Practical Uses of Phase Diagrams – Phase Diagrams (Senior 5 Chemistry) | Ganzaa

Phase Diagrams

Lessons

Senior 5 · Chemistry

Phase Diagrams

4 lessons

Lesson 4 of 4

Practical Uses of Phase Diagrams

25 min

Engineering with phase boundaries

A hospital autoclave sterilises surgical instruments with steam at 121 °C, a temperature that boiling water cannot reach in an open pot. A coffee factory turns brewed coffee into dry granules without ever boiling it. A cooking-gas cylinder holds liquid fuel at room temperature, while methane has to be compressed or chilled to −162 °C. Each of these technologies is a deliberate move across, or along, a line on a phase diagram.

Definition

Gauge pressure: pressure measured above the surrounding atmospheric pressure. Absolute pressure = gauge pressure + atmospheric pressure. Phase diagrams use absolute pressure.

Freeze-drying (lyophilisation): removing water from a frozen material by sublimation at a pressure below the triple point of water.

Supercritical fluid extraction: dissolving a substance out of a material using a fluid above its critical temperature and pressure, most often carbon dioxide.

1. Pressure cookers and autoclaves: moving along the boiling curve

Sealing the vessel lets the vapour pressure build up. Water boils when its vapour pressure equals the pressure inside, so higher pressure means a higher boiling temperature, read off the vaporisation curve. Food cooks faster, and bacterial spores that survive 100 °C are killed in an autoclave at 121 °C, where water's vapour pressure is about 205 kPa (steam tables).

2. Freeze-drying: going around the triple point

Figure 10.4: Freeze-drying on the water phase diagram. Below the triple-point pressure, warming takes ice straight to vapour.

The material is frozen (A → B), the pressure is lowered below 611 Pa (B → C), and gentle heat is applied (C → D). The ice sublimes and the vapour is pumped away and caught on a very cold condenser. Because no liquid water ever forms and the temperature stays low, the product keeps its shape, flavour and aroma, and it rehydrates quickly. Freeze-drying is used for instant coffee, some vaccines and medicines, blood plasma, and food for expeditions.

Freeze-dried instant coffee granules. Brewed coffee is frozen, broken into granules and dried by sublimation, which leaves a light, porous solid that dissolves quickly in hot water. Photo: Sprogz, CC BY 2.0, via Wikimedia Commons

3. Supercritical carbon dioxide: using the critical point

Above 31.0 °C and 7.38 MPa, carbon dioxide is a supercritical fluid. It flows into a porous solid like a gas but dissolves organic substances like a liquid, and its dissolving power can be tuned by changing the pressure. In decaffeination, moist green coffee beans are soaked in supercritical $\mathrm{CO_2}$ , which dissolves caffeine but leaves most of the flavour compounds. When the pressure is released, the $\mathrm{CO_2}$ turns back into gas and leaves no solvent residue. The same method extracts hop oils for brewing, and flavours and fragrances from plants. Carbon dioxide is cheap, non-flammable and non-toxic in small amounts, and its low critical temperature protects heat-sensitive compounds.

4. Liquefying gases: the critical temperature decides

Figure 10.5: A gas can be liquefied by pressure alone only below its critical temperature (critical temperatures from the NIST Chemistry WebBook).

Propane and butane (liquefied petroleum gas, LPG) have critical temperatures far above room temperature. A few hundred kilopascals is enough to liquefy them, so a cooking-gas cylinder holds a lot of fuel as liquid, with vapour above it at the vapour pressure.
Methane (natural gas, also the gas extracted from the deep water of Lake Kivu) has a critical temperature of −82.6 °C. No pressure can liquefy it at room temperature; it is piped as gas, stored compressed, or cooled to −162 °C to make liquefied natural gas.
Oxygen and nitrogen must be cooled below −118.6 °C and −147.0 °C before pressure can liquefy them. A hospital oxygen cylinder therefore contains compressed gas, not liquid.

5. Other uses

Dry ice as a refrigerant for vaccines, samples and food in transport: it sublimes at −78.5 °C, leaving no liquid.
Snow and frost: deposition (gas → solid) on cold nights below 0 °C at high altitude.
Alloys: phase diagrams with two components show, for example, that a tin–lead solder of a particular composition melts sharply at 183 °C.

Worked example

Problem. An autoclave must hold steam at 121 °C, where the vapour pressure of water is 205 kPa. What gauge pressure must it show (a) in Kigali, where atmospheric pressure is about 84 kPa; (b) at sea level, 101 kPa?

Solution. The steam must be at an absolute pressure of 205 kPa. Gauge pressure = absolute − atmospheric.

(a) $205 - 84 = 121\,\mathrm{kPa}$ ; (b) $205 - 101 = 104\,\mathrm{kPa}$ .

Answer: 121 kPa in Kigali, 104 kPa at sea level. Setting a Kigali autoclave to a sea-level gauge reading would give steam below 121 °C, and sterilisation could fail. Equipment instructions should always say whether a pressure is absolute or gauge.

Worked example

Problem. Frozen coffee extract is held at −20 °C in a freeze-drier. The vapour pressure of ice at −20 °C is 103 Pa. (a) What is the highest chamber pressure at which the ice will sublime? (b) Why must the chamber pressure be kept below 611 Pa throughout the process?

Solution. (a) Ice sublimes when its vapour pressure exceeds the pressure of water vapour around it, so the pressure must be held below 103 Pa; in practice it is held well below, so that sublimation is fast. (b) 611 Pa is the triple-point pressure. Above it, warming the ice would cross the melting curve and form liquid water, which would collapse the porous structure and spoil the flavour.

Warning

Pressure vessels are dangerous if misused. Never block the valve of a pressure cooker or open it while it is still under pressure: superheated water can flash into steam and cause severe scalds. Gas cylinders must be stored upright, away from heat: as temperature rises, the vapour pressure of the liquid inside rises steeply.

First aid for scalds (CDC Stacks, First Aid for Burns): get away from the heat; run cool water slowly over the burn for several minutes; cover it with a clean, dry cloth. Do not apply ice, butter, creams or home remedies, and do not break blisters. Get medical help at once if the burn is large, blisters, or is on the face, eyes, hands or feet.

Tip

For any "use of phase diagrams" question, name the line or point involved: pressure cooker, the vaporisation curve; freeze-drying, the triple point and the sublimation curve; supercritical extraction, the critical point; liquefying a gas, the critical temperature.

Applied Science: A close look at supercritical carbon dioxide. Watch dry ice melt to a liquid under pressure in a home-built cell, then watch the liquid–gas boundary disappear as the cell is warmed past the critical point.

✦

Practice

Explain why food cooks faster in a pressure cooker than in an open pot.
The vapour pressure of water at 115 °C is 169 kPa. A pressure cooker in Musanze (atmospheric pressure 81 kPa) is designed to cook at 115 °C. Calculate the gauge pressure it must reach.
Give two reasons why supercritical $\mathrm{CO_2}$ is preferred to dichloromethane for decaffeinating coffee.
Use Figure 10.5 to list the gases that can be liquefied at 25 °C by pressure alone.
Explain why ice must not be allowed to melt during freeze-drying.
A cooking-gas cylinder of butane and a cylinder of methane, both at 25 °C, show pressures of 0.24 MPa and 20 MPa. Explain the large difference.
A company plans to sell liquid oxygen in ordinary steel gas cylinders kept at room temperature. Evaluate this plan.

Show answersHide answers

The pressure inside is above atmospheric, so water boils at a higher temperature (about 118–120 °C); chemical changes in food are faster at higher temperature.
$169 - 81 = 88\,\mathrm{kPa}$ .
Any two: $\mathrm{CO_2}$ leaves no toxic solvent residue; it is non-flammable and cheap; its low critical temperature (31.0 °C) avoids damaging flavour compounds; its solvent power can be tuned by pressure.
Carbon dioxide, propane, ammonia and butane (critical temperatures above 25 °C).
Liquid water would dissolve and move soluble substances and collapse the porous structure; the product would shrink, lose aroma and rehydrate slowly. Sublimation leaves the structure intact.
Butane is below its critical temperature, so it is liquid and the pressure is only its vapour pressure (about 0.24 MPa at 25 °C). Methane is above its critical temperature, so it cannot liquefy; a useful amount can be stored only by compressing the gas to a high pressure.
The plan cannot work: room temperature is far above oxygen's critical temperature (−118.6 °C), so oxygen cannot be liquid in such a cylinder at any pressure. Liquid oxygen must be kept below −118.6 °C, in insulated (vacuum) flasks.

1Engineering with phase boundaries

21. Pressure cookers and autoclaves: moving along the boiling curve

32. Freeze-drying: going around the triple point

43. Supercritical carbon dioxide: using the critical point

54. Liquefying gases: the critical temperature decides

65. Other uses