ASSIGNMENT ADVANCED CHEMICAL

REACTION

FLUIDIZED BED BIOREACTOR

NAME : ABDUL FARIS FIRDAUS BIN ABDUL RAZZAK (2014214574)

: AHMAD MUSLIM BIN NOOR AZMI (2014825712)

1 Introduction

            

Biotechnology is the culmination of more than 8000 years of human experience using living

organisms and the process of fermentation to make products such as bread, cheese, beer and

wine.  Today biotechnology is applied to manufacturing processes used in health care, food and

agriculture, industrial and environmental cleanup, among other applications. The word

biotechnology, contraction of biological technology, came in general use in mid 1970's.  Over

the year’s at least 10 different definitions of biotechnology has been proposed. A widely

accepted definition of Biotechnology is "Application of scientific and engineering principles to

processing of materials by biological agents to provide goods and service".  Some other

definitions replace rather ambiguous word ‘biological agents’ with more specific words such as

microorganisms, cells, plant and animal cells and enzymes. Biotechnology is truly an

interdisciplinary field with contributions from basic life science disciplines such as molecular

and cell biology, biochemistry, genetics and engineering such as chemical, instrumentation and

control. In this section, a brief summary of biotechnological applications, modeling preliminaries

and different types of bioreactors is presented. When a biotechnological process is implemented

on a commercial scale there is every reason to believe that it will the in some bioreactor or

fermenter. The entire process can be divided in three stages.

2. Bioreactors

At every step of the development of a biotechnological process, bioreactor is invariably used.

The sizes of the bioreactor can vary over several orders of magnitudes. The microbial cell (few

mm3), shake flask ( 100-1000 ml), laboratory fermenter ( 1 – 50 L), pilot scale (0.3 – 10 m3) to

plant scale ( 2 – 500 m3) are all examples of  bioreactors. Whatever may be the size of the

bioreactor, the conditions in the bioreactor have to be favorable so  that living microorganisms

can exhibit their activity (specific biochemical and microbial reactions) under defined conditions.

This results in a series of special features in the reaction engineering of biocatalytic processes. 

The reaction rate, cell growth, and process stability depend on the  environmental conditions in

the bioreactor. There are several unique aspects of biotechnological processes, which require

special consideration in design of bioreactors.

2.1 Unique aspects of biological processes

(a)    The concentrations of starting materials (substrates) and products in the reaction mixture

are frequently low; both the substrates and the products may inhibit the process. Cell growth, the

structure of intracellular enzymes, and product formation depend on the nutritional needs of the

cell (salts, oxygen) and on the maintenance of optimum biological conditions (temperature, 

concentration of reactants, and pH) with in narrow limits. 

(b)   Certain substances inhibitors effectors, precursors, metabolic products influence the rate and

the mechanism of the reactions and intracellular regulation. 

(c)    Microorganisms can metabolize unconventional or even contaminated raw materials

(cellulose, molasses, mineral oil, starch, ores, wastewater, exhaust air, biogenic waste), a process

which is frequently carried out in highly viscous, non-Newtonian media. 

(d)   In contrast to isolated enzymes or chemical catalysts, microorganisms adapt the structure

and activity of their enzymes to the process conditions, whereby selectivity and productivity can

change. Mutations of the microorganisms can occur under sub optimal biological conditions. 

(e)    Microorganisms are frequently sensitive to strong shear stress and to thermal and chemical

influences. 

(f)     Reactions generally occur in gas-liquid -solid systems, the liquid phase usually being

aqueous. 

(g)    The microbial mass can increase as biochemical conversion progresses. Effects such as

growth on the walls, flocculation, or autolysis of microorganisms can occur during the reaction. 

(h)    Continuous bioreactors often exhibit complicated dynamic behavior.

2.2 Requirements of bioreactors

         Due to above mentioned demands made by biological systems on their environment, there

is no universal bioreactor. However, the general requirements of the bioreactor are as follows:

(a)    The design and construction of biochemical reactors must preclude foreign contamination

(sterility). Furthermore, monoseptic conditions should be maintained during the fermentation and

ensure containment. 

(b)   Optimal mixing with low, uniform shear

(c)    Adequate mass transfer (oxygen)

(d)   Clearly defined flow conditions

(e)    Feeding of substrate with prevention of under or overdosing

(f)     Suspension of solids

(g)    Gentle heat transfer

(h)    Compliance with design requirements such as: ability to be sterilized; simple construction;

simple measuring, control, regulating techniques; scaleup; flexibility; long term stability;

compatibility with up- downstream processes; antifoaming measures.

2.3 Types of bioreactors

 Bioreactors can be classified according to various different criteria

(a)    Type and form of biocatalyst: free cells in submerged cultures; carried bound or

immobilized cells/enzymes; retention or recirculation of the biocatalyst

(b)   Configuration: tank (height/diameter <3), column (height/diameter > 3)

(c)    Energy input and aeration: liquid phase; gas phase; combined

(d)   Hydrodynamics: perfect mixing; partial mixing; no mixing;

(e)    Mode of operation: batch; continuous; fed-batch.

Mode Of Operations

Bioreactors can be operated in three ways:

Batch reactors are simplest type of mode of reactor operation. In this mode, the reactor is filled

with medium and the fermentation is allowed to proceed. When the fermentation has finished the

contents are emptied for downstream  processing. The reactor is then cleaned, re-filled, re-

inoculated and the fermentation process starts again.

Continuous reactors: fresh media is continuously added and bioreactor fluid is continuously

removed. As a result, cells continuously receive fresh medium and products and waste products

and cells are continuously removed for processing. The reactor can thus be operated for long

periods of time without having to be shut down. Continuous reactors can be many times more

productive than batch reactors. This is partly due to the fact that the reactor does not have to be

shut down as regularly and also due to the fact that the growth rate of the bacteria in the reactor

can be more easily controlled and optimized. In addition, cells can also be immobilized in

continuous reactors, to prevent their removal and thus further increase the productivity of these

reactors.

The fed batch reactor is the most common type of reactor used in industry. In this reactor, fresh

media is continuous or sometimes periodically added to the bioreactor but unlike a continuous

reactor, there is no continuous removal. The fermenter is emptied or partially emptied when

reactor is full or fermentation is finished. As with the continuous reactor, it is possible to achieve

high productivities due to the fact that the growth rate of the cells can be optimized by

controlling the flow rate of the feed entering the reactor.

Comparison of Batch Culture and Continuous Cultivation

In batch cultivation, the bacteria are inoculated into the bioreactor (always stirred tank

bioreactor). Then, under certain conditions (temperature, pH, aeration, etc.) the bacteria go

through all the growth phases (lag, exponential, stationary). At last, the fermentation is stopped

and  the product is collected. Then, after cleaning and sterilization of the fermenter,  the

fermenter is ready for another batch.

In continuous cultivation, the fresh medium flows into the fermentor continuously, and part of

the medium in the reactor is withdrawn from the fermenter at the same flow rate of the inlet

flow. The table below shows the advantages and disadvantages of different modes of operation

of the stirred tank rector. (Ref 1.) 

 

Mode of operation Advantages Disadvantages

Batch Versatile: can be used for

different reactions every day. 

Safe: can be properly

sterilized. 

         Little risk of infection

or strain mutation 

Complete conversion of

substrate is possible

High labor cost: skilled labor

is required 

Much idle time: Sterilization,

growth of inoculum, cleaning

after the fermentation 

Safety problems: when

filling, emptying, cleaning

Continuous Works all the time: low labor

cost, good utilization of

reactor 

Often disappointing:

promised continuous

production for months fails

Often efficient: due to the

autocatalytic nature of

microbial   reactions, the

productivity can be high. 

Automation may be very

appealing 

Constant product quality

due to a. infection. b.

spontaneous mutation of

microorganisms to non

producing strain 

Inflexible: can rarely be used

for other productions without

substantial retrofitting

From the above comparison, although continuous culture has some disadvantage, it can

outperform batch culture by eliminating the inherent down time for cleaning and sterilization and

the long lags before the organisms enter a brief period of high productivity.

Continuous culture is superior to batch culture in several ways for research. Interpretation of

results is difficult for batch culture because of changing concentrations of products and reactants,

varying pH and redox potential, and a complicated mix of growing, dying, and dead cells.

History of Fluidized Bed Reactor

-1 Gasifying coal in a fluidized bed, using oxygen. Not commercially successful (1920)

-2 Catalytic cracking (FCC) process (1940) Converted heavier petroleum cuts into

gasoline.Carbon-rich ”coke" deposits on the catalyst particles and deactivates the catalyst in less

than one second. The fluidized catalyst particles are shuttled between the fluidized bed reactor

and a fluidized bed burner where the coke deposits are burned off, generating heat for the

endothermic cracking reaction.

3. Mineral and metallurgical processes such as drying, calcining, and sulfide roasting (1950s)

4. Fluidized bed processes dramatically reduced the cost of some important monomers (1960s)

Example :Oxy chlorination process for vinyl chloride.

5. Fluidized bed used for synthesis of polyethylene reduced the cost of this important polymer.

The polymerization reaction generates heat and the intense mixing associated with fluidization

prevents hot spots where the polyethylene particles would melt. A similar process is used for the

synthesis of polypropylene. (Late 1970).

Basic principles

The solid substrate (the catalytic material upon which chemical species react) material in the

fluidized bed reactor is typically supported by a porous plate, known as a distributor.[1] The fluid

is then forced through the distributor up through the solid material. At lower fluid velocities, the

solids remain in place as the fluid passes through the voids in the material. This is known as

a packed bed reactor. As the fluid velocity is increased, the reactor will reach a stage where the

force of the fluid on the solids is enough to balance the weight of the solid material. This stage is

known as incipient fluidization and occurs at this minimum fluidization velocity. Once this

minimum velocity is surpassed, the contents of the reactor bed begin to expand and swirl around

much like an agitated tank or boiling pot of water. The reactor is now a fluidized bed. Depending

on the operating conditions and properties of solid phase various flow regimes can be observed

in this reactor.

Applications

Chemical reactors:

Fluidized beds in chemical industry include two main types of reactions:

1. Catalytic gas phase reactions

-Particles are not undergoing any chemical reaction.

-Principal of oil cracking for manufacturing of various chemical substances.

2. Gas-solid reactions

-Fluidized particles are involved in the reactions and undergo a phase change

-Combustion or gasification of coal or bio mass.

Other applications

Fluid bed processing involves

-Drying, cooling, Agglomeration ,Granulation, and Coating of particulate materials. Ideal for a

wide range of both heat sensitive and non heat sensitive products. Uniform processing conditions

are achieved by passing a gas (usually air) through a product layer under controlled velocity

conditions to create a fluidized state

Advantages And Disadvantages of fluidized bed reactor

Fluidized bed reactor (FBR).

Advantages:

1. Uniform Particle Mixing: Due to the intrinsic fluid-like behavior of the solid material,

fluidized beds do not experience poor mixing as in packed beds. This complete mixing

allows for a uniform product that can often be hard to achieve in other reactor designs. The

elimination of radial and axial concentration gradients also allows for better fluid-solid

contact, which is essential for reaction efficiency and quality.

2. Uniform Temperature Gradients: Many chemical reactions require the addition or

removal of heat. Local hot or cold spots within the reaction bed, often a problem in packed

beds, are avoided in a fluidized situation such as an FBR. In other reactor types, these local

temperature differences, especially hotspots, can result in product degradation. Thus FBRs

are well suited to exothermic reactions. Researchers have also learned that the bed-to-

surface heat transfer coefficients for FBRs are high.

3. Ability to Operate Reactor in Continuous State: The fluidized bed nature of these

reactors allows for the ability to continuously withdraw product and introduce new

reactants into the reaction vessel. Operating at a continuos process state allows

manufacturers to produce their various products more efficiently due to the removal of

startup conditions in batch process.

Disadvantages:

1. Increased Reactor Vessel Size: Because of the expansion of the bed materials in the

reactor, a larger vessel is often required than that for a packed bed reactor. This larger

vessel means that more must be spent on initial capital costs.

2. Pumping Requirements and Pressure Drop: The requirement for the fluid to suspend

the solid material necessitates that a higher fluid velocity is attained in the reactor. In order

to achieve this, more pumping power and thus higher energy costs are needed. In addition,

the pressure drop associated with deep beds also requires additional pumping power.

3. Particle Entrainment: The high gas velocities present in this style of reactor often result

in fine particles becoming entrained in the fluid. These captured particles are then carried

out of the reactor with the fluid, where they must be separated. This can be a very difficult

and expensive problem to address depending on the design and function of the reactor.

This may often continue to be a problem even with other entrainment reducing

technologies.

4. Lack of Current Understanding: Current understanding of the actual behavior of the

materials in a fluidized bed is rather limited. It is very difficult to predict and calculate the

complex mass and heat flows within the bed. Due to this lack of understanding, a pilot

plant for new processes is required. Even with pilot plants, the scale-up can be very

difficult and may not reflect what was experienced in the pilot trial.

5. Erosion of Internal Components: The fluid-like behavior of the fine solid particles

within the bed eventually results in the wear of the reactor vessel. This can require

expensive maintenance and upkeep for the reaction vessel and pipes.

6. Pressure Loss Scenarios: If fluidization pressure is suddenly lost, the surface area of the

bed may be suddenly reduced. This can either be an inconvenience (e.g. making bed restart

difficult), or may have more serious implications, such as runaway reactions (e.g. for

exothermic reactions in which heat transfer is suddenly restricted).

Economic price of fluidized bed reactor

Reference

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