1.6. STATE OF THE ART OF BIODIESEL PROCESSING
1.6.3. CATALYSIS ASPECTS
Removal of either reaction products can be an efficient tool in shifting equilibrium toward completion and chances are given for making it in a single reaction contact step.
For such scope selectivity of the solvent must be reinstalled. Barring the reverse reaction by removing one of the products from reaction media could only be done within the frame of economic operations by the proposed use of apolar solvent. The present work addresses colloid chemical basis on this basis. Phase transfer, glycerol rejection and trans-‐esterification kinetics experiments were carried out to de-scribe the phenomena of shifting trans-‐esterification toward completion in a single phase.
When revisiting colloid chemical aspects micro-‐emulsions must also be visited and considered. Micro-‐emulsions with short chain alcohols can yield clear, thermodynamically stable liquid fuel with viscosities in the range of diesel fuel specifications [113]. The special benefit of water and oxygenates containing diesel fuel consists in freezing NOx chemistry by lowering the flame temperature and by such reducing the emissions of this regulated compound [114]. Similar emissions reduction can be achieved with blending water in fossil diesel fuel too [115]. Revisiting this aspect can be reasoned on the ground of ease in atomization of the (micro)-‐emulsified fuels similarly to biodiesel. This effect compensates for the slightly lower heating value in operational diesel engines.
It is to report that the patent related to the present thesis was submitted in 2001 and full grants have been obtained since. This is worth for another ethical comment, Shi and Bao used my technique (and my motives) of hexane dilution without referring to my works. The authors have not even observed a series of advantages of using the hexane solvent and passed next to finding facts, such as: a) necessary reaction time is less than 10 min Shi and Bao used 120 min., b) settling occurs instantaneously and there is no need for settling over 2 h of time, c) ignored the fact that if the crude biodiesel is simply washed with distilled water emulsion formation must be present, d) there is no need for employing 9 times stochiometry, even 2 times stochiometry is excessive under such conditions [116].
levels were only slightly higher than the level commensurable to FFA content. I concluded that steric hindrance of tri-‐glycerides limits the contact of substrate and reagent on an active site of p-‐toluene-‐sulphonic acid containing cation-‐exchange resins and acidic catalysts fixed on the surface of macroporous gels. The kinetics of the reaction requires intimate contact of the reagent and substrate over an active site of the catalyst. Protonation could not have been performed at large distance from the active sites. The steric hindrance is caused by the existence of three long and stretched hydrocarbon chains that form barrier against the contact for the ester link to come in contact with the active site inside the pore of a catalyst. Under high temperature, high pressure conditions the hydrocarbon chain can be pressed into the pores and the necessary contact can be realized. On the other hand the mobile homogeneous catalyst can easily diffuse between the arms of the tri-‐acyl-‐glyceride to come in contact with carbon molecule in ester link. [120] illustrates how the electrons distribution changes in alkali catalyzed exchange (hydrolysis) reaction:
Technologies that employ solid catalyst for trans-‐esterification work under high pressure conditions, at which both the reagent and the substrate are forced into the macro pores where the active sites of the catalyst have been formed. It is to mention, that under such conditions spontaneous, catalyst free trans-‐esterification also takes place. Attempts to employ supercritical methanol requires very severe conditions, that are not possible to get scaled to industrial process, even with allowing FFA in the feed to act right as catalyst (temp>250 °C, p>20 MPa, reagent to substrate rate 20-‐60 times to stochiometry), just as in paraffin oxidation. Wang managed to reduce the severity to 15 MPa reaction pressure even under such circumstances the yield of biodiesel was low (91.4%) with reaction temperature of 350 °C, methanol:oil molar ratio 40:1 and acidity the soybean oil feedstock 15.0 mg of KOH/g [121]. It was revealed by the authors that the trans-‐esterification reaction was controlled by reaction kinetics.
Heterogeneous catalyst based technologies offer neat advantage of ease in crude biodiesel refining operation [122]. The first heterogeneous trans-‐esterification technology was patented by IFP and brought into commercial scale by Axens. Even this technique operates in two consecutive trans-‐esterification steps. Equilibrium controls the progress to completion in individual reaction steps. Hillion [123] prepared a mixed spinnel (Yn+Al) catalyst that operates at a temperature above 200 °C and a pressure of about 10 MPa [124] for the aforementioned system. Severe operational conditions dictate that the scale to economy is about 150 kt/y for heterogeneous trans-‐
esterification plants. Axens received the Kirk Patrick award for this type of process in 2007. Most heterogeneous catalyst developers accept that reaction rates are still low and two step conversion is needed.
Because of the simple reaction chemistry in biodiesel synthesis there is a wide variety of solid catalysts that have been reported to work properly. Compilation of Mittelbach [125] lists mainly simple carbonates as potential catalysts for biodiesel synthesis. Most of the heterogeneous catalyst development works started to reproduce
the catalytic activity of homogeneous alkali (Brönsted) basic catalysts that is formed when an alkali (NaOH, KOH, Na2CO3) and the reagent are metered into the reactor:
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K+OH−+CH3OH⎯ methoxy⎯ ⎯ → ← ⎯ hydrolysis⎯ ⎯ ⎯ CH3O−K++H2O
The alkoxide group attacks the hydroxyl link in the tri-‐acyl-‐glyceride. To avoid unwanted effects of water some technologies employ synthesized, water free alkoxide catalyst, usually dissolved in the reagent methanol. K is the preferred alkali element with a view to utilize the separated salt as fertilizer.
Other approaches followed similar track by choosing basic zeoiltes that act right as heterogeneous alkali catalyst in the form of Broensted basics[126]. Even in this case, the formed catalytic specie is a homogeneous alkoxide. In the case of heterogeneous basic Broensted catalyst such as resin with quaternary ammonium functionality (QN+OH-‐), the positive counter-ions (organic ammonium groups), being bonded are only active if the alkoxide is rented to the homogeneous liquid phase.
Hydrotreatment of tri-‐glyceride bearing stocks result in the most advanced diesel fuel, but because the treatment results in full hydrogenation to mixtures of n-‐ and i-‐
alkanes this approach falls beyond the scope of biodiesel production. This is a typical petroleum refinery process under similarly severe conditions such are present in the Axens technology. The so called hydrodiesel is an excellent high cetane number (around 90) component to improve quality of automotive diesel fuels. Without any reference in technical literature plain hydrodiesel must provoke similar problems as did the ultra low sulfur content variants, mainly because of lack of antiwear properties.
The wear protecting feature of the polar head in the biodiesel must be replenished by adding antiwear and extreme pressure additives in hydrodiesel fuels.
The number of publications on enzyme catalyzed trans-/esterification has been growing for the last 20 years. This biochemical approach is beyond the scope of my interest mainly because of the very slow conversion and difficulties in scaling to industrial technology. While alkali catalysis is preferred in conventional technologies because the trans-‐esterification reactions can be brought to completion within an hour, for acid catalysis this time span can be 12 hours or more, for enzyme catalysis to complete the conversion the necessary contact time extends for more than 2 days.
A new paradigm for biodiesel production has been proposed by Haas [127]. It consists in direct incubation of the oil source with the alcohol and alkali to produce and liberate fatty acid esters from the oilseed. With regard to this technique I feel obligate to report that I proposed and experienced to explore the same technique [128] without knowing about trials in the US Agricultural Research Center. I discontinued to follow this route because of difficulties foreseen for scaling up to industrial technology. The main problem was associated to presence of network solids (protein and fiber) configured into an encapsulated disperse liquid system. These encapsulated disperse structures resulted in very accentuated loss in yield. In a private discussion I learned that the system of Haas consisted of a very slowly rotated (5/min) V-‐shape reactor.
They are using very high excess of methanol. Somehow similar attempts with Jatropha Curcas L. seed as feedstock was attempted at laboratory scale [129]. The optimum reaction condition for biodiesel yield of 98.1% were: temp: 60 °C, reaction time 10 h, methanol to seed ratio [10.5:1, ml/g], catalyst: H2SO4, in rate of 21.8 % (sic!).