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Theory

Equipment

Knock-out vessels

After the first lesson, which looked at safety devices, in this lesson we willexamine the remaining equipment used in the process industry and particularlyin the petrochemical industry.This lesson aims to provide a better insight into a number of important methodsof separating mixtures (phase separation) and will explain the principle ofoperation of the three most important devices used to accomplish separations.These devices are collectively known as knock-out vessels, as the separation isbased on different behaviour of the different phases during a (rapid) passagethrough the device.

Contents of the lesson

1 Phase separation

2 Cyclones

3 Demisters and tower scrubbers

4 Venturi scrubbers

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Lesson

1. Phase separationIn physics, we distinguish three physical states: the liquid state, the gaseous stateand the solid state. These physical states (also referred to as phases) areindicated by the letters L (liquid), G (gas) and S (solid).There are three two-phase mixtures:- liquid-gas mixtures L-G;- liquid-solid mixtures L-S;- solid-gas mixtures S-G.

This lesson is concerned with those cases where a small quantity of one phase ispresent in the other phase.See table 1.

Table 1Mixturesmuch little designationG L mistL S suspensionG S smoke

A small quantity of liquid in a gas is called mist. A small quantity of a solid in aliquid is called a suspension and a small quantity of a solid in a gas is calledsmoke.

In this lesson, we will examine a number of devices by means of which thesesmall quantities of contaminating substances can be separated.This separation is of great importance. A compressor can be severely damaged ifthe gas drawn in is not free from liquid. A contaminated sealing liquid in amechanical seal in rotating equipment can lead to wear. Solids in dischargedgases, which are otherwise clean, can lead to air pollution.

Question 1Which three physical states of substances can we distinguish?

Question 2What is mist; what is a suspension; what is smoke?

- physical states phases

- mixtures

- mist- suspension- smoke

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2 CyclonesA cyclone is a separation device by means of which all three types of separationof importance to us can be carried out. The separation is based on thedifferences in specific density between the phases. Figure 1 shows what acyclone looks like.

The example we will examine is a gas containing a small quantity of solids(smoke). This mixture is introduced tangentially (in a direction touching, but notintersecting the periphery), at high velocity at the top of the cyclone tangent tothe periphery. Due to the tangential inlet, the smoke is forced to perform arotating movement along the cyclone wall, causing the smoke to movedownward in a spiral motion. Consequently, the smoke moves towards thecentre and finally leaves the cyclone via the central pipe. Due to this rotatingmovement of the smoke, the solid particles encounter a centrifugal force. Thesolid particles 'go off the road' as it were, due to their higher specific density.This is what accomplishes the actual separation. The solid particles accumulatealong the outer wall, are decelerated due to the contact with the wall andgradually drop down and collect at the bottom of the cone. There, the dust isdischarged by opening a special valve at regular intervals.

5590-020-001

Figure 1Cyclone

The same explanation applies to suspensions and mists.

Question 3Explain the principle of operation of separation by means of a cyclone.

There are two ways of supplying substances to a cyclone. Either the mixture isdrawn in via the cyclone or the mixture is forced through the cyclone.

If a mixture, a smoke for instance, is drawn in through the cyclone, there is theadvantage that no solid particles enter the fan used to suck in the mixture. Thereis the disadvantage, however, of an underpressure in the cyclone, with the resultthat any small leak in the cyclone will disturb the dust deposit, possibly severelyimpairing the operation of the cyclone.

- centrifugal force

- draw in

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If the mixture, the smoke in this case, is forced through the cyclone, one has thecertainty that greater wear will occur in the fan used to force the mixture to thecyclone, and that in the case of leakage in the cyclone, dust will get into theenvironment. If it is a toxic or harmful substance, this method should always beavoided.

Question 4What is the disadvantage of drawing in a smoke through the cyclone itself?

3 Demisters and tower scrubbersA demister, as the name implies, is a device in which a mist can be separatedinto a dry gas and a liquid. The separation takes place in a wire mesh pad (seefigure 2).

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Figure 2Demister

The mist flows through the wire mesh pad, where, due to the high velocity, thedroplets of liquid collide with the wire mesh and adhere to it. The drops on thewire mesh accumulate and slowly drip downwards through the wire mesh. Thelarge drops fall from the bottom of the wire mesh pad and are discharged underthe demister.

To improve operation, it is possible to let a small flow of liquid descend on thewire mesh pad by means of a sprinkler. In this way, this device can also be usedto clean a small quantity of solids in a gas (smoke) (see figure 3). If a flow ofliquid is used, we speak of a tower scrubber.

- force

- wire mesh pad

- sprinkler

- tower scrubber

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5590-020-003

Figure 3Tower scrubber

Here too, the actual separation takes place in the wire mesh pad. The solidparticles colliding with the wire mesh are washed away by the liquid. The liquidfilm on the wire mesh therefore collects the dust particles.

Due to the operation of the tower scrubber, more and more solids willaccumulate in the water system. It is therefore necessary to continuallydischarge some of this mixture and to replenish with fresh water. The dischargeof water is controlled in such a way that the water level in the vessel is keptconstant.

Question 5Which mixture is separated in a demister?

Question 6When do we speak of a tower scrubber?

4 Venturi scrubbersIn practice, a venturi scrubber is used only to clean smoke (gas + solid). Thesmoke is drawn in by dosing a sharp jet of water in a venturi; the drops of waterof the water jet will catch the particles of dust, as it were. This contaminatedliquid can easily be separated in a cyclone (according to the principle discussedearlier), see figure 4.

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5590-020-004

Figure 4Venturi scrubber

A venturi scrubber therefore consists of a venturi and a cyclone. In the venturi,the actual catching of the dust particles takes place, and the dust caught in wateris separated in the cyclone. The water is therefore an auxiliary substance in theseparation.

Question 7Which two devices are combined in a venturi scrubber?

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SummaryIn this lesson we discussed devices for the separation of mixtures into same-phase substances. The separation is of importance among other things to extendthe life of other equipment or to reduce maintenance to a minimum.The types of mixtures with corresponding separation devices can be summarisedin a table as follows:

mixture name of mixture separation devicesmuch littlegas liquid mist cyclone + demister + tower

scrubberliquid solid suspension cyclonegas solid smoke cyclone + tower scrubber +

venturi scrubbers

A cyclone is suitable for all three mixtures and is based on the principle of thecentrifugal effect.

The demister can operate without external supply of liquid and is then suitablefor the separation of liquid from gas. If the demister is designed with sprinklersover the wire mesh pad, it is also suitable for the separation of smoke and is thencalled a tower scrubber.A venturi scrubber is a combination of a venturi with water injection and acyclone. It is used to separate smoke into gas and solid particles.

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TestExercisesDo not send in your answers for correction

1. What is the disadvantage of forcing a mixture through a cyclone instead ofdrawing it in?

2. Which three physical states of matter do we distinguish and what are theycalled?

3. What is mist, what is a suspension, and what is smoke?

4. When do we speak of a tower scrubber?

5. Which two devices are combined in a venturi scrubber?

Answers to the questions in the lesson1. Solid, liquid and gas.

2. Mist is gas with a (small) quantity of liquid.A suspension is liquid with a (small) quantity of solid matter.Smoke is gas with a (small) quantity of solid matter.

3. Due to the centrifugal effect, the substance with the biggest specific densityis thrown against the cyclone wall.

4. Due to (small) leakages in the cyclone, the formation of the dust deposit canbe disturbed, leading to less than optimal phase separation.

5. Mist or the mixture of gas with liquid.

6. If a demister is fitted with a liquid circulation system with sprinklers over thewire mesh pad.

7. A venturi device and a cyclone.

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Answers to the exercises1. Severe wear of the fan that must build up pressure, and in the case of leaks in

the cyclone, dust will enter the atmosphere.

2. Solid, liquid and gas. These phases are called physical states.

3. Mist is gas with a (small) quantity of liquid, a suspension is liquid with a(small) quantity of solids and smoke is gas with a (small) quantity of solids.

4. If a demister is fitted with a liquid circulation system with sprinklers over thewire mesh pad.

5. A venturi device and a cyclone.

Problems and assignmentsAnswer and send in for correction

1. Describe the principle of separation of mist (gas with liquid).

2. Explain the principle of operation of a venturi scrubber.

3. Which device or devices are used to separate: mist, smoke, a suspension.

4. Explain the principle on which separation by means of a cyclone is based.

5. Which mixture is separated in a demister?

TheoryEquipmentKnock-out vesselsContents of the lesson

LessonPhase separation2CyclonesWhat is the disadvantage of drawing in a smoke through the cyclone itself?

3Demisters and tower scrubbers4Venturi scrubbersSummaryTestExercisesAnswers to the questions in the lessonAnswers to the exercisesProblems and assignments