4 Key excerpts on "Esterification"

• 

  eBook - ePub

  Biodiesel Science and Technology

  From Soil to Oil

  • Jan C.J. Bart, N Palmeri, Stefano Cavallaro(Authors)
  • 2010(Publication Date)
  • Woodhead Publishing

    (Publisher)

  7

  TransEsterification processes for biodiesel production from oils and fats

  Abstract:

  Fatty acid alkyl esters may be manufactured by transEsterification of triglycerides, which is essentially an equilibrium reaction of various consecutive, reversible reactions. Process variables of (catalytic) transEsterification of triglycerides with alcohols, such as reaction temperature, nature of the alcohol, molar ratio of alcohol to oil, catalyst type and concentration, purity of reactants, and mixing intensity are critically evaluated in relation to product yield. Various process intensification methods are described, which take advantage of microwaves, ultrasonics, supercritical fluids, dynamic turbulence or co-solvents. The benefits of in-situ transEsterification are also considered. Production of alkyl esters from vegetable oils via non-catalytic reactions also has been evaluated.

  Key words TransEsterification vegetable oils and animal fats process variables process intensification

  in-situ transEsterification

  7.1 Introduction

  Ester formation constitutes one of the most important classes of reactions in value-added processing of animal fats and vegetable oils. Typical schemes for ester formation include:

  7.1

  7.2

  7.3

  7.4

  Commercially, fatty acid alkyl esters (FAAEs) are manufactured either by direct Esterification of fatty acids or by alcoholysis (also called transEsterification) of triglycerides (TGs). Esterification is carried out batchwise at 473–573 K under pressure; water of reaction has to be removed continuously in order to obtain high yields. Esterification can also be carried out continuously in a countercurrent reaction column using a superheated alcohol [1

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Biobased Lubricants and Greases

  Technology and Products

  • Lou Honary, Erwin Richter(Authors)
  • 2011(Publication Date)
  • Wiley

    (Publisher)

  When the COOH group occurs at the end of a long hydrocarbon chain, the substance is referred to as a fatty acid. Fatty acids are components found mixed with the oils of oilseed plants and present problems in the formulation of biodiesel from these oils. Organic acids tend to be more corrosive than alcohols. Therefore, when oxidation occurs in a lubricant to form an organic acid, the resulting material is more detrimental to the device being lubricated.

  Another class of organic compounds essential to the biolubricants area is a class of compounds called esters. An ester is formed when an organic alcohol reacts with an organic acid. Esters often have very pleasant aromas and are responsible for the “nose” in a glass of wine or the flavor of fruits and vegetables. Wintergreen, for example, is an ester derived from methanol and salicylic acid. If one were to react ethanol with acetic acid, an ester would form (Figure 2.12 ). This compound called ethyl acetate (ethyl from the alcohol, acetate from the acid) is used as a solvent to remove finger nail polish. HOH (water) is lost when an acid and an alcohol react together. This is an important observation. Whenever an ester is formed from an acid and an alcohol, water is the byproduct of the reaction. The entire reaction can be reversed in that water can be added to an ester to regenerate the original acid and alcohol.

  Figure 2.12 Ethyl acetate

  One of the components of biodiesel fuel is methyl oleate. The material is an ester that is formed when methanol is reacted with oleic acid.

  If the -OH group is removed from methanol and the -H is removed from the oxygen atom at the end of the oleic acid molecule and then joined together, water (HOH) is formed. The other substance formed in the same reaction is methyl oleate (Figure 2.13 ) where the -CH3

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Biochemistry

  An Organic Chemistry Approach

  • Michael B. Smith(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  The mechanism for acid-catalyzed Esterification of carboxylic acids is completely reversible, but it is driven to the right (toward the ester product), by application of Le Chatelier’s principle and removal of water. A different application of Le Chatelier’s principle uses a large excess of the alcohol (butanol for the formation of butyl acetate) to shift the equilibrium toward the ester by increasing the probability that it will react with alcohol rather than with water. Note that if butyl acetate is treated with an acid-catalyst and water rather than butanol, the reverse of the mechanism shown will convert the ester back to the acid.

  The Esterification reaction is an acid-catalyzed acyl substitution. Acyl substitution of an ester to give a different ester is possible, where one alcohol unit replaces another. In other words, replacing one OR group in an ester with a new group (OR′) will yield a different ester. This reaction is known as transEsterification . If methyl butanoate is heated with a large excess of ethanol and an acid-catalyst, the product is ethyl butanoate. Analysis of the reaction shows that the OEt unit of ethanol attacks the acyl carbon with loss of methanol. This also requires a proton transfer, and the acid-catalysts suggests formation of an oxocarbenium ion. This acyl substitution must proceed by a tetrahedral intermediate that contain both the OMe (methoxy) and the OEt (ethoxy) units. This sequence is driven toward the ethyl ester because a large excess of ethanol is used (ethanol is the solvent) in accordance with Le Chatelier’s principle. The overall sequence has interchanged OMe for OEt. This is the reason the reaction is called transEsterification.

  In any acid-catalyzed reaction transEsterification there is a potential problem if the OR unit in the alcohol solvent is different from the OR unit of the ester. If, for example, methanol is used as a solvent for the reaction of an ethyl ester, the final product may be a mixture of methyl and ethyl esters. If those esters must be separated, this mixture may be a problem. If the alcohol is the same as the “alcohol part” of the ester, transEsterification generates the same ester. In other words, if methanol is used with a methyl ester and ethanol is used with an ethyl ester there will be no structural changes in the ester unit of the final product.

  8.5 Biosynthesis and Biodegradation of Esters

  Fatty acid biosynthesis has been studied extensively for nearly 60 years. An accepted mechanism is shown in Figure 8.10 .8 a and begins with acetyl-CoA, which reacts with malonyl CoA in the presence of β-ketoacyl-ACP synthase to give the corresponding ACP derivative, the acyl carrier protein. Acetyl-CoA participates in many biochemical reactions in protein, carbohydrate and lipid metabolism where it delivers the acetyl group to the citric acid cycle that can be oxidized for energy production. This protein is an important component in fatty acid biosynthesis with the growing chain bound during synthesis as a thiol ester at the distal thiol of a 4′-phosphoantetheine moiety, an essential prosthetic group of several acyl carrier proteins involved in pathways of primary and secondary metabolism including the acyl carrier proteins (ACP) of fatty acid synthases . Fatty acid synthase (EC 2.3.1.85) is a multienzyme protein that catalyzes fatty acid synthesis. It is not a single enzyme but a whole enzymatic system. The main function is to catalyze the synthesis long-chain saturated fatty acids from acetyl-CoA and malonyl-Co-A, in the presence of NADPH. Indeed, acetyl-S-ACP and malonyl-S-ACP are converted to acyl acetonate-S-ACP in the presence of fatty acid synthase. The stereoselective reduction to the β-hydroxy compound occurs with NADPH, which is reduced to NADP+ (Nicotinamide adenine dinucleotide phosphate is a cofactor used in anabolic reactions that require NADPH as a reducing agent. NADPH is the reduced form of NADP+ ). Elimination of the alcohol moiety gives the conjugated derivatives, and further reaction with NADPH give the butanoyl-S-ACP derivative. This sequence is repeated to generate palmitoleoyl-ACP (Figure 8.10

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Practical Handbook of Soybean Processing and Utilization

  • D. R. Erickson(Author)
  • 2015(Publication Date)
  • Academic Press and AOCS Press

    (Publisher)

  Chapter 16

  InterEsterification

  Michael D. Erickson,     Kraft Food Ingredients Technology Center, 8000 Horizon Blvd., Memphis, TN

  Introduction

  During the synthesis of fats and oils in maturing plants and animals, enzymes attach free fatty acids to glycerol in a specific order. InterEsterification changes this orderly distribution to a random distribution, provided the temperature of the reaction is above the melting point of the oil. There are two exceptions to the rule of randomized distribution: directed interEsterification and enzymatic interEsterification. Although the emphasis of this chapter is on chemical interEsterification, both alternatives are discussed in following sections.

  InterEsterification, often referred to by the appropriately descriptive terms randomization or rearrangement , offers an important alternative for modifying the behavior of fats and oils. The reaction begins when an appropriate catalyst is added to the oil. The “active form” of the catalyst then forms, which promotes the detachment of fatty acids from the glycerol “backbone.” As the reaction proceeds, fatty acids detach and reattach simultaneously at open positions within the same glyceride and at vacant positions on adjacent glycerides. Thus, when the reaction achieves equilibrium, the fatty acids have formed a new mixture of triglycerides that no longer reflects their initial orderly distribution.

  The finished product performance of hydrogenated fats and oils is due in large part to physical changes in the fatty acids (trans isomers). InterEsterification, however, does not change the fatty acid profile of the starting material. The changes in melting and solidification properties of interesterified fats and oils are due to the relative proportions of component glycerides after rearrangement of the fatty acids (1

  Sign up to read

  Learn more about book