Permeability/Fluidisation Apparatus
Instruction Manual W3 ISSUE 6 April 2012
Table of Contents Copyright and Trademarks ...................................................................................... 1 General Overview ....................................................................................................... 2 Equipment Diagrams................................................................................................... 3 Important Safety Information....................................................................................... 4 Introduction.............................................................................................................. 4 The Control of Substances Hazardous to Health Regulations (1988)..................... 4 Water Borne Hazards .............................................................................................. 5 Description .................................................................................................................. 6 Overview.................................................................................................................. 6 Installation ................................................................................................................... 7 Advisory................................................................................................................... 7 Mains Water Supply ................................................................................................ 7 Connection to Drain................................................................................................. 7 Assembly ................................................................................................................. 7 Commissioning ........................................................................................................ 7 Operation .................................................................................................................. 10 Operating the Equipment....................................................................................... 10 Equipment Specifications.......................................................................................... 11 Overall Dimensions ............................................................................................... 11 Equipment Location............................................................................................... 11 Environmental Conditions...................................................................................... 11 Routine Maintenance ................................................................................................ 12 Responsibility ........................................................................................................ 12 General.................................................................................................................. 12 Laboratory Teaching Exercises................................................................................. 13 Index to Exercises ................................................................................................. 13 Viscosity and density of water ............................................................................... 13 Preparation of media and filling the column .......................................................... 13 Exercise A - Permeability .......................................................................................... 15 ii
Table of Contents Exercise B - Fluidisation............................................................................................ 20 Exercise C - Attrition ................................................................................................. 26 Contact Details for Further Information ..................................................................... 29
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Disclaimer This document and all the information contained within it is proprietary to Armfield Limited. This document must not be used for any purpose other than that for which it is supplied and its contents must not be reproduced, modified, adapted, published, translated or disclosed to any third party, in whole or in part, without the prior written permission of Armfield Limited. Should you have any queries or comments, please contact the Armfield Customer Support helpdesk (Monday to Thursday: 0830 – 1730 and Friday: 0830 - 1300 UK time). Contact details are as follows: United Kingdom
International
(0) 1425 478781 (calls charged at local rate)
+44 (0) 1425 478781 (international rates apply)
Email:
[email protected] Fax: +44 (0) 1425 470916
Copyright and Trademarks Copyright © 2012 Armfield Limited. All rights reserved. Any technical documentation made available by Armfield Limited is the copyright work of Armfield Limited and wholly owned by Armfield Limited. Brands and product names mentioned in this manual may be trademarks or registered trademarks of their respective companies and are hereby acknowledged.
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General Overview The flow of a liquid through porous media is a common phenomenon occurring in groundwater flow, seepage and infiltration, dewatering of slurries and sludges in industry, clarification of industrial liquids, fuels and food products, sewage treatment and water purification. In all these cases the flow rate is proportional to the pressure drop (conveniently measured and expressed as head loss), expressed by Darcy's Law, where the constant of proportionality is the permeability. This permeability depends on physical characteristics of the liquid and geometric characteristics of the porous media, expressed by the Kozeny-Carman equation. In some industrial processes, and very importantly in the washing of deep bed filters (as in water purification and sewage treatment), porous granular media are fluidised by upward flow of liquid. The relationships between flow rate, pressure drop (head loss) and degree of expansion during fluidisation are important to the designers and operators of such processes. A semi-empiric equation expresses these relationships. An important characteristic of granular media which undergoes this fluidisationwashing process is that it should be durable, and resist attrition. An accelerated attrition test can be carried out which simulates 3 years' normal working in a 100 hour test. The W3 Permeability/Fluidisation Apparatus enables permeability, fluidisation and attrition testing to be demonstrated using tap water, and suitable porous granular media, usually sieved sand. It can also be used for laboratory testing of various granular media that may be contemplated for use in deep bed filters.
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Equipment Diagrams
Figure 1: Installation drawing for W3 Permeability/Fluidisation Apparatus
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Important Safety Information Introduction Before proceeding to install, commission or operate the equipment described in this instruction manual we wish to alert you to potential hazards so that they may be avoided. Although designed for safe operation, any laboratory equipment may involve processes or procedures which are potentially hazardous. The major potential hazards associated with this particular equipment are listed below.
INJURY THROUGH MISUSE
POISONING FROM TOXIC MATERIALS (EG. MERCURY)
RISK OF INFECTION DUE TO LACK OF CLEANLINESS
Accidents can be avoided provided that equipment is regularly maintained and staff and students are made aware of potential hazards. A list of general safety rules is included in this manual, to assist staff and students in this regard. The list is not intended to be fully comprehensive but for guidance only. Please refer to the notes overleaf regarding the Control of Substances Hazardous to Health Regulations.
The Control of Substances Hazardous to Health Regulations (1988) The COSHH regulations impose a duty on employers to protect employees and others from substances used at work which may be hazardous to health. The regulations require you to make an assessment of all operations which are liable to expose any person to hazardous solids, liquids, dusts, vapours, gases or microorganisms. You are also required to introduce suitable procedures for handling these substances and keep appropriate records. Since the equipment supplied by Armfield Limited may involve the use of substances which can be hazardous (for example, cleaning fluids used for maintenance or chemicals used for particular demonstrations) it is essential that the laboratory supervisor or some other person in authority is responsible for implementing the COSHH regulations. Part of the above regulations are to ensure that the relevant Health and Safety Data Sheets are available for all hazardous substances used in the laboratory. Any person using a hazardous substance must be informed of the following: Physical data about the substance Any hazard from fire or explosion Any hazard to health Appropriate First Aid treatment Any hazard from reaction with other substances How to clean/dispose of spillage
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Important Safety Information Appropriate protective measures Appropriate storage and handling Although these regulations may not be applicable in your country, it is strongly recommended that a similar approach is adopted for the protection of the students operating the equipment. Local regulations must also be considered.
Water Borne Hazards The equipment described in this instruction manual involves the use of water, which under certain conditions can create a health hazard due to infection by harmful micro-organisms. For example, the microscopic bacterium called Legionella pneumophila will feed on any scale, rust, algae or sludge in water and will breed rapidly if the temperature of water is between 20 and 45°C. Any water containing this bacterium which is sprayed or splashed creating air-borne droplets can produce a form of pneumonia called Legionnaires Disease which is potentially fatal. Legionella is not the only harmful micro-organism which can infect water, but it serves as a useful example of the need for cleanliness. Under the COSHH regulations, the following precautions must be observed:
Any water contained within the product must not be allowed to stagnate, ie. the water must be changed regularly.
Any rust, sludge, scale or algae on which micro-organisms can feed must be removed regularly, i.e. the equipment must be cleaned regularly.
Where practicable the water should be maintained at a temperature below 20°C. If this is not practicable then the water should be disinfected if it is safe and appropriate to do so. Note that other hazards may exist in the handling of biocides used to disinfect the water.
A scheme should be prepared for preventing or controlling the risk incorporating all of the actions listed above.
Further details on preventing infection are contained in the publication “The Control of Legionellosis including Legionnaires Disease” - Health and Safety Series booklet HS (G) 70.
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Description Where necessary, refer to the drawings in the Equipment Diagrams section, all numerical references refer to Figure 1.
Overview The apparatus consists of a Perspex column (A) 38mm bore, 500mm long with inlet and outlet connections so that water may flow either upwards or downwards through the column. At the base of the column is a 0.5mm gauze mesh (B.S. 30 sieve mesh) to retain granular media. The top capping piece to the column can be quickly removed by unscrewing the knurled screw. Water is introduced into the apparatus via a constant head tank (F) of 8.3 litres capacity, fitted with an overflow weir which maintains the constant level. This tank should be mounted about 2.5m above the apparatus. The water from the constant head tank enters at the base of a variable area flowmeasuring device (B) (range 50-800 cc/min). The flow rate is indicated by the top edge of the float. A tee connection above the flowmeter has a needle control valve on each branch, the right hand valve (1) to the top of the column, the left-hand valve (2) to the base of the column. The outlet at the top of the column connects to the top of a manifold block (C), and the outlet from the bottom of the column connects to the lower end of the manifold block (C). The upper and lower ends of the manifold block are isolated. The upper end has a drain valve (3), and valves (5) and (7) with connections to the left-hand limbs of the water and mercury manometers respectively. The lower end of the manifold has a drain valve (4) and valves (6) and (8) with connections to the right-hand limbs of the water and mercury manometers respectively. The water manometer (D) has the two limbs joined at the top. The pressure of the air above the water in the two limbs can be adjusted using the bleed screw connected to the top manifold. The mercury manometer (E) has the two limbs joined at the base, forming a mercury U-tube. The valves and tubing connectors are made of chrome plated brass. All tubing is 7mm bore translucent plastic except the constant head tank overflow which is 25mm bore.
Ancillary Apparatus Required (Not Supplied) Glass beaker 500ml capacity. Funnel 100mm diameter. Thermometer -10 to 110°C. Trap tank (500ml wide mouth conical flask with covering of B.S. 44 mesh brass gauze). Washbottle, squeeze type. Mercury for manometers; 350g.
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Installation Advisory Before operating the equipment, it must be unpacked, assembled and installed as described in the steps that follow. Safe use of the equipment depends on following the correct installation procedure. Installation may be completed using a basic tool kit.
Mains Water Supply The equipment requires permanent connection to a cold water supply of 0.75 litres per min. at 0.5 bar (gauge).
Connection to Drain A laboratory drain or sink is required to drain the equipment whilst in use.
Assembly The constant head tank for the apparatus should be mounted 2.5m directly above the site chosen for the framework. The tank has two 10.0mm diameter mounting holes attached to it to enable wall mounting if required. Alternatively the tank can be placed on a suitable support. Connect the smaller tube from the base of the tank to the connection at the base of the flowmeter. (B) Connect the lager tube from the base of the tank to a suitable drain. Connect the tube from the side of the tank to a supply of clean tap water. Connect the tubes from valve (4) at the base of the manifold block and valve (3) at the side of the manifold block to a suitable drain.
Commissioning All numerical references in brackets relate to Figure 1 in the Equipment Diagrams. Fill the clear acrylic column with suitable granular material to a depth of approximately 300 mm (refer to Preparation of media and filling the column for details on preparing the media and filling the column). Ensure that the constant head tank has been installed and connected as described in the Assembly section above. Close all valves (1 – 8) on the W3. Turn on the water supply to the constant head tank and allow the tank to fill until water flows to drain via the overflow pipe. Adjust the flow of water from the supply until a steady flow of water flows via the overflow to drain. Open valve (3) fully then gradually open valve (2) to allow water to flow upwards through the clear acrylic column then to drain. Air at the top of the column can be released through the air-release screw in the cap at the top of the column. Allow air to be displaced from the tubing and column then adjust valve (2) to give a steady reading of 700 cc/min on the flowmeter. Check that a small amount of water is
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Armfield Instruction Manual flowing to drain via the overflow in the constant head tank. If the flow has stopped or the flow is excessive adjust the flow of water from the tap supply as required. Reverse the flow of water through the column by closing valves (2) and (3) then opening valves (1) and (4). Allow air to be displaced from the tubing and column then adjust valve (1) to give a steady reading of 700 cc/min on the flowmeter. Return the column to upward flow by closing valves (1) and (4) then opening valves (2) and (3) as before. Ensure that all air has been removed from the column and tubing then adjust valve (2) to give a steady reading of 700 cc/min on the flowmeter.
Commissioning the Mercury Manometer To ensure safe and accurate operation of the mercury manometer the following priming procedure should be adopted. Before filling the manometer with mercury it will be necessary to prime the manometer with water as follows. Open valves (7) and (8) and allow water to flow through the manometer and connecting tubing (this will only occur when a differential pressure exists because water is flowing through the granular material in the column). Partially unscrew the fitting at the top of each catch pot (at the rear of the manometer) to allow any trapped air to escape. When all air has escaped ensure that these fittings are tightened again. When all air bubbles have been purged from the manometer (including the tubes and catch pots at the rear) close valves (7) and (8) to isolate the manometer from the column. Carefully remove both of the screwed plugs from the top manifold on the mercury manometer. Using a small funnel (not supplied) carefully pour clean mercury (not supplied) into one of the manometer tubes. As the mercury fills the manometer water is displaced from the filling point ensuring that no air is entrained. When the mercury is at the required level, half way up the measuring scale, replace and tighten the two screwed plugs. Open valves (7) and (8) to measure the differential pressure in the column. To ensure that the manometer remains fully primed ensure that valves (7) and (8) are only opened when the column is filled with water and closed before draining the column. Readings are obtained by measuring the difference in height between the two mercury levels in the manometer tubes using the scale on the backplate. Since the surface of the mercury in the manometer tube is not flat (a meniscus forms against the sides of the tube) accurate readings are obtained by taking the measurement to be at the top of each meniscus. Plastic rings attached to the manometer tubes can be used to assist in taking readings. The rings can be pushed along the tubes to any required position and can be used to relate levels in the tube to the scale on the backplate or can be left in position from previous measurements to allow comparison of readings. Note: Mercury is a poison and great care should be used when handling. Any spillages when handling the mercury must be collected immediately.
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Installation The manometer incorporates catch pots to retain the mercury if the range of the manometer is accidentally exceeded. It is suggested that the mercury is collected in a vessel filled with water if it is necessary to recover the mercury from the catch pots. The vessel should be large enough to contain the lower end of the manometer to prevent loss of mercury when the drain plug on the catch pot is unscrewed.
Commissioning the Water Manometer Reduce the flow of water by closing valve (2) to give a reading of 200 cc/min on the flowmeter (excessive flowrate will exceed the range of the water manometer). Open valves (5) and (6) to allow water to flow to the water manometer. Ensure that the tubing to the manometer is full of water and is clear of air bubbles. If air bubbles cannot be removed disconnect the appropriate tubing from the tapping on the manometer and allow water to flow through the tubing until the air bubbles are dispersed. Reconnect the tubing to the manometer. The two levels in the manometer should be located at mid height. If the levels are too low carefully open the bleed screw on the top manifold of the water manometer until the levels rise to the required position then close the bleed screw. If in use the pressure difference exceeds the range of the water manometer (water levels disappear from the top and bottom of the manometer) then valves (5) and (6) must be closed and the mercury manometer used for measurements. If valves (5) and (6) remain open in this condition then the reading on the mercury manometer will be incorrect because water is flowing through the water manometer. Readings are obtained by measuring the difference in height between the two water levels in the manometer tubes using the scale on the backplate. Since the surface of the water in the manometer tube is not flat (a meniscus forms against the sides of the tube) accurate readings are obtained by taking the measurement to be at the bottom of each meniscus. Plastic rings attached to the manometer tubes can be used to assist in taking readings. The rings can be pushed along the tubes to any required position and can be used to relate levels in the tube to the scale on the backplate or can be left in position from previous measurements to allow comparison of readings. Close all valves (1 – 8) and turn off the water supply to the constant head tank. The equipment is ready for use as described in the Laboratory Teaching Exercises.
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Operation Operating the Equipment For details on Operating the Equipment please refer to the Laboratory Teaching Exercises.
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Equipment Specifications Overall Dimensions Height
-
0.79m (not including constant head tank)
Width
-
0.68m
Depth
-
0.25m
Equipment Location The equipment is designed for bench mounting on a firm level surface.
Environmental Conditions This equipment has been designed for operation in the following environmental conditions. Operation outside of these conditions may result reduced performance, damage to the equipment or hazard to the operator. a. Indoor use; b. Altitude up to 2000 m; c. Temperature 5 °C to 40 °C; d. Maximum relative humidity 80 % for temperatures up to 31 °C, decreasing linearly to 50 % relative humidity at 40 °C; e. Mains supply voltage fluctuations up to ±10 % of the nominal voltage; f.
Transient over-voltages typically present on the MAINS supply; NOTE: The normal level of transient over-voltages is impulse withstand (overvoltage) category II of IEC 60364-4-443;
g. Pollution degree 2. Normally only nonconductive pollution occurs. Temporary conductivity caused by condensation is to be expected. Typical of an office or laboratory environment
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Routine Maintenance Responsibility To preserve the life and efficient operation of the equipment it is important that the equipment is properly maintained. Regular maintenance of the equipment is the responsibility of the end user and must be performed by qualified personnel who understand the operation of the equipment.
General When not in use, clean the working section by removing any sand that may have accumulated in the sieves at the top and bottom of the working section. Disconnect the water supply to the constant head tank and drain the contents of the tank.
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Laboratory Teaching Exercises Index to Exercises Exercise A - Permeability Exercise B - Fluidisation Exercise C - Attrition
Viscosity and density of water Calculated from International Critical Tables 1928 and 1929 revised to SI Metric 1972.
Preparation of media and filling the column The media is usually pre-sieved, except possibly for attrition testing, to a uniform size fraction. It should be weighed in the dry state to determine its mass and to provide a permeable bed about 300mm deep in the column (eg. for sand 0.540 kg). The inlet control valves (1) and (2) above the flowmeter together with drain valves (3) and (4) and also all four manometer valves (5, 6, 7 and 8) should be closed. The airrelease screw at the top of the column should be opened so that air can enter and the column drained down to the base unit by opening drain valve (4). The clear acrylic column should be removed from the apparatus by unscrewing the knurled screw on the top cap. The column should be placed upright in a 1 litre measuring cylinder containing water, and the media poured into the column so that it 13
Armfield Instruction Manual falls through water. This will thoroughly wet the media and some stirring by hand with a long rod will help release air. For sand this soaking should take about 5 minutes but with anthracite it should be left to soak, with occasional stirring for at least 48 hours. When all the media has been transferred to the column and is thoroughly wetted, the column should be lifted from the measuring cylinder, excess water allowed to drain and then inserted into the apparatus, ensuring that the rubber O-rings are in place. The cap can then be tightened down with the knurled screw. Water can then be admitted into the base of the column by opening valve (2), allowing air to escape through the air-release screw. When all the air has been expelled the air-release screw should be tightened.
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Exercise A - Permeability The linear relationship between head loss (h) and flow rate, expressed as approach velocity (v a ), is given by Darcy's Law:
…..(1) v a = volumetric flow rate per unit cross-sectional area
k = permeability (usually given at a stated temperature (eg. 10oC or 20oC) Typical values of permeability are: clean gravel
1.0 m/s
coarse sand
10-2 m/s
fine sand
10-5 m/s
silts
10-9 m/s
clays
10-11 m/s
The concept of permeability, which includes characteristics of both the fluid and the porous media was developed further by Kozeny, and later by Carman. They used the analogy that the porous media could be represented by a bundle of tortuous capillary tubes, and that an equivalent hydraulic radius could be developed for granular porous media. The permeability is then related to the physical characteristics as
…..(2) and the Kozeny-Carman equation results:
…..(3) = dynamic viscosity of the fluid = density of the fluid g = acceleration due to gravity = porosity of permeable media (pore volume/total volume) d = diameter of grains constituting the porous media.
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Armfield Instruction Manual If the grains are not spherical the value of d is a notional diameter including a sphericity value (equal to 1.0 for spheres).
Procedure The valves should be set ready for downflow through the column, with both manometers reading: Valve (1) should be closed, but ready for adjustment Valve (2) should be closed Valve (3) should be closed Valve (4) should be closed, but ready for adjustment Valves (5, 6, 7 and 8) should be open. The media should be lightly consolidated by tapping gently along the length of the clear acrylic column with a pencil. The consolidation should be such that any random vibration to the bench or apparatus will not cause the media top level to fall. The drain tube from valve (4) should be inserted into a beaker, which can overflow to drain. A thermometer resting in this beaker will then indicate the temperature of the water leaving the apparatus. The level of the media surface should be read (L) and the manometer water and mercury zero levels noted. (Refer to the Commissioning section for details on how to adjust the levels in the water manometer). By opening valves (1) and (4) water is admitted through the column in a downflow direction, and about 7 settings of flow rate (Q) should be read on the flowmeter, with the manometer levels noted for each flow rate. When the levels in the water manometer approach the scale limits, valves (5) and (6) must be closed to shut off the manometer and prevent circulation of water through it. After 7 readings of increasing flow to the flowmeter scale limit, a further set of readings should be taken with decreasing flow back to zero, switching in the water manometer again for the lower readings. During the experiment observations of water temperature should be made, to obtain the mean water temperature.
Calculations Material:
Well rounded quartz grain sand Sieve size
= B.S. 22-25
Sieve apertures
= 0.710mm - 0.600mm
Mean sieve size d s = 0.655mm
Permeable Bed:
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Mass of sand
= 0.540 kg
Density of sand
= 2650 kg/m3
Length L
= 300mm
Exercise A Diameter
= 38mm
Plan area A = 11.34cm2 Volume = A L
= 300 x 1134 = 0.340 x 106mm3
Solid volume = 0.204 x 10-3m3 = 0.204 x 106mm3 Pore volume
= (0.340 - 0.204) 106 = 0.136 x 106mm3
Porosity = 0.40 Water:
Temperature (mean) = 19oC Density
= 998 kg/m3
Dynamic viscosity = 1.03 x 10-3 kg/ms
Data
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Armfield Instruction Manual * To convert to mm water multiply by 12.6 See 'Permeability Test Data' graph below. From Darcy's Law (Equation 1)
Permeability k From graph h va
= 4.4mm/s
= 4.4 x 10-3m/s Note that the lower part of the graph (up to v a = 7mm/s) is linear with the rising and falling flow values agreeing, so Darcy's Law is confirmed. The upper part curves slightly and the points are more scattered. This departure from Darcy's Law may be due to the onset of non-laminar flow conditions, or small air bubbles coming from solution affecting the porosity. The Kozeny-Carman equation (3)
can be re-written in terms of d (filtration grain diameter)
Using the experimental data dh/dL = 1.0 when v a =4.4 x 10-3m/s
= 0.685 x 10-3m = 0.685mm This compares favourably with the mean sieve size d s = 0.655mm
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Exercise A
Permeability Test Data
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Exercise B - Fluidisation In liquid fluidisation the fluidised mass is homogeneous and there are no discontinuities in the flow at steady state. This is not true of liquid-gas, and gas fluidised beds where slugs of gas pass through the fluidised mass. In the liquid fluidisation of granular media, the liquid initially passes up through the porous bed of grains, such that the upward force exerted by the liquid is less than the downward weight of the grains. Consequently, part of the weight is taken by the support mesh of the fluidisation unit. Under these conditions Darcy's Law (and the Kozeny-Carman equation) applies and head loss is proportional to flow rate. When the upward force (pressure drop times area) equals the weight of the granular media in the liquid, the grains are supported by liquid drag. The bed is then fluidised. As the weight of the media (minus buoyancy) is constant, the liquid drag cannot increase even though the flow rate may increase. Consequently, the pressure drop (ie. head loss) remains constant. Upward force
= pressure difference x area = gh e A
Downward force
= weight of particles in liquid = AL e (1 - e ) ( s - )g
Equilibrium gh e A = AL e (1 - e ) ( s - )g The solid volume of the grains is the same before and during fluidisation. AL (1 -) = AL e (1 - e )
Therefore h e
…..(4)
h e = equilibrium head loss L e = expanded length of the fluidised bed e = porosity of the fluidised bed s = density of grains A = plan area of the fluidised bed In equilibrium the grains are settling at a volocity equal to the rising approach velocity of the liquid. This creates a clear boundary of separation at the upper surface of the fluidised bed in the liquid, which is steady. If the liquid approach velocity is increased the grains can only increase their settling velocity in response, by reducing hindering ie. they move apart. Consequently the bed expands to a new equilibrium position. The relationship between hindered settling velocity (v h ) and concentration of grains (c volume/volume) is given by the equation proposed by Richardson and Zaki, and others.
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Exercise B v h = v t (1 - c)n v t = terminal settling velocity of a single grain. At equilibrium in the fluidised bed v h = v a , and (1 - c) = e
Therefore Noting that L (1 -) = L e (1 - e ), and putting 1/n = x,
…..(5) This relates the expanded length of the bed to the initial conditions (length, porosity), the approach velocity of fluidisation, and the single grain terminal settling velocity, with an empiric exponent x which depends on the grain shape, and the flow regime during fluidisation. As the single grain settling velocity depends on grain size, density and liquid viscosity and density, the expansion during fluidisation will also depend on these physical characteristics. The liquid fluidisation approach velocity at which the bed just becomes fluidised, and the head loss (pressure drop) become constant, is called the critical fluidisation velocity. The foregoing considerations lead to the following graphical relationships for a fluidised bed.
Procedure The valves should be set ready for upflow through the column, with both manometers reading: Valve (1) should be closed Valve (2) should be closed, but ready for adjustment Valve (3) should be closed, but ready for adjustment Valve (4) should be closed Valves (5, 6, 7 and 8) should be open The media should be lightly consolidated by tapping gently along the length of the clear acrylic column with a pencil. The consolidation should be such that any random vibration to the bench or apparatus will not cause the media top level to fall. The drain tube from valve (3) should be inserted into a beaker, which can overflow to drain. A thermometer resting in this beaker will then indicate the temperature of the water leaving the apparatus. The level of the media surface should be read (L) and the manometer water and mercury zero levels noted. (Refer to the Commissioning section for details on how to adjust the levels in the water manometer). 21
Armfield Instruction Manual By opening valves (2) and (3) water is admitted through the column in an upflow direction, and about 8 settings of flow rate (Q) should be made, in an increasing order. During the experiment the flow should not be diminished, and any 'over shooting' cannot be compensated by adjustment of flow downwards. In such a case, the experiment must be restarted, including the initial consolidation of the media. This is because the fluidisation experiment is not reversible, and the media does not return naturally to its original degree of consolidation if flow is reduced. For each flow rate the manometer levels should be read. For sand or less dense media (eg. anthracite) the water manometer alone will give sufficient range of readings. For denser material (eg. ballotini) the water manometer may approach its scale limits, so it should be isolated by closing valves (5) and (6). The head loss readings should then be noted on the mercury manometer. In addition to the manometer readings, the length of the column of media (Le) should be noted. Some readings of the thermometer should be taken, to obtain the mean water temperature.
Calculations Material:
Well rounded quartz grain sand Sieve size
= B.S. 22-25
Sieve apertures
= 0.710mm - 0.600mm
Mean sieve size d s = 0.655mm
Bed for Fluidisation:
Mass of sand
= 0.540 kg
Density of sand
= 2650 kg/m3
Fall velocity v t
= 104mm/s (20oC)
Length L
= 300mm
Diameter
= 38mm
Plan area A = 11.34cm2 Volume = A L
= 300 x 1134 = 0.340 x 106mm3
Solid volume = 0.204 x 10-3m3 = 0.204 x 106mm3 Pore volume 22
= (0.340 - 0.204) 106
Exercise B =0.136 x 106mm3
Porosity = 0.40 Water:
Temperature (mean) = 19oC Density
= 998 kg/m3
Dynamic viscosity
= 1.03 x 10-3 kg/ms
Data
* No mercury manometer readings necessary as all readings were on water manometer scale. See 'Fluidisation Test Data' graph below. From the graph, critical fluidisation velocity = 5.5mm/s equilibrium head loss h e = 295mm water
From equation (4)
= 298mm water The experimental and theoretical values agree well. The expansion equation (5) is
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Armfield Instruction Manual
Fluidisation Test Data
Transposing the equation gives
Consequently, a graph of the two 1n functions should give a slope of x, for the data when the bed is fluidised.
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Exercise B
Column (3) is plotted against column (7) on the accompanying graph 'Fluidisation Expansion Data Logarithmic'.
The slope of the graph gives = 0.28 This is higher than the normally accepted value for sand, of x = 0.22.
Fluidisation expansion data logarithmic
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Exercise C - Attrition The agitation to which the grains of deep bed filters are subjected during the washing process requires that the grains be durable and resist attrition. Opinions differ on how this should be specified, although objectives are the same: the size of the grains should not be materially altered by fracture or attrition during a reasonable lifetime of the media. A realistic specification is to limit the amount of fine material produced, and presumably washed away, during an accelerated washing test which simulates several years washing. On the assumption that washing in practice takes about 6 minutes, then 100 hours of testing represents 1000 washings. If deep bed filters are washed every day, then 1000 washings represent about 3 years of washing operations. If losses due to attrition should not exceed 1% per annum (ie. 10mm in a 1m deep filter), then losses more than 3% in a 100 hour test would be unacceptable, and 1% to 3% should be viewed with suspicion. The grains should be completely fluidised during the entire attrition test, but should not be expanded significantly beyond the critical fluidisation velocity. This will ensure that the grains are in the closest possible contact, while mobile, so that any attrition should be at its most severe.
Procedure The attrition test is a special case of the fluidisation experiment, carried out over an extended period of 4 days (100 hours). The valves should be set ready for upflow through the column, with both manometers reading: Valve (1) should be closed Valve (2) should be closed, but ready for adjustment Valve (3) should be closed, but ready for adjustment Valve (4) should be closed Valves (5, 6, 7 and 8) should be open The drain tube from valve (3) should be inserted into a media trap tank, the simplest form of which is a conical flask, covered with a piece of fine mesh gauze (B.S. 44, 355 microns aperture is appropriate) through which the drain tube passes to the base, as shown.
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Exercise C
Conical flask trap tank
By opening valves (2) and (3) water is admitted through the column in an upflow direction. The flow should be adjusted so that all the media is just fluidised. This can be confirmed by noting that an increase in flow does not materially affect the manometer levels (ie. no increase in head loss). Care must be taken that the water manometer's levels do not go off-scale. If they approach the scale limits, the water manometer must be isolated by closing valves (5) and (6), and the head loss should then be noted on the mercury manometer. The flow rate which achieves the just-fluidised state should be noted, and the level of the top of the fluidised media. Water temperature should be noted as the flow rate to achieve the just-fluidised state depends on temperature. If the water temperature changes significantly during the 4 days of the test the flow rate needs to be adjusted to maintain the level of the top of the fluidised media. The flow rate, media level and temperature should be noted, and adjusted if necessary, every 2 or 3 hours during working hours. The test should be left running overnight during 4 nights. A suitable test commences at 12 noon on Monday, and terminates 1600 hours on Friday, giving 100 hours duration. At the conclusion of the test, the water flow is shut off by closing valve (2). The drain valve (3) is closed, and the manometer valves (5, 6, 7 and 8) are closed. Drain valve (4) is opened, and the top of the column is opened to atmosphere with the knurled screw. Water then drains out of the column leaving the media in a wet state. The column containing the wet media is then removed from the apparatus, making sure that the top fitting is quite free. The column can be up-ended over a sheet of absorbent laboratory paper towelling, so that most of the media falls out. Care must be taken that none is lost. The grains that remain stuck to the inside of the column can be left to dry, when they will fall out readily if the column is tapped. After drying, the media should be weighed. Also any media retained in the trap tank should be dried and weighed. If this exceeds 1% of the original weight it casts doubt on the test as it probably floated over by attached air bubbles, and was not the result of attrition which would be too fine to be retained in the trap tank. Material :
Natal anthracite 27
Armfield Instruction Manual Mass after washing and drying 110oC 0.22500kg (225.00 gwt) Preparation:
Immersed in tap water, occasionally stirred with glass rod to release air for 3 days. Loaded into clear acrylic column. Very little dust released. Unconsolidated length 288mm.
Column drained down, anthracite emptied and dried at 110oC, and weighed. Mass
= 0.22422 kg (224.22 gwt)
Loss of weight
= 0.78 gwt = 0.347%
Material retained in trap, dried and weighed. Mass
= 0.45g
This low percentage loss of weight during 100 hour attrition test indicates satisfactory material.
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US Office:
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[email protected]
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