Filtration and filters

In many industrial processes it is necessary to separate finely divided solid materials from liquids. Filtration is a mechanical or physical operation which is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass. Filtration is also important and widely used as one of the unit operations of chemical engineering. It may be simultaneously combined with other unit operations to process the feed stream. The process of filtration consists in passing the liquid through a porous medium, which retains the solid particles. In some cases the recovery of the solid material is the main object; in others the filtration is done for the sake of purifying the liquid.

The mixture to be separated is called the slurry: the fluid that passes through the septum is called the filtrates and the septum is called the filter medium, and the equipment assembly that holds the medium and provides space for the accumulated solids is called a filter. The fluid may be a gas or a liquid. The solid particles may be very fine, and their concentration in the suspension may be extremely low (a few parts per million) or quite high (>50%).

When the separated solids accumulate in amounts that visibly cover the medium, they are called the filter cake or simply the cake.

Filter media are manufactured from cotton, wool, linen, silk, glass fibre, porous carbon and other solids, rayons and other synthetics, porous rubber, etc. Many types of filters are in use today. They differ in respect to the various materials which are available for filtration, and the construction of the apparatus in which these are applied.

Filters used in chemical processing are classified into two distinct groups: continuous and intermittent. Filters that operate without interruption for weeks to months are classified as continuous filters. The main types are the rotary drum and disk filters and various horizontal designs.

Rotary drum filters are the most widely used continuous filters in the chemical process industries. The design provides means for concentrating slurry solids to dry (moist) cakes, washing solubles from such cakes when required, and producing a clarified effluent.

Many design configurations have been developed to utilize most effectively the basic principle for the varying filtration characteristics of slurries, ranging from extremely fine particles to very coarse particles (50 - 150 mm) and from thin, sticky cakes to thick, fairly dry sludges. These filters are used in some form in practically every process operation involving slurries where solids must be recovered from the liquor for further processing.

Rotary disk filter is a version of the rotary drum filter. A series of parallel disks provides vertical filtering surfaces in place of the cylindrical surface on the rotary drum. Thus, the rotary-disk unit provides more filter area (up to 3.5 times) than a drum type. When the filtering surface operates in a horizontal plane, the filter is designated as a horizontal type.

Filters whose continued vibration is limited by their solid-collecting capacity are known as intermittent filters. Their cycle must be interrupted periodically when the solids accumulation has to be discharged.

3. Answer the questions to the text:

1. What is filtration? What does the process of filtration consist in?

2. Why may filtration be combined with other unit operations?

3. What is the main object of filtration?

4. What is a filter?

5. What is a filter cake?

6. What are filter media manufactured from?

7. What are continuous filters? How do continuous filters operate?

8. What are the main types of continuous filters?

9. What does the design of a rotary drum filter provide?

10. What are intermittent filters? How do intermittent filters operate?

4. Match the words from the text with their synonyms on the right:

1) operation	a) aim	1)
2) simultaneously	b) persistent	2)
3) retain	c) restore	3)
4) object	d) grain	4)
5) coarse	e) at the same time	5)
6) cake	f) construction	6)
7) apparatus	g) action	7)
8) continuous	h) rough	8)
9) design	i) sediment	9)
10) particle	j) settling	10)
11) sludge	k) device	11)
12) recover	l) hold	12)

5. Fill in the gaps with the words from the text and translate them into Ukrainian:

	Ukrainian translation
1. to process the feed __________
2. to provide __________ for the accumulated solids
3. visibly cover __________
4. two __________ groups
5. operate without __________
6. various __________ designs
7. producing __________ effluent
8. must be recovered from __________ liquor
9. solid-collecting __________

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