BTC Biodiesel Workshop Session 2 (1)

8/12/2019 BTC Biodiesel Workshop Session 2 (1)

1/23

BTC PTEC Biodiesel

Workshop

August 7 8, 2006

Session 2 Chemical Background


2/23

Agenda for second session

Biodiesel production

Chemistry background

Chemical compounds Chemical reactions in the production of

biodiesel

Material balance


3/23

Biodiesel Production

Transesterification

Esterification

Pretreatment

Biodiesel

Washing

Biodiesel

Stripping

ASTM Quality

Biodiesel

Gums / Waxes / Insolubles

Glycerine

Neutralization

Glycerine

Stripping

Glycerine

Distillation

USP Grade

Glycerine

Acid Catalyst

Base Catalyst

Water

Oil, Fat or Grease Feedstock

Methanol

Methanol

and Water

Distillation


4/23

Chemical background

Biodiesel is made from a reaction of a

vegetable oil or animal fat with an alcohol

This reaction is called transesterification

and produces an ester plus a glycerol

We will first look at some chemical

structure for compounds of interest in the

making of biodiesel

Then we will look at the reaction


5/23

Chemical compounds

Vegetable oils and animal fats (triacylglycerols)

O

CH2OCR1 O CHOCR2 O CH2OCR3

R groups are from fatty acids

of the form

O

HOCR


6/23

Chemical compounds

Oils and fats are composed of Fatty acids

Saturated fats (no double bonds, C C only)

Good cetane numbers and stability

Poor cold weather properties

Unsaturated fats (one or more double bonds,

C = C)

Can be oxidized Better cold weather properties


7/23

Chemical compounds

Fatty acids

CH3(CH2)14COOHpalmitic acid

CH3(CH2)16COOHstearic acid

CH3(CH2)7-CH=CH-(CH2)7COOH oleic acid CH3(CH2)7-CH=CH-CH2-CH=CH-

(CH2)4COOHlinoleic acid

CH3(CH

2)7-CH=CH-CH

2-CH=CH-CH

2-

CH=CH-CH2-COOHlinolenic acid

CH3(CH2)7-CH=CH-(CH2)11- COOH erucicacid


8/23

Chemical compounds

Things that we will use to make biodiesel are: Alcohols

CH3OHmethanol

CH3CH2OHethanol

CH3CH2CH2OHn-propanol

OH

CH3CHCH3iso-propanol

Bases NaOHsodium hydroxide

KOHpotassium hydroxide

NaOCH3sodium methoxide (sodium methylate, 25% active agent inmethanol)

(We can determine the amount of catalyst needed by titrating asample of the vegetable oil with a base)


9/23

Chemical compounds

The reaction will produce:

Glycerols

CH2OH

CHOH

CH2OH O

Soaps (Naor K)OC - R


10/23

Chemical compounds

And the biodiesel products we want are:

Esters (examples)

O

CH3-C-O-CH3methyl acetate (methyl ester)

O

CH2-C-O-CH2CH3ethyl acetate (ethyl ester)


11/23

Other products

Soaps O O

Na

O

C

R

CH2

O

CR

Mono and diglycerides CHOH OCHOH

Free fatty acids HO - C - R


12/23

The transesterification reaction

O

CH2OCR1 O CH2 - OH

CHOCR2 + 3 CH3OH = 3 CH3OOCRi+ CHOH O CH2 - OHCH2OCR3

Triacylglycerol + alcohol = mixture of fatty acid esters

(biodiesel) + glycerol


13/23

Phases

Biodiesel (upper phase)

Contains esters and some methanol (60:40 split with

glycerine phase)

Water not soluble in this phase Glycerine (lower phase)

Also contains contaminants such as soaps

90+% of soap formed

And unreacted chemicals 95+% of catalyst added

Alcohol split with biodiesel phase


14/23

Fatty acid reactions

Side reactions also occur such as:

Reaction with base to form a soap

RCOOH + KOH = RCOOK + H2O

A pretreatment reaction we might use is a FA

with acid catalyst (H2SO4) and methanol to form

an ester

RCOOH + CH3OH = RCOOCH3+ H2O


15/23

Reactions of esters

Other side reactions may be:

Reaction with bases in water or water to form

free fatty acids and acylates

O O

XOH + RO-C-R = XOR + HO-C-R


16/23

Reaction considerations

Need excess of reactant (100% molar excess of alcohol)

a catalyst (acid or base)

moderate temperature (60 - 65 deg C, 140 150 deg

F) mixing

residence time (2 - 4 hours)

Problems may occur from the presence of

Free glycerol (inhibits reaction) moisture (hydrolysis of FA esters at > 0.5%)

excess catalyst (soap formation)

free fatty acids (soap formation)


17/23

Biodiesel from high FFA feedstocks

To remove free fatty acids (FFA) to

prevent soaps, we can use

Acid catalyzed esterification to reduce FFA to

< 0.5 1% and follow this with

Alkali catalyzed transesterification

Or we can just let them form soaps and

hope for the best (no emulsion formationand not too much loss of product)


18/23

Example mass balance

Reactants 100 pounds of vegetable oil (canola)

23 pounds of methanol (100% excess)

0.4 pounds of sodium hydroxide

Products 100 pounds of ester (assuming 100% yield more

commonly it would be 75% for one step and 98% fortwo steps)

11 pounds of glycerine

12 pounds of methanol (unreacted)

0.4 pounds of sodium hydroxide


19/23

Volume balance

Reactants

13 gallons of vegetable oil (canola)

3.5 gallons of methanol (100% excess)

Products

13 gallons of ester

1 gallon of glycerine

1.7 gallons of methanol (unreacted)


20/23

Transesterification Material Balance

Oil Feedstock

100 lb

Catalyst

0.5 to 1.5 lb

Methanol

10 lb + excess

Acid

Water

1 to 100 lbGlycerine Esters

FFA

0 to 1 lb

Reaction and Separation

WashingAcidulationWaste Water

0 to 100 lb

Excess Methanol

50 to >99%Methanol Removal Methanol Removal

Crude Glycerine

10 lb (pure basis)

Biodiesel

95 to 100 lb


21/23

Other steps in production

Water wash (1 100 pounds)

Methanol recovery

Glycerine recovery or disposal Water treatment and disposal


22/23

Alternative reactant comparison

Alcohol Costs (methanol often cheapest)

Ethanol may be more difficult to recover than methanol, alsoneed more but it is renewable

Propanol and higher alcohol derived biodiesels have lower

freezing points Base catalyst

Sodium hydroxide most common in US due to lower cost

Potassium hydroxide more effective and is common in Europe,residue can be used as a fertilizer

Methoxides used for large scale operations (>5 milliongallons/year) do not form water, most active catalyst

Acid catalyst (sulfuric acid), cheap, does not makesoaps, very slow reaction


23/23

Other chemical issues

Extended storage (>1 year) to result in Oxidation (rancidity)

Polymerization

Reactions catalyzed by metals and favored by contact

with air, water or sunlight Inhibited by anti-oxidants

Microbial attack

Polyunsaturated fatty acids most susceptible tooxidation

Safety Chemicals (Methanol, base, acid)

Disposal of wastes

BTC Biodiesel Workshop Session 2 (1)

Documents

Transcript of BTC Biodiesel Workshop Session 2 (1)