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Separator technology reduces refinery operating costs and downtime

A US refining company contacted us to discuss a separation problem in the feed to an alkylation process. Caustic entrainment from their selective hydrogenation unit was poisoning the hydrofluoric acid catalyst in the alkylation unit. The continuous phase feed to the alkylation unit consisted of ~180m3/hr of mixed C4 hydrocarbons, mainly butanes and butylenes, with varying levels of 10 – 20% caustic solution entrained as a dispersed aqueous phase.

Sulzer was commissioned to design, engineer and manufacture a solution to the problem which would reduce the caustic entrainment to less than 30 ppmv.

The Solution
Examination of the process upstream of the alkylation reactor revealed that separation of the caustic entrainment should have occurred in a gravity separator. Calculations suggested that the gravity separator would be capable of droplet removal down to about 60 microns. However, the low interfacial tension in the hydrocarbon/caustic system was likely to result in much more finely dispersed droplets and coalescer internals were therefore recommended.

Originally planned as a retro-fit project for the gravity separator, the customer decided that potential contamination with hydrofluoric acid presented too many safety issues and a decision was made to replace the vessel with a new skid-mounted coalescer complete with prefilters and caustic flash drum.

The main feature of the package was a 78” diameter horizontal separator vessel complete with Enhanced DC CoalescerTM internals and Enhanced flow distribution device.

The Enhanced DC Coalescer was manufactured as a co-knit composite packing of 316L SS and special multifilament fluoro-polymer fiber (PTFE). The combination of materials with different surface free energies provides accelerated rates of droplet coalescence at each of the thousands of point contacts between the materials – a property of the Sulzer DC Coalescer originally patented by KnitMesh Ltd (now a part of Sulzer Chemtech). The use of the fibrous PTFE multifilament provides exceptional direct interception efficiency for very small droplets, allowing them to accumulate, coalesce and eventually drain to the bulk phase interface in the boot of the vessel.

Two prefilters with 10 micron cut-off size were provided as part of the package to protect the coalescer from particulate contamination in the feed.

Results: The complete package was engineered, manufactured and delivered within a 15 week lead time. Installation of the equipment has reduced the entrainment levels from the original 10-20% to the target levels of < 30 ppmv providing the operator with substantial benefits in terms of:
- Reduced downtime
- Increased catalyst life
- Reduced catalyst cost

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