Chemical Engineering and Processing: Process Intensification
ReviewProcess intensification technologies in continuous biodiesel production
Introduction
The majority of commercial biodiesel is made by transesterification of vegetable oils and animal fats with methanol or ethanol in stirred tank reactors in the presence of base or acids catalysts. There are some challenges related to this process as follows:
- a.
Reaction rate can be limited by mass transfer between the oils and alcohol because they are immiscible;
- b.
Transesterification itself is a reversible reaction and therefore there is an upper limit to conversion in the absence of product removal;
- c.
Most commercial processes are run in a batch mode and thus do not gain some of the advantages of continuous operation.
In order to overcome these problems, current conventional techniques involve long reaction time, high molar ratio of alcohol to oil and catalyst concentration. High operating cost and energy consumption are required to purify biodiesel and recover excess amount of alcohol and catalysts during downstream processing. Significant amounts of toxic waste water may also be produced during downstream purification. Long residence times and downstream processing time incur low production efficiency. Hence, some process intensification technologies have been developed and applied to improve mixing and mass/heat transfer between the two liquid phases in recent years. These technologies either utilize novel reactors or coupled reaction/separation processes. Reaction rate is greatly enhanced and thus residence time may be reduced. Some of the technologies have been applied successfully in commercial production. We do not specifically review intensification technologies involving advanced materials such as new heterogenous catalysts in packed or fluidized bed reactors. The goal of this review is to give an overall summary of process intensification technologies which enhance physical processes including heat, mass, and momentum transfer in the context of biodiesel synthesis.
Section snippets
Static mixers
Static mixers consist of specially designed motionless geometric elements enclosed within a pipe or a column and create effective radial mixing of two immiscible liquids as they flow through the mixer. Recently, they have been used in continuous biodiesel synthesis in combination with other equipment [1], [2].
Thompson and He [3] used a stand-alone closed-loop static mixer system as a continuous-flow reactor to produce biodiesel from canola oil with methanol when sodium hydroxide was used as
Membrane reactors
Membrane reactors integrate reaction and membrane-based separation into a single process. They can increase the conversion of equilibrium-limited reactions by removing some products from the reactants stream via membranes. Dube et al. [32] exploited the possibility of biodiesel production from canola oil and methanol using a two-phase tubular membrane reactor as shown in Fig. 11. The pore size of the carbon membrane used in the reactor was 0.05 μm. The inner and outer diameters were 6 and 8 mm,
Summary
Biodiesel production can be enhanced by process intensification technologies. Each technology has the potential to improve production efficiency and thus reduce operating cost of the process. Table 1 summarizes the advantages of these technologies used in continuous biodiesel production over traditional stirred tank reactors. A rudimentary assessment of reaction time, energy efficiency, operating/capital cost, the difficulty of temperature control, and current status is presented in the table.
Conclusions
Process intensification technologies have significant potential for enhancement of biodiesel production. Enhancement in transport processes and higher reaction rates provide the scope for continuous production. Hence higher conversion yields are possible, under milder conditions and involving reduced molar ratios of alcohol to oil, lower reaction temperature and catalyst concentration than conventional stirred reactors. Some process intensification technologies offer the flexibility to process
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