UNIT OPERATIONS IV: MASS TRANSFER

We learned that heat is transferred if there is a temperature difference in a medium. Similarly, if there is a difference in the concentration of any chemical species in a mixture, mass transfer must occur. Mass transfer is mass in transit as a result of a difference in concentrations of a species in a mixture. Thus, it is important to clearly understand the context in which this term is used in unit operations.

When two phases of different compositions are brought into contact, component transfer may occur from one phase to another, or vice versa. This is the physical basis of mass transfer operations. If the two phases are in contact for a sufficient period of time, they eventually reach equilibrium and thereafter there is no longer a net transfer of components between them.

In most cases of interest to mass transfer operations, the two phases are only partially miscible, so that in equilibrium there are still two phases that can be separated from each other. Usually, these two phases have different compositions and these compositions are also different from those of the phases at the time of initial contact. Therefore, the relative quantities of the components that are transferred between the phases are different, so that a separation between them is achieved.

Under appropriate conditions, repeated contact and phase separation can lead to almost complete separation of the components. The physical basis for the separation processes using multistage equipment – is the difference in phase equilibrium compositions.

Unit operations are pertinent to separative processes that depend only on differences in physical properties and not on chemical behavior. These processes are either based on a difference in the equilibrium phase composition or a difference in the mass transfer rate of the mixture constituents. In the following explanation, we will discuss separation processes that use these two physical bases.

Based on this assumption, the existence of unitary operations involving mass transfer is large. Below we will present separation processes that use these two physical bases.

Distillation

The most widely used separation process in the chemical industry is distillation. This unitary operation is also called fractionation or fractional distillation. The separation of the constituents is based on volatility differences. In distillation, a vapor phase comes into contact with a liquid phase, and there is mass transfer from the liquid to the vapor and from it to that. Liquid and steam generally contain the same components, but in different relative amounts.

The liquid is at its bubble point and the vapor in equilibrium at its dew point. There is simultaneous mass transfer of liquid by vaporization and of vapor by condensation. The end effect is an increase in the concentration of the most volatile component in steam and the least volatile component in liquid.

Distillation is widely used to separate liquid mixtures into more or less pure components. Distillation applications have the widest diversity. Pure oxygen, used in steelmaking, rockets and medical applications, is produced by the distillation of previously liquefied air. Crude oil is initially separated into several fractions (such as light gases, naphtha, gasoline, kerosene, fuel oils, lubricating oils and asphalt) into large distillation columns. These fractions are further processed into finished products and distillation is often used in the intermediate steps of obtaining these final products.

Figure 11: Configuration of a distillation column.
Source: CALDAS, 2007.

Gas absorption and desorption

Gas absorption involves the transfer of a soluble component of a gas phase to a relatively nonvolatile liquid absorber. Desorption is the reverse process, the removal of a component of a liquid by contact with a gas phase.

In the simplest cases of gas absorption, the liquid absorber does not vaporize, and the gas contains only a soluble constituent. For example, ammonia is absorbed from a mixture of air and ammonia by gas contact with liquid water at room temperature. Ammonia is water soluble, but the air is almost insoluble. Water, in turn, hardly vaporizes at room temperature. Therefore, the only mass transfer is from gas phase ammonia to liquid. As the ammonia passes into the liquid, its concentration increases until the dissolved ammonia is in equilibrium with that in the gas phase. Once equilibrium is reached, there is no more effective mass transfer.

In absorption equipment, the liquid absorber is below its bubble point and the gas phase is far above its dew point. Another difference between distillation and gas absorption: the liquid and gaseous phases do not contain all the same components. Thermal effects on absorption are due to the solution heat of the absorbed gas, in contrast to the vaporization and condensation heats that participate in distillation.

Absorption involves the addition of a component to the system (ie, the liquid absorber). In many cases, the solute must be removed from the absorbent. This removal may be done in a distillation column, or in a desorption apparatus, or by another separative process.

Figure 12: Examples of gas absorption column. (a) Bubble column, (b) Descending film, (c) Filling column.
Source: CALDAS, 2007.

Desorption, or extraction, is the opposite operation 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 the equilibrium concentration with the gas. For example, ammonia can be extracted from an aqueous solution by bubbling air through the solution. The air in the inlet contains no ammonia while the liquid contains it; There is then a transfer from liquid to gas.

Absorption and extraction are widely adopted in the chemical industry. Absorption and extraction are performed on multi-stage equipment and, to a lesser extent, on continuous contact equipment.

Liquid-liquid extraction

A liquid mixture can sometimes be separated by contact with a second liquid solvent. The components of the mixture are soluble to varying degrees in the sun. Ideally, the component to be extracted is solvent soluble, and the other components are insoluble. Then the solute is the only component transferred from the initial mixture to the solvent phase. The initial mixture becomes refined as the solute is extracted from it. The solvent phase turns into the extract as it receives the solute.

In practice, all components are possibly soluble to some degree in each other, and separation is only feasible when the solubilities are sufficiently different. In any case, the non-extracted (inert) component must be sufficiently insoluble to form two extractable phases.

Figure 13: Production of essences by liquid-liquid extraction.
Source: http://labvirtual.eq.uc.pt/siteJoomla/index.php?option=com_content&task=view&id=63&Itemid=148#3>.

Liquid-liquid extraction is also called solvent extraction. Separation of one component from a homogeneous solution is by the addition of another insoluble constituent, the solvent, in which the desired component of the solution, the solute, is preferably soluble; In this solvent the solute diffuses at a characteristic rate until equilibrium concentrations are reached in each phase. For example, the acetic acid solute may be separated from an aqueous solution by contact with the isopropyl ether solvent. Although water is slightly soluble in ether, it has the role of refined, practically insoluble.

Solvent extraction is used to remove unwanted components from lubricating oils and other fractions of crude oil, to separate niobium from tantalum, to produce concentrated phosphoric acid, and in many other applications.

Solid-liquid extraction

The components of a solid phase may be separated by selective dissolution of the soluble part of the solid by an appropriate solvent. This operation is also called leaching or washing. The solid must be finely comminuted so that the liquid solvent comes into contact with it all. Usually the desirable component is soluble, and the remainder of the solid is insoluble. The solute must then be recovered from the extract solution in another separation step.

An everyday example of solid-liquid extraction is coffee making. In this case, the soluble constituents of ground coffee are separated from the insoluble fines by solubilization in hot water. If the coffee is boiled for a very long time, the solution will reach equilibrium with the remaining solids. The solution is separated in the strainer from the residual fines.

Source: http://www.nossofoco.eco.br/organicos/producao-de-cafe-organico/

Solid-liquid extraction is also used industrially in the manufacture of soluble coffee to recover soluble coffee from sludge. Other industrial applications include the extraction of soybean oil using hexane as a solvent and the recovery of uranium from low grade ores by extraction with sulfuric acid or sodium carbonate solutions. Because one of the phases is solid and does not flow like a fluid, special types of solid-liquid extraction equipment must be used.

Adsorption

Adsorption involves the transfer of a constituent of a fluid to the surface of a solid phase. To complete separation, the adsorbed constituent must then be removed from the solid. The fluid phase may be either a gas or a liquid. If several constituents are adsorbed to different degrees, it is often possible to separate them into relatively pure states.

Many are the solid adsorbents that are used. Definitely speaking, the concept of adsorbent usually applies to a solid that holds the solute on its surface by the action of physical forces. As an example, we have the adsorption of organic vapors by coal. The lightest fraction of natural gas is industrially separated by a mobile bed of adsorbent. Most other industrial processes use fixed beds and batch or cyclic processes instead of multi-stage equipment as it is difficult to move the solid.

Figure 14: Adsorption columns in industry.
Source: https://derivandoaquimica.wordpress.com/2012/05/04/28/

Molecular sieves consist of special synthetic adsorbents which may be used to separate mixtures by differences in molecular size, polarity or saturation of carbon bonds. For example, water molecules are relatively small and very polar, so they are preferably adsorbed by the adsorbing molecular sieves. Thus, these adsorbents are useful in drying gases and liquids. Molecular sieves are being employed to separate the normal paraffins contained in hydrocarbon streams in petroleum refineries for use in the manufacture of biodegradable detergents.

Other unitary separation processes

Some important unitary operations involve the separation of the components of a mixture, but the operations are not usually grouped together with the usual separation processes.

Evaporation, crystallization and drying involve the simultaneous transfer of heat and mass. On evaporation, a liquid solution is concentrated by vaporizing a part of the solvent.

Evaporation has many industrial applications whenever any solutions have to be concentrated. What is commonly called seawater distillation is actually an evaporative process to recover drinking water.

When a solution is evaporated until it is saturated by the solute, further evaporation or cooling leads to precipitation of solid crystals. This is the physical basis of crystallization, a unitary operation that is used to separate solutes from solution. Crystallization is used industrially in the manufacture of many inorganic salts. It can also be used to separate salt mixtures by fractional crystallization. Drying, in turn, separates a liquid from a solid by vaporizing the liquid.

Figure 15: Forced circulation crystallizer-evaporator, three floors (Swenson);
Source: http://labvirtual.eq.uc.pt/siteJoomla/index.php?option=com_content&task=view&id=42&Itemid=159

All unitary operations seen throughout this series of texts have their particularities and applicability in the industry. They encompass procedures of vital importance to the physicochemical processing industry and are present throughout virtually the entire process. Therefore, they become very important, as well as being indispensable to the knowledge of a chemical engineer, so that he applies the most convenient operation for a given process, both in practical and economic terms.

REFERENCES

– CALDAS, J. N; LACERDA, A. I.; VELOSO, E. Internos de Torres – Pratos & Recheios – 2ª Ed. 2007. Editora Interciência. Rio de Janeiro.

– Foust, Alan et al.; Princípios das Operações Unitárias. LTC: Rio de Janeiro, 2ª Edição, 1982.

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