This column is based on “Acetaldehyde Production from Acetylene,” a report published by Intratec. It can be found at: www.intratec.us/analysis/acetaldehyde-production-cost.
Acetaldehyde (also known as ethanal) is an industrially important, highly reactive aldehyde, mainly used as chemical intermediate in the production of acetic acid, peracetic acid and pyridine bases. Under certain conditions of acetylene price and availability, acetaldehyde can be made economically from that starting material.
The following paragraphs describe a process for acetaldehyde production from acetylene (Figure 1).
Hydration. Fresh acetylene is mixed with recycled water and fed to an evaporator. The resulting vapor is mixed with fresh steam, heated and directed to hydration reactors. The reaction is conducted using four fixed-bed reactors, arranged in such a way that while three reactors are operating, one is in a catalyst-regeneration cycle by air injection (not shown in the diagram).
The preheated feed mixture is split, and part of it is introduced into the first reactor, where it contacts a bed of solid catalyst. The reaction mixture effluent, comprising acetaldehyde product, unreacted acetylene and steam, is withdrawn from the reactor, mixed with steam, quenched by water and fed to the second reactor, along with more fresh feed and steam. The reaction is carried out similarly in the second reactor and, subsequently, in the third reactor.
The effluent from the third reactor is cooled and routed to the crude product compressor.
Separation. The reactor effluent is cooled, partially condensed and fed to a separator, where a flash separation between gases and liquids occurs. The liquid stream from the flash separator (mostly heavy reaction byproducts) is discarded. The gaseous stream, containing acetaldehyde and unreacted acetylene, is compressed in a multi-stage centrifugal compressor. The condensed water is recycled to the hydration reactors for use in quenching the reactor effluents.
The compressed gas is fed to the dehydration column, which further separates unreacted acetylene and acetaldehyde product from water. The column overhead — an acetaldehyde-rich stream plus acetylene — is directed to the acetaldehyde column.
A stream of essentially water is withdrawn from the bottom of the column and recycled, partly to the evaporator, and partly to the hydration reactors, for quenching reactor effluents.
Purification. In this column, a gas stream comprising unreacted acetylene is withdrawn from the overhead and mostly recycled to the hydration reactors. The remainder is discarded as offgas.
A stream of high-purity acetaldehyde product is withdrawn from the column bottom and directed to storage facilities located outside battery limits.
Acetaldehyde is produced at commercial scale mostly by liquid-phase oxidation of ethylene, and to a lesser extent, by the partial oxidation of ethanol and by the hydration of acetylene. Note that the route discussed here is economically feasible only under specific conditions of acetylene price and availability. Figure 2 presents different pathways for acetaldehyde production.
The total estimated operating cost (raw materials, utilities, fixed costs and depreciation costs) to produce acetaldehyde was about $1,640 per ton of acetaldehyde in the third quarter of 2014. The analysis was based on a plant constructed in the U.S. with capacity to produce 70,000 metric tons per year of acetaldehyde.
Edited by Scott Jenkins
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