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

Abstract

This work aims to systematically investigate 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 chemicals by hydrothermal processing of Açaí (Euterpe oleraceae, Mart.) seeds. The hydrothermal carbonization was carried out at 175, 200, 225, and 250 °C at 2 °C/min and a biomass-to-water ratio of 1:10; at 250 °C at 2 °C/min and biomass-to-water ratios of 1:10, 1:15, and 1:20 in technical scale; and at 200, 225, and 250 °C at 2 °C/min and a biomass-to-water ratio of 1:10 in laboratory scale. The elemental composition (C, H, N, S) in the solid phase was determined to compute the HHV. The chemical composition of the aqueous phase was determined by GC and HPLC and the volumetric composition of the gaseous phase using an infrared gas analyzer. For the experiments in the pilot test scale with a 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 showed a smooth growth. By varying the 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 decreased exponentially with increasing water-to-biomass ratio, and that of the liquid phase increased in a sigmoid fashion. For a constant biomass-to-water ratio, the concentrations of furfural and HMF decreased drastically with increasing temperature, reaching a minimum at 250 °C, while that of phenols increased. In addition, the concentrations of CH₃COOH and total carboxylic acids increased, reaching a maximum concentration at 250 °C. For constant process temperature, the concentrations of aromatics varied smoothly with temperature. The concentrations of furfural, HMF, and catechol decreased with temperature, while that of phenols increased. The concentrations of CH₃COOH and total carboxylic acids decreased 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. For the experiments at the laboratory scale with a constant biomass-to-water ratio of 1:10, the yields of solids 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, catechol, and acetic acid) dissolved in the aqueous phase in laboratory-scale study showed the same behavior as those obtained in the pilot-scale study.

1. Introduction

Açaí (Euterpe oleracea Mart.) is a palm native to the Brazilian Amazon [1]. It has an abundant occurrence in the Amazon estuary floodplains [2,3]. The Açaí fruits in nature have 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]. The fruit has a diameter around 10.0 and 20.0 mm [5], containing a large core seed/kernel that occupies between 85 and 95% (vol./vol.) of its volume [3,6].

When the pulp and skin are processed/extracted with warm water to produce a thick, purple-colored juice [3,6], a waste is generated [7,8,9,10,11], namely the Açaí seeds, a rich lignin-cellulosic residue with great potential for energetic use [8,9,12]. In the crop years 2016–2017, around 1200–1274 million tons of Açaí fruits were produced in Brazil;–the state of Pará is the main producer (94%), generating large amounts of solid residues consisting of seeds and fibers [4,5,6,7,8,9].

The residue of Açaí fruit (seeds + fibers) processing has a fibrous outer layer, containing 46.51% (wt.) cellulose and 30.31% (wt.) lignin [11]; this residue (seeds + fibers) is composed of 36.13% (wt.) cellulose, 47.92% (wt.) lignin, 1.57% (wt.) ash, and 16.64% (wt.) extractives [11], representing an important renewable biomass source for energetic applications [8,9,12].

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 [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49]. Li et al. [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–chemical properties of hydrothermal carbonization products (solid, liquid, and gas). In addition, Li et al. [13] reported that the most commonly cited hydrothermal carbonization product parameter was solid (hydro-char) yield (71%), while little attention has been paid to carbon-related information on the liquid and gaseous phases (<18%), which includes the analysis of chemical composition.

In fact, only a few studies systematically investigated the composition of the main chemical compounds, such as aromatic-ring compounds (furfural, HMF, phenols, cresols, catechol, guaiacol, etc.) [14,18,39,40,41], carboxylic acids (HCOOH, CH₃COOH, CH₃CH₂COOH, CH₃CH(OH)COOH, CH₃C(O)CH₂CH₂COOH, etc.) [14,18,39,40,43], alcohols (CH₃OH, CH₃CH₂OH) [14], sugars (glucose, xylose, galactose, fructose, sucrose, mannosan, levoglucosan, etc.) [18,40,43], and BTEX (benzene, toluene, xylenes, and ethylbenzene) [41]. In these studies, the above compounds were dissolved in the process water by hydrothermal carbonization of biomass, including the hydrothermal processing of corn stover [14,18]; Tahoe mix, pinyon/juniper, loblolly pine wood, sugar bagasse, and rice hulls [18]; wheat straw, wheat straw digestate, poplar, garapa, massaranduba, pine wood, α-Cellulose, and D-(+)-xylose [39]; wheat straw, poplar, and α-Cellulose [40]; wheat straw, wheat straw digestate, poplar, garapa, massaranduba, and pine wood [41]; Tahoe mix [43]; and bio-degassed pellets [49]. Recently, Poerschmann et al. [44] investigated the distribution of medium molar mass compounds dissolved in process water by hydrothermal carbonization of glucose, fructose, and xylose by GC-MS and IC, identifying more than 50 compounds, the most abundant being carboxylic acids (formic, acetic, glycolic, lactic, and levulinic acids) and aromatic-ring compounds (furfural, 5-(Hydroxymethyl)-2-furfural).

Another process parameter affecting the physical–chemical properties of hydrothermal carbonization products (solid, liquid, and gas) is the water-to-biomass ratio [19,25,26,27,28,29,30]. In recent years only a few studies have investigated the influence of the water-to-biomass ratio from the hydrothermal processing of biomass, including tomato-peel-waste [26], olive stone [27], microalgae [28], sawdust [29], banana peels [30], wood chips [25], and corn stalks [19], but no study has examined their influence consistently [13], particularly the composition of the main chemical compounds dissolved in the process water.

Açaí (Euterpe oleracea, Mart.) is the only fruit species whose seed’s centesimal and elemental composition is completely different from wood biomass (e.g., poplar, garapa, massaranduba, 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, and wheat straw), or 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], no systematic study has investigated the influence of temperature, biomass-to-water 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 on a 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 the 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 oleracea, Mart.) seeds on a technical scale.

2. Materials and Methods

2.1. 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 chemical composition of main compounds in the liquid phase by HTC of Açaí seeds.

Initially, the Açaí seeds were collected. Afterward, they were subjected to pre-treatments of drying, followed by milling and sieving. The HTC was carried out at 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 the main compounds (furfural, HMF, phenols, catechol, guaiacol, HCOOH, etc.) in the liquid phase was investigated. The composition in the gas phase (CH₄, CO₂, O₂, CO) was determined by infrared spectroscopy, and that of the liquid phase was determined by HPLC and GC-FID.

2.2. Materials, Pre-Treatment, and Characterization of Açaí Seeds in Nature

The charges of Açaí seeds in nature obtained in a small Açaí store of in the city of Belém-Pará, Brazil [12]. The seeds were dried at 105 °C, ground using a knife cutting mill, and sieved using an 18 mesh sieve [12]. Afterward, the seeds underwent physical–chemistry characterization for moisture (AOAC 935.29), volatile matter (ASTM D 3175-07), ash (ASTM D 3174-04), fixed carbon (ASTM D6316-09), lipids (AOAC 963.15), proteins (AOAC 991.20) [12], and fibers according to the literature [50] and for insoluble lignin by the modified method of Klason [51]. The elemental composition (C, H, N, S) of Açaí seeds in the solid phase was also determined. Figure 2 shows the Açaí seeds after drying, milling, and sieving (seeds + fibers (a); comminuted seeds + fibers (b); and sieved seeds + fibers (c)). The HTC experiments were carried out with sieved seeds + fibers, as shown in Figure 2c.

2.3. Experimental Apparatus and Procedures

2.3.1. Experimental Apparatus and Procedures in Pilot Scale

Pilot-scale apparatus is illustrated in Figure 3, described in details elsewhere [14].

The hydrothermal processing of dried Açaí seeds was carried out with hot compressed water at 175, 200, 225, and 250 °C for 240 min with a biomass-to-water ratio of 1:10 and at 250 °C for 240 min with biomass-to-water ratios of 1:10, 1:15, and 1:20, as described in detail elsewhere [14].

2.3.2. Experimental Apparatus and Procedures in Laboratory Scale

The laboratory-scale apparatus is illustrated in Figure 4. The laboratory-scale cylindrical stirred tank stainless steel reactor is 1.0 L (Parr, USA, Model: 4577), with internal diameters of 9.525 cm and 15.748 cm and a weight of 7.257 kg. The reactor contains a mechanical agitation system with a stirrer motor of ¼ hp, 1.81 Nm torque, and 02 impellers (ID = 5.08 cm) with 6 blades, a ceramic movable heater of 2800 W, a modular controller (Parr, Moline, IL, USA, Model: 4848), and 02 type J thermocouples inside a thermos well and operates at maximum conditions of 345 bar and 500 °C. The hydrothermal processing of dried Açaí seeds was carried out with hot compressed water at 200, 225, and 250 °C for 240 min, with a biomass-to-water ratio of 1:10. Initially, the solid moisture was determined, and the mass of water was computed based on the dried solid. A mass of 73.50 g of solid with 25% (wt.) moisture was added to 532.82 g of distilled and deionized water and was 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 was computed from the time the reactor reached the setpoint temperature (τ₀). Afterwards, the reactor was cooled until ambient temperature. The reaction products, including a moist dewatered solid phase and a liquid phase, were determined gravimetrically. Then, the moist solid phase was dried at 105 °C for 24 h. The volume of the gas and its composition were determined, as described elsewhere [14]. The samples of moist dewatered solid, liquid, and dried solid phases were stored for physicochemical analysis.

2.4. Compositional Analysis of Reaction Products

2.4.1. Aqueous Phase

The chemical analysis of volatile low-chain length carboxylic acids (R-COOH, with R₁=CH₃, R₂=C₂H₅, R₃=C₃H_7, R₄=C₄H₉) and alcohols (R-OH, with R₁=CH₃, R₂=C₂H₅, R₃=C₃H₈), as well as selective cellulose/hemicellulose-derived compounds, was done by hydrothermal processing of biomass and then identified by GC while the selective cellulose/hemicellulose-derived phenolic (phenol, cresol, catechol, and guaiacol) and aldehyde (furfural, HMF) compounds were identified by HPLC. The equipment specifications (GC-FID, HPLC), and operating conditions, including the GC-FID temperature program, as well as HPLC procedures, are described in detail elsewhere [14].

2.4.2. Gaseous Phase

The volume of gas, degassed at 25 °C and 1.0 atmosphere, was 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 are described in detail elsewhere [14].

2.5. 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 in a closed thermodynamic system, and the equations are described in detail elsewhere [14].

3. Results

3.1. Hydrothermal Processing of Açaí Seeds

3.1.1. Material Balances, Operating Conditions, and Yields of Reaction Products

Influence of Temperature on the Yields of Reaction Products

The material balances, operating conditions, and yields of reaction products by hydrothermal processing of Açaí seeds in nature at 175, 200, 225, and 250 °C, with an increase of 2 °C/min for 240 min, and a biomass-to-water ratio of 1:10, at a pilot scale, are summarized in

Table 1. Mass balances, processes, operating conditions, and yields of dry solid, liquid, and gaseous products by hydrothermal processing of Açaí seeds with hot compressed H₂O at 175, 200, 225, 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10, at a pilot scale.

		Temperature (°C)
Process Parameters	175	200	225	250
Mass of Açaí Seeds [g]	300.00	299.82	299.98	300.16
Mass of H2O [g]	2997.60	3000.20	3001.30	2999.90
Mechanical Stirrer Speed [rpm]	90	90	90	90
Initial Temperature [°C]	30	30	30	30
Heating Rate [°C/min]	2	2	2	2
Process Time [min]	240	240	240	240
Mass of Slurry [g]	3252.20	3240.20	3216.50	3167.40
Volume of Gas [mL], T = 25 °C, P = 1 atm	0	5290	5590	7470
Mass of Gas [g]	0	7.564	8.231	11.408
Process Loss (I) [g]	45.40	59.82	84.78	132.66
Input Mass of Slurry (Pressing) [g]	3252.20	3240.20	3216.50	3161.70
Process Loss (II) [g]	0.00	0.00	0.00	5.70
Mass of Liquid Phase [g]	2638.53	2615.56	2637.97	2556.96
Mass of Moist Hydro-char [g]	588.10	587.37	557.61	591.29
Process Loss (III) [g]	25.57	37.27	20.92	13.41
Mass of Dry Hydro-char [g]	160.16	118.53	113.052	111.092
(Mass of Liquid Phase + Σ Process Loss + Mass of Moist Hydro-char − Mass of Dry Hydro-char − Mass of Gas) [g]	3137.44	3173.926	3179.997	3177.52
Process Loss (I + II + III) [g]	70.97	97.09	105.70	151.77
Mass of LiquidReaction [g]	139.84	173.726	178.697	177.62
Yield of Hydro-char [wt.%]	53.39	39.534	37.686	37.011
Yield of Liquid Phase [wt.%]	46.61	57.943	59.570	59.188
Yield of Gas [wt.%]	0.000	2.523	2.744	3.801

.

Figure 5 shows the effect of temperature on the yields of reaction products by hydrothermal processing of Açaí seeds in nature with hot, compressed H₂O heated by 2 °C/min to 175, 200, 225, 250 °C for 240 min with a biomass-to-water ratio of 1:10. The exponential function applied to regress the yields of reaction products correlated well with the experimental data for both the solid and liquid phases, with r² (R-Squared) between 0.996 and 0.996. The yield of dry solids shows a smooth first-order exponential decay behavior, while that of the liquid and gaseous phases shows a smooth first-order exponential growth. At 175 °C, hydrothermal carbonization takes place, as the main reaction product is a solid [15]. At 200 °C hydrothermal liquefaction occurs, and the main reaction products are liquids [15]. The hydro-char yields were similar to those reported in the literature for rye straw [16], eucalyptus leaves [17], corn stover [14], sugarcane bagasse [18], and corn stalk [19].

The centesimal composition of Açaí (Euterpe oleracea Mart.) seeds [12] was found to be 40.29% (wt.) cellulose, 5.5% (wt.) hemi-cellulose, 4.0% (wt.) lignin, 29.79% fibers, as well as 6.25% (wt.) protein, 0.61% (wt.) lipid, 10.15% (wt.) moisture, 0.5% (wt.) volatile matter, 0.83% (wt.) fixed carbon, and 0.15% (wt.) ash. The centesimal composition of Açaí fibers in nature is 41.37% (wt.) cellulose, 11.54% (wt.) hemi-cellulose, 40.25% (wt.) lignin, 1.96% ash, and 8.88% moisture, as reported by Tavares et al. [7].

Based on the facts that 61.25% (wt.) cellulose and 15% (wt.) lignin decompose at 200 °C over 24 h with a 1:10 biomass-to-water ratio (according to Falco et al. [16]), that 5.0% (wt.) lipids and 72% (wt.) proteins decompose at 350 °C over 90 min with a 1:5.7 biomass-to-water ratio (as reported by Teri et al. [20]), that hemicellulose degrades 65% (wt.) by a severity factor between 4.5 and 5.0 (as reported by Borrero-López et al. [21]), and that approximately 50% (wt.) cotton fibers decompose at 210 °C over 8.0 h with a 1:41 biomass-to-water ratio (as reported by Zhang et al. [22]), 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 a biomass-to-water ratio of 1:10, using the following equation, computed in

Table 2. Computation of hydro-char yield. Centesimal composition of Açaí seeds and Açaí fibers from nature [7,12], decomposition cellulose and lignin [16], decomposition of lipids and proteins [20], decomposition of hemicellulose [21], decomposition of cotton fibers [22].

		Decomposition
Centesimal Composition	[wt.%]	Cellulose [wt.%]	Lignin [wt.%]	Hemi-Cellulose [wt.%]	Proteins [wt.%]	Lipids [wt.%]	Fibers [wt.%]
		61.25	15.00	65.00	72.00	5.00	50.00
Açaí Seeds [12]		-	-	-	-	-	-
Cellulose	40.29	15.6124	-	-	-	-	-
Lignin	4.00	-	3.40	-	-	-	-
Hemi-cellulose	5.50	-	-	1.925			-
Proteins	6.25	-	-	-	1.82		-
Lipids	0.61	-	-	-	-	0.5795	-
Fibers	29.79	-	-	-	-	-	14.895
Moisture	10.15	-	-	-	-	-	-
Volatile matter	0.50	-	-	-	-	-	-
Fixed carbon	0.83	-	-	-	-	-	-
Ash	0.15	-	-	-	-	-	-
YHydro-char/Seeds [wt.%]	37.287	15.6124	3.400	1.925	1.820	0.5795	14.895
Açaí Fibers [7]		-	-	-	-	-	-
Cellulose	41.37	16.080	-	-	-	-	-
Lignin	40.25	-	34.213	-	-	-	-
Hemi-cellulose	11.54	-	-	4.039	-	-	-
Moisture	8.88	-	-	-	-	-	-
Ash	1.96	-	-	-	-	-	-
YHydro-char/Fibers [wt.%]	16.170	4.775	10.192	1.203	-	-	-

. The Y_{Hydro-char/Cellulose} is computed by multiplying the percentage of cellulose in the centesimal composition of Açaí seeds by the fraction of non-degraded cellulose (Y_{Hydro-char/Cellulose} = 40.29% × (100 − 61.25)/100 = 15.6124%), as summarized in Table 2.

Y_Hydro-char = ∑ Y_{Hydro-char/Cellulose} + Y_{Hydro-char/Lignin} + Y_{Hydro-char/Hemi-Cellulose} + Y_{Hydro-char/Proteins} + Y_{Hydro-char/Lipids} + Y_{Hydro-char/Fibers}

(1)

The yield of hydro-char computed using Equation (1) is very close to the experimental value of 39.534% (wt.), showing a deviation of 2.51%, if the yield of Y_{Hydro-char/Fibers} is computed based on the decomposition of cellulose, lignin, and hemi-cellulose (Y_{Hydro-char/Fibers} = 16.17%), and 5.68%, if the yield of Y_{Hydro-char/Fibers} is computed based on the decomposition of fibers (Y_{Hydro-char/Fibers} = 14.895%). In addition, the computed amount of Açaí seeds fibers (29.79% (wt.)) degraded at 200 °C, 240 min, and the biomass-to-water ratio of 1:10 was 16.17% (wt.), giving a degradation percentage of 54.28%, very close to the results reported by Zhang et al. [22] who stated that approximately 50% (wt.) cotton fibers decompose at 210 °C over 8.0 h with a 1:41 biomass-to-water ratio.

By analyzing the thermal decomposition behavior of cellulose and lignin reported by Falco et al. [16], one observes that the decomposition of cellulose is almost constant between 200 °C and 240 °C (38.75→37.00), showing that degradation/de-polymerization of cellulose is less intense [19], while lignin loses only 7.5% of 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.

The material balances, operating conditions, and yields of reaction products by hydrothermal processing of Açaí seeds from nature at 200, 225, and 250 °C heated at 2 °C/min over 240 min with a biomass-to-water ratio of 1:10, at laboratory scale, are summarized in

Table 3. Mass balances, processes, operating conditions, and yields of solid, liquid, and gaseous products by hydrothermal processing of Açaí seeds with hot compressed H₂O at 200, 225, 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10, in laboratory scale.

	Temperature [°C]
Process Parameters	200	225	250
Mass of Açaí Seeds [g]	73.50	73.50	73.50
Moisture Content of Açaí Seeds [wt.%]	25.23	25.23	25.23
Mass of H2O in Açaí Seeds [g]	18.54	18.54	18.54
Mass of H2O [g]	531.01	531.01	531.01
Mechanical Stirrer Speed [rpm]	90	90	90
Initial Temperature [°C]	30	30	30
Heating Rate [°C/min]	2	2	2
Process Time [min]	240	240	240
Volume of Gas [mL], T = 25 °C, P = 1 atm	1395	2475	3215
CO2 [vol.%]	48.60	44.80	35.80
CH4 [vol.%]	1.90	1.20	0.60
O2 [vol.%]	9.20	9.90	12.10
100 − Σ (CO2 + CH4 + O2) [vol.%]	40.30	44.10	51.50
Mass of Gas [g]	1.94	3.57	4.70
Mass of Dry Hydro-char [g]	30.77	29.43	28.16
Mass of LiquidReaction [g]	34.72	36.01	36.59
Yield of Hydro-char [wt.%]	41.86	40.04	38.31
Yield of Liquid Phase [wt.%]	47.24	48.99	49.78
Yield of Gas [wt.%]	2.64	4.86	6.39

.

Figure 6 compares the yields of solid-phase products as a function of temperature by hydrothermal processing of Açaí seeds from nature with hot compressed H₂O at 175, 200, 225, and 250 °C, heated 2 °C/minovery 240 min, and biomass-to-water ratio of 1:10, in laboratory and pilot scales. One can observe, for the temperature interval 200–250 °C, that the yield of solid-phase products varied between 39.53 and 37.01% (wt.) in pilot scale, while that in the laboratory varied between 41.86 and 38.31% (wt.), showing deviations between 5.56 and 3.39%. The yield of solid-phase products at laboratory and pilot scales are very close, showing that production scales had little effect on hydro-char yield. A linear function was applied to regress the yield of hydro-char at a laboratory scale, correlating very well to the experimental data, with an r² (R-Squared) of 0.999.

Influence of Biomass-to-Water Ratio on the Yields of Reaction Products

Table 4. Mass balances, processes, operating conditions, and yields of solid, liquid, and gaseous phases by hydrothermal processing of Açaí seeds with hot compressed H₂O at 250 °C, 2 °C/min, 240 min, and biomass-to-water ratios of 1:10, 1:15, and 1:20, in pilot scale.

	250 °C
	Biomass/H2O [-]
Process Parameters	1:10	1:15	1:20
Mass of Açaí Seeds [g]	300.16	300.28	300.07
Mass of H2O [g]	2999.90	4502.90	6000.80
Mechanical Stirrer Speed [rpm]	90	90	90
Initial Temperature [°C]	30	30	30
Heating Rate [°C/min]	2	2	2
Process Time [min]	240	240	240
Mass of Slurry [g]	3167.40	4696.50	6217.40
Volume of Gas [mL], T = 25 °C, P = 1 atm	7470	1240	1225
Mass of Gas [g]	11.408	1.905	1.863
Process Loss (I) [g]	132.66	106.68	83.47
Input Mass of Slurry (Pressing) [g]	3161.70	4696.50	6209.30
Process Loss (II) [g]	5.70	0.00	8.10
Mass of Liquid Phase [g]	2556.96	4077.05	5663.60
Mass of Moist Hydro-char [g]	591.29	585.83	518.45
Process Loss (III) [g]	13.41	33.62	35.35
Mass of Dry Hydro-char [g]	111.092	102.25	96.302
(Mass of Liquid Phase + Σ Process Loss + Mass of Moist Hydro-char − Mass of Dry Hydro-char − Mass of Gas) [g]	3177.52	5000.795	6210.805
Process Loss (I + II + III) [g]	151.77	140.30	126.92
Mass of LiquidReaction [g]	177.62	196.125	202.535
Yield of Hydro-char [wt.%]	37.011	34.051	32.093
Yield of Liquid Phase [wt.%]	59.188	65.315	67.286
Yield of Gas [wt.%]	3.801	0.634	0.621

describes in detail 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 pilot scale.

The effect of the H₂O-to-biomass ratio on the yields of reaction products (solid, liquid, and gas) by hydrothermal processing of Açaí seeds in nature is illustrated in Figure 7, and comparison of hydro-char yields with similar data reported in the literature is 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 an r² (R-Squared) between 0.97 and 0.99. The yields of hydro-char and gas decrease with H₂O-to-water ratios, while that of the liquid phase increases. With an increased H₂O-to-biomass ratio, the amount of reaction media (hot compressed H₂O) increases, increasing the number of hydroxonium ions (H₃O⁺) and hydroxide ions (OH⁻) dissociated within the reaction system, thus improving the catalyses of chemical reactions such as hydrolysis and organic compound degradation (e.g., depolymerization, fragmentation) without the aid of a catalyst [23]. In fact, according to the literature [24,25], increasing the H₂O-to-biomass ratio causes a great impact on hydrolysis reactions by hydrothermal processing of biomass.

A compilation of similar data on the effect of the H₂O-to-biomass ratio over hydro-char yields is illustrated in Figure 8. The behavior of the hydro-char yield is similar, showing a decrease in the hydro-char yield as the H₂O-to-biomass ratio increases. The data for Açaí seeds, tomato-peel-waste [26], olive stone [27], and corn stalk [19] were regressed using a dose–response function, showing an r² between 0.941 and 0.969. The experimental data not only resemble similar data reported in the literature for tomato-peel-waste [26], olive stone [27], microalgae [28], sawdust [29], banana peels [30], and wood chips [25] but are close to those of corn stalk [19] carried out at 250 °C and 4.0 h.

By analyzing Figure 8, one can observe that temperature has a combined effect on the hydro-char yield with varying H₂O-to-biomass ratios. At higher temperatures (250 °C), the effect of H₂O-to-biomass is more intense, playing an important role in hydro-char yield. For low–medium hydrothermal processing temperatures, the effect of H₂O-to-biomass on hydro-char yield is secondary, as reported by [26].

3.1.2. Chemical Composition of Gas Reaction Products

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 H₂O at 175, 200, 225, and 250 °C, heated by 2 °C/min over 240 min, and biomass-to-water ratio of 1:10, at pilot scale, is shown in

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 H₂O at 175, 200, 225, 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10, in pilot scale.

	Temperature [°C]
Composition [vol.%]	175	200	200	200
CO2 [vol.%]	0.00	44.40	51.30	60.60
CH4 [vol.%]	0.00	0.40	0.50	1.30
O2 [vol.%]	0.00	1.0	0.30	0.00
100 − Σ (CO2 + CH4 + O2) [vol.%]	0.00	54.20	47.90	38.10
Volume of Gas [mL]	0.00	5290	5590	7470
Volume of CO2 [mL]	0.00	2348.76	2867.67	4526.82
Volume of CH4 [mL]	0.00	21.16	27.95	97.11
Volume of O2 [mL]	0.00	52.90	16.77	0.00
V100 − Σ (CO2 + CH4 + O2) ≈ VCO [mL]	0.00	2867.18	2677.61	2846.07
Mass of Gas[g]	0.00	7.564	8.231	11.408
Mass of CO2 [g]	0.00	4.191	5.117	8.078
Mass of CH4 [g]	0.00	0.014	0.018	0.063
Mass of O2 [g]	0.00	0.068	0.022	0.000
Mass of CO [g]	0.00	3.291	3.074	3.267
Composition of Gas [mol.%]
YCO2	0.00	0.441410	0.510306	0.603507
YCH4	0.00	0.004055	0.004936	0.012943
YO2	0.00	0.009848	0.003017	0.000000
Y100 − Σ (CO2 + CH4 + O2) ≈ YCO	0.00	0.544687	0.481741	0.383550

.

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 al. [14]. Similar studies reported that the volume of gaseous products increases with temperature [18,31,32].

The infrared gas analyzer identified the presence of CO₂, O₂, CH₄, and CO, which was computed by difference [14], as summarized in Table 5, with CO₂ being the most abundant gaseous species produced. This is in accordance with 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, according to Li et al. [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 the decomposition of HCOOH, produced during the hydrothermal degradation of cellulose, and until condensation reactions occur.

The effect of temperature on the chemical composition of gas reaction products is shown in Figure 10. The mole fraction of CO shows a smooth exponential decay behavior, and the mole fraction of CO₂ shows a smooth exponential growth. An increase in 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 al. [37]. By increasing the processing 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 H₂O-to-Biomass Ratio on the Volume of Gas Reaction Products

The effect of H₂O-to-biomass ratio on the volume of gas degassed at 25 °C and 1.0 atmosphere by hydrothermal of Açaí seeds from nature with hot compressed H₂O at 250 °C, heated by 2 °C/min over 240 min, and biomass-to-water ratios of 1:10, 1:15, and 1:20, at a pilot scale, illustrated in

Table 6. Volume of gas and composition of gas products at 25 °C and 1.0 atmosphere as a function of temperature by hydrothermal processing of Açaí seeds with hot compressed H₂O at 250 °C, 2 °C/min, 240 min, and biomass-to-water ratios of 1:10, 1:15, and 1:20, in pilot scale.

	250 °C
	Biomass/H2O [-]
Composition [vol.%]	1:10	1:15	1:20
CO2 [vol.%]	60.60	63.40	59.90
CH4 [vol.%]	1.30	3.10	1.60
O2 [vol.%]	0.00	0.00	0.00
Σ (CO2 + CH4 + O2)	61.90	65.50	61.50
100 − Σ (CO2 + CH4 + O2) [vol.%]	38.10	33.50	38.50
Volume of Gas [mL]	-	-	-
Volume of CO2 [mL]	4526.82	786.16	733.775
Volume of CH4 [mL]	97.11	38.44	19.60
Volume of O2 [mL]	0.00	00.00	00.00
V100 − Σ (CO2 + CH4 + O2) ≈ VCO [mL]	2846.07	415.40	471.625

and Figure 11. By increasing the H₂O-to-Biomass ratio, the volume of gas depletes, indicating that hydrolysis may be the dominant reaction mechanism [24,38].

3.1.3. Chemical Composition of Organic Compounds in the Aqueous Phase

Effect of Temperature on the 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 catechol) and carboxylic acids (CH₃COOH, CH₃CH₂COOH) by hydrothermal processing of Açaí seeds is illustrated in Figure 12, Figure 13 and Figure 14, and the data are summarized in

Table 7. Concentration of aromatics compounds (HMF, furfural, phenol, catechol), carboxylic acids (CH₃COOH, CH₃CH₂COOH), and total carboxylic acids (HAc) in the aqueous phase at 25 °C and 1.0 atmosphere by hydrothermal processing of Açaí seeds with hot compressed H₂O at 175, 200, 225, 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10, at pilot and laboratory scales.

	Temperature [°C]
	Pilot
Concentration of aromatics [mg/L]	175	200	225	250
HMF: CAS: 67-47-0	474.8	4923	1701	24.45
Furfural: CAS:98-01-1	96.83	227.8	119.5	5.05
Phenol: CAS: 108-95-2	11.34	-	60.81	81.24
Catechol: CAS: 120-80-9	63.86	-	-	193.8
Concentration of carboxylic acids [mg/L]	-	-	-	-
Acetic acid: CAS: 64-19-7	700	1280	1410	1680
Total acetic acid (HAc)	773.2	1394	1540	1859
Propionic Acid: CAS: 79-09-4	90	140	160	220
	Laboratory
Concentration of aromatics [mg/L]	-	200	225	250
HMF: CAS: 67-47-0	-	4123	275	4.56
Furfural: CAS:98-01-1	-	311.5	31.82	7.28
Phenol: CAS: 108-95-2	-	121.2	172.5	227.99
Catechol: CAS: 120-80-9	-	156.84	215.4	365.61
Concentration of carboxylic acids [mg/L]	-	-	-	-
Acetic acid: CAS: 64-19-7	-	3420	3690	3810
Total acetic acid (HAc)	-	3797	3996	4286
Propionic Acid: CAS: 79-09-4	-	240	310	410

.

Figure 12 compares the concentration of HMF in the aqueous phase as a function of temperature by hydrothermal processing of Açaí seeds in nature with hot compressed H₂O at 175, 200, 225, 250 °C, heated by 2 °C/min over 240 min, and biomass-to-water ratio of 1:10, at laboratory and pilot scales. We observed, for the temperature interval 175–250 °C, at pilot scale, that the concentration of HMF showed a Gaussian distribution. In the temperature interval 200–250 °C, the concentration of HMF decreased drastically, showing an exponential decay behavior at both pilot and laboratory scales, and was not detectable at 250 °C.

The concentration of HMF showed the same behavior in the temperature interval 200–250 °C at laboratory and pilot scales. However, there is a significant difference between concentration values, showing that production scales had a great effect on HMF concentration.

When the process temperature was increased, the concentrations of furfural, a byproduct of cellulose degradation, decreased exponentially in the temperature interval 200–250 °C and were present at very low concentrations at 250 °C, while the concentrations of phenols and catechol, products of furfural and HMF degradation, increased, as shown in Figure 13.

During 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 ions (H₃O⁺) and increasing the ionic product of reacting media, improving the degradation of biomass [38]. The monosaccharides (glucose, fructose) also undergo dehydration and fragmentation reactions, 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, due to increasing process temperature, the concentrations of furfural and HMF will decrease, while those of catechol and phenols will increase. The results align with similar studies reported in the literature [14,18,39,40,41]. Jung et al. [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 H₂O (HMF → Hydro-char + H₂O), following first-order kinetics $\frac{\mathrm{d}\left[\mathrm{Hydro}-\mathrm{char}\right]}{\mathrm{dt}}=\mathrm{K}\times \left[\mathrm{HMF}\right]$. 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 H₂O at 175, 200, 225, 250 °C, heated by 2 °C/min over 240 min, and biomass-to-water ratio of 1:10, at pilot scale. The concentrations of carboxylic acids, particularly CH₃COOH, the most predominant one, and total carboxylic acids (HAc) increase strongly with temperature. With the hydrothermal processing of biomass, the monosaccharides (glucose, fructose) produced by hydrolysis are decomposed, forming volatile carboxylic acids, including acetic and propionic acids [38]. As reported by Hoekman et al. [18,43] and Machado et al. [14], the concentrations of acetic acid and total organic acids produced by hydrothermal processing of different biomass feedstocks increase with temperature. Poerschmann et al. [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 the degradation of glucose and fructose, respectively, at 220 °C and 2.0 h.

It is known that monosaccharides (glucose, fructose) not only decompose, producing volatile carboxylic acids, but also undergo dehydration and fragmentation reactions, producing furfural-derived compounds (furfural, HMF). According to Kabyemela et al. [45], the reaction mechanism/pathway of cellobiose decomposition in sub- and supercritical H₂O (300 °C/25 MPa, 350 °C/25 MPa, 350 °C/40 MPa, and 400 °C/40 MPa), follows the sequence: hydrolysis of cellobiose to form glucose, followed by pyrolysis to form glycosyl-erythrose and glycosyl-glycol-aldehyde, which undergo hydrolysis to produce erythrose + glucose/fructose and glycol-aldehyde + glucose/fructose. tTat is, glucoseand fructose are intermediate-reaction products, being produced continuously along the hydrothermal process. However, Hoekman et al. [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, and are not detected at 275 and 295 °C. Therefore, one may suppose that the degradation of monosaccharides (glucose, fructose) is not the only reaction mechanism to produce volatile carboxylic acids by hydrothermal processing of biomass as glucose, which according to Falco et al. [16], starts to be produced at 140 °C, reaches a maximum at 200 °C, and then begins to decompose.

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 catechol) and carboxylic acids (CH₃COOH, CH₃CH₂COOH) by hydrothermal processing of Açaí seeds, illustrated in Figure 15 and Figure 16, and the data are summarized in

Table 8. Concentration of aromatics compounds (HMF, furfural, phenol, catechol), carboxylic acids (CH₃COOH, CH₃CH₂COOH), and total carboxylic acids (HAc) in the aqueous phase at 25 °C and 1.0 atmosphere by hydrothermal processing of Açaí seeds with hot compressed H₂O at 250 °C, 2 °C/min, 240 min, and biomass-to-water ratio of 1:10, in pilot scale.

	250 °C
	Biomass/H2O [-]
Concentration of aromatics [mg/L]	1:10	1:15	1:20
HMF: CAS: 67-47-0	24.450	5.188	3.002
Furfural: CAS:98-01-1	5.054	2.972	2.194
Phenol: CAS: 108-95-2	81.24	81.78	89.33
Catechol: CAS: 120-80-9	195.6	193.8	185.9
Concentration of carboxylic acids [mg/L]	-	-	-
Acetic acid: CAS: 64-19-7	1680	1270	1070
Total acetic acid (HAc)	1859	1424	1070
Propionic Acid: CAS: 79-09-4	220	190	20

.

When the H₂O-to-biomass ratio was increased, the concentrations of furfural and HMF become 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 H₂O-to-biomass ratio increases, as illustrated in Figure 16. The concentration is measured in mg/L, so increasing the H₂O-to-biomass ratio increases the volume of reaction, and it is hence expected to decrease the concentration of organic compounds dissolved in processed H₂O.

Effect of Biomass-to-Water Ratio on the Mass Production of Chemicals in the Aqueous Phase

By 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 + Σ Process Loss + Mass of Moist Hydro-char − Mass of Dry Hydro-char − Mass of Gas), described in Table 4, it can be shown that increasing the H₂O-to-biomass ratio has caused an increase in the mass production of chemicals, including phenols and catechol, products of furfural and HMF degradation, as well as acetic acid as shown in Figure 17 and Figure 18.

According to the literature [24,25], increasing the H₂O-to-biomass ratio causes a great impact on the hydrolysis reactions by hydrothermal processing of the 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 (Figure 17). As observed in Figure 17, the mass of furfural and HMF decreases while those of phenols and catechol increase, confirming that furfural and HMF were transformed into phenols and catechol.

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 H₂O at 250 °C, heated by 2 °C/min over 240 min, as the biomass-to-water ratio increases from 1:10 to 1:20.

4. Conclusions

Based on the centesimal composition of Açaí (Euterpe oleracea Mart.) seeds, 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 the biomass-to-water ratio of 1:10. The computed solid-phase yield was 38.562% (wt.), very close to the experimental value of 39.534% (wt.), showing a deviation of 2.51%.

The yields of hydro-char and gas decrease with H₂O-to-biomass ratios, while that of the liquid phase increases, showing that the H₂O-to-biomass ratio causes a great impact on hydrolysis reactions by hydrothermal processing of the 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 from nature.

When the H₂O-to-biomass ratio is increased, 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 H₂O-to-biomass ratio increases. By performing a mass balance, it can be shown that increasing the H₂O-to-biomass ratio caused an increase in the mass production of chemicals, including acetic acid, phenols, and catechol.

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 H₂O at 250 °C, 2 °C/min, 240 min, as the biomass-to-water ratio increases from 1:10 to 1:20.