Practical Design Calculations for Groundwater and Soil Remediation - Chapter 4

Kuo, Jeff "Mass balance concept and reactor design" Practical Design Calculations for Groundwater and Soil Remediation Boca Raton: CRC Press LLC,1999 chapter four Mass balance concept and reactor design Various treatment processes are employed in remediation of contaminated soil or groundwater. Treatment processes are generally classified as physical, chemical, biological, and thermal processes. Treatment systems often consist of a series of unit operations/processes, which form a process train. Each unit operation/process contains one or more reactors. A reactor can be con-sidered as a vessel in which the processes occur. Environmental engineers are often in charge of or, at least, participate in preliminary design of the treatment system. Basically, the preliminary design involves selection of treatment processes and reactor type as well as sizing the reactors. For treatment system design, treatment processes should be selected first by screening the alternatives. Many factors should be considered in selection of treatment processes. Common selection criteria are implementability, effectiveness, cost, and regulatory consideration. In other words, an opti-mum process would be the one that is implementable, effective in removal of contaminants, cost efficient, and in compliance with the regulatory requirements. Once the treatment processes are selected for a remediation project, engineers will then design the reactors. Preliminary reactor design usually includes selecting appropriate reactor types, sizing reactors, and determining the number of reactors needed and their optimal configuration. To size the reactors, engineers first need to know if the desirable reactions or activities would occur in the reactors and what the optimal operating conditions such as temperature and pressure would be. Information from chemical thermo-dynamics, or more practically a pilot study, would provide the answers to these questions. If the desired reactions are feasible, the engineers then need to determine the rates of these reactions, which is a subject of chemical ©1999 CRC Press LLC kinetics. The reactor size is then determined, based on mass loading to the reactor, reaction rate, and type of reactor. This chapter introduces the mass balance concept, which is the basis for process design. Then it presents reaction kinetics as well as types, configu-ration, and sizing of reactors. From this chapter you will learn how to determine the rate constant, removal efficiency, optimal arrangement of reac-tors, required residence time, and reactor size for your specific applications. IV.1 Mass balance concept The mass balance (or material balance) concept serves as abasis for designing environmental engineering systems (reactors). The mass balance concept is nothing but conservation of mass. Matter can neither be created nor destroyed, but it can be changed in form (a nuclear process is one of the few exceptions). The fundamental approach is to show the changes occurring in the reactor by the mass balance analysis. The following is a general form of a mass balance equation: Rateofmass Rateofmass Rateofmass  Rateofmass  ACCUMULATED  IN   OUT   DESTROYED  [Eq. IV.1.1] Performing a mass balance on an environmental engineering system is just like balancing your checkbook. The rate of mass accumulated (or depleted) in a reactor can be viewed as the rate that money is accumulated (or depleted) in your checking account. How fast the balance changes depends on how much and how often the money is deposited and/or with-drawn (rate of mass input and output), interest accrued (rate of mass gen-erated), and bank charges for monthly service and ATM fees imposed (rate of mass destroyed). In using the mass balance concept to analyze an environmental engi-neering system, one usually begins by drawing a process flow diagram and employing the following procedure: Step 1: Draw system boundaries or boxes around the unit processes/op-erations or flow junctions to facilitate calculations. Step 2: Place known flow rates and concentrations of all streams, sizes and types of reactors, and operating conditions such as temper-ature and pressure on the diagram. Step 3: Calculate and convert all known mass inputs, outputs, and ac-cumulation to the same units and place them on the diagram. Step 4: Mark unknown (or the ones to be solved) inputs, outputs, and accumulation on the diagram. ©1999 CRC Press LLC Step 5: Perform the necessary analyses/calculations using the proce-dures described in this chapter. A few special cases or reasonable assumptions would simplify the gen-eral mass balance equation, Eq. IV.1.1, and make the analysis easier. Three common ones are presented below: a. No Reactions Occurring: If the system has no chemical reactions occur-ring, such as a mixing process, there is no increase or decrease of compound mass due to reactions. The mass balance equation would become  Rateofmass  Rateofmass Rateofmass ACCUMULATED  IN   OUT  [Eq. IV.1.2] b. Batch Reactor: For a batch reactor, there is no input into or output out of the reactor. The mass balance equation can be simplified into  Rateofmass  ACCUMULATED = ±GENERATEDor [Eq. IV.1.3]   Examples of using Eq. IV.1.3 will be provided in later sections of this chapter. c. Steady-State Conditions: To maintain the stability of treatment process-es, treatment systems are usually kept under steady-state conditions after a start-up period. A steady-state condition basically means that flow and concentrations at any location within the treatment process train are not changing with time. Although the concentration and/or flow rate of the influent waste stream entering a soil/groundwater system typically fluctuate, engineers may want to incorporate devices such as equalization tanks to dampen the fluctuation. This is espe-cially true for treatment processes that are very sensitive to fluctuation of mass loading (biological processes are good examples). For a reactor under a steady-state condition, although reactions are occurring inside the reactor, the rate of mass accumulation in the reactor would be zero. Consequently, the left-hand side term of Eq. IV.1.1 becomes zero. The mass balance equation can then be reduced to 0 = Rateofmass − Rateofmass ± GENERATEDor [Eq. IV.1.4]  DESTROYED  ©1999 CRC Press LLC Assumption of steady-state is frequently used in the analysis of flow reactors, and examples of using Eq. IV.1.4 will be provided in later sections of this chapter. The general mass balance equation, Eq. VI.1.1, can also be expressed as V dC = åQinCinQ CV out out ±( ´ g) [Eq. IV.1.5] where V is the volume of the system (reactor), C is the concentration, Q is the flow rate, and g is the reaction rate. The following sections will demon-strate the role of the reaction in the mass balance equation and how it affects the reactor design. Example IV.1.1 Mass balance equation — air dilution (no chemical reaction occurring) A glass bottle containing 900 mL of methylene chloride (CH Cl , specific gravity = 1.335) was accidentally left uncapped in a poorly ventilated room (5 m ´ 6 m ´ 3.6 m) over a weekend. On the following Monday it was found that two thirds of methylene chloride had volatilized. For a worst-case sce-nario, would the concentration in the room air exceed the permissible expo-sure limit (PEL) of 100 ppmV? An exhaust fan (Q = 200 ft3/min) was turned on to vent the fouled air in the laboratory. How long will it take to reduce the concentration down below the PEL? Stategy. This is a special case (no reactions occurring) of the general mass balance equation. For this case Eq. IV.1.5 can be simplified into V dC = åQinCinQ C out out [Eq. IV.1.6] The equation can be further simplified with the following assumptions: 1. The air leaving the laboratory is only through the exhaust fan and the air ventilation is equal to the rate of air entering the laboratory (Q = Q = Q) 2. The air entering the laboratory does not contain methylene chloride (C = 0). 3. The air in the laboratory is fully mixed, thus the concentration of methylene chloride in the laboratory is uniform and is the same as that of the air vented by the fan (C = Cout). V dC = −QC [Eq. IV.1.7] ©1999 CRC Press LLC ... - tailieumienphi.vn