The Texaco Gasification Process
The Texaco Gasification Process was developed in the late 1940s. Early research efforts focused on producing syngas from natural gas to produce liquid hydrocarbons via Fischer-Tropsch technology. The first commercial-scale plant based on natural gas as a feedstock was commissioned in 1950 for the production of ammonia. The first commercial-scale use of oil feedstocks occurred in 1956, and early coal work began at about the same time. In the 1970s research efforts were then focused on coal gasification (Weissman and Thone 1995).
During the succeeding 50 or more years over 100 reactors have been licensed for oil or gas service to produce nearly 100 million Nm3/d syngas. One typical reference plant was commissioned in a German chemical plant in the 1960s; with two further expansions it still operates today with a modified product slate for the synthesis gas. Another has been producing 70,000Nm3/h hydrogen for refinery purposes since the mid-1980s.
Commercial plants have been built at pressures of up to 80 bar and experience with unit reactor sizes of up to 3.5 million Nm3/d synthesis gas is now available from the ISAB installation in Sicily.
The oil feedstock is mixed with the moderating steam and preheated in a fired heater. The Texaco burner (Figure 5-24) is of a water-cooled design in which steam and oil are fed together through an annular slit surrounding the central oxygen pipe. The process steam is used to atomize the oil, and mixing is ensured by imparting a counter-rotating vortex motion to the two streams (Pelofsky 1977; Brejc 1989).
The reactor itself is an empty, refractory-lined vessel. The soot make is 1-2 wt% based on feed flow (Appl 1999).
|
Figure 5-24. Texaco Oil Burner (Withpermission: ChevronTexaco) |
Texaco offers two different syngas cooling options: one by direct quenching with water, and another by using a syngas cooler to generate steam (see Figure 5-25).
In the quench mode the hot, raw syngas leaves the bottom of the reactor via a dip — tube into the quench section. The quenched syngas is saturated with water and leaves the quench section with a temperature of about 250°C. At an operating pressure of, say, 80 bar, this corresponds to water loading in the gas of about 2 kg H20 per Nm3 of gas. This high water loading makes the quenched gas suitable for CO shift conversion without further steam addition. The quench mode of syngas cooling is, therefore, Texaco’s preferred mode for hydrogen and ammonia manufacture.
The quench removes the bulk of the solids in the gas, and these are extracted from the quench vessel as a soot-water slurry or “black water.”
Texaco usually uses the syngas cooler mode in applications where a high CO content is required (e. g., oxo synthesis gas) and where the high steam loading of a quenched gas is of no advantage. For intermediate requirements in the H2/CO ratio, such as methanol synthesis gas, a combination of quench and waste-heat boiler cooling is possible (Jungfer 1985).
5-26, which shows the quench configuration, the gas leaves the quench vessel and is then scrubbed with water twice, first in a Venturi scrubber and then in a packed column, to remove final traces of soot. The raw gas is then suitable for subsequent treatment in downstream units, such as CO shift and acid gas removal.