than 40% of South Africa’s liquid fuels and more than 200 other value added polymers, the group directly contributes more than 4% of South Africa’s gross domestic product. This conversion of coal (and natural gas) into synthetic fuels products is achieved through propriety Fischer-Tropsch (FT) technologies unique to Sasol.
In the first part of the process, 46 Mton/y of coal is converted to synthesis (or producer) gas, a combination of carbon monoxide and hydrogen. This is known as Gasification (Figure 2.1). In the gasifier, the coal is pressurized with steam and oxygen and is converted to crude synthesis feed. The gasification condensates once cooled, yields tars, pitches, oils and most importantly purified synthesis feed gas. Gasification is the first part of the Coal to Liquid (CTL) process. Coal is the most important feedstock. However, to reduce carbon emissions, a second feedstock used is natural gas. In this case, the gas is reformed via a process called auto-thermal reforming to produce a comparatively pure synthesis gas, this is known as the Gas to Liquids (GTL) process.
Even though 70 % of the currently used feedstock is coal, it is this Gas to Liquid (GTL) approach that holds the most future promise for the application of FT technology. The main component of natural gas is methane. The conversion of natural gas to synthesis gas is called methane reforming.
For methane reforming, as with gasification, the feedstock usually reacts with steam and oxygen to
University
of Cape
Town
produce hydrogen and carbon monoxide. The cleaner nature of the methane feed (compared to coal) is conducive to the use of high-activity catalysts to enhance the conversion process. The purified synthesis feed gas is then available for conversion either through the high temperature Sasol Advanced Synthol (SAS) or the lower temperature Slurry Phase Distillate (SPD) process.
Figure 2.1, Simplified Block Flow Diagram of the Sasol Process
In the SAS process the synthesis gas reacts under pressure, in a fluidized iron based catalyst bed to yield C1 to C20 hydrocarbons. The product stream is cooled successively to yield liquefied products like petrol, diesel and jet fuel and a methane rich gas that is recycled to the reformer to be again converted to synthesis gas. In the lower temperature Slurry Phase Distillate (SPD) process, the synthesis feed is converted to linear chained hydrocarbons, waxes, paraffin and high quality diesel (Sasol Facts, 2005). Irrespective of the reactor configurations, catalyst and hydrocarbon feedstock, the basic Fischer-Tropsch reaction can be described as follows:
nCO + 2nH2→ CnH2n + nH2O +CH4 (2.1)
The Fischer-Tropsch process produces more water (on a molar basis) than it does actual product (Eq 2.1). The water produced in this reaction, is known as Fischer-Tropsch Reaction Water (FTRW).
University
of Cape
Town
FTRW comprises of C1 to C6 Short Chain Fatty Acids (SCFA). The average COD of this stream is in the order of 18 000mg/L (~20 times the concentration of raw sewage) and at Secunda 29 ML/d is produced (Phillips & Du Toit, 2002).
Currently, the FTRW is treated in an activated sludge plant along with two other wastewater streams, namely API (American Petroleum Industries) which is oily sewer water from the plant and Stripped Gas Liquor (SGL) (Figure 2.2). The former originates from plant drainage and the latter is from the Gasification condensate which has undergone physical processing to recover by products.
Figure 2.2 gives a graphical presentation of the flows and loads on Sasol’s activated sludge treatment system.
Figure 2.2, Current Organic Effluent Treatment System Used at Sasol
The Secunda fully aerobic activated sludge plant treats 128 ML/d of wastewater with a COD load of 677 ton per day, a 7 million person equivalent organic load. At this organic load, 150 ton (TSS) of sludge is produced and 480 tonO2/d is required for aeration. A further 225 tonO2/d and 170 ton/d of coal is required for energy production to supply the 320 MWh of electricity required to run the plant.
University
of Cape
Town
77% of the 677 tCOD/d organic load originates from the FT process (Appendix 1.1). The FTRW consists mostly of C2 to C6 short chain fatty acids (SCFAs), has a low pH (3.77) and TDS (35 mg/L) and little other contaminants.
Aside from the high oxygen, electricity and sludge treatment costs, aerobic treatment of SCFA streams are problematic because of their tendency to produce biomass that flocculates and settles poorly, thus leading to high solids liquid separation costs and an effluent with a high suspended solids after secondary settling (Ekama, 2004). SCFA streams are readily treatable anaerobically, which would lead to much lower sludge production (0.04 gTSS/gCOD), zero oxygen demand and a methane rich biogas stream (Kalyuznhyi & Davlaytshina, 1997a; Sam-Soon et al., 1989). Since the biomass yields of anaerobic micro-organisms is extremely low, most (>95%) of the 522 tonCOD/d of the FTRW will be converted to methane – which can be recycled and reformed and used as synthesis feed (Table 1.1 and Appendix 1.1). Much lower electricity and sludge treatment costs as well as a significantly reduced carbon footprint are therefore expected with the anaerobic treatment of FTRW.