Distillation, batch

Batch operation should be considered when the quantity to be distilled is small when it is produced at irregular intervals when a range of products has to be produced or when the feed composition is likely to vary considerably. 

Batch distillation is often applied to separations in which small amounts of materials are processed or in which the plant does not operate continuously. It is versatile, such that the same equipment can be used for several products of varying composition at different times. The distillate, which is typically much more volatile than the liquid from which it 

Because it is not continuous, the mathematical analysis of batch distillation is based on the total quantities. For a binary system in which the distillate is the desired product, the overall mass balance at the end of a batch run is 

Performing this integration requires that Xd be related to Xb. In single-stage distillation cases where constant relative volatility can be assumed, the equilibrium relationship is  

When constant relative volatility cannot be assumed, either graphical or numerical techniques, such as Simpson s rule, must be applied [2], 

In multistaged systems, Xd and xb are not in equilibrium and, hence, integration of Equation (4.5) requires a different relationship between these product compositions. This is obtained by stage-by-stage calculations. If one assumes negligible hold-up at each stage and at the condenser and accumulator, mass balances can be vwitten for any time t during the batch operation. 

Batch distillation is the oldest separation process and the most widely used unit operation in pharmaceutical and specialty chemical industries. The most outstanding feature of batch distillation is its flexibility. This flexibility allows one to deal with uncertainties in feed stock or product specification. 

The basic difference between batch distillation (rectifier) and continuous distilla- 

Equilibrium curve for the CS2 and CCI4 mixture at 1 atmosphere pressure 

Types of distillation processes (a) batch distillation and (b) continuous distillation. 

In batch distillation the mixture to be distilled is charged as a batch to the still and the distillation carried out till a satisfactory top or bottom product is achieved. The still usually consists of a vessel surmounted by a packed or plate column. The heater may be incorporated in the vessel or a separate reboiler used. Batch distillation should be considered under the following circumstances  

Where the feed composition varies over a wide range. 

Where the choice between batch and continuous is uncertain an economic evaluation of both systems should be made. 

Batch distillation is an unsteady state process, the composition in the still (bottoms) varying as the batch is distilled. 

Fixed reflux, where the reflux rate is kept constant. The compositions will vary as the more volatile component is distilled off, and the distillation stopped when the average composition of the distillate collected, or the bottoms left, meet the specification required. 

Operation of a batch distillation is an unsteady state process whose mathematical formulation is in terms of differential equations since the compositions in the still and of the holdups on individual trays change with time. This problem and methods of solution are treated at length in the literature, for instance, by Holland and Liapis (Computer Methods for Solving Dynamic Separation Problems, 1983, pp. 177-213). In the present section, a simplified analysis will be made of batch distillation of binary mixtures in columns with negligible holdup on the trays. Two principal modes of operating batch distillation columns may be employed  

With constant overhead composition. The reflux ratio is adjusted continuously and the process is discontinued when the concentration in the still falls to a desired value. 

With constant reflux. A reflux ratio is chosen that will eventually produce an overhead of desired average composition and a still residue also of desired composition. 

Enthalpy-Concentration Lines of Saturated Vapor and Liquid of Mixtures of Methanol and Water at a Pressure of 2 atm A basis of 0°C is taken. Enthalpy data for methanol are in Chemical Engineers Handbook (McGraw-Hill, New York, 1984, p. 3.204) and for water in Keenan et al. (Steam Tables SI Units, Wiley, New York, 1978). 

Experimental x-y data are available at 1 and 3 atm (Hirata, 1976, 517, 519). Values at 2 atm can be interpolated by eye. The lines show some overlap. Straight lines are drawn connecting enthalpies of pure vapors and enthalpies of pure liquids. Shown is the tie line for x = 0.5, y = 0.77. 

Enthalpy-Concentration Lines of Satnrated Vapor and Liquid of Mixtures of Methanol and Water at a Pressure of 2 atm 

Although neither as widely used nor as sophistdcated as their continuous counterparts, batch stills play an important role in the diemical process industries. Batch stills are typically used 

Where the compositions of the materials to be separated vary over wide ranges 

Wliere the separation only needs to be performed infrequently, 

Where the materials to be separated are produced in relatively small quantities, such as in semiworks or small-scale commercial 

Where the main product contains only relatively small amounts of light and/or heavy Impurities 

Although gradually diminishing In favor, batch distillation still is tin interesting process to control, and deserves more than casual attention. Like most batch processes, its control system requires special consideration, ultimately bearing only faint resemblance to that of its continuous counterpart. 

A batch separation will require an amount of time inversely proportional to the rate at which heat is introduced. Consequently, if processing time is to be minimized, heat input must be maintained at the maximum permissible level throughout distillation. This feature then fixes one of the variables which was subject to manipulation on a continuous tower. With vapor rate fixed, a material balance can be readily constructed for the batch still shown In Fig. 11.26. 

If distillate flow is selected as the variable to be manipulated for product-quality control, reflux Is then dependent. In the continuous system, product quality was affected by both D/F and V/F. But here, F = 0, so It follows that product quality Is a function of the ratio of the remaining variables, that Is, D/V. In.a sense, a batch still is similar to the enriching section of a continuous tower, part of whose vapor flow is feed. If, in the 

The bottoms composition x at time t can be found from the material balance of the light component  

Now y can be found from x, using the modified Fenske equation. 

If after a certain interval of time the composition of the top product starts to fall, then, if the reflux ratio is increased to a new value R2, it will be possible to obtain the same composition at the top as before, although the composition in the still is weakened to xS2. This method of operating a batch still requires a continuous increase in the reflux ratio to maintain a constant quality of the top product. 

An alternative method of operation is to work with a constant reflux ratio and allow the composition of the top product to fall. For example, if a product of composition 0.9 with respect to the more volatile component is required, the composition initially obtained may be 0.95, and distillation is allowed to continue until the composition has fallen to some value below 0.9, say 0.82. The total product obtained will then have the required composition, provided the amounts of a given purity are correctly chosen. 

One of the added merits of batch distillation is that more than one product may be obtained. Thus, a binary mixture of alcohol and water may be distilled to obtain initially a high quality alcohol. As the composition in the still weakens with respect to alcohol, a second product may be removed from the top with a reduced concentration of alcohol. In this way it is possible to obtain not only two different quality products, but also to reduce the alcohol in the still to a minimum value. This method of operation is particularly useful for handling small quantities of multi-component organic mixtures, since it is possible to obtain the different components at reasonable degrees of purity, in turn. To obtain the maximum recovery of a valuable component, the charge remaining in the still after the first distillation may be added to the next batch. 

The case of a column with four ideal plates used to separate a mixture of ethyl alcohol and water may be considered. Initially there are Si moles of liquor of mole fraction xfl with respect to the more volatile component, alcohol, in the still. The top product is to contain a mole fraction xd, and this necessitates a reflux ratio R. If the distillation is to be continued until there are S2 moles in the still, of mole fraction xs2, then, for the same number of plates the reflux ratio will have been increased to R2. If the amount of product obtained is Db moles, then a material balance gives  

These equations enable the final reflux ratio to be determined for any desired end concentration in the still, and they also give the total quantity of distillate obtained. What is important, in comparing the operation at constant reflux ratio with that at constant product composition, is the difference in the total amount of steam used in the distillation, for a given quantity of product, Db. 

Continuous operation is normally employed when the material to be distilled is large in quantity and is available at a reasonably uniform rate. Under such conditions, it is usually cheaper than batch distillation, but there are a large number of cases which are not suited to continuous operation and which are handled on the batch basis. 

A batch distillation with rectification involves charging the still with the material to be separated and carrying out the fractionation until the desired amount has been distilled off. The overhead composition will vary during the operation, and usually a number of cuts will be made. Some of the cuts will be the desired products, while others will be intermediate fractions that can be recycled to subsequent batches to obtain further separation. 

The equipment employed and the method of operation are similar for batch and continuous distillation, but in the latter the mathematical analysis is based on the assumption that in all portions j he system the coingositions and flow rato aWindepS distilla- 

The batch distillation of binary mixtures will be considered fpr the cases of (1) no rectification, (2) rectification without liquid holdup in the column, and (3) rectification with holdup. 

Ho Rectification Batch distillation without rectification corresponds to the simple distillations of Chap. 6, and the calculations of the concentrations as a function of the amount distilled can be made by Eqs. (6-8) and (6-7). 

The processes discussed so far in this book are all treated as continuous operations. Although dynamics were considered, for the most part those processes were considered as steady state, where the properties or state variables at any given point in the process are considered time invariant. An important separation process that differs in this respect is batch distillation, a time-dependent process. 

A batch distillation apparatus consists of a reboiler attached to the bottom of a trayed or packed column with a condenser at the top. A batch of feedstock is charged to the reboiler, and then the mixture in the reboiler is gradually vaporized. The vapor flows up the column, is condensed in the condenser, and part of the condensate is returned to the column as reflux, flowing countercurrent to the vapor. The rest of the condensate is collected over a period of time as a single distillate product or a series of distillate fractions. Generally, there are no side feeds or side products and the entire column acts as a rectifier. 

The same principles of continuous fractionation apply in the batch rectifier, except in batch distillation the compositions and column conditions change continuously with time. 

The initial distillate cut is the lightest and, as the distillation progresses, the liquid remaining in the reboiler becomes continuously richer in the heavier components, and subsequent distillate cuts become increasingly heavier. The residue remaining in the reboiler after the last distillate cut is the heaviest cut. A multicomponent feed mixture may be separated in one batch distillation column into a number of products with specified purities. Given the required number of trays and reflux ratio, a batch distillation column could, in principle, separate a normal feed mixture (one that is not reactive or azeotrope forming) into its pure constituents. 

The economical design and operation of batch distillation systems must take into 

TABLE 13.4. Molal Heats of Vaporization at Their Normal Boiling Points of Some Organic Compounds That May Need To Be Separated from Water 

Total gas pressure drop, 1.20 kPa Convergence in 8 iterations Segment height, 3.7 cm 

Your objectives in studying this section are to be able to  

Calculate, by modified McCabe-Thiele methods, residue composition and distillation time for binary batch rectification with constant reflux for a given number of ideal stages, boil-up rate, and specified average distillate composition. 

Continuous distillation is a thermodynamically efficient method of producing large amounts of material of constant composition. However, when small amounts of material of varying product composition are required, batch distillation has several advantages. In batch distillation a charge of feed is heated in a reboiler, and after a short startup period, product can be withdrawn from the top of the equipment. When the distillation is finished, the heat is shut off and the material left in the reboiler is removed. Then a new batch can be started. Usually, the distillate is the desired product. 

Assuming a constant boil-up rate in the still and no liquid hold-up in the column, the process can be modeled as 

and y are time dependent. The optimal control problem is to find the control function u t) that maximizes the objective functional 

H = total enthalpy, cal/moles H = HaVa + HbVb + HcVc Hi =MT) 

Normally the accumulation term Cp dMT/dt is very small compcired to the terms on the right side of equation (5.4.1) and is therefore neglected. This is essentially a quasi-steady-state assumption on the energy balance. The energy balance reduces to 

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Batch distillation (see Fig. 3) typically is used for small amounts of solvent wastes that are concentrated and consist of very volatile components that are easily separated from the nonvolatile fraction. Batch distillation is amenable to small quantities of spent solvents which allows these wastes to be recovered onsite. With batch distillation, the waste is placed in the unit and volatile components are vaporized by applying heat through a steam jacket or boiler. The vapor stream is collected overhead, cooled, and condensed. As the waste s more volatile, high vapor pressure components are driven off, the boiling point temperature of the remaining material increases. Less volatile components begin to vaporize and once their concentration in the overhead vapors becomes excessive, the batch process is terrninated. Alternatively, the process can be terrninated when the boiling point temperature reaches a certain level. The residual materials that are not vaporized are called still bottoms. 

The dried malted barley is ground and mashed in a tub, after which the Hquid portion is drained off, cooled, and placed in the fermentor. After fermentation, a batch distillation system is usually used to separate the whisky from the fermented wort. The stiU consists of a copper ketde with a spiral tube or "worm" leading from the top. The dimensions and shape of the stills have a critical effect on the character of the whisky. The product taken off in the first part of the distillation is called foreshots (heads). The middle portion is the high wines and the last portion is the feints (tails). The middle portion is redistilled at the 140—160° proof (70—80%) range and matured in used oak cooperage. 

Spreadsheet Applications. The types of appHcations handled with spreadsheets are a microcosm of the types of problems and situations handled with fuU-blown appHcation programs that are mn on microcomputers, minis, and mainframes and include engineering computations, process simulation, equipment design and rating, process optimization, reactor kinetics—design, cost estimation, feedback control, data analysis, and unsteady-state simulation (eg, batch distillation optimization). 

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. 

Even though the simple distillation process has no practical use as a method for separating mixtures, simple distillation residue curve maps have extremely usehil appHcations. These maps can be used to test the consistency of experimental azeotropic data (16,17,19) to predict the order and content of the cuts in batch distillation (20—22) and, in continuous distillation, to determine whether a given mixture is separable by distillation, identify feasible entrainers/solvents, predict the attainable product compositions, quaHtatively predict the composition profile shape, and synthesize the corresponding distillation sequences (16,23—30). By identifying the limited separations achievable by distillation, residue curve maps are also usehil in synthesizing separation sequences combining distillation with other methods. 

The vapor is thea withdrawa from the stiH as distillate. The changing Hquid composition is most coavenieafly described by foUowiag the trajectory (or residue curve) of the overall composition of all the coexistiag Hquid phases. An exteasive amouat of valuable experimental data for the water—acetoae—chloroform mixture, including biaary and ternary LLE, VLE, and VLLE data, and both simple distillation and batch distillation residue curves are available (93,101). Experimentally determined simple distillation residue curves have also been reported for the heterogeneous system water—formic acid—1,2-dichloroethane (102). 

Although batch distillation is covered in a subsequent separate section, it is appropriate to consider the application of RCM and DRD to batch distulation at this time. With a conventional batch-rectification column, a charge of starting material is heated and fractionated, with a vapor product removed continuously. The composition of the vapor prodiic t changes continuously and at times drastically as the lighter component(s) are exhausted from the stiU. Between points of drastic change in the vapor composition, a cut is often made. Successive cuts can be removed until the still is nearly diy. The sequence, number, and limiting composition of each cut is dependent on the form of... 

FIG. 13-63 Batch distillation paths, (a) Methanol-methyl propionate-water system. 

Batch distillation, which is the process of separating a specific quantity (the charge) of a liquid mixture into products, is used extensively in the laboratory and in small production units that may have to serve for many mixtures. When there are N components in the feed, one batch column will suffice where N — 1 simple continuous-distillatiou columns would be required. 

The progress of batch distillation cau be controlled in several ways ... 

FIG. 13-98 Typical variation in distillate and reboiler compositions with amount distilled in binary batch distillation at a constant-reflux ratio. 

FIG. 13-99 Distillate composition profile for a batch distillation of a four-component mixture. 

Although a number of studies were made and approximate methods developed for predicting the effect of liquid holdup in the period of the 1950s and 1960s, as summarized in the 6th edition of Peny .s Chemical Engineers Handbook, the complexity of the effect of liqmd holdup is such that it is now best to use computer-based batch-distillation algorithms to determine the effect of holdup on a case-bycase basis. 

Therefore, the lower bound on the stiffness ratio at the beginning of batch distillation is given approximately by... 

Example 10 Calculation of Multicomponent Batch Distillation A charge of 45.4 kg mol (100 Ih-mol) of 25 mole percent heuzeue, 50 mole percent monochlorohenzene (MCB), and 25 mole percent orthodichloro-henzene (DCB) is to he distilled in a hatch still consisting of a rehoiler, a column containing 10 theoretical stages, a total condenser, a reflux drum, and a distillate accumulator. Condenser-reflux drum and tray holdups are 0.0056 and... 

FIG. 13-106 Distillate -composition profile for the miilticomponent-batch-distillation example. 

Open-loop behavior of multicomponent distillation may be studied by solving modifications of the multicomponent equations of Distefano [Am. Inst. Chem. Eng. J., 14, 190 (1968)] as presented in the subsection Batch Distillation. One frequent modification is to include an equation, such as the Francis weir formula, to relate liquid holdup on a tray to liquid flow rate leaving the tray. Applications to azeotropic-distillation towers are particularly interesting because, as discussed by and ihustrated in the Following example from Prokopalds and Seider... 

Batch distillation equipment can range from a free-standing column with a reboiler, condenser, receiver, and vacuum system, to the use of a jacketed reactor with a condenser. Distillation often involves the generation of combustible vapors in the process equipment. This necessitates the containment of the vapor within the equipment, and the exclusion of air from the equipment, to prevent the formation of combustible mixtures that could lead to fire or explosion. 

Incorporate batch distillation into written procedure for the process... 

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