Question

Elaborate how solid particles are separated from effluent of oil industry by using filter and thickener?

Answer 1

The oil industries comprise of various unit operations which give rise to a large number of solid wastes which are removed out of the systems as suspensions and slurry. Sometimes, the solid particles are a major source of pollution and must be removed from the suspension making it free for recycling/ reusing. Filteration and sedimentation (thickening) are some of the ways to get rid of the industrial solid wastes from liquid suspensions.

FILTRATION

In this process, industrial effluents (liquid wastes) are allowed to pass through a porous media popularly known as a filter. the filter media is permeable to only the liquid and leaves the solid particle behind, making the liquid free from the suspended solids. An interesting fact to note is that, as the filtration process proceeds, the solid particles start accumulating on the filter media forming a packed bed of solid called the filter cake. This cake provides an additional resistance making the filtration more efficient but slower.

The filtration process is generally pressure-driven (in some cases gravity is used as a driving force for filtration). The filtration process can be operated either as a constant rate (of filtration) process or a constant pressure process. Since the accumulation of solids on media increases the filter media resistance, the filtration becomes slower. In order to maintain a constant rate, the pressure (driving force) must be increased uniformly with time. However, the pressure cannot be increased above the safe operating limits and hence, the filter media needs cleaning.

The cleaning and reassembling of the setup involve the following steps:

1. Cleaning the filter cake by passing fresh water at high pressure.

2. Draining of water

3. Opening of filter and disposal of cake

4. Backwashing of filter media and coating

5. Reassembling of the filter.

For better results, the filter media is coated with materials like kieselguhr.

Depending upon the type of filter cake (compressible or incompressible), the operation duration, operation condition, cleaning procedures changes. For compressible cakes, the porosity decreases with an increase in pressure and hence are preferred to be operated in constant pressure mode rather than a constant rate mode. Heavy backwashing is needed to clean the filter media involving compressible cakes.

SEDIMENTATION (THICKENING)

Industrial separations of solid wastes of high specific gravity and large sizes are done with the help of equipment like batch thickeners, continuous gravity thickeners and centrifuges. The diagram below explains the process of the batch sedimentation process using a thickener.

The slurry is left for a substantial amount of time in a thickener which leads to the formation of compressed/thickened sludge and clear liquid as shown in (d).

A continuous thickener involves similar processes but the sludge in the compressed zone, as well as the clear liquor, is removed continuously with the help of equipment like a pump. Simultaneously, fresh feed is added to the thickener. In this case, the thickener has a large area but the continuity of the process makes the procedure efficient and fast.

The extent up to which a thickener produces a clear liquid free from any particulates is defined as the clarifying capacity of a thickener.

Centrifuges are used for separation of particles when gravity separations in thickeners become ineffective. In oil industries (mostly refineries), the sulphur content in the wastewater is either removed from filters or by centrifuges. These equipment are employed only when the thickener becomes inefficient and slow.