Article pubs.acs.org/IECR
Product Separation after Chemical Interesterification of Vegetable Oils with Methyl Acetate. Part I: Vapor−Liquid Equilibrium Abraham Casas, Maria Jesús Ramos,* and Á ngel Pérez Department of Chemical Engineering, Institute for Chemical and Environmental Technology (ITQUIMA), University of Castilla-La Mancha, Avenida Camilo José Cela s/n, 13071 Ciudad Real, Spain S Supporting Information *
ABSTRACT: Chemical interesterification of triglycerides with methyl acetate yields biodiesel and triacetin. This reaction is highly reversible, which implies the presence of intermediate compounds such as diacetinmonoglycerides. In addition, the use of methanolic potassium methoxide as the catalyst causes the appearance of diacetin, monoacetin, and glycerol. Vacuum distillation becomes an interesting alternative for the separation of biodiesel and triacetin. Vapor−liquid equilibrium (VLE) determination requires information related to the vapor pressures of the different compounds and the nonideality of the liquid phase (activity coefficients). The lack of literature information for some compounds was supplemented with experimental data and predictive models for vapor pressure and activity coefficients (UNIFAC and UNIFAC Dortmund). The suitability of using vacuum distillation was evaluated using the Fenske equation and experimental results from a structured packed distillation column. According to the results, removal of diacetinmonoglycerides can be easily accomplished, unlike the elimination of diacetin, monoacetin, and glycerol, which form azeotropes with triacetin. fractionation of fatty acid esters.9 In a distillation, equilibrium between the vapor and liquid phases is expressed as
1. INTRODUCTION Biodiesel production through the conventional transesterification of vegetable oils with methanol generates crude glycerol as a byproduct. The low price of crude glycerol and the increasing price of raw vegetable oil lead to a reduction of the overall profitability of this process.1 Triacetin can be produced instead of glycerol if methanol is replaced by methyl acetate. This glycerol triacetate is a higher-value product than glycerol and can also be included in biodiesel formulations thanks to their complete solubility.2−4 The reaction between triglycerides and methyl acetate is known as interesterification and consists of three consecutive reversible reactions. The high reversibility of the interesterification reaction, favored by the mutual solubility of reactants and products, involves the presence of intermediate compounds (especially diacetinmonoglycerides, which contain a fatty acid group and two acetic acid groups bonded to a glycerol backbone).5 The main advantages of chemical interesterification,5,6 compared to enzymatic7,8 and supercritical4 interesterification, are the use of cheaper catalysts such as alkali methoxides and milder reaction conditions, respectively. Because methoxide is not properly dissolved in the reaction mixture, one must employ a premixing stage of the alkali methoxide with the vegetable oil5 or with methanolic methoxide to remove the initial induction period. Methanolic methoxide favors the safety of the process and the reaction rate, because the catalyst is completely dissolved from the beginning of the reaction. The presence of methanol in the mixture leads to the appearance of diacetin (glycerol diacetate), monoacetin (glycerol monoacetate), and glycerol.6 Because the main products of the reaction are biodiesel and triacetin, it is necessary to carry out their separation from the reaction mixture by vacuum distillation. This operation is usually employed in the oleochemical industry, especially in the © 2012 American Chemical Society
yP = xiγiPi° i
(1)
where yi and xi are the molar fractions of compound i in the vapor and liquid phases, respectively; P is the total pressure; and γi and P°i are the activity coefficient and vapor pressure, respectively, of compound i. Ideal behavior of the vapor phase can be assumed because of the low pressures used in vacuum distillation (fugacity coefficient, ϕi = 1). Therefore, vapor pressures of the pure compounds are required to determine the vapor−liquid equilibrium. In the literature, data for compounds such as methyl esters10,11 and glycerol12,13 can be found. In the case of triacetin, the data fall in the usual range of pressures used in the oleochemical industry (10−100 mmHg),14−16 so it is necessary to supplement this information with experimental data obtained in the laboratory. Compounds such as diacetinmonoglycerides, diacetin, and monoacetin have not yet been well studied and are not commercially available as pure mixtures. For these compounds, it is necessary to predict the vapor pressure using equations based on group17 or fragment18 contributions. To validate the values predicted by these models, experimental data obtained with mixtures of diacetinmonoglycerides synthesized in the laboratory and with commercial mixtures of diacetin and monoacetin (consisting of triacetin, diacetin, monoacetin, and glycerol in different proportions) can be used.19 Along with this information, the activity coefficients must be evaluated to determine the effects of the liquid-phase nonideality on the vapor−liquid equilibrium. Again, because Received: March 27, 2012 Accepted: May 17, 2012 Published: May 17, 2012 8087
dx.doi.org/10.1021/ie3007903 | Ind. Eng. Chem. Res. 2012, 51, 8087−8094
Industrial & Engineering Chemistry Research
Article
of the lack of literature information and the commercial unavailability of some compounds in pure form, it is necessary to predict these coefficients by methods based on group contributions, such as UNIFAC20 and UNIFAC Dortmund.21 Such methods have already been successfully employed for vapor−liquid equilibrium of the biodiesel−glycerol−methanol system.22 Therefore, the objective of this work was to study the separation of the interesterification reaction products through vacuum distillation. The existing information in the literature related to the vapor−liquid equilibria of these mixtures was completed with data obtained in the laboratory and through prediction and subsequent validation with different methods based on group or fragment contributions. Finally, batch distillations with a structured packed column were carried out to check the results obtained.
16.5 theoretical stages, with an operating pressure of 50 mbar (depending on the hydrodynamic conditions of the column). At the top of the column was placed a glass condenser that used water as the coolant and an electromagnetic gas reflux controller (Afora, Barcelona, Spain). The reflux controller was connected to the pressure-control system of the D1160 semiautomatic distillation apparatus mentioned previously. A Pt(100) probe (±0.1 °C accuracy) was situated between the column and the condenser and connected to a Cole Parmer EW-18200-40 data logger. Finally, a glass manifold was placed in the outlet of the glass condenser to allow sampling of the distillate. The experimental procedure was as follows: First, the distillation flask was filled with 2 L of the mixture to be separated. This mixture was degassed by heating to 70 °C and setting the vacuum system to a pressure of 99.5%) and anhydrous glycerol (>99.5%) were obtained from Sigma-Aldrich (St. Louis, MO). Technical-grade diacetin and monoacetin (∼40%) were provided by Acros Organics (Geel, Belgium). Sunflower oil was supplied by Sovena (Aranda de Duero, Spain), whereas rapeseed, soybean, and palm oils were purchased from Henry Lamotte GmbH (Bremen, Germany). Potassium methoxide (97%) was provided by BASF (Ludwigshafen am Rhein, Germany), and ortho-phosphoric acid (85%) by PANREAC ( B a r c e l o n a , Sp a i n ) . N - M e t h y l - N - ( t r i m e t h y l s i l y l )trifluoroacetamide (MSTFA) and isopropyl acetate were from Sigma-Aldrich (St. Louis, MO). 2.2. Equipment and Procedures. 2.2.1. Vapor−Liquid Equilibrium Determination. The semiautomatic distillation apparatus D1160, supplied by Normalab Analis, was utilized in the determination of vapor−liquid equilibrium. This equipment was adapted for standard test method ASTM D1160 (Standard Test Method for Distillation of Petroleum Products at Reduced Pressure). To improve temperature measurements, the head thermocouple was substituted by a platinum temperature probe (Pt 100, accuracy of ±0.1 °C) connected to a Cole Parmer EW18200-40 data logger. Moreover, the graduated cylinder was replaced by a glass manifold that allowed sampling of 5 mL of distillate for further analysis. To carry out the equilibrium determination, 200 mL of sample was introduced into the distillation flask, and the mixture was degassed at 70 °C and a pressure of
