Jan-1998

Relative economics of mixed C4 processing routes

Ethylene producers are seeking ways to upgrade the mixed C4 stream from ethylene plants to improve operating economics

Stacia M Edwards, Stephen J Stanley and Margaret M Shreehan, ABB Lummus Global

Article Summary

Ethylene plants cracking liquid feedstocks produce a stream rich in valuable unsaturated C4s such as butadiene, isobutylene, butene-1, and butene-2. There are numerous processing routes available to recover these components and/or convert them into final products such as C4 LPG, MTBE, raffinate for alkylation, MEK and propylene.

These processes can be combined in various ways depending on the demand for the products as well as the economics of the region.
As shown in Figure 1, some of the processes and product options available are:
- Butadiene extraction
- Selective hydrogenation
- Total hydrogenation
- MTBE /skeletal isomerisation
- Butene-1 recovery
- Disproportionation (Triolefin process)
- Isobutylene (via MTBE decomposition)
- MEK synthesis
- Secondary butyl-alcohol.
 
The purpose of this article is to present processing options of interest today and to review the estimated economics for those options. The processing routes selected for comparison are:
Scheme 1. Total hydrogenation of the C4 stream to produce either LPG product or recycle feed for the cracking heaters in the ethylene plant.

Scheme 2. Butadiene extraction, with sale of the remaining C4s, hydrogenation of the C4s for recycle to the cracking heaters, production of an LPG product.

Scheme 3. Selective hydrogenation of the butadiene to predominantly butene-1 followed by MTBE production from isobutylene. Additional isobutylene is made in a skeletal isomerisation unit from the unreacted butenes.

Scheme 4. Same as Scheme 3 but with recovery of some of the butene-1 contained in raffinate from the MTBE unit.

Scheme 5. Same as Scheme 4 but with the remaining butene-2 sent to a disproportionation unit for production of propylene. Since butene-2 is a desired product, skeletal isomerisation is not included.

Scheme 6. Selective hydrogenation of the butadiene with the C4s going directly to a disproportionation unit.

While only these six schemes will be evaluated, there are still other processing options. For example, in Schemes 3, 4, and 5, the MTBE can be cracked back to isobutylene for polyisobutylene or MMA (methylmethacrylate) production. Also, the raffinate from the MTBE unit can be used as feed to an alkylation unit and the butene-2, after butene-1 recovery, used to make secondary butyl-alcohol or methyl ethyl ketone.

Process descriptions
A brief description for each processing option follows.

Butadiene extraction
A typical butadiene extraction process consists of two stages of extractive distillation, resulting in a raffinate containing butenes from the first stage overhead and a crude butadiene product from the second stage overhead. The solvent, loaded with acetylenes, is recovered in a degassing system and recycled to extractive distillation.

The crude butadiene is further purified using conventional distillation technology to yield 1,3 butadiene of greater than 99.7 per cent purity.

Selective hydrogenation
A selective hydrogenation process is applied for the hydrogenation of butadiene in mixed C4 streams with a design residual of 10 to 2000 wt ppm butadiene in the product. The catalysts utilised are very selective, offer high yield, and allow for flexibility in the butene-1/butene-2 ratio. Loss to n-butane varies depending on the residual butadiene requirements, but is typically less than 4 per cent with no isobutylene loss.

Selective hydrogenation can be done in a typical fixed-bed unit consisting of one or two adiabatic reactors, depending on the residual butadiene requirement. It can also be accomplished by combining the hydrogenation reaction and distillation tower into a single unit operation called catalytic distillation.

 In the latter case, a debutaniser operates like a conventional debutaniser with two significant differences. The rectifying section of the tower contains a special packing instead of trays or conventional distillation packing. This packing contains conventional hydro-treating catalyst as well as serving as the distillation packing.

These units operate under similar conditions as conventional towers. The estimated cost of a catalytic distillation tower is within 20 per cent of the cost of a conventional distillation tower. One of the main advantages of incorporating hydrogenation in a distillation column is all of the equipment normally associated with a fixed-bed selective hydrogenation unit is eliminated, resulting in lower capital costs.
Total hydrogenation

Total hydrogenation of C4s or C4s and C5s from a steam cracker with recycle to the cracking heaters has been incorporated into more than half the new ethylene plants designed since 1990. Total hydrogenation and recycle of the C4s and C5s reduces fresh feed consumption in a naphtha cracker by 13 per cent and in a propane cracker by 7 per cent.