Article pubs.acs.org/EF
Continuous Packed-Bed Biodiesel Polishing Using Particulate Materials Nilva R. Uliana,† Marshall Palliga,‡ Marintho B. Quadri,† and J. Vladimir Oliveira*,†,‡ †
Department of Chemical and Food Engineering, UFSC, Florianópolis, South Catalina, Brazil Transfertech Innovation Ltda, Erechim, Rio Grande do Sul 99700, Brazil
‡
S Supporting Information *
ABSTRACT: This work reports experimental data on dry-washing of biodiesel produced by enzyme-catalyzed reaction using ion exchange resins, a commercial immobilized enzyme, and a magnesium silicate adsorbent. Experiments were carried out in continuous-mode packed-bed column, varying operating temperature and pressure, and residence time toward reaching the current specifications of the biodiesel samples tested. Besides, the amounts of free fatty acids (FFA), raw material, and products were characterized with regard to the content of monoacylglycerol (MAG), diacylglycerol (DAG), and triacylglycerol (TAG). It is shown that resins and the immobilized enzyme presented, to a certain extent, satisfactory results, in terms of FFA reduction, while the use of the adsorbent allowed reaching “on spec” biodiesel. It was observed that the water content dramatically affected the quality of the final product, which severely hindered one from reaching lower FFA contents. Integration of resins together with silicate adsorbent seems to be an interesting strategy for biodiesel polishing. Pilot plant tests were also conducted using the resins with replicable results from laboratory-scale assays. Results obtained in this work are relevant for the design and operation of industrial-scale plants.
1. INTRODUCTION Nowadays, ∼95% of biodiesel production has been mostly produced through the alkali-catalyzed transesterification of edible oils (mainly palm, soybean, rapeseed, and sunflower oils), using methanol as a substrate.1,2 Nevertheless, the use of lipases has received growing consideration in recent years and is undergoing a rapid development.3 Compared to the conventional alkali-catalyzed production, the enzymatic process is considered a “green route” because it is less energy-intensive and is also highly selective, producing a very-high-purity product with less downstream operations.4−6 In addition, the enzymatic process is very tolerant to the high acid water contents that are present in waste oils and increases the biodiesel yield by avoiding the typical soap formation, because of alkaline transesterification.7 It is well-known that feedstock costs can comprise more than 75% of the overall biodiesel production costs;8 hence, the use of unrefined, less-expensive, high free fatty acid (FFA) content, lower-grade oils and fats would result in a dramatic reduction of the global costs of enzyme-catalyzed biodiesel production.9 Besides, the use of liquid lipases for FAME production can make the entire process cost-efficient, more competitive, and more sustainable. Liquid lipases can be produced and sold at a much lower price (30−50 times lower) and can also be reused after recovery from the glycerin phase.10 On the other hand, biodiesel produced through soluble lipasesfor example, using Eversa lipase (from Novozymes) leads inherently to a relatively high-acid product, usually with an FFA content of ∼3 wt %, and the presence of monoglycerides, diglycerides, and triglycerides (and, obviously, free glycerin) generally above the specifications, which are defined as follows, based on the Brazilian specifications, regulated by ANP (National Agency of Petroleum, Natural © 2016 American Chemical Society
Gas and Biofuels): 0.25 wt % (maximum) FFA, 0.7 wt % monoglycerides, 0.2 wt % (maximum) diglycerides, and 0.2 wt % (maximum) triglycerides.11 In fact, the main problem arising from the presence of acylglycerols in the final product is that they have a tendency to polymerize, forming deposits as well as other impurities that can produce harmful effects on engine performance if not removed.12 Thus, it becomes necessary to go to the further step of biodiesel purification, by wet washing or dry washing. Excellent reports are available in the literature regarding biodiesel purification methods, and, although wet washing is considered very simple and efficient, there has been a consensus that it may cause considerable product losses, additional costs due to wastewater treatment, the introduction of a later stage to remove water (since the maximum allowed is 0.02 wt %), and the possibility of undesired emulsion formation.12−16 There are several materials that are used for biodiesel purification, such as ion-exchange resins (for example, PurolitePD206, BD10 Dry, from Rohm & Haas, as well as resins SP112H, GF101, and GF202, from LanXess), a variety of silicate-based adsorbents such as Magnesol (from Dallas Group of America, Inc.) and Trisyl (from W.R. Grace), starch and cellulose, bleaching clays, zeolites, membranes, and diatomaceous earth, among others.17−26 Although literature is becoming somewhat vast in the field of biodiesel dry washing, using adsorbent materials, the majority of available reported data is related to batch procedures, evaluating the analytical or laboratory-scale performance of such adsorbents. Thus, there is a lack of information on the use Received: September 22, 2016 Revised: December 2, 2016 Published: December 2, 2016 627
DOI: 10.1021/acs.energyfuels.6b02441 Energy Fuels 2017, 31, 627−634
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Energy & Fuels of continuous-mode packed-bed column operation. In this context, the main focus of this work is on the continuous biodiesel dry-washing operation, exploring the use of different strategies with a variety of materials, in laboratory-scale and also in pilot-plant tests, which is aimed at providing support for possible large-scale industrial applications.
2. EXPERIMENTAL SECTION 2.1. Materials. Two commercial resins were used in the experiments: Lewatit SP112H and GF101 ion-exchange resins kindly donated by LanXess, strongly acidic, macroporous, cross-linked polymer-based resins with sulfonic acid groups. A commercial immobilized enzyme, Novozym 435, which was a gift from Novozymes A/S, DK, was also employed as a reaction catalyst. Magnesol R600, which is a well-known commercial magnesium silicate adsorbent, was kindly donated by Dallas Group of America. Three biodiesel samples were used as raw material to be treated: one of them is a commercial standard B100 (biodiesel in accordance with Brazilian regulation standards), and two others coming from the enzymatic process, henceforth denoted as BS1 and BS2, with different characteristics in terms of FFA, monoacylglycerol (MAG), diacylglycerol (DAG), and triacylglycerol (TAG). Methanol that was used in the experiments (purity of >99.5%) was kindly donated by Olfar S.A. (Erechim, RS, Brazil). 2.2. Experimental Procedure. 2.2.1. Materials. Table 1 presents the characterization of the biodiesel samples used in this work, in
Figure 1. Schematic diagram of continuous FFA reduction using packed bed column resins. Legend: 01, reservoir of feeding material (biodiesel + methanol); 02, liquid pump; 03, packed column; 04, sampling flask; and TB, thermostatic bath. shown in Table 2. Samples were taken at every residence time from the columns and followed the same preparation procedure that was mentioned previously.
Table 2. Experimental Conditions of Continuous-Mode Packed Bed Columns with SP112H and GF101 Resinsa temperature (°C) 110 100 90 110 90 110 110 110
Table 1. Characteristics of Biodiesel Samples Employed in This Work raw material
MAG (wt %)
DAG (wt %)
TAG (wt %)
FFA (wt %)
water (wt %)
B100 BS1 BS2
0.230 1.010 0.430
0.137 2.330 0.750
0.035 0.620 0.230
0.17 3.15 2.81
0.018 0.085 0.116
residence time (min) SP112H Resin 30 30 30 30 90 90 60 120 GF101 Resin
90 100 110
terms of FFA content, monoglycerides, diglycerides, and triglycerides, as well as with regard to the amount of water. Note that the biodiesel samples produced by enzymatic catalysis using the Eversa enzyme do not meet the specifications, because of the high content of FFA and acylglycerols (MAG, DAG, and TAG).11 It is also important to note that these two biodiesel samples differ mainly with respect to the content of acylglycerols, which make them interesting toward testing the ability of the materials employed in this work. 2.2.2. Continuous-Mode Packed Bed Resins and Enzyme Tests. The continuous laboratory-scale tests employed a jacketed stainless steel column that had an inner diameter of 24.3 mm and a length of 200 mm, with an approximate volume of 92.8 mL, supported at both ends by 300 mesh wire disks, which were packed with enzyme or resins. In the case of resins, the column bed was conditioned by drying the resins SP112H or GF101 at 110 °C under vacuum for 1 h, and then washed with flowing four bed-volume methanol at 110 °C with no vacuum applied. (See Figure 1.) Based on the density of the resin, the entire column volume, and the resin amount used, the porosity of the bed was estimated to be 0.67, which means a reactor free volume of ∼62.2 mL.27 After column bed conditioning, a vacuum was applied at 110 °C at least for 1 h to eliminate methanol that remained inside the column, and then mixtures of biodiesel and methanol samples at defined proportions at varying temperature and incoming flow rates were charged into the column. Samples were collected after one residence time (τ, which, here, is defined as the reactor free volume per incoming substrate flow rate) had elapsed, subjected to a rotary evaporator to remove methanol and water at 95 °C for 1 h and then stored for analysis of acidity, as well as MAG, DAG, and TAG content. The same procedure was employed for the operation of columns positioned in series: SP112H + SP112H and SP112H + GF101, with substrates flowing through the columns under the pre-established conditions
30 30 30 Series SP112H and SP112H
110
30 Series SP112H and GF101
110
30
a
Initial FFA content of 3.15 wt % and 25 wt % methanol addition to BS1 raw material.
In the case of enzymes, the first column was packed with Novozym 435 (having an estimated porosity of 0.74 -27 and then mixtures of methanol+biodiesel samples were continuously passed through the column at a fixed molar ratio of 1:9 (methanol to the amount of FFA in raw material) and a temperature of 65 °C for all experiments, for residence times of 15 and 60 min. At each residence time, samples were collected and subjected to a rotary evaporator under moderate vacuum at 95 °C for 45 min in order to determine the acidity prior to analysis of monoglycerides, diglycerides, and triglycerides. 2.2.3. Continuous-Mode Packed-Bed Column Tests with Magnesol R600. Previously described laboratory-scale columns were used in the experiments with Magnesol R600; however, in this case, wire disks were replaced by sintered steel plates with glass beads positioned between packing material and plates. Synthetic biodiesel with FFA concentrations of 0.8, 2.6, and 3.5 wt % flowed through the column at 70 °C, and samples were taken at each estimated residence time of 30 min. The true BS1 biodiesel after treatment in the one-step SP112H resin column, with resulting acidity of 0.8 and 1.42 wt %, was also passed through the Magnesol column at 70 °C up to packing material saturation. Samples for analysis were prepared as previously described. Batch assays with Magnesol R600, comprising kinetic 628
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Figure 2. Schematic diagram of the pilot-plant biodiesel resin esterification. (Legend: 01, reservoir of feeding material; 02, pump; 03, heat exchanger; 04, pack bed column; 05, inline window with sampling valve; 06, heat exchanger; 07, Y filter; 08, needle metering valve; 09, flash tank; 10, condenser; 11, refrigeration system; 12, methanol tank storage; 13, vacuum pump; 14, biodiesel storage tank; P, pressure gauge; and T, thermocouple.) KOH solution (mol/L), MFA is the average molar mass of fatty acids (∼282 g/g mol), and ms is the sample mass (g). 2.3.2. Analysis of Monoglycerides, Diglycerides, Triglycerides, Water, and Glycerol. Quantification of monoglycerides, diglycerides, triglycerides, and glycerol was performed by Olfar S.A. (which is an ANP-authorized laboratory, in Erechim, Brazil) using a flame ionization detection (FID) gas chromatography system (Agilent Technologies, Model 7890A, coupled with a Model DB-5 capillary column with dimensions of 30 m × 0.32 mm × 0.1 μm), following ASTM D 6584 and UNE-EN 14105 standardized methods. The water content was determined by Karl Fischer titration, according to AOCS Method Ca 2e-84, using a Mettler−Toledo Model DL 50 titrator.
results are presented as Supporting Information. These experimental data were employed as Supporting Information for continuous packedbed column tests that were performed in this work. 2.2.4. Pilot-Plant Packed-Bed Resin Tests. For the pilot-plant tests with SP112H and GF101 resins, two stainless steel jacketed columns with an inner diameter of 200 mm and a length of 950 mm (yielding an overall volume of ∼29.85 L) were employed, as shown in Figure 2. Two heat exchangers were positioned ahead of the columns, to preheat the entering feed. After the columns, a heat exchanger, followed by two 1/2 in. metering valves (Swagelok, SS-1RS8, São Paulo, SP, Brazil) was arranged to heat materials exiting from the columns, to allow adequate system pressure regulation and reaction products separation in a 600 L flash tank. Access to such a flash tank was provided by a glass window that allowed level inside monitoring, with temperature and pressure gauges on the top of the tank, and valves that were positioned on the bottom and top of the tank permitted vacuum regulation and product withdrawal. A highperformance vacuum pump (Edwards, Model C56) and a set of traps were mounted to recover methanol and store the product. Typically, 23 kg of resins were loaded into the columns, the hot oil was then circulated through the jackets at ∼135 °C, and methanol was then pumped (Bonfiglioli IEC EN 60034) into the column bed at a rate of 15 L/h, up to a total of four flowing bed volumes. Bed porosity was estimated to be ∼0.65, taking into account the density of the resins, the total bed volume, and the characteristics of each resin.27 Afterward, methanol was removed under vacuum until no positive pressure was verified in the columns. Some experimental tests were performed with pressure applied to the systems, i.e., by restriction of the needle valve at the stream exit, in both upflow and downflow experiments. In such cases, the operation pressure was on the order of 5−8 bar, which was above the vapor pressure of pure methanol. However, results were quite similar to those observed when no pressure was applied to the system. 2.3. Analytical Methods. 2.3.1. Determination of Free Fatty Acids (FFA) Content. The determination of the FFA content in the sample solution was carried by titration with KOH, following IUPAC Method 2.201. Briefly, ∼3 g of solution and 3−4 drops of phenolphthalein (1 wt % phenolphthalein in ethanol) were diluted in 50 mL of anhydrous ethanol. Such a solution was then titrated with 0.1 M KOH, under vigorous agitation, until a purple color persisted. The solution acidity was then determined according to the following equation: FFA (wt %) =
VMKOHMFA 10ms
3. RESULTS AND DISCUSSION 3.1. Continuous Packed-Bed Column Tests with Resins SP112H and GF101. Results using resin SP112H applied to BS1 with an initial acidity of 3.15 wt % are presented in Figure 3. As can be seen, generally, for all experimental conditions (see Table 2), there was a sharp reduction in acidity in the first residence times, followed by a near flat regime, which demonstrates the reaction system stability. However, note that the final asymptotic values are much above the limit value established by ANP standard (FFA < 0.25 wt %). Of course, such high values cannot be accepted since product oxidation stability may be seriously affected by the presence of FFA, and the formation of soaps and corrosion is considered quite undesirable.12 According to Teixeira,28 a relevant fact that should also be taken into account is the product storing conditions, especially with regard to the presence of water. Figure 3b shows that, for both temperatures studied after the sharp acidity decrease, the residence time of 90 min leads to a gradual increase in FFA content, which may be explained in terms of moisture content inside the column, since the esters formed may suffer hydrolysis, hence causing an increase in FFA content. In fact, Figure 3c shows a slight increase in the acidity of the outcome material for a residence time of 120 min, compared to that observed for a smaller value (60 min). Note also in Figure 3c that the higher raw material incoming flow rate (and, hence, the corresponding smaller residence time) leads to the best and near flat product acidity over time. Thus, it seems that higher feeding flow rates decrease the amount of water formed, because of the esterification reaction along the column bed, preventing the hydrolysis reaction, to a certain extent. As shown in Figure 3d, the best result found in this case
(1)
where FFA (wt %) denotes the free fatty acids content (the weight percentage of free fatty acids in solution), V is the volume of KOH solution (mL) employed in the titration, MKOH is the molarity of the 629
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Figure 3. Results of FFA reduction in BS1 sample for the experiments listed in Table 2 using (a−d) SP112H and (e) GF101 resin individual columns in a continuous-mode packed-bed reactor.
was observed at 110 °C and within a residence time of 30−60 min, resulting in a final value of ∼0.8 wt %.
To check the saturation point of resin SP112H, two tests were performed, as shown in Figure 3d. For such tests, 630
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The authors analyzed samples that were obtained every 2 h and demonstrated that both resins were able to reduce the glycerol content and remove a cosiderable amount of soap. Product acidity, as well as the MAG, DAG, and TAG content, were also analyzed with a slight increase in acidity, from 0.18 mg KOH/g to 0.21 mg KOH/g, explained in terms of the strong acidity of the resins. They also noticed that the content of MAG, DAG, and TAG practically remained the same as that of the starting raw materials tested. Results obtained in this work show important differences between raw material and product parameters, especially if one considers the starting acidity (3.15 wt %) and starting MAG, DAG, and TAG contents (1.010, 2.330, and 0.620 wt %, respectively), reaching 0.152, 0.722, and 0.420 wt %, respectively, for SP112H, whereas MAG, DAG, and TAG contents of 0.167, 0.650, and 0.272 wt %, respectively, for GF101 resin. The next step in attempting to increase process efficiency was the use of a packed-bed column of SP112H in series with a GF101 column, with detailed operating conditions of which are shown in Table 2. Unfortunately, the product acidity behavior observed was similar to that verified using single, individual, resin columns: from an initial BS1 acidity of 3.15%, a product acidity of 0.78 wt % at 110 °C was observed, the results of which are shown in Figure 4.
residence times of 30 and 60 min were adopted and operation time was run up to progressive loss in resin performance was noted, which was verified to occur in both cases after ∼25 residence times at ∼0.8 wt % acidity. The acidity values of the outcome product when using GF101 are shown in Figure 3e, where a behavior similar to that observed for SP112H can be observed. It is important to call attention to the fact that not only is an esterification reaction between FFA and methanol happening, but it also seems that the hydrolysis of monoglycerides, diglycerides, and triglycerides is underway inside the column when using both heterogeneous catalysts SP112H and GF101. Table 3 shows the results, in terms of MAG, DAG, and TAG, for the tests depicted in Figure 3; as can be seen, starting with a Table 3. Results, in Terms of MAG, DAG, and TAG Contents, for the Experiments Listed in Table 2 and Shown in Figure 3, Using SP112H and GF101 Resins in a Continuous-Mode Laboratory-Scale Packed-Bed Reactora Acylglycerol Content (wt %) sample conditions raw material 90 °C/30 min 100 °C/30 min 110 °C/30 min 90 °C/90 min 110 °C/90 min 110 °C/120 min 110 °C/60 min 90 °C/30 min 100 °C/30 min 110 °C/30 min
MAG 1.010 SP112H Resin 0.280 0.275 0.152 0.446 0.167 0.174 0.189 GF101 Resin 0.212 0.178 0.167
DAG
TAG
2.330
0.620
2.254 2.147 0.722 1.981 0.709 0.586 0.674
0.185 0.407 0.420 0.191 0.158 0.500 0.425
1.375 0.850 0.650
1.056 0.294 0.272
a
Initial raw material acidity of BS1 sample = 3.15 wt %; 25 wt % methanol addition to sample raw material. Results shown are, in fact, the mean of injection measuments performed in triplicate.
raw material presenting values of 1.010, 2.330, and 0.620 for MAG, DAG, and TAG, respectively, an increase in temperature afforded better results for the parameters, reaching values of 0.152 for MAG, 0.722 for DAG, and 0.420 TAG at 110 °C and a residence time of 30 min. An increase in the residence time at a given operation temperature also leads to good results for these parameters and replacing SP112H by GF101, under the conditions of 110 °C and 30 min, also is shown to be the best, in terms of MAG, DAG, and TAG content (0.167, 0.650 and 0.272, respectively), which can be considered to be similar to those obtained with SP112H. After reactions, an amount of 4 bed volumes of methanol was passed through the column, collected, and rotary-evaporated, to determine if MAG, DAG, and TAG could have been trapped somehow in the column resins, instead of being hydrolyzed. However, no remaining material after evaporation was detected in all cases, even by chromatographic analysis of an entire sample, hence indicating the possible hydrolysis of the acylglycerols. This maybe one of the contributing reasons, besides reaction equilibrium, why an acidity lower than ∼0.8 wt % could not be achieved. Berrios and Skelton13 investigated the use of ion exchange resins (PD206 and BD10 Dry) in the biodiesel purification from different sources, following recipes from Rohm & Hass.
Figure 4. FFA results in BS1 with an original FFA content of 3.15 wt %, using a series of SP112H + GF101 and SP112H + SP112H resin columns at a fixed temperature (110 °C), a residence time of 30 min, and 25 wt % methanol addition.
We also tested two columns of SP112H resin in series using the BS1 sample; the results are shown in Figure 4 (see Table 2 for operating conditions). This figure shows that, as in the preceding case, similar results were found when using an individual column: the initial acidity of BS1 raw material of 3.15 wt %, and a final acidity of 1.17 wt % was reached at a temperature of 110 °C and a residence time of 30 min. 3.2. Continuous Packed-Bed Enzyme Column Tests with BS2 and Contaminated B100 Samples. The commercial enzyme Novozym 435 was employed in an attempt to reduce the acidity of two raw material charges (the B100 sample was contaminated with 3.34 wt % FFA and the BS2 sample had 2.81 wt % FFA), using two feeding flow rates at a fixed temperature (65 °C, which is the classical value for this enzyme). As can be seen from Figures 5a and 5b, differences 631
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were achieved, in terms of the MAG, DAG and TAG content, compared to the raw material and BS2 starting points, and equivalent to those found for the resins. Here, comparison with synthetic B100/FFA mixture is important, because even if a biodiesel charge was produced at longer processing times (e.g., 72 h or more) to drastically reduce the final content of MAG, DAG, and TAG using the previously mentioned soluble free enzyme, it seems that treatment with the immobilized enzyme is not capable of reducing the acidity to acceptable values, because of the natural presence of water, which is a characteristic that is intrinsic to the production method. 3.3. Magnesol R600 Packed-Bed Column Tests. Taking into account the satisfactory results obtained in the kinetic study of Magnesol R600 using the B100 sample that was contamined with different amounts of FFA (see the Supporting Information), a real biodiesel (BS1) after it had passed through a SP112H packed column was used in a Magnesol R600 column. In the first step, two biodiesel acidity materials were produced in a SP112H packed column: one with 0.8 wt % FFA (110 °C and a residence time of 30 min) and another with 1.42 wt % FFA (110 °C and a residence time of 90 min). Before passing through the Magnesol column, the charges were carefully rotary-evaporated to eliminate methanol and water almost completely (final value of 0.025 wt %). Results for the outstream of the Magnesol column are shown in Figure 6, where it can be noticed that stable values of ∼0.1 wt % were obtained.
Figure 5. Results of FFA reduction in the BS2 sample with an original acidity of 2.81 wt % FFA and B100 contaminated 3.34 wt % FFA in packed bed of Novozym 435 enzyme at a fixed temperature (65 °C) and a 1:9 molar ratio of FFA in raw material to methanol: (a) residence time = 15 min and (b) residence time = 60 min.
regarding the initial FFA content were not important for the two residence times employed and the enzyme was not effective, since the lowest acidity values were ∼1 wt %, which is far from the ANP standard (0.25 wt %). One should notice the excellent values obtained for a residence time of 60 min: 0.140, 0.184, and 0.054 wt % for MAG, DAG and TAG, respectively, all of which are within the ANP standard (0.7, 0.2, and 0.2 wt %, respectively). Considering the well-known ability of Novozym 435 enzyme to perform transesterifications reactions, BS2 and contaminated B100 samples were chosen, avoiding the high values of acylglycerols present in the BS1 sample. For a residence time of 15 min, the following values were obtained: 0.652, 0.551, and 0.049 wt % for MAG, DAG, and TAG, respectively, which shows an increase in MAG content for this residence time, compared to the original value, 0.430 wt %, at the expense of DAG and TAG, which seems to corroborate the fact that transesterifications, together with hydrolysis reactions, are occurring inside the enzymatic reactor. Overall, good results
Figure 6. Time evolution of the outcome stream in a packed bed Magnesol R600 column using the BS1 sample (initial acidity of 0.8 wt % FFA) and the BS1 sample after it has passed through the SP112H packed bed column (1.42 wt % FFA) under two experimental conditions (see the text).
Faccini et al.14 investigated the purification of biodiesel using the adsorbents Magnesol, silica, Amberlite BD10 DRY, and Purolite PD 206. These authors reported the best acidity to be 0.17 mg KOH/g, from the initial value of 0.33 mg KOH/g. Berrios and Skelton13 also tested Magnesol in batch mode at various concentrations: 0.25, 0.50, 0.75, and 1.00 wt % at 60 °C. Starting from a raw material that presented an acidity of 0.18 wt %, these authors reached a value of 0.11 mg KOH/g. Manique et al.23 employed rice husk ash as an adsorbent in the biodiesel purification produced from used cooking oil and 632
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Energy & Fuels compared that value with those obtained from using Magnesol. In terms of acidity, they started from a value of 0.33 mg KOH/ g and obtained values of 0.19 mg KOH/g using acid wash, 0.25 mg KOH/g when using 1 wt % Magnesol, 0.19 mg KOH/g and 0.13 mg KOH/g, respectively, using 1 and 4 wt % of rice husk ash; such results are similar to those found in this work. Note that the above situations are quite different from those treated in the present work, as much higher acidity are considered and that batch tests were performed, instead of continuous packed-bed column processing (results of batch tests for BS1 biodiesel using Magnesol R600 are presented in the Supporting Information of this work). In fact, operation in a continuous-mode packed-bed column with Magnesol R600 for feeding charges with an acidity of >0.5 wt % is not a simple task, since column plugging, possibly because of soap formation, becomes frequent, making it necessary to tune the incoming flow rate. Despite the good results provided by the use of Magnesol, information on its use in large-scale, industrial processing is rarely found in the open literature. Therefore, it would be quite useful if a comparative economic analysis and disposal of charged adsorbent were available for researchers. 3.4. Pilot-Plant Preliminary Tests. Results from processing the BS1 sample (3.15 wt % original acidity) in continuousmode operation with a GF101 packed bed column are presented in Figures 7a and 7b. In the first case, two residence times were tested in the same experiment30 min and 45 minjust changing the incoming flow rate, at a fixed temperature of 110 °C and 25 wt % methanol addition. A fast decrease in FFA content is observed in the beginning, followed by a slight progressive decrease at ∼1 wt % acidity. Nevertheless, when the flow rate is reduced, i.e., the residence is increased to 45 min, a small but perceptible FFA content increase is noted. This behavior is consistent with the aforementioned fact that some amount of water formed via the esterefication reaction between FFA and methanol is not being extracted from the reactor, because of the low superficial velocity of the incoming flow. In fact, Figure 7b shows that, when the residence time is changed from 30 min to 20 min, the FFA content in the outcome stream is reduced, reaching an acidity of 0.91 wt %. Some other experimental tests are underway in the pilot plant that are directed toward reducing the FFA content in the final product as close as possible to the level required in the regulations. However, it is clear that the starting water content in the raw material may be
