1.1 INTRODUCTION TO MASS TRANSFER OPERATION Mass transfer is a transport of components under a chemical potential gradient. The component moves to the direction of reducing concentration gradient. The transport occurs from a region of higher concentration to lower concentration. Equilibrium reached when the gradient is zero. The transport or migration of one constituent from a region of higher concentration to that of a lower concentration known as mass transfer. Mass transfer operations depend on molecules diffusing from one distinct phase to another and based upon differences in the physico-chemical properties of the molecules, such as vapour pressure or solubility. For interphase mass transfer, there is a concentration gradient between bulk and interface, however under steady state, at interface equilibrium assumed. Mass transfer operation plays an important role in many industrial processes. A group of operations for separating the components of mixtures ased on the transfer of material from one homogeneous phase to another. These methods is covered by the term mass transfer operations which include techniques like gas absorption and stripping, liquid-liquid extraction, leaching, distillation, humidification, drying, crystallization and number of other separation techniques.
Absorption of Gases : 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 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 removed from an ammonia–air mixture by absorption in water.
CONDITIONS OF EQUILIBRIUM BETWEEN LIQUID AND GAS :
When two phases are brought into contact they eventually reach equilibrium. Thus, water in contact with air evaporates until the air is saturated with water vapour, and the air is absorbed by the water until it becomes saturated with the individual gases. In any mixture of gases, the degree to which each gas is absorbed is determined by its partial pressure. At a given temperature and concentration, each dissolved gas exerts a definite partial
Pressure. For dilute concentrations of most gases, and over a wide range for some gases, the equilibrium relationship given by Henry’s law. Can written as:
(1)
where: PA is the partial pressure of the component A in the gas phase, CA is the concentration of the component in the liquid, and H is Henry’s constant.
The absorption process accrue in equipment called absorber or scrubber.
Types of absorber: the absorber can be classified in to two types :
1- Tray tower ( Column) 2- Packed tower
1) PACKED TOWERS FOR GAS ABSORPTION
From the analysis given already of the diffusional nature of absorption, one of the outstanding requirements is to provide as large an interfacial area of contact as possible between the phases. For this purpose, columns similar to those used for distillation are suitable. However, whereas distillation columns are usually tall and thin absorption columns are more likely to be short and fat. In addition, equipment may be used in which gas is passed into a liquid which is agitated by a stirrer.
Figure 1 Countercurrent absorption tower
for mass transfer can be written as:
, (1)
(2)
Height of column based on conditions in the gas film
If Gm = moles of inert gas/(unit time) (unit cross-section of tower),
Lm = moles of solute-free liquor/(unit time) (unit cross-section of tower),
Y = moles of solute gas A/mole of inert gas B in gas phase, and
X = moles of solute A/mole of inert solvent in liquid phase.
and at any plane at which the molar ratios of the diffusing material in the gas and liquid
phases are Y and X, then over a small height dZ, the moles of gas leaving the gas phase
will equal the moles taken up by the liquid
(3)
It may be noted that, in a gas absorption process, gas and liquid concentrations will
decrease in the upwards direction and both dX and dY will be negative
Hence the height of column Z required to achieve a change in Y from Y1 at the bottom
to Y2 at the top of the column is given by:
(4)
which for dilute mixtures may be written as:
(5)
In this analysis it has been assumed that kG is a constant throughout the column, and
provided the concentration changes are not too large this will be reasonably true
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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