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(V semester Sec A/B)

AN INSIGHT INTO MASS TRANSFER OPERATIONS (Notes on Lesson). (V semester Sec A/B). By Dr.M.SUBAS CHANDRA BOSE(Assistant Prof)(BT34), & Mrs.SABARUNISHA BEGUM(Assistant Prof)(BT29) Bio Technology, REC. MASS TRANSFER 0PERATIONS. MASS TRANSFER

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(V semester Sec A/B)

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  1. AN INSIGHT INTO MASS TRANSFER OPERATIONS (Notes on Lesson) (V semester Sec A/B) By Dr.M.SUBAS CHANDRA BOSE(Assistant Prof)(BT34), & Mrs.SABARUNISHA BEGUM(Assistant Prof)(BT29) Bio Technology, REC Dr.M.SUBAS.C.BOSE

  2. MASS TRANSFER 0PERATIONS MASS TRANSFER ● Transfer of material from one homogeneous phase to another. With or without phase change. Dr.M.SUBAS.C.BOSE

  3. MASS TRANSFER 0PERATIONS UNIT - I DIFFUSION & MASS TRANSFER Dr.M.SUBAS.C.BOSE

  4. MASS TRANSFER 0PERATIONS DIFFUSION The term mass transfer Diffusion is used to denote the transference of a component in a mixture from a region where its concentration is high to a region where the concentration is lower. Mass transfer process can take place in a gas or vapour or in a liquid, and it can result from the random velocities of the molecules (molecular diffusion) or from the circulating or eddy currents present in a turbulent fluid (eddy diffusion). Dr.M.SUBAS.C.BOSE

  5. MASS TRANSFER 0PERATIONS A simple example of a mass transfer process is that occurring in a box consisting of two compartments, each containing a different gas, initially separated by an impermeable partition. When the partition is removed the gases start to mix and the mixing process continues at a constantly decreasing rate until eventually (theoretically after the elapse of an infinite time) the whole system acquires a uniform composition. The process is one of molecular diffusion in which the mixing is attributable solely to the random motion of the molecules. The rate of diffusion is governed by Pick's Law, first proposed by FlCK(I) in 1855 which expresses the mass transfer rate as a linear function of the molar concentration gradient. In a mixture of two gases A and B, assumed ideal, Pick's Law for steady state diffusion may be written as: Dr.M.SUBAS.C.BOSE

  6. MASS TRANSFER 0PERATIONS DIFFUSION IN BINARY GAS MIXTURES 10.2.1, Properties of binary mixtures If A and B are ideal gases in a mixture, the ideal gas law, equation 2.15, may be applied to each gas separately and to the mixture: PAV = nART PBV = nRRT PV = nRT where nA and HB are the number of moles of A and B and n is the total number of moles in a volume V, and PA , PB and P are the respective partial pressures and the total pressure. Dr.M.SUBAS.C.BOSE

  7. MASS TRANSFER 0PERATIONS Equimolecular counterdiffusion When the mass transfer rates of the two components are equal and opposite the process is said to be one of equimolecular counterdiffusion. Such a process occurs in the case of the box with a movable partition, referred to in Section 10.1. It occurs also in a distillation column when the molar latent heats of the two components are the same. At any point in the column a falling stream of liquid is brought into contact with a rising stream of vapour with which it is not in equilibrium. The less volatile component is transferred from the vapour to the liquid and the more volatile component is transferred in the opposite direction. If the molar latent heats of the components are equal, the condensation of a given amount of less volatile component releases exactly the amount of latent heat required to volatilise the same molar quantity of the more volatile component. Thus at the interface, and consequently throughout the liquid and vapour phases, equimolecular counterdiffusion is taking place. Dr.M.SUBAS.C.BOSE

  8. MASS TRANSFER 0PERATIONS Mass transfer through a stationary second component In several important processes, one component in a gaseous mixture will be transported relative to a fixed plane, such as a liquid interface, for example, and the other will undergo no net movement. In gas absorption a soluble gas A is transferred to the liquid surface where it dissolves, whereas the insoluble gas B undergoes no net movement with respect to the interface. Similarly, in evaporation from a free surface, the vapour moves away from the surface but the air has no net movement. The concept of a stationary component may be envisaged by considering the effect of moving the box, discussed in Section 10.1, in the opposite direction to that in which B is diffusing, at a velocity equal to its diffusion velocity, so that to the external observer B appears to be stationary. The total velocity at which A is transferred will then be increased to its diffusion velocity plus the velocity of the box. For the absorption of a soluble gas A from a mixture with an insoluble gas B, the respective diffusion rates are given by: NA = - DAB Dr.M.SUBAS.C.BOSE

  9. MASS TRANSFER 0PERATIONS Mass transfer velocities It is convenient to express mass transfer rates in terms of velocities for the species under consideration where: As a result of the diffusional process, there is no net overall molecular flux arising from diffusion in a binary mixture, the two components being transferred at equal and opposite rates. In the process of equimolecular counterdiffusion which occurs, for example, in a distillation column when the two components have equal molar latent heats, the diffusional velocities are the same as the velocities of the molecular species relative to the walls of the equipment or the phase boundary. Dr.M.SUBAS.C.BOSE

  10. MASS TRANSFER 0PERATIONS Thermal diffusion If a temperature gradient is maintained in a binary gaseous mixture, a concentration gradient is established with the light component collecting preferentially at the hot end and the heavier one at the cold end. This phenomenon, known as the Soret effect, may be used as the basis of a separation technique of commercial significance in the separation of isotopes. Conversely, when mass transfer is occurring as a result of a constant concentration gradient, a temperature gradient may be generated; this is known as the Dufour effect. In a binary mixture consisting of two gaseous components A and B subject to a temperature gradient, the flux due to thermal diffusion is given by GREW and IBBS Dr.M.SUBAS.C.BOSE

  11. MASS TRANSFER 0PERATIONS UNIT - II GAS LIQUID OPERATIONS Dr.M.SUBAS.C.BOSE

  12. MASS TRANSFER 0PERATIONS Gas absorption principles Liquid/Gas Absorption ops includes absorption, stripping and desorption soluble vapour absorbed from mixture with a liquid (solute), solute is then regenerated  can also have gas absorption with reaction. Dr.M.SUBAS.C.BOSE

  13. MASS TRANSFER 0PERATIONS The removal of one or more selected components from a mixture of gases by absorption into a suitable liquid is the second major operation of chemical engineering that is based on interphase mass transfer controlled largely by rates of diffusion. Thus, acetone can be recovered from an acetone–air mixture by passing the gas stream into water in which the acetone dissolves while the air passes out. Similarly, ammonia may be removed from an ammonia–air mixture by absorption in water. In each of these examples the process of absorption of the gas in the liquid may be treated as a physical process, the chemical reaction having no appreciable effect Dr.M.SUBAS.C.BOSE

  14. MASS TRANSFER 0PERATIONS The transfer unit The group (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 NOG. 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. Dr.M.SUBAS.C.BOSE

  15. Total condenser Distillation Overhead vapor Reflux drum 1 Distillate Reflux Rectifying section stages 2 Feed Feed Stage f Boilup Stripping section stages N Partial reboiler Bottoms MASS TRANSFER 0PERATIONS UNIT - III VAPOUR LIQUID OPERATIONS Dr.M.SUBAS.C.BOSE

  16. MASS TRANSFER 0PERATIONS Distillation and V-L Equilibria Types of Distillation Action on an Ideal Plate Mass Balance in a Distillation Column Determination of Ideal Number of Plates – McCabe –Thiele Analysis Differential or batch distillation Flash or equilibrium distillation Continuous Rectification – Binary systems Dr.M.SUBAS.C.BOSE

  17. MASS TRANSFER 0PERATIONS Differential distillation: The simplest examples of batch distillation at a single stage. Starting with a still pot, initially full, heated at a constant rate. In this process the vapour formed on boiling the liquid is removed at once from the system. Since this vapour is richer in the more volatile component, with this result the composition of the product progressively alters. Thus, whilst the vapour formed over a short period is in equilibrium with the liquid At the end of the process the liquid, which has been vaporized, is removed as the bottom product. Dr.M.SUBAS.C.BOSE

  18. MASS TRANSFER 0PERATIONS Let S be the number of mols of material in the still and x be the mol fraction of component A. Suppose an amount dS, containing a mol fraction y of A, be vaporised. Then a material balance on component A gives: ydS = d (Sx) = S dx + x Ds The integral on then right-hand side can be solved graphically if the equilibrium relationship between y and x is available. Thus, if over the range concerned the equilibrium relationship is a straight line of the form y= m x + c Dr.M.SUBAS.C.BOSE

  19. MASS TRANSFER 0PERATIONS This process consists of only a single stage, a complete separation is impossible unless the relatively volatility is finite. Application is restricted to conditions where a preliminary separation is to be followed by a more rigorous distillation, where high purifies is not required, or where the mixture is very easily separated Dr.M.SUBAS.C.BOSE

  20. MASS TRANSFER 0PERATIONS Flash & continuous distillation This method is frequently carried out as a continuous process. Consist of vaporizing a definite fraction of liquid feed in such a way that the vapour evolved is in equilibrium with the residual liquid. The feed is usually pumped through a fired heater and enters the still through a valve where the pressure is reduced. The still is essentially a separator in which the liquid and vapour produced is reduced by reduction in pressure with sufficient time to reach equilibrium. The vapour is removed from the top of the separator and is then usually condensed, while the liquid leaves from the bottom. Dr.M.SUBAS.C.BOSE

  21. MASS TRANSFER 0PERATIONS Flash & continuous distillation It is used in petroleum refining, in which petroleum fractions are heated in pipe stills and the heated fluid flashed in to vapour and residual streams, each containing many components. Flash distillation is used most for separating components that boil at widely different temperatures. It is not effective separating components of comparable volatility, which requires the use of distillation with reflux By definition a vapour leaving a plate are brought into equilibrium. Assume that the plates are numbered serially from top down and that the plate under consideration is the nth plate from the bottom. Then the immediately above plate n is plate n-1, and the immediately below is n+1. Dr.M.SUBAS.C.BOSE

  22. Vn-1,yn-1 Ln-2,Xn-2 MASS TRANSFER 0PERATIONS Plate n -1 Vn,yn Ln-1,Xn-1 Plate n Material Balance diagram for plate n: Vn+1,yn+1 Ln ,xn Plate n+1 Dr.M.SUBAS.C.BOSE

  23. MASS TRANSFER 0PERATIONS For example if two fluid enter plate n and two leave it, the liquid, Ln-1 mol/h, from plate n-1 and the stream of vapour, Vn+1 mol/h, from plate n+1 are brought into intimate contact. A stream of vapour, Vn mol/h, rises to plate n-1 and a stream of liquid, Ln mol/h, descends to plate n+1. Since the vapour streams are the V phase, their considerations are denoted by y; the liquid streams are the L phase and their concentrations are denoted by x. Then the concentrations of the streams entering and leaving the nth plate are as follows: Vapour leaving plate, yn Liquid leaving plate, xn Vapour entering plate, yn+1 Liquid entering plate, xn-1 Dr.M.SUBAS.C.BOSE

  24. MASS TRANSFER 0PERATIONS Boiling-Point Diagram showing rectification on ideal plate: Dr.M.SUBAS.C.BOSE

  25. MASS TRANSFER 0PERATIONS Number of Plates Required in a Distillation Column: To develop a method for the design of distillation units to give the desired fractionation, it is necessary to determine the numbers of trays. Before that the heat and material flows over the trays, the condenser and the reboiler must be established Thermodynamic data is required to establish how much mass transfer is needed to establish equilibrium between the stream leaving each tray. The diameter of the column will be dictated by the necessity to accommodate the desired flow rates, to operate within the available drop in pressure, while at the same time affecting the desired degree of mixing of the stream on each tray. Dr.M.SUBAS.C.BOSE

  26. MASS TRANSFER 0PERATIONS Summary of the material balances for two components systems: Let the process be analysed simply for a binary mixture of A and B as follows: Let F be the number of mols per unit of feed of mol fraction xf of A. D be the number of mols per unit time of vapour formed with y the mol fraction of A and B be the number of mols per unit time of liquid with x the mol fraction of A. Then an overall mass balance gives: F = D + B Component A balance FxF = D xD + B xB Dr.M.SUBAS.C.BOSE

  27. MASS TRANSFER 0PERATIONS Eliminating B and D from these equations give the follow: Net flow rates. Quantity D is the difference between the flow rates of the streams entering and leaving the top of the column. A material balance around the condenser and accumulator in the gives: Dr.M.SUBAS.C.BOSE

  28. MASS TRANSFER 0PERATIONS Eliminating B and D from these equations gives the following: The difference between the flow rates of vapour and liquid anywhere in the upper section of the column is also equal to D. This surface includes the condenser and all plates above n+1. A total material balance around this control surface gives: Similar material balances for A give the following equations: Quantity D xD is the net flows rate of the component A upward in the upper section of the column. It too is constant throughout this part of the equipment. In the lower section of the column the net flow rates are also constant but, are in a downward direction. Dr.M.SUBAS.C.BOSE

  29. MASS TRANSFER 0PERATIONS The net flow of total material equals B; that of the component A is BxB. The following equations apply: Operating lines: There are two sections in the column; there are also two operating lines, one for the rectifying section and other for the stripping section. For the first section (rectifying section) the operation line is represented by: Dr.M.SUBAS.C.BOSE

  30. MASS TRANSFER 0PERATIONS Substituting for DxD in the equation above and eliminating Vn+1 For the section below the feed plate, a material balance over control surface II gives Rearranging this equation and taking into account that the slope is the ratio of liquid flow to the vapour flow, and also eliminating Vm+1 Dr.M.SUBAS.C.BOSE

  31. MASS TRANSFER 0PERATIONS Feed Line: The conditions of the vapour rate or the liquid rate may change depending of the thermal condition of the feed. It is related to the heat to vaporize one mole of feed divided by molar latent heat (q) Various type of feed conditions: Cold feed, q>1 Feed at bubble point (saturated liquid), q=1 Feed partially vapour, 0<q<1 Feed at dew point (saturated vapour), q=0 Feed superheated vapour q<0 Dr.M.SUBAS.C.BOSE

  32. MASS TRANSFER 0PERATIONS Feed Line - Diagram: Dr.M.SUBAS.C.BOSE

  33. MASS TRANSFER 0PERATIONS Feed Line: Cold feed : It is assumed that the entire feed stream adds to the liquid flowing down the column. Feed at bubble point: no condensation is required to heat the feed. Feed partial vapour: the liquid portion of the feed becomes part of the L and the vapour portion becomes part of V Feed saturated vapour the entire feed becomes part of the V Feed superheated: part of the liquid from the rectifying column is vaporized to cool the feed to a state of saturated vapour. Feed Line Equation: If xq = xF, and yq =xF then; The point of intersection of the two operating lines lies on the straight line of slope (q/q -1) and intercept (xF,yF) Dr.M.SUBAS.C.BOSE

  34. MASS TRANSFER 0PERATIONS Reflux Ratio: The analysis of fractionating columns is facilitated by the use of a quantity called reflux ratio. Two ratios are used, one is the ratio of the reflux to the overhead product and the other is the ratio of the reflux to the vapour. Both ratios refer to quantities in the rectifying section. The equations for those ratios are If the operation lines equations are divided D, the result is, for constant molar overflow, This equation is an operation line of the rectifying section Dr.M.SUBAS.C.BOSE

  35. MASS TRANSFER 0PERATIONS Reflux Ratio: The y intercept of this line is xD/(RD+1). The concentration xD is set by the conditions of the design. RD, the reflux ratio, is an operating variable that can be controlled at will by adjusting the split between reflux and overhead product or by changing the amount of vapour formed in the reboiler for a given flow rate of the overhead product. A point at the upper end of the operating line can be obtained by setting xn equal to xD in the equation above. A point at the upper end of the operating line can be obtained by setting xn equal to xD in the previous equation. The operating line for the rectifying section then intersects the diagonal at point (xD, xD). Dr.M.SUBAS.C.BOSE

  36. Minimum Reflux MASS TRANSFER 0PERATIONS a e b The operation lines represented by those two equations are plotted with the equilibrium curve on the x-y diagram.Those equations also show that unless Ln and Lm are constant, the operating lines are curved.The lines can be plotted only if the change in these internal streams with concentration is known. Total (Maximum) Reflux xF Dr.M.SUBAS.C.BOSE

  37. Minimum Reflux MASS TRANSFER 0PERATIONS a e b Influence of the Number of Reflux Ratio: Any change in R will therefore modify the slope of the operation line as can be seen from the Figure xF Dr.M.SUBAS.C.BOSE

  38. Minimum Reflux MASS TRANSFER 0PERATIONS a e b If no product is withdrawn from the still (D=0), the column is said to operate under conditions of total reflux and, as seen from equation , the top operating line has its maximum slope of unity, and coincides with the line x=y. The step become very close to the plate above, these conditions are known as minimum reflux and Rm denotes the reflux ratio. Any small increase in R beyond Rm will give a workable system, though a large numbers of plate will be required. The minimum number of plates is required for a given separation at conditions of total reflux There is a minimum reflux ratio below which it is impossible to obtain the desired enrichment, however many plates are used. xF Dr.M.SUBAS.C.BOSE

  39. Minimum Reflux MASS TRANSFER 0PERATIONS a e b Calculation of Minimum Reflux Ratio Rm Based on the previous figure, the slope of the line ad is given by xF Dr.M.SUBAS.C.BOSE

  40. MASS TRANSFER 0PERATIONS McCabe -Thiele principles Construction the operation lines: Locate the feed line Calculate the y-axis intercept xD/(RD + 1) of the rectifying line and plot that line through the intercept and the point (xD, xD) Draw the stripping line through point (xB,xB) and the intersection of the rectifying line with the feed line. Dr.M.SUBAS.C.BOSE

  41. MASS TRANSFER 0PERATIONS Effect the feed condition on feed line: If the feed is a cold liquid, the feed line slopes will be upward and to the right; if the feed is a saturated liquid, the line is vertical; if the feed is a mixture of liquid and vapour, the lines slopes upward and to the left and the slope is the negative of the ratio of the liquid to the vapour; if the feed is saturated vapour the line is horizontal and if the feed is superheated vapour. The lines slope downward and to the left. Dr.M.SUBAS.C.BOSE

  42. MASS TRANSFER 0PERATIONS Feed Plate location After the location of the feed plate the construction of the number of ideal trays is found by the usual step-by step construction. The process can begin at the top and also a total condenser is used. Dr.M.SUBAS.C.BOSE

  43. MASS TRANSFER 0PERATIONS Dr.M.SUBAS.C.BOSE

  44. MASS TRANSFER 0PERATIONS UNIT - IV EXTRACTION OPERATIONS Dr.M.SUBAS.C.BOSE

  45. MASS TRANSFER 0PERATIONS Liquid–Liquid Extraction 13.1. INTRODUCTION The separation of the components of a liquid mixture by treatment with a solvent in which one or more of the desired components is preferentially soluble is known as liquid–liquid extraction—an operation which is used, for example, in the processing of coal tar liquids and in the production of fuels in the nuclear industry, and which has been applied extensively to the separation of hydrocarbons in the petroleum industry. In this operation, it is essential that the liquid-mixture feed and solvent are at least partially if not completely immiscible and, in essence, three stages are involved: (a) Bringing the feed mixture and the solvent into intimate contact, (b) Separation of the resulting two phases, and (c) Removal and recovery of the solvent from each phase. Dr.M.SUBAS.C.BOSE

  46. MASS TRANSFER 0PERATIONS Stage wise Extraction Dr.M.SUBAS.C.BOSE

  47. MASS TRANSFER 0PERATIONS Continuous Extraction Dr.M.SUBAS.C.BOSE

  48. MASS TRANSFER 0PERATIONS Dr.M.SUBAS.C.BOSE

  49. MASS TRANSFER 0PERATIONS General principles of Leaching Leaching is concerned with the extraction of a soluble constituent from a solid by means of a solvent. The process may be used either for the production of a concentrated solution of a valuable solid material, or in order to remove an insoluble solid, such as a pigment, from a soluble material with which it is contaminated. The method used for the extraction is determined by the proportion of soluble constituent present, its distribution throughout the solid, the nature of the solid and the particle size. Dr.M.SUBAS.C.BOSE

  50. MASS TRANSFER 0PERATIONS Factors influencing the rate of extraction The selection of the equipment for an extraction process is influenced by the factors which are responsible for limiting the extraction rate. Thus, if the diffusion of the solute through the porous structure of the residual solids is the controlling factor, the material should be of small size so that the distance the solute has to travel is small. On the other hand, if diffusion of the solute from the surface of the particles to the bulk of the solution is the controlling factor, a high degree of agitation of the fluid is required. Dr.M.SUBAS.C.BOSE

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