High-pressure Sapphire Cell for Phase Equilibria Measurements of CO₂/Organic/Water Systems

The overall goal of this procedure is to evaluate water acetyl nitrile 50 50 phase behavior as a function of CO2 pressure. This is accomplished by first injecting the water acetyl nitrile sample into the sapphire cell. The second step is to pressurize with carbon dioxide.

Next is measuring the volume of each of the phases as a function of CO2 pressure. The final step is to depressurize. Ultimately, a cter is used for volume measurement Compared to sampling methods.

This visual sapphire cell system obtains the composition of each phase of pressure without disturbing the system and therefore the equilibrium. Additionally, this apparatus allows visual insights into the behavior of the system. The sapphire cell is a high pressure apparatus that requires safe handling.

For periodic pressure testing, fill the operating side with water. To initiate a pressure cycle, attach the water filled high pressure syringe pump to the sample inlet connection, and fill the cell completely close the sample inlet valve. Then run the syringe pump so that a few drops of water, leave the high pressure tubing.

Next, attach the tubing to the sapphire cell fitting at the bottom of the sapphire cell. Fill the bottom cell with water to pressurize and monitor the pressure for any possible pressure drop. Gradually increase the pressure to 0.1 megapascals over the setting of the pressure relief valve, collecting the water that is released from the pressure relief valve in a small container.

Now, reduce the pressure to atmospheric. Fill the high pressure syringe pump with water as required by the experimental pressure. Run the high pressure syringe pump, so a few drops of water.

Leave the tubing. Then attach the tubing to the sapphire cell fitting and open the gas inlet valve. Fill the cell with water until the piston level is at a liquid height that may be measured with the cter.

Close the gas inlet valve to pressurize the system. Then attach an airtight syringe to the open sample inlet connection and evacuate the cell by pulling back 10 milliliters. Mass the syringe before addition of the sample.

Then inject a volume of sample using an airtight syringe attached to the sample inlet. Then close the valve if necessary, invert the sapphire cell on the rotating shaft. Now measure the amount of sample by recording the mass of the syringe, accepting a small error associated with sample left in the tubing and fittings.

Next, set the air bath at the desired temperature to equilibrate the sample. Then take a first height measurement with the cter to ensure sample equilibrium was reached. Repeat the measurements until no change is observed for at least three times.

Complete a preliminary study in which the system is observed for an extended period of time. To ensure equilibrium has been achieved, prime the line with carbon dioxide. Add carbon dioxide by first running the high pressure syringe pump to eject any air from the line when it is not attached to the inlet valve.

Now attach the tube to the gas inlet valve. Open the gas inlet valve to the sapphire cell. Measure the amount of CO2 added to the system by recording the volume of the high pressure syringe pump before and after CO2 addition.

Check that the flow rate on the water high pressure syringe pump is zero after equilibrium is reached. To ensure there are no leaks, bring the pressure to the desired value by adjusting the pressurizing fluid. With the high pressure syringe pump, the sapphire cell is then agitated to ensure equilibrium is reached quickly.

Once equilibrium is reached under CO2 pressure, a clear phase split can be seen, and the height of each phase can be measured. Following depressurization, the sample returns to one phase, the measurements recorded using the sapphire cell technique were analyzed using these material balances to construct turny phase diagrams. This turny diagram was obtained with two distinct techniques, a synthetic method using the sapphire cell protocol and an analytical method in which samples of each phase were analyzed separately.

Clearly the data obtained by the two methods are comparable. The teary phase diagram can then be used to calculate phase behavior experimentally. The ability to have a direct visual on the phase behavior is particularly evident here when comparing the solvent system in the absence and in the presence of carbon dioxide.

Don't forget that working with high pressure equipment can be very hazardous, and such as placing a blast shield between the apparatus and the users should always be taken.