Distillation operation

Another possibility to improve selectivity is to reduce the concentration of monoethanolamine in the reactor by using more than one reactor with intermediate separation of the monoethanolamine. Considering the boiling points of the components given in Table 2.3, then separation by distillation is apparently possible. Unfortunately, repeated distillation operations are likely to be very expensive. Also, there is a market to sell both di- and triethanolamine, even though their value is lower than that of monoethanolamine. Thus, in this case, repeated reaction and separation are probably not justified, and the choice is a single plug-flow reactor. 

Separation of mixtures of condensable and non-condensable components. If a fluid mixture contains both condensable and noncondensable components, then a partial condensation followed by a simple phase separator often can give a food separation. This is essentially a single-stage distillation operation. It is a special case that deserves attention in some detail later. 

This technique is useful not only when the mixture is impossible to separate by conventional distillation because of an azeotrope but also when the mixture is difficult to separate because of a particularly low relative volatility. Such distillation operations in which an extraneous mass-separating agent is used can be divided into two broad classes. 

The second class of distillation operation using an extraneous mass-separating agent is extractive distillation. Here, the extraneous mass-separating agent is relatively involatile and is known as a solvent. This operation is quite different from azeotropic distillation in that the solvent is withdrawn from the column bottoms and does not form an azeotrope with any of the components. A typical extractive distillation process is shown in Fig. 3.11. ... 

In addition, one other feature of the prefractionator arrangement is important in reducing mixing effects. Losses occur in distillation operations due to mismatches between the composition of the column feed and the composition on the feed tray. Because the prefractionator distributes component B top and bottom, this allows greater freedom to match the feed composition with one of the trays in the column to reduce mixing losses at the feed tray. 

The scope for integrating conventional distillation columns into an overall process is often limited. Practical constraints often prevent integration of columns with the rest of the process. If the column cannot be integrated with the rest of the process, or if the potential for integration is limited by the heat flows in the background process, then attention must be turned back to the distillation operation itself and complex arrangements considered. 

Table 10.3 provides, for a typical crude, the yields and average properties of the various cuts obtained from a conventional distillation operation. 

Synthetic organic chemical manufacturing industry distillation operations (Subpart NNN)... 

As in the United States, Canadians use com, rye, and barley malt. Their process is essentially the same as the one used by many distiHedes in the United States. Since they have no limitations on distillation proofs, distillers operate their systems for optimum separation and congener concentration. In addition, they are permitted to add blenders or flavoring components up to 9.06% by volume in the final blending after the aging process. 

Distillation Columns. Distillation is by far the most common separation technique in the chemical process industries. Tray and packed columns are employed as strippers, absorbers, and their combinations in a wide range of diverse appHcations. Although the components to be separated and distillation equipment may be different, the mathematical model of the material and energy balances and of the vapor—Hquid equiUbria are similar and equally appHcable to all distillation operations. Computation of multicomponent systems are extremely complex. Computers, right from their eadiest avadabihties, have been used for making plate-to-plate calculations. 

J. P. Boston, H. 1. Britt, S. Jkaphongphan, and V. B. Shah, "An Advanced System for the Simulation of Batch Distillation Operations," in Foundations of Computer-Aided Chemical Process Design, Vol. 2, American Institute of Chemical Engineers, New York, 1981. 

The law of mass action, the laws of kinetics, and the laws of distillation all operate simultaneously in a process of this type. Esterification can occur only when the concentrations of the acid and alcohol are in excess of equiUbrium values otherwise, hydrolysis must occur. The equations governing the rate of the reaction and the variation of the rate constant (as a function of such variables as temperature, catalyst strength, and proportion of reactants) describe the kinetics of the Hquid-phase reaction. The usual distillation laws must be modified, since most esterifications are somewhat exothermic and reaction is occurring on each plate. Since these kinetic considerations are superimposed on distillation operations, each plate must be treated separately by successive calculations after the extent of conversion has been deterrnined (see Distillation). 

AU separation operations require energy input in the form of heat or work. In the conventional distillation operation, as typified in Fig. 13-1, energy required to separate the species is added in the form of heat to the rebouer at the bottom of the column, where the temperature is highest. Also, heat is removed from a condenser at the top of the column, where the temperature is lowest. This frequently results... 

The simple and complex distillation operations just described all have two things in common (1) both rectifying and stripping sections are providea so that a separation can be achieved between two components that are adjacent in volatility and (2) the separation is effected only by the addition and removal of energy and not by the addition of any mass separating agent (MSA) such as in liquid-liquid extraction. 

FIG. 13-2 Complex distillation operations with single columns, a) Use of intermediate heat exchangers, (h) Coupling of intermediate heat exchangers with heat pump, (c) Heat pump with external refrigerant, (d) Heat pump with vapor compression, (e) Heat pump with hottoms flashing. 

FIG. 13-3 Compl ex distillation operations with two or more columns, a) Mnltieffect distillation, (b) SRV distillation... 

A more complex unit is shown in Fig. 13-24, which is a schematic diagram of a distillation column with one feed, a total condenser, and a partial reboiler. Dotted hnes encircle the six connected elements (or units) that constitute the distillation operation. The variables N, that must be considered in the analysis of the entire process are just the sum of the Nfs for these six elements since here Nr = 0. Using Table 13-5,... 

In distillation operations, separation results from differences in vapor-and liquid-phase compositions arising from the partial vaporization of a hquid mixture or the partial condensation of a vapor mixture. The vapor phase becomes enriched in the more volatile components while the hquid phase is depleted of those same components. In many situations, however, the change in composition between the vapor and liquid phases in equihbrium becomes small (so-called pinched condition ), and a large number of successive partial vaporizations and partial condensations is required to achieve the desired separation. Alternatively, the vapor and liquid phases may have identical compositions, because of the formation of an azeotrope, and no separation by simple distillation is possible. 

Residue cui ve maps and distillation region diagrams are very power-Ril tools for understanding all types of catch and continuous distillation operations, particularly when combined with other information such as hquid-liqiiid binod cui ves. Applications include ... 

The first three of these are solely X T.E-based approaches, involving a series of simple distillation operations and recycles. The final approach also relies on distillation (X T.E), but also exploits another physical phenomena, liqnid-hqnid phase formation (phase splitting), to assist in entrainer recovery. This approach is the most powerful and versatile. Examples of industrial uses of azeotropic distillation grouped by method are given in Table 13-18. 

Solvent Effects m Extractive Distillation In the distillation of ideal or nonazeotropic mixtures, the component with the lowest pure-component boihng point is always recovered primarily in the distillate, while the highest boiler is recovered primarily in the bottoms. The situation is not as straightforward for an extractive-distillation operation. With some solvents, the component with the lower pure-component boiling point wih be recovered in the distillate as in ordinaiy distillation. For another solvent, the expected order is reversed, and the component with the higher pure-component boiling point wih be... 

Data on the gas-liquid or vapor-liquid equilibrium for the system at hand. If absorption, stripping, and distillation operations are considered equilibrium-limited processes, which is the usual approach, these data are critical for determining the maximum possible separation. In some cases, the operations are are considerea rate-based (see Sec. 13) but require knowledge of eqmlibrium at the phase interface. Other data required include physical properties such as viscosity and density and thermodynamic properties such as enthalpy. Section 2 deals with sources of such data. 

Introduction Packed columns for gas-liquid contacting are used extensively for absorption, stripping, and distillation operations. Usually the columns are filled with a randomly oriented packing material, but for an increasing number of applications the packing is very care-... 

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