Zeotropic mixture
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 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 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. 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
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 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.Heat transfer coefficient
From low cryogenic to room temperatures, the heat transfer coefficients of zeotropic mixtures are sensitive to the mixture's composition, the diameter of the boiling tube, heat and mass fluxes, and the roughness of the surface. In addition, diluting the zeotropic mixture reduces the heat transfer coefficient. Decreasing the pressure when boiling the mixture only increases the coefficient slightly. Using grooved rather than smooth boiling tubes increases the heat transfer coefficient.Distillation
The ideal case of distillation uses zeotropic mixtures. Zeotropic fluid and gaseous mixtures can be separated by distillation due to the difference in boiling points between the component mixtures. This process involves the use of vertically-arranged distillation columns.Distillation columns
When separating zeotropic mixtures with three or greater liquid components, each distillation column removes only the lowest-boiling point component and the highest boiling point component. In other words, each column separates two components purely. If three substances are separated with a single column, the substance with the intermediate boiling point will not be purely separated, and a second column would be needed. To separate mixtures consisting of multiple substances, a sequence of distillation columns must be used. This multi-step distillation process is also called rectification.In each distillation column, pure components form at the top and bottom of the column when the starting liquid is released in the middle of the column. This is shown in Figure 2. At a certain temperature, the component with the lowest boiling point vaporizes and collects at the top of the column, whereas the component with the highest boiling point collects at the bottom of the column. In a zeotropic mixture, where more than one component exists, individual components move relative to each other as vapor flows up and liquid falls down.
The separation of mixtures can be seen in a concentration profile. In a concentration profile, the position of a vapor in the distillation column is plotted against the concentration of the vapor. The component with the highest boiling point has a max concentration at the bottom of the column, where the component with the lowest boiling point has a max concentration at the top of the column. The component with the intermediate boiling point has a max concentration in the middle of the distillation column. Because of how these mixtures separate, mixtures with greater than three substances require more than one distillation column to separate the components.
Distillation configurations
Many configurations can be used to separate mixtures into the same products, though some schemes are more efficient, and different column sequencings are used to achieve different needs. For example, a zeotropic mixture ABC can be first separated into A and BC before separating BC to B and C. On the other hand, mixture ABC can be first separated into AB and C, and AB can lastly be separated into A and B. These two configurations are sharp-split configurations in which the intermediate boiling substance does not contaminate each separation step. On the other hand, the mixture ABC could first be separated into AB and BC, and lastly split into A, B, and C in the same column. This is a non-sharp split configuration in which the substance with the intermediate boiling point is present in different mixtures after a separation step.Efficiency optimization
When designing distillation processes for separating zeotropic mixtures, the sequencing of distillation columns is vital to saving energy and costs. In addition, other methods can be used to lower the energy or equipment costs required to distill zeotropic mixtures. This includes combining distillation columns, using side columns, combining main columns with side columns, and re-using waste heat for the system. After combining distillation columns, the amount of energy used is only that of one separated column rather than both columns combined. In addition, using side columns saves energy by preventing different columns from carrying out the same separation of mixtures. Combining main and side columns saves equipment costs by reducing the number of heat exchangers in the system. Re-using waste heat requires the amount of heat and temperature levels of the waste to match that of the heat needed. Thus, using waste heat requires changing the pressure inside evaporators and condensers of the distillation system in order to control the temperatures needed.Controlling the temperature levels in a part of a system is possible with pinch analysis. These energy-saving techniques have a wide application in industrial distillation of zeotropic mixtures: side columns have been used to refine crude oil, and combining main and side columns is increasingly used.