1. LITERATURE
3.4 G ENERAL WAYS TO UTILIZE CRUDE GLYCEROL
Benefits and disadvantages of crude glycerol handling processes were collected in Table 3.4.1.
Table 3.4.1 Comparison of general ways to utilize crude glycerol
Waste water treatment Incineration Refining Derivatization
Benefit Disadvantage Benefit Disadvantage Benefit Disadvantage Benefit Disadvantage
No
downstream
operation Expensive No downstream operation
crude glycerol Investment
Almost complete utilization of
capacity needed Large ash volumes
for deposit By-product credit of
purified glycerol Larger volumes of
inorganic salts By-product credit of value-added glycerol derivatives
Larger volumes of inorganic salts Approval by
authorities for
salt release Energy recovery
Formation of toxic
Usually improved salt removal
Biodiesel plant
4 Summary
The aim of the work was to elaborate potential ways for utilization of crude glycerol derived from biodiesel production.
In the Literature chapter of the dissertation different purification processes were delineated considering the problematic steps like distillation of glycerol and transformation of glycerol into high-quality products was reviewed. According to the literature it seems to be imperative to employ a “simple” and reliable process to work-up “crude glycerol”.
In the experimental part purification methods of crude glycerol to different qualities and syntheses of four value-added glycerol derivatives were presented.
Removal of contaminants like soap, pigments, water and inorganic salts were achieved and optimized. Pure commercial glycerol, glycerol purified by the developed process and crude glycerol were the raw materials of the additional experiments. Syntheses of the following value-added glycerol derivatives were studied:
• triacetin from glycerol and acetic acid,
• tripropionin from glycerol and propionic acid,
• glycerol carbonate from glycerol and dimethylcarbonate,
• (2-isobutyl-2-methyl -1,3-dioxolan-4-yl)methanol from glycerol and methyl-isobutyl-ketone.
In case of triacetin and tripropionin the influences of the main reaction variables like quality of raw material, effects of catalysts and azeotropic water removal on esterification of glycerol with acetic/propionic acid were studied.
It was proven that strong acid ion-exchange resins catalized the reactions successfully when raw material was pure glycerol. However, in case of partly purified glycerol ion-exchange resigns are not as effective as in case of pure glycerol which can be explained by the deactivation of active sites of ion-exchange resign by the ions of salts contained in raw material.
Continuous removal of forming water by help of entraining solvents like n-hexane, toluene and methyl-isobutyl-ketone pushed the reaction towards the product formation, however in case of methyl-isobutyl-ketone significant by-product formation was observed when strong homogenous acid catalysts like sulfuric acid was used to catalyze the reaction.
It was shown in this work that a possible way of triacetin and tripropionin synthesis from partly purified glycerol is the sulfuric acid catalysed one, when the by-product, water
is removed by help of entraining solvent like toluene or n-hexane. In these cases the percent yields of triacetin were > 98%.
The most difficult problem in connection with crude glycerol refining is removal of salts which are formed during neutralization of the catalyst (KOH, NaOH). It was established that the presence of salts does not disturb the reaction of glycerol and acetic/propionic acid when homogenous catalyst like sulfuric acid is used to catalyze the reaction and if they are insoluble in triacetin/tripropionin, this problem can be solved by simple filtration and therefore the purification of the products does not require expensive process conditions.
The effect of tripropionin blending on engine performance characteristics and environmental repercussions were studied. According to the good results obtained for reduction of exhaust emissions of CO and smoke, tripropionin represents a promising material, which can be used as fuel additive.
A convenient method has been developed for synthesis of glycerol carbonate from glycerol and dimethyl carbonate using K2CO3 as catalyst. Effects of raw material quality, dimethyl carbonate excess and catalyst concentration were investigated. Due to the fact that solubility of inorganic salts in glycerol carbonate is rather high and purification of glycerol carbonate by over-distillation is not a realistic option, only pure glycerol road was studied in detail. Resulting glycerol carbonate was obtained in almost theoretical yield.
Additionally, a dynamic model of the reactor where the experiments were performed has been worked out.
Reaction of glycerol and methyl-isobutyl-ketone using different acidic heterogeneous and homogenous catalysts were studied. The best results for selectivity and percent yield of (2-isobutyl-2-methyl-1,3-dioxolan-4-yl)methanol synthesis from pure glycerol were obtained in presence of Amberlyst15 catalyst and phosphoric acid. In case of crude glycerol conventional acid catalysts like sulfuric acid were applied successfully, too.
Kinetic parameters of the reaction were determined and a process flow sheet was designed.
The big benefit of this process is that the pure value-added glycerol derivative can be prepared directly from crude glycerol and methyl-isobutyl-ketone using cheap acid catalyst and the purification of the product does not require expensive process. (2-Isobutyl-2-methyl-1,3-dioxolan-4-yl)methanol is a thermally stable material which can be applied as solvent, fuel additive or can be converted into purified glycerol.
Finally material balances for crude glycerol purification process and for the preparations of different value-added glycerol derivatives were calculated. Benefits and disadvantages of elaborated crude glycerol handling processes were compared with the conventional ones.
5 Tézisek
5.1 Kidolgoztam egy újszerű módszert a biodízelgyártás melléktermékeként keletkező nyers glicerin tisztítására.
A teljes tisztítási technológiára vonatkozóan a következőket állapítottam meg:
• A nyers glicerin nagy viszkozitása miatt célszerű a teljes tisztítási technológiát emelt hőmérsékleten (40-80 ºC) üzemeltetni. Ez az összes átadásos műveletben csökkenti a diffúziós gátlást, valamint a keverők és a szivattyúk terhelését.
• Az elegy metanol és illékony komponens tartalmát hígító hatása miatt nem célszerű a technológia elején eltávolítani.
• Célszerű a technológia elején a viszkozitás csökkentése érdekében ~25 %(m/m) vízzel hígítani a feldolgozandó elegyet.
A glicerin tisztítási folyamat általam javasolt lépései a következők:
1. Savazás, melynek célja a szappanok megbontása. Ekkor a zsírsavak kiválnak, és egy elkülöníthető fázisban jelennek meg. Erre a célra a foszforsavval pH=3-ig történő savazást találtuk optimálisnak.
2. Semlegesítés mésztejjel, melynek célja a foszforsav felesleg foszfátsók formájában történő eltávolítása. Megállapítottam, hogy a kálciumfoszfátok eltávolítására legkedvezőbb a pH=5 körüli érték.
3. Aktív szenes derítés, mely célja a részben tisztított glicerin színanyagainak/illatanyagainak eltávolítása. Erre a célra a 2% (m/m) aktív szénpor mennyiséget találtam optimálisnak.
4. Vákuum desztilláció. A glicerin tiszta állapotban való kinyerése legalább két desztillációs lépést igényel. A víz nagyobb részének eltávolítása csak vákuumdesztillációval végezhető el, de ez a lépés nem igényel túlságosan nagy vákuumot. Ezzel a lépéssel a sűrítmény glicerin tartalma 90% (m/m) felettire növekszik. A második lépésben vákuumdesztillációval a glicerint távolítjuk el a nem illékony szerves és szervetlen szennyezők mellől. Ekkor főpárlatként nagy töménységű, 99% (m/m) feletti tisztaságú glicerint kapunk.
A tisztítás egyes lépéseit kombinálva különböző mértékig tisztított glicerin elegyeket állítottam elő. Rámutattam, hogy a további feldolgozási/felhasználási céltól függően egyes lépések kihagyhatók a tiszítási eljárásból.
Kimutattam, hogy a nyers glicerin - előtisztítás nélküli - desztillációs finomítása során a glicerintartalomnak mindössze 25%-a kapható vissza tiszta állapotban. Számos problémára mutattam rá a desztilláció során, melyek jelentős technológiai nehézségeket okoznak:
glicerin bomlás, habzás, jelentős viszkozitás növekedés és sókiválás.
Megállapítottam, hogy a zsír-/olajsavak alkáli sóinak megbontása, és a zsír-/olajsavak savazás utáni elválasztása a desztillációs glicerin-tisztítás elkerülhetetlen előfeltétele.
Kimutattam, hogy az általam javasolt tisztítási eljárással nyert glicerin víztartalma és nem-glicerines szerves anyag tartalma kisebb, mint az egyéb eljárásokkal tisztított nyers glicerineké.
5.2 Bebizonyítottam, hogy a glicerin ecetsav és propionsav észterei, a triacetin és tripropionin nagy tisztaságban (triacetin > 99% (m/m), tripropionin > 96% (m/m)) és jó hozammal előállíthatók az általunk tisztított, szervetlen sókat tartalmazó glicerinből.
Megállapítottam, hogy a biodízelgyártás során felhasznált katalizátor (KOH) semlegesítéséből származó sók jelenléte nem befolyásolja jelentős mértékben a reakciók lefolyását, abban az esetben, ha katalizátorként ásványi savakat használunk. Kísérleti adatokkal igazoltam, hogy a melléktermékként keletkező víz minimális forráspontú azeotrópképző oldószer segítségével a reakcióelegyből könnyen eltávolítható. Rámutattam, hogy a termékből a szervetlen sók egyszerű szűréssel eltávolíthatóak, így nincs szükség a termékek költséges vákuum desztillációjára.
Továbbá megállapítottam, hogy az általunk tisztított, szervetlen sókat tartalmazó glicerin teljes észterezése erősen savas ioncserélő gyanta jelenlétében nem hajtható végre, mivel a szervetlen sók ionjai dezaktiválják a gyanta aktív helyeit, így a termékek jelentős mennyiségű mono- és diacetint, illetve dipropionint tartalmaztak.
5.3 Kidolgoztam egy újszerű módszert glicerin-karbonát - tiszta glicerin- és dimetil-karbonátból történő – előállítására.
Megállapítottam, hogy a glicerin-karbonát előállítás legcélszerűbb módja a tiszta glicerinből történő szintézis. Kísérleti adatokkal igazoltam, hogy a melléktermékként keletkező metanol a reagensként felhasznált dimetil-karbonáttal minimális forráspontú azeotróp elegyet alkot, így megfelelő arányú dimetil-karbonát felesleggel (dimetil-karbonát/glicerin mólarány: 3:1) a reakcióelegyből könnyen eltávolítható.
Továbbá megfogalmaztam egy, a glicerin-karbonát előállítását szimuláló kinetikai modellt.
A modellben a folyadék és gázfázis között lejátszódó transzport folyamatokat is figyelembe vettem. A kinetikai mérések eredményei alapján meghatároztam a reakciók sebességi egyenleteinek kinetikai paramétereit.
5.4 Kidolgoztam egy módszert a (2-izobutil-2-metil-1,3-dioxolán-4-il)metanol nyers glicerinből történő előállítására.
Bebizonyítottam, hogy a reagensként felhasznált metil-izobutil-keton alkalmas a reakció során melléktermékként keletkező víz, valamint a nyers glicerin víztartalmának azeotróp desztillációs eltávolítására. Megállapítottam, hogy a nyers glicerint szennyező komponensek jelenléte nem befolyásolja a glicerin és a metil-izobutil-keton reakcióját ásványi savak jelenlétében, így az IMDM nagy tisztaságban (> 99%) előállítható közvetlenül nyers glicerinből. Ez a nyers glicerin feldolgozó eljárás gazdaságosnak mutatkozik, figyelembe véve az alapanyagok alacsony árát, a technológiai lépések minimális számát és a keletkező hulladékáramok csekély mennyiségét.
Reakciósebességi mérések alapján meghatároztam a reakció főbb kinetikai paramétereit.
Elegyíthetőségi mérésekkel megállapítottam, hogy valószínűleg nincs szükség a termék szerkezetének további módosítására annak érdekében, hogy üzemanyagadalékként felhasználható legyen.
Az általam kidolgozott nyers glicerin feldolgozó eljárás ígéretes lehet az alapanyagok alacsony ára, a technológiai lépések minimális száma és a keletkező hulladékáramok csekély mennyisége miatt.
6 Theses
6.1 A novel process for purification of crude glycerol was elaborated.
The following findings have been made by experiments in connection with purification of crude glycerol:
• The high viscosity of the crude glycerol is hampering the handling of this phase by enhanced force on pums and agitators. The increase of the processing temperature to 40-80 ºC can be a solution for this problem.
• The presence of methanol and other light boiling components is of advantage to reduce the viscosity.
• A dilution of crude glycerol by ~25% (m/m) water was found to be optimal for viscosity reduction.
The purification steps of the elabrated process are the following ones:
1. Acidic treatment to decompose soaps. Since free fatty acids are insoluble in glycerol, these components rise to the top so that they can be skimmed off. For this purpose pH value of 3 was found to be the optimum.
2. Neutralization of excess acid by calcium-hydroxide slurry. The lowest solubilities of phosphates in crude glycerol were obtained to exist near a pH value of 5.
3. Adsorption by activated carbon to remove color bodies. 2% (m/m) of activated carbon allowed to decolorize the partly purified G-phase, moreover the odor was eliminated by activated carbon, as well.
4. Vacuum distillation. Recover of pure glycerol by distillation requires at least two steps. In the first step all water and methanol was removed by distillation under low vacuum. After the removal of the light boiling components, the glycerol concentrationt of this partly purified glycerol went up to > 90% (m/m). In the second step glycerol was purified by distillation from the heavy inorganic compounds under medium vacuum. The purity of the main condensate fraction was
> 99% (m/m) glycerol.
It was shown that by the distillative purification of crude glycerol without any pretreatment step only ~ 25% of the glycerol content could be recovered. Moreover, thermal decomposition of glycerol, enourmous viscosity increase, foaming, salt precipitations and
severe boiling retardations were detected which cause significant technical difficulities in the process.
It was proven that purification of crude glycerol by distillation requires the decomposition of soaps by acidic treatment and removal of fatty acids.
6.2 It has been concluded that acetic and propionic acid esters of glycerol can be prepared with high purity (triacetin > 99% (m/m), tripropionin > 96% (m/m)) starting from partly purified glycerol still containing inorganic salts.
It has been proven that presence of salts is not disturbing the formation of esters using conventional acid catalysts. The forming by-product, water can be removed by azeotropically distillation easily. Since all formed salts can be removed by simple filtration, expensive purification of esters by over-distillation is not necessary. Moreover it has been proven that strongly acidic ion-exchange resigns can not catalyse the esterification reactions properly starting from partly purified glycerol. This could be explained by the deactivation of active sites of the ion-exchange resin by ions of salts, present in that raw material. In those cases products contained mainly mono- and diacetin and dipropionin.
6.3 A novel process for synthesis of glycerol carbonate from pure glycerol and dimethyl carbonate was elaborated.
It has been proven that glycerol carbonate synthesis starting from pure glycerol is the most convenient way of the preparation. It was shown that the forming by-product, methanol forms a minimum-boiling point azeotrope with dimethyl carbonate and it can be removed by azeotropically distillation easily in case of dimethyl carbonate excess (molar ratio of dimethyl carbonate/glycerol: 3:1).
A kinetic model has been developed for synthesis of glycerol carbonate. Mass transport between of the liquid and gas phase was built into the model. By fitting to measured data, the adjustable parameters of kinetic equations of the assumed reactions were assigned.
6.4 A method for preparation of (2-isobutyl-2-methyl -1,3-dioxolan-4-yl)methanol starting from crude glycerol has been elaborated.
It was shown that starting from crude glycerol the reagent, methyl-isobutyl-ketone helps to remove the reaction water during the “ketalization”, but the water content of crude glycerol, as well by azeotropic distillation. It has been proven that presence of inorganic salts and other impurities like soaps is not disturbing the reaction of glycerol and methyl-isobutyl-ketone in presence of conventional acid catalysts. The pure value-added glycerol derivative, IMDM, can be prepared directly from crude glycerol and methyl-isobutyl-ketone with a yield of > 99%. Moreover, purification of the product does not require expensive process. Based on the measured reaction rate data, kinetic parameters of the reaction were determined. It was proven by miscibility tests that (2-isobutyl-2-methyl -1,3-dioxolan-4-yl)methanol is a relatively good co-solvent and further modification of the molecule is not needed to improve it’s solubility in diesel fuel.
This method for IMDM synthesis seems to be an economical way of utilization of crude glycerol, considering the low prices of raw materials, the simple process and the relatively low volumes of waste streams.
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