**5. Simulation**

338 Recent Trends for Enhancing the Diversity and Quality of Soybean Products

To improve the stability of the methanol/oil emulsion, the authors substituted the classical mixing chamber with an emulsificator based on five co-axial rotating ring gears which are able to break the biphasic mixture into very tiny drops. Using this device and starting from two entirely separated liquid phases (oil and methanol) it was possible to obtain a much more stable emulsion. In Fig. 12 a comparison between FFA conversion obtained with the classical mixing chamber and the emulsificator is reported: the better results reached using

Fig. 12. Semi-continuous experiment: FFA conversion % vs. time using crude palm oil as feedstock and having before the catalytic bed: classical mechanic stirring reactor () or emulsificator at 500 rpm (▲). Pump flow = 10 mL min-1; T = 338 K, molar ratio FFA: alcohol

The methanol/oil emulsion lifetime without the emulsificator device, measured in the same way at the exit of the mixing chamber, is of about 15 seconds. Referring to our system, the contact time between the catalyst and the reactant is 1 min for 7 g of catalyst (pump flow = 10 mL min-1): for this reason without the emulsificator device a significant part of the

**4.3 Oil transformation: the transesterification reaction and biodiesel characterization**  The transesterification reaction has been performed by the authors on the raw materials de-

Sodium methoxide (MeONa) was employed as catalyst. MeONa is known to be the most active catalyst for triglycerides transesterification reaction, but it requires the total absence of water (Schuchardt, 1996). For this reason, the unreacted methanol and the reaction water were evaporated from the de-acidified oils before processing them with the

acidified with the esterification process described in the previous paragraph.

The employed experimental setup was the same as displayed in Fig. 2.

the emulsificator are evident.

= 1: 6, weight ratio catalyst: oil = 1: 10.

catalytic bed (about 75%) does not work.

transesterification reaction.

In order to develop a process simulation of the FFA esterification, able to predict the reaction progress, a thermodynamic and kinetic analysis was performed.

### **5.1 Thermodynamic aspects**

The considered reaction system turns out to be a highly non-ideal system, being formed by a mixture of oil, methylester, methanol, FFA and water. The interactions among these molecules are absolutely not ideal, in fact they are only partially soluble and a two phase system is formed if the quantity of methanol is greater than 6-8 wt%.

Indeed, the activity coefficients are used not only for the phase and chemical equilibria calculations, but also for the kinetic expressions. Modified UNIFAC model was used adopting the parameters available in literature and published by Gmehling et al. (2002).

#### **5.2 Reaction kinetics**

The considered reaction is the following (Fig.13).

FFA + methanol methylester + water

Fig. 13. FFA esterification reaction and hydrolysis (indirect reaction) considered in the process simulation.

The oil is considered as non-reacting solvent, being present in large quantity. A pseudohomogeneous model was used for describing the kinetic behaviour of the reaction (Pöpken et al., 2000). The adopted model is displayed in the following equation:

Soybean Oil De-Acidification as a First Step Towards Biodiesel Production 341

Fig. 14. Experimental and calculated (pseudohomogeneous model) FFA conversion % vs. reaction time, rapeseed oil, initial acidity= 3.47% (oleic acid), slurry reactor, T= 338 K,

The use of not refined or waste oils as a feedstock represents a very convenient way in order to lower biodiesel production costs. The main problem associated with the use of this type of low-cost feedstock lies in its high content of FFA, leading to the formation of soaps during the final transesterification step. These materials require therefore to be standardized by the reduction of their acidity and different de-acidification methods have been described in this chapter. Among them, a new technology based on an esterification reaction heterogeneously catalyzed and performed at mild operative conditions, i.e. low temperature (between 303 and 338 K) and atmospheric pressure has been proposed and described. Several kinds of ion-exchange resins, commercially available, have been used as heterogeneous catalysts, different one form the other for what concerns acidity strength, surface area, porosity,

The experimental tests were performed using different reactors (CSTR or PFR), starting oils (in comparison with the results obtained for soybean oil), catalyst/oil ratio and working temperature. All these experimental parameters have been optimized in order to obtain, at the end of the reaction, a concentration of FFA suitable for the transesterification reaction for the biodiesel production (FFA < 0.5 wt%). A crucial parameter for the industrial application is the catalyst lifetime and this parameter has been evaluated by performing ninety consecutive batch de-acidification runs, each lasting 6 hours, with the same catalyst sample (Amberlyst 46®) in a slurry reactor. At the end of the recycles, a decrease of activity of about 25% was observed, to be ascribed to some fragmentation of catalyst's particles, that collide against one another and against the reactor walls. To overcome this problem a packed bed configuration have been adopted and optimized. At last, a process simulation of the FFA esterification, able to predict the reaction progress, through a thermodynamic and kinetic analysis, was successfully performed. A pseudohomogeneous model was used for describing the kinetic behaviour of the reaction, using a modified UNIFAC model for the calculation of the activity coefficients (used not only for the phase and chemical equilibria

weight ratio alcohol/oil=16:100, weight ratio catalyst/oil=1:10.

swelling, characteristics and disposal of acid groups.

calculations, but also for the kinetic expressions).

**6. Conclusions** 

$$\mathbf{r} = \frac{1}{\mathbf{m}\_{\rm cat}} \frac{1}{\upsilon\_{\rm i}} \frac{\mathbf{dn}\_{\rm i}}{\mathbf{dt}} = \mathbf{k}\_1 \mathbf{a}\_{\rm FFA} \mathbf{a}\_{\rm methardorol} - \mathbf{k}\_{-1} \mathbf{a}\_{\rm methyless} \mathbf{a}\_{\rm water}$$

where:

r= reaction rate

mcat= dry mass of catalyst, gr

υi= stoichiometric coefficients of component i

n1= moles of component i

t = reaction time

k1= kinetic constant of direct reaction

k-1= kinetic constant of indirect reaction

ai= activity of component i

The temperature dependence of the rate constant is expressed by the Arrhenius law:

$$k\_i = k\_i^{\,0} \exp\left(\frac{-E\_{a,i}}{RT}\right).$$

where ki 0 and Ea,i are the pre-exponential factor and the activation energy of the reaction i, respectively (i=1 for the direct reaction, i=-1 for the indirect reaction), T is the absolute temperature and R the Universal Gas Constant.

The adopted parameters set is the same reported by Steinigeweg (Steinigeweg & Gmehling, 2003). The absolute values of pre-exponential factors were corrected as reported in Table 5, so to take into account the presence of both a second liquid phase and a different type of catalyst.


Table 5. Kinetic Parameters for the adopted pseudohomogeneous Kinetic Model.

The ratio of pre-exponential factors is: K eq T = k1 0/k-10 = 60.7841.

All the simulations were carried using Batch Reactor of PRO II by Simsci – Esscor.

In the next picture (Fig. 14) the comparison between the experimental and calculated FFA conversion is shown for the esterification reaction of an acid rapeseed oil.

The initial composition (% by weight) entered in the simulation program is: oil (83.2%), FFA (3.47 %), methanol (13.3%).

The model turned out to be able to reproduce qualitatively the behaviour of different systems, characterized by different starting acidities values, at different temperatures and with an high impurities content.

Fig. 14. Experimental and calculated (pseudohomogeneous model) FFA conversion % vs. reaction time, rapeseed oil, initial acidity= 3.47% (oleic acid), slurry reactor, T= 338 K, weight ratio alcohol/oil=16:100, weight ratio catalyst/oil=1:10.

#### **6. Conclusions**

340 Recent Trends for Enhancing the Diversity and Quality of Soybean Products

r ka a k a a

1 FFA methanol 1 methylester water

i

m dt

The temperature dependence of the rate constant is expressed by the Arrhenius law:

*i i*

*k k*

<sup>0</sup> , exp *a i*

respectively (i=1 for the direct reaction, i=-1 for the indirect reaction), T is the absolute

The adopted parameters set is the same reported by Steinigeweg (Steinigeweg & Gmehling, 2003). The absolute values of pre-exponential factors were corrected as reported in Table 5, so to take into account the presence of both a second liquid phase and a different type of

Reaction (i) EA (kJ mol-1)

Esterification (1) 68.71

Hydrolisis (-1) 64.66

In the next picture (Fig. 14) the comparison between the experimental and calculated FFA

The initial composition (% by weight) entered in the simulation program is: oil (83.2%), FFA

The model turned out to be able to reproduce qualitatively the behaviour of different systems, characterized by different starting acidities values, at different temperatures and

T = k10/k-10 = 60.7841.

Table 5. Kinetic Parameters for the adopted pseudohomogeneous Kinetic Model.

All the simulations were carried using Batch Reactor of PRO II by Simsci – Esscor.

conversion is shown for the esterification reaction of an acid rapeseed oil.

*E*

*RT*

0 and Ea,i are the pre-exponential factor and the activation energy of the reaction i,

cat i 1 1 dn

υi= stoichiometric coefficients of component i

temperature and R the Universal Gas Constant.

The ratio of pre-exponential factors is: K eq

(3.47 %), methanol (13.3%).

with an high impurities content.

k1= kinetic constant of direct reaction k-1= kinetic constant of indirect reaction

where:

where ki

catalyst.

r= reaction rate

t = reaction time

mcat= dry mass of catalyst, gr

n1= moles of component i

ai= activity of component i

The use of not refined or waste oils as a feedstock represents a very convenient way in order to lower biodiesel production costs. The main problem associated with the use of this type of low-cost feedstock lies in its high content of FFA, leading to the formation of soaps during the final transesterification step. These materials require therefore to be standardized by the reduction of their acidity and different de-acidification methods have been described in this chapter. Among them, a new technology based on an esterification reaction heterogeneously catalyzed and performed at mild operative conditions, i.e. low temperature (between 303 and 338 K) and atmospheric pressure has been proposed and described. Several kinds of ion-exchange resins, commercially available, have been used as heterogeneous catalysts, different one form the other for what concerns acidity strength, surface area, porosity, swelling, characteristics and disposal of acid groups.

The experimental tests were performed using different reactors (CSTR or PFR), starting oils (in comparison with the results obtained for soybean oil), catalyst/oil ratio and working temperature. All these experimental parameters have been optimized in order to obtain, at the end of the reaction, a concentration of FFA suitable for the transesterification reaction for the biodiesel production (FFA < 0.5 wt%). A crucial parameter for the industrial application is the catalyst lifetime and this parameter has been evaluated by performing ninety consecutive batch de-acidification runs, each lasting 6 hours, with the same catalyst sample (Amberlyst 46®) in a slurry reactor. At the end of the recycles, a decrease of activity of about 25% was observed, to be ascribed to some fragmentation of catalyst's particles, that collide against one another and against the reactor walls. To overcome this problem a packed bed configuration have been adopted and optimized. At last, a process simulation of the FFA esterification, able to predict the reaction progress, through a thermodynamic and kinetic analysis, was successfully performed. A pseudohomogeneous model was used for describing the kinetic behaviour of the reaction, using a modified UNIFAC model for the calculation of the activity coefficients (used not only for the phase and chemical equilibria calculations, but also for the kinetic expressions).

Soybean Oil De-Acidification as a First Step Towards Biodiesel Production 343

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