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introducing reactor design clearly
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Chapter 5
REACTOR CONFIGURATION
Reactor operation in order of precedence
Adiabatic operation
(simplest and cheapest)
Cold shot & Hot shot
(direct contact heat transfer, control reaction rate)
Indirect heat transfer
(for heat integration / heat recovery)
Heat carrier (inert / feed excess / product recycle)
(for change of heat capacity of mixture)
Catalyst profiles
• Very rapid reaction, must be stopped quickly
• Very hot / cold / corrosive products
• Fouling problem in conventional exchanger
Steps inheterogeneous reactions
External diffusion
Internal diffusionAdsorption / Desorption
Chemical reaction
Contacting patterns for gas–liquid reactors
Plug–flow models
(a) Countercurrent packed bed or plate column
Packing / trays (heterogeneoussolid catalyst) for interfacialarea
(b) Cocurrent packed bed (trickle–bed reactor)
For case with gas flowgreater than liquid flow,liquid in films form
(c) In–line static mixer
For case which requiresa short residence time
In–line static mixer structure
In–line static mixer structure
Plug–flow & Mixed–flow models
(d) Spray column
Mixed–flow in gas phase, plug–flow inliquid phase
For case with mass transfer resistancein liquid phase (solid contain or solidformation in the reaction)
(e) Bubble column
Mixed–flow in liquid phase, plug–flow ingas phase
For case with mass transfer resistance ingas phase (solid contain in liquid),disadvantage in high viscous liquid
Mixed–flow models
(f) Agitated tanks
For case with high viscous liquid
Contacting patterns for liquid–liquid reactors
Plug–flow models
(a) Countercurrent packed bed or plate column
Packing / trays (heterogeneoussolid catalyst) for interfacialarea
(b) Multi–stage agitated contactor
A large number of stageswith agitator
(c) In–line static mixer
For case which requiresa short residence time
Plug–flow & Mixed–flow models
(d) Spray column with heavy liquid dispersed
Mixed–flow in light liquid phase, plug–flow in heavy liquid phase
(e) Bubble column with light liquid dispersed
Mixed–flow in heavy liquid phase, plug–flow in light liquid phase
Mixed–flow models
(f) Agitated tank with settler
The more stages that tends tocountercurrent plug–flow
Mass transfer driving force
𝒂 > 𝒃 > 𝒄 > 𝒅 > 𝒆 > 𝒇
Reactor configurations
Tubular reactors
• Suitable to high temperature and high pressure
• Combination to static mixers for good mixing
• Construction is similar to heat exchanger
• Well control of residence time
Stirred–tank reactors
Stirred–tank reactors
• Suitable to both homogeneous and heterogeneous, low
pressure, not high viscous liquid and non–hazardous
chemical
• Suitable to batch or semi–batch operation because of
flexibility
• Combination to jacket, internal coil and flow loop for heat
exchanger
• Better temperature control in case of large volume reactor
Fixed–bed reactors
Fixed–bed reactors
Fixed–bed reactors
• Most designs approximate to plug–flow behavior
• The packed bed can be catalyst, solid reactant or servesonly to good contact phase between gas and liquid
• Combination to cold / hot shot, intermediate cooling /heating, or shell & tube heat exchangers
• Difficult for temperature control
• More popular design is that uses a parallel reactorssystem for catalyst regeneration
Moving–bed reactors
• In case the rate ofdegradation in a fixed bedmight be unacceptable
• Possible to remove thecatalyst continuously forregeneration
Fluidized–bed reactors
Fluidized–bed reactors
• Catalyst or solid reactant in form of fine particles is heldin suspension for good heat transfer and temperatureuniformity, limitation of hot spot formation
• The solid phase tends to be in mixed flow, but the gasphase behaves more like plug flow. Overall performancelies somewhere between mixed flow and plug flow
• Advantage for removing / adding heat to reactorthrough the cycle of catalyst
• One disadvantage is that attrition of catalyst, then lostfrom the system
Kilns
Kilns
Kilns
• For case reactions involving free–flow solid, paste andslurry materials
• The behavior of the reactor usually approximates plug–flow
• High temperature reactions demand refractory linedsteel shell and heated by direct firing
Reactor selection for heterogeneous reactions
The order of precedence
Fixed–bed adiabatic
Fixed–bed with Cold shot & Hot shot
Tubular with indirect heating / cooling
Moving bed
Fluidized bed
Catalyst deactivation
Fixed–bedadiabaticfor veryslow DCC(years)
Fixed–bedwith Coldshot & Hotshot for veryslow DCC(years)
Tubular withindirect heating/ cooling forvery slow DCC(years)
Moving bed forslower DCC(hours, days)
Fluidized bedfor very shortDCC (seconds)
Capital cost
Fixed–bedadiabatic
Fixed–bedwith Coldshot & Hotshot
Tubular withindirectheating /cooling
Moving bed
Fluidizedbed
Trade–offs in reactor selection
Reactor configuration
Reactor size
Catalyst deactivation
Selectivity
Interaction with the rest
process