1. Chap. 4 Process Analysis and Selection  Type of Reactors Batch Reactor. Flow is neither entering nor leaving the reactor Complete-Mix Reactor Complete – mix reactor is assumed that complete mixing occurs instantaneously and uniformly throughout the reactor as fluid particles enter the reactor Plug-Flow Reactor. Fluid particles pass through the reactor with little or no longitudinal mixing and exit from the reactor

2. Application of reactors Type of reactorApplication in wastewater treatment BatchActivated-sludge biological treatment in a sequence batch reactor, mixing of concentrated solutions into working solutions Complete-mixAerated lagoons, aerobic sludge digestion Complete-mix with recycle Activated-sludge biological treatment Plug-flowChlorine contact basin, natural treatment systems Plug-flow with recycle Activated-sludge biological treatment, aquatic treatment system Complete-mix reactors in series Lagoon treatment systems, used to simulate nonideal flow in plug-flow reactors Packed-bed Nonsubmerged and submerged trickling-filter biological treatment units, depth filtration, natural treatment systems, air stripping Fluidized-bedFluidized-bed reactors for aerobic and anaerobic biological treatment, upflow sludge blanket reactors, air stripping

3. C=C C=C 0 t= 0 t=t e -kt Mass-Balance Analysis

4. complete mix reactor

5. 1. General word statement: Rate of accumulation rate of flow of rate of flow of ­ rate of generation of reactant within = reactant into the - reactant out of the + of reactant within the system boundary system boundary system boundary the system boundary (1) (2) (3) (4) (4-2) 2. The corresponding simplified word statement is Accumulation = Inflow - outflow + generation (4-3) (1) (2) (3) (4) Mass-Balance Analysis

6. Preparation of Mass Balances In preparing mass balances it is helpful if the following steps are followed, especially as the techniques involved are being mastered. 1. Prepare a simplified schematic or flow diagram of the system or process for which the mass balance is to be prepared. 2. Draw a system or control volume boundary to define the limits over which the mass balance is to be applied. Proper selection of the system or control volume boundary is extremely important because, in many situations, it may be possible to simplify the mass-balance computations. 3. List all of the pertinent data and assumptions that will be used in the preparation of the materials balance on the schematic or flow diagram. 4. List all of the rate expressions for the biological or chemical reactions that occur within the control volume. 5. Select a convenient basis on which the numerical calculations will be based

7. Application of the Mass-Balance Analysis To illustrate the application of the mass-balance analysis, consider the complete-mix Reactor shown on Figure, the control volume boundary must be established so that all the flows of mass into and out of the system can be identified. On figure the control volume boundary is shown by the inner dashed line. To apply a mass-balance analysis to the liquid contents of the reactor shown on Figure it will be assumed that: 1. The volumetric flowrate into and out of the control volume is constant. 2. The liquid within the control volume is not subject to evaporation (constant volume). 3. The liquid within the control volume is mixed completely. 4. A chemical reaction involving a reactant A is occurring within the reactor. 5. The rate of change in the concentration of the reactant A that is occurring within the control volume is governed by a first-order reaction (re = -kC).

8. 1. Simplified word statement: Accumulations = inflow - outflow + generation (1) 2. Symbolic representation (refer to Figure): (2) Substituting - kC for r c yields (3) (4) (5) Steady-State Simplification Mass-Balance Analysis

9. where dCldt = rate of change of reactant concentration within the control volume, ML - 3 T -l V = volume contained within control volume, L 3 Q = volumetric flowrate into and out of control volume, L 3 T -l C o = concentration of reactant entering the control volume, ML -3 C = concentration of reactant leaving the control volume, ML -3 r c = first-order reaction, (-kC), ML - 3 T -1 k = first-order reaction rate coefficient, T -l Mass-Balance Analysis