“Problems and Theory for Material Balance of Unsteady state Operations” Prepared By: Group.5(130150105038 to 47) B.E. Sem-III(Chemical) Guided By: Prof. D. R. Gandhi Prof. S. R. Banker Chemical Engineering Department Government Engineering College Bhuj-370001

Classification of Chemical Processes Process Classification INDEX Classification of Chemical Processes Process Classification General Material Balance Equation Summary Balance General Procedure for Solving Material Balance Calculations Material Balances on Reactive vs. Nonreactive Processes Example

Classification of Chemical Processes: 1. Batch Processes. 2. Continuous Processes. 3. Semibatch Process.

CLASSIFICATION OF PROCESS Batch Process: Feed is charged at the beginning of the process and the product is collected some time later Continuous Process: In this type of process, raw materials continuously enter and product continuously leaves the process. Semibatch Process: A semi batch process does not fall fully under either “batch” or “continuous” classification.

PROCESS CLASSIFICATION Steady-State: Process variables don’t vary with time, 2. Unsteady-State (Transient):Process variables vary with time Batch and semi batch processes must be transient. Continuous processes may be transient or steady-state.

General Material Balance Equation input + generation – output – consumption = accumulation Input : enters through system boundaries Generation : is produced within the system Output : exits through system boundaries Consumption : is used up within the system Accumulation : is built up within the system

Summary: All batch processes are considered to be unsteady-state if the changes between the initial and final time is required to determine. Continuous processes are operated in the unsteady state at the start-up, then it is operated almost at steady state mode.

Two types of balances may be written: Differential balance: Indicates what happens at a certain moment of time Integral balances : Indicates what happens between two instants of time

1. If the balanced quantity is the total mass, then: Special Cases: 1. If the balanced quantity is the total mass, then: generation = 0 consumption = 0 2. If the balanced substance is a nonreactive species, then: generation = 0 consumption = 0 3. If the system is at steady state, then: accumulation = 0

General Procedure for Solving Material Balance Calculations 1). Choose a basis of calculation. 2). Draw and completely label the flowchart. The flowchart is fully labelled if the composition and flowrate (or amount) of each process stream can be expressed in terms of the labelled quantities 3). Express the result (unknown) of interest in terms of the quantities on the flowchart.

General Procedure for Solving Material Balance Calculations 4). Perform a degree of freedom analysis If the number of degrees of freedom = 0, the problem is solvable. 5). Write down the equations you will solve. Try to write them in an order that will simplify the calculations. 6). Solve the equations.

Material Balances on Reactive vs. Nonreactive Processes When we earlier considered material balances on nonreactive processes (processes in which no chemical reactions are taking place), we wrote the balance equations in terms of the molecular species flowing through the process. We will study two ways of solving material balances for reactive processes. The first, which is often the simplest to implement, is the method of atomic species balances.

Material Balances on Reactive vs. Nonreactive Processes Input of atom i – output of atom i = accumulation of atom I The second approach uses extents of reaction to express flowrates of each reactive species according to the stoichiometries of the reactions taking place. Which approach one chooses to use does not matter in the sense that both yield the same answers

Development of Dynamic Models Illustrative Example: A Blending Process An unsteady-state mass balance for the blending system:

or where w1, w2, and w are mass flow rates. The unsteady-state component balance is:

EXAMPLE In a batch process, the reaction takes place in the presence of an acid medium. The acid is drained from the reaction vessel at the rate of 15ml/s as a result of the density difference of the acid from the reacting component. To avoid wastage of acid, it is recycled to an acid tank which has 1000 L capacity. The acid drained from the rectaion vessel, picks up 50 g/L solids from the reactor.

EXAMPLE Acid is fed once again to the process from acid tank. When the process is started, the acid is almost pure in the tank as a result of filtration. As the reaction proceeds, acid in the tank gets more and more contaminated with the solids. The concentration of the solids should not exceed 100 g/L from the process point of view. The batch time is 16h. Check whether the concentration of the solids will exceed 100g/L during the batch reaction.

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