Sasol to convert natural gas meet demand for diesel

Sasol to convert natural gas to meet demand for diesel - The Globe and Mail

performance liquid fuels such as GTL diesel, naphtha, LPG, jet fuel as well as other premium products. GTL presents GTL also enables gas to be used as a transportation fuel without the need for expensive converting coal and gas into liquid fuels and chemical products . Sasol GTL: proven alternative to meet growing. Sasol is also involved in coal mining and marketing of natural gas and oil products. States that would convert natural gas into 96, bbl/d of diesel and other scheduled to come online in the next few years to meet rising demand for . no longer possible to meet the demand for the product in short supply. When this to convert this gas to liquid fuel, using technology developed on the SASOL plants. .. of the change in the relative demand for petrol and diesel, SASOL 2 was.

The coal 1s present 1n three seams, having mineable heights of 8 to 10 feet 2. The seams are separated by layers of shale, mudstone, and sandstone of varying thickness. The mine 1s under to feet 30 to 60 m of white sandstone. The mining technique used 1s the mechanized room-and-pWar. Mined coal 1s transported via conveyor to primary crushers situated at the bottom of the Inclined coal hauling shafts.

The coal 1s lifted to the surface by conveyor and discharged Into storage bunkers of 12, ton 11, mt total capacity. The final product consists of two coal sizes, less than 0. Coal Is kept damp on the con- veyor belts by water sprays at suitable points to prevent dust formation. The less than 0. The Lurgl pressure gaslfiers are fixed-bed, water-cooled reactors that gasify the coal In the presence of oxygen and steam to yield a syn- thesis gas containing methane, carbon monoxide, hydrogen, carbon dioxide, ammonia, hydrogen sulflde, steam, and numerous other compounds.

Lurgl pressure gaslfiers were selected because they had already been demonstrated 1n smaller sized Installations and had the advantage of being able to work on the rather low grade, high ash coal available to Sasol I.

The fact that they operated at a pressure of approximately ps1 kPa which was also the desired operating pressure for the Flscher-Tropsch plant was an additional advantage.

Thirteen Lurgl gaslfiers consume coal at a total rate of approximately 8, tons mt per day. These gaslfiers are 12 feet 3. On an annual basis, At this level, gas production 1s actually limited not by gasification but rather by gas purification capacities.

The Lurgl gaslfiers operate on the principle of countercurrent flow of coal to steam and oxygen which offers the best conditions for heat and mass transfer and optimum efficiency.

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The overall thermal efficiency of the gaslfler system 1s approximately Raw gas production 13 averages million SCF 9. Oxygen Is produced In one of the world's largest air separation plants. The oxygen Is used 1n the Lurgl gaslffers as well as 1n the partial oxidation methane reforming plant. The nitrogen Is used 1n an ammonia plant. Conventional pulverized fuel boilers are used. The steam generated Is used not only for the gasification of coal, but also 1n the various plants Inside and outside the Sasol complex.

The residual carbon 1s completely burned out of the ash with oxygen 1n the combustion zone at the bottom of the Lurgl gaslflers.

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This exothermic reaction helps supply the heat for the endothermic gasification reaction In the upper part of the gaslflers. The residue 1s essentially burned-out ash which Is transported from the gasification and power plant areas by water In a low velocity sluiceway to the ash dewaterlng unit.

Coarse ash 1s removed by conveyor belts to an ash dump. The fine ash 1s concen- trated 1n a thickener and the concentrated fine ash 1s then dewatered 1n a slimes dam.

The ash contains soluble Inorganic salts that will leach out. The ash system 1s however an evaporative system for water and re- quires water make-up and no purge. Hater drainage from the slimes dam 1s collected and pumped back Into the ash sluiceway system.

To prevent water seepage, the slimes dam was given an Impervious clay layer from clay available on site. The slimes dam was built with an extensive drain- age system to recover all seepage for return to the ash sluiceway system.

Coarse ash contains no excess water and at least the outside of the dump soon dries out to such an extent that It will absorb rain water. No evidence of seepage from the ash dump has been found and no measures are taken against seepage. Success has been achieved In growing grass on the dumps to make them aesthetically acceptable. Regular samples of water from boreholes 1n the vicinity of Sasolburg have been taken over the years and no evidence of underground water pollution has been found.

In addition, the gas contains cyanide compounds, tars, oils, phenols, organic sulfur com- pounds, and numerous other Impurities 1n minor quantities.


The Iron- containing Flscher-Tropsch catalyst Is very sensitive to sulfur, cyanide, and other compounds. Efficient purification of the synthesis gas Is an essential requirement for high Flscher-Tropsch conversion rates. After separation of entrained coal dust, the raw synthesis gas 1s cooled 1n a sequence of waste heat boilers and condensers. The raw gas contains large quantities of undecomposed gasification steam. During gas cooling this steam Is condensed and the resulting aqueous 11quor contains the water-soluble components that were 1n the gas, chiefly phenols and ammonia.

The tars and oils are also separated from the synthesis gas during cooling. The oil and aqueous liquor streams are fed to tar distilla- tion and Phenosolvan plants respectively. In the tar distillation plant, road primer, creosotes, and lighter naphthas fractions are separated.

The naphthas are hydrogenated and distilled to produce benzoles for solvent use and for blending Into gasoline. In the Phenosolvan plant, the aqueous liquor Is treated by solvent extraction with an oxygen-containing organic solvent, butyl acetate, to remove the phenol compounds.

The ammonia Is then recovered by stripping with steam and converted to ammonium sulfate for fertilizer manufacture. Treated liquor 1s used 1n the factory for removal and transport of ash from the gaslflers. Ash acts as an adsorbent, reducing the residual oil content of treated liquor to less than 2.

These Impurities are removed by methanol In the Rectlsol plant. The Rectlsol process 1s based on the capability of one solvent, cold methanol, to absorb all Impurities present In gases from coal gasification tn a single process step.

A typical composition of raw and pure synthesis gas Is shown In Table 2. The extremely pure gas from the Rectlsol process 1s suitable for the sensitive Flscher-Tropsch synthesis catalyst. The main energy consumption In the Rectlsol unit 1s that used to drive the methanol circulation pumps and the refrigeration compressors.

The off-gas containing carbon dioxide and hydrogen sulflde from the Rectlsol plant 1s used as an expansion gas. Before this gas 1s vented to the atmosphere, hydrogen sulflde 1s removed and recovered as elemental sulfur 1n a Stretford sulfur recovery unit. The Stretford unit was Installed In Some hydro- carbons and other Impurities are still present 1n the vent gas.

The purified gas emerging from the Rectlsol plant undergoes the Flscher-Tropsch synthesis which produces hydrocarbons by the catalytic conversion of carbon dioxide and hydrogen according to the following equation: The purified gas from the Rectlsol plant 1s divided Into two streams.

The larger stream Is fed directly to the fixed bed Arge synthesis units where a stationary pellet1zed catalyst Is used. The gas conversion Is not complete. The tall gas from the Arge units contains 1ow-bo1l1ng hydro- carbons and carbon dioxide. These are removed In a Rectlsol wash unit at subzero temperatures.

The washed gas together with the remainder of the fresh gas from the purification plant enters a reforming plant where methane Is reacted with steam and oxygen over a nickel catalyst to produce additional carbon monoxide and hydrogen. After adjustment of the hydrogen- carbon monoxide ratio, the gas 1s fed to the fluid bed Synthol plant where a circulating powdered catalyst 1s used.

The tall gas of this plant Is also recycled to the reforming units after removal of carbon dioxide. For both the Arge and Synthol plants there are recovery and refining plants downstream.

The fixed bed reactor produces In general straight-chain hydrocarbons with a high average molecular weight In the range of dlesel oil and parafln waxes and a relatively low percentage of gasoline, liquefied petroleum gas, and oxygenated compounds alcohols, ketones, organic addsThe fluid bed process produces branched oleflns of a lower average molecu- lar weight 1n the range of liquefied petroleum gas and gasoline, little h1gh-bong material, and some oxygenated products Table 3.

Although the basic chemistry for both reactors Is the same, the different tempera- tures, method of catalyst contacting, recycle ratios, feed gas composi- tions, and hydrogen partial pressures employed 1n the two systems result not only 1n a difference In product selectivity, but also 1n a difference 1n the properties of hydrocarbons within the same boiling range.

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In general, the higher the reaction temperature the higher the content of oleflns and the lower the average molecular weight of the products. Inside each shell there are 2, vertical tubes, 2 Inches 5 cm In diameter, containing the pelletlzed Iron catalyst. The tubes are surrounded on the shell side by a steam jacket. The gas Is passed over the catalyst from top to bottom and the heat released by the exothermic reaction 1s absorbed by boiling the water In the shell.

The reaction temperature Is controlled by controlling the pressure of the boiling water. The Flscher- Tropsch reactions are highly exothermic, and one of the major design problems for both the Arge and Synthol systems Is adequate heat removal from the reactor. Figure 3 Is a schematic drawing of the fixed bed Arge reactor. The reactor operates at pslg kPa. The specific catalyst employed contains a number of promoters Including copper and potassium and has to be partially reduced before It can be used.

It 1s manufactured at Sasol. The fixed bed synthesis accounts for about one-third of Sasol I's plant output. It came on line In with only minor problems and behaved more or less as designed. The major disadvantage of the Arge reactor system 1s Its limited scale-up potential.

New, large synthetic fuel plants would require an Impractical number of such reactors. The synthesis products exit at the bottom of the Arge reactor. The tall gas 1s separated from the heaviest hydrocarbons which are obtained as reactor condensate. The hot gas then exchanges heat with the Incoming feed gas and Is further cooled and washed with sodium hydroxide solution In water-cooled condensers.

The heat exchangers and the condensers produce hydrocarbon and aqueous condensates which are, after pressure release and recovery of the dissolved gas, sent to the refinery. Fixed Bed Arge Reactor 21 The tall gas leaving the condensers Is used for recycle, sold as Industrial gas, or Is sent to a methane reformer.

The tall gas contains low-boiling hydrocarbons up to pentane, Inclusively, and carbon dioxide. The Lurgl gaslflers and both Hscher-Tropsch synthesis processes produce methane as the lowest hydrocarbon and there 1s a tendency for methane to build up 1n the recycle streams. To make full use of the synthesis gas, It Is necessary to reform methane back to hydrogen and carbon monoxide. The nitrogen and argon act as Inerts In the system and have to be removed as a purge gas to keep them within an acceptable level.

The gas from the reforming plant Is fed to the fluid bed Synthol plant. The Synthol process was developed by M.

Kellogg of the United States. Each Synthol reactor consists of a feed system, a reactor tube, product- catalyst separation equipment, and a catalyst recycle hopper Figure 4. This gas-solid mixture comes rapidly to thermal equilibrium and rises up the Synthol reactor where exothermic Flscher-Tropsch and water-gas shift reactions take place.

A significant fraction of the heat liberated Is removed 1n waste heat boilers built Into the reactor. The products are disengaged from the catalyst Initially by gravity and sub- sequently by cyclone separation. The gas leaves the reactor and the recovered catalyst Is collected 1n a settling hopper from which 1t 1s recycled through a stand pipe and slide valves to the feed gas Inlet at the base of the reactor. US oil demand alone has jumped by nearlybarrels of oil per day in recent weeks and the fix is in when it comes to US production growth thanks to the collapse in the rig count.

Long term I think reasonably high oil prices are going to be required to allow for development of enough production to meet the globe's daily thirst for oil. When it comes to natural gas on the other hand I am not bullish at all. The United States has a huge inventory of natural gas drilling locations and on top of that there is significant natural gas produced alongside oil in the shale oil plays.

In fact outside of the Bakken the other major shale oil plays are really "combo" plays that produce a significant amount of associated gas. I therefore believe that there is going to be a long term disconnect between the price of oil and the price of natural gas on an energy equivalency basis.

That could mean there is a significant opportunity for companies with exposure to Gas to Liquids technology. That is why I'm digging into this area where I have a long way to go to get up to speed.

To get some background into the gas to liquids GTL I spent some time on the Shell website which contains a good amount of information on the subject. A has been involved with the process for more than forty years. The gas to liquids process involves converting gaseous hydrocarbons such as natural gas into longer-chain hydrocarbons such as gasoline or diesel fuel.

Way back in two German scientists, Franz Fischer and Hans Tropsch developed the now named Fischer-Tropsch process which is a series of chemical reactions that converts gas into liquid hydrocarbons.

The process played a big role in World War II because Germany had virtually no domestic oil production and access to imports were constrained.

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By the time rolled around the Germans were getting half of their liquid fuel supply from synthetic fuel. A key part of the Allied victory involved shutting down Germany's 13 synthetic fuel plants. The Fischer-Tropsch process also proved to be important to South Africa through Sasol during its isolated apartheid era although instead of natural gas, Sasol used coal as a feedstock.

It is clear that the process works. The GTL process uses catalytic reactions to synthesize complex hydrocarbons from more basic organic chemicals. GTL technology involves the 3-step, indirect conversion of methane to higher molecular weight hydrocarbons ranging from LPG to paraffin waxes, often controlled to peak in the diesel range: The synthesis gas is then converted to hydrocarbons in the Fischer-Tropsch FT section with cobalt with natural gas as feed or iron-based with heavy feeds such as coal catalysts; and Step 3: The liquid products are separated in the final upgrading section, which often also involves mild hydrocracking to convert higher molecular weight waxes and lubes to LPG, naphtha, and diesel.

There isn't much of a middle ground when it comes to getting exposure to gas to liquids technology. There are very large companies that are involved in gas to liquids in some way, but investing in them offers very little leverage to the gas to oil conversion process.

Sasol is one of those companies and it has big exposure to synthetic fuels but the company uses coal as an input for the bulk of its profits. There are also small gas to liquids players which offer huge leverage to the process, perhaps too much leverage for many investors. According to the EIA there are currently only five GTL plants operating globally and they have capacities that range from 2, barrels per day tobarrels per day. Shell operates two of these, one is in Malaysia and the other in Qatar.

Sasol operates one in South Africa and the fifth is a joint venture between Sasol and Chevron in Qatar. The American shale gas boom led a rush of plans to build a GTL plant inside the lower The next one built of course will be the first ever. Sasol usually charges a fee for licensing its technology, but concentrates on establishing a share ownership in the facilities once built - with the aim of ensuring a slice of the long-term profits.

Sasol is now the world's biggest producer of synthetic fuel, having branched out to gas-to-liquid technology too. Its profitability has also been boosted by expanding its end products to include plastics, fertilizers and explosives.

Its larger chemical portfolios include polymers and solvents, and their intermediates, waxes, phenolics and nitrogenous products. The group also has a retail presence - selling liquid fuels and lubricants through a growing network of Sasol convenience stores and Exel service stations.

History The CTL technology dates back to the s, when two German chemists, Franz Fischer and Hans Tropsch, developed a process to convert coal into a gas and then used it to make synthetic fuels. During the Nazi reign, the "Fischer-Tropsch method" was employed in the war effort, as Germany lacked access to sufficient crude oil. International oil companies also experimented with the process, but put it aside because oil was cheaper.

Sasol - prompted by South Africa's apartheid-era isolation, as well as the poor quality of its coal- perfected this technology at its first plant in Sasolburg back in The company was privatised and was listed on the Johannesburg Stock Exchange inbut the government maintains a This heavy subsidisation pattern continues, with this expensive technology requiring similar incentives and loan guarantees elsewhere - resulting in engineers shunning CTL for a long time.

But with the world approaching energy peak, CTL technology is gaining favour.