CE 548 Introduction to Process Analysis and Selection

Reactors used for the treatment Wastewater treatment involving physical, chemical or biological activities are carried out in vessels or tanks commonly known as “reactors”. Types of reactors: Batch reactor: The flow enters, is treated and then is discharged and the cycle repeats. Once the processing commences flow does not enter or leave the vessel. Used for small operations.

Reactors used for the treatment Types of reactors: Complete mix reactor: Complete mixing occurs instantaneously and uniformly throughout the reactor as fluid enters the reactor. Usually squar tanks, L~W. Used with most newer systems.

Reactors used for the treatment Types of reactors: Plug-flow reactor: Fluid passes through the reactor with little or no longitudinal mixing and exit the reactor in the same sequence as they entered. Long rectangular tanks L>>W. Used in most older systems.

Reactors used for the treatment Types of reactors: Complete-mix reactors in series (e) Packed-Bed reactors (f), (g) Fluidized-Bed reactors (h)

Reactors used for the treatment Applications of the reactors: The principle applications of reactors types used for the treatment of wastewater are reported in Table 4-1 Operational factors that are considered in the selection of the type of reactor to be used: The nature of the wastewater to be treated The nature of the reaction The reaction kinetics The process performance requirements

Table 4-1

Reactors used for the treatment Hydraulic characteristics of reactors: Ideal flow in complete-mix and plug-flow reactors for pulse (slug-dose)  = V/Q Where;  = hydraulic detention time, T V = volume of reactor, L3 Q = flowrate, L3

Reactors used for the treatment Hydraulic characteristics of reactors: Ideal flow in complete-mix and plug-flow reactors for step inputs (continuous injection)  = V/Q Where;  = hydraulic detention time, T V = volume of reactor, L3 Q = flowrate, L3

Mass-Balance Analysis Principle: conservation of mass; mass neither created nor destroyed. Rate of accumulation of reactant within the system boundary = Rate of flow of reactant into the system boundary - Rate of flow of reactant out of the system boundary + Rate of generation of reactant within the system boundary (1) (2) (3) (4)

Mass-Balance Analysis Preparation of Mass Balances Schematic Control volume List all data and assumptions List all rate expressions Select a basis for calculation Application of mass-balance analysis Assumptions: Constant flowrate into and out of control volume No evaporation (constant volume) Complete mixing Reaction occurs within reactor Rate of reaction is first-order (rc = -kC)

Mass-Balance Analysis Formulation of mass balance: Accumulation = Inflow – outflow + generation if the reaction is steady state, there is no accumulation (dC/dt = 0), Thus equation (4-6) can be written as:

Modeling Ideal Flow in Reactors Modeling of the hydraulic characteristics of reactors is important because the results can be used to determine the actual amount of time a given volume of water will remain in the reactor. Complete Mix Reactor: Accumulation = Inflow – outflow + generation Using tracer, nothing is being generated; generation = 0 For pulse (slug) input: Integrating with limits C=Co to C=C, and t=0 to t= t yields:

Modeling Ideal Flow in Reactors Complete Mix Reactor: Accumulation = Inflow – outflow + generation Using tracer, nothing is being generated; generation = 0 For step input: Integrating with limits C=C to C=Co, and t=0 to t= t yields:

Modeling Ideal Flow in Reactors Plug-flow Reactor: Accumulation = Inflow – outflow + generation (generation = 0)

Nonideal Flow in Reactors Factors leading to nonideal flow in reactors: (Fig 4-6)

Nonideal Flow in Reactors Need for tracer analysis: Tracer analysis is used to assess the hydraulic performance of the reactor by measuring the residence time. Application of tracer studies include: Assessment of short circuiting in sedimentation tanks and biological reactors Assessment of contact time in chlorine contact basins Assessment of the hydraulic approach conditions in UV reactors Assessment of flow patterns in constructed wetlands and other natural treatment systems Example 4-1 Page 236

Reactions, Rates, and Coefficients Types of reactions: Homogenous: the reactants are distributed uniformly throughout the fluid. (batch, complete-mix, plug-flow) Rate of reaction; Heterogeneous: reaction occur at a specific site. (packed and fluidized bed reactors)

Reactions, Rates, and Coefficients Types of rate expressions: Typical rate expressions for selected processes: Table 4-6 Integration and differential methods used to determine reaction rate coefficients: Table 4-7 Example 4-5.

Modeling treatment process kinetics Batch reactor:

Modeling treatment process kinetics Complete mix-reactor: Graphical solution: Example 4-6

Modeling treatment process kinetics Plug-flow:

Modeling treatment process kinetics Plug-flow with axial dispersion: