Process intensification for the production of the ethyl esters of volatile fatty acids using aluminium chloride hexahydrate as a catalyst

A new process for obtaining the ethyl esters of volatile fatty acids with ethanol by using aluminium chloride hexahydrate as a catalyst is proposed. Aluminium chloride not only exhibits good activity, composition equilibrium is achieved within 4 hours at 343 K, but also induces a phase separation with a convenient distribution of the components. In fact, more than 99 %wt of the ethyl esters, together with most of the unreacted acid and ethanol, were found in the upper layer, which was well separated from the bottom phase, which contained the coformed water and over 97.8 %wt of the catalyst. The intensification of this reaction and separation was thoroughly investigated and the operational conditions optimised. The effects of this separation on the purification of the final ethyl esters is fully investigated. A new configuration of unit operations is designed for the specific production of ethyl acetate, simulated through Aspen Plus V9 and compared with the current industrial process based on sulfuric acid catalysis. The overall production and purification of ethyl acetate is economically competitive, reduces the energy requirements by more than 50 %, and is potentially a zero waste process, resulting in cleaner production.


Introduction
Ethyl esters are non-hazardous organic compounds that have industrial applications as solvents (Hu et al., 2017), fragrances (Saerens et al., 2008), cosmetic products, (Lee et al., 2014) and biofuels (Koutinas et al., 2016). These naturally occurring compounds (fruit flavours) have low toxicity and very limited impact on the environment; they can be easily hydrolysed to ethanol and native acids, which are biodegradable either aerobically (Bernat et al., 2017) or anaerobically (Pagliano et al., 2017). In addition, ethyl esters are bio-derived solvents because they can be produced through the direct esterification of volatile fatty acids (VFAs) and ethanol, both of which can be obtained via the fermentation of renewable biomasses. The production of ethanol through fermentation is a mature technology (Sebayang et al., 2017) and has been optimised for several residual biomasses (Sebayang et al., 2016). Additionally, the production of VFAs is a highly flexible process, in which the desired profile can be achieved by selecting the appropriate operating conditions, such as the type of inoculum and the pH (Wang et al., 2014) or the total solid content (Forster-Carneiro et al., 2008). VFAs or ethanol may be produced from the same fermenter by simply adopting specific operating conditions (Syngiridis et al., 2014). The efficiency and viability of the recovery of VFAs from broad fermentation have been increasingly improved (Singhania et al., 2013). The use of bio-derived VFAs and ethanol in place of fossil sources could contribute to a slowdown of the net increase in greenhouse gases emissions due to the 'short-cycle carbon system' (Kajaste, 2014).
Once isolated, they can react through direct esterification to produce ethyl esters. This process, known as the Fischer reaction (Eq. 1), has been widely studied by academics and industry and is subject to severe kinetic and thermodynamic constraints.
There is only a partial conversion of acids to the relevant esters, and the recovery of pure products in an industrial context is complicated by the coexistence of unreacted acids, ethyl esters, water and ethanol in the crude homogeneous reaction mixture, which requires several further expensive unit operations for purification (Aslam et al., 2010). To simplify the recoverability of the products and to promote equilibrium versus higher conversion, the typically adopted approach consists of removing water from the reactive environment in agreement with the principles of process intensification (Stankiewicz and Moulijn, 2000). Each process that includes the integration of a reaction and a separation represents a typical case of a process-intensifying method. Reactive distillation (using self-crosslinking Nafion-SiO 2 (Deng et al., 2016), or acid ion-exchange resins (Smejkal et al., 2009)) and pervaporation (using a mordenite membrane (Zhu et al., 2016) or zeolites (Tanaka et al., 2001)) completely convert the starting acid to the corresponding ethyl ester in a relatively short time (4-10 h). In addition, microwave-assisted reactive distillation (Ding et al., 2016) and reactive distillation coupled with membrane pervaporation (Lv et al., 2012) also represent good alternatives with improved performance. The chemical sequestration of water, for example through dicyclohexylcarbodiimide (Sano et al., 2011), is also a valid alternative.
Most of these alternatives cannot compete with the present industrial process,

Materials and Methods
All chemical reagents used in this work were of analytical reagent grade and were used directly without further purification or treatment. were performed in triplicate for exhaustive treatment of the data (evaluation of the mean value and the respective error, which always resulted to be within 5 %).

Phase repartition in the esterification of AA with ethanol
The effect of the amount of catalyst on the phase repartition was evaluated on a synthetic mixture with a known thermodynamic composition obtained by reacting

Purification of EA: Process modelling and the optimisation method
Industrial production of EA is nowadays conducted in large plants that have a capacity for manufacturing around 100 000 t of products per year using H 2 SO 4 as a catalyst. The conventional scheme of production of EA reported by Santaella et al. (2015) was considered as the reference case in this study. In order to directly compare this conventional production with the process based on the use of  Table 1.

Table 1
The thermodynamic non-random two-liquid equation The E factor is an immediate measure of the amount of waste generated per kg of product, while E w does not include the water in the waste evaluation.

MI =
Total mass fed as pure reactants kg Mass of product kg 8 The MI factor represents the amount of reagent required to synthesise one kg of the desired product (taking into account the eventual presence of water and excluding it from the computation). This factor is equal to 1 in the cleanest processes, in which the reagents are completely converted to useful products. The greater the MI factor is, the greater the amount of waste produced.
Finally, the MP factor is the inverse of the MI, and represents the mass of the reagent (percentage) converted to products.
These indicators provide an immediate measure of the cleanness of a process in accordance with the principles of green chemistry in terms of waste generated and energy efficiency (Anastas and Eghbali, 2010).
After the simulations met the design criteria, the total annual costs (TAC) were computed considering a 3-year period for return on the investment. Fixed costs were calculated using the method proposed by Douglas (1988) (Fig. 1).

Fig. 1
The direct esterification was monitored in time ( The reactive trends are reported in Fig. 2.

Fig. 2
The experiments were repeated three times, and the respective error bars for each set of data were calculated and represented. The variability of the experimental data was very small (less than 5 %).
The kinetic profiles in  where v is the reaction rate, and k 1 and K eq are the kinetic constants for the forward reaction and the equilibrium constant respectively, and the molar  Fig. 3).

Fig. 3
The values of k 1 are listed in Table 2 together with the K eq calculated using Eq. 16.
K eq = X eq 2 1-X eq 2 16 Table 2 The data in Table 2  where T is the absolute temperature, A is the pre-exponential factor, E a is the activation energy of the reaction, R is the universal gas constant, ΔH° is the reaction enthalpy, or heat of the reaction, and ΔS° is the reaction entropy (Fig. 4).

Fig. 4
The results were collected and are listed in Table 3.

Table 3
The values of E a increased following the order AA > PA > BA (Table 3)   To improve the conversion of the acids, the effect of r was also investigated. In addition to the previously described studies in which r was fixed at 1, the reactions in which r was fixed at 2 and 3 were studied for AA, PA and BA under AlCl 3 •6H 2 O catalysis (Fig. 6).

Fig. 6
Although the final conversion of the initial acid increased with higher yields of the corresponding ethyl ester (conversions increased from 55%-66% for r = 1, to 80%-82% for r = 2, to 82%-85% for r = 3), no phase separations were detected, even when increasing the amount of AlCl 3 •6H 2 O to 5 %mol. With the increase in the value of r, the final conversions for the different acids were more similar than were those when the value of r was 1. This effect could be due to the increasing presence of ethanol, which would influence the K eq of the reaction (Liu et al., 2006).

Effect of AlCl 3 . 6H 2 O on phase separation
The catalysis of AlCl 3 •6H 2 O with pure acids initially resulted in homogeneous solutions for AA, PA, and BA (Fig. 1c). In addition to the changes in the overall compositions due to the formation of the corresponding ethyl esters, bi-phasic systems were demonstrated in all the experiments in which the value of r was fixed to 1 (Fig. 1d). For r = 2 or 3, no separations occurred. To study and describe the bi-phasic system, the overall chemical composition was determined, and the quantification of the two different phases and the distribution of the different species among the two phases were monitored. For a given concentration of the acid and of the catalyst, the phase separation always occurred at the same overall composition, even when appearing at different temperatures (Table 4).

Table 4
The conversion necessary to generate the phase separation decreased with an increase in the length of the alkyl group of the acid ( (1 %mol), a separation of the phases was evident. Next, different catalyst amounts (ranging from 1 to 5 %mol) were added to the synthetic solution (Fig. 7), and the repartition of the phases and the final distribution of the different components were determined.   (Table 5) in the resulting phases were then analysed.

Table 5
According to the data reported in Table 5

Advantages related to the use of AlCl 3 •6H 2 O instead of H 2 SO 4 as the catalyst in producing EA
The industrial production of EA is currently based on the application of the process shown in Fig. 8 (Santaella et al., 2015).

Fig. 8
The most challenging issue related to this industrial production is the downstream purification of the products, which plays a key role in the overall economy of the process. In this process, the reacted homogeneous mixture, composed of EtOH  Table 5. For this reason, a new process can be designed and optimised through a simulation using Aspen Plus V9 ® (Fig. 9).

Fig. 9
In fact, considering that the purification involved the organic phase generated after the reaction, from which water was completely absent, (EtOH (2 542 kg  can also be completely recovered and recycled back to the EC. While the energy required in the reaction was almost the same as that for the conventional process (330 kW), the overall energetic requirement (heating duty) for this purification process was calculated to be 9 780 kW, which is almost one-third that required for the conventional scheme of production.
To evaluate the practicability of the proposed process and to make possible a direct comparison with conventional production of EA, an economic feasibility To evaluate the benefits other than economic feasibility associated with the application of AlCl 3 ·6H 2 O instead of sulfuric acid, a series of sustainability indicators were calculated and are reported in Table 6.

Table 6
All indicators demonstrate that the proposed process is cleaner than the conventional process. For conversion, recovery and productivity, both reactants were considered because they were used in stoichiometric amounts. It is clear that the most important difference occurred with EtOH due to its loss from the aqueous phase generated from the recovery of EA from the ternary azeotrope created with the addition of water in the conventional process (Fig. 8). The EI was also more advantageous because only one-third of the energy was required to sustain the proposed process. The estimated value of the MP factor was close to 0.83, which represents the theoretical maximum achievable for the direct esterification of ethanol and acetic acid (atom economy of the reaction).
Finally, less waste can be produced per kg of product (E), even when water is not included in the estimation of the generated waste (E w ).
Regarding the nature of the waste produced, the conventional process generates an aqueous stream that needs a very expensive treatment due to the presence of a very high concentration of organic compounds. The costs associated with this treatment are not included in the TAC, which results in an underestimation. In addition, sulfuric acid cannot be recycled many times, and new waste is generated, which needs to be disposed of. In contrast, the process based on the use of aluminium chloride generates only one highly contained waste stream (the E factor is 5 times smaller than that for the conventional process). Furthermore, this factor would be cancelled if the aqueous stream of aluminium chloride produced in the proposed process were to find a direct application in WWTP as a flocculant instead of the polychlorides of aluminium.
Under these conditions, the proposed scheme would not only be a potential zerowaste process (E = 0 and perfectly addressing the principles of green chemistry), but it could also be more economically advantageous because it could be sold to WWTPs.

Conclusions
In In addition, taking into consideration that AlCl 3 ·6H 2 O was effectively recoverable in an aqueous phase, which could potentially be used in WWTPs as a coagulant, cogeneration of waste could be eliminated, resulting in a zero-waste process.
All these factors cause the proposed technology to be competitive with the present conventional industrial process for the production of the ethyl esters of VFAs, thus fully satisfying sustainability criteria.