2 Chemical reactor to Reactor safety.pdf

7/25/2019 2 Chemical reactor to Reactor safety.pdf

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ME654: SAFETY IN CHEMICAL INDUSTRY

Even Semester

2013 2014

Department of Mechanical Engineering


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Chemical reaction

A Chemical reaction is a process that results in the conversion ofchemical substances.

The substance or substances initially involved in a chemical reaction are

calledreactants.

These reactants are characterized by a chemical change and theyyieldone or more products.

These products are generallydifferent from the original reactants.

Chemical reactions may be of different nature depending on the type of

reactants, type of product desired, conditions and time of the reaction.


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Chemical reaction

Type of reaction Area of utility

Combination To synthesize new compounds.

Decomposition Breakdown of larger, unuseful compounds/

complexes into smaller useful compounds.

Substitution Salt formation, New compounds formation

/ One small group in a molecule is replaced

by another small group.

Isomerization A chemical compound undergoes a

structural rearrangement without any

change in the atomic composition.


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Chemical reaction

Type of reaction Area of utility

Esterification A reaction between an organic acid and an

alcohol forming an ester and water.

Hydrolysis A large molecule is split into two smaller

molecules in the presence of water.

Hydrogenation Hydrogen is added across a double bond or

a triple bond.


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Chemical reactors

Chemical reactors are vessels designed to contain chemical reactions.

The design of a chemical reactor where bulk drugs would be

synthesized on a commercial scale would depend on multiple aspects of

chemical engineering.

Reactors are designed based on features like mode of operation or

types of phases presentor thegeometry of reactors.

Batch or Continuousdepending on the mode of operation. Homogeneous or Heterogeneous depending upon the phases

present.


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Chemical reactors

Depending upon theflow patternand manner in which the phases make

contact with each other, chemical reactors may also be classified as,

Stirred Tank Reactor

Tubular Reactor

Packed Bed Reactor Fluidized Bed Reactor


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Batch Process

A process in which all the reactants are added togetherat the beginning of

the process and products removed at the termination of the reactionis called a batch

process. In this process, all the reagents are added at the commencement and no

addition or withdrawal is made while the reaction is progressing (Fig. 1). Batch

processes are suitable for small production and for processes where a range of

different products or grades is to be produced in the same equipment for example,

pigments, dye stuff and polymers.


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Continuous Process

A process in which thereactants are fed to the reactor and the products or

byproducts are withdrawn in between while the reaction is still progressing(Fig. 2). Forexample,Haber Processfor the manufacture of Ammonia. Continuous production will

normally give lower production costs as compared to batch production, but it faces the

limitation of lacking the flexibility of batch production. Continuous reactors are usually

preferred forlarge scale production.


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Semi Batch Process

Process that do not fit in the definition of batch or a semi-batch reactor is operated

with both continuous and batch inputs and outputs and are often referred to as semicontinuous or semi-batch.

In such semi-batch reactors,some of the reactants may be added or some of the

products withdrawn as the reaction proceeds.

A semi-continuous process can also be one which is interrupted periodicallyfor

some specific purpose, for example, for the regeneration of catalyst, or for removal

of gas for example, a fermentor is loaded with a batch, which constantly produces

carbon dioxide, which has to be removed continuously.

Another example is chlorination of a liquid.


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Catalytic Processes

Most of the chemical reactions either proceed in the presence of catalysts or increases

their yield in thepresence of catalysts. A catalyst is a substance that, without itselfundergoing any permanent chemical change,increases the rate of a reaction. The rate

of a catalytic reaction is proportional to the amount of catalyst the contact with a fluid

phase reagents. This is proportional to the exposed area, efficiency of diffusion of

reagents in and products out, type of mixing. The assumption of perfect mixing cannot

be assumed. A catalytic reaction pathway is often multistep with intermediates that arechemically bound to the catalyst. Since the chemical binding is also a chemical

reaction, it may affect the reaction kinetics. The behaviour of the catalyst is also a

consideration. Particularly in high temperature petrochemical processes, catalysts are

deactivated by sintering, coking and similar processes.


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Homogeneous Reactions

Homogeneous reactions are those in which the reactants, products and any catalyst

used formone continuous phase; for example, gaseous or liquid. Homogeneous gasphase reactors will always be operated continuously.Tubular (Pipe line) reactors are

normally used for homogeneous gas phase reactions; for example, in the thermal

cracking of petroleum, crude oil fractions to ethylene, and the thermal decomposition of

dichloroethane to vinyl chloride. Homogeneous liquid phase reactors may be batch or

continuous. Batch reactions of single or miscible liquids are almost invariably done instirred or pump around tanks. The agitation is needed to mix multiple feeds at the start

and to enhance heat exchange with cooling or heating media during the process.


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Heterogeneous Reactions

In a heterogeneous reaction two or more phases exist and theoverriding problemsin

thereactor design is to promote mass transfer between the phases.

Liquid-Liquid

Liquid-Solid

Liquid-Solid-GasSolid-Solid

Gas-Solid

Gas-Liquid


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Heterogeneous Reactions


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Reactor Geometry

The reactors used for established processes are usuallycomplex designswhich have

been developed andevolved over a period of years to suit the requirements of theprocess, and are unique designs. However, it is convenient to classify reactor designs

into the followingbroad categories.


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Stirred Tank Reactors

Stirred tank agitated reactors consist of a tank fitted with a mechanical agitator and a

cooling jacket or coils. They are operated as batch reactors or continuous reactors.Several reactors may be used in series.


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Stirred Tank Reactors

The stirred tank reactor can be considered the basic chemical

reactor; modeling on a large scale the conventional laboratory flask.

They are used forhomogeneous and heterogeneousliquid-liquid and

liquid-gas reactions and for reactions that involve freely suspended

solids, which are held in suspension by the agitation.

As the degree of agitation is under the designers control, stirred tank

reactors are particularly suitable for reactions where good mass

transfer or heat transfer is required.


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Tubular Reactors

Tubular reactors are generally used forgaseous reactions, but are

also suitable for some liquid phase reactions.

Ifhigh heat transfer ratesare required small diameter tubes are used

to increase the surface area to volume ratio.

Several tubes may be arranged in parallel, connected to a manifold

or fitted into a tube sheetin a similar arrangement to a shell and tube

heat exchangers.

For high temperature reactions the tubes may be arranged in a

furnace.


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Packed Bed Reactors

There are two basic types of packed bed reactor; those in which thesolid is a reactant

and those in which the solid is a catalyst. In chemical process industries, theemphasize is mainly on the designing of catalytic reactors. Industrial packed bed

catalytic reactors range in size from small tubes, a few centimeters diameter to large

diameter packed beds. Packed-bed reactors are used forgas and gas-liquid reactions.

Heat-transfer rates in large diameter packed beds are poor therefore, where high heat-

transfer rates are required, fluidised beds should be considered.


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Fluidised Bed Reactors

A fluidized-bed reactor is a combination of the two most common,packed-bed and

stirred tank, continuous flow reactors. It is very important to chemical engineeringbecause of itsexcellent heat and mass transfer characteristics. The essential features

of a fluidised bed reactor is that the solids are held in suspension by the upward flow of

the reacting fluid. This promotes high mass and heat transfer rates and good mixing.


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Reactor Design

Fundamentals of Reactor Design

The design of a chemical reactor deals with multiple aspects of chemical

engineering. Chemical reactions, chemical energetics and equations/laws of

thermodynamics play an important role in the selection and design of chemical

reactors.


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Reactor Design

The design of an industrial chemical reactor must satisfy the following requirements.

The chemical factors:The kinetics of the reaction. The design must provide sufficient

residence time for the desired reaction to proceed to the required degree of conversion.

The mass transfer factors: With hetereogeneous reactions, the reaction rate may be

controlled by the rates of diffusion of the reacting species, rather than the chemicalkinetics.

The heat transfer factors: The removal or addition of the heat of reaction.

The safety factors: The confinement of hazardous reactants and products and the

control of the reaction and the process conditions.

Economic factors: Minimum amount of money should be required to purchase andoperate.


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Reactor Design

A general procedure for reactor design is outlined below:

Kinetic and thermodynamic data; Rate of reaction (Pressure, Temperature, Flow

rate, Catalytic Concentration)

Data on physical properties

Rate controlling mechanism (kinetic, mass or heat transfer)

Reactor type (based on experience with similar studies or from the laboratory andpilot plant work)

Selection of optimum reaction conditions

Size of the reactor

Material of Construction

Preliminary mechanical design for the reactor including the vessel design, heattransfer surfaces etc.

Design is optimized and validated

An approximate cost


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Reactor DesignMathematical Models

A model of a reaction process is a set of data and equation that is believed to represent

the performance of a specific vessel configuration (mixed, plug flow, laminar,dispersed, etc.).

Key process variables include:

Residence Time Distribution (RTD)

Volume

Temperature

Pressure

Concentrations of chemical

Heat transfer coefficients


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Commonly used Chemical reactors


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Reaction hazard evaluation and assessment

What is the Hazard?

When processing exothermic chemical reactions including thermally unstable

substances and mixtures, it should be remembered that the hazard comes from

PRESSURE generation.

Pressure can be generated in a closed vessel (or inadequately vented vessel) from:

Permanent gas generatione.g. generation of nitrogen, carbon dioxide, etc from the

desired process or an unexpected event.

Vapour pressure effectscaused by heating, possibly arising from an exothermic

reaction or a process failure condition, thus raising a mixture above its boiling point.

Identification of pressure generation is critically important forvent sizing, the most

common basis of safety in the chemical industry, since the design calculations will

require different data input.


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What is the impact of Scale-Up and why is it so Important?

Firstly, and most obviously, energy is consumed

in heating the REACTION MASS

To retain thermal equilibrium, energy is also consumed

in heating the REACTOR to an equilibrium temperature

Finally, once the outer walls of the vessel are above

the ambient temperature, heat is lost through

the walls to the SURROUNDINGS


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Essentially,as the scale increases, the ability to remove excess heat by

heat loss to the vessel and its surroundings reduces, resulting in a much

higher proportion of the heat retained in the reaction mass.

Heat losses are 10 times higher in the lab scale vessel

Only 2% heat loss to the large scale vessel compared with 19% heat

loss to the small scale vessel

The effects of scale are realand very significant!

Classical laboratory reactor systems are inadequate inproviding this data as they typically have high heat losses. As a

consequence specialist equipment is required to simulate large scale

conditions.


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Reaction hazards assessment comprises of a number of experimental and other

assessment procedures and tools which ultimately fit together to provide a basis ofsafety for any chemical process.

This basisofsafetyis the implemented and documented system that is in place to

either prevent a process running out of control under normal and foreseeable

conditions or provide engineering solutions to control the consequences of run-awayprocess.


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Process Lifecycle Activities

Chemical route selection

Process development and optimization

Pilot (small) scale production

Large scale production


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CHEMICAL ROUTE SELECTION


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Chemical route selection


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PROCESS DEVELOPMENT AND OPTIMIZATION

DIFFERENTIAL SCANNING CALORIMETRY (DSC)To determine the energy associated with the decomposition of a material or

mixturepotentially to screen for explosive properties.

CARIUS (10 G) TUBE SCREENING TEST

The test is designed to provide a preliminary indication of the thermal

behaviour of a material. Exothermic, endothermic and gas generating events are

determined in a semiquantitative fashion. The test can be undertaken on a liquid, solid

or mixture.

ACCELERATING RATE CALORIMETRY (ARC)

The test is normally performed to determine the onset temperature of an

exothermic decomposition and the subsequent kinetics and magnitude of the contained

runaway.


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HEAT FLOW CALORIMETRY (METTLER TOLEDO RC1 REACTION

CALORIMETER)

To determine the heat of reaction under isothermal or isoperibolic conditions

and identify the effect of changes in feed rates, temperatures and concentrations on

the "instantaneous" nature of a reaction system.

ADIABATIC PRESSURE DEWAR CALORIMETRY

To examine the stability of materials under adiabatic (zero heat loss)

conditions.


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The following tests can be conducted in the calorimeter:

Specification of Maximum Safe Handling Temperatures

Collection of Time to Maximum Rate (TMR) Data

Vent Sizing Information Collection for Batch Processes

Vent Sizing Information Collection for Semi-Batch ProcessesTempering Test

Blowdown Test


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Process development and optimization

Impactfor which the BAM Fall hammer test is employed.

Frictionfor which the BAM Friction test is employed.

Burningfor which the USA small scale-burning test is employed

Heatingfor which the DSC (or similar) thermal screening test is employed


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PILOT (SMALL) SCALE PRODUCTION

Examine the existing thermochemical data for obvious hazards inherentin the

process.

Conduct a thorough hazard identification exercise to identify foreseeable (andrealistic) scenarios which may present an over pressurisation hazard.

Hazard and Operability (HAZOP) Studies, Check List Assessments, Informal

what if?Assessments, Failure Modes and Effects Analysis (FMEA), Fault

Tree Analysis


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Identify the consequences of foreseeable deviationsand define the worst case over

pressurisation scenario.

Once a short-list of hazardous scenarios is available, it is necessary toconclusively ascertain whether the consequences of the scenarios are

hazardous or benign. The methods through which this can be done include:

Computational simulation

Estimation based on existing process safety data

Experimental simulation


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Specify and implement safety measures to protect the vessel(s) from all

foreseeable scenarios which may present a risk of over pressurisation.

Once the consequences of all the worst case candidates have been

quantified, the final task is to specify which safety measures are required to

protect the reactor from the consequences or to validate if existing protection

measures and protocols are acceptable.

There are numerous options available including:

Process control

Design for containmentReaction dumping / passive quenching

Reaction inhibition / active quenching

Emergency pressure relief (venting)


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LARGE SCALE PRODUCTION

The procedure for safety evaluation of large scale production would generally follow the

lines of that detailed for pilot scale-up. The most important differences being:

The consequences of a deviation will be more dramatic due to the larger inventory.

This implies the need for a more rigorous and exhaustive hazard identification

exercise. The variability of the plant is likely to be less than for the pilot plant. The need for

instrumented safety systems to comply with best practice will require assessment

of safety systems to international standards such as IEC 61508/11.

2 Chemical reactor to Reactor safety.pdf

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