ArticleslgStudy

physics

Zeotropic mixture

Zeotropic mixture is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Zeotropic mixture rather than just read about it. In short: A zeotropic mixture, or non-azeotropic mixture, is a mixture with liquid components that have different boiling points. For example, a mixture of nitrogen, methane, ethane, propane, and isobutane constitutes a zeotropic mixture.

Zeotropic mixture — main illustration
Zeotropic mixture — illustration

Key takeaways

  • Zeotropic mixture belongs to physics; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Zeotropic mixture to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Zeotropic mixture from memory before moving on to harder problems.

Reference excerpt

A zeotropic mixture, or non-azeotropic mixture, is a mixture with liquid components that have different boiling points. For example, a mixture of nitrogen, methane, ethane, propane, and isobutane constitutes a zeotropic mixture. Individual substances within the mixture do not evaporate or condense at the same temperature as one substance. In other words, the mixture has a temperature glide, as the phase change occurs in a temperature range of about four to seven degrees Celsius, rather than at a constant temperature. On temperature-composition graphs, this temperature glide can be seen as the temperature difference between the bubble point and dew point. For zeotropic mixtures, the temperatures on the bubble (boiling) curve are between the individual component's boiling temperatures. When a zeotropic mixture is boiled or condensed, the composition of the liquid and the vapor changes according to the mixtures's temperature-composition diagram. Zeotropic mixtures have different characteristics in nucleate and convective boiling, as well as in the organic Rankine cycle. Because zeotropic mixtures have different properties than pure fluids or azeotropic mixtures, zeotropic mixtures have many unique applications in industry, namely in distillation, refrigeration, and cleaning processes.

Dew and bubble points

In mixtures of substances, the bubble point is the saturated liquid temperature, whereas the saturated vapor temperature is called the dew point. Because the bubble and dew lines of a zeotropic mixture's temperature-composition diagram do not intersect, a zeotropic mixture in its liquid phase has a different fraction of a component than the gas phase of the mixture. On a temperature-composition diagram, after a mixture in its liquid phase is heated to the temperature at the bubble (boiling) curve, the fraction of a component in the mixture changes along an isothermal line connecting the dew curve to the boiling curve as the mixture boils. At any given temperature, the composition of the liquid is the composition at the bubble point, whereas the composition of the vapor is the composition at the dew point. Unlike azeotropic mixtures, there is no azeotropic point at any temperature on the diagram where the bubble line and dew lines would intersect. Thus, the composition of the mixture will always change between the bubble and dew point component fractions upon boiling from a liquid to a gas until the mass fraction of a component reaches 1 (i.e. the zeotropic mixture is completely separated into its pure components). As shown in Figure 1, the mole fraction of component 1 decreases from 0.4 to around 0.15 as the liquid mixture boils to the gas phase.

Temperature glides Temperature glide is the difference between the bubble point and the dew point. Different zeotropic mixtures have different temperature glides. For example, zeotropic mixture R152a/R245fa has a higher temperature glide than R21/R245fa. A larger gap between the boiling points creates a larger temperature glide between the boiling curve and dew curve at a given mass fraction. However, with any zeotropic mixture, the temperature glide decreases when the mass fraction of a component approaches 1 or 0 (i.e. when the mixture is almost separated into its pure components) because the boiling and dew curves get closer near these mass fractions. A larger difference in boiling points between the substances also affects the dew and bubble curves of the graph. A larger difference in boiling points creates a larger shift in mass fractions when the mixture boils at a given temperature.

Zeotropic vs. azeotropic mixtures

Azeotropic and zeotropic mixtures have different dew and bubble curves characteristics in a temperature-composition graph. Namely, azeotropic mixtures have dew and bubble curves that intersect, but zeotropic mixtures do not. In other words, zeotropic mixtures have no azeotropic points. An azeotropic mixture that is near its azeotropic point has negligible zeotropic behavior and is near-azeotropic rather than zeotropic. Zeotropic mixtures differ from azeotropic mixtures in that the vapor and liquid phases of an azeotropic mixture have the same fraction of constituents. This is due to the constant boiling point of the azeotropic mixture.

Boiling When superheating a substance, nucleate pool boiling and convective flow boiling occur when the temperature of the surface used to heat a liquid is higher than the liquid's boiling point by the wall superheat.

Nucleate pool boiling The characteristics of pool boiling are different for zeotropic mixtures than that of pure mixtures. For example, the minimum superheating needed to achieve this boiling is greater for zeotropic mixtures than for pure liquids because of the different proportions of individual substances in the liquid versus gas phases of the zeotropic mixture. Zeotropic mixtures and pure liquids also have different critical heat fluxes. In addition, the heat transfer coefficients of zeotropic mixtures are less than the ideal values predicted using the coefficients of pure liquids. This decrease in heat transfer is due to the fact that the heat transfer coefficients of zeotropic mixtures do not increase proportionately with the mass fractions of the mixture's components.

Convective flow boiling Zeotropic mixtures have different characteristics in convective boiling than pure substances or azeotropic mixtures. Overall, zeotropic mixtures transfer heat more efficiently at the bottom of the fluid, whereas pure and azeotropic substances transfer heat better at the top. During convective flow boiling, the thickness of the liquid film is less at the top of the film than at the bottom because of gravity. In the case of pure liquids and azeotropic mixtures, this decrease in thickness causes a decrease in the resistance to heat transfer. Thus, more heat is transferred and the heat transfer coefficient is higher at the top of the film. The opposite occurs for zeotropic mixtures. The decrease in film thickness near the top causes the component in the mixture with the higher boiling point to decrease in mass fraction. Thus, the resistance to mass transfer increases near the top of the liquid. Less heat is transferred, and the heat transfer coefficient is lower than at the bottom of the liquid film. Because the bottom of the liquid transfers heat better, it requires a lower wall temperature near the bottom than at the top to boil the zeotropic mixture.

… excerpt ends here. Continue reading the full article.

Illustrations

Zeotropic mixture: Figure 2: Bubble and Dew Curves for Zeotropic Mixtures
Figure 2: Bubble and Dew Curves for Zeotropic Mixtures
Zeotropic mixture: Figure 3 Distillation Column. Feed mixture enters from the middle of the column. Low-boiling component collects in the top rectifying section, while high-boiling component collects in the bottom stripping section.
Figure 3 Distillation Column. Feed mixture enters from the middle of the column. Low-boiling component collects in the top rectifying section, while high-boiling component collects in the bottom stripping section.

Worked examples

Example 1 — a first encounter with Zeotropic mixture

Start with the simplest possible case. Write down what Zeotropic mixture claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Zeotropic mixture before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Zeotropic mixture ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Zeotropic mixture

In research
Zeotropic mixture appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Zeotropic mixture in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Zeotropic mixture is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemical engineering thermodynamics, so understanding it makes those chapters shorter.
In everyday life
Look for Zeotropic mixture outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Zeotropic mixture” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Zeotropic mixture in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Zeotropic mixture means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Zeotropic mixture out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Zeotropic mixture in simple terms?

A zeotropic mixture, or non-azeotropic mixture, is a mixture with liquid components that have different boiling points. For example, a mixture of nitrogen, methane, ethane, propane, and isobutane constitutes a zeotropic mixture.

Why does Zeotropic mixture matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Zeotropic mixture?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Zeotropic mixture.

Tags

  • Chemical engineering thermodynamics

Keep exploring