Enzymatic production of biodiesel from used/waste vegetable oils: Design of a pilot plant
Introduction
The world is witnessing rapid population and economic growths, which result in an increase demand in primary energy around the globe. All countries in the Gulf region are net users of petroleum fuels. In this context, as an indigenous and renewable energy source, the use of biofuels can play a vital role in reducing the dependence on petroleum use and catalyzing the rural economic development [1].
Biodiesel (BD) a methyl (or ethyl) ester of long-chain fatty acids derived from vegetable oils or animal fats, represents a promising alternative fuel for use in compression-ignition (diesel) engines. It comes from renewable sources, biodegradable and less toxic as it is not petroleum-derived. Compared to petroleum-based diesel, BD has a more favourable combustion emission profile, such as low emissions of carbon monoxide, particulate matter and unburned hydrocarbons. In addition, BD has a relatively high flash point (150 °C) [2], [3] that makes it less volatile and safer to transport or handle than petroleum diesel. Furthermore, it provides lubricating properties, which reduce engine wear and extend engine life. At the same time, BD has physical properties and energetic content close to those of petroleum diesel, which allows its efficient function in conventional diesel engines without any modification.
At present, the high cost of BD is the major obstacle to its commercialization and it is mainly due to the cost of the highly purified straight vegetable oil (SVO) used as feedstock and this problem can be overcome by using used/waste vegetable oils that is much cheaper. Another obstacle that may face the production of BD is the rising voices against its production using food resources as feedstock which pushes the food prices high. Both problems can be resolved by using waste/used oil (WO), where no competition with food takes place. In addition, using WO could also help to solve the problem of WO disposal. The main problem facing the conventional processes, using alkali catalyst, is that it is not suitable for use with oils containing high free fatty acids (FFA) contents, such as WO. On the other hand, enzymatic transesterification does not have this limitation and hence can be used with WO. Further, all FFA present in the WO can be converted to BD using this approach.
The objective of the work is to design a plant to produce 1 ton h−1 of BD from WO using the enzymatic approach. Positive results of this work would boost the biodiesel industry.
Section snippets
Reactants and reagents
As mentioned earlier, WO will be used as a feedstock. The lipase-catalyzed transesterification reaction takes place under atmospheric pressure and a temperature of 45 °C, which is reported to be the optimum for the (Novozyme 435) [4].
Methanol is the most commonly used alcohol in BD production, mainly because of its high reactivity and relatively low cost. The effect of alcohol, specifically methanol, on the enzymatic production of BD has been thoroughly discussed in literature. It has been found
Heat exchanger (HE)
The general design equation for heat transfers is presented in Eq (1)where Q is heat-transfer rate, Uo is the overall heat-transfer coefficient based on the outside area of the tubes, ΔTlm is the log mean temperature difference, F is correction factor and Ao is the heat-transfer area based on the outside area of the tubes given by Eq (2),where do is the outside tube diameter and L is the length of the heat exchanger.
The overall heat-transfer coefficient is the sum of several
Capital cost estimation
In this project, the FDs, PBR, LLE and VDC are all considered vessels. Since all the materials handled in the plant are relatively non-corrosive, the material of construction for all equipment in the plant is chosen to be carbon steel, which is cheap and easy to weld. In addition, the strength of carbon steel withstands the operating temperatures and pressures.
The purchase cost, CPo, of all equipment were calculated using Eq (31)where S is the size of the
Conclusion
A pilot plant has been designed to produce 1 ton h−1 biodiesel (BD) from waste/used vegetable oil using enzymatic approach. The amount of feed streams of WO, methanol and tert-butanol required were found to be 1138, 130 and 7.6 kg/h, respectively. The main units in the proposed plant were designed and the economic feasibility of the process was assessed. It was found that the total capital investment required is about US$ 620,000, which will be paid back within four years of operation.
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