Batch Distillation

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MAS-607

Batch Distillation

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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
4.2
4.3
4.4
4.5

General
Choice of batch/continuous operation
Boiling point curve and cut policy
Method of design
Scope of calculations required for design

3
4
5
5
6

5

SIMPLE BATCH DISTILLATION

6

6

FRACTIONAL BATCH DISTILLATION

7

6.1
6.2
6.3
6.4

General
Approximate methods
Rigorous design - use of a computer model
Other factors influencing the design

7
7
7
8

6.4.1
6.4.2
6.4.3
6.4.4
6.4.5
6.4.6
6.4.7

8
8
8
9
9
9
10

Occupation
Choice of Batch Rectification or Stripping
Batch size
Initial estimate of cut policy
Liquid Holdup
Total reflux operation and heating-up time
Column operating pressure


6.5
6.6

Optimum Design of the Batch Still
Special design problems

10
11

7

GENERAL ASPECTS OF EQUIPMENT DESIGN

11

7.1
7.2
7.3

Kettle reboilers
Column Internals
Condensers and reflux split boxes

11
11
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 heatand-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 pointwise (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 computerbased 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-ofmagnitude 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 masstransfer 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 110!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 offcentre load may limit the size of column.

FIGURE 3



FIGURE 3



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
A.1

McCABE - THIELE METHOD - TYPICAL EXAMPLE

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).


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