MOSSGAS AND SYNTROLEUM ECONOMICS COMPARED

In the past, synthetic fuels projects have only been contemplated where specific local and/or political conditions as well as the economies of scale combined to make the project "economically" acceptable. Today special dispensations for synfuels projects are no longer widely available and new synthetic fuels projects must meet the same set of defined economic and financial requirements, as any other energy or fuels project. R. Swanepoel of Bateman Project Holdings Ltd. compared the Mossgas synthetic fuels project in South Africa, which is one of the most recent mega projects to have been built, to a potential similar project based on new technologies, developed by the Syntroleum Corporation, in his presentation to the Monetizing Stranded Gas Reserves Conference held in Houston, Texas, in December.

The Mossgas Project

In the early 1980s, the FA field and some satellites offshore of Mossel Bay were discovered in South Africa. At the time of this discovery, South Africa was politically isolated and under increasing international pressure (including the threat of full-scale oil embargoes) to end its apartheid policies.

The initial assessment of the potential of the gas fields in the area indicated that a gas production rate of 150 million standard cubic feet could be maintained for at least a period of 20 years and probably longer. The gas deposit is sweet, having zero sulfur, and is associated with some 9,000 to 10,000 barrels per day of light condensate. The conceptual design of the offshore facilities called for two pipelines from the platform, one for gas and the other for condensate.

At the end of 1984 a combined study team from Bateman Project Holdings of South Africa and Foster Wheeler Ltd. of the United Kingdom was appointed to perform a 9-month prefeasibility study with the following scope:

"To evaluate the commercially available technologies for the production of motor fuels from natural gas, and to recommend to the government a proposed flowsheet for such a project, together with the expected costs and economics." The required initial product slate was for 50 percent gasoline and 50 percent diesel, and this was subsequently amended to reduce the diesel yield.

Sasolís Synthol technology was selected. Although newer technologies were well under development at the time, the Sasol Synthol process was in fact one of only two commercially proven and available technologies.

The requirement for a relatively large diesel production added a complication because the Synthol process inherently produces relatively small quantities of olefinic (and therefore poor quality) diesel. It was necessary to consider the addition of an oligomerization process to the flowsheet to increase the potential for diesel production from light olefins. This plant would be required to be able to swing between gasoline and diesel production and, when operating in the diesel mode, to produce diesel which would require minimal further work-up.

The final flowsheet for the complex can conveniently be considered in three main sections, namely synthesis gas production, synthesis and refining or product work-up. This is illustrated in Figure 1.

figure1Figure 1

Synthesis Gas Production - The synthesis gas section of the plant comprises the air separation unit producing oxygen for the secondary reformers, the reforming processes (primary and secondary), CO2 removal and hydrogen recovery.

The Synthol process ideally requires an H2/CO ratio of between 2.5 and 3.0. Steam reforming of methane tends to produce a ratio of 3 or higher and partial oxidation (or autothermal reforming) a lower ratio of about 2.0. The resulting choice was to employ combination reforming which utilizes primary steam reforming of a portion of the feed gas followed by autothermal reforming of the remaining feed gas together with the primary reformer product and recycle gas.

Hydrogen required for the downstream refining processes is recovered from a slipstream of the final synthesis gas by means of pressure swing adsorption.

Synthesis Section - While the details of the Synthol process are proprietary, some data has been published. The synthesis plant comprises three parallel streams or reactors. Catalyst can be added on-line, thus maintaining an optimum catalyst activity and overall synthesis performance.

The natural yield of oxygenated chemicals includes a wide spectrum of products including alcohols, aldehydes, ketones and organic acids. Because of the plant design criteria, all of these products with the exception of the water-soluble acids are hydrogenated back to alcohols. The original concept was to blend these alcohols into gasoline (at about 11 to 12 volume percent), but more recently the alcohols have been marketed as byproducts and the gasoline contains no oxygenates.

Tailgas processing involves the recovery of light olefins for use as feed to the alkylation and oligomerization processes and the delivery of a recycle stream of light gas back to the secondary reformers.

Product Yields - The original concept for the plant was designed around the available and sustainable gas flow rate and associated condensate and the market requirements for minimum liquefied petroleum gas, maximum gasoline and distillate (diesel) fuels and no heavy fuel oil.

Excluding the quantity of fuel derived from the associated condensate, the synthetic fuel (including alcohols) produced amounts to some 22,700 barrels per stream day, and this derives from 163.7 million standard cubic feet per day of gas. Thus the approximate liquid yield per MMscf total gas is 138 to 139 barrels. The condensate amounts to 7,500 bpsd.

Capital Costs - In order to make a meaningful comparison of the Mossgas plant concept with that of another technology, Swanepoel restricted the cost to the Inside Battery Limit (IBL) costs for the processes utilized to produce synthesis gas and F-T synthesis.

The overall constructed costs for the IBL units were escalated to second quarter of 1997 and adjusted to reflect United States Gulf Coast costs to yield a total erected cost of $880.4 million.

For ease of comparison, the capital cost can be expressed in terms of the capital investment per daily barrel of syncrude production, which in this case amounts to US$38,780 per daily barrel of capacity.

Operating costs are more difficult to derive because of the complexity of the overall plant and the need to separate the costs and then isolate only those costs associated with the syngas production units. The major components of operating cost are the cost of feed-gas and capital cost recovery (affected by the interest rate and period involved) followed by operations and maintenance costs.

The Syntroleum Process

The Syntroleum process development has been under way since the early 1980s. From the outset, the development was aimed to be broadly applied to the recovery of stranded gas reserves by lowering capital cost, thereby making potential projects economically viable at relatively small plant capacity.

The block flow diagram for Syntroleumís process includes the same basic sections as the Mossgas plant, namely syngas production, synthesis and refining or product work-up (see Figure 1).

Synthesis Gas Production - The synthesis gas section of this plant differs significantly from the Mossgas concept. First, in order to reduce capital requirements, the reforming plant employs only autothermal reforming, designed to operate on air as opposed to oxygen. The reactor is a proprietary design and is simpler than the secondary reformer employed in the Mossgas configuration. This eliminates the need for the air separation plant and the primary reformer. In place of the oxygen plant is the air and feed-gas compression train, powered by gas turbine sets designed specifically to operate on the low-BTU tailgas produced in the F-T synthesis section.

Secondly, because the nitrogen-diluted synthesis gas has the desired H2/CO ratio of close to 2, no CO2 removal facilities are required. If necessary, the synthesis gas ratio is adjusted by the addition of small amounts of steam and/or CO2 to the reactor. The physical volume of gas produced is greater due to the nitrogen dilution of almost 50 percent; however this has relatively little adverse effect on the synthesis section.

Synthesis Section - The basis used for this comparison is the fixed-bed tubular reactor design. The synthesis unit comprises the F-T reactors themselves, a reaction water recovery facility and a fairly simple tailgas separation system. As the plant is designed for once through operation, no sophisticated tailgas recovery and compression system is needed. In order to enhance the overall efficiency of the process, the low-BTU tailgas (low, because of nitrogen dilution) is burned in turbines to drive the air compression train.

The synthesis plant thus differs significantly from the concept employed at Mossgas. The Syntroleum plant operates on a once-through basis, with no recycle gas. This is achieved by the use of two reactors in series with liquid removal between the reactors. In addition, in order to operate in the high nitrogen environment resulting from the use of air in the reformers, Syntroleum has developed F-T catalyst with high activity. Lastly, the tailgas from the synthesis section is not recycled as such, reducing the need for recompression equipment and low-temperature recovery systems.

Product Yields - The product yields are derived as the basic synthetic crude, and not final products, because the final product requirements have a major impact on the degree of refining required. However, because the Syntroleum plant as described here employs a cobalt-based catalyst as opposed to the iron-based catalyst of the Mossgas Synthol system, there is a tendency toward heavier products, well into the heavy wax range beyond the diesel final boiling point. The product distribution is also different, with the F-T liquids being inherently straight-chain, and with a naphtha fraction more suited to a petrochemical feedstock than for gasoline production. The kerosene and diesel fractions are, however, of excellent quality with high smoke point and cetane numbers, respectively.

The products are defined as light F-T liquids (essentially the light ends and naphtha cuts), and heavy F-T liquids containing essentially the diesel and heavier fractions. The total liquid yield from 242.2 MMscfd of gas is 23,600 bpsd. Thus the approximate liquid yield per MMscf feed-gas for the Syntroleum process is 96 to 97 pounds.

Whereas the Synthol process as employed at Mossgas, apart from fuels production, lends itself to the production of high value chemicals derived from the relatively light olefinic products, the Syntroleum process, also in addition to fuels production, lends itself well to an alternative range of high-value products such as waxes and synthetic lubricants.

Capital Costs - The units included in Swanepoelís capital costs for Syntroleum are only those directly concerned with the production of synthesis gas and the syncrude. Costs are given for IBL facilities only; the total erected cost is $535 million.

The specific cost per daily barrel of capacity is in this case much lower at about $22,670 per daily barrel of F-T liquids capacity. One reason for this large reduction in cost is the great simplification of the reforming step. Where Mossgas employs three of the worlds largest steam reformers followed by oxygen-blown secondary reformers, Syntroleum uses a simple, single-stage autothermal reactor.

In the absence of liquid recovery from the tailgas, carbon efficiency, i.e., carbon recovered in a liquid form versus carbon in the feed for the two processes is approximately 44 percent for Mossgas and 53 percent for Syntroleum. With the addition of tailgas recovery these figures increase to 78 percent and 66 percent, respectively.

Break-Even F-T Liquids Costs

The impact of the variations in capital between differing technologies is illustrated in Table 1.

table1

These figures illustrate the importance of keeping capital costs to a minimum and, to a lesser extent, the penalty paid for higher thermal efficiencies. Gas cost could add about $4.00 to $6.00 per barrel of liquid produced for every $0.50 per million BTU. Note that remote associated gas may have a zero or negative value when the valuation includes the production of crude oil as well as natural gas.

Also, capital costs have a major impact on the economics of potential projects and modern processes, as exemplified by Syntroleumís technology, have gone a long way forward in meeting this requirement, notes Swanepoel.


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