3. RESULTS AND DISCUSSIONS
3.2. PHASE BEHAVIOR TESTS
This section deals with equilibrium phase behavior of components employed in trans-‐esterification of vegetable oils8. It has been known that vegetable oil, the substrate of biodiesel synthesis (TG: tri-‐gliceride) does not dissolve (in) methanol (reagent) [166]. If methanol is poured over a vegetable oil the methanol will form a layer above the oil. It has therefore been generally recommended to use intensive mixing and excess amount of reagent to achieve equilibrium conditions in trans-‐esterification within a period of close to ½ hour if conventional conditions: 55°C, 2 times the stochiometric rate of reagent to feedstock and K-‐methoxy catalyst are employed. Since the reagent and the substrate form two distinct immiscible phases intensive mixing is needed to bring them and the catalyst methoxy into intimate contact along the entire reaction step. This intensive mixing does not only promote the forward (methanolysis), but the reverse (hydrolysis) reactions alike. If water is present the forward reaction is slowed down and even frozen. Water is being the byproduct of soap formation and is brought to the system if the catalyst is prepared from K/Na-‐OH and MeOH.
ROOH + KOH ROOK + H2O MeOH + KOH [K+][OMe-‐] + H2O
I proposed the use of an apolar solvent to avoid phase transfer related transport and phase separation difficulties in biodiesel synthesis and refining steps. It must be confessed that the idea came from techniques of acid number test of lubricating oils and from experience gained in refining lubricating oils and separation of aromatics of BTX boiling range gasolines. In contrast to Boobcock’s [167] idea of using polar solvent – tetra-‐hydro-‐furan: THF-‐ to eliminate the mass transfer resistance through the interface I opted for the use of an apolar solvent. My reasons have emerged from considering basic chemical engineering principles:
• the use of a polar solvent makes necessary to employ 16 times the stochiometric rate of methanol,
• because of maintaining a single phase condition all along the biodiesel synthesis full conversion cannot be achieved in a single reaction step. The polar solvent keeps all the components in a single phase and by such the glycerol is being kept readily available for reverse reaction routes (glycerolysis),
• to pass beyond the conversion limit set by the chemical equilibrium (at around 80% conversion progress) to separate the glycerol it is necessary to separate first the solvent by distillation. Only after this can the glycerol be settled or centrifuged. Upon separation of the glycerol the addition of polar solvent and reagent/catalyst mixture is necessary.
In contrast, my intention was to perform the trans-‐esterification to full conversion in a single operational sequence. The phase behavior experience affirmed that the apolar solvent can bring the substrate and reagent into a single phase and rejects the glycerol into a distinct phase as the reaction progresses. This has an additional beneficial effect of expelling water, the most poisonous for methanolysis, from the so called reaction phase into the glycerol phase.
8 One could rationally ask why the system of esterification has not been included in testing the phase behavior? It is because these tests have been performed as a preparatory exercise for trans-esterification.
Beneficial use of hexane (alkanes, ranging from C3 to C9) as an apolar solvent in biodiesel trans-‐esterification has been demonstrated and patent rights have been granted in a series of countries.
To elucidate phase behavior phenomena in biodiesel synthesis in hexane media a series of non-‐catalytic liquid-‐liquid equilibrium tests have been performed.
Mode of operation: liquid-liquid equilibrium tests have been performed in apparatus given in figure 2.2. Feedstock and solvent were introduced from the doping funnel.
Temperature of the experiment was controlled by the thermostated circulator. When the system reached the desired temperature the overhead stirrer was turned on (normal range of operations: 200-300 RPM) and reagent, but no catalyst was added through the doping funnel. The mixture was mixed for 30 min. and left in still (the overhead stirrer turned off) for another 30 min. If the interface between the layers was sharp the contents of the upper and lower layer have been drained separately. Samples of both layers have been taken for headspace GC analysis.
Results are given in tables 3.4 to 3.6. Findings of phase behavior tests:
The test readings can be evaluated for phase behavior, because there were no chemical reaction involved in the equilibrium tests. This conclusion can be easily drawn on the basis of viscosity range of the raffinate streams.
Moderate changes in viscosity of raffinates and moderate color change of the glycerol phase indicated that there were minor –physical -‐refining activities.
The moderate refining effect is shown in refractive indices of the solvent free apolar phase samples. The lower the refractive index, the more apolar the raffinate stream is, hence some removal of polar compounds must have taken place. (Figure 3.6.) We know from other experiments that the glycerol acts as a selective polar solvent. By such the system resembles a duo-‐sol extraction system: hexane dissolves apolar and glycerol dissolves polar compounds of the vegetable oil. Higher hexane rate results in lower overall viscosity, in better rejection of polar compounds from the apolar phase, mainly due to lower viscosity of the medium. On the basis of these data it can be stated that hexane contributes to improving feedstock refining.
Caution must be applied to feedstock pretreatment. Presence of water exerts influence on selectivity of the glycerol solvent. If there is glycerol present, than this can only happen if catalyst (alkali or acidic) was present in the system. If the catalyst was alkalic, than the free fatty acids present in the feedstock are neutralized into soap.
Soaps are surface active agents and phase behavior is influenced by formation of disperse systems. Similarly on colloid chemistry principles salting effect cannot be disclosed if the catalyst is acidic.
Hexane made that water was repelled almost completely into the glycerol phase.
Source of water: the glycerol used contained 5% water. It had influence on MeOH-‐nC6 distribution between phases
Hexane as a solvent had clean effect of avoiding emulsion formation and in breaking emulsion
Hexane is necessary to bring vegetable oil and methanol into a single phase.
Despite earlier myths vegetable oil does not dissolve methanol, but intensive mixing can solubilize some. This solubilized (encapsulated) amount of methanol can initiate trans-‐esterification under optimal conditions. Even in this case phase transfer cannot
be avoided because the catalyst is preferably dissolved in the polar phase and for activation at least 0.5% catalyst must be present in the “reaction phase”. It is shown in Figure 3.9 that small amount of methanol can be solubilized into the vegetable oil even without added solvents. It is interesting to compare and comment trend-‐line matches:
Second order approximation gives perfect match: y = -‐0.8557x2 + 5.6867x -‐
0.4765, R2 = 1; in which y: g methanol in 100 g vegetable oil, x: hexane: methanol rate [g:g]. This trend-‐line is a proof for no methanol dissolution in vegetable oil. This can only be true from strict physical-‐chemistry view points, but not from reaction kinetics.
The hazy appearance of the single phase tells that there is no dissolution, but dispersion (solubilization), but such dispersion would allow initiation of reaction between the substrate and reagent. This is a proof for that experimental technique to mix the substrate with catalyst and add reagent in discrete amounts.
Second order approximation with a line through the origo gives quite accurate match: y = -‐0.7304x2 + 5.1601x, R2 = 0.9989.
Linear approximation gives the poorest match, but indicates that small amount of methanol (2.3g) could have been dissolved (solubilized?) in 100 g vegetable oil. y = 2.2638x + 2.376; R2 = 0.9545
TABLE 3.4 MASS BALANCE OF LIQUID-‐LIQUID EQUILIBRIA IN PHASE TRANSFER EXPERIMENTS NO OIL
[g]
HEXANE
[g]
METHANOL [g]
GLYCEROL [g]
TEMP
[°C]
UPPER PHASE [g]
LOWER PHASE [g]
SR TOTAL
SE
1 100 30 10 10 50 131,2 15,7 30.2 9.8
2 100 20 10 10 50 122,8 15,8 26.3 3.7
3 100 10 10 10 50 111,8 16,8 17.05 2.95
4 100 0 10 10 50 103,8 16,2 8.4 1.6
5 100 0 20 10 50 106,3 23,4 9.6 10.4
6 100 0 30 10 50 35,5 104,5 27.2 2.8
7 100 30 30 10 50 138,5 30,5 54.4 5.6
8 100 30 20 10 50 135,5 23,6 39.7 10.3
9 100 30 10 10 50 132,0 15,0 36.6 3.4
10 100 0 10 0 50 110 0 10 n.a
11 100.4 0 21.3 0 50 9.5 110.5 9.5 10.5
12 100.4 10 21.3 0 50 4.1 125 23.6 4.1
13 100.4 20 21.3 0 50 0 137.8 36.4 0
TABLE 3.5 RAFFINATE CHARACTERISTICS IN PHASE TRANSFER EXPERIMENTS R' LOWER SOLVENT PHASE: SR-ME UPPER SOLVENT SR-HEX NO OIL [G]
VISCOSITY
[mm2/s] [g] MeOH,% nC6,% [g] MEOH,% nC6,%
1 99,8 1.4747 33.8 3.2 1.3375 82 18 21.1 1.3795 18 82 2 99,2 1.4751 34.4 3.1 1.339 83 17 16.9 1.3783 29 71
3 98,9 1.4767 34.7 2.95 1.337 57 43 8.1 1.378 33 67
4 98,9 1.4768 34.4 2.9 1.3335 100 0 0 -‐ -‐ -‐
5 98,8 1.4768 33.9 4.8 1.3375 100 0 0 -‐ -‐ -‐
6 12,4 1.4758 35.7 22.6 1.465 100 0 0 -‐ -‐ -‐
7 99,1 1.4742 34.8 10.8 1.3372 83 17 23.3 1.3781 17 83 8 99,3 1.4749 34.5 7.3 1.3345 85 15 16.2 1.3789 15 85 9 99,5 1.4748 35.4 3.4 1.3375 86 14 26.2 1.3796 13 87
10 100 1.4748 35.4 8.5 1.3371 -‐ 100
11 99.6 1.4749 33.8 9.5 1.3331 100 0 10.5 1.3331 100 0 12 99.1 1.4742 33.7 3.6 1.3369 82 18 18.4 1.3792 18 82 13 99.2 1.4738 33.5 4.6 1.3376 81 19 36.4 1.3572 52 48
TABLE 3.6 EXTRACT CHARACTERISTICS IN PHASE TRANSFER EXPERIMENTS SOLVENT
NO GLYCE
-ROL [g]
[g] MEOH,% C6,%
1 1.4012 8.2 1.4718 5.9 1.3391 98 22
2 1.4018 7.6 1.4704 6.3 1.339 98 2
3 1.4016 9.2 1.4716 6 1.3345 99 1
4 nd 8.2 1.4693 5.5 1.3329 100 0
5 nd 7.9 1.4708 4.8 1.3331 100 0
6 nd 99.9 1.4716 4.6 1.3335 100 0
7 nd 7.5 1.4723 20.3 1.3336 99 1
8 nd 8.2 1.4718 16.2 1.3365 98 2
9 nd 8.1 1.4717 7 1.3391 97 3
10 nd
11 1.3402 0.5 9.6 1.3388 97 3
12 1.3417 0.5 3.4 1.3376 98 2
13 nd
€
nD20
€
nD20
€
nD20
€
nD20
€
nD20
€
nD20
FIGURE 3.6 HIGHER HEXANE:METHANOL RATES ARE SHOWING EVIDENCE OF REFINING
FIGURE 3.7 HEXANE BRINGS METHANOL INTO VEGETABLE OIL PHASE
Either of the second order trend-‐line can be expected a correct. This can be interpreted as there must be a limit to hexane usage to promote trans-‐esterification, to maintain as high methanol concentration in reaction phase as possible. See: figure 3.15.
y = -7E-05x2 - 0.0006x + 1.4769 R² = 0.89608
1.474 1.4745 1.475 1.4755 1.476 1.4765 1.477 1.4775
0 0.5 1 1.5 2 2.5 3 3.5
RI of rafinate
hexane:MeOH [g:g]
0 1 2 3 4 5 6 7 8 9 10
0 0.5 1 1.5 2 2.5 3 3.5
MeOH in substrate phase [g]
hexane:methanol [g:g]
FIGURE 3.8 THERE IS AN OPTIMAL RATE OF HEXANE USAGE
Lack of hexane makes that less methanol is available for getting into contact with the substrate and with an increase of excess to stochiometry makes the addition less efficient. This is why in commercial technologies a 6:1 [mol:mol], i.e. double to stochiometry rate is applied, with the exception of the Biox process in which as high as 16:1 [mol:mol] rate is employed. See figure 3.9.
FIGURE 3.9 25-‐30% OF THE METHANOL CAN EASILY BE WASTED
The amount of available methanol can significantly increased if hexane is present, see figure 3.10. In such case it is not necessary to differentiate between solution (dissolved) or dispersed (solubilized) methanol amounts. Positive influence of hexane is clearly seen, even though there is a less strict correlation than in previous case.
y = -7.3042x2 + 51.602x R² = 0.99983
0 10 20 30 40 50 60 70 80 90 100
0 1 2 3 4 5 6
available MeOH for reaction [%]
hexane:methanol [g:g]
y = 2.8333x2 - 13.5x + 39.667 R² = 1
0 5 10 15 20 25 30 35
0 0.5 1 1.5 2 2.5 3 3.5
available MeOH for contract [%]
excess to stochiometry [times]
MeOH available if hexane is not used
FIGURE 3.10 HEXANE BRINGS METHANOL INTO REACTION PHASE
Lack of hexane resulted in difficulty in mixing and in formation of emulsion like dispersion, and even switching position of phases. High excess to stochiometry by operational units can be reasoned on this ground to reduce viscosity for boosting mixing of insoluble phases.
All these observations are correct for the system of hexane diluted fresh oil, methanol reagent and catalyst that is soluble in the reagent or in the oil. It had to be learned later, that the phase equilibrium tests are not valid for the system containing sulfuric acid as a catalyst.
Observations are also valid for solid catalyst (including ion exchange resin) systems is all the liquid constituents form a homogeneous phase.
0 10 20 30 40 50 60 70 80 90 100
0 0.5 1 1.5 2 2.5 3 3.5
methanol available for contact [%]
hexane: MeOH [g:g]