5 - Reactor Design

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