2 Chemical Reactor To Reactor Safety.pdf 2o5q6

Department of Mechanical Engineering Even Semester 2013 – 2014

ME654: SAFETY IN CHEMICAL INDUSTRY

Chemical reaction A Chemical reaction is a process that results in the conversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants. These reactants are characterized by a chemical change and they yield one or more products. These products are generally different 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.

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.

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.

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 present or the geometry of reactors. • Batch or Continuous depending on the mode of operation. • Homogeneous or Heterogeneous depending upon the phases present.

Chemical reactors Depending upon the flow pattern and manner in which the phases make with each other, chemical reactors may also be classified as, • Stirred Tank Reactor • Tubular Reactor • Packed Bed Reactor • Fluidized Bed Reactor

Batch Process A process in which all the reactants are added together at the beginning of the process and products removed at the termination of the reaction is 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.

Continuous Process A process in which the reactants are fed to the reactor and the products or byproducts are withdrawn in between while the reaction is still progressing (Fig. 2). For example, Haber Process for 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 for large scale production.

Semi Batch Process •

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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 semi continuous 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 periodically for 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.

Catalytic Processes Most of the chemical reactions either proceed in the presence of catalysts or increases their yield in the presence of catalysts. A catalyst is a substance that, without itself undergoing any permanent chemical change, increases the rate of a reaction. The rate of a catalytic reaction is proportional to the amount of catalyst the 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 are chemically 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.

Homogeneous Reactions Homogeneous reactions are those in which the reactants, products and any catalyst used form one continuous phase; for example, gaseous or liquid. Homogeneous gas phase 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 in stirred 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.

Heterogeneous Reactions In a heterogeneous reaction two or more phases exist and the overriding problems in the reactor design is to promote mass transfer between the phases. Liquid-Liquid Liquid-Solid Liquid-Solid-Gas Solid-Solid Gas-Solid Gas-Liquid

Heterogeneous Reactions

Reactor Geometry The reactors used for established processes are usually complex designs which have been developed and evolved over a period of years to suit the requirements of the process, and are unique designs. However, it is convenient to classify reactor designs into the following broad categories.

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.

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 for homogeneous and heterogeneous liquid-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.

Tubular Reactors • Tubular reactors are generally used for gaseous reactions, but are also suitable for some liquid phase reactions. • If high heat transfer rates are 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 sheet in a similar arrangement to a shell and tube heat exchangers. • For high temperature reactions the tubes may be arranged in a furnace.

Packed Bed Reactors There are two basic types of packed bed reactor; those in which the solid is a reactant and those in which the solid is a catalyst. In chemical process industries, the emphasize is mainly on the deg 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 for gas and gas-liquid reactions. Heat-transfer rates in large diameter packed beds are poor therefore, where high heattransfer rates are required, fluidised beds should be considered.

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 engineering because of its excellent 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.

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.

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 chemical kinetics. 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 and operate.

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 and pilot plant work) • Selection of optimum reaction conditions • Size of the reactor • Material of Construction • Preliminary mechanical design for the reactor including the vessel design, heat transfer surfaces etc. • Design is optimized and validated • An approximate cost

Reactor Design – Mathematical 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

Commonly used Chemical reactors

Reaction hazard evaluation and assessment What is the Hazard? When processing exothermic chemical reactions including thermally unstable substances and mixtures, it should be ed that the hazard comes from PRESSURE generation. Pressure can be generated in a closed vessel (or inadequately vented vessel) from: • Permanent gas generation e.g. generation of nitrogen, carbon dioxide, etc from the desired process or an unexpected event. • Vapour pressure effects caused 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 for vent sizing, the most common basis of safety in the chemical industry, since the design calculations will require different data input.

Reaction hazard evaluation and assessment 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

Reaction hazard evaluation and assessment 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 real – and very significant! Classical laboratory reactor systems are inadequate in providing this data as they typically have high heat losses. As a consequence specialist equipment is required to simulate large scale conditions.

Reaction hazard evaluation and assessment Reaction hazards assessment comprises of a number of experimental and other assessment procedures and tools which ultimately fit together to provide a basis of safety for any chemical process. This “basis of safety” is 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-away process.

Reaction hazard evaluation and assessment Process Lifecycle Activities    

Chemical route selection Process development and optimization Pilot (small) scale production Large scale production

Reaction hazard evaluation and assessment

Reaction hazard evaluation and assessment

Reaction hazard evaluation and assessment CHEMICAL ROUTE SELECTION

Reaction hazard evaluation and assessment Chemical route selection

Reaction hazard evaluation and assessment PROCESS DEVELOPMENT AND OPTIMIZATION DIFFERENTIAL SCANNING CALORIMETRY (DSC) To determine the energy associated with the decomposition of a material or mixture – potentially 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.

Reaction hazard evaluation and assessment 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.

Reaction hazard evaluation and assessment 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 Processes Tempering Test Blowdown Test

Reaction hazard evaluation and assessment Process development and optimization Impact – for which the BAM Fall hammer test is employed. Friction – for which the BAM Friction test is employed. Burning – for which the USA small scale-burning test is employed Heating – for which the DSC (or similar) thermal screening test is employed

Reaction hazard evaluation and assessment PILOT (SMALL) SCALE PRODUCTION • Examine the existing thermochemical data for “obvious” hazards inherent in the process. • Conduct a thorough hazard identification exercise to identify foreseeable (and realistic) 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

Reaction hazard evaluation and assessment PILOT (SMALL) SCALE PRODUCTION • Identify the consequences of foreseeable deviations and define the worst case over pressurisation scenario. Once a short-list of hazardous scenarios is available, it is necessary to conclusively 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

Reaction hazard evaluation and assessment PILOT (SMALL) SCALE PRODUCTION • 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 containment Reaction dumping / ive quenching Reaction inhibition / active quenching Emergency pressure relief (venting)

Reaction hazard evaluation and assessment 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.