**12. Adsorbers operation**

#### **12.1 Bleaching tanks**

Manuale et al. (2011) used bleaching silicas for the removal of FFA in biodiesel in a series of tests in a stirred tank reactor under varying temperature and pressure conditions (70 and 110 °C, 760 and 160 mmHg). Their results confirm the pattern already seen in the case of the silica refining of edible oils. For the same adsorbent and in the presence of vacuum the influence of temperature is low. For example for TriSyl 3000 in vacuo, after 90 min, and from a similar initial acidity level (1.5%), the adsorbate load at two different temperatures is: *q70 °C*=99.3%, *q110 °C*=75.0%. Similarly, for TriSyl 300B, 90 min bleaching time, 1.7-1.9% initial acidity: *q70 °C*=82.0, *q110 °C*= 69.0%. The trend is clear. Higher temperatures lead to lower

Adsorption in Biodiesel Refining - A Review 451

"total" bleaching time. No benefits can then be got from the multi-tank countercurrent bleaching operation. The only possibility of multiple units is that of parallel bleaching tanks

Fig. 10. Adsorbent load as a function of time and the number of countercurrent bleaching

*Lead-lag operation.* Most liquid phase packed beds are operated in series. This means passing all of the flow through one column bed, a lead column, and then passing flow through another similar sized column bed, the lag vessel. This method offers several advantages over a single column. The series configuration allows the maximum use of the adsorbent throughout the entire bed. This assumes that the MTZ is contained within a single properly sized packed bed. By placing two or more columns in series, the MTZ is allowed to pass completely through the first (lead) bed as the leading edge of the MTZ migrates into the second (lag) bed. By allowing this to happen, the maximum contaminant concentration is allowed to come into contact with adsorption sites in the lead vessel that require a greater concentration gradient to hold additional contamination. When the MTZ exits the lead vessel, that vessel is then exhausted, and requires change out with virgin or regenerated adsorbent. Even though the adsorption capacity of the lead vessel is exhausted, treatment continues in the lag vessel. Then, during change out, the lead vessel is taken off-line and the lag vessel is placed in the lead position. The former lead vessel is then replenished with adsorbent and then becomes the lag vessel and brought on-line. Further insights on the operation of serial and parallel adsorbers can be found elsewhere (Sigrist et al., 2011). *Regeneration.* For the removal of glycerol and to a lower extent of MGs and DGs, the methanol concentration in the fluid is important. Methanol adversely affects the adsorption capacity because it increases the activity of glycerol and glycerides in the liquid phase. This was studied by Yori et al. (2007) with the method disclosed by Condoret (1997) and Bellot (2001). The method is based on the knowledge of the curves describing the variation in the glycerol activity with respect to its concentration, established separately for each phase (solid and liquid). Henry's constants were obtained from the slope of the isotherms in the diluted range using the UNIFAC method for calculating the liquid phase activity coefficients. The results are shown in Fig. 11 and indicate that for all practical purposes the

working long times (e.g. 2 h) in order to increase the adsorbent usage.

steps (1, 2 and 3) (Manuale, 2011). Process conditions as in Figure 9.

adsorption of glycerol over silica is null at high methanol concentrations.

**12.2 Packed beds** 

adsorption capacities. This is related to the fact that adsorption is exothermal and thus adsorption equilibrium is favored at low temperatures. In the absence of vacuum, adsorption is very low, one order of magnitude the value at 160 mmHg. Water adsorption reportedly inhibits the diffusion and adsorption inside the pore network of the silicas. At 90 °C or higher temperatures water desorption from an adsorbent dipped in oil can only proceed to a non-negligible extent in the presence of vacuum. Therefore if the adsorbent is not previously dehydrated, dehydration occurs simultaneously with adsorption during the bleaching experiment. In some cases the release of water from the silica goes directly into the biodiesel phase and the water content of the oil phase is increased.

These results indicate that surface diffusion of FFA over several adsorbents is very slow and the limiting step of the whole adsorption process. This leads to two negative consequences: (i) if a high level of FFA removal and a short bleaching time is required then big amounts of adsorbent must be used and these adsorbents are only partially used; (ii) if a total utilization of the adsorbent is desired, unconveniently high bleaching times must be used.

Fig. 9. Biodiesel acidity as a function of time and the number of countercurrent tank bleaching steps (1, 2 and 3) (Manuale, 2011). Adsorbent load=2%, initial biodiesel acidity= 2%, *KLDF*= 0.0188 min-1. Left: Linear adsorption (Henry's law, *H*=37.6). Right: Irreversible adsorption (square isotherm). Dotted line: FFA European standard EN 14214, FFA limit.

Manuale et al. (2011) discussed the conditions for total total adsorbent utilization and for quasi complete FFA removal. They used the LDF model with both linear and square isotherms. They tested by simulation the use of serial cocurrent and countercurrent bleachers in order to assess their bleaching performance. The results are presented in Figure 9. In the case of the linear adsorption isotherm the use of countercurrent bleachers does not lead to a reduction of the adsorbent consumption. An effective reduction only occurs when the isotherm is square. These conclusions hold independently of the number of serial bleachers. Therefore when adsorption is strong and irreversible, spent adsorbents can be used advantageously to bleach streams highly contaminated while fresh adsorbents can be used to polish bleach the most lean streams. In the case of the linear isotherm the modulation of the adsorption capacity results in an operation that depends only on the bleaching time (all traces in Figure 9-left coincide at the end of the bleaching cycle).

Figure 10 is a plot of *q(t)* for a train of countercurrent beds packed with adsorbents having a linear isotherm. The results show that all traces for the multistep operation are practically parallel to the 1-bleacher trace. Hence the adsorption capacity *q* is only a function of the

adsorption capacities. This is related to the fact that adsorption is exothermal and thus adsorption equilibrium is favored at low temperatures. In the absence of vacuum, adsorption is very low, one order of magnitude the value at 160 mmHg. Water adsorption reportedly inhibits the diffusion and adsorption inside the pore network of the silicas. At 90 °C or higher temperatures water desorption from an adsorbent dipped in oil can only proceed to a non-negligible extent in the presence of vacuum. Therefore if the adsorbent is not previously dehydrated, dehydration occurs simultaneously with adsorption during the bleaching experiment. In some cases the release of water from the silica goes directly into

These results indicate that surface diffusion of FFA over several adsorbents is very slow and the limiting step of the whole adsorption process. This leads to two negative consequences: (i) if a high level of FFA removal and a short bleaching time is required then big amounts of adsorbent must be used and these adsorbents are only partially used; (ii) if a total utilization

the biodiesel phase and the water content of the oil phase is increased.

of the adsorbent is desired, unconveniently high bleaching times must be used.

Fig. 9. Biodiesel acidity as a function of time and the number of countercurrent tank bleaching steps (1, 2 and 3) (Manuale, 2011). Adsorbent load=2%, initial biodiesel acidity= 2%, *KLDF*= 0.0188 min-1. Left: Linear adsorption (Henry's law, *H*=37.6). Right: Irreversible adsorption (square isotherm). Dotted line: FFA European standard EN 14214, FFA limit.

bleaching time (all traces in Figure 9-left coincide at the end of the bleaching cycle).

Figure 10 is a plot of *q(t)* for a train of countercurrent beds packed with adsorbents having a linear isotherm. The results show that all traces for the multistep operation are practically parallel to the 1-bleacher trace. Hence the adsorption capacity *q* is only a function of the

Manuale et al. (2011) discussed the conditions for total total adsorbent utilization and for quasi complete FFA removal. They used the LDF model with both linear and square isotherms. They tested by simulation the use of serial cocurrent and countercurrent bleachers in order to assess their bleaching performance. The results are presented in Figure 9. In the case of the linear adsorption isotherm the use of countercurrent bleachers does not lead to a reduction of the adsorbent consumption. An effective reduction only occurs when the isotherm is square. These conclusions hold independently of the number of serial bleachers. Therefore when adsorption is strong and irreversible, spent adsorbents can be used advantageously to bleach streams highly contaminated while fresh adsorbents can be used to polish bleach the most lean streams. In the case of the linear isotherm the modulation of the adsorption capacity results in an operation that depends only on the "total" bleaching time. No benefits can then be got from the multi-tank countercurrent bleaching operation. The only possibility of multiple units is that of parallel bleaching tanks working long times (e.g. 2 h) in order to increase the adsorbent usage.

Fig. 10. Adsorbent load as a function of time and the number of countercurrent bleaching steps (1, 2 and 3) (Manuale, 2011). Process conditions as in Figure 9.

#### **12.2 Packed beds**

*Lead-lag operation.* Most liquid phase packed beds are operated in series. This means passing all of the flow through one column bed, a lead column, and then passing flow through another similar sized column bed, the lag vessel. This method offers several advantages over a single column. The series configuration allows the maximum use of the adsorbent throughout the entire bed. This assumes that the MTZ is contained within a single properly sized packed bed. By placing two or more columns in series, the MTZ is allowed to pass completely through the first (lead) bed as the leading edge of the MTZ migrates into the second (lag) bed. By allowing this to happen, the maximum contaminant concentration is allowed to come into contact with adsorption sites in the lead vessel that require a greater concentration gradient to hold additional contamination. When the MTZ exits the lead vessel, that vessel is then exhausted, and requires change out with virgin or regenerated adsorbent. Even though the adsorption capacity of the lead vessel is exhausted, treatment continues in the lag vessel. Then, during change out, the lead vessel is taken off-line and the lag vessel is placed in the lead position. The former lead vessel is then replenished with adsorbent and then becomes the lag vessel and brought on-line. Further insights on the operation of serial and parallel adsorbers can be found elsewhere (Sigrist et al., 2011).

*Regeneration.* For the removal of glycerol and to a lower extent of MGs and DGs, the methanol concentration in the fluid is important. Methanol adversely affects the adsorption capacity because it increases the activity of glycerol and glycerides in the liquid phase. This was studied by Yori et al. (2007) with the method disclosed by Condoret (1997) and Bellot (2001). The method is based on the knowledge of the curves describing the variation in the glycerol activity with respect to its concentration, established separately for each phase (solid and liquid). Henry's constants were obtained from the slope of the isotherms in the diluted range using the UNIFAC method for calculating the liquid phase activity coefficients. The results are shown in Fig. 11 and indicate that for all practical purposes the adsorption of glycerol over silica is null at high methanol concentrations.

Adsorption in Biodiesel Refining - A Review 453

effluents and the sparing of washing, oil-water separation and wastewater treatment units. Other advantages are small capital expenditure, robustness and easiness of operation. Cost-effective means for the scale-up of packed bed adsorbers for biodiesel refining seem to be accurate models for flow and adsorption and scaled-down RSCCTs. Accurate models for flow and adsorption can be solved in their full complexity only with the aid of numerical calculations but analytical solutions for rapid design and sensitivity analysis can be got using approximations, such as the use of square and linear isotherms and LDF models.

The operation of adsorbers should minimize the consumption of adsorbent. From this point of view countercurrent bleaching tank arrays should be used but this mode of operation cannot be exploited in the case of adsorbents with linear isotherms. In the case of packed bed adsorbers common lead-lag setups of 2 or more serial columns are recommended.

This work was financed with the support of Universidad Nacional del Litoral (Grant PI-60-

Allara, D.L. & Nuzzo, R.G. (1985) Spontaneously organized molecular assemblies. 1.

Anderson, D., Masterson, D., McDonald, B. & Sullivan, L. (2003). Industrial Biodiesel Plant

*(PIPOC)*, 24-28 August 2003, Putrajaya Marriot Hotel, Putrajaya, Malaysia. Bellot, J.C., Choisnard, L., Castillo, E. & Marty, A. (2001). Combining solvent engineering

Bondioli, P., Gasparoli, A., Bella, L. D. & Tagliabue, S. (2002). Evaluation of biodiesel storage

Bournay, L., Casanave, D., Delfort, B., Hillion, G. & Chodorge, J. (2005). New heterogeneous

Busto, M., D'Ippolito, S.A., Yori, J.C., Iturria, M.E., Pieck, C.L., Grau, J.M. & Vera, C.R.

*Fuels*, Vol 20, No 6, pp. 2642-2647, ISSN 0887-0624.

Formation, dynamics, and physical properties of n-alkanoic acids adsorbed from solution on an oxidized aluminum surface. *Langmuir*, Vol 45, No 1, pp. 45-52. Álvarez-Ramírez, J., Fernández-Anaya, G., Valdés-Parada, F.J. & Ochoa-Tapia, J.A. (2005).

Physical Consistency of Generalized Linear Driving Force Models for Adsorption in a Particle. *Industrial & Engineering Chemistry Research*, Vol. 44, No 17, pp. 6776-

Design and Engineering: Practical Experience. *Chemistry and Technology Conference, Session Seven: Renewable Energy Management, International Palm Oil Conference* 

and thermodynamic modeling to enhance selectivity during monoglyceride synthesis by lipase-catalyzed esterification. *Enz. Microb. Technol.*, Vol. 28, No 4-5,

stability using reference methods. *Eur. J. Lipid Sci. Tech.*, Vol. 104, No 12, pp. 777-

process for biodiesel production: A way to improve the quality and the value of the crude glycerin produced by biodiesel plants. *Catalysis Today*, Vol. 106, No. 1-4, pp.

(2006). Influence of the axial dispersion on the performance of tubular reactors during the non-catalytic supercriticaltransesterification of triglycerides. *Energy &* 

298, CAI+D 2009) and CONICET (National Research Council of Argentina).

Further approximations can be obtained for low Biot and high axial Péclet numbers.

**14. Acknowledgements** 

6783, ISSN 08885885.

pp. 362-369, ISSN 0141-0229.

784, ISSN 1438-9312.

190-192, ISSN 0920-5861.

**15. References** 

Fig. 11. Silica adsorption isotherms for the Gly-FAME (squares) and Gly-MeOH (triangles) systems. *H* values calculated from the slope of the traces. Yori et al. (2007).

The elution of 4 bed volumes of methanol through the exhausted packed bed is reported to restore the adsorbent capacity. Elimination of the adsorbed methanol from the silica was done by blowing nitrogen through the bed but could be performed using any other gas. Elimination of the solvent produced a decrease of the bed temperature because methanol evaporation needs 1104 J g-1. This translates to 200 kJ kgsilica-1 for the fully saturated silica and hence provisions should be made in order to maintain the bed temperature and prevent biodiesel flow problems at unconvenient low temperatures. In this sense flushing the bed with a hot gas seems the most suitable means for desorbing methanol.

$$\left(H \, \slash\, H^{0}\right) = e^{-\frac{\Delta H}{R} \left[\frac{1}{T\_1} - \frac{1}{T\_2}\right]}\tag{32}$$

The thus recommended way of regenerating the silica bed seems superior to other means used for regeneration of adsorbent packed beds, notably the thermal swing. A thermal swing with purified hot biodiesel could be used to regenerate the bed. Manuale et al. (2011) found that the silica adsorption of oleic acid from biodiesel has a heat of adsorption of -5.7 KCal mol-1. This is similar to reported values for similar systems (Sari & Iþýldak, 2006). In order to decrease the adsorption capacity 100 times (*H/H°*=0.01, Eq. 32) the thermal swing should be *T*=480 °C. For mild regenerations with *H/H°*=0.1 and *H/H°*=0.25, the required thermal swings are still high, *T*=200 °C and *T*=140 °C. The results indicate that though for the silica-FFA system adsorption is weak enough to yield a linear isotherm, the heat of adsorption is too high and discourages the use of a thermal swing for regeneration.

#### **13. Conclusions**

Adsorption is a robust and reliable operation for the refining of biodiesel and its feedstocks. Hydrophillic adsorbents seem the best choice, because most of the undesired impurities are polar. In this sense silicas offer a high saturation capacity (10-15%) for glycerol and glycerides, and enough affinity for soaps, FFA, metals and salts.

One advantage of adsorption units for the removal of glycerol, glycerides, soaps, phosphatides and metals from biodiesel and its feedstocks, is the reduction in wastewater

Fig. 11. Silica adsorption isotherms for the Gly-FAME (squares) and Gly-MeOH (triangles)

The elution of 4 bed volumes of methanol through the exhausted packed bed is reported to restore the adsorbent capacity. Elimination of the adsorbed methanol from the silica was done by blowing nitrogen through the bed but could be performed using any other gas. Elimination of the solvent produced a decrease of the bed temperature because methanol evaporation needs 1104 J g-1. This translates to 200 kJ kgsilica-1 for the fully saturated silica and hence provisions should be made in order to maintain the bed temperature and prevent biodiesel flow problems at unconvenient low temperatures. In this sense flushing the bed

*T*=480 °C. For mild regenerations with *H/H°*=0.1 and *H/H°*=0.25, the required

(32)

*T*=140 °C. The results indicate that though for

*H*

*RT T HH e*

The thus recommended way of regenerating the silica bed seems superior to other means used for regeneration of adsorbent packed beds, notably the thermal swing. A thermal swing with purified hot biodiesel could be used to regenerate the bed. Manuale et al. (2011) found that the silica adsorption of oleic acid from biodiesel has a heat of adsorption of -5.7 KCal mol-1. This is similar to reported values for similar systems (Sari & Iþýldak, 2006). In order to decrease the adsorption capacity 100 times (*H/H°*=0.01, Eq. 32) the thermal swing

the silica-FFA system adsorption is weak enough to yield a linear isotherm, the heat of

Adsorption is a robust and reliable operation for the refining of biodiesel and its feedstocks. Hydrophillic adsorbents seem the best choice, because most of the undesired impurities are polar. In this sense silicas offer a high saturation capacity (10-15%) for glycerol and

One advantage of adsorption units for the removal of glycerol, glycerides, soaps, phosphatides and metals from biodiesel and its feedstocks, is the reduction in wastewater

systems. *H* values calculated from the slope of the traces. Yori et al. (2007).

with a hot gas seems the most suitable means for desorbing methanol.

glycerides, and enough affinity for soaps, FFA, metals and salts.

should be

**13. Conclusions** 

thermal swings are still high,

<sup>0</sup> (/ )

*T*=200 °C and

adsorption is too high and discourages the use of a thermal swing for regeneration.

effluents and the sparing of washing, oil-water separation and wastewater treatment units. Other advantages are small capital expenditure, robustness and easiness of operation.

Cost-effective means for the scale-up of packed bed adsorbers for biodiesel refining seem to be accurate models for flow and adsorption and scaled-down RSCCTs. Accurate models for flow and adsorption can be solved in their full complexity only with the aid of numerical calculations but analytical solutions for rapid design and sensitivity analysis can be got using approximations, such as the use of square and linear isotherms and LDF models. Further approximations can be obtained for low Biot and high axial Péclet numbers.

The operation of adsorbers should minimize the consumption of adsorbent. From this point of view countercurrent bleaching tank arrays should be used but this mode of operation cannot be exploited in the case of adsorbents with linear isotherms. In the case of packed bed adsorbers common lead-lag setups of 2 or more serial columns are recommended.

#### **14. Acknowledgements**

This work was financed with the support of Universidad Nacional del Litoral (Grant PI-60- 298, CAI+D 2009) and CONICET (National Research Council of Argentina).

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