Transitioning SA's Petrochemical Value Chain

Secunda’s final products consist of approximately 60% synfuel and 40% chemicals:

• Synthetic fuels components of the FT output are upgraded and marketed together with conventional fuels produced in a crude oil refinery. Refining of this fraction of the output from the FT processes is comparable in many respects to crude oil refining, and includes fractionating, separating, treating, cracking and conversion of hydrocarbons to different fuels and fuel components. Fuel and fuel components are subsequently blended to meet fuel specifications.

• Chemicals are either sold directly to customers or transferred to Sasolburg for further processing.

A more detailed diagram of the process and description of the product suite can be found below.

There is flexibility at the Secunda and Sasolburg sites to allow them to respond to changes in the market for fuels and chemicals through process and production modifications.

Utilities: Coal and natural gas, as well as methane-rich gas produced from the process, are used for onsite electricity generation in coal boilers and gas engines or turbines at Secunda. The site has the capacity to generate 600 MW from coal (via steam) and 240 MW from natural gas and methane rich gas. The onsite gas power plant utilises approximately 10 PJ of gas per annum. Onsite power generation infrastructure provides around 50% of Secunda’s electricity requirements, with the balance being purchased from Eskom. Coal is also used to produce steam for the gasifiers.

Secunda produces liquid fuels and a range of chemical products. The key inputs to the facility are coal and natural gas: 

• Coal provides a source of carbon which is a building block for several products from the production process. Coal is also used for electricity generation and steam production.
• Natural gas is used as a source of hydrogen, as well as for generation of electricity on site.

The Secunda production process is centred around the conversion of syngas to liquid fuels in the Fischer-Tropsch (FT) process. The production of the feed syngas with the required composition for FT requires a number of upstream processes. 

A high-level overview of the production process is as follows:
 

1. Coal, produced primarily from Sasol’s own mines, is gasified with oxygen and steam under high pressure in coal gasifiers to produce a mixture of gases known as synthesis gas or syngas, which includes hydrogen, carbon monoxide, carbon dioxide and methane.
2. Hot raw gas leaving the gasifiers is quenched or cooled to remove solids and heavy tars which are taken for further processing.
3. The cooled gas stream, which contains dissolved oil, phenols, tar acids, organic acids, and ammonia, is further processed to recover valuable by products.
4. The raw syngas stream is also purified by scrubbing it with cold methanol in the Rectisol plant, to remove further by-products (tar naphthas) and unwanted acid gases, mainly CO2 and H2S/COS.
5. At this point the hydrogen content of the syngas is still too low, and so further hydrogen is produced through steam reforming of methane and also recovered from elsewhere in the process.
6. The gas from the Rectisol plant is mixed with the additional hydrogen to provide a feed with a H2:CO ratio 1 of 1.7-2.0, which is that required for Fischer-Tropsch synthesis. Further adjustment is achieved through the water gas shift reaction. A concentrated stream of carbon dioxide is produced during the ratio adjustment process of the water shift reaction, which is the biggest contributor of CO2 emissions to the atmosphere.
7. Finally, the purified syngas with a specific ratio of hydrogen to carbon monoxide is sent for Fischer-Tropsch synthesis. Here, the syngas is converted using an iron-based catalyst into a mixed hydrocarbon stream sometimes known as “syncrude”. Syncrude is sent for downstream processing to produce a range of synthetic fuel components, hydrocarbon fuels and chemical feedstocks used for further manufacturing. The Secunda site has eight Fischer-Tropsch reactors.