Basic Separation Theory

Competence development

Basic separation theory Printed

Mar 2010

Book No

EPS004-E-1

Alfa Laval reserves the right to make changes at any time without prior notice. Any comments regarding possible errors and omissions or suggestions for improvement of this publication would be gratefully appreciated. Copies of this publication can be ordered from your local Alfa Laval company. Published by:

Alfa Laval Tumba AB Competance Development SE - 147 80 Tumba Sweden

© Copyright Alfa Laval Tumba AB 2010. Original instructions

Contents 1

Fundamental concepts ........................................... 5 1.1 1.2 1.3 1.3.1 1.4 1.4.1 1.4.2 1.4.3 1.5 1.6

2

Improving separation efficiency ..................... 13 2.1 2.1.1 2.1.2 2.2 2.2.1 2.2.2 2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6

3

Gravity separation .......................................... ..........5 Laminar and turbulent flow .......................... .........6 Capacity flow..................................................... .........7 Settling velocity ..................................................... ..........8 Parameters in Stokes law ............................ ..........9 Particle size ........................................................... ..........9 Density difference.................................................. ..........9 Viscosity................................................................. ..........9 Stokes' law ...................................................... ..........10 Separation efficiency vs flow and area . ..........11

Baffle plates flow .......................................... ..........13 Why baffle plates matter...................................... ..........14 Summing up ........................................................ ..........15 Centrifugal separation.................................. .........16 Redesigning the vessel ....................................... ..........17 From gravitational to centrifugal force................. ..........18 High speed separator (HSS) design ........ ..........19 The Disc stack..................................................... ..........19 Flow between discs............................................. ..........20 The tank............................................................... ..........21 The divided tank .................................................. ..........22 Adding oil in the divided tank.............................. ..........23 Increasing sufficiency with centrifugal force ...... ..........26

Bowl and application .............................................. 27 3.1 3.2 3.2.1 3.2.2 3.3 3.4 3.5 3.6 3.6.1 3.6.2 3.7

The separator ................................................. ..........27 Basic separation principles ........................ .........28 Separator............................................................. ..........28 Clarifier / Purifier .................................................. ..........29 Operational problems .................................. ..........30 Purifier bowl.................................................... ..........31 Clarifier Bowl ................................................... .........32 Factors affecting the interface ................. .........33 Interface moving inwards .................................... ..........33 The interface moving outwards ........................... ..........34 How to find the right gravity disc ............ ..........35

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DNVPS / ALFA LAVAL FUEL OIL TREATMENT

4

Separation efficiency .............................................. 37 4.1 4.2 4.2.1 4.2.2 4.3 4.4

5

Summary .......................................................................... 43 5.1 5.2 5.3 5.4 5.4.1

4

Separator vs Filter ........................................ ..........37 Particles we separate/ don´t separate ... ..........38 Components in oils not affected .......................... ..........38 Components in oils affected ................................ ..........39 ISO standard 8217 ........................................ ..........40 Catfines ............................................................. .........41

Optimum interface ........................................ ..........43 Temperature ................................................... ..........43 Stoke’s law. ..................................................... ..........44 Separator limitations .................................... .........45 Density limits........................................................ ..........45

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BASIC SEPARATION THEORY

1 FUNDAMENTAL CONCEPTS

1 Fundamental concepts The aim of the course is to present some basic concepts of separation and explain how the High Speed Separator works. We will start with reviewing some fundamental concepts concerning separation, which will lead us to the formulation of Stokes' law.

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Gravity separation

We will also look into how we can improve separation efficiency, describe the bowl and application and in the end we have a look at separation efficiency.

•

Starting point- A liquid with sludge

•

The density differs

•

The heavier particles sink because of the

ρ particle > ρ liquid

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1.1 Gravity separation Let’s start to look at a batch tank with a liquid that contains solid particles, i.e. sludge. What is the root cause to the fact that the sludge particles settle? The settling is due to the difference in density between the particles and the liquid. Density is denominated (”rho”). The heavier particles sink to the bottom of the tank because of the force of gravitation.

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We can also identify two important parameters in this simple setup. The longer the time, the greater the separation. We can also see that different particles settle at different speeds. The smaller the particles, the more time required before they settle. This important phenomenon is a part of Stokes' law, which will be discussed later on.

force of gravitation.

•

The longer the time, the greater separation

•

The smaller the particles , the more time required to settle

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BASIC SEPARATION THEORY

1.2 Laminar and turbulent flow If we instead set up a continuous flow tank where the sludge remains at the bottom of the tank. The flow enters the tank to the left and exits to the right. Q denominates the flow rate. But why is it that the particles stay at rest after that they have reached the bottom? Why are they not swept away towards the outlet due to the liquid flow? Furthermore, does sludge always settle irrespective of the flow rate or the tank design? The answer to these questions is important for our understanding of how a separator works

Fig.1.4. Laminar flow

The red arrows indicate the speed of the liquid flow at different depths. The closer we get to the bottom of the tank, the more slowly the liquid phase moves. This is the natural behavior of what is known as “laminar flow”, i.e. a relatively slow and steady flow. (Fig. 1.4) The tank to the right illustrates a much higher Fig.1.5. Turbulent flow flow rate. When the flow rate is increased to a certain level, “turbulent flow” arises. (Fig. 1.5) In a turbulent flow, the liquid constantly sweeps the particles along due to the quite fierce liquid movement throughout the tank. Summing up One of the things to be understood from this is that we have to limit the flow in a continuous separator so that the flow stays laminar. Laminar flow will allow the sludge to settle in the tank.

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At moderate flow rates, the particles will fall to rest in a separation tank due to the fact that the liquid flow will vary along the vertical axis of the tank, i.e. the flow varies depending on how deep we are looking in the tank.

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1 FUNDAMENTAL CONCEPTS

1.3 Capacity flow Separation capacity is important for any kind of separator and for a continuous tank we will here set up an equation for it. Let’s say that we have a certain demand for purity at the outlet. There is then a flow limit at which that purity can be reached. Let’s call this limit flow rate Qc, where the subscripted “c” stands for capacity. A flow rate higher than Qc will give less separation than required. Then we can formulate the following expression for the capacity flow, Qc. Qc = vg ⋅ A

The equation states that the capacity of a continuous gravity separator is proportional to the settling velocity (vg) and the area (A=width·length=w·l) of the tank. The settling velocity, is the vertical velocity with which the particle is approaching the bottom of the tank.

Inlet

‘ Qc = vg ⋅ A Outlet •

Qc= Flow= throughput capacity (m²/s)

•

Vg =Gravitational settling velocity (m /s)

•

A= Settling area ( l x w) ( m²)

W L

Fig. 1.6. Capacity of a continuous tank EPS004-E-1

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1.3.1 Settling velocity To be able to use the equation for the separator’s capacity flow, we need to explore the term settling velocity (vg) a bit more. Let’s pick one of the sludge particles and take a look at its movement.(fig. 1.7.) As is indicated, this particle is moving diagonally at this moment. It is important to stress the fact that was mentioned on the last page. It is only the vertical portion of this movement that is represented in the formula for Qc. To sort this out, we can recognize that the diagonally movement is due to two forces acting on it: Firstly, we have the gravitational force, making the particle to move downwards – this is also the vertical vg that you find in the formulas. Secondly, the particle is also moving due to the flow.

Fig. 1.7

Before we move on, let’s make a comment on the terms and concepts that we use: The term “velocity” will sometimes be replaced with the term “speed” as we go along. The term speed can be used when direction of the motion is not an issue for understanding the sentence. In this course we will not go into details when it comes to forces as such in equations etc., instead we will focus on the velocities (which of course are due to the action of forces). In Fig. 1.8 we see our particle in the flow again. What is it that makes this particle to move diagonally down as we saw in Fig. 1. 7? Firstly, we have the gravitational force that will make the particle sink towards the bottom. This is the “gravitational settling velocity”, vg, we have pointed out before. If gravity were the only force, i.e. if we had no flow, the particles would move vertically towards the bottom. Due to the flow, the particles will also be forced to move in the flow direction. This movement illustrated by the arrow pointing towards the outlet. If gravity were turned off for a second (which of course is not possible) the particle would move in this direction only. The resulting velocity, thus its resulting movement is the sum of the two velocities. 8

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Fig. 1.8

BASIC SEPARATION THEORY

1 FUNDAMENTAL CONCEPTS

1.4 Parameters in Stokes law The gravitational settling velocity, vg, decides how fast the settling during gravitational separation is. But the settling velocity is not decided by gravity only. Let’s discuss the parameters that decide the settling velocity before we look into the Stokes’ law equation itself.

1.4.1 Particle size We have already mentioned in the beginning of the course that particle size does affect the settling – the larger the particle, the faster it settles. Take for instance grains of granite, one with a diameter of a couple of millimeters and the other of a diameter of some tenth of a millimeter (just visible). If you let them sink in a volume of water, the settling speed will differ noticeably.

1.4.2 Density difference Thinking about it, it is quite natural that the density difference between particles and liquid affects how fast the settling takes place. Granite has more than 2.5 times greater density compared to water. Thus it settles quickly when put in water.

1.4.3 Viscosity Viscosity is a measure of the flow resistance of a fluid. As an example, we can feel the flow resistance in a viscous liquid (e.g. a bucket with fuel oil) when we stir it. The flow resistance due to viscosity is higher in fuel oil compared to water and this will affect the settling velocity.

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1.5 Stokes' law Stokes’ law is written as seen in the grey square on the right.. The parameters in the equation are: •

vg, which is the gravitational settling velocity. Velocity is measured in meters per second.

•

Particle diameter. You can see that the particle diameter will affect the settling velocity by the power of two. This means that a particle double the size will settle four times faster, and a particle treble the size will settle nine times faster. Diameter is measured in meters.

•

Difference in density between the particle and liquid. Density is denominated by the Greek letter rho (ρ). Density is measured in kilogram per cubic meter.

•

Viscosity, which is denominated by the Greek letter eta (η). Viscosity is commonly measured in cP, “centipoise”. Conversion to SI-units: 1 cP = 1 Pa · s ·10-3 = 1 kg/(m·s) ·10-3 =1000 cP = 1 kg/(m·s)

•

10

Gravitational acceleration constant. Acceleration is measured in meter per second to a potent of two.The dimension for acceleration is meter per square second.

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Stokes law:

2 d (ρ – ρ ) p l v = ------------------------------ g g 18η v g = gravitational settling velocity ( m/s ) d = particle diameter (m) ρ = particle density ( kg / m³ ) p ρ = liquid phase density ( kg / m³ ) l η = liquid phase viscosity (cP) g = gravitational accelaration (m /s²)

BASIC SEPARATION THEORY

1 FUNDAMENTAL CONCEPTS

1.6 Separation efficiency vs flow and area Let’s conclude this chapter by reconnecting to the capacity again. We can illustrate the term separation efficiency as a function of flow in the separator tank by a simple graph. (Fig. 1.9) If you have a requested separation, e.g. 60%, you can read from the graph how large the maximum flow could be for this separation efficiency. From experiments and from field tests with High Speed Separators, one can get data to draw graphs where separation efficiency is a function of particle size, density difference and viscosity, respectively. We will not cover this in this course, but you should know that those relationships of course is of importance when deciding which model should be used for a specific application.

Fig. 1.9. Seperation efficiency vs flow

When we look another graph ( Fig. 1.10) we can see that separator efficiency increases proportional to area. The larger area we have the higher is the separation efficiency. So the conclusion is; Separation efficiency is proportional to settling area and inversely to Q.

Fig. 1.9. Seperation efficiency vs area

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BASIC SEPARATION THEORY

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BASIC SEPARATION THEORY

2 IMPROVING SEPARATION EFFICIENCY

2 Improving separation efficiency In this section we will discuss how separation efficiency can be improved. This is the basis for understanding the design of our High Speed Separators.

2.1 Baffle plates flow Let’s return to the settling tank again. There is a simple, but important, change in its design to radically improve the separation efficiency.

Fig. 2.1 Continuous gravity separation vessel

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To sort this out, we need to go back to the discussion concerning particle movement and how the liquid flows that we had in the beginning.

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The design change is to add baffle plates. The baffle plates can be placed horizontally or tilted. In this case we look at the tilted alternative. The greatest improvement with baffle plates can be described as that the settling distance for particles in the sludge is shortened.

Fig. 2.2. Enlarged settling area by means of baffle plates

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BASIC SEPARATION THEORY

2.1.1 Why baffle plates matter In Fig. 2.2 we will look at the flow between the baffle plates a bit closer in order to see how they can improve the separation efficiency. The key concept behind this design change is that the flow between the baffle plates will vary depending on how close to the plates the liquid passes. 1

This is the liquid flow profile showing how quickly different portions of liquid move between the plates. Long arrows indicate high liquid flow rate and short arrows indicate low liquid flow rate. This kind of flow profile yields for laminar flow.

2

Near the baffle plate surfaces, the liquid flow diminishes and the shear force on settled particles is small. Let’s review this stepwise: Here we have three particles. The resulting velocity for each particle will be a sum of two contributing velocities, represented by each pair of arrows:

The upwards directed arrows represents the velocity portion that the particle gets from the surrounding liquid’s motion. For the three particles, you can see that this flow velocity differs depending on where they are in the laminar flow.

b

The downwards directed arrows represent the velocity portion that the particle gets due to gravity. This is the same for the three particles.

4

For the particle in the middle of the stream, the resulting movement at this specific moment is according to the length and direction of the red arrow. This particle will at this moment follow the flow upwards, but not entirely parallel to the flow direction.

5

For the second particle, the movement at this moment is directed almost horizontal due to the fact that the liquid flow is slower around this particle.

6

The particle near the wall will fall downwards. The movement of this particle is almost entirely determined by gravity. As you remember, the liquid movement upwards is almost zero in the vicinity of the plate. And when the particle reaches the baffle, it follows the baffle wall downwards.

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3

a

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BASIC SEPARATION THEORY

2 IMPROVING SEPARATION EFFICIENCY

2.1.2 Summing up By means of the baffle plates, the settling distance for the particles is reduced. Take your time to try to see this from the all arrows in the slide. Each pair of black arrows represent the velocity contributions from a

the force of gravity and

b

the liquid movement.

The red arrows represent the direction and speed that the particles have at this specific moment – the red arrows explain why the particles move so differently depending on where they are in the flow between the plates they are.

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The smooth and simple shape of the laminar flow profile in this example is valid for relatively low flow rates and simple designs. In a high speed separator, the laminar flow profile will be much more complicated and it will also vary throughout the disk stack. Nevertheless, the underlying principle we have discussed here is the same.

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BASIC SEPARATION THEORY

2.2 Centrifugal separation So far we have looked at a settling tank where the separation is only influenced by gravity, i.e. 1g, which of course is constant (9,81 m/s2). To get a higher settling speed, we can create a centrifugal force acting on the particles by spinning the vessel. The heavier particle will then move towards the periphery instead of downwards (the gravitation force is still present but is quite minute compared to the high centrifugal force we can get due to the high rotating speed. High Speed Separators create centrifugal forces corresponding to 5000 -15000 g. The magnitude of the driving force acting during centrifugal separation depends on the rotating speed and the radius. This is shown in the formula. Compared to Stokes' law for gravity separation, gravity g is replaced with the term r2.

Rotating speed. In the formula, the rotating speed is expressed as angular speed (). Angular speed is measured in (radians)/sec.

The angular speed is as high near the centre as it is in the periphery. As angular speed is represented as 2 in the formula, doubling the rotating speed will give four times higher settling speed. Three times the angular speed gives nine times higher settling speed, etc. Here it is useful to know that energy consumption for getting the separator up to speed is also rising exponentially with the angular speed. r2 is measured in [m·(radians)2/s2]. As radians is defined without dimension, the dimension for vc turns out to be m/s, which, as we would have expected, is the dimension for velocity.

Radius, r In gravity separation, the settling speed is the same throughout the volume of the vessel. But when it comes to centrifugal separation, the settling speed varies with the distance to the periphery a particle has at each moment. The greater the radius, i.e. the closer to the periphery, the higher the settling speed for the particles.

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•

•

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BASIC SEPARATION THEORY

2 IMPROVING SEPARATION EFFICIENCY

2.2.1 Redesigning the vessel Let’s look at the redesign of the separation vessel we have done so far. We started our discussions with the most simple kind of vessel.

2

By introducing baffle plates, we managed to improve the separation efficiency quite a lot.

3

The third design displayed is a new design introduced here. The purpose of this design is only to bridge over to the design of the High Speed Separators. This design has the same functions as the second one. The inlet lies above the outlet creating a flow through the vessel and you can see that the baffle plates are in place.

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1

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2 IMPROVING SEPARATION EFFICIENCY

2.2.2 From gravitational to centrifugal force By taking the re-designed vessel turning it 90× &and setting the vessel in rotation we have an illustration of a centrifugal separation vessel with a disk stack. The centrifugal force in a High Speed Separator corresponds to up to 7000 g. Please note how the particles are moving towards the lower sides of the disks on their way to the outer wall of the bowl. The particles’ paths are determined by the strong centrifugal force, which is directed outwards.

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.

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BASIC SEPARATION THEORY

2 IMPROVING SEPARATION EFFICIENCY

2.3 High speed separator (HSS) design 2.3.1 The Disc stack Fig. 2.7 shows a exploded view of a bowl. Inside the bowl we find the disc stack. The discs act as buffer plates in the bowl and are situated in the center of the bowl. Each disc has distance pieces to keep the proper distance to the next disc. The distance pieces are called caulks. The distance between the discs is quite small and lie normallybetween 0.4-0.8 mm depending on the application. Separators used in some applications (e.g. Fish and Meat) are loaded with mixtures containing very sticky solids. In these separators, the distance can be 1-2 mm between discs to avoid clogging of the disc stack. The disc stack is one of the most important parts of the separator. It is here that separation of dirt particles takes place. Should the disc stack get dirty, or blocked in any way, the separation efficiency will be drastically reduced. Here we find the forces working on the particles during separation – g force and flow. Bowl Short settling paths, large settling area.

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Disc- stack

Particles and forces during separation Caulk(s) 0,4-0,8 mm I 0 01 06 2

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Fig. 2.7. Disc stack overview

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2.3.2 Flow between discs

If we imagine a particle going into the bowl and into the opening between the discs, the particle will be in the middle of the flow due to the high flow forces working on it, the G force and the force of the flow. When the particle enters the bowl and the discs, the force in the bowl starts to influence the particle which will start moving out of the main flow and into the lower flow underneath the discs due to the forces working at different angles. We remember this from the earlier presentation on settling velocity for particles. When the particle has moved close to the disc the flow forces are lower than the g-forces and the particle now moves against the flow and outwards into the periphery of the bowl i.e. into the sludge space. The g-force is higher at the periphery than in the middle of the bowl, which makes it easier to 1. G-forces 2. Flow catch particles the further they get from the centre of the bowl. One effect of this is that if the flow is too high in the bowl no particles will Fig. 2.10. Particles in the discs be caught and the lower the flow the more particles will be caught. 20

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In Fig. 2.8 we look inside the disc stack at two discs and the opening between them. This opening depends on the thickness of the caulks 0,5 - 0,8 mm and can vary from 0, 4 to 0,8mm depending on the application. We will now look at the forces working on a particle passing between the discs. There are two forces working on the particle between the discs – the flow and the g-force. You remember this from the earlier slide. Let’s look at the flow first; Inside the discs we have what we call a parabolic velocity profile, meaning the flow is higher in the middle than on the sides close to the discs. This is the same effect we find in a pipe or in a river. The flow in the middle of the river can be Fig. 2.9. Parabolic velocity profile.Friction close to very high, but near the banks the flow can be wall causes lower velocity. Friction close to centre of very low and in some cases actually go in the tube causes higher velocity. opposite direction. We use this difference in flow to maximize the separation process.

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2.3.3 The tank

When we fill the bowl with water we still have only the small natural g-force acting on the particles. (Fig. 2.12)

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We have been looking at separation in a simple vessel. Let us go one step further and use the separator to illustrate the product flow and separation. As we can see in Fig. 2.11, the separator turned 90 ° looks much like a tank.

Fig. 2.11. Tank

G-force

Fig. 2.12. Tank with liquid Long settling distance Short settling distance I 00 10 36 C

We are only able to separate particles from one liquid when we have a separator/vessel in this configuration.( Fig. 2.14) How can we separate two liquids and particles? If we try to add more than one liquid into the separator now with a different density, the heaviest liquid will settle at the bottom and eventually fill the tank and flow out. We need to control both the light and heavy liquid when we have continuous separation, but how?

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When we look at the shape of the bowl the particles have a long way to travel in the deep end and a short way to travel in the shallow end. This means that it will take longer time for the particles in the deep end to settle. (Fig. 2.13)

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Fig. 2.13. Tank with liquids and solids

Fig. 2.1 4 . Separation of liquids and solids

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2.3.4 The divided tank We introduce buffer plates! ( Fig. 2.15 ) These buffer plates will divide two liquids with different densities and we can control the flow out of the separator. We will have one heavy phase and one light phase. But we need a way to control the interface between the two liquids to control the discharge of the liquids through different outlets.

If we add more water into the bowl ( Fig. 2.17), the water starts to flow out of the water outlet/ heavy phase outlet. We can see that the level in the U-tube is not rising; all the water we add will just flow out of the water outlet. We can now have continuous separation of one liquid and particles, but we are interested in separation of two liquids, water and oil and particles. Let’s see what happens when we add oil to the separator.

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Fig. 2.15. Install baffle plates

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U-tube

Fig. 2.16. Water sealed filled

U-tube

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In fig. 2.16 we can see the water level filling up until it flows over in the heavy outlet. We have now set the water seal in the separator and made sure the oil is not coming through the wrong outlet. We introduce the U-tube to follow the hydraulic balance in the bowl. We have the same level in the U-tube as we have in the separator and you can see the water is up to the water outlet, we now have the maximum water level in the Utube and the bowl.

U-tube

Fig. 2.16. Water sealed filled. More water added.

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2 IMPROVING SEPARATION EFFICIENCY

2.3.5 Adding oil in the divided tank When we add the oil to the separator we start to define the interface between the water and the oil. We establish the oil column height and we have the same height in the U tube as in the separator. We establish the water column height and also this will be the same height in the u-tube as in the separator. This interface is what we control to position the light phase and heavy phase in the separator. When we filled the oil to the U-tube we removed some of the water from the vessel, but since the oil has a lower density than the water it will float on top of the water. We have now created two levels in the u-tube and the separator. The difference in the oil column height and the water column height is the density difference. The greater the density differences between the two liquids the greater the difference in column height.

Oil column height

Water column height

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Density difference

Fig. 2.18. Separation of oil/water and solids

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It is possible to change the output level at two different points -at the water outlet and at the oil outlet. ( Fig. 2.19) We don’t change the interface if we change the oil output level, only increase or reduce the oil column height. This could be a solution if the oil was homogenous and the density never changed. But the density of fuel oil is changing all the time moving the interface up and down. We need a tool to make sure the interface is in the correct position, make sure water is going through the water outlet and the oil in the oil outlet. We leave the oil height weir in a fixed position and use the heavy side weir to control the interface.We control the position of the interface with an adjustable weir in the heavy phase outlet; also called gravity disc in the separator!

“Fixed oil surface level weir”= The level ring “oil surface level weir”

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Fixed

Fig. 2.19. Divided levels

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We adjust the position of the interface, up or down, with the height of the weir (in the separator we use different diameters on the gravity disc). ( Fig. 2.20) It is important to have control over the interface. If it becomes too high, water can come through with the oil and if too low will send oil out with the water. The height of the weir depends on the density of the oil. The higher density of the oil the higher the weir. The oil will displace more water when it has a high density and we increase the height of the weir to keep the interface in the same position in the separator and make sure oil is not coming out with the water. If that happens we have lost the water seal and have no control of the process. If the oils density is lower we get the opposite effect. The oil displaces less water and the interface is rising and we can have a situation where water comes out with the oil.. In this case we install a higher density weir without changing the density of the oil. We can see the interface is moving upwards in the bowl and in the u-tube. When we do this the water column becomes higher and therefore displaces more of the oil from the U-tube. This increase in the water column height corresponds to the change in the density weir, gravity disc!

Fig. 2.20. Divided levels EPS004-E-1

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When we have established the interface and the process is running properly, we still only have a small production due to just one G is influencing the particles. So what can we do to increase the production? If we look back to the beginning of this presentation we talked about efficiency. We mentioned that increasing the area would also increase the efficiency. So we add some buffer plates which increases the settling area in the separator, these buffer plates are called discs in the separator. ( Fig. 2.21) G-force This will increase the efficiency and thereby the production, but we need to increase production. Fig. 2.21. Adding Buffer plates (discs) How can we increase production further?

2.3.6 Increasing sufficiency with centrifugal force We raise the vessel and turn it into a rotating separator! We are now replacing G force with r² (Centrifugal force) in Stokes’ law. This means that the force acting on the particles is increased by thousands. In some of our separators the Gforce is increased to 7000g. We can increase the flow and produce enough for engine consumption by choosing the right size of separator.

Thousands of G replacing 1G

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Rotation of the tank

Fig. 2.22. Increasing sufficiency with centrifugal force

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3 BOWL AND APPLICATION

3 Bowl and application In this section we will look at the bowl, its major components, purifier/clarifier and limitations of the conventional separator. We will also look at some common problems of operating a conventional separator, and have a look at how to choose the right gravity disc.

3.1 The separator In Fig. 4.1 we can see the different parts in the separator: The feed to the separator, oil, particles and water.The light phase; where clean oil is ejected. The heavy phase; where water is ejected.

feed to separator Light phase out Heavy phase out

Gravity disc

Level Ring

I0 01 03 6I

Solids out

Fig.4.1. The separator

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3.2 Basic separation principles Let’s have a look at the conventional separator, purifier and clarifier! The conventional separators were always run in series, first the purifier and then the clarifier.

3.2.1 Separator First a separator set up as a purifier ( Fig. 4.2); dirty oil and water is come in one end, go through the separator, and clean oil comes out through the clean oil outlet. Here we find the back pressure valve, pressure gauge, and pressure sensors. Water comes out of the open water outlet and down into the sludge outlet where the ejected sludge and water also end up. This process, removing water and solids from the oil is called “Dewatering of oil”

Oil/ Water/solids inlet

Cleaned oil outlet

Water outlet

S 00 10 04

Oil is cleaned from water and solids “ Dewatering of oils”

Fig. 4.2. Separator for purification

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3.2.2 Clarifier / Purifier

We call the separation process in a clarifier “polishing the oil”.

Oil/ Water/solids inlet

Cleaned oil outlet

No Water outlet! Water outlet is closed.

Solids are removed from Oil “ Polishing of oils”

S0 01 00 5

When we look at the clarifier (Fig. 4.3 ); oil comes from the purifier into the clarifier. The clarifier is removes the particles the purifier couldn’t remove, acting as a safety net in the process. We have the same set up in the clean oil outlet as on the purifier, back pressure valve, pressure gauge and pressure sensor. There one large difference from the purifier in the setup, the water outlet is closed! This is done by replacing the gravity disc with a clarifier disc, the disc with the smallest inner diameter. When the water outlet is closed the separators ability to remove water is limited, it will only remove water by discharge.

Fig. 4.3. Separator for clarification

There are some limitations as to the type of oil the separators can handle.The purifier has a density limit of 991kg/m³ and a viscosity limit of 600 cSt. Clarifiers have no density limit due to the use of the clarifier disc closing the water outlet. Let’s look at the applications where we can use the conventional separators; We use the purifier for: •

fuel oil cleaning

•

lube oil cleaning

•

hydraulic oil cleaning

We use the clarifier for : •

fuel oil polishing

•

hydraulic oil polishing

As you can see here we are not using the clarifier in lube oil systems! And there is a good reason for that! There is one thing missing in a clarifier that we have in a purifier - a water seal. There is no water added to the bowl in a clarifier which would keep the sludge in a liquid state making it easy to discharge.

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3.3 Operational problems

The lubricating oil contains small amounts of Calcium sulphate, produced by the neutralizing sulphuric acid when it forms in the engine. The acid is from burning fuel sulphur. Some of the sulphur is converted to sulphuric acid. Most of the marine lubricants are over based, meaning they contain base calcium carbonate.The reaction of calcium carbonate with sulphuric acid results in calcium sulphate, water and carbon dioxide. Gypsum is a hydrated form of calcium sulphate, CaSO4*2H2O, and there is Fig. 4.4. Lube oil contamination sources usually a fare amount of water in the oil. This gypsum will dry out due to the lack of water in a clarifier (no water seal , therefore no water is added into the bowl at start-up) and will subsequently create a risk for only partial removal smudge during a sludge discharge sequence. If only a part of the sludge cake is discharged, the remainder of the sludge in the bowl can create an uneven distribution of this sludge in the bowl and the result will be a severe unbalance of the bowl, a heavy side unbalance. This heavy side unbalance can lead to severe damage to the separator and to injury of personnel operating the separator.

As to the LOPX or S type separators, which are basically clarifiers, this risk is eliminated by the introduction of conditioning water into the bowl. Conditioning water is fed into the bowl prior to the opening of the oil 3 way feed valves This water will create a thin water layer in the bowl periphery and all the sludge removed from the oil will have to pass through this water layer, thereby absorbing a little water and thus obtaining a “soft “consistence, will not dry out and thus will not have the risk of gypsum formation and uneven sludge discharge. 30

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In lubricating oil cleaning systems a clarifier type separator may not be used for the following reason:

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3.4 Purifier bowl When we look inside a purifier (Fig. 4.5. ) there are some things we need to keep track of, the top disc - the interface and the gravity disc. If we start with the top disc; this is the buffer plate keeping the oil and water separated. The interface is the borderline between the water and the oil. The interface is not a clear line, emulsified oil is present on both the waterside and oil side of the interface. The movement of the interface controls the separation result; we will look into this in the following slides. The gravity disc is what controls the position of the interface depending on the density of the oil, temperature and flow in the bowl. We will come back to this when we talk about how to find the right gravity disk. Here we can see the water seal inside the bowl, the interface is set just inside the top disc and the water accumulating moves outwards and up to the gravity disc. Accumulation of water will make the interface move inwards due to the higher density of the water. If we have the right size gravity disc the interface will move only a short distance before the water starts coming over the gravity disc and be removed from the bowl.

Oil inlet Clean outlet Closed water outlet

Gravity Disc Top Disc Interface

I0 01 0 35 B

Sludge outlet

Fig. 4.5. The Purifier Bowl

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3.5 Clarifier Bowl If we look inside a clarifier bowl we find the same parts as in the purifier.(Fig. 4.6)The only exception is that the gravity disc is replaced by a clarifier disc. This has a huge influence on the hydraulic balance in the bowl. There is no water seal in the clarifier and the reason is pure physics. If we tried to set a water seal in a clarifier where the adjustable weir, if you remember the tank earlier in the presentation, is high, water inside the bowl will displace the oil due to the higher density. The interface will be inside the disc stack and we lose control over the separation process. This also means that the clarifier cannot remove water, only accumulate water in the sludge space and eject it from the bowl during a discharge. Let’s look at some of the problems that can arise during separation with a conventional separator.

Oil inlet Clean outlet Closed water outlet

Gravity Disc Top Disc

I0 0 10 35 D

Sludge outlet

Fig. 4.6. The clarifier bowl

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3.6 Factors affecting the interface 3.6.1 Interface moving inwards

What happens when the interface is too far into the disc stack? On the discs we have distribution holes which are used to distribute the oil evenly through the disc stack. If the interface reaches these holes it will block the holes and interrupt the oil distribution through the disc stack! Only a small part of the disc stack will be open for the oil to pass thru and what happens with the flow if we have the same flow but through a much smaller area! The flow through the open discs will be so high that the separation efficiency will be zero! Oil is flowing thru so fast that even the high g-forces inside the separator can’t make a difference.

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The interface will move inwards when: •

The gravity disc is too small

•

The density of oil decreases

•

The viscosity of oil decreases

•

The temperature of oil increases

•

The flow rate decreases

This will cause: •

Water blockage in disc stack

•

Bad separation efficiency

I0 01 03 5B

Let’s assume that we have a running separator with a well positioned interface. Something happens that disturbs the balance in the bowl and the interface moves inwards into the disc stack. (Fig. 4.7) One of the changes that can move the interface is change of fuel oil. Using lower density will move the interface inwards. A lower viscosity will also have this effect on the interface. This will happen if the gravity disc is chosen at a wrong temperature. When the temperature is increased the viscosity decreases and interface moves inwards. The interface will also move inwards if we lower the flow through the bowl. This is because the back pressure in the clean oil outlet will decrease. One of the most common reasons for this problem is that the gravity disc used is too small! In some extreme cases we have found the clarifier disc installed in a purifier, making the purifier act like a clarifier, no water removal and the interface in the wrong position. They have actually changed the purifier into a pump!

Fig.4.7. Interface moving inwards

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How can we detect this problem? We can’t! And this is the biggest problem when this happens. There is no way to tell the customer that something is wrong in the separator! There will be no alarm, no indication of anything wrong until problems arise in the engine due to the separator not removing impurities from the oil i.e. cat-fines. So what can we do? The only way to prevent this from happening is to test the separator, to see if we have the right gravity disc installed. This is a time consuming and tedious work but it is the only way to make sure the separator is working correctly. We shall come back to how to find the right gravity disc later in this presentation.

3.6.2 The interface moving outwards

Both the problems are annoying but the first one gives no indication when it happens and can be dangerous for the engine operation! From this we can see that the conventional separator with gravity disc will be time consuming to reach maximal efficiency and the operator has to be well educated and knowledgeable to do so. It is important to keep the separator under close control and always check when some of the parameters are changed, such as a new oil batch, different flow etc.

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The interface will move outwards when: •

The gravity disc is too big

•

The disc stack is dirty

•

The density of oil increases

•

The viscosity of oil increases

•

The temperature of oil decreases

•

The flow rate increases

This will cause: •

Oil in water outlet and a broken water seal

I0 0 10 35 A

This is another problem that can happen in a purifier, the interface moves outwards and breaks the water seal. ( Fig. 4.8 ) The reason for this is: higher oil density, higher viscosity, lower temperature, higher flow into the separator or that the gravity disc is too big. There is also an additional reason for this to happen; the disc stack can be dirty. If the disc stack is dirty the channels leading the oil are smaller, the pressure outside the disc stack is increasing and the interface move outwards. This situation; where the interface moves outwards and past the end of the top disc is annoying but not dangerous. When this happens we get an alarm and we then know that something is wrong and can fix it! When the alarm sounds we know what the problem is and know what to do to fix it.

Fig. 4.8. Interface moving outwards

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3.7 How to find the right gravity disc The gravity disc is important for the operation and efficiency of the separation process . To keep the interface in the correct position we need the correct gravity disc. To find this we have to consider the density of the oil, the temperature of the oil and the flow in the bowl. All conventional separators have a full setup of gravity discs that are specific for their type and size. They have also a specific nomogram, a diagram to help us choose the right gravity disc (Fig. 4.9). First we look at the density of the oil, this is stated in the bunker delivery note or the bunker test answer from the laboratory. 1

Take the density and match it with the density in the left side of the left graph. Now follow the curved graph to the right separation temperature, normally 98°C for HFO and 9095°C for lube oil.

2

The next step is to draw a horizontal line to the flow through the separator, found on the bottom of the right diagram. Now we have a starting point, a suggestion for the size of the gravity disc.In this case we end up in the border between 53,3 and 55,5.

3

To make sure we have the interface as close to the end of the top disc as possible, start with the largest suggested gravity disc. We mount the 55,5 and start the separator to test if the water seal will hold.

4

To make sure we don’t have any problems with the interface later we have to make sure that the oil temperature is correct, separation temperature is correct and the flow is correct during this test. We will never be able to find the right size gravity disc if the temperature or flow varies.

5

If all the operational parameters are ok and the water seal is holding we try an even larger gravity disc. We do this until we find the gravity disc where the water seal is breaking.We then go back one size. This is the optimum gravity disc for this oil at this temperature and flow.

If any of these parameters changes we may have to change the gravity disc again. Having the separator in the most efficient operation mode all the time demands a lot of attention from the operators. In many cases they choose to fit a small gravity disc to avoid alarms from the separator but this can create the situation where the interface is inside the disc stack and all the problems that come with that scenario .

1

S0 0 10 07

2

Fig. 4.10. A Nomogram which makes us able to choose the correct gravity disc. EPS004-E-1

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4 Separation efficiency In this section we look at separator vs filter, we talk about what we are removing from the oil and what we not remove from the oil. We look at into ISO 8217, the standardization of fuel purchase and we look at cat fines.

4.1 Separator vs Filter First we compare cleaning effect between separators and filters. If we first look at particles larger than 4µ we can see that the separator removes between 65 to 85% of the particles, while the filter removes 5 to 10% of the particles. We have the same effect when we talk about our number one enemy in HFO Cat-fines. The separator removes 60 to 90% of the particles, and the filter removes approximately 5%. If for instance we look at iron and sodium, the separator removes 40 to 60% while the filter removes up to 5%. This goes to show that the separator is there to clean the oil and the filter is there just for protection, stopping nuts and bolts from entering the engine.

Particles

Separator removal (%)

Filter removal (%)

Particles under 4 μm

65-85

5-10

Catfines

60-90

~5

Iron

40-60

~5

Sodium

40-50

<5

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4.2 Particles we separate/ don´t separate 4.2.1 Components in oils not affected There are some components in the oil that are not affected by the separator process. Some because they have a lighter density than the oil and others because they are part of the oil.The separation process does not affect: •

The Density of the oil; even if we remove some of the particles in the oil.

•

The Viscosity of the oil ; we change the density with the heater.

•

The CCAI . CCAI stands for Calculated Carbon Aromaticity Index; a way to measure the HFO’s willingness to ignite inside the cylinder.

•

The Flash point. This is a parameter indicating at what temperature the light parts of the oil start to evaporate and create dangerous gases

•

The Pour point. It indicates what temperature it is possible to pump the oil.

•

The Micro carbon residue.It’s an indication of the amount of carbon dissolved in the oil.

•

Sulphur and vanadium. They’re part of the oil and can’t be removed by the separator.

•

Asphaltenes are also part of the oil and will not be removed by the separator.

For more information and definitions regarding the different part of the oil, see appendix 1.

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4.2.2 Components in oils affected The separator is good for removing the dangerous parts in the oil; let’s have a look at some of the particles we can remove with the separator. •

Water; it can be salt water or fresh water coming from different sources but the density is much higher than oil and are therefore relatively easy to remove from the oil.

•

Sodium is of course a part of sea water and can therefore be moved easily. In some cases the oil contains some sodium naturally but then as a part of the oil, making it impossible to remove.

•

Aluminium and silicon; also called cat-fines are residue from the refining process and are really dangerous for the engine operation if they are not removed. Cat-fines are extremely hard. On a scale from 1 to 10 with diamond as 10 the cat-fines are at 8,2, making them very hard and thereby dangerous to the engine if not removed. Luckily the particles can easily be removed with the right tuning of the separator.

•

Iron, magnesium, ash and calcium are part of contamination in the oil and can be removed by separation due to higher density than the oil.

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4.3 ISO standard 8217 We can blame the ISO 8217 that we are using for fuel purchase for much of the problems we have on board the ships regarding HFO, and also thank it for needing the separator to clean the oil after it is delivered on board the ship. If we take a look at the values the supplier is allowed to include in the oil when we buy the HFO, we can see the potential for huge problems and costs. First look at the water, they are allowed to give us 0,5% water in the delivery. This may not look like a big problem, water is water and there is usually a fuel plant on board with separators to take care of the water. But, you are paying 500$ + per ton of water (bunkering 2000t of oil gives 10t of water to a cost of 5000$), you don’t know if it is sea water including salt, and the problems with high temp corrosion, or fresh water with less impact on the process. If the separators are not maintained and operated correctly the water is still in the oil when it reaches the engine. We come back to this later in the presentation.

40

Another big trouble maker is the Al/Si content allowed in the delivery. They are allowed 80ppm cat fines, and can give you this amount every time you buy fuel. The engine manufacturers demand not higher than 15 to 20ppm cat-fines maximum entering the engine. This means that the separator plant has to be in a good condition to remove enough cat fines to reach the engine manufacturer’s specification. It is impossible to day to find the actual size of the particles through fuel testing. A fuel test will only reveal the level of contamination and not the actual size of the particles. Smaller particles, smaller than 4-5 micron are not as harmful to the engine as larger particles due to the thickness of the oil film. It could be that if the particles are small, less than 4 micron and smaller, we could deliver 80ppm after the separator without damaging the engine. We are working on a solution to make sure the particle size living the separator is so small that they can’t harm the engine.

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4.4 Catfines Fig. 5.1. shows real cat-fines and Dyno particles magnified 2000 times. As you can see the catfines come in all kinds of shape and sizes. Some of the particles are big and easy to remove through separation, others are small and more difficult to remove. The Dyno particles are exactly the same size, 5 micron, and the same weight. We used these particles to test the S range for CFR. For more information read the Marine diesel engines, catalytic fines and a new way to ensure safe operation, SPS booklet. Fig.5.1. Catfines and Dyno particles

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5 Summary 5.1 Optimum interface In order to get the optimum separation result: •

Water must never enter the disc stack! It should be kept in-between the end of the top disc and outside the disc stack. Otherwise operational problems can result.

•

Use the Correct gravity disc!

•

Keep the disc-stack clean!

•

Keep the oil properties constant! If the density or the viscosity is changed most likely the gravity disc has to be changed too.

•

Keep a constant flow rate! If the flow is changed the interface is changes accordingly and we have to change the gravity disc also.

•

Keep a constant separation temperature! Fluctuations in the separation temperature can move the interface inwards or outwards depending on the change. What we actually are changing with the temperature is the viscosity of the oil. A little change in the temperature can have a large impact on the separation efficiency. Let’s look at some examples regarding temperature changes.

5.2 Temperature If we have HFO 380cSt oil and we change the temperature from 98°C to 95°C, this doesn’t sound so bad only 3°C. If we look at the change in viscosity on the oil with a 3°C change, the viscosity changes from 26cSt to 29cSt. This doesn’t look like much but it is actually a 10% change in viscosity. If we have the same oil and change the temperature from 98°C to 90°C, it still doesn’t sound like a big change, but the viscosity is changed from 26cSt to 35cSt! This is a 30% change in the viscosity and will have a big impact on the separation effect. EPS004-E-1

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5.3 Stoke’s law. To sum up the effect of Stokes’ law; •

Stokes law:

The bigger the particles the higher the separation effect.

2

•

The bigger the density difference the higher the separation effect.

•

The higher the viscosity the lower the separation efficiency

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d ( ρp – ρl ) = ---------------------------- g 18η

BASIC SEPARATION THEORY

5 SUMMARY

5.4 Separator limitations These are the limitations and drawback with the conventional separator system; •

Purifier and clarifier in series. Two separators in series means a rather expensive installation compared to a single parallel system with ALCAP separators.

•

Gravity disc. As we have shown earlier in this presentation having a separator with a gravity disc means a lot of work to achieve the optimum separation effect.

•

Maximum 600cSt and density 991kg/m³ limits. Setting a limit on the oil the system can handle, oil with this characteristics can be more expensive.

•

Manual adjustment. Having a conventional separator system means a lot of work, every time the oils characteristic is changed a manual adjustment most be made on the separator.

•

Optimum Separation hard to achieve. To have the separator in optimum setup all the time is rather difficult. Even a experienced operator have to spend a lot of time fine adjusting the separator since the oil characteristics change all the time.

•

Need of qualified attention for optimum result. The operators have to be well educated and experienced to operate the separator and achieve a good separation result. This shouldn’t be a job the newest guy on board are set to do, then the result newer will be optimal.

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5.4.1 Density limits We mentioned earlier that the conventional separator have a density limit at 991kg/m³ but this only apply to separators manufactured after 1984/85. All older separators have a density limit at 985kg/m³ at 15°C. The reason for this is that the refinery process changed much in the 1980’s, new methods where introduced to remove more of the light products from the oil and the density of the HFO became higher. We did some changes to the bowl design and got a new density limit at 991 kg/m³. You can’t handle HFO med 991kg/m³ if you have a separator before 84/85, then you have to by oil with 985kg/m³.

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