Oct-2012

Production of biolubricant base stock

Using microwave technology in biolubricants production enhances product yields and reduces reaction time

Shalini Gupta, Rajeev Kumar, Sudha Tyagi and Peddy V C Rao
Bharat Petroleum Corporation

Article Summary

Biofuels have already been accepted around the world for their advantages over conventional petroleum fuels, including the opportunity for energy independency. Now, similar growth is expected for biolubricants, which are derived from renewable vegetable oils for different niche applications. Recently, the idea of producing vegetable oil-based biolubricants has led researchers to develop process technologies for their commercialisation.

Biolubricants are esters of heavy alcohols derived from vegetable oil- based feedstock and have lubricating properties similar to those of mineral oil-based lubricants. Even though biolubricants are priced twice as high as conventional petroleum lubricants,¹ industries are investing in R&D towards increasing oil recovery from seeds, reducing the costs of processes and exploring niche application areas. The high cost of biolubricant today is mainly due to the non-availability of low-cost vegetable oil feedstock and lower-value realisation of the by-products.

Vegetable oils are being investigated as a potential source of environmentally favourable lubricants because of their combination of biodegradability, renewability and excellent performance as lubricants. However, their relatively low resistance to oxidation and thermal stability, poor low-temperature properties and narrow range of available viscosities limit their application as industrial lubricants.

Traditionally, biolubricants are produced from vegetable oils in a reaction vessel, where the mixture is heated by means of coil jacketing using steam/hot water under laminar conditions for an extended period. Some of the key parameters that affect the transesterification reaction are: reaction temperature, reaction time, amount of catalyst and alcohol-to-oil ratio. In view of this, the present work is mainly focused on developing a cost-effective, homogeneous catalytic route as an alternative to the conventional synthesis of biolubricant base stock. The technique under development involves the influence of microwave radiation3 (3000-30 000 MHz) to reduce chemical reaction times from hours to minutes and to reduce side reactions to help in improving the overall yield.^2,3,4

In the present study, various vegetable oils have been explored to synthesise higher esters using heavy alcohols and catalysts. Process variables1,5 such as the molar ratio of alcohols to oils, higher homologues of alcohols, catalyst-to-oil ratio, reaction temperature, reaction time and feedstock quality are investigated. The effect of variables was monitored with the help of HPLC and FT-IR analysis techniques.6 Furthermore, the synthesised lubricant base stocks were characterised for essential properties such as density, viscosity index (VI), pour point, ester content, and so on. These properties of vegetable oil-based lubricants can be tailored by modifying the operating parameters and using suitable additive formulations and some chemical modifications for end-use applications.

Challenges in production and commercialisation
For biolubricants to be economically viable, both the lubricating characteristics and the method of synthesis are important. Biodegradable esters are derived from the transesterification of vegetable oils with higher homologues of alcohols. These products have been developed for various biodegradable lubricant applications, including metal working fluids and food-grade lubricants. Vegetable oils have an excellent lubricity, but poor oxidation and low temperature stabilities. In order to use plant-based oils as lubricants, special additives and chemical modifications are necessary for end- use applications.

Keeping these factors in view, the reaction parameters for the synthesis of biolubricant base stock by microwave and conventional routes were optimised. A study of the effect of residence time for both the conventional route and the microwave route has been carried out and the results compared. Microwave processing provides a distinct and substantial advantage over conventional processes primarily due to the manner in which microwave radiation interacts with the matter and transfer of energy.

The data gathered could be used for optimisation of the processing parameters for the synthesis of biolubricant base stocks using a microwave system at shorter duration. The conceptualisation of the process scheme has been developed for scale-up and pilot studies towards the commercial production of biolubricant base stocks.

Selection of feedstock, synthesis  and characterisation of base stock
Vegetable oils (karanja, jatropha, soya bean and linseed) were obtained from local commercial suppliers and were used without further purification. A CEM make MARS 5 microwave system was used for the transesterification reactions. The system was equipped with a round-bottom flask and reflux condenser with magnetic stirrer bar and non-contact infrared continuous feedback temperature system, which allows for the required constant temperature with continuous stirring.

Solvents such as methyl tertiary butyl ether (MTBE) and n-hexane, used as the mobile phase for high- performance liquid chromatography (HPLC), were of the appropriate grade and were degassed prior to use. HPLC analysis was carried out on a Shimadzu advanced Vp HPLC system at a column temperature of 30°C. Separations were made on a Princeton silica column with a particle size of 5 microns and dimensions of 250 mm x 4.6 mm. The mobile phase consisted of a mixture of n-hexane and MTBE in the ratio 90:10 by volume at a flow rate of 0.8 ml/min. A RID 10A detector was used for the detection of eluting components. The optimum detector cell temperature was kept at 30°C and the injection volume was 10 µl. Data acquisition and analysis was carried out on Class Vp software. FT-IR spectra were recorded using an ATR crystal for characterisation of the prepared ester samples on a Thermo Nicolet Nexus instrument.

Typical properties measured for the vegetable oils used in the present study are shown in Table 1. The feedstocks were characterised in terms of their viscosity and pour point according to ASTM D 445 and D97 respectively. Total acid number was estimated according to ASTM D664.
 
Conventional synthesis⁵
The vegetable oils were mixed with an alcohol-catalyst mixture in a glass reactor with a reflux condenser and magnetic bar and subjected to conventional heating for two to six hours.