Бурение грунтовых зондов, установка энергетических колодцев

There are considerable attractions to producing liquid hydrocarbon fuels from remote sources of natural gas. On the one hand, it provides a means of bringing energy resources from remote locations to the market, in a form that is not limited by the small number of receiving terminals, as is the case for LNG, but that allows the utilization of the existing large and flexible infrastructure in place for the transport of liquid hydrocarbons.

On the other hand, the quality of Fischer-Tropsch products enable them to be sold at a premium price. All FT products are sulfur-free (typically <1 ppm), but particularly the high-quality diesel cut with no aromatic content and a cetane index of over 70 can contribute significantly to achieving the U. S. Environmental Protection Agency (EPA) standards valid from 2006 (Mulder 1998; Agee 2002).

The basic Fischer-Tropsch process produces a mixture of straight-chain hydrocarbons from hydrogen and carbon monoxide according to the reaction

CO+ 2 H2=-[CH2]-+ H20 -152MJ/kmol (7-5)

where -[СЕЩ — is the basic building block of the hydrocarbon molecules. The prod­uct mixture depends on the catalyst, the process conditions (pressure and tempera­ture), and the synthesis gas composition. The product slate follows the Schulz-Flory distributions. The selectivity of two typical processes is shown in Table 7-6. See also Tables 7-7 and 7-8.

Different FT syntheses require different H2:CO ratios in the syngas. Furthermore, additional hydrogen is often required for product work-up. These differences demand an individual approach to syngas generation for each project, depending on

the synthesis process selected, the desired product slate, and the product work-up scheme (Higman 1990).

Proven and operating syngas production routes from natural gas include partial oxidation and combined reforming (steam reformer followed by an oxygen-blown secondary reformer as used for methanol production). Both routes are described in Higman (1990). In principle, an autothermal reforming or gas-heated reformer — based scheme can also be applied, although no plant of this nature has yet been proven at the sizes likely to be required for a world-scale GTL facility. In conform­ity with the scope of this book, however, our example is partial-oxidation-based.

A typical specification for Fischer-Tropsch syngas is shown in Table 7-9. From this we can see that the gas must be sulfur-free, since sulfur is a catalyst poison. Whereas with catalytic reforming processes the desulfurization must be performed upstream of gas generation to protect the reforming catalyst, with partial oxidation one also has the option of desulfurizing in the syngas. In fact, syngas desulfurization has a number of advantages over natural gas desulfurization. First, organic sulfur

species in the natural gas are converted to H2S (and traces of COS) in the partial oxi­dation reactor, thus obviating the need for an upstream hydrogenation stage com­plete with hydrogen recycle. Second, the syngas from a typical partial oxidation unit has a very high CO partial pressure and high Boudouard equilibrium temperature (about 1050°C), which makes it an extremely aggressive metal dusting agent. Syngas desulfurization leaves the sulfur in the syngas while it is in the dangerous temperature range. The sulfur is therefore able to act as an effective corrosion inhibitor. And third, sulfur-free synthesis gas can be subject to spontaneous methanation at temperatures above about 400°C, given the right conditions. Against these benefits is the fact that the volume of the synthesis gas to be treated is about three times that of the natural gas feed. The level of desulfurization is therefore less, and equipment will tend to be larger. Nonetheless, in applications with waste-heat recovery, which would be typical for a Fischer-Tropsch environment, the authors would recommend syngas desulfurization.

The Fischer-Tropsch synthesis produces methane and other light fractions that, depending on the tail gas recovery arrangement, may contain olefins. Some of this stream may be recycled to the partial oxidation unit for reprocessing to synthesis gas. A 100% recycle is, however, not possible, since the inerts (argon and nitrogen) will build up excessively in the recycle loop. In a reformer-based system it is pos­sible to create an inerts purge by using FT tail gas as reformer fuel. In the partial oxidation scenario, hydrogen production provides the opportunity for a purge, as shown in Figure 7-8. Care should be taken, however, with FT tail gas as reformer feed. The olefin content will tend to coke on the reformer catalyst. CO in the tail gas may methanate on the catalyst for olefin hydrogenation. Nonetheless, with suitable pretreatment, tail gas can be used as reformer feed.

The overall flowsheet can be seen in Figure 7-8. Natural gas is fed directly to the partial oxidation unit undesulfurized. (In this context, it is assumed that any bulk sulfur removal, LPG recovery, or the like has been conducted at the wellhead. Clearly this has to be taken into account in the economics of a project for handling remote gas, for which no wellhead treatment would otherwise be available. See Chapter 8.) Desulfurization takes place on a zinc-oxide/copper-oxide adsorber bed in the syngas stream before the latter is fed to the synthesis unit. In the tail gas

recovery unit, light, gaseous products are recovered and partially recycled. The rest is used for hydrogen manufacture. Heavy oils and waxes are then hydrotreated as part of the product work-up.

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