Process analysis of main organic compounds dissolved in aque- ous phase by hydrothermal processing of Açaí (Euterpe Oleraceae, Mart.) seeds: Influence of process temperature and biomass-to-water ratio

This work aims to investigate systematically the influence of process temperature and biomass-to-water ration on the chemical composition of aqueous and gaseous phases and mass production of chemical by hydrothermal processing of Açaí (Euterpe Oleraceae, Mart.) seeds. The hydrothermal carbonization carried out at 175, 200, 225, and 250 °C, 2 °C/min, biomass-to-water ratio of 1:10, and at 250 °C, 2 °C/min, and biomass-to-water ratios of 1:10, 1:15, and 1:20, in technical scale. The chemical composition of aqueous phase determined by GC and HPLC and the volumetric composition of gaseous phase by using an infrared gas analyzer. For constant biomass-to-water ratio of 1:10, the yields of solid, liquid, and gaseous phases varied between 53.39 and 37.01% (wt.), 46.61 and 59.19% (wt.), and 0.00 and 3.80% (wt.), respectively. The yield of solids shows a smooth exponential decay with temperature, while that of liquid and gaseous phases a smooth growth. By variation of biomass-to-water ratios, the yields of solid, liquid, and gaseous reaction products varied between 53.39 and 32.09% (wt.), 46.61 and 67.28% (wt.), and 0.00 and 0.634% (wt.), respectively. The yield of solids decreases exponentially with increasing water-to-biomass ratio and that of liquid phase increases in a sigmoid fashion. For constant biomass-to-water ratio, the concentrations of Furfural and HMF decrease drastically with temperature, reaching a minimum at 250 °C, while that of phenols increases. In addition, the concentrations of CH3COOH and total carboxylic acids increase, reaching a maximum at 250 °C. For constant process temperature, the concentrations of aromatics vary smoothly with the temperature. The concentrations of furfural, HMF, and cathecol decrease with temperature, while that of phenols increases. The concentrations of CH3COOH and total carboxylic acids decrease exponentially with temperature. Finally, for the experiments with varying water-to-biomass ratios, the productions of chemicals (furfural, HMF, phenols, cathecol, and acetic acid) in the aqueous phase is highly dependent on the biomass-to-water ratio.


Introduction
. Açaí (Euterpe oleracea Mart.) is palm native to the Brazilian Amazon [1]. It has abundant occurrence in the Amazon estuary floodplains [2][3]. The Açaí fruits in nature have a great economic importance for the agroindustry, as well as extractive activities of rural communities of the Brazilian Amazonian state of Pará [4]. The fruit is a small dark-purple, berry-like fruit, almost spherical, weighing between 2.6 to 3.0 g [5]. It has a diameter biomass-to-water ration on the chemical composition of aqueous and gaseous phases and mass production of chemicals by HTC of Açaí seeds in technical scale.
This work aims to investigate systematically the influence of process temperature and biomass-to-water ration on the chemical composition of main chemical compounds, such as aromatic-ring compounds, carboxylic acids, and alcohols, dissolved in process water, the gaseous phase composition, and mass production of chemicals by hydrothermal processing of Açaí (Euterpe Oleraceae, Mart.) seeds in technical scale.

Experimental apparatus and procedures 2.2.1. Experimental apparatus
The technical scale apparatus illustrated in Figure 1, described in details elsewhere [14].

Gaseous phase
The volume of gas, degassed at 25 °C and 1.0 atmosphere, measured with a gas flow meter, while an infrared gas analyzer was used to determine the volumetric composition of gaseous products [14]. The equipment's specifications and procedures described in details elsewhere [14].

Steady state material balance by hydrothermal carbonization
The yields of reaction products (solid, liquid, and gaseous phases) were determined by applying the mass conservation principle within the stirred tank reactor, operating in batch mode, closed thermodynamic system, and the equations described in details elsewhere [14].
By analyzing the thermal decomposition behavior of cellulose and lignin reported by Falco et. all. [16], one observes that decomposition of cellulose is almost constant between 200 °C and 240 °C (38.7537.00), showing that degradation/depolymerization of cellulose is less intense [19], while lignin loses only 7.5% its initial mass (85%77.5%), thus making it possible to explain the small differences for the solid phase yields between 200 °C and 250 °C. Table 2 describes in details the material balances, operating conditions, and yields of reaction products by hydrothermal processing of Açaí seeds in nature at 250 °C, 2 °C/min, 240 min, and biomass-to-water ratios of 1:10, 1:15, and 1:20. Table 2. Mass balances, process and operating conditions, and yields of solid, liquid, and gaseous phases by hydrothermal processing of Açaí seeds with hot compressed H20 at 250 °C, 2 °C/min, 240 min, and biomass-to-water ratios of 1:10, 1:15, and 1:20. The effect of H20-to-Biomass ratio on the yields of reaction products (solid, liquid, and gas) by hydrothermal of Açaí seeds in nature, illustrated in Figure 3 (a) and comparison of hydro-char yields with similar data reported in the literature, shown in Figure 3 (b). At 250 °C hydrothermal liquefaction is dominant, as the main reaction products formed are liquids [15]. The yields of reaction products, illustrated in Figure 3 (a), were regressed using a dose-response function, showing r 2 (R-Squared) between 0.97 and 0.99. The yields of hydro-char and gas decrease with H20-to-water ratios, while that of liquid phase increases. By increasing the H20-to-Biomass ratio, the amount of reaction media (hot compressed H20) increases, increasing the number of hydroxonium ion (H3O + ) and a hydroxide ion (OH -) dissociated within the reaction system, thus improving the catalyzes of chemical reactions such as hydrolysis and organic compounds degradation (e.g. depolymerization, fragmentation) without aid a catalyst [23]. In fact, according to the literature [24][25], increasing the H20-to-Biomass ratio causes a great impact on hydrolysis reactions by hydrothermal processing of biomass.

°C
A compilation of similar data on the effect of H20-to-Biomass ratio over hydro-char yields illustrated in Figure 3 (b). The behavior of hydro-char yields is similar, showing a decrease on the hydro-char yields as the H20-to-Biomass ratio increases. The data for Açaí seeds, tomato-pell-waste [26], olive stone [27], and corn Stalk [19], were regressed using a doseresponse function, showing r 2 (R-Squared) between 0.941 and 0.969.
The experimental data are not only according to similar data reported in the literature for tomato-pell-waste [26], olive stone [27], microalgae [28], sawdust [29]; banana peels [30], wood ships [25], but close to that of corn Stalk [19], carried out at 250 °C and 4.0 h. By analyzing Figure 3 (b), one observes that temperature has a combined effect on the hydro-char yield with varying H20-to-Biomass ratios. At higher temperatures (250 °C), the effect of H20-to-Biomass is more intense, playing an important role on hydro-char yield. For low-medium hydrothermal processing temperatures, the effect of H20-to-Biomass on hydro-char yield is secondary, as reported by [26].  (a) (b) Figure 3. Effect of H20-to-Biomass ratio on the yields of reaction products (solid, liquid, gas) (a) and comparison of hydrochar yields with similar data reported in the literature (b).

Chemical composition of gas reaction products
The volume of gas degassed at 25 °C and 1.0 atmosphere by hydrothermal processing of Açaí seeds with hot compressed H20 at 175, 200, 225, 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10 is shown in Figure 4 (a) and the volumetric composition of gaseous products in Figure 4 (b). The volume of gas increases exponentially as the process temperature increases and the same behavior was reported for the hydrothermal carbonization of corn Stover by Machado et. all. [14]. Similar studies reported that volume of gaseous products increases with temperature [18,[31][32].   The infrared gas analyzer identified the presence of CO2, O2, CH4, and CO was computed by difference [14], as summarized in Tables 3 and 4, being CO2, the most abundant gaseous specie produced. This is according to similar studies on the evaluation of gaseous products and compositions by hydrothermal processing of biomass [14,18,31,[33][34]. The presence of high volumetric concentrations of CO2 in the gaseous phase indicates that decarboxylation is probably one of the dominant reaction mechanisms/pathways by hydrothermal processing of Açaí seeds in nature, being according to Li et. all. [35]. In fact, according to the literature [36], by hydrothermal processing of biomass, decarboxylation takes place, yielding CO2, but other sources can also produce CO2, including de decomposition of HCOOH, produced during the hydrothermal degradation of cellulose, and until condensation reactions.
The effect of temperature on the chemical composition of gas reaction products is shown in Figure 4 (b). The mole fraction of CO shows a smooth exponential decay behavior and the mole fraction of CO2 a smooth exponential growth. An increase on CO2 concentration in the gaseous phase by hydrothermal processing of biomass may be explained by analogy to the mild torrefaction process of biomass, as reported by Wannapeera et. all. [37]. By increasing the process temperature, the oxygen functional groups in the Açaí seeds are decomposed resulting not only in higher amounts of gas formed, but also in higher yields of CO2. Table 3. Volume of gas and composition of gas products at 25 °C and 1.0 atmosphere by hydrothermal processing of Açaí seeds with hot compressed H20 at 175, 200, 225, 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10. The effect of H20-to-Biomass ratio on the volume of gas degassed at 25 °C and 1.0 atmosphere by hydrothermal of Açaí seeds in nature with hot compressed H20 at 250 °C, 2 °C/min, 240 min, and biomass-to-water ratios of 1:10, 1:15, and 1:20, illustrated in Figure  5. By increasing the H20-to-Biomass ratio, the volume of gas depletes, indicating that hydrolysis may be the dominant reaction mechanism [24; 38].

Chemical composition of organic compounds in the aqueous phase
The effect of temperature on the concentration profile of aromatic-ring compounds (Furfural, HMF, Phenols, and Cathecol) and carboxylic acids (CH3COOH, CH3CH2COOH) by hydrothermal processing of Açaí seeds, illustrated in Figure 6 (a) and (b), and the data summarized in Table 5. Table 5. Concentration of aromatics compounds (HMF, furfural, phenol, cathecol), carboxylic acids (CH3COOH, CH3CH2COOH) and total carboxylic acids (HAc) in aqueous phase at 25 °C and 1.0 atmosphere by hydrothermal processing of Açaí seeds with hot compressed H20 at 175, 200, 225, 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10. By increasing the process temperature, the concentrations of furfural and HMF, by products of cellulose degradation, decreases exponentially, being present at very low concentrations at 250 °C, while the concentrations of phenols and cathecol, products of furfural and HMF degradation, increase, as shown in Figure 5 (a).
By hydrothermal processing of biomass, as cellulose hydrolyzes, it forms glucose, being transformed by isomerization reactions into fructose [38]. The decomposition of monosaccharides (glucose, fructose) produces volatile carboxylic acids, dissociating within the reaction media, thus producing hydroxonium ion (H3O + ) and increasing the ionic product of reacting media, improving the degradation of biomass [38]. The monosaccharides (glucose, fructose) also undergo dehydration and fragmentation reaction producing furfural-derived compounds (furfural, HMF), as well as acids and aldehydes [38]. Finally, as temperature increases, furfural-derived compounds (furfural, HMF) suffer degradation, producing acids, aldehydes, and phenols [38]. In this context, based on the reaction mechanism described by Sevilla and Fuertes [38], it is expected that, by increasing the process temperature, the concentrations of Furfural and HMF will decrease, while those of cathecol and phenols increase. The results are according to similar studies reported in the literature [14,18,[39][40][41]. Jung et. all. [42], studied the growth mechanism of hydro-char and the kinetic model of fructose degradation by hydrothermal carbonization, concluding that HMF degrades forming hydro-char and H20 (HMF Hydro-char + H20), following a first-order kinetics ]. This is according to the results for hydro-char yields in Table 1, that is, the higher the concentration of HMF, the higher the yield of hydro-char. Figure 6 (b) shows that temperature has a great effect on concentrations of carboxylic acids (CH3COOH, CH3CH2COOH) and total carboxylic acids (HAc) by hydrothermal processing of Açaí seeds with hot compressed H20 at 175, 200, 225, 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10. The concentrations of carboxylic acids, particularly CH3COOH, the most predominant one, as well as the concentration total carboxylic acids (HAc), increase strongly with temperature. By hydrothermal processing of biomass, the monosaccharides (glucose, fructose) produced by hydrolysis of biomass are decomposed forming volatile carboxylic acids, including acetic and propionic acids [38]. As reported by Hoekman et. all. [18,43], and Machado et. all. [14], the concentrations of acetic acid and total organic acids produced by hydrothermal processing of different biomass feedstocks increases with temperature. Poerschmann et. all. [44], investigated the distribution of main medium molar mass compounds dissolved in process water by hydrothermal carbonization of glucose, fructose and xylose at 180, 220, and 250 °C by GC-MS and IC, reporting acetic acid concentrations of 4560 and 3920 for degradation of glucose and fructose, respectively, at 220 °C and 2.0 h It is known that monosaccharides (glucose, fructose) decompose, producing not only volatile carboxylic acids, but also undergo dehydration and fragmentation reaction producing furfural-derived compounds (furfural, HMF). According to Kabyemela et. all. [45], the reaction mechanism/pathway of Cellobiose decomposition in sub and supercritical H20 (300 °C /25 MPa,350 °C/25 MPa,350 °C/40 MPa,and 400 °C/40 MPa), fallows the sequence: hydrolysis of Cellobiose to form glucose, followed by pyrolysis to form glycosylerythrose and glycosyl-glycol-aldehyde, which undergo hydrolysis to produce erythrose + glucose/fructose and glycol-aldehyde + glucose/fructose, that is, glucose/fructose are intermediate-reaction products, being produced continuously along the hydrothermal process. However, Hoekman et. all. [43], reported that concentrations of glucose/xylose and total sugars decrease with increasing process temperature (215,235,255,275, 295 °C) from 1.02% (wt.) to 0.08% (wt.) and 1.41% (wt.) to 0.22% (wt.), respectively, being not detected at 275 and 295 °C, such that, one may suppose that degradation of monosaccharides (glucose, fructose) are not the only reaction mechanism to produce volatile carboxylic acids by hydrothermal processing of biomass, as glucose, according to Falco et. all. [16], starts to be produced at 140 °C, reaching a maximum at 200 °C, where it begins to decomposes. The effect of biomass-to-water ratio on the concentration profile of aromatic-ring compounds (Furfural, HMF, Phenols, and Cathecol) and carboxylic acids (CH3COOH, CH3CH2COOH) by hydrothermal processing of Açaí seeds, illustrated in Figure 7 (a) and (b), and the data summarized in Table 6. By increasing the H20-to-Biomass ratio, the concentrations of furfural and HMF are very low and decrease smoothly, while that of phenols shows a smooth first-order exponential growth behavior, as shown in Figure 7 (a). In addition, the carboxylic acids (CH3COOH, CH3CH2COOH) and total carboxylic acids (HAc) also decrease as the H20-to-Biomass ratio increases, illustrated in Figure 7 (b). In a first look, Figure 7 (a) and (b) do not say much, as the concentration was measured in mg/L, so that, increasing the H20-to-Biomass ratio, the volume of reaction media increases, and hence it is to expect a decrease on the concentration of main organic compounds dissolved in process water, but performing a mass balance by multiplying the concentration of main organic compounds dissolved in process water, described in Table 6, and the volume of process water (Mass of Liquid Phase +  Process Loss + Mass of Moist Hydro-char -Mass of Dry Hydro-char -Mass of Gas), described in Table 2, it can be shown that increasing the H20-to-Biomass ratio has caused an increase on the mass production of chemicals, as shown in Figure 8. According to the literature [24][25], increasing the H20-to-Biomass ratio causes a great impact on hydrolysis reactions by hydrothermal processing of biomass, so that, the remaining cellulose in biomass is hydrolyzed, producing monosaccharides (glucose, fructose), and the decomposition of monosaccharides (glucose, fructose) produces volatile carboxylic acids, particularly acetic acid, confirmed by Figure 8 (a). It may be concluded that hydrolysis is probably the dominant reaction mechanism, but not the only one, by hydrothermal processing of Açaí seeds with hot compressed H20 at 250 °C, 2 °C/min, 240 min, as biomass-to-water ratio increase from 1:10 to 1:20.

Conclusions
The yield of solids shows a smooth first-order exponential decay behavior, while that of liquid and gaseous phases a smooth first-order exponential growth. At 175 °C hydrothermal carbonization takes places, as the main reaction product is a solid [15]. From 200 °C, hydrothermal liquefaction occurs, as the maim reaction products are liquids [15].
Based on the centesimal composition of Açaí (Euterpe oleracea Mart.) seeds [12], one may perform a centesimal mass balance to compute the approximate theoretical mass degradation of Açaí seeds at 200 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10, obtaining for the solid phase yield 41.01% (wt.), very close to the experimental value of 39.534% (wt.), showing a deviation of 3.73%.
The yields of hydro-char and gas decrease with H20-to-water ratios, while that of liquid phase increases. Increasing the H20-to-Biomass ratio causes a great impact on hydrolysis reactions by hydrothermal processing of biomass.
The presence of high volumetric concentrations of CO2 in the gaseous phase indicates that decarboxylation is probably one of the dominant reaction mechanisms/pathways by hydrothermal processing of Açaí seeds in nature, being according to Li et. all. [35].
The concentrations of furfural and HMF, decreases exponentially, being present at very low concentrations at 250 °C, as temperature increases, while the concentrations of phenols and cathecol increase.
By increasing the H20-to-Biomass ratio, the concentrations of furfural and HMF are very low and decrease smoothly, while that of phenols shows a smooth first-order exponential growth behavior. In addition, the carboxylic acids (CH3COOH, CH3CH2COOH) and total carboxylic acids (HAc) also decrease as the H20-to-Biomass ratio increases. Performing a mass balance, it can be shown that increasing the H20-to-Biomass ratio has caused an increase on the mass production of chemicals, particularly acetic acid.
It may be concluded that hydrolysis is probably the dominant reaction mechanism, but not the only one, by hydrothermal processing of Açaí seeds with hot compressed H20 at 250 °C, 2 °C/min, 240 min, as biomass-to-water ratio increase from 1:10 to 1:20.