Unit Operations Laboratory — Kinetics Experiment
Winter 2008

 

Contact:  Daniel Gulino, gulino@ohio.edu

Background

Your company has recently begun to manufacture crystal violet dye (CVD).  In polar solvents, it forms a bright purple solution.  The continuously-discharged wastewater stream from the process has the following characteristics:

 

Parameter

Mean

Standard Deviation

flow rate, gallons per day

4500

200

temperature, °F

100

3

pH

8.0

0.1

CVD concentration, molar

7.0 x 10-4

0.3 x 10-4


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 if the discharge to the ponds is bright purple.  You have been asked to propose a process to decolorize 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 6.5 and 7.5 to preserve the microorganism population.

 

Internal company literature provides the following information:


Crystal Violet Dye (CVD) is a dark powder of formula weight 408.0 g/mole.  CVD in aqueous solution strongly absorbs light at 592 nm, allowing the use of ultraviolet-visible (UV/Vis) spectroscopy to measure concentrations below about 2.5 x 10–5  M.  In aqueous solution, OH– reacts with CVD, turning the purple solution clear. The reaction is believed to be first-order in both CVD and OH–.

A recently-deceased research engineer was testing the reaction of CVD with NaOH in water.  Her laboratory notebook was also consumed in the spontaneous combustion event which tragically ended her life. However, she left two Excel files: "Calibration December 2001.xls" and "Experiment December 2001.xls".  It is suggested that you use the information in these files to help you design your experiments.

 

suggested references:    (1)  Turgeon, J. D., and LaMer, V. K., J Amer. Chem. Soc., 74, 5988 (1952).

                                      (2)  Corsaro, G., J. Chem Ed., 50(8), 575 (1973).

 

 

Design Objective

 

Produce a preliminary design for a reactive process to use NaOH to decolor the wastewater stream for discharge into the treatment ponds.  Include all operations/equipment 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.  The cost of the reactor(s) should be a major consideration.  Specify the approximate size of the reactor and basic operating variables such as temperature, pressure, feed flow rate, and feed composition.  (You do not need to size the other equipment in your proposed process.)  In order to produce this preliminary design, you will need some quantitative information about the CVD + NaOH reaction. You should decide what this information is (your experimental objective) and determine it experimentally.

 

 

Equipment and Supplies

 

Available experimental equipment includes a jacketed, stirred reactor (3 liter total volume) that is to be run in continuous mode, a diode-array UV/Vis spectrometer with a flow-through cell, and a constant-temperature water bath.  Reaction mixture may be continuously withdrawn from the reactor, passed through the spectrometer cell, and returned to the reactor using small-diameter Tygon tubing and a "sipper" pump supplied for this purpose.  Available supplies include tap water, distilled water, solid CVD, 1 N NaOH in aqueous solution, methanol, 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.

 

Emphasis

  • 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.

 

Prelab Guidelines

 

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). Also include an appropriate design equation for a continuous reactor showing how the rate constant can be calculated.  Discuss how the parameters in this equation are used to design a reactor.

·  Experimental Methods
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 with water, then with methanol and then with water again.  Allow 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.  Include the concentrations of the reactants, temperature, flow rates, and reactor volume for each run in your test matrix.

·  Expected Data and Results
Include a plot of [CVD] vs. time for each run until steady-state is achieved.  With the steady-state concentration and the other reactor parameters, you should be able to determine the rate constant.  Run at several different temperatures, you should be able to determine the Arrhenius equation parameters necessary to calculate the rate constant at any temperature. 

·  Prelab Meeting
Significant sample calculations are expected. Remember that, in continuous mode, significant quantities of reactant can be consumed.  You might need to do some literature searching to come up with a possible magnitude for the rate constant so as to be able accurately predict needed flow rates, concentrations, and residence times.  Bring your calculations to the prelab meeting.

 

Hints

  1. The reactor can be run in batch or CSTR mode.  For this experiment, you are to run in CSTR mode.
  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 elementary.
  4. 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 is significant compared to the reactant lifetime, the data become unreliable, because a significant portion of the reaction happens in the tubing instead of in the stirred tank reactor. In this case, you must change the reaction conditions to slow down the reaction.  (You cannot change the flow through the tubing.)
    • As a rule of thumb for batch mode, it should take at least 5 minutes for the absorbance to fall from ca. 2 to ca. 0.1. Experience has shown that NaOH concentrations greater than about 0.05 M lead to trouble.
  5. 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.  (Set the stirrer at about 10). 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. Leave the UV/Vis running while you dislodge the air bubble. You will still be able to use the data from before and after the air bubble appeared.
  6. It is nearly impossible to dissolve more than 0.5 g of CVD in a liter of water.


 

 

(Last modified by D. Gulino on January 7, 2008)