František Skopal

Kinetics and mechanism of the KOH — catalyzed methanolysis of rapeseed oil for biodiesel production

The reaction of rapeseed oil with methanol catalyzed by KOH is described by a model consisting of two sequences of consecutive competitive reactions. The first sequence expresses the methanolysis of rapeseed oil to methyl esters (biodiesel) whereas the second sequence describes the always present side reaction-saponification of glycerides and methyl esters by KOH. The proposed chemical model is described (after rational simplifications) by a system of differential kinetic equations which are solved numerically by two independent computing methods. The thus obtained theoretical kinetic and equilibrium results are compared numerically and/or graphically with the experimental parameters. The latter were obtained by the determination of the relevant components in the actual reaction mixture by analytical methods. According to the experimental results, the proposed reaction scheme is fulfilled with the probability of ca. 78%. The optimal average rate constants and equilibrium constants of individual reaction steps of the discussed scheme are introduced. The limitations of the proposed reaction model are discussed.

Treatment of glycerol phase formed by biodiesel production

Glycerol is a by-product of biodiesel produced by transesterification and is contained in the glycerol phase together with many other materials such as soaps, remaining catalyst, water, and esters formed during the process. The content of glycerol is approximately 30-60 wt.%. In this paper, treatments of the glycerol phase to obtain glycerol with a purity of 86 wt.% (without distillation) and a mixture of fatty acids with esters (1:1) or only a mixture of fatty acids with a purity of 99 wt.% are presented. The treatment was carried out by removing of alkaline substances and esters. Fatty acids were produced by saponification of the remaining esters and subsequent neutralization of alkaline substances by phosphoric, sulfuric, hydrochloric, or acetic acids. Salts are by-products and, in the case of phosphoric acid can be used as potash-phosphate fertilizer. The process of treatment is easy and environmentally friendly, because no special chemicals or equipment are required and all products are utilizable.

The effect of the acidity of rapeseed oil on its transesterification.

The aim of this work is to study the transesterification of vegetable oil with a high acid number at unchanged reaction conditions. Rapeseed oil was used as the raw material and its acid number was changed by the addition of oleic acid (from 0.89 to 12.25 mg KOH/g). Methanol was used for transesterification (molar ratio of oil to methanol 1:6) and potassium hydroxide was used as a catalyst. After the reaction time, the residue of the catalyst was neutralised by gaseous carbon dioxide and the methanol excess was removed. After the separation of two phases, each of them was analyzed (in the ester phase: yield, content of methyl ester and acid number; in the glycerol phase: yield, density, viscosity, content of glycerol, soaps, methyl ester, potassium carbonate and hydrogen carbonate). The obtained data was compared with theoretical material balances and the effect on the saponification of oil was discussed. The results show that the yield of methyl ester (biodiesel) is significantly affected by a higher acid number, as well as enhanced soap formation. On the other hand, the conversion of the oil and acid number of the ester phase remain at constant values in studied borders.

Biodiesel fuel from rapeseed oil, methanol, and KOH. Analytical methods in research and production

The enumeration of the analytical methods used in the production of biodiesel from rapeseed oil and methanol catalyzed by KOH and published till 1997 is given. Some of our original methods for individual or simultaneous determination of the main components in the reaction mixture are described. All these methods can be also used to analyse the non-equilibrium complex and heterogencous mixture.

Ethanolysis of rapeseed oil – Distribution of ethyl esters, glycerides and glycerol between ester and glycerol phases

The distribution of ethyl esters, triglycerides, diglycerides, monoglycerides, and glycerol between the ester and glycerol phase was investigated after the ethanolysis of rapeseed oil at various reaction conditions. The determination of these substances in the ester and glycerol phases was carried out by the GC method. The amount of ethyl esters in the glycerol phase was unexpectedly high and therefore the possibility of the reduction of this amount was investigated. The distribution coefficients and the weight distributions of each investigated substance were calculated and compared mutually. The distribution coefficients between the ester and glycerol phase increase in this sequence: glycerol, monoglycerides, diglycerides, ethyl esters, and triglycerides. Soaps and monoglycerides in the reaction mixture cause a worse separation of ethyl esters from the reaction mixture. The existence of a non-separable reaction mixture was observed also, and its composition was determined.

Study of effects of some reaction conditions on ethanolysis of rapeseed oil with dispergation

The alkaline-catalyzed (KOH) ethanolysis of rapeseed oil with the help of a mechanical disperser was researched in this study. The effects of chosen reaction conditions (independent variables: the reaction temperature and time, the amount of catalyst, the molar ratio ethanol to oil and the rotation frequency of the disperser) on the ethanolysis process and ways for the improvement of the transesterification and separation process were studied. The transesterification of oil, the separation of the ester phase from the glycerol phase and qualitative properties of the ester phase were monitored by many dependent variables (e.g. the yield of the ester phase, the weight concentration of glycerides, the content of potassium ions, etc.). The measured data was analyzed by multi-linear regression with the help of many statistical tests (significance of parameters, exclusion of outliers, etc.). The created mathematical models describing the relations between the independent and dependent variables were verified by several independent experiments.

Relationships among flash point, carbon residue, viscosity and some impurities in biodiesel after ethanolysis of rapeseed oil

Many samples of rapeseed oil ethyl ester were prepared by alkaline-catalyzed transesterification at various conditions (reaction time, temperature, amount of catalyst, the molar ratio of ethanol to oil, the rotations of a disperser and the purification by water). The concentrations of the key impurities for biodiesel quality (the concentrations of monoglycerides, diglycerides, triglycerides, free glycerol, ethanol, free fatty acids, water) and some qualitative parameters (flash point, carbon residue, kinematics viscosity at 40 degrees C) were determined and then the relationships among them were found out. The relationships were characterized by the linear or non-linear statistical models. The found models enable the better understanding of the significance of the qualitative parameters and estimate them from the concentrations of impurities. The temperature dependence was also measured in the case of the viscosity of ethyl ester and used rapeseed oil.

Combined effect of water and KOH on rapeseed oil methanolysis.

This paper deals with the effect of water and catalyst (KOH) amount on the quantity and quality of transesterification products of rapeseed oil by methanol, the methyl ester phase (i.e. yield, conversion), and the side-product, the glycerol phase (i.e. density, viscosity, the mass fraction of glycerol, esters, soaps). The dependencies were described by statistical models. The transesterification was carried out at constant reaction conditions (90 min reaction time, 400 rpm, 60 degrees Celsius). Twelve experiments with the independent factors, amount of potassium hydroxide (0.65-0.9 mg per gram of oil) and total amount of water (0.24-1.42 mg per gram of oil) naturally present in the reaction components or formed by the neutralisation reaction of free fatty acids and of added water. The data were analyzed by linear regression with respect to regression triplet (complex critical analysis of the model, data and regression method). The analysis resulted in a set of linear and/or quadratic models consisting of statistically proven terms at a statistical significance level of 0.05 and demonstrated that ester in the glycerol phase increases with increasing amount of soaps.

Effect of phase separation temperature on ester yields from ethanolysis of rapeseed oil in the presence of NaOH and KOH as catalysts.

The effects of phase separation temperatures (5-90°C) on losses of higher fatty acid (C(16) and C(18)) ethyl esters in the glycerol phase were investigated. Losses of ethyl esters produced from ethanolysis of rapeseed oil were 30-60% higher when NaOH rather than KOH was used as homogeneous catalyst. The losses decreased with an increase in separation temperature, resulting in an increase in the yield of the ester phase. The concentration of impurities (e.g. alkali metals, free glycerol and glycerides) in the ester phase increased with increasing separation temperature due reversible transesterification and reciprocal solubility of the compounds. Carbonates formed during neutralization of the catalysts are also transesterification catalysts and they cause reverse reaction. The ethyl ester bound in the glycerol phase during NaOH-mediated catalysis can be extracted by heating the separated glycerol phase to 60-90°C. The ester yield is increasing with increasing separation temperature, however with decreasing quality.

Continuous biodiesel production in a cascade of flow ideally stirred reactors.

The continuous methanolysis of rapeseed oil catalyzed by KOH in a cascade of 4 flow stirred reactors at a steady state of 60 degrees C was studied. By comparing of the determined steady state concentrations of rapeseed oil, biodiesel and KOH in the reactors (under various initial concentrations of these components and feeding) with the assumed kinetic model the rate constants of the relevant differential rate equations for rapeseed oil consumption and biodiesel production were calculated.