Operating Constraints Within a Refinery Environment

It is important to consider the integration of an ETBE unit within an operating refinery complex. In particular, only feeds that are readily available can be used and consideration must be given to the life-cycle of a unit (i.e. the time required between periodic maintenance or catalyst replacement).

High purity isobutene is generally not available within a refinery complex, nor is it necessarily desirable as it makes control of the reactor temperature difficult (more heat is released per unit of feed for high purity feeds). However, isobutene is present in significant concentrations in the products of several refining processes, particularly steam cracking and catalytic cracking. The other components present in the products of these units are usually other C4 hydrocarbons and mostly olefinic. A typical steam cracker product will contain 40% isobutene, 30% n-butenes and 30% butadiene while a typical catalytic cracker C4 product will contain 15% isobutene, 35% n-butenes, 40% isobutane and 10% n-butane (Miracca, 1996; Jensen, 1996). These compositions will vary somewhat with the crude source, and also the type and activity of the catalyst used in catalytic cracking. Isobutane dehydrogenation can also be the source of isobutene and very high purities (95%+) are sometimes achievable, in which case dilution with another C4 stream is desirable for etherification.

Ethanol, whether sourced from renewable material (biomass) or from ethylene, is usually manufactured to a high purity to essentially eliminate water. Azeotropic ethanol is widely available but is not recommended for etherification processes as the water reacts quickly with isobutene to form isobutanol While isobutanol can itself be used as an octane enhancer and oxygenate, ils properties make it less suitable than ETBE for gasoline blending. The commercial value of isobutanol is also lower than ETBE, so that the hydration reaction should always be minimised.

The fluids involved in the ETBE process are mostly non-fouling and non-corrosive so that the life-cycle of a unit would most likely be determined by the degradation of catalyst activity in either a fixed-bed reactor or the reactive distillation column. The catalyst is poisoned by salts, basic compounds such as nitriles, and sulphur compounds that may be found in potential hydrocarbon feed streams. Consequently, a guard chamber (or similar pretreatment equipment) is essential to remove contaminants before they reach the reactor(s). This equipment would take on added importance in a reactive distillation unit as the task of replacing the catalyst inside a column is significantly more difficult.

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