Unit Operation and Unit Process PDF

UNIT OPERATION AND UNIT PROCESS - The entire chemical engineering can be classified into two groups; unit operations or unit processes. o The concept of unit operations was introduced in 1915 by Dr. Arthur D. Little. o The concep...

UNIT OPERATION AND UNIT PROCESS - The entire chemical engineering can be classified into two groups; unit operations or unit processes. o The concept of unit operations was introduced in 1915 by Dr. Arthur D. Little. o The concept of unit processes was introduced in 1923 by P.H. Groggin. A. Unit Operation - A unit operation is defined as a process which does not involve any chemical reaction. - Unit operations only deal with physical changes of the materials involved in the process. o They are equipment which cause the materials to undergo physical changes. o The physical changes are carried out for variety of purposes. - Generally, unit operations steps are carried out before subjecting the materials to chemical reactions so that chemical reactions happen smoothly. - The physical changes can imply phase changes such as; evaporation, condensation, crystallization etc. o Thus, Distillation is a unit operation step because condensation and evaporation happen inside the column. o Evaporators, and crystallizers are also unit operations equipment. - Unit operation equipment are also responsible for mechanical operations which involves size reduction, physical separations, mixing, and grinding. - The mass transfer, heat transfer process all may happen together. Chemical reaction doesn’t happen. 1. Distillation 4. Leaching 2. Evaporation 5. Absorption 3. Crystallization 6. Adsorption Drying The unit operations are classified in the following manner: Fluid flow operations: Pumping, compression, and fluidization. Mechanical operations: Size reduction, size enlargement, mixing, agitation, blending, filtration, classification-separation, etc. Mass transfer: Distillation, evaporation, crystallization, leaching, absorption, adsorption, extraction, etc. Heat transfer: When materials are handled the heat transfer can take place by any fundamental mechanism; conduction, convection, or radiation. Usually, two fundamental mechanisms occur simultaneously. B. Unit Processes - Chemical reaction is at the heart of the unit processes. - A chemical reactor is an equipment which falls under the category of unit processes. - Literal chemical change takes place inside the equipment wherein the chemical structure of the material changes and it transforms and forms an entirely new material.  o All kinds of chemical reactions carried out in industrial equipments comes under this category. o Some examples of such chemical reactions are; sulphonation, nitration, halogenation, alkylation, hydrolysis, hydrogenation, polymerization, oxidation, reduction, etc. - The examples can be innumerable as there can be innumerable compounds which can be formed by reaction of two or more materials or by decomposition of the material itself. - Thus, classification can be similarly endless. 1. Alkylation 5. Nitration 2. Halogenation 6. Oxidation 3. Hydrolysis 7. Sulphation 4. Hydrogenation - Most of the chemical reactions are irreversible in nature. - A chemical reaction step in a process forms the heart of the process, often governing the economics of the entire process. Thus, the unit processes are often subject of optimization. - Often unit operations have to be carried out before unit processes to give proper shape to the materials so that they will be able to react optimally. - Also, unit operations are often needed even after the chemical reaction has happened, they are needed for the purpose of separation of the primary product from the secondary or tertiary products. - All the unit operation processes wherein chemical reaction is made to take place to improve the efficiency of the equipment also falls under unit processes. o For example, reactive distillation, reactive extraction, reactive absorption, etc. FUNDAMENTAL TRANSPORT PROCESSES 1. Momentum transfer. This is concerned with the transfer of momentum which occurs in moving media, such as in the separation processes of fluid flow, sedimentation, mixing, and filtration. 2. Heat transfer. In this fundamental process, we are concerned with the transfer of heat from one place to another; it occurs in the separation processes of drying, evaporation, distillation,and others. 3. Mass transfer. Here mass is being transferred from one phase to another distinct phase; the basic mechanism is the same whether the phases are gas, solid, or liquid. This includes distillation, absorption, liquid–liquid extraction, membrane separation, adsorption, crystallization, and leaching.  CONSERVATION OF MASS AND MATERIAL BALANCES 1.5A Conservation of Mass - One of the basic laws of physical science is the law of conservation of mass. o This law, stated simply, says that mass cannot be created or destroyed (excluding, of course, nuclear or atomic reactions). - Hence, the total mass (or weight) of all materials entering any process must equal the total mass of all materials leaving plus the mass of any materials accumulating or left in the process: input = output + accumulation - In the majority of cases there will be no accumulation of materials in a process, and then the input will simply equal the output. Stated in other words, “what goes in must come out.” We call this type of process a steady-state process: input = output (steady state) 1.5B Simple Material Balances - In this section we do simple material (weight or mass) balances in various processes at steady state with no chemical reaction occurring. - We can use units of kg, lbm, lb mol, g, kg mol, and so on, in our balances. - When chemical reactions occur in the balances (as discussed in Section 1.5D), one should use kg mol units, since chemical equations relate moles reacting. - To solve a material-balance problem, it is advisable to proceed by a series of definite steps, as listed below: 1. Sketch a simple diagram of the process. - This can be a simple box diagram showing each stream entering by an arrow pointing in and each stream leaving by an arrow pointing out. - Include on each arrow the compositions, amounts, temperatures, and so on, of that stream. All pertinent data should be on this diagram. 2. Write the chemical equations involved (if any). 3. Select a basis for calculation. - In most cases the problem is concerned with a specific amount of one of the streams in the process, which is selected as the basis. 4. Make a material balance. - The arrows into the process will be input items and the arrows going out output items. - The balance can be a total material balance in Eq. (1.5-2) or a balance on each component present (if no chemical reaction occurs). - Typical processes that do not undergo chemical reactions are drying, evaporation, dilution of solutions, distillation, extraction, and so on. - These can be solved by setting up material balances containing unknowns and solving these equations for the unknowns. 1.5C Material Balances and Recycle - Processes that have a recycle or feedback of part of the product into the entering feed are sometimes encountered. o For example, in a sewage treatment plant, part of the activated sludge from a sedimentation tank is recycled back to the aeration tank where the liquid is treated. o In some food-drying operations, the humidity of the entering air is controlled by recirculating part of the hot, wet air that leaves the dryer. o In chemical reactions, the material that did not react in the reactor can be separated from the final product and fed back to the reactor. 1.5D Material Balances and Chemical Reaction - In many cases the materials entering a process undergo chemical reactions in the process, so that the materials leaving are different from those entering. - In these cases, it is usually convenient to make a molar and not a weight balance on an individual component, such as kg mol H2 or kg atom H, kg mol CO 3 - ion, kg mol CaCO3, kg atom Na+ kg mol N2, and so on. For example, in the combustion of CH4 with air, balances can be made on kg mol of H2, C, O2, or N2. 1.5 D Heat Balances - The most common important energy form is heat energy and the conservation of this can be illustrated by considering operations such as heating and drying. - In these, enthalpy (total heat) is conserved and as with the mass balances so enthalpy balances can be written round the various items of equipment. or process stages, or round the whole plant, and it is assumed that no appreciable heat is converted to other forms of energy such as work. Enthalpy (H) - always referred to some reference level or datum, so that the quantities are relative to this datum. - Working out energy balances is then just a matter of considering the various quantities of materials involved, their specific heats, and their changes in temperature or state (as quite frequently latent heats arising from phase changes are encountered). Figure 4.3 illustrates the heat balance. - Heat is absorbed or evolved by some reactions in processing but usually the quantities are small when compared with the other forms of energy entering into food processing such as sensible heat and latent heat. A. Latent heat is the heat required to change, at constant temperature, the physical state of materials from solid to liquid, liquid to gas, or solid to gas. B. Sensible heat is that heat which when added or subtracted from materials changes their temperature and thus can be sensed. - The units of specific heat are J/kg K and sensible heat change is calculated by multiplying the mass by the specific heat by the change in temperature, (m x c x ΔT). - The units of latent heat are J/kg and total latent heat change is calculated by multiplying the mass of the material, which changes its phase by the latent heat. - Having determined those factors that are significant in the overall energy balance, the simplified heat balance can then be used with confidence in industrial energy studies. - Such calculations can be quite simple and straightforward but they give a quantitative feeling for the situation and can be of great use in design of equipment and process.