Clean Gas Shift
Conventional (high temperature) shift uses an iron oxide-based catalyst promoted typically with chromium and more recently with copper. The operating range of these catalysts is between 300 and 500°C. Much above 500°C sintering of the catalyst sets in and it is deactivated. HT shift catalyst is tolerant of sulfur up to a practical limit of about 100 ppm v, but it is likely to loose mechanical strength, particularly if subjected to changing amounts of sulfur.
An important aspect in the design of CO shift in the gasification environment, where inlet CO contents of 45% (petroleum residue fed) to 65% (coal) are common, is the handling of the heat of the reaction, particularly under end-of-run conditions where an inlet temperature of 350°C or more may be necessary. On the one hand, the reaction must be performed in several stages to avoid excessive catalyst temperatures and to have an advantageous equilibrium. On the other hand, optimum use must be made of the heat.
One such arrangement is shown in Figure 8-12. Desulfurized syngas containing about 45 mol% CO, which leaves the AGR at about 54 bar and ambient temperature, is heated and water saturated at a temperature of about 215°C by water that has been preheated with hot reactor effluent gas. The saturated gas is further preheated to the catalyst inlet temperature of between 300°C and 360°C. The steam loading from the saturator is such that only the stoichiometric steam demand for the reaction is required to be added from external sources. In the first stage, the CO is reduced to a
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Figure 8-12. CO Shift with Saturator-Desaturator Circuit (Source: Higman 1994) |
level of about 7-8 mol% at an outlet temperature of about 500°C. The outlet gas is cooled to a temperature of about 380°C in the gas and water preheaters before entering the second catalyst bed. Here the residual CO is reduced to about 3.2mol%. The gas is then cooled in a direct-contact desaturator tower. There are a number of different designs, particularly for the first reactor, that incorporate the gas-gas heat exchanger as an internal. In such reactors, the exchanger is arranged centrally inside an annular catalyst bed with an axial (Lurgi) or axial-radial (Casale) gas-flow pattern. Alternative methods of controlling the catalyst outlet temperature include interbed condensate injection (e. g., Toyo). The use of an isothermal steam raising reactor has been proposed, and although such a solution has been employed in a steam reformer plant, none is recorded at the high CO inlet concentrations involved in a gasification plant.
Typical catalyst lifetime for the first bed in a gasification situation is two to three years, which is considerably shorter than for a steam-reforming situation. This is generally attributed to the high operating temperatures associated with high CO concentrations in the inlet gas. On a moles-converted basis over the lifetime of the catalyst, the performance in the gasification context is comparable with that of steam reforming.
Low Temperature (LT) Shift
Low temperature shift operates in the temperature range 200°C to 270°C and uses a copper-zinc-aluminum catalyst. It is used in most steam reforming-based ammonia plants to reduce residual CO to about 0.3 mol%, a requirement for a downstream methanator, but has generally not been applied in gasification-based units. On the one hand, it is highly sulfur-sensitive, and even with 0.1 ppmv H2S in the inlet gas, will over time become poisoned. A second reason for its lack of use particularly in oil — gasification plants, is the effect of the higher pressure on the water dewpoint in the gas. Operation near the dewpoint will cause capillary condensation and consequent damage to the catalyst. With a dewpoint of about 215°C and a temperature rise of 25-30°C, there is not much margin for error below the upper temperature limit of 270°C when recrystallization of the copper catalyst begins. The first application of low temperature shift at high pressure was in Shell’s Pernis gasification facility, which has now performed successfully for several years (de Graaf et al. 2000).
Medium Temperature (MT) Shift
An improved copper-zinc-aluminum catalyst able to operate at higher exit temperatures (300°C) than conventional LT shift has been developed, particularly for use in isothermal reactors. No application in gasification plants is known.