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Overview
� Case Study: Batch Reactor
� Case Study: Batch Distillation
� Summary of Selection Considerations
� Equipment Available for Continuous Processing
� Summary
Case study: Batch Reactor
A � B � C, 10000 tpa of product C, k1 = k2 = 1 hr -1
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Batch Reactor
� Reaction type: A → B → C
� Reaction time of 5 hours is sufficient
dCA/dt = – k1CA
dCB/dt = k1CA – k2CB
dCC/dt = k2CB
0.0
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1.0
0 2 4 6 8 10
Batch Time
Conce
ntrat
ion
A
B
C
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� Concentration in the reactor is equal to outlet concentration
� For same selectivity, ~50 hours of residence time required
� 5 hours residence time, selectivity to ‘C’ will be much lower
Continuous Reactor – CSTR
CAIN
CAO
CBO
CCO
F(CAIN – CAO) = k1CAOV
F(CBO) = k1CAOV – k2CBOV
F(CCO) = k2CBOV
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0 10 20 30 40 50Residence Time
Conce
ntrat
ion
A
B
C
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Implications on Productivity, CAPEX
� Production rate 10000 tpa, (1250 kg/hr, 1.5 m3/hr)
� Batch process: fill, heat, reaction, drain, cool: 10 hours cycle
� 2 batches per day, 330 days (660 batches per year)
� Total quantity to be produced = 1.5 x 8000 = 12000 m3
� Batch reactor volume = 12000/660 = 18 m3
� Volume productivity = 1250 / 18 = 69.4 kg hr -1 m3
� Volume of CSTR required = 50 x 1.5 = 75 m3
� Volume productivity = 1250 / 75 = 16.7 kg hr -1 m3
� Plug flow reactors can improve productivity substantially
� Separate Heat exchangers, pumps required for Continuous
� CAPEX: equipment, instrumentation higher for Continuous
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Implications on Utility
� Consider ∆H = -100 kcal/mol
� Batch: 18 m3 reactor, peak rate of reaction 10 kmol/m3hr
� Peak Heat generation=10x18x1000x100=18 Mkcal/hr(6000TR)
� Heat generation rate drops to zero towards the end of batch
� Continuous: Rate is constant and much lower
� For a residence time of 50 hours: rate = 15 kmol/hr
� Heat generation rate=15 x1000x100=1.5 Mkcal/hr (500TR)
� Utility sizing for batch = 18/1.5 = 12 times bigger
� Utility designs based on Peak demand: Large CAPEX, OPEX
� Utility designs based on average demand: thermal runaway
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Implications on Process Engineering
� Batch: peak heat load=18Mkcal/hr,18m3 reactor(18m2
Area), U∆T required = 1Mkcal/hrm2G unmanageable
� CSTR: heat load = 1.5Mkcal/hr, 75m3 reactor (75m2 area),
U∆T required = 20000 kcal/hrm2 G manageable
� Consider hydrogenation reactions
� Batch: Peak hydrogen flow=180kmol/hr= 4000 Nm3/hr
� Fermentation Operations (4000/60/18 = 3.7 VVM)
� Continuous: 15 kmol/hr=340 Nm3/hr (340/60/75=0.075VVM)
� Batch: Piping system, control valves, instrumentation
designed for peak flow
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Implications on Process Engineering
� Gas liquid mixing: Batch = 3.7VVM, Continuous =0.075VVM
� Batch: 18m3 vessel, Continuous: 75m3 vessel
� Agitator diameter: Batch = 0.9m, CSTR = 1.5m
� Gas dispersion batch = 5 kW/m3, CSTR = 2 kW/m3
� Power required batch: 90 kW, CSTR: 150 kW
� Impeller speed Batch: 222 rpm, CSTR = 95 rpm
� Torque, Batch: 4070 N-m, CSTR = 15000N-m
� Demands on agitation system substantially higher for CSTR,
primarily because of large volume
� Use of plug flow continuous reactors helps
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Implications on SHE
� Batch Process
– Peak Concentrations of ‘A’ and ‘B’ are high
– Rate of reaction, heat release is significantly higher
� In CSTR: negligible ‘A’ and ‘B’, rate is constant
� Important SHE implications
– Bhopal Disaster
– Safety systems: RDs etc. designed for higher capacity
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Learnings G
� Both types have their merits
� It would be wonderful to combine advantages of batch and
continuous systems
� This can be done by continuous process with lower reactor
volumes (continuous systems: plug flow reactors)
� New types of equipment required, new ways of conducting
reactions have to be looked at, possibly new chemistry G
� This will be covered in some of the subsequent talks
Case study: Multi-component,
vacuum distillation
Batch: 18 m3 (15000 kg) to be distilled in 10 hours
Continuous: 1250 kg/hr feed to distillation
Composition: 50% solvent (low boiler), 48% product, 2%
impurity (high boilers)
Relative volatility: solvent-product = 3, product-impurity = 1.5
Requirements on Purity: Solvent, product 99%
Pressure: solvent @ 300 mmHg, Product @ 30 mm Hg
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Batch Distillation
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Profiles of Variables
Time
Reflux Ratio
Top Purity
Top Level
Still LevelBoilup Rate
Still Temperature
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Fundamentals
Mole Fraction in Liquid
Mole
Fra
ctio
n in V
apor
� Modes of Operation:
– Constant product Quality, Increasing Reflux
– Constant Reflux, deteriorating product quality
Only
Rectification
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Batch Distillation Design
� 1st Cut (3 hrs) Start of cut: 50% solvent in still to 99%
solvent at top, end: < 1% solvent in still, 99% at top
– Start of cut: min. reflux ratio=0.96, stages at 2Rmin= 6
– End of cut: min reflux ratio= 49, stages at 2Rmin = 10
– Average distillation rate= 7500/3 = 2500 kg/hr
� 2nd cut (5 hrs): start of cut: 95% product in still to 99%
product at top, end: 5% product in still to 99% product at top
– Start of cut: min reflux ratio=0.64, stages at 2Rmin=2.3
– End of cut: min reflux ratio=19.7, stages at 2Rmin=13
– Average distillation rate = 7200/5 = 1800 kg/hr
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Batch: Hardware and Utility
� 1st cut: distillation rate = 2500 kg/hr, stages = 10, Let R = 4
– Liquid flow = 10000 kg/hr, vapor flow=12500 kg/hr, 300mmHg,150oC
– Dia = 1.5 m, HETP = 0.6m, height = 6 m, ∆P=9 mm Hg
– Packed volume = 11 m3
� 2nd cut: distillation rate = 1800 kg/hr, stages = 13, Let R = 5
– Liquid flow = 9000 kg/hr, vapor flow=10800 kg/hr, 30mmHg, 150oC
– Dia = 2.5 m, HETP = 0.65m, height = 8.5m, ∆P=14mmHg
– Packed Volume = 42 m3 (we have to select larger of the two)
� QC&QB=12500x100=1.25Mkcal/hr, ACond=1.25x106/(500x70) = 35m2, Astill
= 1.25x106/(300x30) = 140 m2, CW = 125 m3/hr, steam = 2500 kg/hr
� Column Cost = 42m3 � 42 Lakhs, exchangers = 175m2 �175 Lakhs,
Total = 220 Lakhs
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Batch: Additional Constraints
� Huge heat transfer area required in the still, difficult to
accommodate in the batch still
� Liquid level falls, heat transfer area reduces with time,
Boiling point increases with time, vapor generation rate
drops with time, distillation rate drops with time
� Column demands change significantly with time: danger of
flooding / improper wetting � loss of packing efficiency
� High hydrostatic head in the batch still, boiling occurs only
near the liquid surface
� Inter-cuts required, several product receivers required,
product recovery is lower
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Continuous Distillation Fundamentals
Mole Fraction in Liquid
Mole
Fra
ctio
n in V
apor
Equilibrium
Feed
Rectification
Stripping
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Continuous Distillation Design
1250 kg/hr
50%S, 48%P, 2%I
630 kg/hr
99%S, 1%P
620 kg/hr
96%P, 4%I
595 kg/hr P
25kg/hr I
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Continuous Distillation Hardware
� 1st Column: min reflux ratio = 0.96, stages @ 2Rmin= 14
– Distillate = 630 kg/hr, max liquid flow = reflux + feed =
2510 kg/hr, max vapor flow = reflux+distillate = 1890kg/hr
– Column dia = 0.6m, HETP = 0.6m, height = 8.4m, ∆P =
10.5 mm Hg, Packed Volume = 2.4 m3
� 2nd Column: min reflux ratio = 1, stages @ 2Rmin= 30
– Distillate = 595 kg/hr, max liquid flow = reflux + feed =
1810 kg/hr, max vapor flow = reflux+distillate = 1785kg/hr
– Column dia = 1.0m, HETP = 0.65m, height = 19m, ∆P =
30 mm Hg, Packed Volume = 15 m3
� Cost of columns = 2.4+15 = 17.4 m3 � Rs. 20 Lakhs
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Continuous Distillation Utility
� 1st Column: QC & QB = 1890 x 100 = 0.19Mkcal/hr,
– Acond = 0.19x106/(500x70)=5.4m2
– Areboiler=0.19x106/(300x30)=21m2
– CW = 19m3/hr, steam = 380 kg/hr
� 2nd Column: QC & QB = 1810 x 100 = 0.18 Mkcal/hr,
– Acond = 0.18x106/(500x70)=5.1m2
– Areboiler=0.18x106/(300x30)=20m2
– CW = 18m3/hr, steam = 360 kg/hr
� Costs of all exchangers = 52m2 � 52 Lakhs
� Total = 70 Lakhs, Savings over batch = 150 Lakhs
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Selection Considerations
� Scale of Operation
� Quality Considerations
� Process Variables
� Process Measurement and Control
� Equipment
� Recycle Streams
� Utilities
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Scale of Operation� Small scale of operation – batch
� Large scale of operation – continuous
� For the same production rate
– Batch plant – large inventory, big vessels, filling, heating,
discharge
– Continuous – smaller volumes to be handled all the time
� For hazardous chemicals – continuous plants preferred
� e.g. if Production rate = 10000 tpa, is this small/large ?
– Continuous plant flow rates 1200 kg/hr
– Batch plant vessel size (12 hr batch) = 20 KL
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Quality Considerations
� Batch – charge, heat, react, discharge – in series
� Continuous - charging, reacting, discharging, simultaneously
� Batch – Fill it, shut it, forget it. – what does it do to quality?
� Continuous – operates at steady state
� Batch – process conditions just right for every batch
� Continuous – maintain desired conditions all the time
� Quality control is much better in continuous plants
� Batch plants require close monitoring for ensuring quality
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Process Variables
� Batch – volume, composition change with time
� Physicochemical properties and handling conditions change
with time
� Continuous – compositions constant
� Handling is easier
� Minimize and correct for disturbances
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Process Measurements and Control
� Batch – low, control with time, more manual in nature
� Continuous – measurements and control against disturbances
need to be in place at all times
� This necessitates on line control
� Once the measurements and control systems are in place,
continuous processing is easier than batch
� Continuous – can not handle large disturbances
� Batch – can handle large disturbances
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Equipment
� Batch – Multifunctional equipment
� Batch plants can be run with different grades, campaigns
� Continuous – dedicated equipment needed for each step
since all steps happen simultaneously
� Each equipment can be much smaller in size
� Batch – design is not critical, batch time can be changed to
get desired production rate
� Continuous – Equipment design has to be perfect
� Batch – can have good turndown
� Continuous – tends to have only a narrow turndown
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Recycle Streams
� Batch – recycle is difficult, separate storage may be needed
for recycle streams, recycle streams tend to be inconsistent
(volume as well as composition) causes plant upsets
� Continuous – recycle stream is more consistent, much
easier to handle, no separate storage needed
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Utilities
� Batch – Utility load changes with time, cause disturbances
in utility plants, Utility plant has to be designed for maximum
load, can represent a lot of over design and high cost
� Heat integration is difficult
� Continuous – Utility load constant, utility plants can be
designed and operated with highest efficiency at the desired
point, no need for over design
� Heat integration is easy
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Information needed from Laboratory
� Eliminate all Mixing and mass transfer issues (high rpm,
results should be independent of rpm)
– Kinetics of reactions (rate constant, order)
– Heats of reaction
– Solubility
– Vapor-Liquid Equilibrium (vapor pressures)
– Distribution Coefficients
� Using the above information equipment can be designed for
batch and continuous plant, choose whichever is attractive
from Economics point of view
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Rules of thumb� Reaction (continuous process is preferred ifG)
– Any component is toxic, hazardous (e.g. CS2, H2S)
– Reaction is fast (reaction time is comparable to charge,
preheat, and drain time)
– Product consistency is important
– Recycle is needed
– Reactants are gaseous (gas – solid reactions)
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Rules of thumb
� Distillation (continuous process is preferred ifG)
– More than one cut is needed
– Vacuum operation is needed
– Boiling point differences are small
– High purity cut needed (impure reflux at start of cut)
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� Evaporation & Drying (continuous process is preferred ifG)
– Vacuum Operation is needed
– Degradation occurs at high temperatures
– Viscosity is high
– Steam economy is needed (Multiple effect)
� Crystallization (continuous process is preferred ifG)
– Precipitation type
– Nucleation is needed
– Crystal growth rates are fast
– Evaporative type
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Equipment for Continuous Processing
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� Gas – Liquid – Solid Contacting
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Catalyst
Particles
GAS IN
GAS
OUT
LIQUID OUT
LIQUID IN
TRICKLE BED
Packings
LIQUID
IN GAS OUT
GAS IN LIQUID OUT
PACKED COLUM�
Gas in Liquid out
FALLI�G FILM REACTOR
Gas out
Liquid in
Thin Liquid Film
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Conclusions
� Continuous processing offers advantages in terms of better
quality, lower costs, etc.
� Several types of equipment are available for various
operations
� For design of these equipment laboratory experiments may
be needed to identify fundamental process parameters,
physico-cochemical properties, kinetics etc.
� Any batch operation can potentially be replaced by
continuous operation
