شیمیست چمران

Mass Transfer

When two phases of different compositions are brought into contact, a transfer of components may occur from one phase to the other, and vice versa. This is the physical basis of mass-transfer operations. If the two phases are allowed to remain in contact for a sufficient time, they will reach an equilibrium condition where there is no further net transfer of components between phases. In most cases of interest in mass-transfer operations, the two phases are only partially miscible, so that at equilibrium there still exist two phases that can be separated from each other. Usually, these two phases have compositions different from each other and also different from the compositions of the two phases that were initially contacted. As a result, the relative amounts of components transferred between phases are different, so that a separation is achieved. Under appropriate conditions, repeated contacting and separation of phases can lead to an almost complete separation of components. The dissimilar compositions of equilibrium phases are the physical bases for the separation processes utilizing multistage equipment.

Separation processes: the term separation processes includes those unit operations involving separation of components by transfer of mass between phases. When faced with the problem of separating components out of a homogeneous mixture, the engineer utilizes differences in the properties of the constituents of the mixture to effect the separation. The various chemical and physical properties of the constituents of the mixture are examined to determine which properties offer the greatest difference among components, because a greater difference in a property will generally permit an easier, more economical separation. Of course, the engineer must consider many other factors in arriving at a choice of separation processes. The energy requirements, the cast and availability of process and construction materials, and the integration step in the overall chemical process all contribute to determining which separation process is economically most attractive.

Unit operation is concerned with those separation processes that depend upon differences in physical properties, rather than chemical behavior. Such processes depend either upon a difference in composition of phases at equilibrium or upon a difference in the rate of mass transfer of constituents of a mixture.

Distillation: The mast widely used separation process in the chemical industry is distillation. This unit operation is also referred to as fractionation or fractional distillation. Separation of constituents is based upon differences in volatility. In distillation, a vapor phase contacts a liquid phase, and mass is transferred both from the liquid to the vapor and from the vapor to the liquid. The liquid and vapor generally contain the same components but in different relative quantities. The liquid is at its bubble point, and the vapor in equilibrium is at its point. Mass is transferred simultaneously from the liquid by vaporization and from the vapor by condensation. The net effect is an increase in concentration of the more volatile component in the vapor and of the less volatile component in the liquid. Vaporization and condensation involve the latent heats of vaporization of the components, and heat effects must therefore be considered in distillation calculations. In an ideal solution (such as a mixture of benzene and toluene), the volatility can be related directly to the pure-component vapor pressure of each component. In non ideal solutions (such as a mixture of ethanol and water), no simple relationship exists. Distillation is widely used to separate liquid mixtures into more or less pure components. Because distillation involves vaporization and condensation of the mixture, large quantities of energy are required.

A great advantage of distillation is that no additional component need be added to effect the separation. Many other separation processes require the addition of another component, which must then be removed later in another separation step. The temperature and the volume of materials being boiled depend on the pressure. Elevated pressure may be used to decrease volumes and/or to increase temperatures to facilitate condensation; decreased pressures may be needed to lower the boiling point below the point of thermal decomposition.

Applications of distillation are tremendously diverse. Pure oxygen, for use in steel-making, in rocket, and for medical applications, is produced by the distillation of air that has been liquefied. Crude petroleum is initially separated into a number of fractions (such as light gases, naphtha, gasoline, kerosene, fuel oil, lubricating oil, and asphalt) in large distillation columns. These fractions are further processed into finished products, and distillation is frequently used in the intermediate steps in the manufacture of the final products.

Distillation is frequently carried out in multistage equipment; continuous-contact  equipment is also used.

Gas Absorption and Desorption. Gas absorption involves the transfer or a soluble component of a gas phase into a relatively nonvolatile liquid absorbent. Desorption is the reverse process: removal of a component of liquid by contact with a gas phase.

In the simplest case of gas absorption, none of the liquid absorbent vaporizes, and the gas contains only one soluble constituent. For example, ammonia is absorbed from an air-ammonia mixture by contacting the gas with liquid water at room temperature. Ammonia is soluble in water, but air is almost insoluble. The water does not vaporize to an appreciable extent at room temperature. As a result, the only mass transfer is of ammonia from the gas phase to the liquid. As ammonia is transferred to the liquid, its concentration increases until the dissolved ammonia is in equilibrium with that in the gas phase. When equilibrium is reached, there is no further net mass transfer.

In more complex cases of absorption, several components may be absorbed, and part of the absorbent may vaporize.

In absorption equipment, the liquid absorbent is below its bubble point and the gas phase is well above its dew point. A further difference between distillation and gas absorption is that the liquid and gas phases usually do not contain all of the same components. The heat effects in absorption are due to the heat of solution of the absorbed gas, in contrast to the heats of vaporization and condensation involved in distillation.

Absorption involves the addition of a component to the system (i.e., the liquid absorbent). In many cases, the solute must be removed from the absorbent. This removal may require a distillation column, a desorber, or some other separation process.

Desorption, or stripping, is the opposite of absorption. In this case, the soluble gas is transferred from the liquid to the gas phase, because the concentration in the liquid is greater than that in equilibrium with the gas. For example, ammonia can be stripped from an aqueous solution by bubbling fresh air through the solution. The entering air contains no ammonia and the liquid does, so transfer is from the liquid to the gas.

Absorption and stripping are widely used in the chemical industry. Hydrochloric acid is produced by the absorption of hydrogen chloride gas in water. Aerobic fermentation of sewage sludge requires the absorption of air. Carbonation of soft drinks involves the absorption of carbon dioxide; some desorption occurs as the bottle is opened and the pressure is reduced.

Both absorption and stripping are carried out in multistage equipment and to a lesser extent in continuous-contact equipment.

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