Chemical Reactors

Chemical Reactors blog contains following aspects 

• Chemical Reactors Introduction
• Factors influencing the choice among the different reactor types
• Typical reactors used in environmental application
• Suspended-Growth Reactors
• 1. Batch reactors
• 2. Sequencing Batch Reactor (SBR)
• Continuous-flow stirred-tank reactor(CSTR)
• Comparison of CSTR and PFR
• Practical Aspects of Reactor Design

Chemical Reactors Introduction

1. Many different types exist for environmental engineering

2. Generally designed to emphasize suspended growth or biofilms

3. That make use of suspended growth are also called:

• Suspended-floc
• Dispersed-growth
• Slurry reactors

4. That make use of biofilms are also called:

• Fixed-film
• Attached-growth
• Immobilized reactors

Engineer must understand 

1. Kinetics of substrate removal by different types of microorganisms 
2. Fundamental properties of different reactor types

Factors influencing the choice among the different reactor types : 

• Physical & chemical characteristics of the waste being considered 
• Concentration of contaminants being treated 
• Presence or absence of oxygen 
• Efficiency of treatment and system reliability required 
• Climatic conditions under which the reactor will operate 
• Number of different biological processes involved in the overall treatment system 
• Skills & experience of those who will operate the system 
• Relative costs at a given location and time for construction and operation of different possible reactor configurations

The aim of this chapter :

• How to construct mass balances for different reactors
• How to use of mass balances to derive basic equations that describe the relationship between reactor size and treatment performance.

Typical reactors used in environmental application

Basic reactors

Biofilm reactors

Chemical Reactors Types

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Suspended-Growth Reactors

A. Batch reactors: 

• The simplest suspended-growth reactor 
• Biochemical reactions take place without new additions until the reaction is complete 
• Commonly used in laboratory-scale 
• Kinetics of contaminant removal is similar to that of an ideal plug-flow reactor

B Sequencing Batch Reactor (SBR)

• Cyclic operation in a single reactor : 

1) Fill, 

2) React (aerobic/anoxic or anoxic/aerobic), 

3) Settle, 

4) Draw, 

5) Idle

• SBR can also employ several batch reactors operated in parallel

Advantages of SBR: 

1) Total capital costs are significantly reduced due to the elimination of clarifiers and recirculation facilities. 

2) Operating flexibility is greatly increased, since the cycle format can be easily modified at any time to offset i)change in process conditions, ii)influent characteristics or iii)effluent objectives. 

3) Process reliability is greatly improved because the SBR process is not affected by hourly, daily, or seasonal feed variations. 

4) Since only one vessel is used for all proces operations, plant extension is simplified. 

5) Better resistance to sludge bulking, since the biomass undergoes cyclic feast-famine conditions, which have been proven to produce better settling sludge than continuous flow.

Continuous-flow stirred-tank reactor(CSTR), or completely mixed reactor :

Used to culture organisms or to study basic biochemical phenomena in

laboratory (chemostat)

Liquid or slurry stream is continuously introduced, and liquid contents are

continuously removed from the reactor

Concentration of substrates and microorganisms are the same everywhere

throughout the reactor (Ideal CSTR) ; it makes analysis of CSTR comparatively simple.

Sometimes referred to tubular reactor or piston-flow reactor.

In the ideal PFR,the flow moves through the reactor with no mixing with

earlier or later entering flows.

Hence if one knows the flow rate to the reactor and its size,

the location of the element at any time can be calculated.

Unlike the CSTR, the concentartions of substrates and microorganisms vary

throughout the reactor.

An ideal PFR is difficult to realize in practice, because mixing in the direction

of flow is impossible to prevent .

Comparison of CSTR and PFR

1) The high rate of substrate utilization at the entrance of reactor in PFR because the substrate concentrations are highest at the entrance. 

If other conditions are the same, a higher S gives a higher rate of reaction. So a PFR generally produces a higher conversion of S in a given volume than a CSTR. (advantage of PFR). 

It exceeds the ability to supply sufficient oxygen (high DO demand at the entrance and low DO demand at the exit) in an aerobic system. Thus the aerators for PFR should be designed to provide more oxygen in the inlet region. (disadvantage of PFR) 

It results in excess organic acid production and pH problems , e.g., destruction of methanogens at low pH in an anaerobic system. (disadvantage of PFR)

2) In CSTR, the S in the reactor is the same as S in the effluent.

So the fresh feed is immediately dispersed into an environment of low S.

In PFR, the S decreases along the length of reactor.

If no biomass enters PFR, no biological reaction would occur and the reactor

washes out.

On the other hand, the influent to a CSTR is mixed with reactor fluid

containing biomass so that a CSTR can be sustained even in the absence

of biomass in the feed.

Processes for in situ biodegradation of contaminants in ground waters often

operate similar to PFR.

Here, mixing in the direction of flow (longitudinal) is generally small,

making plug flow the natural outcome.

 3) The CSTR is more stable than a PFR in response to toxic and shock loadings. 

If a concentrated pulse of a toxic substance enters a PFR, the concentration remains high as it moves along the PFR. 

Because of high concentration, the toxic substance may destroy an appreciable quantity of the biomass in the system and cause a long term upset in PFR performance. 

With a CSTR, the pulse of toxin is dispersed rapidly throughout the CSTR and its concentration level is reduced so that the metabolic processes of microorganisms may be only slightly affected by the diluted toxin. 

In general, a CSTR gives a more uniform effluent under varying feed conditions.

4) The CSTR and PFR are idealized models that are difficult to achieve in large scale biological reactors.

n actual CSTR, short-circuiting of fluid and stagnant zones may occur because of incomplete mixing with the bulk of the reactor fluid.

In PFR, aeration of the fluid causes longitudinal mixing and a distribution of residence times. Thus, long biological reactors with aeration are often better simulated by an axial dispersion model or a CSTR in series model.

Tracer techniques are useful in establishing an appropriate hydraulic model for a biological reactor.

Practical Aspects of Reactor Design

•The deviation from two idealized flow patterns : 

1) Dead Zone (Stagnant zone) 

2) Channeling of fluid 

3) Short-circuiting caused by i) density current in plug-flow reactor ii) inadequate mixing in a CSTR

• This type of flow should be avoided since it always lowers the performance of the unit.

• The problems of non-ideal flow are intimately tied to those of scale-up.

• Often the uncontrolled factor differs widely between large and small units. Therefore ignoring this factor may lead to gross errors in design.

Reactor arrangements

1) Reactors in series : 

When different types of treatment are needed. 

For example, 

Organic oxidation (1st reactor) 

Nitrification (2nd reactor)

Denitrification (3rd reactor)

To create plug-flow characteristics. 

2) Reactors in parallel : 

To provide redundancy in the system so that some reactors can be out of service, while others on a parallel track remain in operation.

When the total flow to be treated far exceeds the capacity of the largest practical units available. 

It maintains more of a completely mixed nature, compared to the more plug.  Flow nature of reactors in series.

Mass Balances

Reactor design

1) Mass balance is the key to design and analysis of microbiological processes. 

It provides the critical information on what must be added to and removed from the process.

It makes determine the amount of chemicals to satisfy the energy, nutrient, and environmental needs of the microorganisms.

For example: process for biological denitrification

0.02mole bacteria represents and is called ; sludge, waste biomass, waste biosolids, excess biosolids

2) System boundary ; a control volume

Figure - Three possible control volume

A component may enter /or leave the control volume

3. Reaction rates affect the size of the treatment system.

The mass balance is defined in terms of rates of mass change in the control volume.

Each component must have its own mass balance components : COD, TOC, biomass, oxygen, electron acceptor, nitrate, ammonium etc.

In the development of equations useful for a reactor system, mass balances on several different components of interest and around several different control volumes sometimes are required.

Rate of mass accumulation in control volume = rate(s) of mass in - rate(s) of mass out + rate(s) of mass generation

Accumulation : (total mass of the component) or

( the reactor volume x the concentration ; d(VC)/dt)

Mass in / out : mass crossed the control-volume boundaries

Generation 

Formation of the component of interest within the control volume

Negative – component destroyed rather than being formed – endogenous respiration or predation 

Positive – bacteria cells produced through consumption

A Batch Reactor

• A batch reactor operated with mixing

Only component concentrations changing with time. The rate of mass accumulation = Vdc/dt

Constant reactor liquid volume with time

The control volume consists of the entire reactor

Uniformly distribution of components throughout the reactor

• Selection of the components

- Bacteria 

- Limiting substrate 

– The electron donor

Assumption

- Sufficiently high concentration of all other bacterial requirements

such as electron acceptor and nutrients

- At time=0, microorganisms’ concentration in the reactor = X0 (mg/l), rate limiting substrate in the reactor = S0 (mg/l)

Typical reactors used in environmental application

Mass balance for substrate

Rate of mass accumulation in control volume =

rate of mass in - rate of mass out + rate(s) of mass generation

: mass of substrate accumulating = mass of substrate generated <-- if no substrate added or removed

If rate of substrate utilization follows Monod kinetics

Mass balance for microorganisms

Rate of mass accumulation in control volume = rate of mass in - rate of mass out + rate(s) of mass generation

If the organism growth rate follows Monod kinetics,

Initial conditions

Remark: 

1) Interdependence between Xa and S, both of which vary with time 

2) In order to solve for Xa and S as functions of time, eq 5.4, 5.7and 5.8 should be considered simultaneously. 

3) Due to the nonlinear Monod forms, the systems of eq 5.4, 5.7and 5.8 cannot be solved analytically. It must be done with a numerical solution. 

4) if organism decay is considered to be negligible (b =0 in eq 5.7), an analytical solution can be obtained. This is reasonable for cases of batch growth where organism decay is small while they are growing rapidly.

Assumption : organism decay is negligible while the microorganisms are growing rapidly

Initial conditions

When t is known, S can be solved by eq 5.11. 

And also Xa can be solved by eq 5.9

affecting bacterial growth and the substrate concentration

- The higher the intial concentration of biomass, the lower the substrate utilization time.

- For the lowest initial organism con., a lag period occurs before the onset of significant substrate utilization.

- The increase of biomass between t=0 and t = t at S=0 is the same in all cases

A chemical reactor is a process equipment where in chemicals are fed in order to make them chemically react with each other for the purpose of making a desired product. 

Chemical reactors are designed in such a way to increase the net present value for a given reaction and it is done by ensuring highest efficiency to output the desired product.

Types of Reactors

1. Batch Reactor
2. Continuous Stirred Tank Reactor (C.S.T.R)
3. Plug Flow Reactor (P.F.R)
4. Semi-Batch Reactor

1. Batch Reactor

A batch reactor is a closed vessel in which reactions happen and it is a non-continuous type of reactor. The reactants are fed in to the reactor all at once initially. 

The vessel contains an agitator. The purpose of the agitator is to mix the reactants thoroughly so that the contact makes them react together efficiently and produce products.

In order to handle exothermic reactions the batch reactor is often equipped with cooling coils. In order to work with endothermic reactions the batch reactor has provisions for heating the reaction mixture.

The batch reactor is a non-steady, transient reactor. It means the extent of conversion within the reactor depends on time. Due to agitator the batch reactor is highly uniform in nature. 

It means the extent of conversion does not depend on location within the reactor. At a given time the extent of reaction at any location of the volume of the reactor will be equal to each other.

Advantage

The greatest advantage of operating a batch reactor is its versatility. 

Same batch reactor can be used to chemically react quite different variety of reactants. 

Batch reactors are especially used in cases where the reaction produces lots of products. 

Batch reactors are often used in labs to study kinetics of the liquid phase reaction systems.

Disadvantage

The disadvantage of batch reactor is that it requires lots of labour force to constantly charge reactants, discharge products and then to clean the reactor for the next batch.

2. Continuous Stirred Tank Reactor (C.S.T.R)

A continuous stirred tank reactor (C.S.T.R) is also often called a mixed flow reactor (M.F.R). In this reactor also the reaction occurs in a closed tank. 

The tank also has agitator in order to mix the reactants thoroughly. 

It is different from batch reactor in the sense that the name itself indicates it is continuous type of equipment.

The reactants enter the reactor at a certain mass flow rate, the react inside the vessel for sometime dictated by the space time of the reactor and then they form products. 

The products flow out of the reactor at the same mass flow rate. One space time is the time required to process one reactor volume.

The C.S.T.R is steady sate equipment. It means the extent of conversion does not depend on the time. The agitator makes the concentration uniform throughout the reactor. 

It means the extent of conversion does not depend on the location also. The extent of conversion depends on the volume of the reactor.

Advantage

The biggest advantage of using a C.S.T.R in industries is that it can produce a large amount of products and being a continuous steady state reactor the reactor can keep on operating hours on end.

Disadvantage

The disadvantage is that a C.S.T.R cannot be used for reactions which have very slow kinetics because it will require a reactor of very large volume. 

The fabrication and operational cost of the reactor may make it infeasible. Batch reactor is used in this case.

3. Plug Flow Reactor (P.F.R)

A plug flow reactor (P.F.R) is also sometime called a continuous tubular reactor (C.T.R). 

In an idealised model, the profile of the reaction mixture can be considered to be made up of a number of plugs and each plug having a uniform concentration.

The idealized P.F.R model has an assumption that there is no axial mixing. It means that there is no back mixing inside the reactor.

Advantage

The advantage of P.F.R over C.S.T.R is that for same space time and same level of conversion, the volume of the P.F.R is relatively smaller than a C.S.T.R, 

It means a smaller space is needed for the reactor also for same volume of reactor the level of conversion is higher in P.F.R than in C.S.T.R. Often the P.F.R are used to study kinetics of gas phase catalytic reactions.

Disadvantage

The disadvantage is that if we carry out an exothermic reaction in a P.F.R then the temperature gradients are difficult to control. The operational and maintenance cost of a P.F.R are also greater than a C.S.T.R.

4. Semi-Batch Reactor

A semi-batch reactor is a semi-flow reactor. It is a modification of batch reactor. 

It is also a closed vessel which contains agitator for the purpose of mixing the reactants thoroughly. 

The difference is that one of the reactants is charged completely initially in the reactor and the other reactant is charged continuously in the reactor as the time progresses.

Advantage

The advantage of using a semi-batch reactor is that if we are carrying out multiple reactions then we have a greater control over yield or selectivity of the products. 

This reactor is extremely useful when we are carrying out an exothermic reaction as the continuous flow of the other reactant can be varied to better control the exothermic reaction.

Disadvantage

If we want to scale up the semi-batch process then disadvantage over the continuous process reactors (C.S.T.R and P.F.R) is that capital costs per unit scales up relatively a lot. 

Greater man power is required to charge and discharge the contents of the reactor, to clean blades, to clean reactors etc.

An agitator is a device which is used to import motion in the form of stirring to the liquids or semi solids. 

The agitator is a device which contains a shaft and an impeller/propeller. 

The shaft is connected to the gear box and the assembly is driven by motors using electricity.

The purpose of agitators can be for mixing liquids, solids, slurries, pastes, mixing liquids and gases, mixing solids and gases, promote chemical reactions, promote heat transfer, etc. 

An agitator is installed in the vessel to ensure that the contents in the vessel become uniform and homogeneous and remain in a proper mixed state. 

Since they have such wide variety of uses, the agitators are used in a multitude of industries such as chemical, process, food, pharmaceuticals, cosmetics, metal extractions, ink, paint etc.

Agitators are used in a variety of industrial processes such as for carrying out a chemical reaction, for mixing operations, for filtration, for drying, for heat exchange, etc. 

The agitators are able to cater to such wide variety of applications because there are a variety of impellers which come in different shapes and sizes.

Types of Agitators

1. Turbine Agitators
2. Paddle Agitators
3. Anchor Agitators
4. Propeller Agitators
5. Helical Agitators

1. Turbine Agitators

These agitators have an axial input and the output is radial. 

Turbine agitators are very versatile; they are able to handle a wide variety of mixing operations because these agitators can create turbulent movement of the fluid due to the combination of rotational as well as centrifugal motion. 

They are popularly used in metal extraction industries and also for chemical reactions and have a very high blending efficiency. These are mostly used in chemical, pharmaceuticals, grease, cosmetics industries.

2. Paddle Agitators

It is one of the most basic types of agitator. It contains blades which are paddle shaped; the blades stretch throughout the vessel and reach the walls of the vessel. 

They are primarily used in applications where a uniform laminar flow of the liquids and little shearing is desired. This type of agitator is especially used for viscous materials. 

These are mostly used in food, chemicals, pharmaceuticals, sorbitol etc industries

3. Anchor Agitators

As the name indicates the shape of the impeller resembles that of the anchor. 

These agitators also extend and spread throughout the vessel such that there is very less clearance between the blades and the walls of the vessel. 

These agitators are also used when laminar flow conditions are desirable. These impellers sweep the whole batch because the blades are almost in physical contact with the walls of the vessels. 

These agitators are used in ink, paint and adhesive industries.

4. Propeller Agitators

These have impellers that similar to marine type propellers. They have blades which taper towards the shaft to minimize centrifugal force and to promote axial flow. 

It means when this agitator operates the motion of fluid is such that the inlet flow is parallel to the shaft and the outlet flow is also parallel to the shaft, the ideal flow is axial in nature. 

They are primarily used in applications to stir low viscosity liquids. They are used in pharmaceuticals industries and also other industries which use suspensions as the agitators don’t let the solid particles settle.

5. Helical Agitators

The blades of the helical agitators are arranged in a structure of helix. 

The appearance is similar to how a threaded screw looks. The motion of the liquids in this type of agitator is also axial in nature due to the way the blades or the ribbons move while helical agitator is in operation. 

There is a vigorous motion of the fluids within the vessel when the agitator is in operation. It is used in polymer industries and other industries which require the use of quite viscous materials.

Different types of jackets used for reaction vessels are

1. Plain jacket. 
2. Half pipe coil jacket. 
3. Dimple jacket.
4. Plain jacket

It can be termed as an extra covering all around the vessel or on some part of the vessel. 

The annular space between the vessel wall and jacket wall is used for circulation of heating or cooling medium. 

Plain jackets are suitable for small capacity vessels and for operations where pressure inside the vessel is more than twice the jacket pressure. 

1. Plain jacket

It can be termed as an extra covering all around the vessel or on some part of the vessel. 

The annular space between the vessel wall and jacket wall is used for circulation of heating or cooling medium. 

Plain jackets are suitable for small capacity vessels and for operations where pressure inside the vessel is more than twice the jacket pressure.

It is most suitable where heating is to be done with steam. Jacket height is usually up to the liquid height in the vessel. 

Jacket can be fabricated in one piece or can be divided into number of parts and all parts operating in parallel. For higher flow sentimental jacket is preferred when downstream vapors are used as heating medium, plain jacket is preferred.

Spiral baffling can be provided in the jacket.. 

It helps to induce turbulence and increase the heat transfer coefficient. A vessel with plain jacket and spiral baffles.

2. Half pipe coil jacket

Pipe is cut into two pieces and half pipe coil is welded to the vessel wall. 

It helps to provide high velocity and high turbulence. 

It also helps to provide strength to the vessel wall and thereby reduce the cost of vessel. It provides structural rigidity which is an advantage for high temperature operation. 

To have flexibility and high efficiency the half coil jacket can be divided into multiple zones.

Half coil jacket is usually made from carbon steel. Stainless steel, monel. Inconel and other alloys can also be used for the fabrication of half coil jacket. Shows the half coil jacket welded to shell.

If jacket pressure is the controlling parameter in estimating vessel wall thickness, then half coil jacket is preferred. 

These are used for high capacity vessels and where high velocities for circulating hot oils, glycols etc. is required. In these jackets by passing, short circulation is completely avoide

3. Dimple jack

It can be fabricated by using thin sheets. 

It is useful for high jacket pressure operation. 

Dimple jacket can induce turbulence even at very low flow velocity. 

It can be used for circulating steam and hot Oil.etc.

A vessel can be provided either a jacket on limpet coil for heat transfer. Which alternative should be selected depends upon the process requirements, ease of operation and the cost involved. 

Jacketed vessel ensures more heat transfer as compared to limpet coil but the heat transfer coefficient is less in jacket than in limpet coil. Pressure drop is less in jacket than in limpet coil.

Channeling of heating or cooling medium is not possible in jacket, unless spiral baffles are provided. Pumping cost is less for jacket as compared to limpet coil for the same operating pressure jacket requires more thickness than limpet coil.

Construction of jacket is easy as compared to limpet coil because in limpet coil construction lot of cutting, bending, welding is involved. Therefore jacket construction is cheaper than limpet coils.

If the heat transfer areas for jacket and limpet coils are compared it can be seen than jacket gives more surface area as compared to limpet coils.

A vessel can be provided with a jacket or a limpet coil for heat transfer in the vessel. The selection of any of the two depends upon the requirements of the process, ease of operation and the cost involved.