Biodiesel Fuel

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Introduction

Biodiesel fuel is mono alkyl esters of long chain fatty acids derived from a renewable lipid feedstock, like animal fat or vegetable oil. The word ‘Bio’ represents its renewability and biological source in contrast to traditional petroleum-based diesel fuel; and “diesel” refers to its use in diesel engines. As an alternative fuel, biodiesel can be used in neat form or mixed with petroleum-based diesel.

Biodiesel, is an alternative fuel, has many merits. It is renewable, domestic resource, which is non-toxic and biodegradable. Biodiesel when compared with petroleum-based diesel, it more favorable in the release of low emissions of particulate matter, carbon monoxide and unburned hydrocarbons. Carbon dioxide released by combustion of biodiesel can be recycled by photosynthesis, thereby minimizing the greenhouse effect. Biodiesel has a relatively high flash point (150 °C) and less volatile, which makes it to handle and use as a transport fuel or than petroleum diesel. It provides lubricating properties that can reduce engine wear and extend engine life. These properties of biodiesel made it to use as an alternative to petroleum-based fuel

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Biodiesel is produced by transesterification, which refers to a catalyzed chemical reaction involving vegetable oil and an alcohol to yield biodiesel and glycerol. Vegetable oil, consist of three long chain fatty acids esterified to a glycerol backbone. When triacylglycerols react with an alcohol (methanol), the three fatty acid chains are released from the glycerol skeleton and combine with the alcohol to yield fatty acid alkyl esters (fatty acid methyl esters or FAME). Glycerol is produced as a by-product. Methanol is the most commonly used alcohol because of its low cost.

Transesterification reactions can be acid-catalyzed, alkali-catalyzed, and enzyme-catalyzed. As for the enzyme-catalyzed system, it requires a much longer reaction time than the other two systems.

Biodiesel produced from virgin oil costs much more than petroleum-based diesel. Instead of using virgin oil to produce biodiesel, waste cooking oil can be used to produce biodiesel is an effective way to reduce the raw material cost because it is estimated to be about half the price of virgin oil. In addition, using waste cooking oil could also help to solve the problem of waste oil disposal

Biodiesel Environmental benefits

  • By the use of Biodiesel carbon monoxide (CO) emissions can be reduced by 50 % and carbon dioxide emissions by 78 % approximately and sulfur emissions (SO2), because biodiesel does not contain sulfur.
  • Biodiesel usage reduces 65 % of the emission of small particles of solid which can reduce cancer risks up to 94 %.
  • NOx emissions released from Biodiesel are less compared to petro diesel. The release in NOx emissions may be due to the higher cetane rating of biodiesel.
  • Biodiesel has higher cetane rating than petro diesel, ignites more rapidly when injected into engine.
  • Tests conducted by the United States Department of Agriculture confirm that biodiesel is less toxic than table salt and biodegrades as quickly as sugar.
  • Petroleum diesel (64 °C) has low flash point compared to biodiesel (>150 °C). Depending on the proportion of different types of esters the gel point of biodiesel. Biodiesel has higher gel and cloud point than petroleum diesel [1].

Biodiesel feed stocks

Feedstocks of biodiesel include Food grade cooking oils like sunflower, canola, peanuts and rapeseed etc. Waste vegetable oils, including lard, chicken fat, Animal fats and fish oils are the sources of biodiesel [2].

Biodiesel production processes

Biodiesel can be produced from the Transesterification reactions like acid-catalyzed, alkali-catalyzed and enzyme-catalyzed. As for the enzyme-catalyzed system, it requires a much longer reaction time than the other two systems.

Alkali-catalyzed system

Alkali-catalyzed process is sensitive to the purity of reactants; the alkali-catalyzed system is very sensitive to both water and free fatty acids. The presence of water may cause ester saponification under alkaline conditions. Free fatty acids can react with an alkali catalyst to produce soaps and water. Saponification not only consumes the alkali catalyst, but also the resulting soaps can cause the formation of emulsions. Emulsion formation creates difficulties in downstream recovery and purification of the biodiesel. Thus, dehydrated vegetable oil with less than 0.5 wt. % free fatty acids, an anhydrous alkali catalyst and anhydrous alcohol are necessary for commercially viable alkali-catalyzed systems. This requirement is likely to be a significant limitation to the use of waste cooking oil as a low-cost feedstock. Usually the level of free fatty acids in waste cooking oil is greater than 2 wt.% recommended , a pretreatment step to reduce the free fatty acid content via an esterification reaction with methanol in the presence of sulfuric acid catalyst. Glycerin was employed as a liquid entraining agent to purify the refined oil. After such a treatment, the oil phase, having a low level of free fatty acids (less than 0.5 wt %), was subjected to the alkali-catalyzed transesterification. Such a pretreatment step was applied to the alkali-catalyzed process using waste cooking oil in the present study.

Pretreatment of waste oils by Acid-catalyzed process for prior to alkali-catalyzed production of biodiesel

Alkali-catalyzed process using waste cooking oil

Esterification

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The esterification reaction was carried out at 70 °C, 400 kPa and a 6:1 molar ratio of methanol to crude oil. The fresh methanol stream (128 kg/h), the recycled methanol stream (188 kg/h) and the H2SO4 stream (10 kg/h) were mixed before being pumped into esterification reactor by pump. The waste cooking oil stream (1050 kg/h), containing 6% free fatty acids, was heated in exchanger to 60 °C before entering esterification reactor. In esterification reactor, all the free fatty acids were converted to methyl esters. After being cooled to 46 °C, stream was forwarded to glycerin washing column to remove the sulfuric acid and water.

Glycerine washing

The resulting water and acid catalyst (H2SO4) from esterification reactor must be removed completely before proceeding to the alkali-catalyzed transesterification. By adding 110 kg/h of glycerine at 25 °C and 200 kPa, all of the resulting water was removed from oil stream, after three theoretical stages of washing. Stream from glycerine washing was sent to downstream transesterification unit. On the other hand, oil stream (336 kg/h) contained 60% unreacted methanol, 33% glycerol, 3% sulfuric acid, 3% oil, 1% water and traces of esters. Recovering most of the methanol in this stream for reuse in esterification reactor was a logical step, which was realized in methanol recovery column.

Methanol recovery

In methanol recovery column, five theoretical stages and a reflux ratio of 5 were used. At 28 °C and 20 kPa, 94% of the total methanol fed to the column was recovered in the distillate at the rate of 188 kg/h. It contained 99.94% methanol and 0.06% water and was recycled to esterification reactor. At 70 °C and 30 kPa, bottom stream (147 kg/h) was composed of 75% glycerol, 8% methanol, 7% sulfuric acid, 7% oil and 3% water. Due to the presence of sulfuric acid, this stream was not reused and was treated as waste. Nevertheless, neutralizing the sulfuric acid and then recovering the glycerol is a feasible alternative to reduce waste. Despite the decrease in raw material cost by using waste oil, the addition of a pretreatment unit to reduce the content of free fatty acids in the feedstock oil in the process.

Acid-catalyzed process using waste cooking oil

An acid-catalyzed continuous process from waste cooking oil appears to be a promising alternative to the alkali process.

Transesterification

The reaction conditions were set to a 50:1 molar ratio of methanol to oil, a 1.3:1 molar ratio of sulfuric acid to waste oil, a reaction temperature of 80 °C and a pressure of 400 kPa. Fresh methanol (216 kg/h), recycled methanol (1594 kg/h) and sulfuric acid (150 kg/h) were mixed first and fed to transesterification reactor by pump. Waste cooking oil (1030 kg/h) entered transesterification reactor after being heated to 60 °C in exchanger. In transesterification reactor, 97% of the oil was assumed to be converted to FAME after 4 h.

Methanol recovery

Because of the large excess of methanol in stream, methanol recovery was the first step following the reaction in order to reduce the load in the downstream units. In methanol distillation column, five theoretical stages, a reflux ratio of 2 and vacuum distillation were employed. 94% methanol recovery rate was achieved in stream and recycled to. Bottom stream was forwarded to acid removal unit.

Acid removal

For acid removal, the design principle was the same as for alkali removal. In acid removal unit, sulfuric acid was completely removed in a neutralization reaction by adding calcium oxide (CaO) to produce CaSO4 and H2O. Calcium oxide was used primarily due to its low-cost relative to other alkali substances. Also the water produced would be absorbed by the resulting CaSO4 to form CaSO4·2H2O. However, since absorption of water by CaSO4 is relatively slow, in our current simulations CaSO4 rather than CaSO4·2H2O was considered as a solid waste. A gravity separator was employed to remove the CaSO4. The resulting stream (1247 kg/h) consisting of 79% FAME, 9% glycerol, 8% methanol, 2% unconverted oil and 2% water proceeded to water washing column.In terms of equipment sizing and operating conditions, the remaining water washing column and purification units (i.e., FAME purification column and glycerol purification column )

Other considerations

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There are some other methods to increase oil conversion to the esters. Based on our recent experimental results, a 99% oil conversion to FAME was observed after 4 h when a 245:1 molar ratio of methanol to oil and 80 °C were used. A qualified biodiesel product was achieved, as well as the reduction of unconverted oil waste. However, such a huge excess of methanol in the system resulted in a very large increase in the sizes of the reactor, methanol distillation column and other separation units. [3]

Conclusion

The alkali-catalyzed process using virgin oil had the lowest total capital investment because of the relatively small sizes and carbon steel construction of most of the process equipment. For a plant producing 8000 tonne/year biodiesel, the total capital investment in was approximately half of that in the other processes. When waste oil of low cost was used as the raw material, alkali-catalyzed process required a pretreatment unit to reduce the content of free fatty acids. The cost associated with this pretreatment unit, including the cost for extra solvent, more than balanced the credit of using waste oil. [4].

References

  • http://www.biofuels.ru/biodiesel/what_bd/
  • F. Ma, M.A. Hanna / Bioresource Technology 70 (1999) 1-15
  • Bioresource Technology Volume 89, Issue 1, August 2003, Pages 1-16
  • BioresourceTechnology Volume 90, Issue 3, December 2003, Pages 229-240