Batch Distillation

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Process Engineering Guide: GBHE-PEG-MAS-607

Batch Distillation Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Batch Distillation CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 BACKGROUND TO THE DESIGN 3 4.1 General 3 4.2 Choice of batch/continuous operation 4 4.3 Boiling point curve and cut policy 5 4.4 Method of design 5 4.5 Scope of calculations required for design 6 5 SIMPLE BATCH DISTILLATION 6 6 FRACTIONAL BATCH DISTILLATION 7 6.1 General 7 6.2 Approximate methods 7 6.3 Rigorous design - use of a computer model 7 6.4 Other factors influencing the design 8

6.4.1 Occupation 8 6.4.2 Choice of Batch Rectification or Stripping 8 6.4.3 Batch size 8 6.4.4 Initial estimate of cut policy 9 6.4.5 Liquid Holdup 9 6.4.6 Total reflux operation and heating-up time 9 6.4.7 Column operating pressure 10



6.5 Optimum Design of the Batch Still 10 6.6 Special design problems 11 7 GENERAL ASPECTS OF EQUIPMENT DESIGN 11 7.1 Kettle reboilers 11 7.2 Column Internals 11 7.3 Condensers and reflux split boxes 11 8 PROCESS CONTROL AND INSTRUMENTATION IN

BATCH DISTILLATION 12 9 MECHANICAL DESIGN FEATURES 12

10 BIBLIOGRAPHY 15 APPENDICES A McCABE - THIELE METHOD - TYPICAL EXAMPLE 16



FIGURES 1 SCHEMATIC OUTLINE OF A TYPICAL BATCH

DISTILLATION UNIT 4

2 TRUE BOILING POINT FOR A TERNARY MIXTURE 5 3 DIAGRAM FOR A TYPICAL MULTI-PURPOSE STILL 13 4 EQUILIBRIUM DIAGRAM FOR ETHANOL - WATER

SYSTEM 18

5 McCABE - THIELE DIAGRAMS xD = 0.8 19 6 PLOT OF 1/(xD – xW) AGAINST Xw 20 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 21



0 INTRODUCTION/PURPOSE Batch distillation in conceptually similar to continuous distillation in terms of the equipment and mode of operation generally employed. However the design of a batch distillation system is more complicated because of the transient nature of the operation and because there may be several ways in which the overall separations required may be accomplished. 1 SCOPE This Engineering Guide sets out the factors which can influence the design of a batch still facility. However, no single document can fully reflect the known diversity of experience and technique available. It follows that this guide can provide, at best, a checklist by which an optimum design can be evolved whilst avoiding some of the more common pitfalls. For the reader who wishes only to be reminded of the traditional McCabe - Thiele based methods applied to a batch still problem, a typical example is set out in Appendix A. 2 FIELD OF APPLICATION This Guide applies to the process engineering community in GBH Enterprises worldwide. 3 DEFINITIONS For the purposes of this Guide no specific definitions apply.



4 BACKGROUND TO THE DESIGN 4.1 General Fractionation requirements may range from a single theoretical plate (i.e. a reboiler) to say a hundred plates. Operating pressures vary from below 1 mb to several bars. At the start of the design exercise the engineer should satisfy him himself that distillation is likely to be the best way of performing the separation. The most common constraints are due to either a low relative volatility between key components or to thermal instability of the feedstock. These problems can be tackled (by, for example, the use of an azeotroping agent or by the use of a thin-film evaporator at very low pressure) but the techniques involved can be very specialized. The final design of a batch still facility can be influenced by a large number of factors. A typical configuration is shown in Figure 1 in which a batch charge to the reboiler is rectified into one or more distillate fractions.



FIGURE 1 SCHEMATIC OUTLINE OF A TYPICAL BATCH DISTILLATION UNIT

All equipment rated to 180°C, 30 mb 4.2 Choice of batch/continuous operation Separation by batch distillation is widely used, especially in the Fine Chemicals area, where any of the following features may weigh against continuous operation: (a) a low rate of production (for example below 3,000 te/a), (b) where several product streams are to be resolved from a feedstock and where either the feedstock or product quality may change,



(c) where the adjacent processing stages are batch operations and where batch identity may be important,

(d) the need for operational flexibility in a multiproduct unit, particularly when

detailed knowledge of the vapor-liquid equilibria is not available, (e) where the contributory cost of energy is low (use of utilities such as hot oil

may favor continuous processing when interchange is possible). In considering the economic implications of these features there are no simple prior criteria for establishing whether or not separations should be performed batchwise; each case should be looked at on its own merits and each may require preliminary design studies. For example, scale of production can vary from a single batch for evaluation purposes up to several thousand te/a. Equally where the associated processes are continuous and there is no operational need for flexibility, serious consideration should be given to providing small continuous units which could be cheaper to build and run than a batch still with its associated storages. 4.3 Boiling point curve and cut policy Measurement and consideration of the boiling point curve of the feedstock can give a useful indication of the distillation requirements to achieve separation, particularly when the detailed vapor-liquid equilibria are known. For example, the true boiling point test (see Bibliography, ref 4) in which the material is batch distilled in a column with high plateage (up to 100) and high reflux (up to 100:1) typically results in a plot of the overhead vapor temperature against percentage distilled of the form indicated in Figure 2. The art of batch distillation lies in deciding at what points in the distillation process the intermediate cuts should be taken containing mixtures of components and how to subsequently reprocess these mixtures to improve the overall recovery. For example, a cut fraction could either be worked up separately or recharged with the next batch of feedstock. In many multicomponent systems the shape of the curve can be enormously more complex than that of Figure 2 and the range of cut point options is correspondingly greater; in such cases the practice is to accumulate "fores" and "tails" fractions about the major (or most valuable) component for subsequent reworking.



FIGURE 2 TRUE BOILING POINT FOR A TERNARY MIXTURE

4.4 Method of design The design of batch distillation systems may be based largely on either an empirical approach or by a simplified manual calculation or, where correlated vapor-liquid equilibria are available, by means of a computer simulation. With the widespread use of batch stills in multi-product plants it is rare for a new design to be undertaken without some prior experience of performing the separation on a trial campaign. Thus the design and mode of operation of many existing batch stills has evolved on a empirical basis with only limited reference to conventional distillation engineering calculation. Whilst this is a particularly valid approach when the vapor-liquid equilibria are unknown, it relies on both experience and availability of suitable plant to get a practical separation scheme. In many cases, laboratory tests conducted at high reflux (say 10:1) are sufficient to define purity targets and appropriate cut points which can be directly tested on the plant. Manual methods of design are based on the McCabe-Thiele analysis of the fractionation of two key components. Whilst providing a graphic account of the distillation process, these methods are both approximate and tedious (particularly when more than 10 theoretical plates are required) and have tended to be replaced by computer-based methods. An example of a manual calculation is included in Appendix A.



Where reliable correlations for the vapor-liquid equilibria are available, the use of an appropriate computer simulation is virtually mandatory in all but the simplest of cases (ie binary systems of high relative volatility) particularly where tight design and operation are critical. When insufficient time or material is available for an empirical approach, the use of a computer program for solving the unsteady state distillation problem permits a quantitative optimization of still design and operation. Indeed it is possible, given the vapor-liquid equilibria, for the engineer to evolve a sensible operational policy with regard to splitting (and possibly reworking) a feedstock in order to guide laboratory scale experimentation at a very early stage in product development. An increasingly frequent problem involves separation of an aqueous feedstock containing both organics and salts (substantial salting out effects may be localized in the reboiler); the current range of software cannot handle this case. 4.5 Scope of calculations required for design All distillation column designs require solutions to the phase equilibria, the heat-and-mass balances through the column (usually on a theoretical tray basis) and then the hydraulics and mass-transfer characteristics of the tower internals. Whilst these solutions specify the performance of a continuous still, batch distillation calculations should additionally solve the unsteady state (compositions throughout the unit changing as material is withdrawn) which in turn depends on the holdup of liquid and throughput characteristics of the kettle, column and auxiliaries. In many instances the liquid holdup pattern will determine the achievable splits as well as having implications for control of the column. For example, holdup and drainings from the column will determine the level of volatile impurity for a product retained in the kettle; holdup in the reflux drum will dominate distillate purity when a volatile product is a minor component in the feedstock. Furthermore, a multicomponent distillation may be effected with changes of reflux ratio, reboiler duty or operating pressure as specific cuts are taken. The compositions of intermediate cuts may well also be parameters which have to be optimized with regard to rework operation. Finally, the optimum purities of products should be assessed in the context of the overall flowsheet requirements since adjacent processing stages may be influenced by trace contaminants or solvents. It follows therefore that, in the case of a multicomponent feedstock, the number of variables open to optimization can be extremely large. Design is therefore inevitably an iterative process.



5 SIMPLE BATCH DISTILLATION The simplest case of binary batch distillation involves a reboiler and condenser with no column fractionation. This was first considered by Rayleigh whose solution to the equilibrium and mass balance equations assuming constant relative volatility is as follows:

This type of distillation is generally restricted to duties where the relative volatility is say above 5, and where neither high recoveries nor high purities are crucial. It may also be used with an existing facility where the layout precludes the installation of a column. In such a case it may be necessary to undertake repeated Rayleigh-type distillations to improve recovery and purity in products, but this carries penalties in terms of both energy and occupation. 6 FRACTIONAL BATCH DISTILLATION 6.1 General Simple batch distillation produces only a limited separation of components and in most cases a fractionating column is used to improve separation. The fractionation requirements (plateage and reflux ratio) can be calculated either by approximate methods (usually manual) or by a rigorous computer-based approach. 6.2 Approximate methods Where fractionation is required it has been possible to extend the McCabe-Thiele analysis for both constant-and-variable-reflux operation by looking at the point-wise (in time) separation achieved (see Bibliography, ref 2). This method is basically limited to binary systems with fairly high relative volatilities requiring say less than 10 theoretical plates and reflux ratios of below 5:1.



Wide-boiling multicomponent systems may be regarded as sequential binary distillations between key components (see Bibliography, ref 2). The tedious nature of the manual calculations required together with drastic simplifying assumptions (in particular the methods are for binary systems only and ignore holdup) combine to ensure that such calculations are only undertaken when a rigorous computer method is not accessible. An example of a distillation conducted at constant reflux to give a specified bulk distillate is worked through in Appendix A. 6.3 Rigorous design - use of a computer model For the simulation of any multicomponent batch distillation, several computer-based methods are available which obviate many of the assumptions of manual calculations. These are invariably theoretical plate methods which require an adequate knowledge of the vapor-liquid equilibria of the system. Some commercially available computer programs simulate batch distillation of a multicomponent, non-ideal system and has the following facilities: (a) a wide range of cut point options . (b) handles reboiler, column and reflux drum holdup. (c) uses direct resubstitution methods which are extremely stable and

particularly suited to the fast-changing composition profiles which can occur in multicomponent batch distillation.

(d) allows the user to change the reflux ratio, reboiler load, or column

pressure through the distillation and to add/remove a continuous feed from the still.

(e) interfaces to a reaction routine (to allow for reactions in the reboiler) and

also to FRI column internals routines are possible (but not generally available).

Some programs requires the user to specify the number of theoretical plates, reflux ratio and boilup rate, some initial estimate of these design variables is required. Treating the system as pseudobinary (ie topping-off each component in turn), variations on the Fenske equation for minimum number of stages and the Underwood equation for minimum reflux ratio can be used to get suitable starting values (see GBHE-PEG-MAS-603).



The following typical plateage and reflux requirements have been cited in the open literature for overhead product purities between 300 and 8000 ppm (see Bibliography ref 5).

In general, as a first estimate for the typical multi-purpose still, 10 theoretical plates can be used. In turn the boil-up required can be worked out from the batch size and a notional cycle time (from the assumed occupation and production rate). Once initial guesses of plateage, reflux and boil-up have been made it will generally be quicker to proceed using the computer program direct rather than re-estimating these variables from Fenske-Underwood since it will rapidly become evident what other changes in operation are necessary to maintain sensible operation (e.g. changing reflux ratio/column pressure as various cuts are taken). 6.4 Other factors influencing the design 6.4.1 Occupation The proportion of time for which the plant can be used for distillation activities (i.e. charging, operation and discharging) has to be assessed at the start of the design. For a dedicated still with a steady feedstock composition and with no serious cross-contamination problems an assumed occupation of up to 70% can be used for initial sizing purposes. Where a specific production rate is required from a feedstock of variable quality the occupation should reflect the worse case (particularly when reworked fractions are involved). For a multi-purpose still where cleanouts between batches and reworking of various cuts are undertaken it would be unwise to assume that the still can be used at more than 50% occupation.



6.4.2 Choice of Batch Rectification or Stripping The most common arrangement is to charge the feed to the reboiler and operate the column as a rectifier. However, in some cases the nature of the operation may favor batch stripping in which the feedstock is charged to the reflux drum and product is withdrawn continuously from the base of the column. Batch stripping is especially suited to the partial removal of traces of low boilers, particularly when these are prone to thermal instability. 6.4.3 Batch size Several factors may influence the preferred batch size: (a) the overall or instantaneous production rate, (b) the relationship with adjacent production stages, (c) the frequency of operator attention, especially if needed at critical cut

points and if the operator has other duties, (d) the thermal stability of the feedstock and residues, (e) the toxic/inflammable inventory of the batch still and associated storages. The designer may have to evaluate several batch sizes in some detail before coming to an suitable arrangement. Batch sizes are usually between 2 and 50 m3. 6.4.4 Initial estimate of cut policy Where a simple solvent recovery is required, the cut policy to be adopted may be almost self evident from the feedstock and product specifications. However, with a greater number of products to be resolved the cut policy can become more complex; particularly when substantial intermediate cuts are involved. There are three possible ways of dealing with intermediate cuts, these being: (a) Distil the cuts separately; this may lead to extended overall cycle times or

large storage requirements. A typical practice would be to retain the heels from several distillations before finally discharging the accumulated heel for disposal.



(b) Recharge the cut in with next batch; this is operationally simple but is inefficient on both cycle time and utilities. The cut could alternatively be added part-way through the next batch.

(c) Recharge the cut back to the reboiler; this has been done in one instance

where the compositions matched up. When using a simulation model for a new multicomponent problem with cut policy and column design to be optimized, the user is advised to start off using cut options based on total distillate taken before evolving strategies based on stream compositions. This is because it is easy in initial calculations to make incompatible demands on the cut patterns, e.g. when switching from one cut on distillate composition to a cut based on kettle composition (which may already have been exceeded). Once a feasible cut pattern has been evolved this can then be translated into a policy which more clearly corresponds to likely practice - viz: cuts determined by column top temperature, kettle temperature, etc. 6.4.5 Liquid Holdup Holdup in the column will determine the residual level of volatile impurity of a product drawn (with column drainings) from the still. Similarly, holdup in the reflux drum will dominate distillate purity, especially when the volatile product is a minor component in the feedstock. These effects can only be realistically predicted using a simulation model. Since holdup in the system can have drastic consequences for product quality some initial guess at holdup is required even before a hydraulic design is undertaken. For a column with 10 theoretical stages the total column holdup may be initially taken as 2% of batch charge; for a greater number of theoretical plates the column holdup should be scaled in direct proportion. The holdup in the condenser/reflux system may initially be taken as 1% of the batch charge. Once a feasible plateage/cut policy has been evolved a preliminary hydraulic design (together with tray/packing efficiency) should be undertaken to re-assess the holdup pattern. For a tray column the holdup is essentially the clear liquor depth on the tray. For packed columns the various holdup correlations in the literature (see GBHE-PEG-MAS-612) should be regarded as giving order-of-magnitude estimates only. Similarly the holdup in the condenser/reflux systems will have to be re-estimated; this is usually done on the basis of holdup time in the reflux drum (which may need to be substantial if the drum also serves to separate two-liquid phases).



6.4.6 Total reflux operation and heating-up time Batch distillation columns are often operated under total reflux for some time prior to product take-off to ensure purity specifications at the start of take-off. The time required to equilibrate the column can be a substantial part of the overall cycle time and is typically one to three hours. Quite often the time will be specified on the basis of experience in an existing column or operational convenience (where the operator has other duties). Some methods have been evolved to estimate column equilibration times (see Bibliography, ref 2). The batch may be heated up while charging though this is not advisable when charging from drums. Charging times rarely exceed one hour and from the initial estimates of boil-up rate the heat-transfer facilities will dictate the time required to bring the batch to the boil. If a viscous high-boiling point feedstock is charged cold the variation in overall heat-transfer coefficient whilst heating up may be substantial. 6.4.7 Column operating pressure As in continuous operation, the choice of operating pressure is usually dependent on available utilities to heat and cool the still and condenser and to make the minimum use of expensive utilities such as refrigeration or hot oil. In some cases the thermal stability of the feedstock and residues may force the designer to opt to drive the still with desuperheated low pressure steam or hot water. Relative volatilities generally improve at lower pressures. If the feedstock components are of wide boiling range (say above 30°C) or if the recovery of a volatile component is to be maximized the distillation may be undertaken with a reducing pressure profile. In practice the reduction of pressure has to be undertaken gradually to avoid column flooding. Usually the heat source will be turned off and the column put under total reflux whilst the pressure is reduced. The maximum permissible rate of pressure reduction could in principle be estimated by doing an adiabatic flash on the still contents and checking the vapor load. Similar considerations will apply when considering the pressure control arrangements since poor control can lead to massive entrainment of the reboiler contents.



6.5 Optimum Design of the Batch Still Once the designer has set up an initial simulation, the subsequent evolution is an iterative process in which several variables (reflux ratio, top pressure, cut points, etc.) are manipulated. A cut point policy is usually evolved around the recovery and purity of the major (most valuable) product stream. In virtually all cases simplicity of the process sequence and its control will be critical to both minimal capital and to successful operation. For instance, the total number of cut fractions is usually kept to a minimum to minimize storage requirements for intermediate fractions. Off-spec material may be accumulated simply as "fores" and "tails" cuts for reworking. Obviously, simulation of the overall cycle has to include the rework distillations together with correction for any recovered material that is recharged with the main batch. There is a general preference to adopt a cut policy (viz. changes of reflux ratio, etc.) that is as simple as possible using essentially step changes in operation rather than ramped changes. For example, a ramped reflux policy to maintain a fixed distillate composition may only be feasible with direct on-line analysis and would probably be too unreliable for most practical applications in multi-purpose plant. Similarly, various cuts may be deliberately "overdone" in order to avoid a length analysis. Deliberate cycling operation (ie. repeated intermittent take-off, possibly including draining the reflux drum, after a period at total reflux) is not a generally adopted first design practice on plant-scale units though some applications have been reported in Petrochemicals Division (see Bibliography, ref 1). As the design of the still and its auxiliaries evolves, an increasing number of details should be reconsidered in case they influence the process requirements. Examples include: (a) column hydrodynamics at all stages of the distillation (e.g. varying mass-

transfer efficiency and column pressure drop in vacuum duties), (b) suitable instruments and controls for all stages of the distillation (e.g. the

need for specialized instruments on nominated cuts; fast response valves for small cut fractions),

(c) thermal degradation of the still contents (especially of any reworked

heels), (d) heat-transfer characteristics of the reboiler due to varying batch viscosity

and liquor level,



(e) the number of product receivers required and analytical time involved where a product specification has to be guaranteed,

(f) the area available in the still for disengagement of any foam. This is

almost impossible to characterize and may occur at any stage of the distillation restricting the permissible boilup rate.

6.6 Special design problems It is possible, using the facilities within CHEMCAD, for continuous feed addition within the program to solve two quite common cases: (a) where a reaction in the reboiler generates a volatile product, (b) in azeotropic distillation where the distillate splits into two phases, one of

which is returned to the column as reflux. If a batch distillation is to be simulated where the effective number of theoretical plates in the columns changes significantly through the distillation (due to changing physical properties or column hydrodynamics) then CHEMCAD will have to be run repeatedly on two (or more) cuts. 7 GENERAL ASPECTS OF EQUIPMENT DESIGN The general configuration of a typical multi-purpose batch still is shown in Figure 1. 7.1 Kettle reboilers Kettle reboilers are almost invariably used on multi-purpose stills. The tube bundle should be mounted low in the kettle to ensure good heat-transfer when high boil-downs (e.g. up to 90% of charge) are required. This consideration involves mechanical constraints as noted below and may force tight tube spacing (thus limiting heat-transfer performance) to keep the coils submerged. In some cases a dual-exchange bundle is installed so that the upper bundle can be turned off part way through the boil-down. Finned tubes have been proposed in some non fouling duties but have not been widely adopted in the industry.. The designer should ensure sufficient disengagement area to avoid priming the still contents into the column; this can be particularly difficult at the end of a distillation if accumulated tars promote priming.



If the final residue of the kettle is viscous or is subject to either decomposition or charring on exposed coil surface the above factors may combine to favor an external forced circulation reboiler designed for minimum inventory. Thin film evaporators should be considered for low pressure applications - falling-film evaporators are suitable in the 10-50 mb range and wiped-film evaporators for 1- 10!mb. Where the feedstock is viscous or a slurry, preference may be given to using an agitated vessel or reactor as the reboiler; the agitation requirements should be checked through the normal batch cycle and any likely upset conditions (e.g. an over-distilled batch). 7.2 Column Internals The same general hydrodynamic consideration apply in selecting internals as for continuous columns (see GBHE-PEG-MAS-612 and GBHE-PEG-MAS-611). Packings are usually adopted up to 1 m column diameter or in corrosive environments. Great care may be required with the specification of hold-down plates to withstand the transient vapor loads caused by flashing of the still contents (plates are usually fixed for metal packings or weighted for ceramics). 7.3 Condensers and reflux split boxes Condensers and reflux split boxes used in batch distillation are usually of conventional design and external to the column. As discussed previously, the holdup in the reflux drum may limit the achievable separation of a column and various low inventory reflux splitter arrangements are available (see Bibliography, ref 2). If timed reflux splitting is adopted the pulse rate should reflect the (notional) holdup on the top tray to avoid pulsing the column profile. Swinging bucket reflux splitters have been used in columns up to 1 m diameter. Use of pumped reflux is not common since it is frequently desirable to drain the overheads by gravity to storage. For small-scale columns or high vacuum duties (say below 20 mm Hg head pressure) there may be considerable advantage in using an internal condenser; however, reflux flow control (particularly at high reflux) may be more difficult. This may be simplified by using an internal dephlegmator (to provide reflux) together with an external condenser (the reflux rate being essentially fixed by the cooling water supply to the dephlegmator). In small duties the reflux drum may be combined with the condenser bonnet. The "Spotton" reflux divider is one commercially available splitter which has proved satisfactory in a number of applications since it provides reproducible splits over a wide range of flows with only a small holdup (see Bibliography, ref.6).



It is almost impossible to ensure tight design of a two-liquid phase separator for azeotropic distillation where one phase is refluxed because of the transient loads and because the coalescence characteristics may change through the batch cycle. To ensure adequate settling these separators are usually grossly oversized (say for hour holdup). 8 PROCESS CONTROL AND INSTRUMENTATION IN BATCH

DISTILLATION In a typical batch distillation, the process control requirements are determined by the cut-point policy and by the product specifications and operator attention implicit in that policy. Whilst some of the standard control techniques of continuous distillation can be copied direct into batch practice the attendant requirements of simplicity and robustness (especially for multipurpose plant) are such that usually only simple control functions are adopted. Thus reflux ratios, for example, may be pre-set to a fixed value for each cut. The most common practice is to fix a reboiler steam rate and to switch cuts depending on some unambiguous observation such as accumulated distillate or temperature in the column top, midpoint or still, or alternatively the loss of boil-up at a given column and steam pressure. A wide variety of analytical methods have also been used to characterize distillate cut-points including density, refractive index, crystallizing point, gas chromatography and on-line NMR. A number of schemes for boil-up control have been described in ref 2 (see Bibliography) ranging from control of steam pressure, steam flow, reboiler temperature differential or column pressure drop; the latter being particularly suited to packed columns. Exceptions will occur when a particularly fast response is required but such cases are more commonly avoided by changing the cut pattern A Line Diagram for a typical multipurpose still is shown in Figure 3.



9 MECHANICAL DESIGN FEATURES The main features of mechanical design which are particular to batch stills include: (a) Kettle reboiler heads where the tube bundle may need to be located close

to the knuckle radius when a high boil down is required. (b) Careful checks on thermal shock (especially for glass-lined or graphite

equipment), when charging a cold feed to a hot still. (c) The risk of implosion when a cold feed is charged to a still which is full of

hot vapors. For normal materials of construction the entire still system should be designed to withstand full vacuum since this is preferable to providing a vacuum protection facility.

(d) Where a column is to be mounted directly onto an agitated vessel the off-

centre load may limit the size of column. FIGURE 3 DIAGRAM FOR A TYPICAL MULTI-PURPOSE STILL



FIGURE 3 DIAGRAM FOR A TYPICAL MULTI-PURPOSE STILL



10 BIBLIOGRAPHY REF. 1 D J Facey

Petrochemicals Division. Process Design Methods No 3/4 15 Jan 1973

2 R W Ellerbe Chapter 1.3 (Batch Distillation)

in Handbook of Separation Techniques for Chemical Engineers, (Ed) P A Schweitzer, McGraw Hill 1979

3 Users' Specification for the Program CHEMCAD 4 Perry R H and Chilton C H Chemical Engineers' Handbook.

5th Edition. McGraw Hill 1973 5 O Frank Chem. Eng. March 14 1977 pp 111-128 6 Spotton reflux divider. SSC



APPENDIX A McCABE - THIELE METHOD - TYPICAL EXAMPLE A.1 DESIGN OF BATCH RECTIFICATION AT CONSTANT REFLUX BY A

MANUAL METHOD Reference 4 (see Bibliography) cites an example (originally due to Block, Chem. Eng. 68,88. Feb 6, 1961) of a batch rectification at constant reflux to produce a bulked distillate of specified quality. This case is worked through in detail below to give an indication of the labor and precision required. The McCabe-Thiele method is used assuming no column/overheads holdup and with a steady boil-up rate. There were some anomalies in the original calculations and the source of equilibria data was not specified; these are clarified below. A.2 PROBLEM A feedstock of 520 moles containing 18% molar ethanol in water is to be distilled at a boil-up rate of 75 moles/hr to produce a bulked distillate of 80% molar composition. The column and reboiler are equivalent to 7 theoretical plates. Estimate the amount of distillate recovered and the time required. A.3 BACKGROUND EQUATIONS AND DATA The following relationships are pertinent (see Bibliography, ref 4):

Where W i, W f are the initial and final still contents

x w, x d are the still and corresponding distillate compositions



The average distillate composition is given by:

The time required for distillation is given by:

Where R is the reflux ratio

V is the boil-up rate Q is the integral defined by equation 1

The vapor-liquid equilibria for ethanol water were determined in a comprehensive series by Otsuki & Williams (Chem. Eng. Prog. Symp. Ser. No 6, 49, 55-67, 1963) and a selection of data for atmospheric pressure are as follows:



A.4 SOLUTION The equilibrium diagram (see Figure 4) shows that to produce distillate D from feed F the reflux ratio has to exceed not only the line DF but also (because the curve is inflected ) the tangent to the curve from D. This point of tangency is located approximately at (0.600, 0.695) and the minimum L/V ratio is thus:

The operating (L/V) ratio is taken as being about 1.5 times the minimum; for these calculations we will assume an (L/V) ratio of 0.75 which corresponds to a reflux ratio of 3. A series of McCabe-Thiele plots is then made for a number of distillate compositions in the range of interest. An example of one of these for xD = 0.800 is given in Figure 5 using a reflux ratio of 3 with 7 theoretical plates which correspond to a still composition (xW) of 9.06% ethanol. The results of these diagrams are as follows:

A plot of 1/(xD > xW) against xW is then prepared (see Figure 6). A trial and error procedure is then used to satisfy the equations 1 and 2 by integrating between the initial liquid composition (0.18) and various lower limits until the equations are satisfied and an average distillate composition of 0.8 is reached. The results are as follows:



The final still composition will then be about 4% ethanol and will correspond to a distillate recovery of 520-424, i.e. 96 moles. From equation 3 the required time for distillation will be:



FIGURE 4 EQUILIBRIUM DIAGRAM FOR ETHANOL - WATER SYSTEM



FIGURE 5 McCABE - THIELE DIAGRAMS xD = 0.8



FIGURE 6 PLOT OF 1/(xD - xW) AGAINST xW

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: ENGINEERING GUIDES GBHE-PEG-MAS-603 Short Cut Methods of Distillation Design (referred to in

6.3). GBHE-PEG-MAS-611 Design and Rating of Trayed Distillation Columns

(referred to in 7.2). GBHE-PEG-MAS-612 Design and Rating of Packed Distillation Columns

(referred to in 6.4.5 and 7.2).

Batch Distillation

Technology

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