CE 548 Introduction to Process Analysis and Selection

1. CE 548 Introduction to Process Analysis and Selection

2. Processes Involving Mass Transfer • Definition: a process involving the transfer of mass from one homogenous phase to another. • The process is designed to reduce the concentration of a given component in one stream and increase it in another stream. • The deriving force of the transfer of material is a pressure or concentration gradient. • Important for a number of wastewater treatment processes(activated sludge, RBC, TF, digestion, etc.), common application transfer of O2 • Table 4-9 gives the common operations and processes in wastewater treatment involving mass transfer.

4. Processes Involving Mass Transfer • Gas-liquid mass transfer: • Several mass-transfer theories have been proposed to explain the mechanism of gas transfer: • The two-film theory (simple and commonly used)  • The penetration model (more theoretical (complex)) • The surface-renewal model (more theoretical (complex)) • The Two-Film Theory: • The theory is based on a physical model in which two films exist at the gas-liquid interface. • Two conditions can exist, (a) “absorption”: gas is transferred from the gas phase to the liquid phase and (b) “desorption”: gas is transferred from the liquid phase into the gas phase.

5. Flux from gas phase into interface: • rg= kg (PG – Pi) • Flux from interface into liquid phase: • rL= kL (Ci– CL) • r = kg (PG – Pi) = kL (Ci– CL) eq(4-122) • Because it is difficult to measure kLand kg at interface, the overall coefficient KG and KL can be used: • r = KL (CS – CL) eq(4-124) • From eq (4-122) and (4-124) • r=KL (CS – CL)=kg (PG – Pi)=kL (Ci– CL) • From Henery’s law at interface: • PG = H CS and Pi = H Ci Where: CS = conc. at interface in equilibrium with partial pressure in the bulk gas phase CL = conc. In the bulk liquid phase

6. The overall driving force (CS – CL) can be written as: • (CS – CL) = (CS – Ci) + (Ci –CL) • Substituting for PG and Pi in eq (4-122) and combining eq (4-125) and (4-124), the following is obtained where the liquid film controls mass transfer: • If mass transfer is controlled by the gas film: Where: CS = conc. at interface in equilibrium with partial pressure in the bulk gas phase CL = conc. In the bulk liquid phase

7. To estimate the flux from gas to liquid phase: • Where; r= rate of mass transfer per unit area • Where; = rate of mass transfer per unit volume • = volumetric mass transfer coefficient • = area of mass transfer per unit volume • Using eq(4-130) to describe the mass transfer through the surface of a basin: • Integrating between the limits of C=Co and C=Ct and t=0 and t=t, • (Example 4-10)

8. Introduction to Process Selection • Process selection involves detailed evaluation of the various factors that must be considered when evaluating unit operations and processes to meet the treatment objectives. • The purpose of the process analysis is to select the most suitable unit operations and processes and the optimal operational criteria. • Important factors in process selection: (Table 4-11) • Process Applicability. The ability of the process to get the job done in terms of eliminating the pollutant. Reflects directly upon the skill and experience of the design engineer. Resources include: data from operating installations; published information in technical journals, manuals of practice (MOPs) from the WPCF; process manuals from EPA and lab and pilot plant studies. • Applicable Flow Range and Variability. For example, stabilization ponds are not suitable for extremely large flowrates, if the flow variation is too great, equalization may be required. • Wastewater Characteristics. Affect the type of processes to be used: physical, chemical, biological. • Many others. Climate, sludge processing, complexity.

9. Introduction to Process Selection • Process selection based on reaction kinetics: Emphasis is placed on defining the nature of the reaction occurring within the process, the appropriate values of the kinetic coefficients, and the selection of the reactor type. • Nature of the kinetic reactions: • Should know reaction order (zero, first, second, etc..) • Affect selection of type and size of reactor • Selection of appropriate kinetic rate coefficients: • Selection of coefficients is based on: literature, similar systems, or pilot-plant studies.

10. Introduction to Process Selection • Selection of reactor type: Factors that must be considered in reactor selection include: • The nature of wastewater • The nature of the reaction kinetics • Special process requirements • Local environmental conditions • Selection based on mass transfer: • Processes involving mass transfer include: aeration (addition of oxygen), removal of VOCs, stripping of constituents (e.g. ammonia stripping), exchange of dissolved constituents (ion exchange) • Lots of literature and experience is available on theses subjects.

11. Introduction to Process Selection • Process design based on loading criteria: • If mathematical expressions (reaction rate and/or mass transfer coefficients) cannot be developed, loading criteria are often used. • For example, activated sludge systems based on lb BOD/1000ft3 of aeration tank capacity. Knowing the BOD loading, e.g. 200 mg/l in the influent, then yields a tank volume. • Limits of loading criteria are seldom defined. • With the new activated-sludge biological treatment process variations and new aeration equipment, the loading factors based design should be avoided.

12. Introduction to Process Selection • Bench tests and pilot –plant studies: • Bench or pilot-scale tests are conducted where the applicability of a process for a given situation is unknown. • The purpose is to establish the suitability of the process in the treatment of a specific wastewater under specific conditions in order to obtain the necessary data for full-scale design. • Factors that should be considered in planning pilot-plants studies are given in Table 4-12 • Pinch-scale tests are conducted in the laboratory with small amounts of wastewater. • Pilot-scale tests are conducted with flows that are 5 to 10 percent of the design flow.

14. Introduction to Process Selection • Reliability considerations in process selection: • Treatment performance and reliability in meeting permit requirements is an important factor in process selection. • Because wastewater treatment effluent quality can be variable for a number of reasons, it is necessary to ensure that the treatment system is designed to produce effluent quality of equal or less than the permit limit. • Impact of variability: • Almost all kinetic and empirical factors are based on constant wastewater flowrate and loading conditions. • Pumping and Piping. The critical design factor is the maximum hourly flowrate. Variations may lead to flooding or overflowing. • Secondary Sedimentation. The critical design factor is minimum flowrate and detection time. Variations may lead to rising sludge. • Statistical analysis are used to assess reliability: the probability that a system can meet performance criteria consistently over extended period of time.

15. Elements of Conceptual Process Design • Design Period : • Elements that can be easily expanded lend themselves to a shorter design time than difficult to expand structures. • A longer design will avoid untimely expansions, but on the other hand, longer design periods involve larger facilities and increased capital costs. Almost all funding agencies, city, state, feds seek to avoid capital costs. • An under-time designed facility may be overloaded in a short period of time. STPs have been constructed that have been overloaded the day they went on line. On the other hand, a grossly over-time designed facility may not meet current needs. Typical strategies involve building less but setting up for future expansion in terms of larger pump buildings but not placing the pumps, construction 2 tanks but leaving room and connection for 4 tanks. • Hydraulics 20-40 years. Pumping equipment 10-25 years.

16. Elements of Conceptual Process Design • Flow Diagrams, Design Criteria, Preliminary Sizing • Flow diagrams are graphical representations. Although limitless permutations are possible, however, most modern day urban designs would involve a completely mixed activated sludge plant with anaerobic digestion of sludge and chlorination of the effluent. For reuse purposes, tertiary treatment is usually added in the form of filtration. It is a good idea to start simply and add elements based on the site-specific needs of the installation. • Design Criteria. Should be evaluated for the future target date as well as the present. An example of design criteria would be a minimum detention time of 90 minutes for a primary sedimentation tank. Design criteria in a sense is the heart of design because this information leads to the size and arrangement of the facilities that will eventually be constructed. • Preliminary Sizing. Maintenance and Repair considerations are important. Flexibility in terms of extra valves, by-pass lines, choice of operations is of paramount importance. • Others. Include: solids balance, layout and hydraulics.