Notes - Chapter 6.pdf - CIV ENG 2B04 Principles of Environmental Engineering Chapter 6 Reactor Theory 1 Reactor Theory Reactors Fluid containing

Notes - Chapter 6.pdf - CIV ENG 2B04 Principles of...

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Unformatted text preview: CIV ENG 2B04 - Principles of Environmental Engineering Chapter 6 Reactor Theory 1 Reactor Theory Reactors - Fluid containing vessels (water & wastewater treatment processes) - Environmental systems in which fluid is confined (e.g., river, lake, indoor air) Analysis based on mass balance Develop expression for a control volume 2 Mass Balance Nonconservative substance: Accumulation rate = input rate – output rate ± transformation rate The rate of reaction: 3 Batch Reactor Fill-and-draw type No inflow or outflow Unsteady reaction Uniform contents at any instant in time Example: BOD bottle 4 Example A batch reactor is to be designed remove organic compounds from the wastewater that contains organic compounds. A treatability study indicated that the BOD5 level decreased from 300 mg/L on Day 1 to 122 mg/L on Day 10. Assuming a first-order reaction, determine the retention time necessary to achieve a COD removal efficiency of 90%. 5 Solution 6 Example A batch reactor is partially filled with oxygenfree water. The remainder of the reactor contains air with 21% O2. The air and water are independently well mixed. How does the dissolved oxygen content of the water change with time? 7 Solution 8 Notes on Batch Reactor Small reactor volume, easy to operate - e.g., small-scale industrial wastewater treatment Reaction time depends on inflow and outflow characteristics, as well as the reaction kinetics. Is the reaction time dependent on reactor volume? Reaction Order Rate Expression Reaction Time Zero First 9 Completely Mixed Flow Reactor (CMFR) Also called continuous-flow stirred tank reactor (CSTR) Fluid particles entering reactor are perfectly dispersed throughout reactor Fluid exiting reactor has the same concentration as the fluid in the reactor Example: wastewater treatment Inflow Outflow 10 CMFR – Case A No reaction A step increase in inflow concentration Initial and boundary conditions: - t = 0, Cin = 0 - t > 0, Cin = C1 Inflow Cin Outflow Cout 11 Hydraulic Detention Time Detention time or retention time The time that fluid particles remains in the reactor - : Flow rate of reactor : Volume of fluid in reactor The degree to which the reaction goes to completion depends on the hydraulic detention time 12 CMFR – Case B No reaction A step decrease in inflow concentration Initial and boundary conditions: - t = 0, Cin = C1 - t > 0, Cin = 0 Inflow Cin Outflow Cout 13 CMFR – Case C First-order reaction A step increase in inflow concentration Initial and boundary conditions: - t = 0, Cin = 0 - t > 0, Cin = C1 Inflow Cin Outflow Cout 14 CMFR – Case D First-order reaction Steady state conditions Initial and boundary conditions: - Cin = C1 - Inflow Cin Outflow Cout 15 Series of CMFR Replacing one large CMFR with a series of smaller CMFR can help improve the efficiency C0 C1 C1 Assumption: - Steady state conditions - First-order reactions CMFR 1: 1 1 C2 C2 C3 CMFR 2 ?, … , CMFR n? 16 C3 Example Compare the outflow concentration for - a) 1 CMFR; θ = 1.0 d; k = 2 d-1; C0 = 200 mg/L - b) 4 CMFRs; θ = 1.0 d (total); k = 2 d-1; C0 = 200 mg/L 17 Solution 18 Notes on CMFR For relatively large water flow rates (normally > 150 m3/d) Resistant to upsets due to significant changes in influent quality Sufficient mixing is needed in practice Performance can be improved by using CMFR in series 19 Plug Flow Reactor (PFR) Long, narrow reactor “Plug” elements of the fluid No longitudinal mixing Ideally mixed within each element T= t1 T= t2 T= t3 Example: pipe, aeration tank 20 PFR – Mass Balance Perform a mass balance (on species M) on an incremental volume A ∆ A: cross-sectional area of the reactor ∆ : width or thickness of the element Question: why can’t we choose the whole reactor for the control volume, like we did for batch reactors and CMFRs? 21 PFR – Mass Balance (cont’d) ∆ ∆ | | ∆ ∆ ∆ ∆ →0 | ∆ | ∆ ∆ Using steady-state assumption: 22 Example A plug flow reactor is to be used to carry out the reaction M → N. The reaction is first order, and the rate is characterized as rM = kCM . Determine the steady state effluent concentration as a function of hydraulic detention time. 23 Solution 24 Example An activated sludge tank, 9 x 9 x 60 m is designed as a plug flow reactor, with an influent BOD of 200 mg/L and a flow rate of 4 million L/day. a) If BOD removal is a first‐order reaction, and the rate constant is 2.5 d‐1, what is the effluent BOD concentration? b) If the same system operates as a CMFR, what must its volume be (for the same BOD reaction)? How much bigger is this, as a percent of the plug‐flow volume? c) If the plug‐flow system was constructed and found to have an effluent concentration of 27.6 mg/L, the system could be characterized as a series of CMFRs. How many? 25 Solution 26 Notes on PFR Long, narrow reactor with large length-towidth ratios (normally > 50:1 ) Requires a smaller reactor volume compared to a CMFR Weakness: susceptibility to significant change in water quality. E.g., a step change in toxic substances would kill the microorganisms within the reactor. 27 ...
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  • Fall '16
  • Rate equation, Chemical reactor, Batch reactor,  Analysis

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