Process Tech

Environmental process technology

• ·         Introduction& objectives
• ·         Process and Reactorsdesign
• ·         Types of reactors
• ·         Chemical kinetics/reaction kinetics

Environmental Process Technology

Key aspects

1.      Environment

            Is a natural world of simply nature or the surroundings which could be inform of conditions or situations that can or cannot support life.

2.      Process

            Is the treatment that transforms inputs(reactants) to outputs(products) usually of the desired quality.

3.      Technology

            Is the art or science that is applied through design, monitoring and construction so as to solve man’s problems

Environmental process technology is the study of application of science to transform the environmental conditions and nature/surroundings in to a desired state that is essential or conducive to human life.

Objectives of the study Environmental Process Technology

·         To enlighten the reluctant people about the importance of environmental conservation and the need to moderate the activities that result in uncontrolled release of pollutants in to the environment

·         To train and produce personnel who will be able to manage environmental issues effectively and efficiently and be able to identify environmental problems and their sources, gather relevant data, design solutions, strength and weaknesses using modern technology

·         To carry out environmental processes that can reduce on health hazards emanating from man’s activities.

·         To be able to collect data to monitor the environmental quality and designing the environmental processes and clean up.

Therefore the best knowledge of environmental process technology will enable learners to build up solutions that will solve environmental problems.

Process and Reactor Design

A design is a creative activity which involves synthesis or manufacturing that puts together ideas so as to achieve a desired purpose.

Steps involved in design process

1.      The designer starts with specific objective in mind or establishes the need for design.

2.      Tie designer considers the best ways of achieving these objectives through data collection.

3.      Development and generation of possible designs.

4.      Evaluating and monitoring the number of possible designer by evaluating the consequence /limitations involved.

Objectives

A designer creates a design for the system or reactor to fulfill a particular need. In a design for a particular process, the need is a public need to solve a problem.

Collection of data

To proceed with the design, the designer must first ensemble all relevant facts, information and data required.

This will include equipment performance and physical data about the material to be collected and the different design.

This is the most frustrating and time consuming part. The national standerds may also work as a source of designing and method of data.

Generation of possible design

This is the creative part of the design process different ideas are collected to come up with a particular design.

Here the designer will largely rely on his previous experience and that of others.

Evaluation and selection

Identification of possible design:- these are designs that lead to meet the external consequences or limitations

External consequences (constraints) are limitations which are set outside the designers influence.

Identifications of playssible designs(internal consequences). These are designs which meet the internal consequences, ie These are designs which are selected using

Internal consequences are limitations which the designer may have influence to the designer e.g choice of the process, environment etc.

Identification of probable design. These are designs under personal judgment.

Identification of the best design. This is the design that is judged basing on un optimization.

Fixed(invariable) consequences. This are the ones which don’t vary with time of from design to design.

Unfixed(variable) consequences. These are variations which change from time to time or from design to design.

Factors considered during selection of a design.

The cost of operation

Safety and health

Efficiency of the reactor

The capacity of the reactor

The economic costs

The minimum pollution

TYPES OF REACTORS

1.      Batch reactor

Here all the reacting agents are added at the commensment(start) reaction proceeds as decomposition changes with time then reaction stopped when the conversion has been achieved and the product as withdrawn.

Advantages.

·         It is majorly used for small scale products.

·         It is important where high quality products are needed or products of high purity.

·         It is possible to produce a wide range of products i.e one equipment/container can be reused to produce another different product.

Disadvantages

·         It is highly labour intensive.

·         It is only limited to small scale production.

2.      Continuous process reactor

Here the reactant are fed to the reactor as products are being withdrawn simultaneously. It is suitable for handleing large scale production eg in cement and manufacturing factory.

Continuous processes are assumed to operate at a steady state. ie. Process variable such as flowrates donot change with time. This implies that there is no accumulation of materials within the reactor.

Advantages

·         It is suitable for large scale production.

·         It is less labour intensive, ie low production costs.

Disadvantages

·         It cannot be used to produce a wide range of products.

·         There is high cost of installation.

Nb; continuous process reactors can also be categorized or classified depending on the flow of materials through the reactor.

Examples of continuous process reactors.

        i.            Plug/tabular flow reactors

This is characterized by the fact that the flow is orderly and there is no elemental overtaking of materials ie. First in first out. As the law materials are fed at one end of the tube and as it passes through the tube different transformations takeplace.

If good transfer rates are required small diameter tubes are used to increase the surface area to volume ratio. Therefore several tubes are arranged in parallel and connected to the main folder fitted in form of a sheet.

      ii.            Mixed flow reactor/ Continous stirred tank reactor.

Here the materials are continuously stirred to ensure uniformly of the material.

    iii.            Packed bed reactors.

There are two basic types of packed bed reactors ie. Those in which the solid is a reactant and those in which the solid is a catalyst.

This kind of reactor consists of either tube or tank packed with either solid reactants or catalyst through which the fluid can be passed. Eg Sand filters.

     iv.            Fluidized bed reactors

The essential feature of a fluidized reactor is that solids are held in suspension by the up flow of the reacting fluid. This promotes high mass transfer rates and hence better mixing.

CHEMICAL KINETICS / REACTION KINETICS.

For any reaction, there are two governing factors i.e kinetics and thermodynamics

Thermodynamics deals with the heat changes of reactions and the equilibrium of reactions while kinetics deals with the rate of reaction and reaction mechanism.

Kinetics of reactions depend on the kind of reactions formed and the reactions can be classified depending on the phases of reacting species. Ie homogeneous and heterogeneous phases.

The reaction is said to be homogeneous if it takes place in one phase and it is also said to be heterogeneous if it takes place in two phases.

Eg. N­_2(aq) + 3H_2(g)                                                            2NH_3(g)        this is one phase.

      CaCO_3(s)                    CaO_(s) + CO_2(g)                      this is two phase.

Factors affecting rate of reaction.

These are:- temperature, pressure and concertration

Temperature.

If the above reaction is endothermic, increase in temperature favours a forward reaction and reduction in temperature favours a backward reaction.

Presure.

Consider a reaction N_2(g) + 3H_2(g)                                                     2NH_3(g)

Sudden increase in pressure will favour a forward reaction and the reverse is true.

Concertration

Consider a reaction A + B               C + D

Sudden increase in the amount of C will favour a backward reaction and the reverse is true.

Rate of reaction.

This can be defined as the rate of disappearance of the reactant or the rate of appearance of a product.

The rate expression is given a negative sign if the expression is given in terms of the reactants and a positive sign if the expression is given in terms of the products.

Consider a reaction A + B               C

The rate of reaction with respect to A can be given as:-

                                       Rate = _AC_B

                                                                                                _{= ­A­­­}C­­­­­_B

                                                                _{= ­A­­­}C­­­­­_B  (reactants)

_{this is the rate law equation.}

_{= = ­A­­­}C­­­­­_B

_{= ­A­­­}C­­­­­_B  (products)

Non elementary or complex reactions.

For complex reactions, there are two factors to consider. Ie yields and selectivity.

Yield refers to the fraction of the reactant that has been converted in to a specific product while selectivity refers to the ratio of the production of one product to that of the other product or products.

Material Balances.

Fundamentals of material balance.

Material balance is the application of the law of conservation of mass which states that matter can neither  be created nor destroyed but changes from one state to another.

Why material balance?

·         To improve efficiency

·         To maintain production

·         To reduce environmental discharges

Material balance generally helps one to understand what Is taking place with in a plant. Here one has to define the area of interest and this area of interest is known as a system.

A system is any arbitrary portion that you want to consider for analysis.

Material balance establishes relationships between flows and compositions in different parts of the plant.

When you apply law of conservation in a system.

(input of materials) + (generation within the system) – (Output of materials) – (consumption of material within the system) = (accumulation of materials within the system)

This equation may be applied in different ways according to the precise definition of material and the way in which the process is operated.

If the balance is applied to the total mass or if there is no chemical process, the terms, generations and consumptions becomes zero

(Input of materials) – (Output of materials) = (Accumulation of materials with in system)

For steady state processes, there is no accumulation

(input of materials) – (output of materials) = 0

Inputs = outputs

Systematic procedures taken when handling material balance.

1.      Draw a schematic diagram of the process or system

2.      Fix on the diagram all the data available

3.      Denote by numbers or letters on all the streams available

4.      Select the basis of calculation of mass balance, component balance or energy balance or by considering a limiting reactant.

A basis is a reference chosen by you for the calculations you plan to make in any particular problem. This makes it easier to solve.

One chooses a basis when no specific amount of material is specified.

5.      If a reaction is involved, write the chemical equation and makesure the equation is balanced.

6.      Make the necessary calculations carefully and check the althemetic units as you proceed.

7.      Summarize in an orderly manner all results obtained including their dimensions.

8.      Check the results in an independent way

9.      Determine whether the answers seem reasonable in view of your experience in these types of calculations.