ChE 416 - Unit Operations Lab II 
Dr. Russell Chen 
Kinetics Experiment

Background (Return to top)

Your company has recently begun to manufacture Crystal Violet Dye (CVD).  In polar solvents, it forms a bright purple solution.  The wastewater stream from the process (2000 gallons per day at 70 °F and pH 7.0) contains 5 x 10^-5 M CVD.  Your plant has an environmentally-friendly biological treatment facility for wastewater, which consists of a series of outdoor ponds.  These ponds are populated with microscopic organisms that metabolize CVD and similar molecules, releasing CO₂ and N₂O.  Although this system has been approved by the appropriate regulatory agencies, the company is concerned about public perception, since the treatment ponds nearest the plant will no doubt be bright purple.  You have been asked to propose a process to decolor the wastewater before it is discharged to the treatment ponds.  Discharge to the ponds must have a temperature between 40 and 90 °F and a pH between 6 and 8 to preserve the microorganism population.

Internal company literature provides the following information.
Crystal Violet Dye (CVD) is a dark powder of formula weight 408 g/mole.  CVD in aqueous solution strongly absorbs light at 592 nm, allowing the use of UV/Vis spectroscopy to measure concentrations below about 2 x 10^-5  M.  Below about 2 x 10^-5 M. , the concentration and the absorbance of the solution are directly related. (Absorbance is defined as -log(fraction of light transmitted).) At higher concentrations, the solution is practically opaque to the available UV/Vis instrument. A BASIC program called "CALIB" has been prepared to convert absorbance to concentration; it has been thoroughly tested for absorbances between 0.2 and 1.9.  In aqueous solution, OH- reacts with CVD, turning the purple solution clear.

A research engineer tested the reaction of CVD with NaOH.  He was summarizing his findings in an e-mail memo, and had just written,

"The reaction CVD + NaOH ---> Products has been studied using an excess of NaOH, resulting in kinetics that are apparently first order in both reactants.  The reaction rate coefficient at room temperature is  "

when he and his laboratory notebooks were tragically consumed by spontaneous human combustion.  It happens sometimes, people just explode -  natural causes.

Produce a preliminary design for a reactive process to decolor the wastewater stream for discharge into the treatment ponds.  Include all operations necessary to make the specified wastewater stream meet the criteria for discharge to the bioponds. Select from the three ideal reactor types (plug flow, continuous stirred tank, stirred batch) for your design and justify your selection. Determine the information you need about the CVD reaction with NaOH by experiment.

Available equipment includes a jacketed, stirred reactor (3 liter total volume) that may be run in either batch or continuous mode, a diode-array UV/Vis spectrometer with a flow-through cell, a hand-held pH meter and a constant-temperature water bath.  Available supplies include distilled water, solid CVD, 1 N NaOH in aqueous solution, typical laboratory glassware and a balance accurate to 0.0001 g.  You may also use any other equipment or supplies available in the unit operations laboratory.

• A thorough understanding of the relationship between intrinsic properties of a reaction (rate coefficient, reaction order, Arrhenius parameters) and the equations describing the operation of idealized reactors (stirred batch, continuous stirred tank, plug flow).
• Careful planning of experiments to efficiently obtain both results and a quantitative measure of their accuracy.
• Experimental technique.  The bright purple CVD provides ample evidence if you are careless in the laboratory.

These are in addition to the standard guidelines in the syllabus.
 

Introduction

Include a rate equation for the reaction, written as a differential equation in [CVD] (i.e., the left side of the equation should be d[CVD]/dt, where [CVD] is the concentration of crystal violet dye, and t is time). How are the parameters in this equation used to design a reactor?
 

Experimental Methods

Remember that the late research engineer's results were preliminary, and consider the possibility that they are incorrect or incomplete.  It is best to use excess NaOH, as he did.  Be prepared to explain why.  How can you verify that the reaction is first order in both CVD and NaOH? Consider how best to measure each component of the reactor feed to minimize experimental error.  CVD is a very impressive dye.  Do not use plastic labware.  Glassware can be cleaned by rinsing thoroughly with water, then pouring in methanol and letting it sit for a few minutes, then rinsing with water again.  Make sure your experimental plan allows time at the end of lab to clean up.  Do not include the step-by-step procedure for UV/Vis operation.  Assume anyone trying to repeat your work can read these instructions.  Do specify anything which requires your decision.  When will you collect a blank?  What cycle time will you choose? Your test matrix should show the concentrations of the reactants for each run and the amounts of each reactant added for each run. Typically, students are able to complete 5-6 batch runs per lab period.

Expected Data and Results

Include a sketch of expected UV/Vis data (absorbance vs. reaction time) for a typical batch experiment.  I will ask you what the units on the rate coefficient are.  I will ask you how your data will be affected by changes in temperature, reactant concentration, and reaction order.

These are in addition to the standard guidelines in the syllabus.

Present a flow diagram of your process and a brief description.  Present a block diagram, not a detailed P&ID.  Include all operations necessary to make the specified wastewater stream meet the criteria for discharge to the bioponds. Label all the streams with their approximate temperature, flow rate, and composition.  Specify the type(s) and size(s) of reactor(s) required.  Select from the three ideal reactor types (plug flow, continuous stirred tank, stirred batch) for your design and justify your selection. Details of other equipment need not be specified.  (For example, if your process needs a pump, include a pump in your diagram, but you do not need to specify the type of pump, the power, etc.)  Assume the reactors represent the major contribution to capital costs for this project.

1. The reactor can be run in batch or CSTR mode. In theory, it should be possible to collect more data more quickly in CSTR mode. In practice, students have been unsuccessful in achieving and maintaining steady state in CSTR mode.
• If you want to run in CSTR mode, it is best to pump the feeds and maintain the level of the reaction mixture with a siphon. There are calibration curves available for the pumps.
• If you want to run in batch mode, take precautions to prevent carryover of reaction mixture from one run to the next. Also, think carefully about how to mix the reactants so that the effect of dilution does not confound your observations of concentration changes due to reaction
2. Remember that the rate expression includes the concentrations in the reactor, not the concentrations of the feed solutions.
3. Assume that the reaction between CVD and NaOH is an elementary reaction.
4. CALIB is unreliable for absorbances outside the range 0.2 - 1.9. Within the range 0.2 - 1.9, you may assume the uncertainty on concentrations from CALIB is +/- 2 %. Use the CVD concentrations you get from CALIB rather than the ones you might calculate from the volumes added to the reactor.
5. Remember that reaction is happening in the flow cell and its associated tubing, as well as in the reactor. When the residence time in the flow cell becomes significant compared to the rate of reaction, the data become unreliable. In this case, you must change the reaction conditions to slow down the reaction.
• As a rule of thumb for a CSTR, the average residence time in the reactor should be at least 5 minutes.
• As a rule of thumb for a batch reactor, it should take at least 5 minutes for the absorbance to fall below 0.2.
6. You cannot quantify CVD concentrations greater than about 2 x 10^-5 M by UV/Vis. Also, experience has shown that NaOH concentrations greater than about 0.05 M lead to trouble. (Why might this be?)
7. An air bubble in trapped in the flow cell can produce a sudden jump in the absorbance. To avoid air bubbles, don't run the sipper pump unless the inlet tube is submerged, and avoid excessive stirrer speed. Gently tapping the flow cell and/or changing its orientation will free a trapped air bubble. Gently is the key. Rough handling will break the seal in the cell and air will leak in constantly.
8. It is nearly impossible to dissolve more than 0.5 g of CVD in a liter of water.
9. Warm up the UV/Vis for 30 minutes before you collect the blank spectrum and start your first run.

(Last modified on 1/3/03)