Kinetics of Condensation Reaction of Crude Glycerol With Acetaldehyde in a Reactive Extraction Process

8/3/2019 Kinetics of Condensation Reaction of Crude Glycerol With Acetaldehyde in a Reactive Extraction Process

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temperature, mole ratio of the reactants, and agitation was carried out to get for maximumconversion & high yield of glycerol. The experimental results were found to fit a 2nd order rate

equation for the forward reaction. The influence of temperature on the kinetic constants was

determined & the activation energy was obtained.

Introduction

Glycerol is produced as a side product in the transesterification of vegetable oils during

biodiesel production. The crude glycerol contains methanol, water, alkali, soap and organicmatter. It is conventionally refined through filtration, chemical additions and distillation to yield

an industrial grade glycerol with purity close to 90%. When glycerin is used in food, cosmetics,

and drugs, further purifications such as bleaching, de-odorizing, and ion exchange treatment are

needed to remove all trace impurities and get purity close to 99.9% [1]. With increased production of biodiesel market more and more crude glycerol is continuously generated.

Research is therefore needed to explore effective ways of the glycerol utilization. Moreover

separation and recovery of glycerin through processes that are less energy intensive as compared

to the existing technologies need to be developed.

Presently biodiesel production using crude vegetable oils involves two stages. An acidcatalyzed esterification step followed by a base catalyzed transesterification step [2-5]. The

esterification is carried out to convert the high concentration of free fatty acids present in the oil

to alkyl ester and the transesterification is carried out to convert the triglycerides to the

corresponding alkyl ester. The commonly used catalysts being sulfuric acid for esterification, potassium hydroxide and sodium hydroxide for transesterification reaction and the most

preferred alcohol methanol [6]. After the final processing is completed the product is obtained in

two layers: the methyl ester layer and the glycerol layer. The product layers are separated,weighed and analyzed after which the biodiesel layer is washed with water and dried to get

methyl ester with high purity and yield [7]. The purification steps involved in the treatment of

the biodiesel glycerol is largely based on the requirement of the final purity of glycerol. Thecrude glycerol contains glycerol 33%, methanol 49.6%, fatty matter 0.07% and free fatty acids.

In some biodiesel processes the transesterification reaction is done in two stages wherein the first

stage the glycerol layer is obtained. In the second stage the glycerol formed during the reaction ispresent in the biodiesel layer and no separate layer is formed. This glycerol lost during the

washing of the biodiesel layer ranges from 4-6% by weight.

In the present work, the washed water effluent layer from biodiesel washing stage iscombined with the crude glycerol stream as a result the methanol and the glycerol from the

biodiesel layer are recovered during glycerol recovery. Due to the addition of the water effluent

layer the concentration of glycerol in the stream is reduced greatly.

Conventionally recovery of glycerol from aqueous solutions is usually achieved in

multiple effect evaporators or distillation. The heat required per kg of recovered material is highand the purification steps increase with increase in impurity levels. Different methodologies for

achieving pure glycerol other than the conventional approach may prove to be more energy

efficient. An alternate easier recovery approach is by reactive extraction. The present study is

therefore focused mainly on recovery of glycerol from aqueous streams by reversible reactions.


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Compared to evaporation and distillation, extraction process is advantageous as the energy

requirement is lower [8, 9].

The recovery of pure glycerol from aqueous effluent streams via reversible reactions is

studied in the present work. The reactive extraction route followed by hydrolysis is adopted for

the present process. The process utilizes an acid catalyzed condensation reaction of glycerol withacetaldehyde to form glycerol acetal. The acetal is hydrolyzed to recover the glycerol. As both

the reactions are equilibrium limited, the reactions are forced to completion by removing the

reaction product continuously.

Research on extraction of poly hydroxyl compounds was carried out by Tink et al [10].

The authors had applied the technique to extract sugars and sugar alcohols from dilute solutions

by cyclic acetal formation. Robert et.al [11] studied the recovery of 1, 2-propanediol and 1,3-propanediol from a dilute aqueous solution through cyclic acetalization with formaldehyde and

acetaldehyde. The acetals obtained in the reaction were separated from the solution by

distillation or extraction. Atul et al [12] and Chopade et al [13] reported the recovery of 1,2-

propanediol and ethylene glycol from an aqueous solution via reactive distillation; theacetalization reaction was also carried out with formaldehyde or acetaldehyde. Malinowski et al

[14] reported the recovery of PDO by cyclic acetalization with acetaldehyde to form 2-methyl-1,3-dioxane; the acetal was recovered by o-xylene, toluene, or ethylbenzene extraction, but the

hydrolyzation of the acetal was not studied.

Broekhuiset et al [15] compared reactive distillation and reactive extraction viaacetalization with formaldehyde or acetaldehyde to conventional separations for the recovery of

PG from dilute solution. Chopade et al [16] studied the recovery of EG from aqueous solution

via acetalization with formaldehyde using ion exchange resins as catalysts in batch andcontinuous reactive distillation. Chopade et al [17] have also generated the vapor pressures of the

acetals and vaporliquid equilibrium data for 24DMDwater and 2MDwater. Reaction

equilibrium constants for acetalization were measured via simple batch experiments. Moreoverthe same authors [18] have developed a method for recovery of EG or PG from aqueous glycol

streams produced via epoxide hydration.

The present work is a part of the work related to the process development for biodiesel

production using crude vegetable oils. In the present work the recovery of glycerol is carried via

reversible reactions. The condensation reaction of glycerol with acetaldehyde to form glycerol

acetal, followed by hydrolysis of acetal to recover the glycerol. The glycerol acetal formedduring the condensation reaction is continuously extracted using toluene as the solvent. Present

work is focused on the kinetics of this reactive extraction process. The new route for recovery of

pure glycerol from crude glycerol stream is less energy intensive than the existing conventionalpurification processes.

Experimental


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The crude glycerol stream after the addition of the effluent water streams is alkaline in

nature because of the alkali catalyst. The pretreatment step involves acidification with sulfuric

acid to pH 3-4. The soap and alkali present in the mixture is converted to FFA in this stage. TheFFA being insoluble separates out at the top as a layer and is filtered out. The salt remains

dissolved in the mixture. The remaining mixture is subjected to distillation to recover the alcohol

present in the system. The crude glycerol then contains glycerol 42%, water 46 %, fatty matter6% and dissolved salts 6%. The reversible reactions are carried out after the pretreatment step to

recover glycerol in pure form.

Chemicals

Acetaldehyde (28 %), toluene (99.8 %), and catalysts sulphuric acid and amberlyst-15

were purchased from SD fine chemicals, India. The chemicals were used without further

purification and amberlyst -15 was used in acidic form.

Experimental procedure

Preparation of the glycerol acetal from crude glycerol was carried out in a 500 ml four

necked jacketed glass reactor equipped with a condenser, overhead stirrer, provision fortemperature measurement and sample withdrawal. The temperature in the reactor was maintained

by circulating hot fluid through a thermostat. The condenser temperature was maintained bycirculating chilled water.

A typical run was started by charging the reactants and extracting solvent into the reactor.

The reaction was initiated with the addition of the catalyst after the desired temperature wasattained. Samples were drawn periodically and analyzed. The acetal formed was continuously

extracted from the aqueous layer by the addition of a solvent like toluene. Water formed in the

reaction is retained in the aqueous layer. After completion of the reaction the two product layerswere separated, weighed and analyzed. The final reaction mixture was also analyzed to get

values for the equilibrium constant at various temperatures.

Analysis

The layers obtained were analyzed for the distribution of the acetal in both the phases.

The analysis was carried out on a Gas Chromatograph GC-17A, FID detector. An FID detectorwas utilized for analysis of organic layer. The column used was 5% SE-30, initial temperature

130 oC, final temperature 210 oC, rate 10 oC/min, injector temperature was 220 oC and detector

temperature 230 oC. Nitrogen was used as carrier gas. After equilibrium was attained also the

analysis was carried out. The aqueous layer was analyzed for glycerol concentration by, sodiumperiodate method.

Experimental details

Experiments were carried out at four different temperatures (5, 10, 15, 20 & 25 oC

respectively), with 1:1 mole ration of glycerol to acetaldehyde. The effect of temperature on the

conversion of glycerol and the extraction of glycerol acetal was determined. Based on theconversion time data the kinetics were established.

Results and Discussion


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Thermo chemistry

The feasibility of the condensation reaction with acetaldehyde is checked by calculating

the free energy. The standard heat of reaction at 25 oC was calculated to be 91.6 kJ/mol, Gibbsfree energy of reaction at 25 oC was 125 kJ/mol. The estimated values indicate that the

conversion of glycerol to glycerol acetal using acetaldehyde is an endothermic in nature and

therefore the conversions are expected to increase with increase in temperature. However as theboiling point of acetaldehyde is 21 oC and it is volatile in nature all the reactions are conducted at

temperatures lower than 25 oC. The equilibrium constant K was calculated from the experimental

data at different temperatures using the expression given below.

]][[

]][[

deacetaldehyglycerol

wateretalglycerolacK= . (1)

Effect of Temperature

The conversion of glycerol to glycerol acetal increased with increase in

temperature and the trend is depicted in Figure 2. As is evident from the plot the maximum

conversion attained is 88%.

0 5 10 15 20 25 30

0

10

20

30

40

50

60

70

80

90

100

%Conversionofglycerol

Temperature0C

Figure: 2 Effect of temperature on conversion of glycerol for the condensation reaction of glycerol withacetaldehyde.

The effect of temperature on the distribution ratio of toluene is given in Figure 3.


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5 10 15 20 25 30

0

2

4

6

8

10

KD

Temperature,0

C

Figure: 3 Effect of temperature on the distribution ratio for acetaldehyde-glycerol reaction

The extraction efficiency calculated based on the weight fraction of the product in

extractant layer to that of the raffinate layer when obtained at different temperatures showedmarked increase in the efficiency with increase in temperature as is shown in Figure 4.

5 10 15 20 25 30

30

40

50

60

70

80

90

100

Temperature0C

Figure: 4 Effect of temperature on extraction efficiency for acetaldehyde-glycerol reaction.


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Kinetic Interpretation

The following reaction takes place during the condensation reaction of glycerol withacetaldehyde

WateretalGlucerolacdeAcetaldehyGlycerol ++

All the reactions were carried out at 1:1 molar ratio of glycerol to acetaldehyde hence asecond order elementary kinetic expression was assumed.

SRBAACCkCCkr

21= . (2)

Where CA, CB, CR & CS represents the concentrations of glycerol, acetaldehyde,glycerolacetal and water respectively. The equilibrium constant K is defined as:

2

1

k

kK = . (3)

Solving the above rate equation gives

tCkKXXK

XXKAO

AA

AA22

)1(

)1(ln =

+

.. (4)

Where XA represents the conversion of glycerol and CAO represents the initial

concentration. The value of K is obtained from equation (1). The experimental data of X A vs. t isconfronted with the above equation to get the value of k2 while the rate constant k1 is calculated

form the equation (3).

A plot of equation (4) at a tmeperature of 10 oC is shown below

0 1 2 3 4

0.00

0.75

1.50

2.25Y = 0.5899*t

R2= 0.92

Y

Time, t

Figure: 5 Plot of Y vs. t, where Y is defined as

AA

AA

XXK

XXK

+

)1(

)1(ln in the equation (4)


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The slope obtained from the above graph gives the value of k2. The rate constant k1 is

calculated by the equation (3). Similar calculations were done at other temperatures and the

corresponding rate constants are shown below

Temperature (oK) k1 (lit/mol.hr) k 2 (lit/mol.hr)

278 0.0244 0.0385283 0.0445 0.0531

288 0.09 0.0686

293 0.147 0.0951

The Arrhenius plot for the forward reaction is shown below

0.00340 0. 003 44 0. 00348 0.003 52 0.0 035 6 0.00360

-3.6

-3.2

-2.8

-2.4

-2.0ln k

1= -9917.6*(1/T) + 31.964

lnk

1

1/T, (/oK)

Figure: 6 Arrhenius plot for the reaction

From the above graph the slope (-Ea/R) = -9917.6.Activation energy (Ea) = 9917.6*1.984 = 19676.52 cal/mol

Conclusions

The recovery of glycerol in its purest form from aqueous solutions containing impurities

such as salts and solvents is achieved in this paper. The glycerol in the aqueous solution is

converted to glycerol acetal by reaction with acetaldehyde, extracted into an organic layer and

hydrolyzed back to glycerol. Chemical equilibrium limitations do not allow the reaction toproceed to an appreciable extent therefore simultaneous extraction of the product acetal into the

toluene pushes the reaction forward. The kinetics of the reaction showed that the activationenergy is 19.67 kcal/mol, which indidates that the reaction proceeds well in the low temperature

region. The reactive extraction approach is an attractive alternate route for attaining pure

glycerol.


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Kinetics of Condensation Reaction of Crude Glycerol With Acetaldehyde in a Reactive Extraction Process