Jump to content

Zeotropic mixture

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by MissAndrea (talk | contribs) at 08:34, 19 February 2017 (Moved "Refrigeration" from Sandbox to Wikipedia). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Template:Distinguish2

VLE of a zeotropic mixture; liquid and vapor composition vs. temperature[1]
VLE of a zeotropic mixture; vapor vs. liquid composition[1]

A zeotropic mixture, or nonazeotropic mixture, is a mixture with components that have different boiling points[2]. For example, nitrogen, methane, ethane, propane, and isobutane constitute a zeotropic mixture[3]. Individual substances within the mixture do not evaporate or condense at the same temperature as one substance[4]. 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[4]. When boiling a zeotropic mixture, there is a state between when the liquid and the vapor are completely saturated due to the difference in evaporation temperatures[citation needed]. On graphs, this temperature glide can be seen as the temperature difference between the bubble point and dew point[5]. For zeotropic mixtures, the temperatures on the bubble (boiling ) curve are between the individual component's boiling temperatures[6].

When a zeotropic mixture is boiled or condensed, the composition of the liquid and the vapor changes[citation needed]. In addition, the mixture's enthalpy is not directly proportional to the temperature as the phases change[7].

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

Figure 1: Temperature-Composition diagram of a zeotropic mixture

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[5]. 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 a isothermal line connecting the dew curve to the boiling curve as the mixture boils[5]. 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[6]. Unlike azeotropic mixtures, there is no azeotropic point at any temperature on the diagram where the bubble line and dew lines would intersect[5]. 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.

An azeotropic mixture that is near its azeotropic point has negligible zeotropic behavior and is near-azeotropic rather than zeotropic[6].

A larger difference in boiling points between the substances affects the dew and bubble curves of the graph[5]. A larger gap between the boiling points creates a larger temperature glide between the boiling curve and dew curve at a given mass fraction[5]. In addition, a larger difference in boiling points creates a larger shift in mass fractions when he mixture boils at a given temperature[5]. However, with any difference in boiling points, the temperature glide decreases when the mass fraction of a component nears 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[5].

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[8].

Nucleate Pool Boiling

The characteristics of pool boiling are different for zeotropic mixtures than that of pure mixtures[9]. 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[9]. Zeotropic mixtures and pure liquids also have different critical heat fluxes[9]. In addition, the heat transfer coefficients of zeotropic mixtures are less than the ideal values predicted using the coefficients of pure liquids[9]. 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[9].

Convective Flow Boiling

Zeotropic mixtures have different characteristics in convective boiling than pure substances or azeotropic mixtures[9]. Overall, zeotropic mixtures transfer heat more efficiently at the bottom of the fluid, whereas pure and azeotropic substances transfer heat better at the top[9]. 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[9]. In the case of pure liquids and azeotropic mixtures, this decrease in thickness causes a decrease in the resistance to heat transfer[9]. Thus, more heat is transferred and the heat transfer coefficient is higher at the top of the film[9]. The opposite occurs for zeotropic mixtures[9]. The decrease in film thickness near the top causes the component in the mixture with the higher boiling point to decrease in mass fraction[9]. Thus, the resistance to mass transfer increases near the top of the liquid[9]. Less heat is transferred, and the heat transfer coefficient is lower than at the bottom of the liquid film[9]. 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[9].

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[3]. In addition, diluting the zeotropic mixture reduces the heat transfer coefficient[3]. Decreasing the pressure when boiling the mixture only increases the coefficient slightly[3]. Using grooved rather than smooth boiling tubes increases the heat transfer coefficient[10].

Distillation

Distillation column

The ideal case of distillation uses zeotropic mixtures[11]. Zeotropic fluid and gaseous mixtures can be separated by distillation due to the difference in boiling points between the component mixtures[11][12]. This process involves the use of vertically-arranged distillation columns[12]. 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[12]. In other words, each column separates two components purely[11]. If three substances are separated with a single column, the substance with the intermediate boiling point will not be purely separated[11], and a second column would be needed[11]. To separate mixtures consisting of multiple substances, a sequence of distillation columns must be used[12]. This multi-step distillation process is also called rectification[12].

In each distillation column, pure components form at the top (rectifying section) and bottom (stripping section) of the column when the starting liquid (called feed composition) is released in the middle of the column[12]. At a certain temperature, the component with the lowest boiling point (called distillate or overhead fraction) vaporizes and collects at the top of the column, whereas the component with the highest boiling point (called bottoms or bottom fraction) collects at the bottom of the column[12]. 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[12].

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[11]. For example, a zeotropic mixture ABC can be first separated into A and BC before separating BC to B and C[11]. On the other hand, mixture ABC can be first separated into AB and C, and AB can lastly be separated into A and B[11]. These two configurations are sharp-split configurations in which the intermediate boiling substance does not contaminate each separation step[11]. 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[11]. 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[11].

In a concentration profile, the position of a vapor in the distillation column is plotted against the concentration of the vapor[12]. 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[12]. The component with the intermediate boiling point has a max concentration in the middle of the distillation column, so in mixtures with greater than three component substances, more than one distillation column is needed to separate the mixtures[12].

When designing distillation processes for separating zeotropic mixtures, the sequencing of distillation columns is vital to saving energy and costs[13]. In addition, other methods can be used to lower the energy or equipment costs required to distill zeotropic mixtures, including combining distillation columns (which uses as much energy as the most energy-consuming separated column), using side columns (saves energy by preventing different columns from carrying out the same separation of mixtures), combining main columns with side columns (saves equipment costs by reducing the number of heat exchangers needed), and re-using waste heat for the system[13]. Re-using waste heat requires the amount of heat and temperature levels of the waste to match that of the heat needed[13]. Thus, using waste heat requires changing the pressure inside evaporators and condensors of the distillation system in order to control the temperatures needed[13]. Controlling the temperature levels in a part of a system is possible with Pinch Technology [14]. 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[13].

Examples of distillation for zeotropic mixtures can be found in industry. Refining crude oil is an example of multi-component distillation in industry that has been used for more than 75 years[11]. Crude oil is separated into five components with main and side columns in a sharp split configuration[11]. In addition, ethylene is separated from methane and ethane for industrial purposes using multi-component distillation[11].

Separating aromatic substances requires extractive distillation, for example, distilling a zeotropic mixture of benzene, toluene, and p-xylene[11].

Refrigeration

Research has proposed using zeotropic mixtures as substitutes to halogenated refrigerants due to the harmful effects that hydrocholorofluorocarbons (HCFC) and chlorofluorocarbons (CFC) have on the ozone layer and global warming[4]. Researchers have focused on using new mixtures that have the same properties as past refrigerants to phase out harmful halogenated substances, in accordance to the Montreal Protocol and Kyoto Protocol[4]. For example, researchers found that zeotropic mixture R-404A can replace R-12, a CFC, in household refrigerators[15]. However, there are some technical difficulties for using zeotropic mixtures[4]. This includes leakages, as well as the high temperature glide associated with substances of different boiling points[4], though the temperature glide can be matched to the temperature difference between the two refrigerants when exchaning heat to increase efficiency[6]. Replacing pure refrigerants with mixtures calls for more research on the environmental impact as well as the flammability and safety of refrigerant mixtures[4].

Relevance

Zeotropic mixtures can be separated by normal distillation. The separation factor never becomes exactly 1, but might be very close to 1, making a separation by distillation very difficult or even technically impossible[citation needed].

Some specific zeotropic mixtures are used as refrigerants (Code names are R-4xxx)[citation needed]. Zeotropic mixtures have the disadvantage that their composition changes during the repeated evaporation and condensation processes[citation needed]. This causes a temperature glide – a change in the boiling temperature at constant pressure[citation needed]. Additionally, this leads to changing thermodynamic properties, notably, heat of vaporization and heat capacity which are the most important in the refrigeration process[citation needed]. Azeotropic mixtures on the other hand have constant properties at their azeotropic composition and behave like pure components[citation needed]. If zeotropic blends are used, it is common to select mixtures with small separation factors (so called near-azeotropic mixtures).[16]

Occurrence

Pressure dependency of the azeotropic composition

Zeotropy occurs when the boiling difference is large, or when the chemicals in a blend are similar, e. g. two ketones or two amines[citation needed]. Azeotropy occurs if the boiling point difference is smaller and the chemicals contain different functional groups (e. g. chloroform and methanol)[citation needed].

A mixture can be both zeotropic and azeotropic because the azeotropic composition changes with temperature and pressure[citation needed]. A good example is the mixture of ethanol and water, which is azeotropic above approximately 305 K (32 °C; 89 °F) and 12 kilopascals (1.7 psi)[citation needed]. Below that temperature and pressure, the mixture is zeotropic[citation needed]. The separation factor at the limiting temperature is still very small, and thus the differences between liquid and vapor composition is too small to be of use for distillation[citation needed].

References

  1. ^ a b Tamir A., Wisniak J., "Vapor-Liquid Equilibria of Isobutanol-n-Butanol and Isopropanol-sec-Butanol Systems", J.Chem.Eng.Data, 20(4), 391-392, 1975
  2. ^ Gaspar; Pedro Dinis; da Silva; Pedro Dinho (2015). Handbook of Research on Advances and Applications in Refrigeration Systems and Technologies. IGI Global. p. 244. ISBN 978-1-4666-8398-3. Retrieved 23 January 2017.
  3. ^ a b c d Barraza, Rodrigo; Nellis, Gregory; Klein, Sanford; Reindl, Douglas. "Measured and predicted heat transfer coefficients for boiling zeotropic mixed refrigerants in horizontal tubes". International Journal of Heat and Mass Transfer. 97: 683–695. doi:10.1016/j.ijheatmasstransfer.2016.02.030.
  4. ^ a b c d e f g Mohanraj, M.; Muraleedharan, C.; Jayaraj, S. (2011-06-25). "A review on recent developments in new refrigerant mixtures for vapour compression-based refrigeration, air-conditioning and heat pump units". International Journal of Energy Research. 35 (8): 647–669. doi:10.1002/er.1736. ISSN 1099-114X.
  5. ^ a b c d e f g h Herold, Keith; Radermacher, Reinhard; Klein, Sanford (2016-04-07). Absorption Chillers and Heat Pumps, Second Edition. CRC Press. pp. 23–63. doi:10.1201/b19625-4. ISBN 9781498714341.{{cite book}}: CS1 maint: date and year (link)
  6. ^ a b c d Sweeney, K.A.; Chato, J.C. (May 1996). "The Heat Transfer and Pressure Drop Behavior of a Zeotropic Refrigerant Mixture in a Microfinned Tube" (PDF). Air Conditioning and Refrigeration Center. {{cite web}}: Cite has empty unknown parameter: |dead-url= (help)
  7. ^ Liu, Zhaoyong; Zhao, Li; Zhao, Xuezheng; Li, Hailong. "The occurrence of pinch point and its effects on the performance of high temperature heat pump". Applied Energy. 97: 869–875. doi:10.1016/j.apenergy.2011.12.061.
  8. ^ Atkins, Tony; Escudier, Marcel (2013). A Dictionary of Mechanical Engineering. Oxford University Press. ISBN 9780199587438.
  9. ^ a b c d e f g h i j k l m n o Radermacher, Reinhard; Hwang, Yunho (2005). Vapor compression heat pumps with refrigerant mixtures. Boca Raton, Florida: Taylor & Francis. pp. 237–244. ISBN 9781420037579.
  10. ^ Zhang, Xiaoyan; Ji, Changfa; Yuan, Xiuling (2008-10-01). "Prediction method for evaporation heat transfer of non-azeotropic refrigerant mixtures flowing inside internally grooved tubes". Applied Thermal Engineering. 28 (14–15): 1974–1983. doi:10.1016/j.applthermaleng.2007.12.009.
  11. ^ a b c d e f g h i j k l m n o Górak, Andrzej; Sorensen, Eva (2014). Distillation: Fundamentals and Principles. Elsevier. pp. 271–300. ISBN 978-0-12-386547-2.
  12. ^ a b c d e f g h i j k Stichlmair, Johann (2000). Distillation, 1. Fundamentals. Wiley-VCH Verlag GmbH & Co. KGaA. ISBN 9783527306732.
  13. ^ a b c d e Stichlmair, Johann (2000-01-01). Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH Verlag GmbH & Co. KGaA. doi:10.1002/14356007.o08_o02. ISBN 9783527306732.
  14. ^ Asprion, Norbert; Mollner, Stephanie; Poth, Nikolaus; Rumpf, Bernd (2000-01-01). Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH Verlag GmbH & Co. KGaA. doi:10.1002/14356007.b03_12.pub2. ISBN 9783527306732.
  15. ^ Dincer, Ibrahim (2000-01-01). Kirk-Othmer Encyclopedia of Chemical Technology. John Wiley & Sons, Inc. doi:10.1002/0471238961.1805061819090212.a01.pub2. ISBN 9780471238966.
  16. ^ Jones J.A., "Near Azeotropic Mixture Substitute for Dichlorodifluoromethane", US-Patent, Pat.No. PCT/US91/01905, 1-13, 1991
Retrieved from "https://en.wikipedia.org/enwiki/w/index.php?title=Zeotropic_mixture&oldid=766278001"