3. RESULTS AND DISCUSSIONS
3.3. ESTERIFICATION
TABLE 3.7 DEGUMMING IN ESTERIFICATION OF SUNFLOWER OIL DOPED WITH OLEIC ACID NO CATA
-LYST
OIL:CATALYST:
MEOH[g:g:g] CATALYST
[%FFA] YIELD, [%] YIELD
-GAIN[%] P,
[mg/kg] FFA, [%]
0 -‐ -‐ -‐ 100 65.16 12.5
Conditions: temperature: 50 °C, residence time 10 minutes
1 H2SO4 20:0.0145:1.1855 1 96.5 59 17.34 10.5
2 H2SO4 20:0.029: 1.1855 2 99.1 61.6 21.24 10.3
3 H2SO4 20:0.0435: 1.1855 3 94.8 57.3 23.24 9.16
4 H2SO4 20:0.058: 1.1855 4 96.5 59 21.22 5.03
5 H2SO4 20:0.0725: 1.1855 5 97.0 59.5 24.45 1.35
6 H2SO4 20:0.11: 1.1855 5 98.3 60.8 24.82 2.36
7 H3PO4 20:0.0145:1.1855 1 93.1 55.6 25.12 12.2
8 H3PO4 20:0.029: 1.1855 2 93.9 56.4 27.30 12.2
9 H3PO4 20:0.0435: 1.1855 3 90.5 53 25.81 11.8
10 H3PO4 20:0.0435: 1.1855 4 90.1 52.6 26.11 11.4
11 H3PO4 20:0.0435: 1.1855 5 88.7 51.2 25.48 10.9
Conditions: temperature: 50 °C, residence time 20 minutes
12 H2SO4 20:0.11: 1.1855 5 98.3 60.8 21.31 1.58
Conditions: temperature: 50 °C, residence time 30 minutes
13 H2SO4 20:0.11: 1.1855 5 97.8 60.3 17.44 1.72
Conditions: temperature: 50 °C, catalyst: 5%
Catalyst oil:MeOH, [g:g] τ, min
14 Amberlyst 15 D 20:1.1855 30 99 61.5 67.88 1.28
15 Amberlyst 15 D 20: 1.1855 60 97.5 60 17.4 0.35
This series of experiments proved that there is no need for (super-)degumming upstream to (trans-‐)esterification because the esterification treatment reduces the phosphrous content efficiently. Any additional degumming treatment would only contribute to loss in product yield on solvent extraction principles basis. The reaction of esterification eliberates polars that are transferred to the extract phase and these polars attract additional components with adequate polarity into the polar phase. This is the overall mechanism by such the esterification executes degumming too. This exclusion of polars from the refined phase is expressed by the change in phosphorous content of esterified streams. This characterization is just like the practice in assessing the performance of super degumming. Data clearly show that majority of phosphorous compounds are excluded into the extract phase. The most favorable kinetic of degumming has been achieved with sulfuric acid, the best overall performance with ion exchange resin.
As known from practice degumming is considered beneficial to trans-esterification without specific comparison of cases in technical literature. Figures 3.12-3.15 compare trans-esterification kinetics of algae oil with and without esterification pretreatment. All the tests have been performed with algae oil diluted with 42% hexane, 1% KOH catalyst and 1.8:1 stochiometric rate of reagent to feedstock. The results prove the statement that degumming is beneficial to trans-esterification kinetics. The two sets of curves clearly indicate that kinetic conditions for the reaction have been improved under acid catalyzed treatment. The level of improvement is differential for the different stages of the sequential
system of reactions. The most straightforward action is exerted on the course of mono-glyceride kinetics. A plausible explanation can be associated to lack of saponification reactions.
On the other hand, it must be stressed that this series of experiments have been done with algae oil and many of the required properties by fuel standards have not been satisfied even with the pre-esterified, trans-esterified algae based biodiesel samples.
FIGURE 3.11 ESTERIFICATION KINETIC OF SUNFLOWER OIL DOPED WITH OLEIC ACID
FIGURE 3.12 MONO-GLICERIDE PATTERN, TRANS-ESTERIFICATION KINETICS OF ALGAE OIL
0 2 4 6 8 10 12 14
0 30 60 90 120 150 180 210 240
FFA, %
time, min 42%
30%
no solvent
0 0.5 1 1.5 2 2.5 3 3.5
0 20 40 60 80 100 120
MG, %
time, min without esteri•ication
with esteri•ication
FIGURE 3.13 DI-GLICERIDE PATTERN, TRANS-ESTERIFICATION KINETICS OF ALGAE OIL
FIGURE 3.14 TRI-GLICERIDE PATTERN, TRANS-ESTERIFICATION KINETICS OF ALGAE OIL
Upper curve: without pre-esterification, lower curve: with pre-esterification
Sulfuric acid proved to be efficient degumming agent. Degumming efficiency of sulfuric acid was equal or better than with the standard industrial degumming reagent phosphoric acid.
Sulfuric acid has been even better than phosphorous acid, because of acting as an efficient acidic catalyst for esterification. In degumming with phosphoric acid there was practically no conversion of FFA to FAME.
Amberlyst 15 D is efficient catalyst equal to sulphuric acid in degumming and is better by recorded benefits in yield.
0 0.5 1 1.5 2 2.5 3 3.5
0 20 40 60 80 100 120
DG, %
time, min
without esteri•ication
with esteri•ication
0 1 2 3 4 5 6 7 8
0 20 40 60 80 100 120
TG, %
time, min
without esteri•ication
with esteri•ication
The reaction of esterification is both kinetically (phisically) and reaction rate limited.
Both the mass transfer and the reaction mechanism influence the slow progress in esterification. To overcome this the catalyst is given (as in industrial practice) much at higher rate than to limit the activity to catalysis, but must have a role in extracting the reaction product.
Solvent extraction/adsorption feature of the catalyst proved to be an important characteristic of esterification, and this is why I am confident that the need for high dosage of catalyst must have other than reaction kinetics reason and must be part of the industrial myths. Demonstration of this follows later.
Advanced esterification experiments have been performed with yellow grease, because this proved to be the most difficult feedstock for this operation. The difficulty must deriver from complex colloid character. This series made evident that not only the concentration and composition of FFA, but the matrix of the feedstock plays important role in progress of chemical reaction. The present series and further experiments made also evident that the main benefit of using the apolar solvent is its rejection and separation of polar reaction components, water and phospholipids difficult to transfer into the water phase. Very slow settling times in absence of an apolar solvent was turned into a step of complete settling achieved within minutes.
Reaction mechanism/kinetics. By addressing reaction mechanism one looks for the path and/or sequence of reaction steps that occur in performing a chemical conversion. Kinetics is dealing with exploring how the reaction takes place, what are the conditions that influence the promotion or retardation of the process. In catalytic reaction system of high FFA feedstock beside the basic process steps: the diffusion to the active site, the adsorption onto the active site by the reagents, desorption of the reaction products and diffusion from the active site the mass transfer specifics between the distinct phases must also be taken into consideration.
The role of solvent extraction and adsorption was challenged by adding an ad-‐
sorbent to the reaction environment. These experiments were conducted at atmospheric pressure under total reflux. The methanol:FFA rate was 6 times the stochiometry. The evidence of positive role of a fuller earth -‐ natural bentonitic -‐
adsorbent is presented in terms of FFA conversion kinetic in Figure 3.15 for distillers corn oil, and Figure 3.16 for yellow grease. The role of addition of adsorbent is clearly shown in figure 3.17. The interdependence of adsorbent, catalyst and solvent reflects the colloid character of the system. In kinetics of yellow grease esterification addition of adsorbent to the catalyst resulted in significant increase in rate of reaction. By taking into account the counter effect of addition of the apolar solvent a plausible explanation can be that a quasi heterogeneous system was created and the mechanism of esterification resembles such a system. The major drawback in these series of reaction was the very slow settling time. In the series without the addition of an apolar solvent the use of a centrifuge could not have been omitted. On the contrary, settling was efficient and rapid when hexane was added. In such system with adsorbent and apolar solvent the rejection of the catalyst into the polar phase formed physical barrier against collision of the substrate, reagent and catalyst. The apolar solvent prevented the diffusion of components onto the active site, but promoted the reverse direction transfer. This concluded in characteristic kinetic limitation of the esterification reaction.
FIGURE 3.15 ESTERIFICATION KINETIC OF DISTILLERS CORN OIL AS A FUNCTION OF ADDITION OF APOLAR SOLVENT
Conditions: Temperature: 75C, Methanol:FFA=[6:1],[mol:mol]
0%H: no hexane added, 30%H: 30% hexane is added to the feedstock, 42%H: 42% hexane is added to the feedstock .
FIGURE 3.16 ESTERIFICATION KINETIC OF YELLOW GREASE AS A FUNCTION OF ADDITION OF APOLAR SOLVENT
Conditions: Temperature: 75C, Methanol:FFA=[6:1],[mol:mol]
0%H: no hexane added, 30%H: 30% hexane is added to the feedstock, 42%H: 42% hexane is added to the feedstock .
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0
0 30 60 90 120 150
FFA, %
time, min 0% H 30%H 42%H
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0
0 60 120 180 240 300
FFA, %
time,min 0% H 30%H 42%H
FIGURE 3.17 ESTERIFICATION KINETIC OF YELLOW GREASE AS A FUNCTION OF ADDITION OF ADSORBENT AND APOLAR SOLVENT
Conditions: Temperature: 75C, Methanol:FFA=[6:1],[mol:mol]
1%K: conventional esterification with 1% sulfuric acid catalyst, no solvent, no adsorbent. 1%K,0.1%P:
conventional esterification, with 1% sulfuric acid catalyst and 0.1% adsorbent, 1.1+30%H: as the case before and 30% hexane solvent, 1.1+42%H: as the case before but 42% hexane.
It can be concluded, that the addition of the apolar solvent in esterification can be clearly beneficial for promotion of phase separation. Improving the rate of esterification is a different matter. The main factor in this respect can be attributed to the colloid chemical influence. Addition of an adsorbant has clear positive influence on boosting the rate of esterification as well as the increase of reagent:feedstock rate. Because of the preference of the catalyst to stay in the polar phase made me to revisit my general conclusion that the use of apolar solvent can be a universal tool in biodiesel processing. Even though, the FFA content in treated streams could have been lowered below the desired level of 1%
without excessive dilution with reagent and excessive times on stream.
On this basis the only possible explanation to this set of curves is that the esterification mechanism is not a phase transfer reaction, but a reaction taking place at the interface. The marked difference between kinetics of the distillers corn oil and of the yellow grease accentuates the influence of the colloid structure, on how difficult is the transfer of components onto and from the interface.
Beneficial feature of shifting the reaction toward esterification by the use of 3A molecular sieve in an attached column element right between the reaction vessel and the reflux condenser produced slightly better esterification conversion, but still not significantly better, and even slightly inferior than in the case of using the fuller earth adsorbent dried at 105 °C. This is in line with the proposed influence of the colloid structure. A possible explanation to lower than expected efficiency is that the sulfuric acid strongly competed for water. When the 3A molecular sieve was immersed into the reaction media, the boosting efficiency was similar to the one achieved with the adsorbant. The positive impact could have been reasoned by deposition of a catalytic sulfuric acid layer on the solid surface of the adsorbent and an associated reaction mechanism similar to heterogeneous catalysis. The association contact surface is formed with the solid particles onto which the acid and intermediary acidic components can adhere by ionic forces.
Reaction partners can also come into contact after being diffused onto the active site of
0 2 4 6 8 10 12 14 16 18
0 30 60 90 120 150 180 210 240
FFA,%
time, min 1,1+42%H
1,1+30%H 1%K, 0,1%P 1%K
the adhered acid on the adsorbent surface. This mechanism was indicated by the amount of water that was rejected into the distinct phase, being significantly lower the calculated amount of water formed in esterification reaction. The difference was adsorbed by the adsorbent. The adsorbent filtered showed evidence of adsorption of gum components.
Accordingly only part of the water can be extracted in the extractive distillation and part of the water resides with the acid, and by dilution weakens the catalytic activity and increases the amount of sludge formed. It still remains to understand and reason the beneficial effect of addition of higher rate of catalyst for efficient conversion. NB: in practice the rate of catalyst addition can be as high as 5%. This cannot be accepted for a low carbon footprint process. Even though efficiencies of esterification under short contact time are higher with 1% catalyst than with 2% catalyst indifferent of the extent of dilution with the apolar solvent. For an efficient conversion addition of an apolar solvent is beneficial, but the need for high catalyst dosage can be doubtful. Efficiencies of esterification under short contact time are higher with 1% catalyst than with 2% catalyst indifferent of the extent of dilution with the apolar solvent. This behavior can be beneficially explored in counter current operations with addition of tinier amounts of catalyst to the system. This makes the downstream process steps more economic by necessitating less base for neutralization and less specific consumption of catalyst. On this ground it can be explained that the use of high catalyst rate in conventional system serves nothing else, but assisting in breaking the dispersion of the reaction products.
Because the esterification reactions have been performed well after my successful trans-esterification trials in a counter current setup trans-esterification of sunflower oil doped to 12.5%
FFA by adding oleic acid was performed in the 2.5 m heigh glass apparatus. One might ask why haven’t I continued with the ion exchange resin experiments. The simple answer is that because of the special interets of an engineering company to introduce the process in America. A potential buyer9 expressed interest to use the technique, but with the condition that I develop the system for sulfuric acid catalysis to offset the high investment cost related to filling two ion exchange columns.
With 1.00 kg/h feed rate of 30% hexane containing 12.5% FFA containing refined sunflower oil reacted with 0.2 kg 10% sulfuric acid containing methanol at 53oC the resulted a raffinate that contained 0.33% FFA. An indicative FFA level decrease is illustrated in table 3.8, figure 3.15 along the esterification column. The influence of dilution is shown in figure 3.19, in which the second and the third set of data have been collected with dilution rate of 42% and 50%. Samples have been withdrawn at different moments after the system showed signes of stability (steady state conditions). This indicate that esterification reactions progress according to known kinetic features (figure 3.18) in the counter current section and in the rising section of the column alike. This was the case of a system close to real solutiion of FFA in oil. When distillers corn oil of FFA level of 12.5% and yellow grease of FFA level of 15.5% were submitted to counter current esterification the column height of 2.5m proved to be too small for the desired conversion. The column had to be enlarged to 3 m and even doubed with a total height of close to 5 m. The conversion was according to expectation for the simpler case of distillers corn oil, the FFA content could have been reduced to close to 1%. (figures 3.20, 3.21) The yellow grease presented even signs of fouling.
9 Aker Solutions of UK dealt with planning on the order of Blue Norther Energy company from USA.
TABLE 3.8 FFA REDUCTION ALONG THE COLUMN HEIGHT IN COUNTER CURRENT ESTERIFICATION OF SUNFLOWER OIL DOPED WITH OLEIC ACID
Point 1 2 3 4 5 6 7
Height, cm
0 30 75 115 150 180 Top
FFA, % 12.5 9.8 7.35 3.24 1.88 0.62 0.33
FIGURE3.18 ESTERIFICATION PROFILE ALONG THE COLUMN HEIGHT
FIGURE 3.19 ESTERIFICATION PROFILE ALONG THE COLUMN HEIGHT: SUNFLOWER OIL DOPED
WITH OLEIC ACID
Conditions: Temperature: 53°C, Methanol:FFA=[3.6:1],[mol:mol], catalyst 1.4 % sulfuric acid with reference to feed. Apolar solvent added to the feedstock: 30% (top),42% (middle), 50% (lower curve points
0 2 4 6 8 10 12 14
0 50 100 150 200 250
FFA, %
column height, cm
FIGURE 3.20 ESTERIFICATION PROFILE ALONG THE COLUMN HEIGHT- DISTILLERS CORN OIL Conditions: Temperature: 53°C, Methanol:FFA=[4.4:1],[mol:mol] (upper curves) Methanol:FFA=[6.2:1], [mol:mol] (lower curve) 1.7 % sulfuric acid with reference to feed),
Counter current esterification in the given setup was repeated to learn about stability of the system and to check whether the residence time can be reduced further. It is necessary to explain why the feed rate was selected to be about 1 kg/h. In solvent extraction experiments for lubricating oil refining I used the same diameter column and this range of column loading. Those results have been proved to be in harmony with results obtained in the real industrial column. Those lessons learned in solvent refining of lubricating oil basestocks constituted the guiding rules for the present experiments. Table 3.9 contains these results along with the mass balance of the esterification in the counter current setup. Under normal loads the acidity could have been reduced below the target level, if the column was overloaded the conversion was not sufficient. Bacuse of kinetic considerations the reason must in shorter than necessary residence time.
TABLE 3.9 COUNTER CURRENT ESTERIFICATION OF SUNFLOWER OIL OF 12.5% FFA NO FEED,
g/h
R+C, g/h ESTERIFIED, g/h
REJECTED,
g/h FFA, % P, [mg/kg]
1 1000 192.5 1096 96.5 0.57 16.24
2 1000 195 1062 133 0.98 12.10
3 1000 200 1070 130 0.72 13.12
4 1000 225 906 319 1.08 12.41
5 1900 375 1933 341 2.9 15.90
6 1400 340 1025 315 1.42 16.10
7 1200 240 1180 260 1.04 15.92
8 1200 260 1145 315 1.16 13.80
9 2500 450 2540 410 2.81 15.45
10 3000 550 3107 443 3.24 16.25
Even though these experiments made evident that the major obstacle in preforming an efficient esterification of the yellow base stock addition energy must be conveyed to the system for breaking up the colloid structure. Raising reraction temperature was a rational option for this case. In a moderate pressure stainless steel apparatus the temperature could
0 2 4 6 8 10 12 14
0 50 100 150 200 250 300
FFA, %
column height, cm
have been raised to 72ºC at 8 bar pressure. The temperature was limited by inefficiency of the insulation, the conversion was limited by relaively short residence time (relatively short column). The feed and reagent rates were raised according to the scale (see table 3.10) because of the inefficiency of esterification in a single contact, it remained the only applicable solution to recycle the estrified stream for a second treatment with the corn oil to reduce the FFA content below 1%. For this the solvent containing esterified stream was fed into the column at the lower feeding point and the reagent+catalyst mixture feed through the normal metering ports, at rates of the third metering port in the first step. The result is somehow cheaty because of applying increased rate of reagent in the second step.
FIGURE 3.21 ESTERIFICATION PROFILE ALONG THE COLUMN HEIGHT- YELLOW GREASE
upper curve: Temperature: 53°C, Methanol:FFA=[4.4:1],[mol:mol], 1.7 % sulfuric acid
middle curve: Temperature: 53°C, Methanol:FFA=[8.0:1],[mol:mol], 1.7 % sulfuric acid
lower curve: Temperature: 53°C, Methanol:FFA=[8.0:1], [mol:mol] 5 % sulfuric acid catalyst.
It can be concluded, that the matrix in which the free fatty acid is residing has the most definite influence on sulfuric acid catalyzed esterification. The most difficult feedstock studied was the yellow grease. For efficient esterification conversion the technology must respond to accordingly and as different the feedstock is as different the conditions must be.
The matrix effect must have been controlled by colloid chemistry principles. If compared the esterification kinetic curves along the column height it seems to be evident, that by increase of reagent:feed, feed dilution and catalyst rates the conversion kinetic can be boosted. The more complex the feedstock, the more the kinetics approaches a pattern that can be described by closer to linear than polynomial relationship. These observations constitute the basis for the conclusion that the mechanism is best described by an interfacial reaction model. To this interfacial reaction model the emulsion breaking mechanisms are superposed. The role of sulfuric acid is not only to catalyze the esterification, but to promote the breaking of emulsion structure and to absorb the water.
In an attempt to learn about the specifics of the kinetic of esterification yellow grease was contracted in an extended length (3m) co-current column with exaggerated amount of reagent -Methanol:FFA=35.6:1-and catalyst –H2SO4: 8%-. The yellow grease experienced degradation in stay and the FFA level increased from 15.5 to 18.8%. Without adding apolar solvent to the feedstock the FFA level could only be reduced to 11.7% (a conversion inferior
2 4 6 8 10 12 14 16 18
0 50 100 150 200 250
FFA, %
column height, cm
to 40%). With the use of n-hexane the FFA level could have been reduced to 1.8 % (coversion: 90%). Even though the efficiency of co-current contact is much inferior to counter current mode of contact the pilot plant, in which the operation can be performed at higher pressure to raise the temperature of reaction above 100 °C a provision of feeding the streams in co-current has been planned. Special colloid features have been observed both in the esterification and in the downstream neutralization column. In the esterification experiment without the addition of the apolar hexane in phase splitting could have been visualized in sections free of packing. This phase splitting could have been the reason for inferior conversion. In hexane containing mode of operation this splitting was not visible, even though the samples taken along the column height experienced fast splitting into distinct liquid phases. Some drops of gums have been also accumulated in counter current flow against the co-current stream. In downstream neutralization column the saponification produced soap produced tough plugs of emulsions in sections not filled with packing. This caused fouling of the column. Hexane contributed to controlling emulsion caused fouling to a given extent.
To explore these results for the sake of specifying the final technological regimes a series of esterification with YG were performed in mixer settler with added technical means. Table 3.11 represents some experimental data of atmospheric esterification trials with some exaggerated rates of catalyst and reagent to feed. The relatively wide range is due to different rates of methanol:hexane. Esterification in the glass mixer settler reactor (see figure 2.2) was performed under moderate mixing for 90 min reaction and 30 min. settling.
The range of reaction temperature was between t=57-65°C and was controlled by total refluxing of the solvent-reagent mixture. In such device the temperature of the treatment can be increased to this extent because of the refluxing, that cannot be done in the counter current device without perturbing the phase behavior in the column due to intensive bubbling. The temperature of the reaction could have been raised by executing reaction in a vapor-liquid equilibrium condition device (see figure 2.5). The rise of bubbles in counter current mode of operation, entrains any compponents without selectivity and makes that the system resembles a system in foam. I had to admit, that under atmospheric pressure the acid number cannot reduced to the desired level within the desired rate of reagent addition and residence time of not longer than 1.5 hours.
TABLE 3.10 ESTERIFICATION, 2.5 M GLASS COUNTER CURRENT REACTOR EXTRACTOR FEEDSTOCK (F) REAGENT, CATALYST RATES, MOL:MOL FINAL
COMPONENT RATE, kg/h
COMPONENT RATE, kg/h
R:F,mol/mol CATALYST,% FFA,% Sunflower(F’)
Hexane
0.70 0.30
Methanol(R) Sulfuric acid(C)
0.19 0.01
3.6:1 1.4 0.33
Corn oil (F) Hexane
0.58 0.42
Methanol(R) Sulfuric acid(C)
0.19 0.01
4.4:1 1.7 4.87
Corn oil (F) Hexane
0.58 0.42
Methanol(R) Sulfuric acid(C)
0.38*
0.02
8.8:1 3.4 0.72
YG (F) Hexane
0.58 0.42
Methanol(R) Sulfuric acid(C)
0.38*
0.02
8.9:1 2.9 2.08
*: reagent/catalyst mixture fed in series in two columns