83 The Nature Of Synthetic Membranes

Membranes used for the pressure-driven separation processes, microfiltration, ultrafiltration and reverse osmosis, as well as those used for dialysis, are most commonly made of polymeric materials'1. Initially most such membranes were cellulosic in nature. These are now being replaced by polyamide, polysulphone, polycarbonate and a number of other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most...

The mechanical classifier

Several forms of classifier exist in which the material of lower settling velocity is carried away in a liquid overflow, and the material of higher settling velocity is deposited on the bottom of the equipment and is dragged upwards against the flow of the liquid, by some mechanical means. During the course of the raking action, the solids are turned over so that any small particles trapped under larger ones are brought to the top again. The mechanical classifier is extensively used where it is...

45 Packed Columns

Since packed columns consist of shaped particles contained within a column, their behaviour will in many ways be similar to that of packed beds which have already been considered. There are, however, several important differences which make the direct application of the equations for pressure gradient difficult. First, the size of the packing elements in the column will generally be very much larger and the Reynolds number will therefore be such that the flow is turbulent. secondly, the packing...

82 Classification Of Membrane Processes

Industrial membrane processes may be classified according to the size range of materials which they are to separate and the driving force used in separation. There is always a degree of arbitrariness about such classifications, and the distinctions which are typically drawn are shown in Table 8.1. This chapter is primarily concerned with the pressure driven processes, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). These are already well-established...

1954 Process design and optimisation

The design of a production chromatograph is a complex exercise because of the many process variables involved. Some of the published work on optimisation proceeds from the unsatisfactory notion that a large-scale chromatograph is little more than a scaled-up analytical one into which large samples are injected. Thus the sample size is commonly chosen as the largest which does not excessively degrade resolution. While this approach simplifies scale-up in preparative work, it does not lead to...

2957

A still is operated at 18 kN m2 and steam is blown continuously into it. Estimate the temperature of the boiling liquid and the composition of the distillate if liquid water is present in the still. 11.9. The following values represent the equilibrium conditions in terms of mole fraction of benzene in benzene-toluene mixtures at their boiling-point Liquid 0.51 0.38 0.26 0.15 Vapour 0.72 0.60 0.45 0.30 If the liquid compositions on four adjacent plates in a column are 0.18, 0.28, 0.41 and 0.57...

134Angles of repose and of friction

A rapid method of assessing the behaviour of a particulate mass is to measure its angle of repose. if solid is poured from a nozzle on to a plane surface, it will form an approximately conical heap and the angle between the sloping side of the cone and the horizontal is the angle of repose. When this is determined in this manner it is sometimes referred to as the dynamic angle of repose or the poured angle. In practice, the heap will not be exactly conical and there will be irregularities in...

2022 Principles and advantages of process intensification

Process intensification, pioneered by Ramshaw(8) in the 1980's, may be defined as a strategy which aims to achieve process miniaturisation, reduction in capital cost, improved inherent safety and energy efficiency, and often improved product quality. In recent years, process intensification has been seen to provide processing flexibility, just-in-time manufacturing capabilities and opportunities for distributed manufacturing. In order to develop a fully intensified process plant, it is...

114 The Fractionating Column 1141 The fractionating process

Fractionating Process

The operation of a typical fractionating column may be followed by reference to Figure 11.11. The column consists of a cylindrical structure divided into sections by Figure 11.11. Continuous fractionating column with rectifying and stripping sections Figure 11.11. Continuous fractionating column with rectifying and stripping sections a series of perforated trays which permit the upward flow of vapour. The liquid reflux flows across each tray, over a weir and down a downcomer to the tray below....

The gyratory crusher

The gyratory crusher shown in Figure 2.6 employs a crushing head, in the form of a truncated cone, mounted on a shaft, the upper end of which is held in a flexible bearing, whilst the lower end is driven eccentrically so as to describe a circle. The crushing action takes place round the whole of the cone and, since the maximum movement is at the bottom, the characteristics of the machine are similar to those of the Stag crusher. As the crusher is continuous in action, the fluctuations in the...

682 Fluidised Bed Catalytic Cracking

The existence of a large surplus of high boiling material after the distillation of crude oil led to the introduction of a cracking process to convert these materials into compounds of lower molecular weight and lower boiling point in particular into petroleum spirit. The cracking was initially carried out using a fixed catalyst, although local variations in temperature in the bed led to a relatively inefficient process, and the deposition of carbon on the surface of the catalyst particles...

1523 Crystal nucleation

Nucleation, the creation of crystalline bodies within a supersaturated fluid, is a complex event, since nuclei may be generated by many different mechanisms. Most nucleation classification schemes distinguish between primary nucleation - in the absence of crystals and secondary nucleation - in the presence of crystals. Strickland-Constable(27) and Kashchiev(28) have reviewed nucleation, and Garside and Davey(29) have considered secondary nucleation in particular. Classical theories of primary...

102 Mass Transfer In Leaching Operations

Mass transfer rates within the porous residue are difficult to assess because it is impossible to define the shape of the channels through which transfer must take place. It is possible, however, to obtain an approximate indication of the rate of transfer from the particles to the bulk of the liquid. Using the concept of a thin film as providing the resistance to transfer, the equation for mass transfer may be written as where A is the area of the solid-liquid interface, b is the effective...

Dc d2c

Net accumulation rate due to diffusion from boundaries in axial direction Diffusion in radial direction at radius r Corresponding rate at radius r + 8r Net accumulation rate due to diffusion from boundaries in radial direction - 2itr8l e DR + 2ic(r + Sr)Sl e DR + 8r dr V or dr2 8r+r8r + (8r)2 dr dr2 dr2 Thus, from equations 4.33, 4.30, 4.31 and 4.32 (2nr8r8l)e -uc(2nr8r) 81 + 2nr8re)DL -8l+2n8leDR dt dl dl2 --h uc Dl - + -Dr r Longitudinal dispersion coefficients can be readily obtained by...

173 Adsorption Equilibria

Much of the early work on the nature of adsorbents sought to explain the equilibrium capacity and the molecular forces involved. Adsorption equilibrium is a dynamic concept achieved when the rate at which molecules adsorb on to a surface is equal to the rate at which they desorb. The physical chemistry involved may be complex and no single theory of adsorption has been put forward which satisfactorily explains all systems. Fortunately for the engineer, what is needed is an accurate...

811 Reverse Osmosis Water Treatment Plant

The largest scale applications of membrane separation processes are those which form the key step in the desalination, or more generally demineralisation, of brackish water in the production of drinking water. In this section an outline is given of such a plant capable of producing 70,000 m3 day of drinking water for a large city in the Middle East, as described by Finlay and Ferguson(25) . The water to be processed is obtained from a deep well with a total dissolved solids (TDS) content of 1.4...

233 293 386

Adiabatic conditions may be assumed and heat transfer between phases neglected. 12.16. A bubble-cap column with 30 plates is to be used to remove n-pentane from solvent oil by means of steam stripping. The inlet oil contains 6 kmol of n-pentane 100 kmol of pure oil and it is desired to reduce the solute content of 0.1 kmol 100 kmol of solvent. Assuming isothermal operation and an overall plate efficiency of 30 per cent, what is the specific steam consumption, that is kmol of steam required kmol...

224 Methods of operating crushers

Diagram Close Circuit Grinding

There are two distinct methods of feeding material to a crusher. The first, known as free crushing, involves feeding the material at a comparatively low rate so that the product can readily escape. Its residence time in the machine is therefore short and the production of appreciable quantities of undersize material is avoided. The second method is known as choke feeding. In this case, the machine is kept full of material and discharge of the product is impeded so that the material remains in...

52 Sedimentation Of Fine Particles 521 Experimental studies

The sedimentation of metallurgical slimes has been studied by Coe and Clevenger , who concluded that a concentrated suspension may settle in one of two different ways. Figure 5.1. Sedimentation of concentrated suspensions (a) Type 1 settling (b) Type 2 settling Figure 5.1. Sedimentation of concentrated suspensions (a) Type 1 settling (b) Type 2 settling In the first, after an initial brief acceleration period, the interface between the clear liquid and the suspension moves downwards at a...

62 Liquidsolids Systems 621 Bed expansion

Liquid-fluidised systems are generally characterised by the regular expansion of the bed that takes place as the velocity increases from the minimum fluidisation velocity to the terminal falling velocity of the particles. The general relation between velocity and volumetric concentration or voidage is found to be similar to that between sedimentation velocity and concentration for particles in a suspension. The two systems are hydrody-namically similar in that in the fluidised bed the particles...

2VMalv cos AA

Equation 17.44 is known as the Kelvin equation, and is considered to be valid for desorption. Figure 17.10. The capillary condensation equation applied to a cylindrical pore (a) for desorption (b) for Figure 17.10. The capillary condensation equation applied to a cylindrical pore (a) for desorption (b) for For adsorption on to existing layers of adsorbate on the cylindrical surface of the pores dV d n(rp - r2)Lp -2nrLp dr dA d 2nrLp 2nLp dr where r'p is the net radius of the pore. Equation...

1232 Application of mass transfer theories

The preceding analysis of the process of absorption is based on the two-film theory of Whitman(1). It is supposed that the two films have negligible capacity, but offer all the resistance to mass transfer. Any turbulence disappears at the interface or free surface, and the flow is thus considered to be laminar and parallel to the surface. An alternative theory described in detail in Volume 1, Chapter 10, has been put forward by Higbie(2), and later extended by Danckwerts(3) and Danckwerts and...

Selection of economic reflux ratio

Optimum Reflux Ratio

The cost of a distillation unit includes the capital cost of the column, determined largely by the number and diameter of the plates, and the operating costs, determined by the steam and cooling water requirements. The depreciation charges may be taken as a percentage of the capital cost, and the two together taken as the overall charges. The steam required will be proportional to Vm, which may be taken as Vn where the feed is liquid at its boiling point. From a material balance over the top...

Dt dc JH

In general, the concentration of particles will be a function of position and time and thus Conditions of constant concentration are therefore defined by Substituting in equation 5.34 gives the following relation for constant concentration Since equation 5.36 refers to a constant concentration, df dC is constant and uw( dH dt) is therefore also constant for any given concentration and is the velocity of propagation of a zone of constant concentration C. Thus lines of constant slope, on a plot...

105 Calculation Of The Number Of Stages

The solid residue obtained from a batch leaching process may be washed by mixing it with liquid, allowing the mixture to settle, and then decanting the solution. This process can then be repeated until the solid is adequately washed. Suppose that, in each decantation operation, the ratio R' of the amount of solvent decanted to that remaining in association with insoluble solid is a constant and independent of the concentration of the solution, then, after the first washing, the fraction of the...

66 Mass And Heat Transfer Between Fluid And Particles

The calculation of coefficients for the transfer of heat or mass between the particles and the fluid stream requires a knowledge of the heat or mass flow, the interfacial area, and the driving force expressed either as a temperature or a concentration difference. Many early investigations are unsatisfactory in that one or more of these variables was inaccurately determined. This applies particularly to the driving force, which was frequently based on completely erroneous assumptions about the...

E Electrostatic precipitators

Electrostatic Precipitator Figure

Electrostatic precipitators, such as that shown in Figure 1.63, are capable of collecting very fine particles, < 2 m, at high efficiencies. Their capital and operating costs are high, however, and electrostatic precipitation should only be considered in place of alternative processes, such as filtration, where the gases are hot or corrosive. Electrostatic precipitators are used extensively in the metallurgical, cement and electrical power industries. Their main application is in the removal...

104 Countercurrent Washing Of Solids

Where the residual solid after separation is still mixed with an appreciable amount of solution, it is generally desirable to pass it through a battery of washers, arranged to give countercurrent flow of the solids and the solvent as shown in Figure 10.10. If the solids are relatively coarse a number of classifiers may be used and, with the more usual case of fine solids, thickeners are generally employed. In each unit a liquid, referred to as the overflow, and a mixture of insoluble residue...

Drying of a granular material according to the capillary theory

Pipe Fitting Formula Bend Deflection

If a bed of uniform spheres, initially saturated, is to be surface dried in a current of air of constant temperature, velocity and humidity, then the rate of drying is given by where Pw0 is the saturation partial pressure of water vapour at the wet bulb temperature of the air, and Pw is the partial pressure of the water vapour in the air stream. This rate of drying will remain constant so long as the inner surface of the stationary air film remains saturated. As evaporation proceeds, the water...

47 References

Les Fontaines publiques de la ville de Dijon. Exposition et application suivre et des formules a employer dans les questions de distribution d'eau Victor Dalamont, 1856 . 2. Eisenklam, P. Chapter 9 Porous Masses in Cremer, H. W. and Davies, T. Chemical Engineering Practice, Vol. 2 Butterworths, 1956 . 3. Brinkman, H. C. Appl. Scient. Res. 1A 1948 81-86. On the permeability of media consisting of closely packed porous particles. 4. Dupuit, A. J. E. J. Etudes theoriques et...

5

What power would be required to crush 1 kg s of a material of compressive strength 45 MN m2 from a feed of minus 45 mm, plus 40 mm to a product of 0.50 mm average size 2.3. A crusher reducing limestone of crushing strength 70 MN m2 from 6 mm diameter average size to 0.1 mm diameter average size, requires 9 kW. The same machine is used to crush dolomite at the same output from 6 mm diameter average size to a product consisting of 20 per cent with an average diameter of 0.25 mm, 60 per cent with...

Mno

Whilst the Schildknecht column is essentially a laboratory-scale unit, a melt-crystalliser of the wash-column type was developed by Phillips Petroleum Company in the 1960s for large-scale production of -xylene. The key features of this Phillips pulsed-column crystalliser, as described by McKay etal. 88 , are shown in Figure 15.27. A cold slurry Figure 15.27. Phillips pulsed-column crystalliser1-88-1 Figure 15.27. Phillips pulsed-column crystalliser1-88-1 feed, produced in a scraped-surface...

1473 Natural circulation evaporators

Horizontal Tube Evaporator

Whilst each of the previous types of evaporator is of considerable importance in a given industry, it is the steam-heated evaporator that is the most widely used unit in the process industries and this is now considered in detail. In Chapter 9 of Volume 1, it is shown that the movement of the liquid over the heating surface has a marked influence on the rate of heat transfer, and it is thus convenient to classify evaporators according to the method of agitation or the nature of the circulation...

Centrifugal attrition mills

Figure Attrition Mill

This mill shown in Figure 2.19, consists of a series of pushers which cause heavy cast iron balls to rotate against a bull ring like a ball race, with the pressure of the balls on the bull ring being produced by a loading applied from above. Material fed into the mill falls on the bull ring, and the product is continuously removed in an upward stream of air which carries the ground material between the blades of the classifier, which is shown towards the top of the photograph....

Batch type vacuum filters

Batch vacuum filters were developed from gravity filters and, in essence, Buchner funnels as used in laboratories and Nutsche type filters as used in industry are similar to gravity filters, except that they feature a vacuum pump or some other vacuum generating equipment to reduce the pressure under the filter medium, thereby increasing the driving force across the filter medium. Simple filters operating in this manner are often referred to as pan filters, and they incorporate certain features...

Overall transfer coefficients and transfer units

The driving force over the gas film is taken as yi y and over the liquid film as x xi . If ye is the concentration in the gas phase in equilibrium with concentration x in the liquid phase, then ye y is taken as the overall driving force expressed in terms of y. Similarly x xe is taken as the overall driving force in terms of x, where xe is the concentration in the liquid in equilibrium with a concentration y in the vapour. The overall driving forces Ay o and Ax o may then be written as Ay 0 ye...

2025 Twophase flow in a centrifugal field Introduction

As described by Bisschops et al. 20'21 centrifugal technology, using countercurrent contact of the process liquid with micrometer-range adsorbent particles constitutes a new technique for carrying out adsorption and ion-exchange processes in a centrifugal field. Because the use of very small particles results in large interfacial areas and short distances for diffusion, the mass-transfer rates are extremely fast, and centrifugal adsorption equipment is usually much more compact than that used...

1642 Capillary theory of drying Principles of the theory

The capillary theory of drying has been proposed in order to explain the movement of moisture in the bed during surface drying. The basic importance of the pore space between granular particles was first pointed out by Slichter 16 in connection with the movement of moisture in soils, and this work has been modified and considerably expanded by Haines 17 . The principles are now outlined and applied to the problem of drying. Considering a systematic packing of uniform spherical particles, these...

178 Adsorption Equipment

The scale and complexity of an adsorption unit varies from a laboratory chromatographic column a few millimeters in diameter, as used for analysis, to a fluidised bed several metres in diameter, used for the recovery of solvent vapours, from a simple container in which an adsorbent and a liquid to be clarified are mixed, to a highly-automated moving-bed of solids in plug-flow. All such units have one feature in common in that in all cases the adsorbent becomes saturated as the operation...

1659 Turboshelf dryers

Simple Figure Turbo Type Dryer

The handling of sticky materials can present difficulties, and one type of dryer which is useful for this type of material is the turbo-dryer. As shown in Figure 16.30, wet solid is fed in a thin layer to the top member of a series of annular shelves each made of a number of segmental plates with slots between them. These shelves rotate and, by means of suitably placed arms, the material is pushed through a slot on to a shelf below. After repeated movements, the solid leaves at the bottom of...

1279 The transfer unit

The group f dy ye y , which is used in Chapter 11, has been defined by Chilton and Colburn 52 as the number of overall gas transfer units N0G. The concept of the transfer unit is also introduced in Volume 1, Chapter 10. The application of this group to the countercurrent conditions in the absorption tower is now considered. Over a small height dZ, the partial pressure of the diffusing component A will change by an amount dPAG. Then the moles of A transferred are given by change in mole fraction...

737 Washing of the filter cake

When the wash liquid is miscible with the filtrate and has similar physical properties, the rate of washing at the same pressure difference will be about the same as the final rate of filtration. If the viscosity of the wash liquid is less, a somewhat greater rate will be obtained. Channelling sometimes occurs, however, with the result that much of the cake is incompletely washed and the fluid passes preferentially through the channels, which are gradually enlarged by its continued passage....

734 Delayed cake filtration

In the filtration of a slurry, the resistance of the filter cake progressively increases and consequently, in a constant pressure operation, the rate of filtration falls. If the build-up of solids can be reduced, the effective cake thickness will be less and the rate of flow of filtrate will be increased. In practice, it is sometimes possible to incorporate moving blades in the filter equipment so that the thickness of the cake is limited to the clearance between the filter medium and the...

Fluid energy mills

Another form of mill which does not give quite such a fine product is the jet pulveriser, in which the solid is pulverised in jets of high pressure superheated steam or compressed air, supplied at pressures up to 3.5 MN m2 35 bar . The pulverising takes place in a shallow cylindrical chamber with a number of jets arranged tangentially at equal intervals around the circumference. The solid is thrown to the outside walls of the chamber, and the fine particles are formed by the shearing action...

85 Crossflow Microfiltration

The solids-liquid separation of slurries containing particles below 10 m is difficult by conventional filtration techniques. A conventional approach would be to use a slurry thickener in which the formation of a filter cake is restricted and the product is discharged continuously as a concentrated slurry. Such filters use filter cloths as the filtration medium Membrane flux is denoted by J, the usual symbol in the literature on membranes. It corresponds with uc, as used in Chapters 4 and 7 for...

Nab

Bed inversion a Complete segregation b Complete and partial segregation' The relative rates at which the bed densities change as the fluidising velocity is increased may be obtained by differentiating equations 6.42a and 6.42b with respect to uc, and dividing to give _ dpbH j _ dpbL _ dpbH _ psH - p uql_ V n _ PsH - P eH As eH gt eL and psH gt psL, then from equation 6.44, r, which is independent of fluidising velocity, must be greater than unity. It is thus the bed of heavy...

610 References

Chem. Eng. Prog. 44 1948 201. Fluidization of solid particles. 2. Jackson, R. Trans. Inst. Chem. Eng. 41 1963 13, 22. The mechanisms of fluidised beds. Part 1. The stability of the state of uniform fluidisation. Part 2. The motion of fully developed bubbles. 3. Murray, J. D. J. Fluid Mech. 21 1965 465 22 1965 57. On the mathematics of fluidization. Part 1. Fundamental equations and wave propagation. Part 2. Steady motion of fully developed bubbles. 4. Leva, M.,...

1510 References

W. 'Crystallization' in Kirk-Othmer Encyclopedia of Chemical Technology, Volume 7, 3rd. edition John Wiley amp Sons, New York, 1979 2. Mullin, J. W. 'Crystallization and Precipitation' in Ullmann's Encyclopedia of Industrial Chemistry, Volume B2 VCH Verlagsgesellschaft mbH, Weinheim, 1988 3. Mullin, J. W. Crystallization 4th. edn. Butterworth-Heinemann, Oxford, 2001 4. Feilchenfeld, H. and Sarig, S. Ind. Eng. Chem. Process. Prod. Res. Dev. 24 1985 130-133. Calcium chloride...

The hydraulic jig

Denver Jig

The hydraulic jig operates by allowing material to settle for brief periods so that the particles do not attain their terminal falling velocities, and is therefore suitable for separating materials of wide size range into their constituents. The material to be separated is fed dry, or more usually in suspension, over a screen and is subjected to a pulsating action by liquid which is set in oscillation by means of a reciprocating plunger. The particles on the screen constitute a suspension of...

The Dodge jaw crusher

Dodge Crusher

In the Dodge crusher, shown in Figure 2.5, the moving jaw is pivoted at the bottom. The minimum movement is thus at the bottom and a more uniform product is obtained, although the crusher is less widely used because of its tendency to choke. The large opening at the top enables it to take very large feed and to effect a large size reduction. This crusher is usually made in smaller sizes than the Stag crusher, because of the high fluctuating stresses that are produced in the members of the...

Working Of Stag Jaw Crusher

Fixed Jaw Face Swing Jaw Face Swing Jaw Stock Toggle Seating Front Toggle Plate Toggle Seating Back Toggle Plate Springs and Cups Swing Jaw Shaft Eccentric Shaft 14. Flywheel grooved for V rope drive 19. Cheek Plates top 19A. Cheek Plates bottom 21. Swing Jaw Shaft Bearing Caps 22. Eccentric Shaft Bearing Caps 27. Eccentric Shaft Bearing Bush bottom 28. Eccentric Shaft Bearing Bush top Figure 2.4. Typical cross-section of Stag jaw crusher Stag crushers are made with jaw widths varying from...

The Denver DR flotation machine

Denver Sub Aeration Mechanism

The Denver DR flotation machine, which is an example of a typical froth flotation unit used in the mining industry, is illustrated in Figure 1.47. The pulp is introduced through a feed box and is distributed over the entire width of the first cell. Circulation of the pulp through each cell is such that, as the pulp comes into contact with the impeller, it is subjected to intense agitation and aeration. Low pressure air for this purpose is introduced down the standpipe surrounding the shaft and...

Electronic particle counters

Electronic Particle Counters

A suspension of particles in an electrolyte is drawn through a small orifice on either side of which is positioned an electrode. A constant electrical current supply is connected to the electrodes and the electrolyte within the orifice constitutes the main resistive component of the circuit. As particles enter the orifice they displace an equivalent volume of electrolyte, thereby producing a change in the electrical resistance of the circuit, the magnitude of which is related to the displaced...

154 The hydrocyclone or liquid cyclone

In the hydrocyclone, or hydraulic cyclone, which is discussed extensively in the literature1-29-35-1, separation is effected in the centrifugal field generated as a result of introducing the feed at a high tangential velocity into the separator. The hydrocyclone may be used for a separating particles suspended in a liquid of lower density by size or density, or more generally, by terminal falling velocity Figure 1.36. NEI Delta sizer ultrafine classifier Figure 1.36. NEI Delta sizer ultrafine...

12 Particle Characterisation 121 Single particles

The simplest shape of a particle is the sphere in that, because of its symmetry, any question of orientation does not have to be considered, since the particle looks exactly the same from whatever direction it is viewed and behaves in the same manner in a fluid, irrespective of its orientation. No other particle has this characteristic. Frequently, the size of a particle of irregular shape is defined in terms of the size of an equivalent sphere although the particle is represented by a sphere...