CIVL375: Water and Wastewater Technology Experiment 1: Dispersion Coefficient of a Plug Flow Reactor Introduction The design of reactors is a very important aspect in the field of water and wastewater treatment Reactors could be batch, completely stirred, plug-flow, packed bed, or fluidized packed bed Plug flow reactors are classified as ideal plug flow (no dispersion), or dispersed plug flow In practical, the former is rarely found although sometimes assumed to ease the calculations especially if little mixing occurs during the flow In the dispersed plug flow reactors, any input stimuli (e.g a tracer) injected at the inlet will exit from outlet after dispersion Figure shows how an injected pulse of a chemical will appear in the effluent (output) of the reactor When dispersion is zero (σ2=0), the chemical will appear suddenly at the outlet When dispersion is large (large σ2), the chemical spreads and appears at different levels as time elapsed Fig 1: Plug flow reactor and effluent concentration showing different levels of dispersion The dispersion coefficient (D) of a plug flow reactor can be determined experimentally using moment analysis of an injected conservative tracer This is done by determining the effluent concentration versus time after injecting the tracer From such data one can determine the followings: t = The centroid (actual retention time) t C σ2 = ∑ i i −t2 ∑σC2 i σθ = t The variance ∑ tiCi ∑ Ci The normalized variance The normalized variance is related to the dispersion number by σ θ2 = D D − 2( ) (1 − e −uL / D ) uL uL Thus, the dispersion number (D/uL) is determined by trial and error The dispersion coefficient can then be determined given that L and u (=Q/A) are known from the applied experimental conditions Objective The objective of this experiment is to determine the dispersion coefficient (D) of a plug flow reactor in the laboratory Procedure A reactor shown in Fig will be used to conduct the experiment on and determine its dispersion coefficient NaCl water Tap water Inlet Gate L H Outlet Fig 2: Plug flow reactor prototype Determine the dimensions of the reactor (L, W, and H) Use a pump that gives 100 ml/min flow Determine the flow accurately by a graduated cylinder and a stop watch (Q=V/t) Prepare liters of 5000 mg/l NaCl solution using tap water Measure the TDS of the prepared solution Measure the TDS of the tap water alone Fill the reactor with Tap water with the inlet gate closed Fill the inlet tank with the NaCl solution with the inlet gate closed 8 Start injecting the NaCl solution, open the gate and the outlet valve at the same time Also, start the stop watch Take a sample from the effluent every 5-10 minutes and measure TDS 10 Stop injecting solution after 30 minutes by closing outlet valve, stopping the pump, and place the gate back Also stop the watch 11 Empty the inlet tank from the NaCl solution and fill with tap water 12 Attach the pump to the tap water line and continue pumping with tap water, open the gate, and open the outlet valve, continue the stop watch 13 Continue sampling the outlet water for TDS every 5-10 minutes till the outlet concentration reaches that of tap water 14 Correct TDS readings by subtracting the value of TDS for tap water from the readings Requirements: Fill out the provided experiment data sheet and carry out the required calculations If you are requested to submit a report of this experiment then your report should contain (a) an introduction that ends with the objective (b) methodology including sample identification (c) results and discussion (d) conclusion (e) references and (f) an appendix that contains the filled data sheet Experiment Data Sheet Dispersion Coefficient of a Plug Flow Reactor Name ID Experimental conditions Date the experiment was conducted TDS of NaCl solution (mg/l) TDS of tap water (mg/l) Reactor length (L) in cm Reactor width (W) in cm Reactor depth (H) in cm Flow rate (Q) in ml/min Injection duration (min) Effluent concentration (mg/l) versus time (min) after injection Effluent Corrected Effluent Time Time TDS TDS TDS Corrected TDS Required Calculations: Plot the effluent TDS versus time Determine the experimental retention time and compare with the theoretical value Determine the dispersion coefficient of the reactor