747 The tube press

One of the major problems in coal preparation is the dewatering of fine coal to a moisture content sufficiently low enough both to meet market requirements and to ease the problems of handlability encountered at some collieries and there is an interest in different types of dewatering equipment in addition to the conventional rotary vacuum filter. As a result, a tube press built by the former English China Clays Limited, now Imerys, was installed at a colliery where it was used to treat raw...

O

Chromatogram for stepwise elution of bovine serum albumin on a Vistec diethyl aminoethyl cellulose ion-exchanger, using stepwise increases in sodium chloride concentration in the mobile phase to achieve selective desorption. Proteins 1, serum fraction not adsorbed by column (includes y-gobulin) 2,3, transferrin, and so on 4,5 albumin1-57-1 For large-scale separations, a further operational change is commonly adopted. Maximum production requires that the width of the protein band...

131 General characteristics

The properties of solids in bulk are a function of the properties of the individual particles including their shapes and sizes and size distribution, and of the way in which the particles interact with one another. By the very nature of a particulate material, it is always interspersed with a fluid, generally air, and the interaction between the fluid and the particles may have a considerable effect on the behaviour of the bulk material. Particulate solids present considerably greater problems...

Population balance

Growth and nucleation interact in a crystalliser in which both contribute to the final crystal size distribution (CSD) of the product. The importance of the population balance(37) is widely acknowledged. This is most easily appreciated by reference to the simple, idealised case of a mixed-suspension, mixed-product removal (MSMPR) crystalliser operated continuously in the steady state, where no crystals are present in the feed stream, all crystals are of the same shape, no crystals break down by...

Sieving 50 m

Sieve analysis may be carried out using a nest of sieves, each lower sieve being of smaller aperture size. Generally, sieve series are arranged so that the ratio of aperture sizes on consecutive sieves is 2, 21 2 or 21 4 according to the closeness of sizing that is required. The sieves may either be mounted on a vibrator, which should be designed to give a degree of vertical movement in addition to the horizontal vibration, or may be hand shaken. Whether or not a particle passes through an...

Solution

Taking a material balance for the whole throughput and for the ammonia gives D + W 1.0 0.995D + 0.1 W (1.0 x 0.3) Thus D 0.22 kg s The enthalpy-composition chart for this system is shown in Figure 11.29. It is assumed that the feed F and the bottom product W are both liquids at their boiling points. Nm for minimum reflux is found by drawing a tie-line through F, representing the feed, to cut the line x 0.995 at Nm. length AL (1952 - 1547) (1547 - 295) Figure 11.29. Enthalpy-composition diagram...

77

By interpolation, yr yro 0.5 when f 4.4 h. Because of the difficulty of ensuring that the solid moves steadily and at a controlled rate with respect to the containing vessel, other equipment has been developed in which solid and vessel move together, relative to a fixed inlet for the feed and a fixed outlet for the product. Figure 17.29 shows the principle of operation of a rotary-bed adsorber used, for example, for solvent recovery from air on to activated-carbon. The activated-carbon is...

1762 Pore sizes

Having obtained a measure of surface area, a mean pore size may be calculated by simplifying the pore system into np cylindrical pores per unit mass of adsorbent, of mean length Lp and mean pore radius rp. Hence Pore volume Vp npnr2Lp This expression has been generalised by Everett and Stone(8) for any shape of capillary by including a shape factor y which takes a value that depends on the geometry of the capillary. This is unity for parallel sided fissures as well as for cylindrical pores....

106 Number Of Stages For Countercurrent Washing By Graphical Methods

It is sometimes convenient to use graphical constructions for the solution of countercurrent leaching or washing problems. This may be done by a method similar to the McCabe-Thiele method for distillation which is discussed in Chapter 11, with the overflow and underflow streams corresponding to the vapour and liquid respectively. The basis of this method is now given, although a generally more convenient method involves the use of triangular diagrams which will be discussed in some detail. For...

136 Flow of solids through orifices

The discharge rate of solid particles is usually controlled by the size of the orifice or the aperture at the base of the hopper, though sometimes screw feeders or rotating table feeders may be incorporated to encourage an even flowrate. The flow of solids through an orifice depends on the ability of the particles to dilate in the region of the aperture. Flow will occur if the shear force exerted by the superincumbent material exceeds the shear strength of the powder near the outlet. The rate...

Draughttube agitated vacuum crystallisers

A Swenson draught-tube-baffled (DTB) vacuum unit is shown in Figure 15.17. A relatively slow-speed propellor agitator is located in a draught tube that extends to a small distance below the liquor level. Hot, concentrated feed-stock, enters at the base of the draught tube, and the steady movement of magma and feed-stock to the surface of the liquor produces a gentle, uniform boiling action over the whole cross-sectional area of the crystalliser. The degree of supercooling thus produced is less...

1532 Evaporating crystallisers

If the solubility of a solute in a solvent is not appreciably decreased by lowering the temperature, the appropriate degree of solution supersaturation can be achieved by evaporating some of the solvent and the oldest and simplest technique, the use of solar energy, is still employed commercially throughout the world(66). Common salt is produced widely from brine in steam-heated evaporators, multiple-effect evaporator-crystallisers are used in sugar refining and many types of forced-circulation...

771

In applying this equation to evaporators, there may be some difficulty in deciding the correct value for the temperature difference because of what is known as the boiling point rise (BPR). If water is boiled in an evaporator under a given pressure, then the temperature of the liquor may be determined from steam tables and the temperature difference is readily calculated. At the same pressure, a solution has a boiling point greater than that of water, and the difference between its boiling...

10000

Carman's graph of R pu2 against Re1 The form of equation 4.16 is similar to that of equation 4.17 proposed by Forchheimer(8) who suggested that the resistance to flow should be considered in two parts that due to the viscous drag at the surface of the particles, and that due to loss in turbulent eddies and at the sudden changes in the cross-section of the channels. Thus The first term in this equation will predominate at low rates of flow where the losses are mainly attributable to...

2

If za is the value of z0 which corresponds to the position where the free surface is at the axis of rotation (r0 0), then Equations 9.1 and 9.2 correspond with equations 2.80 and 2.79 in Volume 1, Chapter 2. Taking the base of the bowl as the origin for the measurement of z0, positive values of za correspond to conditions where the whole of the bottom of the bowl is covered by liquid. Negative values of za imply that the paraboloid of revolution describing the free surface would cut the axis of...

1

Where NL is the number of liquid film transfer units and Hl is the height of a liquid film transfer unit, which for distillation applications is presented in Table 11.6, taken from the work of Gilliland and Sherwood(73), as a function of type and size of packing. Table 11.6. Values of HL for distillation(73) Table 11.6. Values of HL for distillation(73)

421 Darcys law and permeability

The first experimental work on the subject was carried out by Darcy1 in 1830 in Dijon when he examined the rate of flow of water from the local fountains through beds of sand of various thicknesses. It was shown that the average velocity, as measured over the whole area of the bed, was directly proportional to the driving pressure and inversely proportional to the thickness of the bed. This relation, often termed Darcy's law, has subsequently been confirmed by a number of workers and can be...

Example 117

A mixture of benzene and toluene containing 40 mole per cent benzene is to be separated to give a product containing 90 mole per cent benzene at the top, and a bottom product containing not more than 10 mole per cent benzene. The feed enters the column at its boiling point, and the vapour leaving the column which is condensed but not cooled, provides reflux and product. It is proposed to operate the unit with a reflux ratio of 3 kmol kmol product. It is required to find the number of...

11102 Factors determining column performance

The performance of a column may be judged in relation to two separate but related criteria. First, if the vapour and liquid leaving a tray are in equilibrium this constitutes a theoretical tray and provides a standard of performance. Secondly, the relative performance of, say, two columns of the same diameter must be considered in relation to their capacity for liquid and vapour flow. The main features are (a) Liquid and vapour velocities. (b) Physical properties of the liquid and vapour. (c)...

Rosens solutions

Rate equations, such as equation 17.85, make no attempt to distinguish mechanisms of transfer within a pellet. All such mechanisms are taken into account within the rate constant k. A more fundamental approach is to select the important factors and combine them to form a rate equation, with no regard to the mathematical complexity of the equation. In most cases this approach will lead to the necessity for numerical solutions although for some limiting conditions, useful analytical solutions are...

1441 General principles

If an evaporator, fed with steam at 399 K with a total heat of 2714 kJ kg, is evaporating water at 373 K, then each kilogram of water vapour produced will have a total heat content of 2675 kJ. If this heat is allowed to go to waste, by condensing it in a tubular condenser or by direct contact in a jet condenser for example, such a system makes very poor use of steam. The vapour produced is, however, suitable for passing to the calandria of a similar unit, provided the boiling temperature in the...

08

Cumulative oversize distribution Fu(d) as a function of F(d) for various particle sizes (d) Figure 1.11. Cumulative oversize distribution Fu(d) as a function of F(d) for various particle sizes (d) Grade efficiency in a centrifugal separator The behaviour of suspended particles in a centrifugal field is considered in detail in Chapter 3. It is, however, convenient to consider here the extension of the preceding treatment of grade efficiency to centrifugal separators. In the case of...

Directcontact cooling

The occurrence of crystal encrustation in conventional heat exchangers can be avoided by using direct-contact cooling (DCC) in which supersaturation is achieved by allowing the process liquor to come into contact with a cold heat-transfer medium. Other potential advantages of DCC include better heat transfer and lower cooling loads, although disadvantages include product contamination from the coolant and the cost of extra processing required to recover the coolant for further use. Since a...

533 Solids flux in batch sedimentation

In a sedimenting suspension, the sedimentation velocity uc is a function of fractional volumetric concentration C, and the volumetric rate of sedimentation per unit area or flux f is equal to the product ucC. Thus f ucC uc(1 e) (5.88) Then, if the relation between settling velocity and concentration can be expressed in terms of a terminal falling velocity (u0) for the particles, substituting for u0 using equation 5.71 From the form of the function, it is seen that f should have a maximum at...

Info

0.01 0.02 0.04 0.08 0.2 0.3 0.5 0.8 1 0.03 0.06 0.1 0.4 0.6 0.01 0.02 0.04 0.08 0.2 0.3 0.5 0.8 1 0.03 0.06 0.1 0.4 0.6 Figure 6.7. Bed expansion for the fluidisation of 0.5-0.6 mm lead shot in water in a 100 mm tube It is, however, surprising that a reduction in particle size should also cause increased deviations from the ideal system. It may be noted from Figure 6.7 that, over a wide range of liquid velocities, the mean voidage of the bed is less than that predicted by equation 6.31. This...

2013 The role of the chemical engineer

In a further paper, Edwards(6) classifies the processes and operations which occur in the manufacture and supply of products using an appropriate length scale as follows Chemical engineers are well versed in the design and sizing of unit operations such as reactors, mixing vessels, heat exchangers and separation units, operating on a length scale appropriate to the equipment of around 1 metre. Chemical engineers are also able to integrate individual operations to create an entire plant or...

1112 Further Reading

Process Plant Design (Heinemann Educational Books, London, 1973). Billet, R. Distillation Engineering (Heyden and Sons Ltd., 1979). Buckley, P. S., Luyben, W. L. and Shunta, J. P. Design of Distillation Column Control Systems (Edward Arnold, New York, 1985). Hoffman, E. J. Azeotropic and Extractive Distillation (Interscience Publishers, Inc., New York, 1964). Holland, C. D. Fundamentals of Multicomponent Distillation (McGraw-Hill Book Co., New York, 1981)....

Xl

1 Xtt is strongly dependent on the mass fraction of vapour y. The density and viscosity terms give a quantitative correction for the effect of pressure in the absence of nucleate boiling. Eighty-five per cent of the purely convective data for two-phase flow were correlated to within 20 per cent by the expression Similar results for a range of organic liquids are reported by Guerrieri and Talty(10), though, in this work, hL is based on the point mass flowrate of the unvaporised part of the...

B10h14

Naphthalene Quinoline, 6-decyl-1, 2, 3, 4-tetrahydro-naphthalene, 2-butyl-l-hexylindan Size limit for Linde sieve 10X about here ( 0.8 nm) Size limit for Linde sieve 13X about here ( 1.0 nm) oxidised at temperatures in excess of 1000 K in atmospheres of materials such as steam or carbon dioxide. Activated carbon has a typical surface area of 106 m2 kg, mostly associated with a set of pores of about 2 nm in diameter. There is likely to be another set of pores of about 1000 nm in diameter, which...

Filter press

Using a filter press with n frames of thickness b m, the total time, for one complete cycle of the press (if + 120n + 240) s, where if is the time during which filtration is occurring. Overall rate of filtration --- 1.25 x 1CT4 m3 s where Vf is the total volume of filtrate per cycle. The volume of frames volume of cake deposited by unit volume of filtrate, v, is given by But Vf (3.38 x 10-10) x (1.75 x 105)(2n x 0.3 x 0.3)2if (from equation 7.11) That is 25.8b2 + 0.015n + 0.030 0.629nb n is a...

5000

Volume as function of time for delayed cake and constant pressure filtration as a function of The concentration of solids in the slurry in the feed vessel to the filter at any time can be calculated by noting that the volumetric rate of feed of slurry must be equal to the rate at which filtrate leaves the vessel. For a rate of flow of filtrate of dV dt out of the filter, the rate of flow of slurry into the vessel must also be dV dt and the corresponding influx of solids is (1 e0) dV...

1536 Batch and continuous crystallisation

Continuous, steady-state operation is not always the ideal mode for the operation of crystallisation processes, and batch operation often offers considerable advantages such as simplicity of equipment and reduced encrustation on heat-exchanger surfaces. Whilst only a batch crystalliser can, in certain cases, produce the required crystal form, size distribution, or purity, the operating costs can be significantly higher than those of a comparable continuous unit, and problems of product...

Example 146

For the concentration of fruit juice by evaporation it is proposed to use a falling-film evaporator and to incorporate a heat pump cycle with ammonia as the medium. The ammonia in vapour form will enter the evaporator at 312 K and the water will be evaporated from the juices at 287 K. The ammonia in the vapour-liquid mixture will enter the condenser at 278 K and the vapour will then pass to the compressor. It is estimated that the work for compressing the ammonia will be 150 kJ kg of ammonia...

Plate efficiency in terms of liquid concentrations

With the same concept for tray layout as in Figure 11.55, relations for Eml and EMl may be derived. Assuming that the vapour concentration does not change in a horizontal plane, a similar analysis to that above gives Emi 1 exp( N0l) (11.134) The efficiencies Emv and Eml may be related by using the relation between N0G and Nol given in equation 11.130 to give ln(1 Emi) e ln(1 Emv) (11.135)

0105

From equation 11.103 ln(Sj S2) 1.1 and (S1 S2) 3.0. Product obtained, Db S1 S2 (100 100 3) 66.7 kmol. Amount of ethanol in product xjSj x2S2 (0.55 x 100) (0.105 x 33.3) 51.5 kmol Thus average composition of product (51.5 66.7) 0.77 mole fraction ethanol. The heat required to provide the reflux (4000 x 2.1 x 66.7) 560,380 kJ. Heat required to provide reflux per kmol of product (560,380 66.7) 8400 kJ. Thus in Example 11.12 the total heat required per kmol of product is (5320 + 4000) 9320 kJ and...

422 Specific surface and voidage

The general structure of a bed of particles can often be characterised by the specific surface area of the bed SB and the fractional voidage of the bed e. SB is the surface area presented to the fluid per unit volume of bed when the particles are packed in a bed. Its units are (length)-1. e is the fraction of the volume of the bed not occupied by solid material and is termed the fractional voidage, voidage, or porosity. It is dimensionless. Thus the fractional volume of the bed occupied by...

425

The solid is usually in contact with the hot metal for 6-15 s, short enough to prevent significant decomposition of heat sensitive materials, and heat transfer coefficients are 1-2 kW m2 K. When the temperature of the drying material must be kept as low as possible, vacuum drying is used, and one form of vacuum dryer is shown in Figure 16.18. The dried material is collected in two screw conveyors and carried usually to two receivers so that one can be filled while the other is emptied.

1342 Cocurrent contact with immiscible solvents

In this case, which is illustrated in Figure 13.12, triangular diagrams are not required. If the initial solution contains a mass A of solvent A with a mass ratio Xf of solute, then the selective solvent to be added will be a mass S of solvent S. On mixing and separating, a raffinate is obtained with the solvent A containing a mass ratio X1 of solute, and an extract with the solvent S containing a mass ratio Y1 of solute. A material balance on the solute gives

1144 Location of feed point in a continuous still

From Figure 11.20 it may be seen that, when stepping off plates down the top operating line AB, the bottom operating line CE cannot be used until the value of xn on any plate is less than xe. Again it is essential to pass to the lower line CE by the time xn xb. The best conditions are those where the minimum number of plates is used. From the geometry of the figure, the largest steps in the enriching section occur down to the point of intersection of the operating lines at x xq. Below this...

334 Rising velocities of light particles

Although there appears to be no problem in using the standard relations between drag coefficient and particle Reynolds number for the calculation of terminal falling velocities of particles denser than the liquid, Karamanev, Chavarie and Mayer(12) have shown experimentally that, for light particles rising in a denser liquid, an overestimate of the terminal rising velocity may result. This can occur in the Newton's law region and may be associated with an increase in the drag coefficient CD'...

522 Flocculation Introduction

The behaviour of suspensions of fine particles is very considerably influenced by whether the particles flocculate. The overall effect of flocculation is to create large conglomerations of elementary particles with occluded liquid. The flocs, which easily become distorted, are effectively enlarged particles of a density intermediate between that of the constituent particles and the liquid. The tendency of the particulate phase of colloidal dispersions to aggregate is an important physical...

162 General Principles

The moisture content of a material is usually expressed in terms of its water content as a percentage of the mass of the dry material, though moisture content is sometimes expressed on a wet basis, as in Example 16.3. If a material is exposed to air at a given temperature and humidity, the material will either lose or gain water until an equilibrium condition is established. This equilibrium moisture content varies widely with the moisture content and the temperature of the air, as shown in...

91 Introduction

There is now a wide range of situations where centrifugal force is used in place of the gravitational force in order to effect separations. The resulting accelerations may be several thousand times that attributable to gravity. Some of the benefits include far greater rates of separation the possibility of achieving separations which are either not practically feasible, or actually impossible, in the gravitational field and a substantial reduction of the size of the equipment. Recent...

2014 Green chemistry

Increasing concern for the need to conserve and to use effectively world reserves of raw materials and, at the same time, to reduce the quantities of waste materials which are likely to have an adverse effect on the environment has led to pressure for the increased use of renewable resources and so-called 'green chemistry'. The principles of green chemistry have been enunciated by Hamley and Poliakoff1-7 as follows molecular level micro level unit operations factory supply chain Length (m) lO-l...

3360 4000 4760

The efficiency of screening is defined as the ratio of the mass of material which passes the screen to that which is capable of passing. This will differ according to the size of the material. It may be assumed that the rate of passage of particles of a given size through the screen is proportional to the number or mass of particles of that size on the screen at any instant. Thus, if w is the mass of particles of a particular size on the screen at a time t, then where k is a constant for a...

Simple and vacuum sublimation

Simple sublimation is a batch-wise process in which the solid material is vaporised and then diffuses towards a condenser under the action of a driving force attributable to difference in partial pressures at the vaporising and condensing surfaces. The vapour path between the vaporiser and the condenser should be as short as possible in order to reduce masstransfer resistance. simple sublimation has been used for centuries, often in very crude equipment, for the commercial production of...

Expressions for plate efficiency related to mass transfer

By assuming that the vapour issuing from slots is in the form of spherical bubbles, Chu(66), Geddes(67), and Bakowski(68) derived methods for expressing the efficiency E in terms of transfer coefficients, kg, kl and tray parameters such as the slot dimensions. These methods have proved very difficult to use because of the unreliability of data for calculating transfer coefficients, and the greater problem of calculating the interfacial areas. Probably the most successful analysis for...

E

Fixed bearing whereas the other has an adjustable spring-loaded bearing. Since the rollers rotate at different speeds, size reduction is effected by a combination of compressive and shear forces. The roller mill is extensively used in the flour milling industry and for the manufacture of pigments for paints.

1123 Relative volatility

The relationship between the composition of the vapour yA and of the liquid xA in equilibrium may also be expressed in a way, which is particularly useful in distillation calculations. If the ratio of the partial pressure to the mole fraction in the liquid is defined as the volatility, then Volatility of A and volatility of B The ratio of these two volatilities is known as the relative volatility a given by Substituting PyA for PA, and PyB for PB This gives a relation between the ratio of A and...

1382 Aqueous twophase systems

The possibility of a three component system consisting of water and two organic components separating into two phases has been known for some time, though it is only since biochemical processes, including fermentation, have been used for the production of expensive highly complex molecules that it has assumed such a great industrial importance. Two-phase aqueous systems are now extensively used for the separation of product from the associated cell debris. Essentially, the addition of a...

Forcedcirculation crystallisers

A Swenson forced-circulation crystalliser operating at reduced pressure is shown in Figure 15.15. A high recirculation rate through the external heat exchanger is used to provide good heat transfer with minimal encrustation. The crystal magma is circulated from the lower conical section of the evaporator body, through the vertical tubular heat exchanger, and reintroduced tangentially into the evaporator below the liquor level to create a swirling action and prevent flashing. Feed-stock enters...

1943Liquid chromatography LC

Many forms of liquid chromatography have been developed in order to separate different types of compound, as shown in Table 19.2. Bonded-phase chromatography (BPC) is an improved form of the now virtually extinct liquid-liquid chromatography. The problem of mutual dissolution of the two liquid phases, and hence progressive loss of stationary phase during service, is solved by chemically bonding the stationary liquid as a monomolecular layer to the surface of a solid support. The support is...

93 Centrifugal Pressure

A force balance on a sector of fluid in the rotating bowl, carried out as in Volume 1, Chapter 2, gives the pressure gradient at a radius r Unlike the vertical pressure gradient in a column of liquid which is constant at all heights, the centrifugal pressure gradient is a function of radius of rotation r, and increases towards the wall of the basket. Integration of equation 9.3 at a given height gives the pressure P exerted by the liquid on the walls of the bowl of radius R when the radius of...

11105 Plate efficiency

The number of ideal stages required for a desired separation may be calculated by one of the methods discussed previously, although in practice more trays are required than ideal stages. The ratio n np of the number of ideal stages n to the number of actual trays np represents the overall efficiency E of the column, which may be 30-100 per cent1-4). The main reason for loss in efficiency is that the kinetics for the rate of approach to equilibrium, and the flow pattern on the plate, may not...

222 Mechanism of size reduction

Whilst the mechanism of the process of size reduction is extremely complex, in recent years a number of attempts have been made at a more detailed analysis of the problem. If a single lump of material is subjected to a sudden impact, it will generally break so as to yield a few relatively large particles and a number of fine particles, with relatively few particles of intermediate size. If the energy in the blow is increased, the larger particles will be of a rather smaller size and more...

1522 Solubility and saturation

A solution that is in thermodynamic equilibrium with the solid phase of its solute at a given temperature is a saturated solution, and a solution containing more dissolved solute than that given by the equilibrium saturation value is said to be supersaturated. The degree of supersaturation may be expressed by where c and c* are the solution concentration and the equilibrium saturation value respectively. The supersaturation ratio, S, and the relative supersaturation, p are then and p Ac c* S -...

1861 Staged operations

In the simple batch process, conservation of counter-ions leaving the liquid may be written as V(Co - C) Rv(CS - Cso) (18.24) where V and Rv refer to initial volumes of liquid and resin. If the batch process behaves as an equilibrium stage, the phases in contact will achieve equilibrium. If the equilibrium relationship is known, then and equations 18.25 and 18.26 may be solved. In Figure 18.5 it is assumed that V and Rv remain constant. It is sometimes convenient to use the fractional...

Plate efficiency in terms of transfer units

The process of mass transfer across a phase boundary is discussed in Volume 1, Chapter 10. A resistance to mass transfer exists within the fluid on each side of the interface, and the overall transfer rate of a component in a mixture depends on the sum of these resistances and the total driving force. The concept of a transfer unit for a countercurrent mass transfer process, introduced in Volume 1, is developed further for distillation in packed columns in Section 11.11. The number of transfer...

Colloid mills

Colloidal suspensions, emulsions and solid dispersions are produced by means of colloid mills or dispersion mills. Droplets or particles of sizes less than 1 m may be formed, and solids suspensions consisting of discrete solid particles are obtainable with feed material of approximately 100-mesh or 50 m in size. As shown in Figure 2.33, the mill consists of a flat rotor and stator manufactured in a chemically inert synthetic abrasive material, and the mill can be set to operate at clearances...

L L

Concentration of adsorbate in equilibrium with C s Specific heat capacity of the gas phase Specific heat capacity of the adsorbent with adsorbate Specific heat capacity of the wall Diffusivity Average diffusivity Effective diffusivity Knudsen diffusivity Longitudinal diffusivity Molecular diffusivity Surface diffusivity Surface diffusivity in standard conditions Total diffusivity Bed diameter Pellet diameter Energy of activation in surface diffusion Energy of activation of desorption from empty...

0033

Whilst the solubility of a gas is not substantially affected by the total pressure in the system for pressures up to about 500 kN m2, it is important to note that the solubility falls with a rise of temperature. Thus, for a concentration of 25 per cent by mass of ammonia in water, the equilibrium partial pressure of the ammonia is 30.3 kN m2 at 293 K and 46.9 kN m2 at 303 K. In many instances the absorption is accompanied by the evolution of heat, and it is therefore necessary to fit coolers to...

17 Further Reading

Particle Size Measurement, Volumes 1 and 2. 5th edn. (Chapman and Hall, London, 1997) Beddow, J. K. Particle Characterization inTechnology, Vol 1 Application and Micro-analysis, Vol 2 Morphological Analysis. (CRC Press, Boca Raton, FL, 1984) Bohren, C. F. and Huffman, D. R. Absorption and Scattering of Light by Small Particles (Wiley, New York, 1983) Bradley, D. The Hydrocyclone. (Pergamon Press, Oxford, 1965) Clarke, A. N. and Wilson, D. J. Foam Flotation, Theory and Applications....

1542 Multistageprocesses

A single-stage crystallisation process may not always achieve the required product purity and further separation, melting, washing, or refining may be required. Two approaches are used a) a repeating sequence of crystallisation, melting, and re-crystallisation b) a single crystallisation step followed by countercurrent contacting of the crystals with a relatively pure liquid stream. The first approach is preferred if the concentration of impurities in the feedstock is high, and is essential if...

1863 Moving beds

In principle, all the moving bed devices discussed in Chapter 17 may be used for ion exchange. Ion exchange is largely a liquid-phase phenomenon in which the solids may be made to flow relatively easily when immersed in liquid. A method of moving resin discontinuously has been developed by Higgins, as shown in Figure 18.8, and described in Kirk-Othmer(18) and by Setter etal.(19). For a short period in a cycle time, ranging from a few minutes to several hours, the resin is moved by pulses of...

65 Heat Transfer To A Boundary Surface 651 Mechanisms involved

The good heat transfer properties of fluidised systems have led to their adoption in circumstances where close control of temperature is required. The presence of the particles in a fluidised system results in an increase of up to one-hundredfold in the heat transfer coefficient, as compared with the value obtained with a gas alone at the same velocity. In a liquid-fluidised system the increase is not so marked. Many investigations of heat transfer between a gas-fluidised system and a heat...

95 Sedimentation In A Centrifugal Field

Centrifuges are extensively used for separating fine solids from suspension in a liquid. As a result of the far greater separating power compared with that available using gravity, fine solids and even colloids may be separated. Furthermore, it is possible to break down emulsions and to separate dispersions of fine liquid droplets, though in this case the suspended phase is in the form of liquid droplets which will coalesce following separation. Centrifuges may be used for batch operation when...

828

(Dryer inlet 700 K, dryer outlet 365 K) (Dryer inlet 700 K, dryer outlet 365 K) In these systems, the total collection efficiencies of the dry product are 85 per cent for the drying vessel, 90 per cent for the cyclone collector and 98 per cent for the scrubber-condenser. The net efficiency of the system may be as high as 99.97 per cent if the scrubber effluent is considered as product. All the runs are based on 1.25 kg s product and 0.75 kg s evaporation at an elevation of 300 m above sea...

Streamline and turbulent flow

Equation 4.9 applies to streamline flow conditions, though Carman(7) and others have extended the analogy with pipe flow to cover both streamline and turbulent flow conditions through packed beds. In this treatment a modified friction factor R pu is plotted against a modified Reynolds number Re . This is analogous to plotting R pu2 against Re for flow through a pipe as in Volume 1, Chapter 3. The modified Reynolds number Re1 is obtained by taking the same velocity and characteristic linear...

86 Ultrafiltration

Ultrafiltration is one of the most widely used of the pressure-driven membrane separation processes. The solutes retained or rejected by ultrafiltration membranes are those with molecular weights of 103 or greater, depending mostly on the MWCO of the membrane chosen. The process liquid, dissolved salts and low molecular weight organic molecules (500-1000 kg kmol) generally pass through the membrane. The pressure difference applied across the membrane is usually in the range 0.1-0.7 MN m2 and...

243 Size enlargement processes

Processes commonly used for size enlargement are listed in Table 2.4, taken from Perry(30). For comprehensive overall reviews, reference may be made to Perry and to the work of Browning(31). (a) Spray drying (as discussed in Chapter 16). In this case, particle size is largely determined by the size of the droplet of liquid or suspension, which may be controlled by a suitably designed spray nozzle. The aggregates of dried material are held together as a result of the deposition of small amounts...

Example 142A Forwardfeed

4 kg s (14.4 tonne hour) of a liquor containing 10 per cent solids is fed at 294 K to the first effect of a triple-effect unit. Liquor with 50 per cent solids is to be withdrawn from the third effect, which is at a pressure of 13 kN m2 ( 0.13 bar). The liquor may be assumed to have a specific heat of 4.18 kJ kg K and to have no boiling point rise. Saturated dry steam at 205 kN m2 is fed to the heating element of the first effect, and the condensate is removed at the steam temperature in each...

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...