Most process industries require the separation of solid and liquid phases, and several techniques are utilized to accomplish this. Figure below shows several types of liquid-solid separation techniques.
Gravity or centrifugal force can be used to separate materials based on their different densities, or in the case of filtration, the particle size and shape. The best method to apply will be specified by the solids concentration, feed rate, size, composition of the solid particles, the objective of separation (clear liquid or solid product), and degree of dryness of the solid required.
Figure below illustrates the variety of equipment and technique uses in relation to slurry concentration and particle size.
[1] Thickeners and Clarifiers
Both thickening and clarifying are sedimentation processes, and both processes use similar equipment. While clarifying removes a tiny amount of fine particulates to produce a clear liquid effluent, thickening’s primary objective is to concentrate a comparatively significant amount of suspended materials. When treating huge amounts of liquid, the processes of thickening and clarifying are generally inexpensive.
Flocculating agents are frequently added to thickeners to improve their separation ability.
[2] Filtration
In filtration processes, the slurry is passed through a porous filter medium in order to separate the solids from the liquid. In the chemical industry and other process industries, filtration is a common separation technique.
Woven cloth is the most popular type of filter medium, although there are many other types as well. Filter aids are frequently employed to speed up the filtration of challenging slurries. They either are applied to the filter cloth as a precoat or are mixed with the slurry and deposited with the solids to help create a porous cake.
When choosing filtration equipment, it is important to consider:
- The type of slurry and cake formed.
- The feed’s solids concentration.
- The required throughput.
- The composition and physical characteristics of the liquid, including viscosity, flammability, toxicity, and corrosiveness
- The need for cake washing.
- The required cake dryness.
- Whether it is accepted for the solid to be contaminated by a filter aid.
- Whether the valued product is a solid, liquid, or a combination of both.
Table below provides recommendations for filter selection based on slurry properties.
[3] Centrifuges
Centrifuges are categorized according to the mechanism used for solids separation:
- Sedimentation centrifuges: These centrifuges separate materials based on the density difference between the solid and liquid phases (solid heavier).
- Filtration centrifuges: which use filtration to separate the phases. The liquid filters through the cake of deposited solids and is extracted due to the porous walls of the centrifuge basket.
The type of feed and product requirements will determine whether to use a centrifuge for sedimentation or filtering.
Table below provides an overview of the key variables to consider. Sedimentation centrifuges are typically used to produce a cleared liquid, and filtration centrifuges are typically used to produce a pure and dry solid.
| Factor | Sedimentation | Filtration |
|---|---|---|
| Solid size, fine | x | |
| Solid size, > 150 micron | x | |
| Compressible cakes | x | |
| Open cakes | x | |
| Dry cake required | x | |
| High filtrate clarity | x | |
| Crystal breakage problems | x | |
| Pressure operation | ||
| High-temperature operation | will depend on the type of centrifuge used | will depend on the type of centrifuge used |
[4] Hydrocyclone
In addition to solids classification and liquid-liquid separation, hydrocyclones are utilized for solid-liquid separations. The centrifugal force is produced by the motion of the liquid in this centrifugal device, which has a stationary wall. Similar to a gas cyclone, the gas cyclone operates on much the same principles. Hydrocyclones are inexpensive, reliable separators that work with particle sizes ranging from 4 to 500 micron.
[5] Pressing
For some specialized applications, pressing—in which the liquid is squeezed (expressed) from a mass of solids via compression—is used. Pressing requires a lot of energy and should not be utilized unless there are no other viable separation options. However, there are some situations when drying and dewatering by pressing can be equally effective.
Screw presses and hydraulic batch presses are the two main categories of presses. Fruit juices are extracted using hydraulic presses, and items like paper pulp, trash, and manure are dewatered using screw presses.
[6] Solids Drying
Drying is the removal of water, or other volatile liquids by evaporation process. Most solid materials need to be dried at some point during manufacture.
Drying equipment can be categorized based on the following operational and design characteristics:
- Batch or continuous.
- Physical state of the feed: liquid, slurry, wet solid.
- Method of conveyance of the solid: belt, rotary, fluidised.
- Heating system: conduction, convection, radiation.
Tray, band, rotary, fluidized, pneumatic, drum, and spray dryers are the basic types of dryers utilized in the chemical process industries.
Table below demonstrates the fundamental characteristics of the major solids dryer types utilized in the process industries.
This post is quite long, I know. I wrote about this post because currently I am working on different project. I usually doing oil and gas project, but now it’s quite different, so I need to learn.
I hope you find this post useful as always.
Reference:
Sinnot, R.K., Chemical Engineering Design Volume 6 Fourth Edition, Elsevier Butterworth-Heinemann, 2005.
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