By: Muhammad Mufti Azis
Background of Separation Process
Study about separation process has been developed since the 18th century. By the time goes on, it could be seen that the onrushing development exist in many aspect of separation. Whilst, process improvement of separation rapidly grown in accordance with development of industries. Various kinds of chemical product have insisted company to achieve higher quality of product than market specification. Consequently, the experts are imposed to continuously improve the separation method due to fulfill market specification.
Separation processes is main part of unit operation. Unit operation has responsibilities in forefront and finale processes in Chemical Plant Industries. The forefront unit has responsibility preparing raw material so that it can fulfill the reactor specification. Normally, the equipment widely used in this unit consists of size-reduction equipment, screening equipment, mixing equipment, and so forth. The material then processed throughout chemical reaction occurs inside of chemical reactor.
The output products of reactor still contain large impurities (inert compound), undesired product (side product), and main product. For this reason, unit operation takes important role in order to increase the amount of main products by separating it from undesired product. Therefore, separation processes come to the forth to achieve the demand. Separation processes widely found in Chemical Process Industries (CPI) are distillation, extraction, absorption, adsorption, and so forth. Each of them has undergone many improvements in calculation method and application. All the things are done in order to keep pace with the development of processes in chemical industries.
As mentioned above, distillation is one of the separation processes usually being put on final unit of CPI. The main function of distillation unit is to separate desired products from undesired products. It can be involved by two or more components. The main principle here, distillation is separation process that uses heat as separating agent. It can be concluded that distillation process will utilize mass balance, heat balance, and equilibrium, respectively. All of those things become part of Chemical Engineering principles, well known as Chemical Engineering Tools.
The concept of distillation is based on (generally) equilibrium concept. Consequently, the graphical method, which interprets equilibrium data of particular system at specific pressure and temperature, is widely used in the past. Although the numerical and rigorous method is feasible to be computed nowadays, however, for quick calculation we can have more sensible result by using graphical method. The x-y system for binary system and residual curve (for ternary system) have been popular in solving distillation cases.
Azeotropic Solution
The term of azeotrope is derived from the Greek meaning “to boil unchanged’. Literally, azeotrope occurs when vapor boiling from a liquid has same composition as the liquid. An azeotrope solution is a liquid mixture that exhibits maximum or minimum boiling points of surrounding mixture compositions. Boiling points of the pure components in the mixture must be sufficiently close to permit formation of an azeotrope. A mixture of close-boiling components may form an azeotrope when only small deviation from ideal liquid solution occur. Azeotrope occur infrequently for mixture composed of components whose boiling points differ by more than 30oC (54oF).
Azeotropic system may be classified broadly in relation to the character of the azeotrope (maximum or minimum-boiling point), number of components in the system, and whether one or more liquid phase are formed (homogenous and heterogenous). Lecat (Comptes Rendous) proposed to divide azeotropes into : 1. Positive Azeotropes which has main characteristic minimum-boiling temperature at constant pressure; 2. Negative Azeotropes, which having maximum boiling temperature at constant pressure. There are some methods to determine azeotropes range and other analyzes about azeotropes solution.
The existence of azeotrope phenomena has been a subject study of some experts. The problem of separation azeotropic solution is mainly caused by the low purity of desire products. Unfortunately, most of organic and inorganic solutions are classified as azeotropic solution. Hence, some technique should be carried out when we encounter separation process of azeotropic solution. The technique is well-known as Enhanced Distillation.
Enhanced Distillation2
The separation technique of azeotropic solution is classified by Stichlmair,Fair,and Bravo as enhanced distillation. Enhanced Distillation is carried out when the relative volatility of two components may be below 1.10 (different boiling points less than 500C).
The techniques included as Enhanced Distillation are:
a. Extractive Distillation
b. Salt Distillation
c. Pressure-Swing Distillation
d. Homogenous Azeotropic Distillation
e. Heterogenous Azeotropic Distillation.
Here, each of them will be described more detail.
Extractive Distillation
Extractive distillation was first introduced by Dunn in connection with the commercial separation of toluene from paraffin using phenol as a solvent. This method uses a large amount of a relatively high-boiling solvent to alter the liquid phase activity of mixture, so that the relative volatility of the key components becomes more favorable.
Generally, extractive distillation is used to separate azeotrope and other mixtures that have key components with relative volatility below 1.1 over appreciable range of concentration. If the feed is minimum boiling point, solvent with a lower relative volatility is added above the feed stage and below top of the column. It is purposed so that the solvent is down flowing with the liquid at the bottom of the column and only a little amount of solvent is lost with the overhead vapors. If the feed is maximum boiling point, solvent enters the column with the feed.
In selecting solvent for extractive distillation, a number factors are considered including availability, cost, corosivity, vapor pressure, thermal stability, heat of vaporization, reactivity, toxicity, infinite-dilution activity coefficient in the solvent of the components to be separated, and ease of recovery for recycle. In addition, solvent should not form azeotropes. Initial screening is based on the measurement or prediction of infinite dilution activity coefficient.
Berg
3 points out that all successful solvents for extractive distillation for extractive distillation are highly hydrogen-bonded liquids, such as (1) water, amino, alcohols, amides, and phenols that form three-dimensional networks of strong hydrogen bonds, and (2) alcohols, acids, phenols, and amides that are composed of molecules containing both active hydrogen atoms and donor atoms (oxygen, nitrogen, and fluorine). In general, it is very difficult or impossible to find suitable solvent to economically separate components having the same functional groups.
The disadvantages of extractive distillation are the using of large amount of solvent and, even though the solvent is introduced into the column several trays below the top tray, enough solvent is stripped by vapor traffic, and consequently it lowers the purity of distillate products.
Salt Distillation
The backwards of extractive distillation are the utilization large amount of solvent and appreciable amount of solvent which is stripped to the overhead vapor can reduce the purity of distillate. For this reason, Othmer
4 introduces the use of concentrated Calcium Chloride Brine as a solvent. Not only does Calcium Chloride, which is highly soluble in water, reduce the relative volatility of water, but it also has strong affinity.
An even earlier patent by Van Raymbeke
5 describes the extractive distillation of ethanol from water by using solution of Calcium Chloride, Zinc Chloride, or Potassium Carbonate in Glycerol. Rather than using a solvent that contains a dissolved salt, the salt can be added as a solid or melt directly into the column by dissolving it in the liquid reflux before it enters the column. This technique was demonstrated experimentally by Cook and Furter
6 for the separation of ethanol from water using Potassium Acetate. At salt concentration below saturation and between 5 and 10 mol%, an almost pure ethanol distillate was achieved. The salt, which must be soluble in the reflux, is recovered from the aqueous bottoms by evaporation and crystallization.
In an aqueous alcohol solution, both salting out and salting in have been observed by Johnson and Furter. They observed vapor-liquid equilibrium in (a). sodium nitrates salts out methanol (b). mercuric chloride salts in methanol. Even low concentration of potassium acetate, however, it can eliminate the ethanol-water azeotrope. In Germany, mixed acetate salts have been widely used for the separation of ethanol and water.
Salt distillation can also be applied to the separation of organic compounds that have little capacity alone for dissolving inorganic salts by using a special a special class by using a special class of organic salts called hydrotropes. Typical hydrotropic salts are alkali and alkaline-earth salts of the sulfonate of toluene, xylene, or cymene, and the alkali benzoates, thiocyanates, and salycilates. For example, Mahapatra, Gaikar, and Sharma
7 showed that the addition of aqueous solution of 30 and 66 wt% p-toluenosulfonic acid to mixture of 2,6-xylenol and p-cresol at 1 atm increased the relative volatility from approximately 1 to about 3. Hydrotropes can also be used to enhance separation by liquid-liquid extraction.
Pressure Swing Distillation
System which forms homogenous azeotropes, either minimum or maximum boiling or both, can not be separated by simple fractional distillation. If the azeotrope composition is “pressure sensitive” or varies by at least 4 to 5 % over a nominal change of total pressure, it is possible to separate the azeotrope solution by using two-column fractionation. This process is referred to as pressure-swing distillation. Knapp and Doherty
8 have listed 36 pressure sensitive binary azeotropes, which is taken mainly from Horsley
9.
Van Winkle describes the process of pressure swing distillation. Fig 1 and fig 2 show the operation for minimum and maximum boiling point azeotrope.
Fig 1. Distillation Sequence for minimum boiling azeotropes. (P1
10 . These conditions are based on the rule that for potential entrainer, E, the two components, A and B, to be separated, or any product azeotrope, must lie in the same distillation region of the residue-curve map. Thus, a distillation boundary cannot be connected to the A-B azeotrope. Furthermore, A or B, but not both, must be a saddle.
The maps suitable for a sequence that includes homogenous azeotropic distillation together with ordinary distillation are classified into five groups. For more detail explanation, it can be read in ”Basic Principles of Separation” by Henley and Seader.
It is often difficult to find an entrainer for a sequence involving homogenous azetropic distillation and ordinary distillation. However, azeotropic distillation can also be incorporated into a sequence involving separation operation other than distillation. In that case, some of the restriction for the entrainer and the resulting residue-curve map may not apply. For example, the separation of benzene and cyclohexane using acetone as the entrainer. The system violates the restrictions for a distillation-only sequence because the ternary system involves only two minimum-boiling binary azeotropes. However the separation can be achieved by: (1) Homogenous azeotropic distillation with acetone entrainer to produce bottom product of nearly pure benzene and a distillateclose in composition to the minimum-boiling binary azeotrope of acetone and cyclohexane, (2) liquid-liquid extraction of distillate with water to give a raffinate of nearly pure cyclohexane and an extract of acetone and water, and (3) ordinary distillation of extract to recover the acetone for recycle.
Heterogenous Azeotropic Distillation
The requirement for a distillation sequence based on homogenous azeotropic distillation, that A and B must lie in the same distillation region of the residue-curve map with entrainer, E, is so restrictive that is usually difficult, if not impossible, to find feasible entrainer.
An alternative technique that does find wide industrial application is heterogeneous azeotropic distillation, which is used to separate close boiling binary mixtures and minimum boiling binary azeotropes by employing entrainer that forms a binary and/or ternary heterogeneous azeotrope. The overhead vapor from the column is close to the composition of the heterogeneous azeotrope. When condensed, two liquid phases form in decanter downstream of the condenser. After separation in the decanter, most or all of the entrainer-rich liquid phase is returned to the column as a reflux, while most or all of the other liquid is sent to the next column for further separation. Because these two phases usually lie in different distillation regions of the residue-curve map, the restriction that usually dooms distillation sequences based on homogeneous azeotropic distillation is overcome. Thus, in heterogeneous azeotropic distillation, the components to be separated need not lie in the same distillation region.
Heterogeneous azeotropic distillation has been practiced for almost a century, first by batch and then by continuous processing. Two of the most widely used applications are (1) the use of benzene or one of a number of other entrainers to separate the minimum boiling azeotrope of ethanol and water, and (2) the use of ethyl acetate or one of a number of other entrainers to separate close-boiling mixture of acetic acid and water. Other applications, cited by Widagdo and Seider
11, include dehydration of isopropanol with isopropylether,sec-butyl-alcohol with disec-butyl-ether, chloroform with mesityl oxide, formic acid with toluene, and acetic acid with toluene. Also dehydration of tanker-transported feed stocks such as styrene and benzene as major application.
A variety of column sequences for heterogeneous azeotropic distillation have been proposed. Three of these that utilize only distillation is taken from the study of Ryan and Doherty
12. Most common is the usage of three column sequences. Three columns consist of : (1)preconcentrator column ; (2) azeotropic column which is completed by decanter to send back entrainer rich-liquid phase as reflux; (3) entrainer recovery column. Ryan and Doherty also proposed to use four columns, which the fourth column is water removal column. On the other hand, Pham and Doherty
13 found no advantage for the four-column sequence over three-column sequence. A novel two columns sequence is described by Lynn, Ryan, and Doherty. The feed is sent to the column 2, which is a combined preconcentrator and entrainer recovery column. The distillate from this column is the feed to the azeotropic column. The bottom products of column 1 and 2 are nearly pure ethanol and nearly pure water respectively. For feeds that are very dilute in ethanol, Ryan and Doherty found that two-column sequence has a lower investment cost, but a higher operating cost, than the three-column sequence. For feeds that are richer in ethanol, these two sequences are economically comparable.
The convergence of rigorous calculation for heterogeneous azeotropic distillation can be extremely difficult, especially when the convergence of the entire sequence is attempted. For calculation purposes, it is preferable to uncouple the column by using a residue curve-map to establish, by material balance calculations, the flow rates and composition of the feeds and products for each column. Among the most successful approaches for the most difficult cases are the boundary-value tray-by-tray method of Ryan and Doherty, the homotopy-continuation method of Kovach and Seider
14, and the collocation method of Swartz and Stewart
15.
CONCLUSIONS
1. Extractive distillation, salt distillation, pressure swing distillation, homogeneous azeotropic distillation, and heterogeneous azeotropic distillation are enhanced distillation technique to be considered when separation by ordinary distillation is uneconomical or impossible.
2. Extractive distillation, using low volatility solvent that enters near the top of the column, is widely used to separate azeotropes and very close boiling mixtures. Preferably, the solvent should not form an azeotrope with any component with any component in the feed.
3. Certain salts, when added to a solvent, reduce the volatility of the solvent and increase the relative volatility between the two components to be separated. In this process, called salt distillation, the salt is dissolved in the solvent or added as a solid or melt to the reflux.
4. Pressure-swing distillation, utilizing two columns operating at different pressures, can be used to separate an azeotropic mixture when the azeotrope can be made to disappear at some pressure. If not, the technique may still be practical if the azeotropic composition changes by 5 mol% or more over a moderate range of pressure.
5. In homogeneous azeotropic distillation, an entrainer is added on a stage, usually above the feed stage. A minimum or maximum-boiling azeotrope, formed by the entrainer with one or more feed components, is removed from top or the bottom of the column, respectively. Unfortunately, the application of this technique for difficult separation of mixtures are not common because of limitations due to distillation boundaries.
6. A more common and useful technique is heterogeneous azeotropic distillation, in which the entrainer forms, with one or more components of the feed, a minimum boiling-boiling heterogeneous azeotrope. When condensed, the overhead vapor splits into organic-rich and water-rich phases. The azeotrope is broken by returning one liquid phase as reflux, with the other sent on as distillate for further processing.
REFERENCES
Van Winkle, M., 1967, Distillation, pp. 381-389, McGraw-Hill, New York.
Perry, R.H., and Green, D.W.,1984, Perry’s Chemical Engineers’ Handbook, 6 ed, pp. 13-53 – 13-65, McGraw-Hill International Editions, New York.
Henley and Seader, 2006, Separation Process Principle, 2nd ed, John Wiley & Sons, Massachusets
[1] JD. Seader wrote Chapter 13: Distillation in Perry’s Chemical Engineer’s Handbook 6th edition
2 Based on Henley and Seader“Basic Principles of Separation”.
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