7862
Ind. Eng. Chem. Res. 2008, 47, 7862–7867
Component Distribution between Light and Heavy Phases in Biodiesel Processes Renzo Di Felice,*,† Danilo De Faveri,† Paola De Andreis,‡ and Piero Ottonello‡ Dipartimento di Ingegneria Chimica e di Processo “G.B. Bonino”, UniVersita` degli Studi di GenoVa, Via Opera Pia 15, 16145 GenoVa, Italy, and Chemtex Italia srl - Gruppo Mossi & Ghisolfi, Strada SaVonesa 9, 15050 RiValta ScriVia, Italy
Knowledge of the component distribution in the biodiesel production process is fundamental both during the transesterification reaction, where reactants are partially miscible, and during the recovery of the final products, which exist in two separate phases, a heavy one containing nearly all of the glycerol and a light one containing nearly all of the biodiesel. In this article, a simple methodology able to predict the product distribution at equilibrium between the heavy and light phases is suggested. Experimental equilibrium data for mixtures of two (biodiesel and glycerol), three (biodiesel, glycerol, and methanol), and four (biodiesel, glycerol, methanol, and water) components are collected and correlated with the Wilson activity coefficient model. The approach is based on thermodynamic data already available in the literature, without the addition of any empirical fitting parameter. A small effect brought about by the presence of water at low concentrations on methanol distribution between the two phases is pointed out and qualitatively supported by the model predictions. The influence of the addition of electrolyte contaminants (soap or catalysts) is also considered. Introduction Awareness in the industrialized world of the depletion of crude oil reserves is pushing toward the search for renewable alternative energy sources. Painless and comprehensive solutions are not, by any means, readily available at present. However, certain actions that are already available and are planned for the immediate future, as also evidenced by European Directive 2003/30/EC,1 can contribute to alleviate this problem. The use of biodiesel as a transportation fuel is one such action, and this explains the unprecedented interest in this alternative form of energy. Biodiesel, the common name for fatty acid methyl esters (FAMEs), is a liquid fuel obtained from the transesterification of vegetable (or animal) oils. The process has been known for a long time and certainly cannot be defined as “high-tech”, as practically anyone can do it in his or her own kitchen, without the use of any sophisticated equipment or material. It involves simply the reaction, under very mild conditions, between vegetable oil and typically a large excess of alcohol of methanol, in the presence of an acid or basic catalyst, which produces FAMEs as the main product and glycerol as a byproduct.2 The separation of the transesterification reaction products is then greatly facilitated by the formation of two immiscible liquid layers: a heavy one containing nearly all of the glycerol and a light one containing nearly all of the biodiesel, with the excess alcohol being distributed between the two phases. Further biodiesel cleaning processes are then carried out to obtain a product of the required purity and characteristics, so that it can be used, alone or more commonly mixed with diesel of fossil origin, as a transportation fuel.3 The simplicity of the process seems to have been transferred to the industrial level, without any further refinement. Indeed, the industrial process is currently carried out based mostly on the specific knowledge and experience of the plant operators, very much like a craftsman who treasures his know-how and * To whom correspondence should be addressed. Tel.: +39 0103532924. Fax: +39 0103532586. E-mail:
[email protected]. † Universita` degli Studi di Genova. ‡ Chemtex Italia srl - Gruppo Mossi & Ghisolfi.
resists any change that aims at rationalizing and improving the overall process. Quantification of the basic parameters governing the biodiesel production process is the first step toward better knowledge, and better operation as a direct consequence, and it represents the aim of the work in which we are currently involved. In this article, we concentrate on a very specific aspect, namely, the separation of the products after the transesterification reaction and the washing with water of the resulting biodiesel-rich streams. These operations are carried out in the liquid phase by exploiting the formation of two immiscible phases of quite different densities. Consequently, the two phases, in terms of component distribution, must conform to the thermodynamic liquid-liquid equilibrium that is established between them. A simple methodology is suggested here that is capable of predicting the product distribution at equilibrium between the heavy and light phases. The components that must be considered for the current problem are essentially biodiesel, methanol, glycerol, water, soap, and catalysts. First, we limit the study to the behavior of a mixture made up of all of the nonelectrolyte products: biodiesel, glycerol, methanol, and water. Then, we consider the effects on the obtained results due to the addition of soap or catalysts. This approach allows a simple predictive model to be put forward for the nonelectrolyte mixture. It has to be stressed that such an approach is based on thermodynamic data already available in the literature, without the introduction of any empirical fitting parameter. Theoretical Analysis The situation addressed here considers n components distributed between two liquid phases. The component distribution, at equilibrium conditions, is governed by the chemical potential, so that the following relationship must hold for every component i (γixi)L ) (γixi)H
(1)
In this equation, x is the component molar fraction; γ is its activity coefficient; and H and L represent the light and heavy phases, respectively. The activity coefficient for the ith component is generally a complex function of the molar fraction of
10.1021/ie800510w CCC: $40.75 2008 American Chemical Society Published on Web 09/25/2008
Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7863 Table 1. Wilson Paired Energy Parameters λij (J/mol)
biodiesel methanol glycerol water
biodiesel
methanol
55507
33387 46287 21788
a a
glycerol
water
a
a
-36337 16569
3478 53299 -
a
Because biodiesel is practically immiscible with glycerol or water, these parameters are not necessary.
each component in the respective phase, as well as the temperature. Numerous efforts have been made to express such a function with workable numerical expressions having some physical basis. The generalized Wilson relationship4 is one such effort, and it is expressed as
(∑ ) n
ln γi ) -ln
n
xjΛij + 1 -
j)1
∑ k)1
xkΛki n
∑xΛ j
(2)
kj
j)1
which contains only binary interaction parameters, Λij, given by Λij )
( )
νj λij exp νi RT
(3)
where ν is the component molar volume and λij is a numerical constant to be determined experimentally. It follows that knowledge of λij is the only input needed to be able, in principle, to quantify the component distribution between two phases in thermodynamic equilibrium for any given temperature, the only difficulty being numerical, as it involves the solution of a system of highly nonlinear equations. For the specific case considered here, we are interested in predicting how biodiesel, methanol, glycerol, and water would distribute between the light and heavy phases. This prediction therefore requires knowledge of the pair of binary interaction parameters for all six possible component pairs. To simplify the present approach, in this case, we assumed that no biodiesel was present in the heavy phase and that no water or glycerol was present in the light phase. As discussed later, these assumptions are based on both experimental results from this work and literature evidence.5,6 In this way, the number of required binary interaction parameters was reduced to four pairs. They were all retrievable from the open literature, inferred in all cases from measurements of vapor-liquid equilibrium, and they are summarized in Table 1. It has to be stressed that, as reported by Orye and Prausnitz,10 the Wilson equation has two features that make it particularly useful for engineering applications. First, it has a built-in temperature dependence that has some theoretical significance, as the quantities λi,j are independent of temperature. The values of these parameters, obtained from data at one temperature, can be used to predict activity coefficients at other temperatures, which is an important advantage. Second, Wilson’s model for a multicomponent solution requires only parameters that can be obtained from binary mixture data, thus reducing the required amount of experimental work. Even if other local-composition models, such as NRTL and UNIQUAC, have the same features, in this study, the Wilson equation was used because of its greater simplicity as well as the availability in the open literature of the parameters involved in the present investigation. However,
because Wilson’s equation is valid only for nonelectrolytes, the effects of soap and catalyst could not be included in the model predictions. Experimental Procedure Materials. Biodiesel (palm oil methyl esters) was kindly supplied by Mythen SpA, Ferrandina (MT), Italy. Its purity and bound glycerol and free glycerol contents are >98.5%, < 0.8%, and
