Supercritical Water Gasification of Lactose as a Model Compound for

Sep 10, 2015 - Lactose was used in this work as a model compound of dairy effluents for .... Energy Conversion and Management 2016 110, 296-306 ...
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Supercritical Water Gasification of Lactose as a Model Compound for Valorization of Dairy Industry Effluents Sonil Nanda,† Sivamohan N. Reddy,‡ Howard N. Hunter,§ Ian S. Butler,∥ and Janusz A. Kozinski*,† †

Department of Earth and Space Science and Engineering, and §Department of Chemistry, York University, Toronto, Ontario M3J 1P3, Canada ‡ Department of Chemical Engineering, Indian Institute of Technology, Roorkee, Uttarakhand 247667, India ∥ Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada ABSTRACT: The dairy industry effluents, including whey waste and milk-based residues, are enriched in lactose and minor amounts of glucose that could potentially be converted to biofuels and biochemicals. Lactose was used in this work as a model compound of dairy effluents for gasification in supercritical water using a continuous flow tubular reactor. Four parameters impacting supercritical water gasification were studied, namely, temperature (550−700 °C), residence time (30−75 s), feed concentration (4−10 wt %), and catalyst concentration (0.2−0.8 wt %). The best total gas yields, carbon gasification efficiency, H2 yields, and other major gases (CO2 and CH4) were obtained at 700 °C using a feed concentration of 4 wt % lactose and a residence time of 60 s at fixed pressure of 25 MPa. Furthermore, catalytic lactose gasification involving 0.8 wt % Na2CO3 resulted in maximum H2 yield (22.4 mol/ mol) compared to those obtained by 0.8 wt % K2CO3 (21.5 mol/mol) and noncatalytic gasification (16 mol/mol).

1. INTRODUCTION The adverse effects of climate change resulting from increasing greenhouse gas (GHG) emissions and overpowering consumption of fossil fuels are well-known. In addition, the pollution of natural resources, such as water, air, and soil, by refractory industrial wastes has also become an important environmental concern globally. Industrial effluents with a high organic load that are produced through microbial composting or anaerobic digestion result in the release of methane (CH4), which is nearly 25 times more potent as a GHG than is CO2.1 Methane is the primary GHG emitted from the putrefaction of ruminant-generated (e.g., cattle, cow, sheep, and goats) products, as it originates from enteric fermentation occurring during feed digestion.2 The effluents from dairy industries are one of such wastes that require proper attention prior to disposal. Dairy wastes do not usually contain any of the toxic chemicals listed in the EPA’s (US Environmental Protection Agency) Toxic Release Inventory.3 However, these effluents have a relatively high biological oxygen demand (BOD) and chemical oxygen demand (COD) in the range of 0.1−100 kg/ m3 with a biodegradability index (BOD5/COD) between 0.4 and 0.8.4 BOD5 is determined by incubating a water sample (or liquid effluent containing degradable organic matter) saturated with oxygen for 5 days at 20 °C in the dark.5 Dairy effluents are composed chiefly of spoiled and/or spilled milk, yogurt, cream, cheese whey, fat, etc. As per the FAO’s definition of milkderived dairy wastes, losses occur due to spillage during industrial milk treatment (i.e., pasteurization) and processing to dairy products (e.g., cheese, yogurt, condensed milk, etc.).6 © 2015 American Chemical Society

Improper handling procedures at postharvest facilities as well as indecorous storage at distribution and household levels also account for dairy wastes. At the consumer level, milk is the major dairy waste comprising about 40−65% of the total food waste globally.6 Dairy effluents are characterized by greater levels of milk carbohydrate (i.e., lactose), whey protein, phosphoproteins (i.e., casein), amino acids (e.g., glycine and glutamic acid), fats, lactic acid, and minerals (e.g., ammonia and phosphates). A typical cheese effluent is composed of minerals (0.5−10%), phosphorus (0.01−0.5 kg/m3), nitrogen (0.01−1.7 kg/m3), fats (0.08−10.6 kg/m3), total suspended solids (0.1−22 kg/m3), protein (1.4−33.5 kg/m3), and lactose (0.18−60 kg/m3) making the total organic load in the range of 0.6−102 kg/ m3.4 Lactose is a disaccharide sugar derived from the condensation of glucose and galactose that form a β-1→4 glycosidic linkage, and it has the systematic name β-Dgalactopyranosyl-(1→4)-D-glucose. Lactose (C12H22O11), rich in mammalian milk, is hydrolyzed by the lactase enzyme in the intestine into absorbable sugars, namely, glucose and galactose.7 Currently, the treatment of dairy wastes relies on three major approaches. As dairy effluents (e.g., cheese whey) contain more than 50 g/L lactose and 10 g/L proteins,4 the first approach concentrates on their recovery by nanofiltration, ultrafiltration, reverse or forward osmosis, and thermolysis for use as dietary Received: Revised: Accepted: Published: 9296

July 16, 2015 September 2, 2015 September 10, 2015 September 10, 2015 DOI: 10.1021/acs.iecr.5b02603 Ind. Eng. Chem. Res. 2015, 54, 9296−9306

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Figure 1. Schematics of the continuous flow tubular reactor in supercritical water gasification.

supplements.8 The second approach is related to dairy waste fermentation for generating value-added products, such as lactic acid;9 propionic acid;10 butyric acid and acetic acid;11 acetone, butanol, and ethanol;11 glycerol;12 biohydrogen and methane;10 and single-cell proteins.13 The third approach deals with chemimechanical techniques for recycling dairy waste through coagulation, flocculation, adsorption, and membrane separation.3,14 Owing to significant levels of lactose and casein protein, the aerobic digestion of dairy effluents by microorganisms is usually malodorous, and attracts insects and pests.14 In contrast, anaerobic digestion of dairy wastes by methanogenic bacteria has been widely investigated in the past few years because of the end product CH4. However, the intermediate products obtained during lactose hydrolysis, such as organic acids, fatty acids, and alcohols can cause partial inhibition in the methanogenesis phase.15 Antonopoulou et al. performed a two-stage anaerobic digestion to produce biohydrogen and CH4 from cheese whey. 10 The process simultaneously facilitated the acidogenesis and methanogenesis phases to yield biohydrogen and CH4, respectively, without much product inhibition. As one of the preliminary attempts on the thermochemical conversion of dairy wastes, Muangrat et al. performed gasification of whey in near-critical water.16 The subcritical water gasification involved partial oxidation of whey with H2O2 in the presence of an alkaline catalyst, that is, NaOH. This process, which was operated at 300−390 °C and 9.5−24.5 MPa for up to 120 min, resulted in ∼40% H2 in the gas products and >80% NH3 in the liquid effluents. Supercritical water (SCW) treatment at temperatures greater than 374 °C and pressures greater than 22.1 MPa has found wide application in waste biomass gasification for the production of synthesis gas or syngas (H2 and CO).17 The high lactose concentration in dairy wastes has tremendous potential to produce H2-rich syngas at SCW conditions. Supercritical water gasification (SCWG) uses water as the gasification medium and is advantageous in the way that dairy effluents can be used directly as the feedstock for gasification. As lactose and other organics are dissolved in the dairy effluents, SCWG can directly use these liquid wastes, thus

eliminating the additional process expenditures involved in feedstock drying, pretreatment, and component separation. The temperature and pressure above the critical point of water, that is, 374 °C and 22.1 MPa weakens the H-bonding between the organic molecules present and contributes to H2 release through a series of reactions, such as the water gas shift (WGS) reaction, methanation, hydrogenation, and the steam reforming reaction.18 Free-radical chemistry plays a significant role in SCWG. In such reactions, water neither acts as reactant nor catalyst for the organic compounds, although it appears to be a medium for incorporating oxygen functionalities to the hydrocarbons.19 The syngas can be used directly as a fuel or converted to diesel, ethanol, or other hydrocarbons through gas-to-liquid (GTL) technologies, such as Fischer−Tropsch process and syngas fermentation.20 However, the yield of syngas from SCWG depends on several factors, for example, temperature, pressure, residence time, feed concentration, and catalyst concentration. With the depletion of fossil fuels, there is a growing interest in utilizing lignocellulosic biomass and alternative organic wastes for the production of biofuels, biochemicals, and biomaterials.21−24 Although the biological conversion of dairy wastes has been attempted for CH4 generation, there is minimal literature available on their thermochemical conversion. Besides, the chemistry and degradation behavior of lactose as a model dairy waste compound is also lacking. With the intention of valorizing dairy wastes, the study presented here was aimed at converting lactose as a model carbohydrate compound of dairy industry effluents into biofuels through SCWG. Additionally, the effects of temperature, feed concentration, residence time, and catalyst concentration were systematically studied.

2. MATERIALS AND METHODS 2.1. Supercritical Water Gasification Reactor Assembly. The SCWG experiments were performed in a stainlesssteel continuous flow tubular reactor (CFTR). The SCW CFTR reactor, located in the Lassonde School of Engineering at York University, Canada, was designed and tested by Nanda et al.25 The schematics of the reactor assembly are shown in Figure 1. The assembly sequentially consisted of a high9297

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varying their concentrations from 0.2 to 0.8 wt % at a constant (or optimal) temperature, pressure, feed concentration, and residence time. The total gas yield, individual gas yield, and carbon gasification efficiency (CGE) were obtained from the volumetric flow rate of gas products, and quantitative data were subsequently obtained by the gas chromatography analysis. The total gas yield (L/g of lactose) was determined as the volume (L) of total gas products (i.e., hydrogen, H2; carbon dioxide, CO2; carbon monoxide, CO; methane, CH4; and ethane, C2H6) collected at room temperature and pressure per gram of lactose. The individual gas yields (mol/mol of lactose) were calculated as the moles of each gas per mole of lactose. The CGE was calculated as the total moles of carbon in product gases per total moles of carbon in lactose eq 1. The total moles of carbon in the product gas can be obtained by the summation of carbon moles in CO, CO2, CH4, and C2H6.27 The lower heating values (LHV) of gases were calculated using eq 2, as described by Chen et al.28

pressure pump, pressure gauge, relief valve, check valve, preheater, tubular flow reactor, thermocouple, tube cooler, 2 μm filter, back-pressure regulator, gas−liquid separator cylinder, and mass flow meter with other necessary valves and tube connectors. All the tubes and connections used in the assembly were of stainless-steel (SS316) made with pressure ratings more than 35 MPa. The tubes, fittings, filters, valves, and tubing accessories were purchased from Swagelok (Swagelok Central Ontario, Canada). The feed (lactose) solution was passed into the reactor through the high-pressure LabAlliance Prep pump (Scientific Systems Inc., Pennsylvania, USA). The preheater was placed prior to the SCWG reactor to heat the feed solution to a desired temperature range of 80 to 100 °C. The preheater was 12 in long and had an outer diameter of 0.25 in and an inner diameter of 0.12 in. The tubular flow reactor was 18 in long and had an outer diameter of 0.5 in and an inner diameter of 0.37 in. It was placed inside an electronically heated ATS series 3210 furnace (Applied Test Systems, Pennsylvania, USA). The preheater and furnace temperatures were monitored by an ATS Type K, T/C temperature control system (Applied Test Systems, Pennsylvania, USA) and recorded using the Omega USB 4718 portable data acquisition module (Spectris Canada Inc., Quebec, Canada). The water-cooled tube cooler helped the products exiting the tubular flow reactor to condense below room temperature. The pressure inside the SCWG assembly was maintained by a TESCOM 26-1700 series back-pressure regulator (Tescom Corporation, Minnesota, USA). The gas−liquid separator installed after the back-pressure regulator allowed separation of the condensed gasification products (both liquid effluents and gases). The gas products entered through a desiccator (Praxair Canada Inc., Ontario, Canada) to retain any moisture present. The moisture-free gases were passed through Delta Smart II mass flow meter (Brooks Instrument, Pennsylvania, USA) to measure their flow rates before being collected for analysis. Finally, the gas was collected in 500 mL gas-sampling Tedlar bags (Environmental Sampling Supply, California, USA) for analysis by gas chromatography (described below). After each experiment, 2 M H2O2 solution was purged through the entire reactor assembly at room temperature and pressure to remove any organic components that may reside in the reactor.26 Prior to starting a new experiment, the reactor assembly was cleaned by passing deionized water through it at a flow rate of 3 mL/min for an hour. 2.2. Feedstock, Catalysts, and Parametric Studies. DLactose monohydrate was used as the milk sugar or model carbohydrate compound of dairy effluents for supercritical water gasification. All the chemicals used in this study were purchased from Sigma-Aldrich Canada Co., Ontario, Canada. The SCW gasification experiments for lactose solution were performed at 25 MPa to explore the impacts of temperature, feed concentration, residence time, and catalyst concentration. Temperatures in the range of 550−700 °C were used to determine the impact on gas yields and composition. The feed (lactose) concentration was varied from 4 to 10 wt % to understand its effect on the conversion to gases. After identifying the optimal temperature and feed concentration, the effect of residence time was changed from 30 to 75 s by varying the feed flow rate at a constant temperature. Finally, homogeneous alkali catalysts, such as Na2CO3 and K2CO3, were used in an attempt to enhance the gasification efficiency. A comparative evaluation of their effectiveness was made by

CGE (%) = ((total number of carbon moles in CO, CO2 , CH4and C2H6)/(number of carbon moles in feedstock)) × 100

(1)

LHV (kJ/Nm 3) = [(119950.4 × nH 2) + (10103.9 × nCO) + (50009.3 × nCH4)]/V

(2)

where, nH2, nCO, and nCH4 are the molar yields of H2, CO, and CH4, respectively, and V is the volume of gas. 2.3. Gas Chromatography (GC) Analysis. The gas samples were analyzed using an Agilent gas chromatography system 7820A (Agilent Technologies, California, USA). The gaseous products especially H2, CO2, CO, CH4, C2H6, and C2+ were quantified. The GC was equipped with a thermal conductivity detector (TCD), three packed columns, and one capillary column. The carrier gas for TCD was argon. The CO2 and C2+ components were analyzed using an Ultimetal Hayesep T 80/100 mesh column while H2, CO, and CH4 were analyzed using an Ultimetal HayesepQ T 80/100 mesh column. Any N2 and O2 in the gas products were identified through an Ultimetal molsieve13 80/100 mesh column. All the columns (outer diameter: 0.125 in) were maintained at 60 °C. The data were analyzed using Agilent OpenLAB CDS ChemStation software. 2.4. Nuclear Magnetic Resonance (NMR) Analysis. The liquid effluents collected from the gas−liquid separator after each parametric SCWG experiment were analyzed by NMR spectroscopy to determine any residual carbohydrate and resulting hydrocarbons. The NMR analyses were performed using a Bruker AVANCE III NMR spectrometer (Bruker Corporation, Milton, Ontario, Canada) operating at a frequency of 700.28 MHz. The spectrometer was equipped with a 5 mm TXI 1H/13C/15N CPTCI probe operating at 25 °C. The Bruker proprietary TopSpin software (v. 3.2) was used in the analysis. The samples were prepared by combining 540 μL of liquid effluent with 60 μL of D2O in a 5 mm NMR tube. The quantification was completed by the addition of known amounts of standard samples and comparing peak heights to those of the reaction solutions. Water suppression in the 1D (one-dimensional) spectra was achieved using the standard excitation sculpting with a 15 s relaxation delay.29 The 2D (two-dimensional) NMR analyses that were employed to identify the proton (1H) and carbon-13 9298

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Table 1. Effects of Temperature, Feed Concentration, and Residence Time on Total Gas Yield, Carbon Gasification Efficiency, Lower Heating Value of Gases, and pH of the Liquid Effluentsa parameters temp (°C) [60 s, 4 wt % lactose]

feed concn (wt %) [700 °C, 60 s]

residence time (s) [700 °C, 4 wt % lactose]

a

550 600 650 700 4 6 8 10 30 45 60 75

total gas yield (L/g)

carbon gasification efficiency (%)

lower heating value (kJ/Nm3)

pH of liquid effluent

0.43 0.76 1.07 1.69 1.69 1.51 1.36 1.26 1.19 1.41 1.69 1.68

41 61 75 85 85 87 90 93 68 78 85 89

1855 2462 2817 3513 3513 3278 2964 2719 3263 3365 3513 3444

3.39 3.44 3.49 3.53 3.53 3.31 3.20 3.15 3.23 3.41 3.50 3.61

Note: The data presented here are the average of triplicate measurements with standard deviation less than 5%.

(13C) shifts with molecular connectivity were 2D 1H−13C HSQC (heteronuclear single quantum coherence), 2D 1H−13C HMBC (heteronuclear multiple-bond correlation), 2D 1H−1H COSY (correlation spectroscopy) and 2D 1H−1H TOCSY (total correlated spectroscopy). All the pulse programs were used from a standard pulse program library without any modification.

3. RESULTS AND DISCUSSION 3.1. Effects of Temperature. Lactose at a concentration of 4 wt % was gasified in SCW at a constant pressure of 25 MPa and residence time of 60 s to determine the effects of different temperatures (550, 600, 650, and 700 °C). The total gas yields, CGE, and LHV at different gasification temperatures are given in Table 1. With an increase in temperature, the SCWG of 4 wt % lactose showed an increase in the total gas yields from 0.43 L/g (at 550 °C) to 1.07 L/g (at 700 °C). The CGE also increased from 41% (at 550 °C) to 85% (at 700 °C). The results indicate that temperature has a positive impact on total gas yields and CGE. With the increase in temperature, the organics decompose into gases, thus increasing the total gas yields. High temperatures implicate the breaking of C−C bonds, which result in high concentrations of single carbon compounds, such as CO, CO2, and CH4, thereby enhancing the CGE.26 The LHV of gases, which is also dependent on the concentrations of H2, CO, and CH4, increased from 1855 kJ/ Nm3 at 550 °C to 3513 kJ/Nm3 at 700 °C. Water near its critical point serves as an eco-friendly solvent, a reactant, and a catalyst in the reactions involving organic degradation.19 At near-critical temperatures of water, the ionic mechanisms prevail leading to the breakdown of complex organic molecules to simple gases. 28 The free-radical mechanisms are dominant at higher temperatures causing maximum gas yields.20 While organic compounds enjoy high solubility in near-critical water, they experience complete miscibility in supercritical water. Since gases are also miscible in SCW, this environment provides a medium to conduct single-fluid phase reactions unlike multiphase systems under conventional conditions. The advantage of such a single-fluid phase SCW medium is that higher concentrations of reactants are attained with no interphase mass transfer limitations.19 The individual gas yields for H2, CO, CO2, CH4, and C2H6, which were generated from 4 wt % lactose at 550−700 °C, are shown in Figure 2. The H2 yield improved dramatically from

Figure 2. Effect of temperature on gas yields from supercritical water gasification of 4 wt % lactose at 25 MPa with 60 s residence time.

1.8 to 16 mol/mol with an increase in temperature from 550 to 700 °C, which is nearly a 9-fold rise. Similarly, the respective CO2 and CH4 yields increased from 1.5 to 6 mol/mol and 0.6 to 2 mol/mol. While at higher temperatures, the WGS reaction favors higher yields of H2 and CO2, methanation reactions promote the CH4 yields.18,30 In contrast, the CO yields declined from 2.5 mol/mol at 550 °C to 1.5 mol/mol at 700 °C. This difference is because of the enriched WGS reaction and its forward equilibrium shift at higher temperatures that increases the H2 and CO2 yields at the expense of CO.31 The C2H6 yields also increased to 0.36 mol/mol at 700 °C from 0.15 mol/mol at 550 °C. At 700 °C, the yields on the trend of gases produced decreased as H2 > CO2 > CH4 > CO > C2H6. Owing to higher yields of H2 (16 mol/mol), the optimal temperature for lactose gasification in SCW was 700 °C. The liquid effluents exiting the SCW CFTR reactor were collected after cooling prior to determining the carbonaceous components. The nature of the dissociated hydrocarbons and the nondissociated carbohydrates were established by 1H NMR spectroscopy. The NMR analysis of the liquid samples obtained from 550 to 700 °C indicated that with the increase in temperature the number of components produced decreased. Table 2 summarizes the 1H NMR chemical groups of liquid products obtained from the SCWG of 4 wt % lactose at 550− 700 °C and 25 MPa with 60 s of residence time. 9299

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Table 2. 1H NMR Integration of Specific Chemical Groups in the Liquid Effluents Obtained from 4−10 wt % Lactose Gasification at 550−700 °C and 25 MPa Pressure for 30−75 s of Residence Timea Component

effluent from 550 °C effluent from 700 °C effluent from 4 wt % (4 wt % lactose and (4 wt % lactose and lactose (700 °C and 60 s) 60 s) 60 s)

effluent from 10 wt % lactose (700 °C and 60 s)

effluent from 30 s (4 wt % lactose and 700 °C)

Eeffluent from 75 s (4 wt % lactose and 700 °C)

34 15.7 0.5 0.03 2.88 1.31 ND ND 0.44 7.6 0.15 0.15 0.29 3.48 0.34 0.22

10.4 5.0 3.62 0.01 0.33 0.19 ND ND 0.08 3.46 0.09 0.08 0.11 1.71 0.08 0.09

Concentration [M] × 10−3 acetaldehyde 1,1-ethanediol acetone acetic acid ethanol formic acid 2-furaldehyde hydroxymethylfurfural hydroxyquinone methanol m-cresol o-cresol 3-pentanone phenol propionic acid tert-butanol a

44.6 22.3 0.2 1.58 3.24 0.29 0.53 0.42 0.21 1.5 0.05 0.01 0.42 0.02 0.61 0.13

19.3 9.3 0.35 0.23 0.48 0.49 0.02 ND 0.15 5.73 0.06 0.06 0.15 1.27 0.43 0.08

8.4 4.2 0.36 0.02 0.28 0.45 ND ND 0.07 3.31 0.06 0.05 0.09 1.31 0.07 0.03

47.9 25.1 1.43 0.1 0.95 0.76 ND ND 1.05 23.3 0.39 0.4 0.85 4.21 1.42 0.43

Note: Not detected, ND.

The 2D NMR methods (i.e., 1H−13C HSQC, 1H−13C HMBC, 1H−1H COSY, and 1H−1H TOCSY) were used to identify the main components in 1D 1H NMR spectra. Figure 3

analysis, the principal components were assigned and presented in Table 2. The liquid effluents formed at 550 °C contained significantly higher levels of several components, including acetaldehyde, acetic acid, propionic acid, ethanol, tert-butyl alcohol, 2furaldehyde, hydroxymethylfurfural (HMF), hydroxyquinone, and 3-pentanone. The formation of HMF and 2-furaldehyde at lower temperatures (550 °C) is due to autocatalysis by the acid products, such as acetic acid, formic acid, and propionic acid. The lower pH of the effluents at 550 °C relative to that at 700 °C also confirmed the formation of acids as the degradation products of the SCWG of lactose (Table 1). Acetaldehyde was present in both effluents as an equilibrium mixture of the aldehyde and 1,1-ethanediol (acetaldehyde hydrate),32 although there was a significant reduction in the intensity of aldehydeprotons at 700 °C. A few components were found to be present in substantial amounts in the effluents at 700 °C, namely, formic acid, m-cresol, o-cresol, methanol, phenol, and acetone. There was a noticeable increase in methanol and phenol after increasing the SCWG temperature to 700 °C. Acetic acid and propionic acid are usually the intermediate degradation products of the SCW oxidation of food-derived wastes.33 These authors observed a substantial amount of acetic acid in the effluents at lower temperature that is in accordance with our findings. As the temperature increased, these intermediate degradation products are dissociated and become completely oxidized to gases. These organic acids usually undergo autocatalysis33 and pyrolysis34 due to the presence of free radicals of water (·H and ·OH) under supercritical conditions to generate phenols, alcohols, aldehydes, ketones, and aromatics. While the WGS reaction results in H2 and CO2, the secondary reactions, such as methanation and hydrogenation, lead to the formation of CH4. 3.2. Effects of Feed Concentration. Different concentrations of lactose (i.e., 4, 6, 8, and 10 wt %) were gasified in SCW at the optimal temperature of 700 °C and residence time of 60 s. The effects of lactose concentration on the total gas yields, CGE, LHV, and pH are shown in Table 1. The total gas

Figure 3. 2D 1H−13C HMBC NMR spectra of liquid effluents collected from the reactions involving 4 wt % lactose, 25 MPa, and 60 s residence time at 550 and 700 °C.

illustrates the identification of the 1H and 13C nuclei using multiple bond J-coupling NMR spectroscopy. The multiplicity of the proton resonances and the chemical shifts of both the 1H and 13C nuclei assisted in the identification of the molecular fragments. The advantages of 2D NMR are that it permits the linking and identification of different segments of a molecule based upon both 1H and 13C chemical shifts. When used in concert with integration from 1D 1H NMR, the molecules are much easier to assign. Each of the identified chemical components was quantified for the liquid effluents generated at 550 and 700 °C. After completion of the 2D NMR spectral 9300

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Industrial & Engineering Chemistry Research yield decreased from 1.69 to 1.26 L/g and the CGE increased slightly from 85 to 93% with the variation in the concentration from 4 to 10 wt %. The gas yields and CGE are inversely proportional to feed concentration.35 Figure 4 shows the individual gas yields with respect to different lactose concentrations. It can clearly be seen that the

Figure 4. Effect of feed concentration on gas yields from supercritical water gasification of lactose at 700 °C and 25 MPa with 60 s of residence time.

Figure 5. 2D 1H−1H TOCSY NMR spectra of liquid effluents collected from the reactions at 700 °C, 25 MPa, and 60 s of residence time with 4 and 10 wt % lactose concentration.

H2, CO2, and CH4 yields decrease, whereas that of CO increases with increments in feed concentration. The H2 yields decreased from 16 to 8.7 mol/mol with an increase in lactose concentration from 4 to 10 wt %. The CO2 yields also declined from 6 to 4.9 mol/mol at 10 wt % lactose concentration. The decrease in H2 and CO2 yields is attributed to poor WGS reactions at higher feed concentrations, which result in lower reaction rates because of the decrease in water molecules and the increase of feed molecules.18 Additionally, the reforming reactions involving water also have adverse impacts on gas yields.36 The CO yields increased from 1.5 mol/mol at 4 wt % lactose to 3 mol/mol at 10 wt % lactose. Owing to the deprived WGS reaction rates at higher feed concentrations, the maximum amount of CO remains unreacted. However, there was no significant variation in CH4 yields at 4 wt % lactose (2.04 mol/ mol) and 10 wt % lactose (2.1 mol/mol). Furthermore, the yield of C2H6 increased from 0.15 to 0.4 mol/mol with the rise in lactose concentration up to 10 wt %. Similar results were reported by Susanti et al. from the SCWG of glucose over the concentration range of 1.8 to 15 wt %.26 These researchers obtained maximum H2 yields at lowest glucose concentration of 1.8 wt % and 740 °C, 25 MPa, and 60 s. On the basis of the H2 yields, the LHV of gases was relatively higher at 4 wt % lactose (3513 kJ/Nm3) than at 10 wt % lactose (2719 kJ/Nm3) (Table 1). Lv et al. suggested that weaker thermal cracking and steam reforming at higher feed concentrations results in minimum LHV.36 Our results indicate that a lower feed concentration (4 wt % lactose) is favorable for maximum H2 yield (16 mol/mol) and total gas yields (1.69 L/ g) with higher CGE (85%) and LHV (3513 kJ/Nm3). The information obtained from the combination of 2D NMR spectroscopy with 1H−1H geminal and vicinal coupling studies of the effluents generated from 4 and 10 wt % lactose is indicated in Figure 5. The 1H−1H 2D TOCSY spectroscopy assisted in confirming the identity of the components listed in

Table 2. The pH of the liquid effluents generated at 700 °C, 25 MPa, and 60 s residence time varied with different lactose concentrations as 4 wt % (3.53) < 6 wt % (3.31) < 8 wt % (3.2) < 10 wt % (3.15). This was also evident with the higher concentration of organic acids, such as acetic acid, formic acid, and propionic acid in the effluents being generated at 700 °C with 10 wt % lactose. Moreover, the 10 wt % lactose concentration also showed the occurrence of higher levels of acetaldehyde (including 1,1-ethanediol), hydroxyquinone, 2pentanone, acetone, ethanol, tert-butyl alcohol, methanol, phenol, m-cresol, and o-cresol. It is worth noting that when the lactose concentration increased by 2.5 times from 4 to 10 wt %, the concentration of many of the components augmented by roughly a 6-fold factor or more (Table 2). 3.3. Effects of Residence Time. The effect of different residence times (i.e., 30, 45, 60, and 75 s) on the SCWG of lactose at the optimal temperature of 700 °C, optimal feed concentration of 4 wt % and constant pressure of 25 MPa was also investigated. As a result of the increase in residence time from 30 to 75 s, the CGE and total gas yields were enhanced considerably at 60 s with no major difference being noted at 75 s (Table 1). The total gas yields increased from 1.19 L/g at 30 s to 1.69 g/L at 60 s and 1.68 L/g at 75 s. The CGE elevated from 68% at 30 s to 85% in 60 s and 89% in 75 s. In the same way, the LHV of the gas products was greatest at 60 s (3513 kJ/ Nm3) than at 75 s (3444 kJ/Nm3) and 30 s (3263 kJ/Nm3). The SCWG of isooctane has also resulted in a similar effect with respect to residence time.37 These authors reported that CGE showed an asymptotic behavior (reaching to 52.5%) from 18 to 33 s, while the total gas yields improved significantly from 1.1 to 2.9 L/g with isooctane gasification. The individual gas yields from 4 wt % lactose operated at 700 °C with different residence times are illustrated in Figure 6. The H2 yield increased from 10.2 mol/mol (in 30 s) to 16 mol/mol (in 60 s), after which it reduced to 15.4 mol/mol (in 70 s). An 9301

DOI: 10.1021/acs.iecr.5b02603 Ind. Eng. Chem. Res. 2015, 54, 9296−9306

Article

Industrial & Engineering Chemistry Research

The reduction in the yield of CO together with the increase in the amounts of CH4 can be attributed to the greater occurrence of methanation reactions. As the residence time is increased, the reforming reactions, the WGS reaction, and methanation reactions proceed more efficiently, thus enhancing the gas product yields.18,41 However, longer residence times cause the gases (i.e., H2, CO2, and CO) to react among themselves and take part in methanation and hydrogenation reactions to produce predominantly CH4.41,42 On the basis of our observations, the optimal residence time was found to be 60 s at 700 °C using 4 wt % lactose, thereby obtaining a H2-rich gas product (16 mol/mol). Figure 7 shows the 1D 1H NMR spectra of the liquid effluents generated from 4 wt % lactose at 700 °C and 25 MPa with residence times of 30 and 75 s. A clear qualitative distinction can be seen in the peak intensities between the spectra. The concentration of all the organic components detected including ketones, aldehydes, alcohols, phenols, and organic acids decreased with an increase in residence time to 75 s (Table 2). The pH of the effluents showed a drop with the rise in residence time (Table 1), which was attributed to the consumption of organic acids and their degradation to gas products. This observation indicates that longer residence times lead to secondary reactions (viz., methanation, hydrogenation, and WGS reaction) resulting in the formation of gaseous products (primarily H2, CO2, CO, and CH4) leaving behind less organics in the effluents. 3.4. Effects of Catalyst Concentration. The above investigations suggest that there is a possibility of enhancing the H2 yield in the gas products by employing catalysts, which could suppress the undesired reactions (i.e., tar and char formation) in large-scale operations. Owing to the changes in the dielectric constant, there is a limitation in the solubility of

Figure 6. Effect of residence time on gas yields from supercritical water gasification of 4 wt % lactose at 700 °C and 25 MPa.

optimum residence time is required for maximum H2 yields beyond which there is no significant improvement in the yields.38 The highest LHV of 3513 kJ/Nm3 in 60 s was attributed to the highest H2 yield (Table 1). The CO2 yield was also highest (6.03 mol/mol) at 60 s compared to 30 s (4.6 mol/ mol). In addition, the CO yield increased up to 1.63 mol/mol at 45 s and thereby decreased to 1.41 mol/mol at 75 s. On the other hand, the yields of CH4 and C2H6 were highest at 75 s (2.4 and 0.4 mol/mol, respectively) compared to 30 s (1.8 and 0.3 mol/mol, respectively). During the reforming reactions of methanol, glucose, and glycerol in SCW, Byrd et al. noticed higher CH4 levels in the gas products as a result of secondary methanation reactions at longer reaction times.39,40

Figure 7. 1D 1H NMR spectra of liquid effluents collected from the reactions involving 4 wt % lactose at 700 °C and 25 MPa along with residence times of 30 and 75 s. 9302

DOI: 10.1021/acs.iecr.5b02603 Ind. Eng. Chem. Res. 2015, 54, 9296−9306

Article

Industrial & Engineering Chemistry Research an applied alkali metal catalyst in SCW.27 This also imposes a constraint on the concentration of alkali metal catalysts in SCWG of biomass and its components. Alkali carbonates have the potential to improve the WGS reaction; hence K2CO3 and Na2CO3 were used in this study. Different concentrations ranging from 0.2 to 0.8 wt % of these homogeneous catalysts were tested in an effort to improve the yield of H2 at the optimum conditions, namely, 700 °C for a 4 wt % lactose concentration at a residence time of 60 s. The total gas yields, CGE, and LHV, for the different concentrations of K2CO3 and Na2CO3 are presented in Table 3. With increasing concentration of K2CO3 from 0.2 to 0.8 wt Table 3. Effect of Catalyst Concentration on Total Gas Yield, Carbon Gasification Efficiency, Lower Heating Value of Gases and pH of the Liquid Effluentsa catalysts [700 °C, 60 s, 4 wt % lactose]

concn (wt %)

total gas yield (L/g)

carbon gasification efficiency (%)

lower heating value (kJ/Nm3)

pH of liquid effluent

K2CO3

0 0.2 0.5 0.8 0 0.2 0.5 0.8

1.69 1.83 1.96 2.13 1.69 1.84 2.01 2.2

85 87 91 96 85 88 92 97

3513 3625 3661 3715 3513 3601 3702 3741

3.53 6.07 5.28 4.92 3.53 6 5.3 5.01

Na2CO3

a

Note: The data presented here are the average of triplicate measurements with standard deviation less than 5%.

%, the total gas yields improved from 1.83 to 2.13 L/g. Correspondingly, the CGE also improved from 87% to 96% with the rising K2CO3 concentration. In the presence of 0.8 wt % K2CO3, the respective yields of H2 and CO2 increased to 21.48 and 8.01 mol/mol (Figure 8a). K2CO3 improves the total gas and H2 yields by stimulating the WGS reaction.28 Compared to the noncatalytic SCWG of 4 wt % lactose (1.45 mol/mol), the application of K2CO3 substantially increased the CO yields (0.05−0.01 mol/mol). Furthermore, there was an upturn in CH4 yield at 0.2 wt % K2CO3 (2.42 mol/mol) through to 0.8 wt % K2CO3 (2.64 mol/mol) indicating an active methanation reaction compared to noncatalytic gasification. Along with H2 yields, the CO2 yields increased at the expense of CO. The addition of Na2CO3 resulted in higher total gas yields along with improved CGE and LHV of the gas products. The total gas yields escalated from 1.84 to 2.2 L/g with an increase in Na2CO3 from 0.2 to 0.8 wt % (Table 3). Similarly, the CGE also enhanced from 88 to 97% as the Na2CO3 concentration increased from 0.2 to 0.8 wt %. The comparatively higher activity of WGS reaction with 0.8 wt % Na2CO3 resulted in the highest H2 and CO2 yields of 22.4 and 8.15 mol/mol, respectively (Figure 8b). Although the yields of CO (0.01− 0.06 mol/mol) and CH4 (2.4−2.61 mol/mol) were on a par with that of K2CO3, they were significantly greater than were the noncatalytic gasification yields (CO: 1.45 mol/mol and CH4: 2.04 mol/mol). Similarly, C2H6 yields were similar in both scenarios of K2CO3 (0.4−0.45 mol/mol) and Na2CO3 (0.39−0.44 mol/mol). The LHV of gas product from 0.8 wt % Na2CO3 was highest (3741 kJ/Nm3) compared to 0.8 wt % K2CO3 (3715 kJ/Nm3) and noncatalytic gasification (3513 kJ/ Nm3) of 4 wt % lactose at 700 °C and 60 s residence time. This

Figure 8. Effects of (a) K2CO3 and (b) Na2CO3 concentrations on gas yields from supercritical water gasification of 4 wt % lactose at 700 °C and 25 MPa with 60 s of residence time.

result was evidently due to the maximum H2 yield in the case of 0.8 wt % Na2CO3 (22.4 mol/mol) followed by 0.8 wt % K2CO3 (21.48 mol/mol) and noncatalytic lactose gasification (15.97 mol/mol). It can be inferred from the current observations that Na2CO3 is more efficient than K2CO3 in terms of H2 yields, total gas yields, and CGE from lactose gasification. However, during subcritical water gasification of real food wastes, Muangrat et al. reported that K2CO3 resulted in higher yields of H2 compared to that of Na2CO3.43 K2CO3 generates potassium formate (HCOOK) as an intermediate product in SCW.44 Furthermore, HCOOK produces H2 and potassium bicarbonate (KHCO3) during the course of WGS reaction. The decomposition of KHCO3 releases CO2 and K2CO3. Formic acid (HCOOH) is another intermediate product from SCWG of organics that undergoes decarboxylation and dehydration.45 Xu et al. suggested that Na2CO3 increases H2 yield by accelerating the decarboxylation of formic acid.27 Nevertheless, the most widely used alkali homogeneous catalysts are KOH, NaOH, K2CO3, and Na2CO3 due to their ability in breaking C−C bonds together with the simultaneous 9303

DOI: 10.1021/acs.iecr.5b02603 Ind. Eng. Chem. Res. 2015, 54, 9296−9306

Article

Industrial & Engineering Chemistry Research promotion of the WGS reaction.27,30,41 The benefits of using homogeneous catalysts are low cost, faster and higher conversion rates, and flexibility for use in continuous operations.46 In contrast, heterogeneous catalysts reveal a predilection for recovery and reuse with additional expenditures. Although the homogeneous catalysts cannot usually be recovered or reused, the waste stream can be recycled as the process feed. 3.5. Mechanistic Overview of the SCWG for Model Compounds. On the basis of our current findings, an overall reaction mechanism of lactose gasification in SCW was elucidated. Regardless of the biomass species, two reaction mechanisms exist in SCWG, namely ionic reaction mechanism and free-radical mechanism. While ionic reaction mechanism is preferred at near-critical pressures and lower temperatures (