Options for vacuum equipment in bleaching and deodorizing

Abstract:

When designing a refinery, it is necessary to decide what operating pressure to use for the bleaching and deodorizing systems and what type of vacuum system is most appropriate. There are large variations in capital and running costs between the different possibilities.

The bleaching process is not very sensitive to the pressure in the bleaching vessel. Depending on the design, a pressure much lower than about 100 mbar risks making the oil too dry.

Deodorizer vacuum system come in four main types:

1. The ‘traditional’ system of steam jet ejectors and direct contact spray condensers in an open circuit with a cooling tower.

2. ‘Clean water’ systems similar to No.1 except that process vapors are isolated from the cooling water.

3.‘Chilled water’ systems where the water entering the spray condensers has been cooled to about 5°C to reduce the overall energy requirement.

4.‘Sublimator’ or ‘Dry Condensing’ vacuum sets freeze the water vapor coming from the process. The overall energy consumption is much lower than other systems but the capital cost is relatively high.

In deodorization the amount of steam that has to be injected into the oil falls as the pressure falls but at the same time the energy required per kg of injection steam used rises. The balance between these two effects means that for systems 1, 2 and 3 the most economical deodorizer pressure is about 3 mbar and for system 4 about 2 mbar. The exact pressure depends on the overall design and the relative costs between steam and electricity. There is no benefit in operating below the most economical pressure.