Removal of Residual Catalyst from Simulated Biodiesel’s Crude Glycerol forGlycerol Hydrogenolysis to Propylene Glycol

Chuang-Wei Chiu,† Mohanprasad A. Dasari,† Willam R. Sutterlin, ‡ and Galen J. Suppes*,†

Department of Chemical Engineering, W1013 Lafferre Hall, UniVersity of Missouri, Columbia, Missouri 65211,and Renewable AlternatiVes LLC, 410 South Sixth Street, Suite 203, N. Engineering Building, Columbia,Missouri 65211-2290

The removal of sodium from glycerol solutions by crystallization/precipitation of hydroxyapatite (HAP) throughthe co-addition of lime [Ca(OH)2] and phosphoric acid was evaluated as a means to remove soluble catalystfrom the glycerol byproduct of biodiesel production. Phosphate ions precipitated as hydroxyapatite uponreacting with calcium and hydroxide ions. Seed crystals and pH impacted crystallization. The yield decreaseddue to the polymerization of glycerol at high pH values (pHg 11). The continuous removal of phosphate bya lime packed column method was also evaluated for process scale-up considerations. Higher temperaturesfavored the phosphate removal efficiency with higher temperatures raising the pH and the supersaturationregion of the respective effluents to the desired level for HAP crystallization/precipitation. The suitability ofthe resulting product was evaluated as a hydrogenolysis feedstock for producing propylene glycol. The yieldof propylene glycol increased with increasing filtrate pH.

Introduction

With the annual world production of biodiesel expected toincrease to over four billion liters by the end of this decade,the projected amount of the crude glycerol byproduct of theprocess will increase to over 400 million liters per year. Forlarger biodiesel facilities that refine and sell glycerol, theincreased glycerol supply has resulted in lower glycerol prices.Many smaller plants simply discard the glycerol byproduct asa waste. A primary reason for discarding the glycerol is the5-15% (water-free basis) of soluble salts that can be costly toremove.

The traditional method of removing salts from crude glycerolis to evaporate the glycerol from nonvolatile salts in a flash-separation process. While flash-separation processes are effec-tive, they present capital, maintenance, and utility costs. Thepurpose of this paper is to evaluate alternative salt removalmethods and to evaluate the compatibility of these removalmethods with converting the glycerin to propylene glycol overa copper-chromite catalyst.

In the production of biodiesel, a catalyst is used to promotetransesterification, producing methyl esters (biodiesel) and aglycerol byproduct along with soaps from residual free fattyacids and water. The catalysts are typically base catalysts suchas sodium hydroxide or other alkali metal hydroxides.1-4 Abiodiesel plant that utilizes base catalysis can be described as asuccession of different sections and is presented in Figure 1.At high conversions, the biodiesel and glycerol phases areimmiscible. Most unreacted catalysts and soaps (base-neutralizedfatty acids) are preferentially distributed into the glycerol phase.5

After reaction, the biodiesel is typically decanted from theglycerol phase. For the biodiesel’s crude glycerol byproduct,the treatment phase generally involves neutralization andrecycling of the unreacted methanol, either of which could occurbefore or after decanting the biodiesel from the glycerol.

Hydrochloric and sulfuric acids are commonly used to neutralizethe catalyst after reaction to reduce the amount of soaps(potassium or sodium salts of free fatty acids) that adverselyimpact separation and represent a loss of yield.

Larger biodiesel facilities often refine the glycerol for salein the commodity glycerol market. However, the price ofglycerol is already (in 2005) about half the price of past averagesin Europe, where biodiesel production exceeds 1600 millionliters per year. Increased biodiesel production is expected tofurther suppress glycerol prices, and so, conversion of glycerolto other value-added consumer products is desirable. Thehydrogenolysis of biodiesel’s crude glycerol to propylene glycolis one process being evaluated to increase the profitability ofbiodiesel production.

Hydrogenolysis Catalysts.Proplyene glycol can be producedby hydrogenating glycerol only with a highly selective hydro-genolysis catalyst. In general, the alcohol groups are more stableagainst hydrogenolysis than carbonπ-bonds and do not readilyreact at normal hydrogenating conditions. In a previous study,the authors showed that copper-containing catalysts of differentcomposition are potentially good catalysts for this purpose.6

These catalysts exhibit poor hydrogenolytic activity toward C-Cbonds and efficient activity for C-O bond hydro-dehydroge-nation.7,8 However, these catalysts are very sensitive to typicalcatalyst poisons such as S, Cl, and P.9

The salts found in biodiesel’s crude glycerol typically act ashydrogenolysis catalyst poisons, causing deactivation. Theprimary objective of the research described in this paper wasto identify cost-effective methods (alternative to refining) toneutralize or remove the catalyst and/or salts from biodiesel’scrude glycerol in a manner that does not lead to hydrogenolysiscatalyst deactivation.

The chlorides can be removed with a chloride absorbent. Thesulfates can be eliminated by addition of barium hydroxide toform insoluble barium sulfate. While it is technically feasibleto remove chlorides and sulfates, it is economically prohibitive.Phosphates are possibly the easiest and most economical anionsto remove from solution and were the emphasis of the currentstudy.

* To whom correspondence should be addressed. Tel.:+1-573-884-0562. Fax:+1-573-884-4940. E-mail address: suppesg@missouri.edu.

† University of Missouri.‡ Renewable Alternatives LLC.

791Ind. Eng. Chem. Res.2006,45, 791-795

10.1021/ie050915s CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 11/15/2005

Phosphate Crystallization and Precipitation.Considerableworldwide research has been undertaken on phosphate removaltechnologies. The technical feasibility of phosphate crystalliza-tion and precipitation as a unitary process for wastewatertreatment has been demonstrated by Zoltek;10 Hirasawa, Shi-mada, and Osanai;11 Joko;12 and Van Dijk and Braakensiek.13

This same approach should also be effective for removingphosphate salts from the biodiesel’s crude glycerol in theexisting biodiesel facilities.

Crystallization can be categorized into two processes: nucle-ation and growth. For precipitation, both nucleation and growthtake place simultaneously where there are only small concentra-tions of seed crystals; this is also referred to as spontaneous orhomogeneous crystallization.14 Crystallization/precipitation ofhydroxyapatite (HAP), Ca5(PO4)3OH, in an aqueous solutionis fundamental to this phosphate removal method and issummarized by eq 1. The relative insolubility of HAP is due toits thermodynamic stability at pH’s above 6.8.15

Kaneko et al.16 reported the special affinity that crystals havefor phosphate. The result is explained by a chemical reactionbetween the phosphate ions and the surface of the seed materials.This crystallization/precipitation of HAP on a seed crystal iscommonly influenced by the nature of the seed crystal, thephosphate concentration, the calcium ion concentration, and thepH value. Research work was conducted to remove thephosphate anions from an aqueous glycerol solution by acrystallization/precipitation reaction with calcium ions as theseed crystal material coexisting in the solution. Several typesof HAP salts will form that incorporate sodium, and so, this isan effective means to remove both the phosphorus and thesodium from the system.

To determine the optimal operation parameters for effectivephosphate removal from aqueous glycerol solutions for subse-quent hydrogenolysis of glycerol to propylene glycol, sets of50 g of phosphate-containing glycerol solutions were contactedwith lime [Ca(OH)2] by a batch-stirred reactor and a continuouspacked column. The neutralized glycerol solutions were sub-jected to an autoclave reactor to perform the glycerol hydro-genolysis reaction using a copper-chromite catalyst at a hydro-gen pressure of 200 psi and a temperature of 200°C.

In the broader sense, apatite salts are a category of calcium-phosphate salts known to have low solubility. In this paper, wehypothesized that calcium-sodium-phosphate salts can be formedthat have low solubilities and processabilities. In this study,sodium hydroxide was neutralized with phosphoric acid inaqueous glycerol solutions by the crystallization/precipitation

of HAP using lime. The susceptibility of the glycerol was thenevaluated in a hydrogenolysis reaction.

Experimental Section

Materials. Glycerol (99.9%), sodium hydroxide pellets,calcium hydroxide, and phosphoric acid (85%) were purchasedfrom Fisher Scientific Co. (Fairlawn, NJ). Sodium monobasicphosphate (98%), sodium dibasic phosphate (98%), copper-chromite catalyst, and lime had an approximately mean particlesize of 100 mesh and were purchased from Sigma-Aldrich(Milwaukee, WI). High purity grade hydrogen was obtainedfrom Praxair.

Experimental Procedures.An 80% glycerol solution with20% water was mixed with 4% sodium hydroxide in a glassflask for about 30 min at 50°C. An 85% phosphoric acidsolution was added to the mixture to neutralize it until a pH of5.5 was reached. The phosphate-containing glycerol solutionwas contacted with excess lime through batch reactions andlime-packed columns in order remove the phosphate throughcrystallization/precipitation.

The batch crystallization/precipitation experiments were car-ried out in 200 mL glass flasks. Varying amounts of lime wereadded to a 50 g phosphate-containing glycerol solution as theseeding material and to adjust the pH of the glycerol solution.The change in pH with time was monitored with a pH meter.Glycerol solutions were maintained above constant pH valuesby the addition of lime, and the addition volume was recorded.The solution was continuously stirred at a constant speed of250 rpm with a magnetic stirrer at a constant temperature of 50°C. After stirring for predetermined times, the solution wasvacuum-filtered.

Column removal experiments were carried out in a stainlesssteel column (i.d. 30 mm, length 150 mm) equipped with anexternal heating tape for the heating system. The column packedwith 15 g of lime was connected to a peristaltic high-performance liquid chromatography (HPLC) pump. The 50 gphosphate-containing glycerol solution was pumped in a down-ward direction through the column. The temperature of thecolumn was controlled by the CAMILE 2000 control and dataacquisition system using TG 4.0 software. The residence timewas adjusted by proper control of the flow rate.

After the glycerol solutions were treated through the batchor column methods, they were placed into the autoclave for thesubsequent hydrogenolysis of glycerol to form propylene glycol.All reactions were carried out in a stainless steel multi-autoclavereactor capable of performing eight reactions simultaneously.Each reactor has a capacity of 150 mL and is equipped with astirrer, a heater, and a sample port. The temperatures of thereactors were controlled by the CAMILE 2000 control and data

Figure 1. Example block flow diagram of biodiesel production.

3HPO42- + 5Ca2+ + 4OH- f Ca5(PO4)3OH + 3H2O (1)

792 Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006

acquisition system using TG 4.0 software. The reactors wereflushed several times with nitrogen followed by hydrogen. Then,the system was pressurized with hydrogen to the necessarypressure and heated to the desired reaction temperature. Thespeed of the stirrer was set to be constant at 100 rpm throughoutthe reaction. The copper-chromite catalyst used in this studywas reduced prior to the reaction by passing a stream ofhydrogen over the catalyst bed at 300°C for 4 h.

Analytical Methods. Reaction product samples were takenafter 24 h of reaction time, cooled to room temperature, andcentrifuged using an IEC (Somerville, MA) Centra CL3Rcentrifuge to remove the catalyst. These samples were analyzedwith a Hewlett-Packard 6890 (Wilmington, DE) gas chromato-graph equipped with a flame ionization detector. Hewlett-Packard Chemstation software was used to collect and analyzethe data. A Restek Corp (Bellefonte, PA) MXT WAX 70624gas chromatography (GC) column (30 m× 250µm × 0.5µm)was used for separation.

A solution of n-butanol with a known amount of internalstandard was prepared a priori and used for analysis. Thesamples were prepared for analysis by adding 0.1 mL of productsample to 1 mL of stock solution in a 2 mLglass vial. A 2µLportion of the sample was injected into the column. The oventemperature program consisted of the following segments: startat 45 °C (0 min), ramp at 0.2°C/min to 46°C (0 min), andramp at 30°C/min to 220°C (2.5 min). Using the standardcalibration curves that were prepared for all the components,the integrated areas were converted to weight percentages foreach component present in the sample.

For each data point, the theoretical yield of propylene glycolwas calculated. The theoretical yield is defined as the ratio ofthe number of moles of propylene glycol produced to thetheoretical number of moles of propylene glycol that would beproduced at 100% conversion. Conversion of glycerol is definedas the ratio of the number of moles of glycerol consumed inthe reaction to the total moles of glycerol initially present.

Results and Discussion

Reaction Profiles of Hydrogenolysis of Glycerol to Pro-pylene Glycol.Earlier work in our group has demostrated thatcopper or copper-based catalysts exhibit higher selectivitytoward propylene glycol with little or no selectivity towardethylene glycol and other degradation byproducts.6 Figure 2shows the reaction profiles of glycerol conversion and the yieldof propylene glycol with time at a temperature of 200°C and200 psi hydrogen pressure for the copper-chromite catalyst usingan 80% glycerol solution. It can be seen that an equilibrium

glycerol conversion of 54.8% was reached at 24 h with a totaltheoretical yield of 46.6%. Figure 2 also provides a baselinefor the copper-chromite catalyst in the absence of all salts.

Effect of Residual Salts on Glycerol Hydrogenolysis.Toevaluate the effect of residual salts from the biodiesel processon the glycerol hydrogenolysis reaction, reactions were carriedout by simulating crude glycerol by the addition of sodiumhydroxide, phosphoric acid, sodium phosphates (Na2HPO4 andNaH2PO4), and lime. Table 1 provides the summary of theconversions of the 80% glycerol solution with different salts at200°C and 200 psi hydrogen pressure using the copper-chromitecatalyst. As expected, trace amounts of phosphate ions in theglycerol solution negatively affected the hydrogenolysis reactiv-ity of the copper-chromite catalyst. There was no conversionobserved with the addition of small amounts of sodiumphosphates and phosphoric acid. This indicates that phosphatesreact with or irreversibly adsorb onto active sites to deactivatethe catalyst. The presence of sodium hydroxide decreased theyield of propylene glycol due to the formation of degradationreaction products resulting in the polymerization of glycerol athigh pH values. The data in Table 1 also show that the additionof lime, owing to its low solubility in glycerol solution, mayalso reduce the hydrogenolysis activity of copper-chromite dueto catalyst site blockage with physical adsorption of the insolublecalcium component.

Lime was selected for the phosphate removal material becauseit contains water-soluble calcium which reacts with the phos-phate ion to form insoluble crystalline calcium phosphates,mainly HAP, and also because it can be a seeding crystalmaterial due to its fine particle size. Experiments wereperformed in the batch mode to evaluate phosphate removalfor the phosphate-containing glycerol solution with 1 wt %straight phosphoric acid by the addition of lime, as shown inTable 2. Lime effectively neutralizes the phosphoric acid, asshown by an increased yield of propylene glycol to 37.6% inthe absence of sodium salts. These data indicate that phosphoricacid and lime can be used to improve the viability of crudeglycerol as a hydrogenolysis feedstock.

Removal of Phosphate in Batch Reactors.The effects ofthe filtrate pH and the lime addition on the HAP crystallization/

Figure 2. Reaction profiles of glycerol conversion and yield of propyleneglycol for copper-chromite catalyst at 200°C and 200 psi hydrogen pressure.

Table 1. Effect of the Contaminants from the Biodiesel Process onthe Formation of Propylene Glycol from Glycerola

contaminant pH % yield

none 46.61 wt % H3PO4 1.25 02 wt % NaH2PO4 4.2 01 wt % NaH2PO4 4.2 3.31 wt % Na2HPO4 8.9 3.91 wt % NaOH 12.5 14.41 wt % Ca(OH)2 11.5 18.3

a All the reactions were performed using an 80% glycerol solution at200 °C and 200 psi hydrogen pressure for 24 h.

Table 2. Summary of the Glycerol Hydrogenolysis Results with theAddition of Ca(OH) 2 in the Phosphate-Containing Glycerol SolutionPrepared by 1 wt % Phosphate Acida

Ca(OH)2 (g) filtrate pH % yield

0 1.25 01.37 5 15.31.5 7 24.51.64 10.5 37.6

a All phosphate removal experiments were performed in the batchmethod. All glycerol hydrogenolysis reactions were performed using an80% glycerol solution at 200°C and 200 psi hydrogen pressure for 24 h.

Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006793

precipitation system were investigated by determining the yieldof propylene glycol on hydrogenolysis of glycerol.

A. Effect of Filtrate pH. Figure 3 shows the effect of thepH value on the HAP crystallization/precipitation system forphosphate removal. The yield of propylene glycol is plotted asa function of the batch reaction time with different pH valuesof 7.5, 9, and 10. Both the yield and the reaction rate increasedwith increasing pH.

The yield of propylene glycol from glycerol hydrogenolysisis increased with increasing pH of the HAP crystallization/precipitation system from 14.3% at pH 7.5 to 32.2% at pH 10.5after 120 min. An explanation for these trends is that the HAPcontinues to poison the catalystseventually poisoning all activesites. Higher pH’s drive the precipitation of the HAP at theexpense of increasing soluble base concentrationssapparentlythe soluble base (being low due to the low solubility of Ca-(OH)2) is less detrimental than the soluble anions of HAP.

B. Effect of Lime Addition. Table 3 summarizes the glycerolhydrogenolysis results of propylene glycol formation withdifferent amounts of lime addition in the batch HAP crystal-lization/precipitation system. The amounts of lime added toobtain the indicated pH levels of mixtures containing 50 g ofthe phosphate-containing glycerol solution in the batch HAPcrystallization/precipitation system after 120 min of mixing arealso provided.

In general, a higher yield of propylene glycol can be obtainedat a higher dosing of lime since the phosphate removal throughHAP precipitation is enhanced with a high calcium concentrationand a raised pH level.17 However, the yield of propylene glycolincreased until 29.45 g (pH 10.5) of lime was added and beganto decrease as the dosing was increased further. This decreasein the yield of propylene glycol with calcium hydroxide dosingover 30 g (pHg 11) is due to glycerol polymerization at highpH values.18

Removal of Phosphate by a Packed-Column Method.Dueto the low solubility of lime, it is possible to remove phosphatefrom solution by passing the solution through a column packedwith lime. In these experiments, the effects of the residencetime and the column temperature were determined.

In these studies, the glycerol was passed through a columncontaining sodium hydroxide that had been neutralized withphosphoric acid. The objective was to form HAP in the columnwhich would then precipitate from solution. Hydrogenolysis wasthen performed on the column effluent to evaluate howeffectively the more soluble sodium phosphate salts had beenremoved.

A. Effect of Residence Time.In Figure 4, the yield ofpropylene glycol is plotted as a function of glycerol that hadflowed through the column at different flow rates to inducedifferent residence times for the precipitation process. Thecolumn temperature was 180°C, and the hydrogenolysisconditions are the same as those previously used.

A gradual increase in the yield of propylene glycol wasobserved as the column residence time increased to 10 minasymptotically approaching a yield of 28%. This maximum yieldis similar to that obtained for the batch results of Figure 3.

B. Effect of Column Temperature.The temperature of 180°C, as used for the data reported in Figure 4, was determinedby a series of screening studies through the column. In thesescreening studies, the glycerol solutions were passed through aheated column at temperatures of 50, 100, 120, 150, 170, and180°C. Glycerol hydrogenolysis reactions were preformed withthe effluent glycerol solutions to identify the impact of tem-perature on the crystallization/precipitation of HAP in thecolumn. Table 4 shows the hydrogenolysis results of the effluentglycerol solutions through the column at different temperatures.The yield of propylene glycol increased with increasing columntemperature. A 26.9% yield of propylene glycol was obtained

Figure 3. Summary of the glycerol hydrogenolysis results with differentpH values in the batch HAP crystallization/precipitation system. All glycerolhydrogenolysis reactions were performed using an 80% glycerol solutionat 200°C and 200 psi hydrogen pressure for 24 h.

Table 3. Summary of the Glycerol Hydrogenolysis Results withDifferent Amounts of Ca(OH)2 Addition in the Batch HAPCrystallization/Precipitation Systema

Ca(OH)2 (g) filtrate pH % yield Xb

0 5.5 0 0.004.88 6.5 4.9 1.008.39 7.5 14.3 1.70

15.04 9 26.6 1.7729.45 10.5 32.2 1.0939.23 11 22.1 0.56

a All glycerol hydrogenolysis reactions were performed using an 80%glycerol solution at 200°C and 200 psi hydrogen pressure for 24 h.b Theefficiency factor (X) is the ratio of grams of propylene glycol produced pergram of lime used in preparing the reagent.

Figure 4. Summary of the glycerol hydrogenolysis results of the effluentglycerol solutions that passed through the column with different residencetimes at a constant column temperature of 180°C. All glycerol hydro-genolysis reactions were performed using an 80% glycerol solution at 200°C and 200 psi hydrogen pressure for 24 h.

Table 4. Summary of Glycerol Hydrogenolysis Results of theEffluent Glycerol Solutions That Passed through the Column with15 min of Residence Time at Different Temperaturesa

column temp (°C) effluent glycerol pH % yield Xb

50 5.98 3.9 0.26100 6.9 10.6 0.71120 7.65 11.5 0.77150 8.86 15.5 1.03170 10.01 23 1.53180 10.57 26.9 1.79

a All glycerol hydrogenolysis reactions were performed using 80%glycerol solution at 200°C and 200 psi hydrogen pressure for 24 h.b Theefficiency factor (X) is the ratio of grams of propylene glycol produced pergram of lime used in preparing the reagent.

794 Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006

when the phosphate-containing glycerol solution flowed througha 180°C column.

These results indicate that high phosphate removal efficiencycan be obtained from a column with the removal efficiencyhighly dependent on the precipitation temperature. High tem-peratures increase the solubility of lime and possibly the rateof solution, resulting in higher pH values.

The formation of HAP in aqueous solutions takes placefollowing the development of supersaturation. Also, the crystal-lization of HAP should occur in the metastable supersaturatedregion of HAP.19 Kaneko et al.16 described the operatingconditions that should be set up in the metastable supersaturatedregion close to the super solubility curve in order to inducephosphate crystallization on the seed crystal. However, increas-ing temperature contributes to the solution supersaturationdevelopment and to a metastable supersaturated region, becausethe sparingly soluble HAP has a reverse solubility. In otherwords, a driving force that provides a pH and solutionsupersaturation adjustment is created by high-temperatureoperation to crystallize the phosphate on the lime bed.

Efficiency Factor Comparison. The following expression(eq 2) was used to quantify the efficiency of lime consumptionfor phosphate removal as an easy comparison of the experi-ments.

The efficiency factorX was calculated from the batch andcolumn results with a high value ofX indicating more effectiveuse of the lime. At a pH value of 9 in the batch and columnexperiments, anX value of 1.03 was obtained at a residencetime of 15 min and 150°C in the column experiment comparedto 1.77 with 120 min in the batch experiment. TheX valuegradually increased as the column temperature increased (seeTable 4). A maximumX value of 1.79 was achieved at thecolumn temperature of 180°C.

The column precipitation method exhibited an advantage overbatch precipitation with respect to the efficiency of limeutilization. In the batch experiments (Table 3),X increasedinitially with increasing pH but, then, reached a maximum asthe pH was increased further. The decrease ofX from pH 9 to10.5 is due to the relatively higher amount of lime that is neededto maintain a desired pH value in the high alkalinity region. Alow value ofX of 0.56 at pH 11 in the batch study is due toglycerol polymerizing to polyglycerol during the glycerolhydrogenolysis.

Conclusions

Sodium was removed from glycerol by first neutralizing themixture with phosphoric acid and then precipitating an insolublesalt by contacting the mixture with lime to form hydroxyapatite(HAP). Lime performed several roles in this separation, includ-ing supplying the calcium ions, controlling pH, and nucleatingcrystals.

The success of the glycerol cleanup was measured by theability to hydrogenate the product over a copper-chromitecatalyst to propylene glycol. In the batch experiments with aconstant temperature, increasing the pH value from 7.5 to 10.5improved hydrogenolysis yields by a separation method includ-

ing HAP crystallization/precipitation. However, at pH valuesg 11, the excess base promoted polymerization.

The effectiveness of separation over a packed column of limewas a strong function of temperature. A temperature of 180°Cprovided a balance of separation rates and sufficiently lowdegradation of the glycerol. This study demonstrated the viabilityof using the crystallization/precipitation of HAP method for theremoval of residual catalysts from the biodiesel’s crude glycerolas a means to improve the quality of glycerol as a hydrogenoly-sis reagent.

Acknowledgment

This material is based upon work supported by the NationalScience Foundation under Grant No. 0318781 and The MissouriSoybean Merchandising Council.

Literature Cited

(1) Sprules, F. J.; Price, D. Production of Fatty esters. U.S. Patent2,494,366, 1950.

(2) Suppes, G. J.; Dasari, M. A.; Doskocil, E. J.; Mankidy, P. J.; Goff,M. J. Transesterification of Soybean Oil with Zeolite and Metal Catalysts.Appl. Catal., A2004, 257, 213.

(3) Dasari, M. A.; Goff, M. J.; Suppes, G. J. Noncatalytic alcoholysiskinetics of soybean oil.J. Am. Oil Chem. Soc. 2003, 80 (2), 189.

(4) Canakci, M.; Gerpen, J. V. Biodiesel Production from Oils and Fatswith High FFAs.Trans. ASAE2001, 44 (6), 1429.

(5) Chiu, C. W.; Goff, M. J.; Suppes, G. J. Distribution of Methanoland Catalysts between Biodiesel and Glycerol.AIChE J. 2005, 51 (4), 1274.

(6) Dasari, M. A.; Kiatsimkul, P.; Sutterlin, W. R.; Suppes, G. J. Low-pressure Hydrogenolysis of Glycerol to Propylene Glycol.Appl. Catal., A2005, 281 (1-2), 225.

(7) Runeberg, J.; Baiker, A.; Kijenski, J. Copper catalyzed aminationof ethylene glycol.Appl. Catal.1985, 17 (2), 309.

(8) Montassier, C.; Giraud, D.; Barbier, J. Polyol conversion by liquid-phase heterogeneous catalysis over metals.Stud. Surf. Sci. Catal.1988,41, 165.

(9) Rase, H. F.Handbook of Commercial Catalysts HeterogeneousCatalysts; CRC Press: New York, 2000.

(10) Zoltek, J., Jr. Phosphorous Removal by Orthophosphate Nucleation.J. Water Pollut. Control Fed. 1974, 46, 2498.

(11) Hirasawa, I.; Shimada, K.; Osanai, M. Phosphorus removal processfrom wastewater by contact crystallization of calcium apatite.Pac. Chem.Eng. Congr., [Proc.] 1983, 4 (3rd), 259.

(12) Joko, I. Phosphorous Removal from Wastewater by the Crystal-lization Method.Water Sci. Technol. 1984, 17 (2/3), 121.

(13) Van Dijk, J. C.; Braakensiek, H. Phosphate Removal by Crystal-lization in the Fluidized Beds.Water Sci. Technol. 1984, 17 (2/3), 133.

(14) Momberg, G. A.; Oellermann, R. A. The Removal of Phosphateby Hydroxyapatite and Struvite Crystallization in South Africa.Water Sci.Technol. 1992, 26 (5/6), 987.

(15) Boskey, A. L.; Posner, A. S. Formation of Hydroxyapatite at LowSupersaturation.J. Phys. Chem. 1976, 80, 40.

(16) Kaneko, S.; Nakajima, K. Phosphorous Removal by CrystallizationUsing a Granular Activated Magnesia Clinker.J. WPCF1988, 60, 1239.

(17) Song, Y.; Hahn, H. H.; Hoffmann E. The effects of pH and Ca/Pratio on the precipitation of calcium phosphate.Chemical Water andWastewater Treatment VII, Proceedings of the Gothenburg Symposium,10th, Gothenburg, Sweden; IWA Publishing: London, 2002; p 349.

(18) Garti, N.; Aserin, A.; Zaidman, B. Polyglycerol ester: optimizationand techno economic evaluation.J. Am. Oil Chem. Soc. 1981, 58, 878.

(19) Nancollas, G. H.; Mohan, M. S. The growth of hydroxyapatitecrystals.Arch. Oral Biol. 1970, 15, 731.

ReceiVed for reView August 8, 2005ReVised manuscript receiVed October 17, 2005

AcceptedOctober 19, 2005

IE050915S

X ) yield of propylene glycollime consumption

(2)

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