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BY DR YUSUF R.O & DR O.A.A. ELETTA Department of Chemical Engineering SEPARATION PROCESS II

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COURSE OUTLINE Gas Absorption, Solvent Extraction, Multicomponent gas absorption, Extractive and azeotropic distribution, Evaporation, Multiple effect evaporator, Adsorption, Crystallization, Ion Exchange, Reverse Osmosis, Membrane separation Processes. Core; 3 Credits Pr CHE 442 Recommended Texts Chemical Engineering Volume II by Richardson and Coulson Unit Operations of Chemical Engineering by Mc Cabe and Smith 2

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THINGS TO NOTE Note: Attendance is compulsory at lectures. Only students with minimum of 75 % attendance will be allowed to sit for exams Assessment CA comprising Tests and Quizzes30% Examination70% 3

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GAS ABSORPTION Introduction It is a mass transfer operation in which a soluble vapour is absorbed from its mixture with an inert gas by means of a liquid in which the solute gas is more or less soluble. Desorption or gas stripping is the reverse of gas absorption. Comom apparatus employed is the packed column 4

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I t consists of a cylyndrical column equipped with a gas inlet and distributing space at the bottom; a liquid Inlet and distributor at the top; gas outlet at top and liq outlet at the bottom; and a supported mass of inert packings Working of the absorption tower The solute containing gas, enters the column from the base of the column and flows up tru the interstices in the packing counter – current to the flow of the liquid ( with intimate contact) 5

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The solute in the gas is absorbed by the fresh liquid entering the column and the lean gas leaves the top of the tower. The enriched liquid flows down the tower and is discharged at the bottom of the column TYPES OF PACKING 3 principal types Randomly dumped: these are made of cheap, inert materials (clay, porcelain, plastics), thin – walled metals rings or aluminium. The packing allows high void spaces and large passages for the fluids are achieved by making the packing units unit irregular 6

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Or hollow.eg ceramic Berl saddles and Rashing rings Pressure drop and limiting flowrates Stacked by hand Structured or ordered packing:: has ordered geometry evolved Definition of terms Channeling Flooding Loading Point 7

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Pressure Drop and Limiting Flow Rates The fig below shows a typical data for the pressure drop in a packed tower. The pressure drop per unit packing depth comes from the fluid friction which is plotted on logarithmic coordinates against the gas flow rate Gy which is expressed in mass of gas per hr per unit of cross sectional area based on the empty tower. The flowrate is related to the superficial gas velocity by the eqn 8

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9 For dry packing, the line so obtained is straight and has a slope of about 1.8 which implies that the pressure drop increases with the one eighth power of the velocity. For wet packing, ( ie packing is irrigated by a constant flow of liquid), the pressure drop is greater than that in dry packing cos the liq in the tower reduces the space available for gas flow. At moderate gas velocities, the line for irrigated packing gradually becomes steeper as, the gas now impedes the down flowing lq and the liq hold uo increases with the gas rate. The pont at which liq hold up increas is called the loading point

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10 Increasing the gas velocity further leads to a more rapid increase in pressure drop, the lines become almost vertical. For an operating column, the gas velocity must be lower than the flooding velocity. As flooding is approached, most of the packing surface is wetted The choice of velocity must be far enough from the flooding velocity to ensure a safe operation but not Too low as to warrant a much larger column. (pg 713)

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11 Flooding velocity depends strongly on the type and size of packing and the liquid mass velocity. Packing characteristics are accounted for by a packing factor F p which decreases with increasing packing size or increasing void fraction. Eg A tower packed

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13 An empirical equation for the limiting pressure drop is ------1 where Fp = Packing factor and, it is dimensionless eqn 1 can be used for packing factors from 10 to 60. For higher values of Fp, the pressure drop at flooding can be taken as 2.0

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16 Principles of Absorption The diameter of a packed absorption column depends on the quantities of gas and liquid handled, their properties and the ratio of one stream to the other. The height of the tower and hence the total volume of packing, depends on the magnitude of the desired concentration changes and on the rate of mass transfer per unit of packed volume. Calculations of the tower height, therefore, rest on material balances, enthalpy balances, and estimates of driving force and mass – transfer coefficients.

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18 Material Balance over control volume gives

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19 Operating line equation x and y rep the bulk compositions of the liq and gas, resp in contact with each other at any given section through the column. Assumption The composition at any given elevation are independent of position in the packing The absorption of a soluble component from a gas mixture makes the total gas rate V decrease as the gas passes tru the column, and the flow of L increase

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20 This results in the operating line slightly curved Limiting gas – liq ratio Eqn 6 shows that the avg slope of the operating line is L/V, ie the ratio of the molal flows of liq and gas For a given gas flow, a reduction in liq flow decreases the slope of the operating line. Consider the line ab in the figure below. Assume that the gas rate and the terminal concs xa, ya and yb are held fast and the liq flow L decreased. The upper end of the operating line and xb, the conc of the strong liquor increases. The max possible liquor

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