Optimize Batch Distillation

Proper design depends upon an understanding of key relationships.

By John E. Edwards, P & I Design

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A sound modelling strategy is to define the still condition by specifying the reboiler duty or boil-up rate to sustain the overhead fixed or variable composition condition specified. As the distillation proceeds and less of the mvc remains in the system, the reboiler duty requirement will increase to sustain the same boil-up rate and satisfy the increasing reflux ratio if operating at constant top composition.

Consider the following when setting up the simulation:

• Distillate temperature, at constant pressure, sets distillate composition.

• At constant boil-up rate, the reflux ratio sets the distillate withdrawal rate.

• The reflux ratio and distillate rate can’t be set independently.

• The reboiler and condenser duties can’t be set independently.

To set the initial conditions, flash the batch charge with a zero vapor fraction at the operating pressure to give the bubble point of the mixture. The first step should start from total reflux to establish equilibrium and a composition profile. All subsequent steps should begin with current column content. The steps can be stopped based on time, temperature, composition, quantity or reflux ratio as required. A simple one-stage batch distillation without rectification is modelled by selecting two stages and a reflux ratio of zero; the reboiler and condenser each represent a stage.

For the desired separation, ensure the number of stages selected satisfies NMIN. For heterogeneous azeotropes, select “total condensation with decant,” which requires specification of the fraction (0.0–1.0) of light phase or heavy phase removed to a decanter and mandates use of the simultaneous correction method.

Figures 4-6 show the residue curves for the ethanol/benzene/water system starting from compositions in each of the three regions and verify the residue curve mapping behavior predicted.

(Note: I used CC-BATCH software by Chemstations to generate the process flow schematic, residue curves, binodal and batch distillation plots.)

GET MORE DETAILS
This article is based on material that appears in “Chemical Engineering in Practice,” 2nd edition. The 550-page book, which also includes a CD covering all the cases discussed, is available from P & I Design, http://pidesign.co.uk/publication.htm or via Amazon.


John Edwards is senior consultant at P & I Design Ltd., Stockton-on-Tees, U.K. E-mail him at   jee@pidesign.co.uk.


REFERENCES

1. U. M. Diwekar, “Batch Distillation: Simulation, Optimal Design, and Control,” 2nd ed., Taylor & Francis, London (2011).

2. J. E. Edwards, “Keep Cool when Designing Batch Reactors,” Chemical Processing, September 2005, www.ChemicalProcessing.com/articles/2005/554/

3. I. Smallwood, “Solvent Recovery Handbook,” 2nd ed., Blackwell, Oxford, U.K. (2002).

4. R. E. Rooks, “Draw Insights on Distillation,” Chemical Processing, May 2006, www.ChemicalProcessing.com/articles/2006/071/

5. F. G. Shinskey, “Process Control Systems: Application, Design and Tuning,” 4th ed., McGraw-Hill, New York City (1996).

6. J. E. Edwards, “Chemical Engineering in Practice,” 2nd ed., P&I Design, Stockton-on-Tees, U.K. (2011) [see sidebar].

NOMENCLATURE

A surface area

a, b, c Antoine coefficients for each pure solvent

C total moles in accumulator

D distillate flow, mole/h

d reactor diameter

F separation factor

f fluid flow

H specific heat of liquid

h film coefficient for heat transfer

I impeller diameter

J jacket fluid mass

K empirical constant

k thermal conductivity

L reflux flow, mole/h

M mass of still bottoms fluid

m slope of operating line

N number of theoretical stages

Nu Nusselt number

n empirical constant

Pr Prandt number

p vapor pressure of component, mm Hg

Q heat duty,

q empirical constant

R reflux ratio

Re Reynolds number

r impeller rotation rate

S total moles in reboiler

s cross-sectional area

t temperature, °C

T temperature, K

U overall heat transfer coefficient

V boil-up rate, moles/h

W wetted perimeter for heat transfer

w still equipment weight

X mole fraction of more volatile component

x material thickness

subscripts

b bulk

bot bottoms fluid

C in accumulator

D in rundown

e natural log

equ equivalent

f fouling

fi inside fouling

fo outside fouling

g glass

hf heat transfer fluid

i inside

j jacket

m metal

mean mean

min minimum

o outside

pf process fluid

S in reboiler

still still material of construction

sub condenser subcool

w wall

1 time 1 or as specified in text

0 time 0 or as specified in text

Greek letters

α relative volatility based on Raoult’s law

θ time

λ latent heat of vaporization

μ viscosity

ρ density

 

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