Reactors for Liquid Liquids Systems Final

Institute Of Chemical Technology, Mumbai

Reactors for Liquid-Liquid Systems

Chemical Reaction Engineering

Submitted by: Ms. Bhumika Patil M. Tech., Green Technology


INDEX

Introduction 2

Reactors for Liquid-Liquid Systems 2

1. Batch Reactors with Two Liquid Phases 3

2. Continuous Reactors 4

a. Spray Columns 4

b. Sieve Plate Columns 5

c. Packed Columns 7

d. Rotating Disc Contactors 8

e. Continuous Stirred Tank Reactors 9

Comparison of Liquid-Liquid Continuous Reactors 10

[1]


Introduction

Chemical reactor is a heart of the chemical process plant. It is any device used to conduct a

chemical reaction, that is, one where molecular compounds are transformed into other

molecular compounds. Chemical reaction, heat transfer and mass transfer processes occur in

a reactor. The type of reactor depends upon the process conditions, capacity, conversion and

the quality of product.

In a reactor, chemical reaction occurs between different phases:

1. Single phase systems where reaction medium consists of either liquid or gas phase.

2. Multiphase or heterogeneous systems where reaction medium consists of different phases

as:

a. Gas + Liquid

b. Liquid + Liquid

c. Gas + Solid

d. Liquid + Solid

e. Gas + Liquid + Solid

f. Fluid + Solid

Reactors for Liquid-Liquid Systems:

Liquid-liquid reactors are extensively used for physical extraction with a

solvent, but these devices can also be used when two liquid phases reacts

chemically with one another. The design of liquid-liquid reactors is same

as gas-liquid reactors. One of the liquids may serve as the catalyst, or the

liquids may react with one another across the interface. In the latter case,

the product may be soluble in one of the liquids or precipitate out as a

solid.

[2]

Fig. 1. Batch reactor with two liquid phases.


Liquid-liquid systems require dispersion of one of the liquid phases to provide sufficient

interfacial area for mass transfer. This can be achieved by the use of static mixers, jets, or

mechanical means such as in a CSTR.

Broad classification of reactors for liquid-liquid systems on the basis of mode of operation

can be done as:

1. Batch Reactors: Stirred Tank Reactors/Mechanically Agitated Reactors

2. Continuous Reactors:

a) Spray Column

b) Sieve Plate Column

c) Packed Column

d) Rotating Disk Contactor

e) Continuous Stirred Tank Reactor (CSTR)

1. Batch Reactor with two liquid phases:

Batch reactor is the simplest type of reactor. Generally, batch reactors are closed systems.

These systems are used for the liquids which are usually not miscible. The transport of

reactants determines the specific reaction rate. In stirred batch reactors, the contact between

the reacting phases is maintained by the use of agitators.

Applications:

i. This type of reactor is useful for substrate

solutions of high viscosity and for

immobilized enzymes with relatively low

activity.

ii. They are suitable for small capacity

operations.

iii. These type of reactors are used to carry out

fermentation and in pharmaceuticals.

[3]


Advantages:

i. Batch reactors give high conversion per unit volume for one pass.

ii. They are easy to operate.

iii. They require less instrumentation.

iv. Product quality can be well controlled.

v. These reactors give flexibility of operation.

vi. Same reactor can be used to carry out different types of reactions without breaking the

containment.

Disadvantages:

i. The cost of operation for the batch reactors is high.

ii. It is difficult to maintain the same product quality in various batches.

2. Continuous Reactors:

a. Spray Column:

Spray column is simplest form of liquid-liquid reactors. In a continuous column, perforated

plates or series of nozzles are used to introduce counter-current dispersed phase in the

column. The dispersed drops flow through the continuous phase and are collected at the

bottom of the column. This results in the formation of a homogeneous layer which is

continuously removed from the column.

Depending on whether the dispersed phase is lighter or heavier than the continuous phase, it

is introduced either from the top or bottom of the column.

[4]


Hydrodynamics of spray columns:

In spray columns, two types of flow regions occur depending upon the properties and flow

rates of the two phases. These two flow regions have different holdup values as

Loose bed having a holdup value, ԐD < 0.20 with clearly separate and independent

drops.

Dense bed having a holdup value, ԐD > 0.40 where drops are often close or packed

and move as a group.

The holdup, interfacial area and mean drop diameter can be correlated using the equation:

A = 6 ԐD / dB

This is the standard equation to determine the interfacial area in a reactor.

Backmixing:

In a spray column, the backmixing of continuous phase occurs due to axial and transverse

movement of dispersed phase droplets, which entrain the continuous phase as they move

about.

Advantages:

i. Simple in construction

ii. Highly economical

[5]

Fig. 2. Spray Column.


iii. Relatively unaffected by the presence of media containing high concentrations of

solids.

Disadvantages:

i. Contact efficiency is not more than average.

ii. Sometimes, backmixing is significant.

iii. Spray interfacial area is much lower.

b. Sieve Plate Column:

There are two types of sieve plate columns:

The static column which contains no moving parts. The dispersed phase moves from

stage to stage through a series of sieve plates. The continuous phase crosses each

compartment horizontally and moves vertically from one stage to the next in

downward direction.

The pulsed column in which the fluids moves across the plates with a reciprocating

motion. In industrial reactors, the liquid phases are pulsed by means of a reciprocating

piston in the continuous phase.

[6]

Fig. 3. Sieve Plate Column (a)Static Column (b)Pulsed Plate Column


Dispersed phase holdup can be determind by

ug (1 - ԐD) = +

In case of static columns, above equation can be written as

ԐD (1 - ԐD) =

Where, VSD, VSC = superficial velocity of dispersed and continuous phases respectively.

ug = relative velocity or slip velocity.

Backmixing:

In static columns, backmixing is nearly non-existant as compared to pulsed columns. Also,

the compartments between the plates can be considered perfectly mixed for the continuous

phase in static columns.

Advantages:

i. High capacity

ii. Simplicity of operation

iii. Low cost of the process equipment.

iv. Backmixing effects are much weaker.

Disadvantages:

i. Lack of flexibility

ii. Limited number of theoretical stages

iii. Plugging can occur in the presence of solids

iv. Corrosion may occur.

c. Packed Columns:

In packed columns, packing material is used because of

which, backmixing effects are much lower than in other

[7]


types of liquid-liquid reactors. Depending upon the type of pacing material used in the

column, highly variable values for transfer coefficients, interfacial area and holdup can be

obtained. The general plate column is shown in fig. 4.

Wetting of the packing material by either continuous or

dispersed phase is most important. If the dispersed

phase wets the packing, it will flow as a film through

the packing. If continuous phase wets the packing, the

dispersed phase will flow as deformed drops.

For a packed column,

ug = +

where, ԐC, ԐD = volume fraction of reactor occupied by continuous and dispersed phase

respectively.

ԐB = volume fraction of reactor occupied by the packing material.

It can be written as, ԐB + ԐC + ԐD = 1

Advantages:

i. Packed columns are resistant to corrosion.

ii. These are inexpensive.

Disadvantages:

i. They have relatively low efficiency.

ii. There is high risk of plugging in presence of solids.

d. Rotating Disc Contactor:

[8]

Fig. 4. Packed Column


A rotating disc contactor consists of a column divided into compartments partially enclosed

by annular discs. A solid disc rotates in the center of each compartment. The construction and

installation of rotating disc contactors is relatively simple as the diameter of discs is smaller

than the openings in the stator.

Holdup in rotating disc contactors can be correlated by

following equation:

ug (1 - ԐD) = +

The backmixing is very significant in rotating disc

contactors. The construction of these reactors requires

large compartment heights. Also, the openings between

compartments are such that circulation between them is

inevitable.

Advantages:

i. Relatively unaffected by the presence of solids

ii. Highly flexible in operation.

Disadvantages:

i. The major disadvantage of rotating disc contactor is its significant backmixing.

ii. Energy requirement is more, thereby increase in operating costs.

iii. Scale-up of the columns is difficult.

e. Continuous Stirred Tank Reactors (CSTR):

Continuous stirred tank reactors, CSTRs are also

known as mixed flow reactors. In CSTR, unlike

batch reactors, one of the two liquid phases is made

[9]

Fig. 5. Rotating Disc Contactor


continuous. In case of liquid-liquid continuous stirred tank reactor systems, a separator is

needed to separate the two liquid phases.

Advantages:

i. CSTRs are more efficient as compared to STRs.

ii. Easy control of temperature.

iii. Low operating cost

Disadvantages:

i. Equipment is slightly more complicated.

ii. They give lowest conversion per unit

volume.

iii. By-passing and channelling is possible

because of poor agitation.

Comparison of Liquid-Liquid Continuous Reactors:

Sr. Type of Reactor

Maximum Flow

m3/(h.m2)

H, height maximum

(m)

Cost of Reactor Corrosion Viscous Products

1 Spray Column

20 - Low No No

2 Sieve plate column

70 12 Average Average No

3 Packed 30-100 15 Dependent of Suitable No

[10]

Fig. 6. Continuous Stirred Tank Reactor


Column packing mateerial used

4 RDC 35 15 Average Suitable No

[11]

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Reactors for Liquid Liquids Systems Final