Prelab

1. Print this datasheet and bring at least one copy per group to the lab.

2. Bring a USB key to this session.

3. Define the terms saturation temperature and saturation pressure and explain them as they relate to the lab below.

Purpose

To compare theoretical and actual saturated temperatures and pressures using a control volume and closed system.

Theory

At standard atmospheric temperature and pressure, water boils at 100°C. Boiling occurs when the vapor pressure of a liquid exceeds the atmospheric pressure. The balance of these pressures relates the rate at which molecules of the fluid leave the surface of the fluid vs rejoin the surface of the fluid. If more molecules are leaving the liquid phase, the substance is boiling, if more are joining the liquid phase then fluid is condensing. The saturation temperature is also known as the boiling temperature, and describes the maximum heat a fluid can contain, hence saturation. A liquid cannot be heated beyond its saturation point without transitioning to the gaseous phase. For water, this temperature is 100°C, boiling water will never exceed 100°C under standard conditions. Any excess energy is carried away in the phase transition from liquid to gas in the steam that leaves the liquid surface.

If we alter the surrounding conditions, in particular, by increasing the ambient pressure, we can also increase the boiling point of the liquid. This is the case because there is a greater opposing pressure balancing the vapor pressure, and molecules are more reluctant to leave the liquid phase. This allows us to heat the water beyond 100°C and thus transfer more heat to whatever is in the water. This is the basic principle behind the use of pressure cookers.

The graph below describes the trend in the saturation temperature of water vs ambient pressure:

In this lab we will verify this relation experimentally. We will also compare the efficiency of this system to to those systems analysed in Lab 1.

Problem

AP Engineering Inc. is developing a reaction vessel for a chemical process proposed by Arnold Chemical. The reactants should reach at least 115 °C in order for the reaction to go to completion. The reactants are in the aqueous phase (dissolved in water) and a scale model of the reaction vessel has been provided to your team for evaluation. Your team has been tasked with determining the pressure requirements required to have a closed system bring water to at least 115°C and to determine the energy used in the process.

Procedure

1. Weigh the pressure cooker pot (without the lid). Weigh the stopper.

2. Fill the pot about 1/3 full with water and weigh the pot and water.

3. Lock the lid in place and place the stopper over the center hole

   • make sure the bare wire of the thermocouple, that is inside the pot, doesn’t touch the bottom of the pot but rests in the water.

4. Place the pot on the burner element.

5. Start your computer along with the pDaq Thermocouple Meter software (if not already done) and:

   • • Confirm the thermocouple types are set correctly (type K).

     • Make sure the temperature of the water in the pot is displayed and record it once it stabilizes.

6. Turn the element to high and start a timer (phone, watch, web etc...).

7. Record the temperature of the water (shown top right) and pressure inside the pot (from the gauge on the lid) every minute on paper.

   • Note that the pressure gauge measures gauge pressure and not absolute pressure.

   • The pressure will not start rising until steam is generated.

   • Because the temperature inside the pot changes quickly, always record this temperature first.

8. Record the electrical power into the hot-plate (Watt meter).

9. Continue to record the temperatures and pressures as the water heats up.

10. Record any qualitative observations as the water approaches the boiling point.

11. Steam will initially escape from the safety lock mechanism (at the base of the handle) but the locking pin should rise and plug the hole.

    • Be careful (the steam is HOT).

12. Continue to record the temperatures and pressures until they stabilize; once they do, turn the element off.

13. Do not try to open the pot, stopper, or otherwise vent the steam. Doing so will result in steam burns.

Results

• Calculate the additional pressure created by the pressure cooker; the diameter of the hole is 3.48mm.

• Plot the temperature versus pressure.

• State the maximum recorded pressure and the corresponding temperature from your lab.

• Calculate the theoretical saturation pressure (P_atm + P_stopper)

  • Find the corresponding saturation temperature from the saturated water tables.

  • Use interpolation if you do not have a value for the pressure you obtained in the lab and comment on the result.

• Calculate the energy required to heat the water from the initial temperature to the final temperature and the energy consumed by the hotplate.

  • Calculate the net efficiency of the hot-plate heating the water as was done in Lab 1.

  • Comment on this efficiency vs the lid on, and lid off trials in Lab 1.

• Is the scale model tested an appropriate choice for the requirements from Arnold Chemical?

  • In what ways would you change the setup to improve performance or capabilities?

• If the process requires a 10% factor of safety, what pressure must the tank be rated for if your scale model is representative of the real world version.

Questions

1. Why might the efficiency of the pressure cooker be higher than that from Lab 1?

2. How would a change in atmospheric pressure (say p_atm was 95 kPa or 105 kPa rather than 101.3kPa) change the saturation temperature and pressure?