Thermodynamic Properties of Propanol and Butanol as Oxygenate Additives to Biofuels Thermodynamic Properties of Propanol and Butanol as Oxygenate Additives to Biofuels

Alternative and renewable energy technologies are being sought throughout the world to reduce pollutant emissions and increase the efficiency of energy use. Oxygenate second-generation biofuels fuels lead to a reduction in pollutant emissions and their thermodynamic and transport properties allow that the facilities for transport, storage and distribution of fuels could be used without modification. Higher alcohols, like propanol and butanol, enhance the octane number, boosting the anti-knock effect in gasoline. Then the compression ratio of the engines can be increased without risk of knocking, leading to higher delivery of power. From the combustion point of view, the production of carbon monoxide and volatile hydrocarbons from the combustion of alcohols is less than the one of gasoline. This chapter covers mixtures of butanol and propanol with hydrocarbons. The properties reviewed are excess volume or density (VE), vapour-liquid equilibrium (VLE), and heat capacity (C p ). on thermodynamic properties be available to check and develop predictive empirical equations, models and simulation programs. Industrial processes as storage, transport, separation and mixing processes also reliable data for its design. As a result, the experimental literature reviews on properties of pure compounds and its mixtures with characteristic hydrocarbons can provide valuable information about the fluid behaviour under various temperature and pressure conditions. The paper presents the literature review of available data on thermodynamic properties (density, vapour-liquid equilibrium, specific heat,) of the mixtures of 1-propanol, 1-butanol, TBA and its mixtures with hydrocarbons representatives of gasoline. Density has to do with the volumetric behaviour of the mixtures under pressure and temperature conditions and is the primary data to check equations of state. The vapour-liquid equilibria, which allows the calculation of the Gibbs function, deal with the equilibrium between the liquid and vapour phase under fixed pressure and temperature conditions. And the heat capacity gives information related to the sensible energy storage of the liquids. The review includes only the interval of temperature and pressure of every property reported. The wider is the range of pressure and temperature of the measured properties, so it would be the reliability of the applications of predictive and equations and models. Discussion of further data (uncertainties, experimental apparatus, etc.) would require more space than available. Interested readers should access the literature references to check these issues.


Introduction
Biofuels, as environmental friendly fluids, have been paid much attention over the last decades. They contribute to diminish the greenhouse gas emissions due to its neutral carbon dioxide balance. Moreover, some oxygenated compounds are used as biofuel additives as they lead to a reduction in pollutant emissions and to an increase in the energy efficiency of vehicle engines [1,2].
Some alcohols and ethers, as oxygenated compounds additives, are added to present gasoline with the aim of reducing the emission of gases that produce environmental impact. The advantages of these oxygenates can be classified into several categories. First, they can be obtained from renewable, agricultural and raw materials, reducing the dependence of fossil sources [3]. Second, they enhance the octane number, boosting the anti-knock effect in gasoline. Then, the compression ratio of the engines can be increased without risk of knocking, leading to higher delivery of power. From the combustion point of view, the production of carbon monoxide and volatile hydrocarbons from the combustion of alcohols is less than the one of gasoline. Amongst the thermodynamic properties, the heat of vaporization of alcohols is high and leads to a reduction in the peak temperature of combustion, which means lower emissions of nitrogen oxides.
Alternative and renewable energy technologies are being sought to reduce pollutant emissions and increase the efficiency of energy use. Propanol and butanol have been proposed as an alternative to conventional gasoline and diesel fuels [4,5]. They are higher member of the series of alcohols with each molecule containing three or four carbon atoms rather than two as in ethanol. The EN standards of the European Union (EU) and the World-Wide Fuel Charter (WWFC) for gasoline include, for example, 2-propanol, 2-methyl-2 propanol (also known as tert-butyl alcohol, TBA), and 2-methyl-1 propanol [6,7] as gasoline components.
The traditional production and consumption of bioethanol have found an alternative with the second-generation biofuels, such as biobutanol. For example, 85% ethanol, E85, needs some modification of the internal combustion engines specifications, unlike butanols, which can work directly in present engines. The energy content per volume unit of butanol is similar to the one of gasoline, and higher than the same for ethanol. Concerning the contribution to the anti-knocking effect, butanol behaves almost the same as other alcohols like methanol or ethanol. And in the presence of water, the mixture butanol/gasoline shows lesser tendency to separation of phases than the mixture ethanol/gasoline. Then, all the facilities for transport, storage and distribution of fuels can be used without modification. Butanol, which can be synthesized chemically or biologically, is an alternative transportation fuel since it has properties that would allow its use in existing engines with minor hardware modifications [5]. For practical purposes, ASTM D7862 [8] gives specifications for blends of butanol with gasoline at 1-12.5% in volume for automotive spark ignition engines. Three butanol isomers are covered by the specification, 1-butanol, 2-butanol, and 2-methyl-1-propanol, while specifically excludes 2-methyl-2-propanol (TBA).
Besides its use as fuel component, its industrial uses covers a broad range of applications as solvent or as reactive for the production of other chemicals. Applications, chemicals and products that use butanol include solvents, plasticizers, coatings, chemical intermediate or raw material, textiles, cleaners, cosmetics, drugs and antibiotics, hormones, and vitamins.
Since the 1950s, most butanol is obtained from fossil sources [6]. 1-butanol and/or 2-butanol could be obtained from reduction of butyraldehyde with hydrogen, which is previously obtained by hydroformulation reaction of propene (propyelene). Meanwhile, propylene oxide production leads to isobutene, from which TBA could be derived. Butanol from biomass is called biobutanol [9], and it can be used in unmodified gasoline engines. Biobutanol can be produced by fermentation of biomass by the ABE process [9,10]. The process uses the bacterium Clostridium acetobutylicum, the bacterium for the production of acetone from starch. The butanol was a by-product of this fermentation. Other by-products as acetic, lactic and propionic acids, isopropanol and ethanol, as well as a certain amount of H 2 , are generated by the process. Ralstonia eutropha can also be used to produce biobutanol, by means of an electrobioreactor and the input of carbon dioxide and electricity.
According to DuPont [11], existing bioethanol plants can be converted to biobutanol production with low economic cost. The main modification could affect to the fermentation process, with minor changes in distillation, as both alcohols use the same stocks: food energy crops (sugar beets, sugar cane, corn grain, wheat, etc.), non-food energy crops (switchgrass, cellulose, etc.) and agricultural by-products (straw, corn stalks, etc.).
Biopropanol is a rarely discussed biofuel. Tough propanol is included as regular component of gasolines [6], its frequent use as chemical solvent makes it rare to consider it as a fuel. Biopropanol could be produced from microbial fermentation of biomass (cellulose), but the process is extremely inefficient [12]. The issues with microbial production of biopropanol are analogous to the issues with microbial production of biobutanol, so if biobutanol becomes a more practical biofuel to produce, then biopropanol will also become more feasible.
This paper concerns thermodynamic properties of 1-propanol, 2-propanol, 1-butanol, 2butanol and TBA. Accurate experimental data on thermodynamic properties should be available to check and develop predictive empirical equations, models and simulation programs. Industrial processes as storage, transport, separation and mixing processes also need reliable data for its design. As a result, the experimental literature reviews on properties of pure compounds and its mixtures with characteristic hydrocarbons can provide valuable information about the fluid behaviour under various temperature and pressure conditions. The paper presents the literature review of available data on thermodynamic properties (density, vapour-liquid equilibrium, specific heat,) of the mixtures of 1-propanol, 1-butanol, TBA and its mixtures with hydrocarbons representatives of gasoline. Density has to do with the volumetric behaviour of the mixtures under pressure and temperature conditions and is the primary data to check equations of state. The vapour-liquid equilibria, which allows the calculation of the Gibbs function, deal with the equilibrium between the liquid and vapour phase under fixed pressure and temperature conditions. And the heat capacity gives information related to the sensible energy storage of the liquids. The review includes only the interval of temperature and pressure of every property reported. The wider is the range of pressure and temperature of the measured properties, so it would be the reliability of the applications of predictive and equations and models. Discussion of further data (uncertainties, experimental apparatus, etc.) would require more space than available. Interested readers should access the literature references to check these issues.

The literature review
Thermodynamic properties of liquid propanol and butanol and its liquid mixtures with some hydrocarbon have been obtained from the literature search using online library databases (Web of Science © , Scopus © , NIST © Standard Reference Database) and high impact electronic journals.
Special attention is given to alcohol + hydrocarbon mixtures. As stated, 1-propanol, 1-butanol and TBA have been selected as alcohols. As representative of hydrocarbons, n-heptane, 2,2,4 trimethylpentane (iso-octane), cyclohexane, methyl-cyclohexane, benzene, toluene and 1hexene have been chosen. They represent linear, branched and cyclic alkanes, aromatics, as well as olefins, which are regular components of gasoline. Concerning properties, there is a huge amount of available thermodynamic data for pure compounds. With respect to the mixtures, density data are shown in Table 2 for binary mixtures alcohol (1) + hydrocarbon (2). Tables 3 and 4 present the vapour-liquid equilibria selected for mixtures alcohol (1) + hydrocarbon (2) and alcohol (1) + hydrocarbon (2) + hydrocarbon (3). Finally, heat capacity data for binary mixtures alcohol (1) + hydrocarbon (2) are provided in Table 5.
Only three references were found reporting VLE data of ternary mixtures, as shown in Table 4, at atmospheric or lower pressures. Temperatures were moderate, with maximum at 344 K measured in Ref. [98]. No ternary mixture with 1-propanol was found.

Heat capacity of mixtures 1-propanol, or 1-butanol, + hydrocarbon
Only eight references reporting heat capacity of binary mixtures alcohol (1) + hydrocarbon (2) are cited. Three of them correspond to the binary mixture 1-propanol (1) + heptane (2) at atmospheric pressure and at moderate temperatures (up to 300 K). No other mixture of 1propanol with the any of selected hydrocarbons was found.
While the heat capacity of 1-butanol with heptane, 2,2,4 trimethylpentane, cyclohexane, toluene and 1-hexane was measured by several authors. It must be pointed out that some measurements [86,160,161] have been performed at pressures up to 25 MPa and temperature of 313 K.

Conclusion
The literature review on thermodynamic properties of liquid mixtures of 1-propanol, 1-butanol and TBA with representative hydrocarbons has been reported. Seven hydrocarbons (linear, branched and cyclic alkanes, aromatics, and olefins) have been selected as representative of present and future unleaded gasoline. The review covers density, vapour-liquid equilibrium and heat capacity of mixtures.
The review of density data shows a big amount of data at low pressure and moderate temperatures. Only two references report data above 30 MPa at a maximum temperature of 333 K. And at temperatures above 450 K, the maximum pressure is 5.5 MPa. With respect to the vapour-liquid equilibrium, only one reference shows measurements over 555 K at 5 MPa. Heat capacity data of mixtures are very scarce, tough some high pressure and high temperature data can be found for some alcohol + hydrocarbon mixtures.
The performance of fuels and biofuels in engines and other devices shows a trend of increasing pressure and temperature, which leads to the need of more reliable predictive models for complex mixtures at such conditions. Availability of high pressure and high temperature thermodynamic properties is then a requisite for the implementation of these equation and models. The review shows a lack of reliable data at high pressure and high temperature thermodynamic data, which serve as a basis for the development of predictive equations and models.