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

: This work aims to investigate systematically the influence of process temperature, biomass-to-water ratio, and production scales (laboratory and pilot) 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, as well as at 200, 225, elemental The chemical composition aqueous the volumetric composition using an infrared gas analyzer. For the in pilot scale with 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 and gaseous phases a smooth growth. By varying the biomass-to-water ratios, the yields solid, gaseous reaction varied between 53.39 and 32.09% (wt.), and 67.28% (wt.), and 0.00 and 0.634% (wt.), respectively. The yield and that of liquid in sigmoid the HMF dras-tically with increasing reaching a minimum at 250 °C, while that of phenols increases. the yields of solid ranged between 55.9 and 51.1% (wt.), showing not only a linear decay with temperature, but also a lower degradation grade. The chemical composition of main organic compounds (furfural, HMF, phenols, cathecol, and acetic acid) dissolved in the aqueous phase in laboratory scale shows the same behavior of those in obtained in pilot scale. and


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 around 10.0 and 20.0 mm [5], containing a large core seed/kernel that occupies between 85-95% (vol./vol.) of its volume [3,6].
The thermo-chemical transformation of lignin-cellulose rich biomass with H2O in the sub-critical or supercritical state is a promising technique, and the literature reports several studies on the subject . Li et. all. [13], applied statistical methods to investigate the role of process conditions (temperature, reaction time, biomass-to-water ratio) and chemical raw material characteristics on the physical-chemistry properties hydrothermal carbonization products (solid, liquid, and gas). In addition, Li et. all. [13], reported that most commonly cited hydrothermal carbonization product parameter was solid (hydrochar) yield (71%), while little attention has been paid to carbon-related information to the liquid and gaseous phases (< 18%), which includes the analysis of chemical composition.
Açaí (Euterpe oleracea, Mart.) seed is the only fruit specie, whose centesimal and elemental composition is completely different from wood biomass (Poplar, Garapa, massa-randuba, Tahoe mix, Pinyon/Juniper, Loblolly pine, and pine wood) [18,[39][40][41]43], agriculture residues of cereal grains (corn Stover, corn Stalk, rice hulls, wheat straw), agriculture residues of sugar cane (sugar bagasse) [18]. Although, hydrothermal treatment has been applied to enhance enzymatic hydrolysis of Açaí seeds in aqueous-H2SO4 at 121 °C [47], until the moment no systematic study investigated the influence of temperature, biomass-towater ratio, and production scales (laboratory and pilot) on the chemical composition of aqueous and gaseous phases, as well as on the mass production of chemicals by hydrothermal carbonization of Açaí seeds in technical scale.
This work aims to investigate the influence of process temperature, biomass-to-water ratio, and production scales (laboratory and pilot) 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.

Methodology
The process flow sheet shown in Figure 1 summarizes the applied methodology, described in a logical sequence of ideas, chemical methods, and procedures to analyze the the chemical composition of main compounds in the liquid phase by HTC of Açaí seeds. Initially, the Açaí seeds are collected. Afterwards, subjected to pre-treatments of drying, followed by milling and sieving. The HTC carried out in laboratory and pilot scales to investigate the influence of temperature (175, 200, 225, 250 °C) and biomass-to-water ratio (1:10, 1:15, 1:20) on the yields of hydro-char, H2O, and gas, as well as on the chemical composition in the liquid phase. Finally, the influence of production scales on the composition of main compounds (Furfural, HMF, Phenols, Cathecol, Guaiacol, HCOOH, etc.) in the liquid phase investigated. The composition in the gas phase (CH4, CO2, O2, CO) determined by infrared spectroscopy, and that of liquid by HPLC and GC-FID.

Experimental apparatus and procedures 2.3.1. Experimental apparatus and procedures in pilot scale
Pilot scale apparatus illustrated in Figure 3, described in details elsewhere [14]. The hydrothermal processing of dried Açaí seeds carried out with hot compressed at 175, 200, 225, and 250 °C, 240 minutes, biomass-to-water ratio of 1:10, and at 250 °C, 240 minutes, and biomass-to-water ratios of 1:10, 1:15, and 1:20, as described in details elsewhere [14].

(a) (b) (c)
The laboratory scale apparatus illustrated in Figure 4. The laboratory scale cylindrical stirred tank stainless steel reactor of 1.0 L (Parr, USA, Model: 4577), with internal diameter of 9.525 cm and 15.748 cm, weights 7.257 kg. The reactor contains a mechanical agitation system with a stirrer motor of ¼ hp, 1.81 N.m Torque, and 02 impellers (ID=5.08 cm) with 6-blades, a ceramic movable heater of 2800 W, a modular controller (Parr, USA, Model: 4848), 02 type J thermocouples inside a thermos well, operates at maximum 345 bar and 500 °C. The hydrothermal processing of dried Açaí seeds carried out with hot compressed at 200, 225, and 250 °C, 240 minutes, biomass-to-water ratio of 1:10. Initially, the solid moisture determined and the mass of water computed based on the dried solid. 73.50 g of solid with 25% (wt.) moisture added to 532.82 g of distilled and deionized water introduced in the reactor. The operating temperatures (200, 225, and 250 °C) set for a heating rate of 2.0 °C/min. The reaction time computed from the time the reactor reaches the set point temperature (τ0). Afterwards, the reactor cooled down until ambient temperature. The reaction products, a moist dewatered solid phase and a liquid phase, determined gravimetrically. Then, the moist solid phase dried at 105 °C for 24 h. The volume of gas and its composition determined, as described elsewhere [14]. The Samples of moist dewatered solids, liquid phase, and dried solid phase stored for physicochemical analysis.

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 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].

Decomposition
The material balances, operating conditions, and yields of reaction products by hydrothermal processing of Açaí seeds in nature at 200, 225, 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10, in laboratory scale, are summarized in Table 3. Figure 6 makes a comparison for the yields of solid phase products as a function of temperature by hydrothermal processing of Açaí seeds in nature with hot compressed H20 at 175, 200, 225, 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10, in laboratory and pilot scales. One observes, for the temperature interval 200-250 °C, the yield of solid phase products varied between 39.53 and 37.01% (wt.) in pilot scale, while that in laboratory varied between 41.86 and 38.31% (wt.), showing deviations between 5,56 and 3,39%. The yield of solid phase products in laboratory and pilot scale are very close, showing that production scales had little effect on hydro-char yield. A linear function applied to regress the yield of hydro-char in laboratory scale, correlating very well the experimental data, with r 2 (R-Squared) of 0.999. Table 3. Mass balances, process and operating conditions, and yields of solid, liquid, and gaseous products by hydrothermal processing of Açaí seeds with hot compressed H20 at 200, 225, 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10, in laboratory scale.   Table 4 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, in pilot scale. Table 4. 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, in pilot scale. 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 7 and comparison of hydro-char yields with similar data reported in the literature, shown in Figure 8. At 250 °C hydrothermal liquefaction is dominant, as the main reaction products formed are liquids [15]. The yields of reaction products, illustrated in Figure 7, 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 8. 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 8, 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].  [19,[26][27][28][29].

Influence of temperature on the 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, in pilot scale, is shown in Table 5. Table 5. 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, in pilot scale.   Figure 9 illustrates the effect of process temperature on the volume of gas degassed at 25 °C and 1.0 atmosphere and the volumetric composition of gaseous products shown in Figure 10. 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 5, 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 shown in Figure 10. 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.

Influence of H20-to-Biomass ratio on the volume of gas reaction products
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,  Figure 11. Effect of H20-to-Biomass on the volume of gas degassed at 25 °C and 1.0 atmosphere by hydrothermal processing of Açaí seeds with hot compressed H20. 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 Figures 12,13,and 14, and the data summarized in Tables 7 .   Table 7. 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, in pilot and laboratory scales scale.  Figure 12 makes a comparison for the concentration of HMF in aqueous as a function of temperature by hydrothermal processing of Açaí seeds in nature with hot compressed H20 at 175, 200, 225, 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10, in laboratory and pilot scales. One observes, for the temperature interval 175-250 °C, in pilot scale, the concentration of HMF shows a Gaussian distribution. In the temperature interval 200-250 °C, the concentration of HMF decreases drastically, showing an exponential decay behavior, in both pilot and laboratory scales, being not detectable at 250 °C. The concentration of HMF shows the same behavior in the temperature interval 200-250 °C, in laboratory and pilot scales. However, there is a significant difference between concentration values, showing that production scales had a great effect on HMF concentration.
By increasing the process temperature, the concentrations of furfural, a byproduct of cellulose degradation, decreases exponentially, in the temperature interval 200-250 °C, being present at very low concentrations at 250 °C, while the concentrations of phenols and catechol, products of furfural and HMF degradation, increase, as shown in Figure 13.
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 14 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, in pilot scale. 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. 3.1.3.2. Effect of biomass-to-water ratio on the chemical composition of organic compounds in the aqueous phase 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 15 and 16, and the data summarized in Table 8.  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 15. In addition, the carboxylic acids (CH3COOH, CH3CH2COOH) and total carboxylic acids (HAc) also decrease as the H20-to-Biomass ratio increases, illustrated in Figure 16.
In a first look, Figures 15 and 16 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  8, 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 3, it can be shown that increasing the H20-to-Biomass ratio has caused an increase on the mass production of chemicals, as shown in Figures 17 and 18 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 17.
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.

Author Contributions:
The individual contributions of all the co-authors are provided as follows: Conceição de Maria Sales da Silva contributed with formal analysis and writing-original draft preparation, Douglas Alberto Rocha de Castro contributed with formal analysis and writing-original draft preparation, Marcelo Costa Santo contributed with formal analysis and software, Hélio da Silva Almeida contributed with formal analysis, software, and visualization, Ulf Lüder contributed with investigation and validation, Maja Shultze contributed with investigation and methodology, Judi A. Libra with funding acquisition, Jan Mumme contributed with funding acquisition, Thomas Hofmann contributed with resources and project administration, and Nélio Teixeira Machado contributed with supervision, conceptualization, and data curation. All authors have read and agreed to the published version of the manuscript.