Crystallization of Salicylic Acid via Chemical Modification

Organic (sodium salicylate, NaSAL) and acid (sulfuric acid, 0.25 M = 0.50 N) solutions will be fed to the crystallizer. Make sure to wear latex gloves when handling NaSAL, salicylic acid or their solutions, and the 0.25 M sulfuric acid.

The entire system is controlled from a PC using a commercial distributed controller with an interface similar to the one in Figure 1. All on-off or 3-way solenoid valves and controller set points can be operated and changed using this interface. A schematic shows trends of the analog values (flow rates, temperature) associated with the unit.

1. Starting up the Crystallizer

At the start of a run, all continuous controllers should be in manual mode, and all solenoid valves should be either closed (on-off) or in recycle (3-way) mode.

1. Be sure the crystallizer is full to the overflow level, (~4.15 L) indicated on the stirred tank, with water and salicylic acid slurry (in crystallization, the slurry is often called "magma"). If not full, add these through the addition port.
2. Turn on the agitator for the crystallizer and thethermostated for the bath and pumps.
3. Set the temperature controller for the bath temperature to AUTO and the setpoint to the desired temperature. The recommended temperature is ~53°C for a 50°C crystallizer.
4. Set the pump speeds using the interface (e.g., 30% open). Knowing the concentrations of the feeds, set the flow rates for stoichiometric equivalence based on Equation (1).
5. Confirm that the product tank is not full and that the drain valve is closed.
6. Power up the spectrometer equipment and establish communication with a link provided in the control console. Spectrometer procedures are detailed in the operating manual (SpectraSuite). Calibration of the spectrometer is provided.

2. Operating the Crystallizer

1. Raise the pump output as necessary to achieve the desired flow rates. For the acid solution, this is ~25-35 mL/min. For the NaSAL, it is determined by stoichiometric equivalence.
2. Switch to feed mode on both 3-way valves. This is time zero for an experiment.
3. Periodically check the overflow line. Under certain conditions it may block up. If so, use a piece of steel tubing to ream out the line entering the product tank.
4. Collect five samples directly from the crystallizer through the sample port using a wide-mouth pipette and transfer them to 15 mL test or centrifuge tubes. Take two sets of samples about 10-15 min apart.
5. Repeat at two other widely-spaced residence times, controlling (residence time) by varying the volumetric flow rates, but maintaining stoichiometric equivalence.

3. Shutting Down the Crystallizer

1. To shut down the system, set the 3-way valves to recycle and the pump outputs to 0 %.
2. Return the temperature controller to MANUAL at 0% output, and shut off the pumps, agitators and thermostated bath.
3. If using the spectrophotometer, remember to shut off the lamps.

4. Analysis

Dissolved NaSAL and salicylic acid concentrations can be measured simultaneously by UV/Vis spectroscopy.The absorbances of dissolved salicylate and salicylic acid can be assumed additive because the same chromophore is observed. Further instructions are included in Appendix A. The salicylic acid concentration can also be determined gravimetrically in units of kg/m³ slurry.

1. Centrifuge the 15 mL tubes for 5 min and record the volume of liquid sample retrieved by decanting. The decanted liquid can be used for NaSAL spectrophotometric analysis.
2. Dry the test tubes containing the solids upright in the convection oven at 70ºC, for two days.
3. Weigh, clean out the tubes, then re-dry briefly before re-weighing to get the weight of the crystals.