Fractional batch distillation

10 Pages·0.814 MB·English

by  Seidl D., Wusterhausen M., Fricke A.

Fractional
 batch
 distillation

Problem
 &
 solution
 principle

A
 mixture
 of
 three
 substances
 is
 to
 be
 separated
 by
 means
 of
 distillation.
 Afterwards,
 the

ingredients
 (benzol,
 toluene
 and
 o-­‐xylene)
 are
 supposed
 to
 be
 available
 in
 almost
 pure
 form.

The
 possibility
 of
 fractional
 batch
 distillation
 is
 presented
 in
 this
 document
 for
 this
 purpose.

Batch
  distillation
  also
  allows
  the
  economic
  separation
  of
  smaller
  quantities
  of
  substance

mixtures
 with
 at
 the
 same
 time
 high
 flexibility
 regarding
 the
 design
 specification.
 Due
 to
 the

temporal
 dependence
 of
 the
 distillation
 process
 caused
 by
 the
 substance-­‐specific
 boiling
 range,

batch
 distillation
 is
 a
 discontinuous
 distillation
 process.

A
 batch
 column
 with
 5
 collection
 tanks
 is
 used
 to
 accumulate
 the
 respective
 distillate.
 A
 so-­‐
called
 time
 switch
 between
 the
 tanks
 and
 the
 column
 ensures
 allocation
 to
 the
 respective

product
 tank.

Fig.
 1
 shows
 the
 layout
 of
 the
 process.

Fig.
 1

 Process
 flow
 chart

èfocused
 on
 process
 simulation
Implementation
 of
 the
 batch
 distillation
 in
 CHEMCAD

To
 implement
 the
 process
 in
 CHEMCAD,
 the
 substance
 mixture
 must
 be
 defined,
 the
 flow
 sheet

generated
 and
 the
 design
 specifications
 established.

Once
 the
 substances
 benzol,
 toluene
 and
 o-­‐xylene
 have
 been
 selected
 from
 the
 substance

database
 and
 the
 unit
 operation
 placed
 on
 the
 flow
 sheet,
 the
 still
 pot
 of
 the
 column
 is
 defined.

Fig.
 2
 shows
 the
 column
 options
 and
 fig.
 3
 the
 feed
 composition
 after
 the
 sub-­‐item
 [Pot
 Charge]

has
 been
 selected.

Fig.
 2

 Batch
 column
 options
  Fig.
 3

 Feed
 composition

The
 column
 properties
 are
 listed
 in
 the
 sub-­‐item
 [Batch
 Column].
 Fig.
 4
 shows
 the
 settings

options
 for
 the
 distillation.
 As
 three
 main
 fractions
 and
 two
 intermediate
 fractions
 are
 to
 be

obtained,
 the
 "Number
 of
 Operation
 Steps"
 is
 set
 to
 5.
 The
 number
 of
 stages
 is
 set
 to
 101.
 The

condenser
 pressure
 is
 set
 to
 atmospheric
 pressure.
 Optionally
 it
 is
 possible
 to
 define
 pressure

losses
 within
 the
 column,
 or
 simulate
 a
 volume,
 mass
 or
 mol
 hold-­‐up.

1
 Here,
 the
 selection
 of
 the
 design
 and
 column
 parameters
 is
 based
 on
 experience;
 alternatively,
 a
 heuristic
 layout
 or

performing
 a
 sensitivity
 study
 is
 also
 a
 possibility

èfocused
 on
 process
 simulation

  Page
 2
 of
 10
Fig.
 4

 Design
 parameter
 settings
 for
 the
 column

The
  options
  sub-­‐item
  [Operation
  Parameters]
  is
  selected
  to
  define
  the
  cycle.
  Now
  five

programme
 windows
 appear
 in
 which
 the
 stop
 values
 of
 the
 respective
 fraction
 can
 be
 set.

Fig.
 5
 shows
 the
 settings
 of
 the
 1st
 main
 fraction.
 The
 reflux
 ratio
 and
 the
 distillation
 rate
 are

set
 to
 101
 and
 100
 kmol/h
 for
 all
 fractions,
 and
 the
 step
 size
 of
 the
 integration
 is
 set
 to
 0.005
 h.

At
 the
 start
 of
 distillation,
 almost
 only
 the
 benzol
 boils,
 which
 is
 why
 the
 stop
 value
 of
 the

fraction
 is
 set
 to
 95
 mol%
 in
 the
 first
 tank.

Fig.
 5

 Design
 parameters
 of
 the
 1st
 main
 fraction

èfocused
 on
 process
 simulation

  Page
 3
 of
 10
Fig.
 6

 Design
 parameters
 of
 the
 1st
 intermediate
 fraction

As
  the
  boiling
  ranges
  of
  benzol
  and
  toluene
  overlap,
  the

1st
 intermediate
 fraction
 is
 stopped
 once
 the
 toluene
 concentration
 in
 the
 distillate
 is
 high

enough
 (95
 mol%)
 to
 generate
 the
 2nd
 main
 fraction
 (see
 fig.
 6).

This
  is
  collected
  in
  the
  third
  tank.
  Here
  the
  process
  is
  stopped
  analogue
  to
  the

1st
 main
 fraction
 once
 the
 toluene
 concentration
 in
 the
 tank
 sinks
 and
 reaches
 95
 mol%.
 (See

fig.
 7)

Fig.
 7

 Design
 parameters
 of
 the
 2nd
 main
 fraction

èfocused
 on
 process
 simulation

  Page
 4
 of
 10
Fig.
 8

 Design
 parameters
 of
 the
 2nd
 intermediate
 fraction

The
  2nd
  intermediate
  fraction
  consisting
  of
  toluene
  and
  o-­‐xylene
  is
  stopped

analogue
 to
 the
 1st
 intermediate
 fraction
 once
 the
 o-­‐xylene
 concentration
 is
 sufficiently
 high

with
 95
 mol%
 in
 the
 distillate
 flow
 (see
 fig.
 8).

A
 time
 limit
 of
 one
 hour
 is
 selected
 as
 the
 stop
 option
 for
 the
 last
 main
 fraction
 o-­‐xylene
 (see

fig.
 9).
 At
 a
 distillation
 rate
 of
 100
 kmol/h
 and
 a
 feed
 volume
 of
 100
 kmol,
 batch
 distillation
 is

thus
 complete
 after
 one
 hour.

The
 distillation
 process
 has
 now
 been
 specified
 in
 sufficient
 detail.

Fig.
 9

 Design
 parameters
 of
 the
 3rd
 main
 fraction

èfocused
 on
 process
 simulation

  Page
 5
 of
 10
Assessment
 of
 the
 simulation
 results

12
 different
 parameters
 for
 recording
 during
 the
 simulation
 can
 be
 selected
 in
 the
 options

menu
 of
 the
 batch
 column
 (see
 fig.
 2)
 at
 [Set
 Screen
 Information].
 The
 mol
 break
 of
 the

components
  in
  the
  tanks
  is
  selected
  in
  order
  to
  be
  able
  to
  reproduce
  the
  concentration

progression
 of
 the
 individual
 components.

The
 concentration
 values
 in
 the
 tanks
 are
 now
 plotted
 parallel
 to
 the
 simulation.

Fig.
 10.
 a
 shows
 the
 concentration
 progression
 in
 the
 first
 collection
 tank.
 As
 you
 can
 see
 in
 the

figure,
 the
 benzol
 initially
 flows
 into
 the
 collection
 tank
 with
 a
 concentration
 of
 100%,
 and
 is

stopped
 after
 0.335
 h
 at
 95
 mol%
 as
 defined
 by
 the
 stop
 option.

a
  b

Fig.
 10

 Concentration
 progression

 a

 1st
 main
 fraction;

b

 1st
 intermediate
 fraction

The
 concentration
 progression
 of
 the
 1st
 intermediate
 fraction
 in
 the
 second
 tank
 is
 shown
 in

fig.
 10.
 b.
 The
 accumulation
 of
 the
 two-­‐substance
 mixture
 benzol
 –
 toluene
 is
 stopped
 once
 the

toluene
 concentration
 reaches
 95
 mol%.

Fig.
 10
 shows
 that
 the
 concentration
 progressions
 in
 the
 containers
 do
 not
 progress
 steadily
 in

the
  case
  of
  a
  sequence.
  For
  example,
  the
  toluene
  concentration
  at
  the
  end
  of
  the
  first

accumulation
 is
 approx.
 6
 mol%,
 but
 starts
 with
 a
 concentration
 of
 approx.
 67
 mol%
 in
 tank
 2.

This
 is
 due
 to
 the
 short
 distillation
 times
 between
 the
 accumulations,
 in
 which
 the
 batch

column
 is
 operated
 with
 full
 reflux
 ratio
 in
 order
 to
 achieve
 the
 next
 minimum
 concentration
 in

the
 top.

èfocused
 on
 process
 simulation

  Page
 6
 of
 10
Fig.
 11.
 a
 shows
 the
 progression
 of
 the
 mol
 concentrations
 in
 the
 third
 container.
 Analogue
 to

the
 1st
 main
 fraction,
 it
 becomes
 evident
 that
 the
 2nd
 main
 fraction
 consists
 of
 toluene
 and

that
 the
 stop
 option
 is
 once
 again
 95
 mol%.

The
 progression
 of
 the
 concentrations
 of
 the
 2nd
 intermediate
 fraction
 (see
 fig.
 11.
 b)
 is

analogue
 to
 the
 concentration
 progressions
 of
 the
 1st
 intermediate
 fraction.
 However,
 here
 o-­‐
xylene
  is
  the
  component
  with
  the
  higher
  concentration,
  and
  the
  toluene
  concentration

decreases
 steadily.

a
  b

Fig.
 11

 Concentration
 progressions

 a

 2nd
 main
 fraction;

b

 2nd
 intermediate
 fraction

The
 fifth
 and
 last
 container
 is
 fed
 with
 the
 3rd
 main
 fraction,
 o-­‐xylene
 (see
 fig.
 12).

Accumulation
 ends
 once
 the
 batch
 distillation
 has
 been
 run
 for
 one
 hour.

 Fig.
 12

 Concentration
 progression
 of
 the
 3rd
 main
 fraction

èfocused
 on
 process
 simulation

  Page
 7
 of
 10
CHEMCAD
  makes
  it
  possible
  to
  record
  the
  concentration
  progression
  steadily
  across
  the

distillate
 flow.
 The
 option
 [Plot]
 à
 [Dynamic
 Plots]
 à
 [Batch
 Column
 History]
 is
 selected
 for

this
 purpose.
 Afterwards,
 the
 batch
 column
 is
 selected
 (see
 fig.
 13).

 Fig.
 13

 Dynamic
 column
 plot

This
 is
 followed
 by
 the
 selection
 of
 the
 components
 to
 be
 recorded
 together
 with
 a
 variable

(see
 fig.
 14).
 The
 mol
 fraction
 in
 the
 distillate
 flow
 is
 now
 being
 observed.
 The
 result
 is
 output

immediately
 (see
 fig.
 15).

 Fig.
 14

 Setting
 of
 the
 dynamic
 column
 plot

èfocused
 on
 process
 simulation

  Page
 8
 of
 10
Fig.
 15

 Concentration
 progression
 in
 the
 distillate
 flow

Instead
 of
 the
 concentration
 progressions
 in
 the
 tanks,
 a
 steady
 concentration
 progression
 of

the
 components
 is
 displayed
 now.
 By
 comparing
 the
 respective
 concentration
 with
 the
 end
 and

start
 tank
 concentrations,
 the
 time
 periods
 can
 be
 determined
 with
 full
 reflux
 ratio
 and
 closed

time
 switch.

The
 simulation
 results
 are
 thus
 fully
 recorded.

èfocused
 on
 process
 simulation

  Page
 9
 of
 10
Optimisation
 of
 batch
 distillation

Batch
 distillation
 can
 be
 optimised
 with
 respect
 to
 several
 parameters,
 such
 as
 the
 energy
 input

in
 the
 column,
 the
 distillation
 time
 or
 the
 heating
 costs.
 In
 view
 of
 the
 product
 specification,
 it
 is

however
 expedient
 to
 optimise
 the
 distillation
 with
 regard
 to
 the
 purity
 of
 the
 product.
 As

absolute
 purity
 can
 only
 be
 approximated,
 the
 aim
 is
 to
 achieve
 a
 compromise
 between
 the

desired
 concentrations
 and
 realistic
 energy
 expenditure.

In
 addition,
 batch
 distillation
 can
 be
 adapted
 to
 a
 constant
 reflux
 ratio
 or
 a
 constant
 top

concentration.

In
 case
 the
 feed
 pot
 heats
 up
 excessively,
 it
 is
 possible
 to
 switch
 to
 a
 total
 reflux
 ratio
 and
 thus

approach
 a
 new
 duty
 point
 without
 loss
 of
 distillate.
 The
 energy
 expenditure
 can
 be
 optimised

with
 a
 good
 column
 insulation.

Operating
 and
 cost
 data
 can
 also
 be
 included
 in
 the
 simulation
 in
 real
 time.
 The
 tutorial

"Mapping
 in
 CHEMCAD"
 at
 www.chemstations.eu
 provides
 further
 information
 on
 this
 topic.

The
 simulation
 discussed
 in
 this
 document
 was
 generated
 in
 CHEMCAD
 6.5.3
 and
 can
 be
 used
 in

all
 versions
 as
 of
 CHEMCAD
 5.

Are
 you
 interested
 in
 further
 tutorials,
 seminars
 or
 other
 solutions
 with
 CHEMCAD?

Then
 please
 contact
 us:

Mail:
 [email protected]

Phone:
 +49
 (0)30
 20
 200
 600

www.chemstations.eu

Authors:

Daniel
 Seidl

Meik
 Wusterhausen

Armin
 Fricke

èfocused
 on
 process
 simulation

  Page
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