Apparatus and procedure

Figure 1 shows the SCWO system that was employed in this study[18]. The system was jointly designed by the Shanghai Institute of Applied Physics (China) and Supercritical Fluid Technologies (USA) and manufactured by Supercritical Fluid Technologies. This system mainly comprises a feeding system, preheater, reactor, heat exchanger, pressure control valve, and gas–liquid separator. Among them, the reactor is made of Inconel 625 (volume = 200 mL), while the preheater is made of Inconel 625 (volume = 250 mL). The facility is designed to withstand a temperature and pressure of up to 600 °C and 28.4 MPa, respectively. The pressure of the entire system was controlled by a back-pressure valve, and CV was <0.03%. The preheater and reactor were heated by electrical heating wires that were coiled around its outer wall. The feedback controls of the temperature and pressure were achieved with a proportional integral derivative (PID) electric actuator. The feeding system includes two high-pressure metering injection pumps, which were employed to feed the water and organic phases a flow rate ranges of 1–100 and 0–36 mL/min, respectively. Additionally, an online pH probe was set before the gas–liquid separator (the front panels allow access to the pressure vessels, valves, fittings, and electronics). Moreover, two rupture disc assemblies offered mechanical protection against the overpressure of the system as an additional safety precaution (one was incorporated with the pipes between the water pump and preheater, and the other was placed between the cooler and back-pressure regulator.

Fig. 1Full-screen View

Schematic of the SCWO system

Before each experiment, the temperatures and pressures of the preheater and reactor were increased to the desired values with deionized water. Thereafter, the fluid comprising H₂O and H₂O₂ was first introduced into the preheater via Pump 1, after which the feed solution was directly introduced into the reactor via Pump 2. The effluent (exiting the top of the reactor) was cooled rapidly after passing through the heat exchanger (cooler) and was depressurized to ambient pressure through the back-pressure regulator. Afterward, the effluent was introduced into the gas–liquid separator. Next, the gas products were transported to a gas chromatograph for composition analysis. The liquid products were sampled with tubes, and each experiment was performed three times within 60 min after 20 min of stable running.

Materials and analytical methods

TBP (AR, 98.5%), H₂O₂ (AR, 30%, W/W), NaNO₃ (AR, 99%), Na₂S₂O₃·5H₂O (AR, 99%), KClO₄ (AR, 99.5%), and K₂Cr₂O₇ (AR, 99.8%) were purchased from Sinopharm Chemical Reagent Co., China. Deionized water (18.2 MΩ) was prepared employing a Milli-Q ultrapure water purification system with a 0.22 µm filter.

TOC of the liquid effluent was analyzed employing a TOC analyzer (Shimadzu TOC-L CSH, Japan). Three samples were measured, and the average was utilized to calculate the rate of TOC removal. Furthermore, the gas composition was determined by a gas chromatograph (Agilent GC 7890A, Agilent Technologies, Inc. USA), which was equipped with 1). a thermal conductivity detector (TCD) and a G3591-80013 Q packed column (helium was employed as the carrier gas at a flow rate of 40 mL/min); 2). flame ionization detector (FID) and a G3591-80013 Q packed column (N₂ was employed as the carrier gas at a flow rate of 40 mL/min). The column, TCD, and FID temperatures were maintained at 50, 250, and 300 °C, respectively.

The amount of feed added was calculated, following the IAPWS-IF97 guide (http://www.iapws.org/relguide/IF97-Rev.html).

Conditions

In this study, the feed comprised two phases, namely the water and organic phases. Among them, the water phase was H₂O + H₂O₂ in proportion, and the organic phase was the solution, which was prepared, as presented in Table 1. To minimize the influence of high temperature (preheating) on the oxidants and improve the reliability, NaNO₃, Na₂S₂O₃, KClO₄, and K₂Cr₂O₇ were first added to TBP and stirred continuously for 12 h under sealed conditions at room temperature (25 °C) until they were completely mixed. The composition of the feed is listed in Table 1.

Table 1Full-screen View

Composition of the feeds

Feed	Composition
1	TBP	1000 ml	-	-
2	TBP	1000 ml	NaNO3	849.9 mg (10 mmol)
3	TBP	1000 ml	Na2S2O3	2481.8 mg (10 mmol)
4	TBP	1000 ml	KClO4	1385.5 mg (10 mmol)
5	TBP	1000 ml	K2Cr2O7	2941.8 mg (10 mmol)
Feed 1	Feed 2	Feed 3	Feed 4	Feed 5

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Here, all the experiments were performed at a pressure of 25 MPa (3625 ± 50 psi) and a temperature range of 450–550 °C (±1 °C). Before each experiment, the SCWO system was fully rinsed with deionized water to eliminate any possible interference.

Calculation

The residence time (t) is approximately the period from when the feed entered the reactor to when it left. It depends on the feed rate (Q), reaction temperature, and pressure and can be calculated employing Eq. (1): $t=\frac{V_0}{Q} \times \frac{V}{V_{\text{r}}},$ (1) where V₀ is the reactor volume (200 mL in this case); Q and V are the volumetric flow rate and specific volume of the liquid effluent under room temperature (25 °C) and pressure (0.1 MPa) conditions, respectively; and V_r is the specific volume of the feedstock at the reaction temperature and pressure (25 MPa).

The feed concentration refers to the volume ratio of the organic phase in the SCWO system. It can be employed to evaluate the processing capabilities of the system. The feed concentration (c) was determined from Eq. (2): $c =\frac{Q_{\text{o}}}{Q_{\text{w }}+ Q_{\text{o}}},$ (2) where $Q_{\text{o}}$ and $Q_{\text{w}}$ are the volumetric flow rates of the organic and aqueous phases, respectively.

TOC was determined to evaluate the degradation efficiency of the organics in the SCWO system. The TOC of the liquid effluent was measured by a TOC analyzer, and the TOC removal of the liquid effluent was calculated via Eq. (3): $\text{TOC removal }=\left(1-\frac{TO{C}_{\text{L}}}{TO{C}_0}\right) \times 100\%,$ (3) where TOC_L and TOC₀ are the concentrations of the TOC in the liquid effluent and feedstock, respectively, considering the dilution effects of H₂O and H₂O₂.

The oxidant stoichiometric ratio (OSR) of H₂O₂ was calculated by Eq. (4), as follows: $\text{OSR} =\frac{{[H_2{O}_2]}_r}{{[H_2{O}_2]}_0}\times 100\text{\%,}$ (4) where ${[{\text{H}}_2{\text{O}}_2]}_0$ is the theoretical concentration that is required for the complete oxidation of the organic matter, following the stoichiometry of the reaction, and ${[{\text{H}}_2{\text{O}}_2]}_{\text{r}}$ is the specific concentration employed in each experiment. Further, [H₂O₂] is determined by the following equations (see Eqs. (5)–(7)) considering that H₂O₂ is completely decomposed into oxygen during preheating[18, 28]: ${\text{2H}}_{\text{2}}{\text{O}}_{\text{2}}\to {\text{H}}_{\text{2}}{\text{O + O}}_{\text{2}}$ (5) ${\left({\text{C}}_{\text{4}}{\text{H}}_{\text{9}}\right)}_{\text{3}}{\text{PO}}_{\text{4}}{\text{+ 18O}}_{\text{2}}\to {\text{12H}}_{\text{2}}{\text{O + 12CO}}_{\text{2}}{\text{+ H}}_{\text{3}}{\text{PO}}_{\text{4}}$ (6) That is, ${\left({\text{C}}_{\text{4}}{\text{H}}_{\text{9}}\right)}_{\text{3}}{\text{PO}}_{\text{4}}{\text{+ 36H}}_{\text{2}}{\text{O}}_{\text{2}}\to {\text{30H}}_{\text{2}}{\text{O + 12CO}}_{\text{2}}{\text{+ H}}_{\text{3}}{\text{PO}}_{\text{4}}$ (7)

TOC Removal employing SCWO

In SCWO, the reaction temperature is among the most critical factors that affect the degradation of organic matter. Based on our previous work[19], the degradation efficiencies of the organic matters were studied under the following conditions: a temperature range of 450–550 °C, 4% (V/V) TBP, and a residence time of 45 s with/without additional oxidants. The injection volume of H₂O₂ was the theoretical amount required for the complete degradation of TBP according to Equation (4), and the results are presented in Table 2.

Figure 2 shows the degradation of TBP under different conditions. Figure 2a shows the TOC value of the effluent, and 2b shows the TOC removal of the influent. The figure shows that temperature plays a very crucial role in the degradation of organic matter. TOC of the effluent decreased significantly, TOC decreased rapidly from >7500 to <1500 ppm, and the TOC removal increased from <70% to >90% as the temperature increased, especially from 450 to 500 °C. Thermodynamically, the increase in the temperature would activate more molecules whose energy exceeds the apparent activation energy (E_a), thereby promoting the reactions and causing the further degradation of by-products. Regarding the kinetics, the increase in the temperature causes an increase in the apparent rate constant (K_a), following the Arrhenius equation, and this accelerates the degradation reaction [29, 30]. Additionally, temperature also causes a change in the density of the reaction medium, which consequently affects the degradation process of organic matters.

Fig. 2Full-screen View

Degradation of TBP under different oxidation conditions: a. TOC of the effluent with temperature b. TOC Removal of influent with temperature

Most of the organic matt wereer degraded when the temperature reached ≥500 °C, and the effect of the temperature on TOC was no longer significant. However, the TOC of the effluent remained roughly stable. It is very likely that some substances, which cannot be readily degraded or exhibit very slow degradation kinetics, have accumulated in the system. Thus, such substances were analyzed via gas chromatography–mass spectrometry (GC–MS). Meanwhile, it is possible that the addition of an extra oxidant exerted a certain effect on the removal of the organic matter. At 475 °C, the additional oxidant reduced TOC from ~5500 to ~3000 ppm. Put differently, the addition of NaNO₃, Na₂S₂O₃, KClO₄, or K₂Cr₂O₇ enhanced the degradability of the SCWO system. Namely, under the same degradation effects, the addition of an oxidant can effectively lower the required temperature of the system, thus reducing the energy consumption and the negative impact of high temperature on the equipment. Further, at >500 °C, the addition of NaNO₃, KClO₄, or K₂Cr₂O₇ reduced TOC of the effluent from ~1200 to <500 ppm, obtaining increased degradation efficiency.

Study of SCWO capacity

Regarding an SCWO system that is coupled with an ionic oxidant, the change in the capacity of the oxidant also requires elucidation, and no relevant study has been conducted to date. To further study the effect of additional oxidants on the SCWO system, the degradation behavior of SCWO at different TBP concentrations was studied under the following conditions: 550 °C, 45 s, and 25 MPa, and the results are presented in Table 3.

Figure 3 shows the degradation of TBP at different feed concentrations. The degradation of each system began to diverge as the TBP feed increased. TOC of the effluent clearly increased sharply from ~550 to ~9500 ppm, and the TOC removal decreased from ~98% to ~85% as the TBP concentration increased from 4% to 10% without an extra oxidant. When the TBP concentration increased to >10%, an insoluble organic matter appeared in the effluent. Compared with Feed 1, TOC decreased from >4000 to <1000 ppm owing to the addition of NaNO₃, Na₂S₂O₃, KClO₄, or K₂Cr₂O₇ by increasing the TBP concentration to 6%, indicating that an additional oxidant could effectively improve the removal of the organic matter at this concentration. However, as the feed concentration increased to 8%, the effect of KClO₄ on the removal of the organic matter reduced, and TOC increased to >5000 ppm. Meanwhile, the addition of NaNO₃, Na₂S₂O₃, or K₂Cr₂O₇ could still maintain a low level for TOC of the effluent in this situation. When the feed concentration increased to 10%, the additions of NaNO₃ and K₂Cr₂O₇ maintained the TOC of the effluent at <500 ppm and that of Na₂S₂O₃ was ~1800 ppm. By increasing the feed grains to 12%, the addition of K₂Cr₂O₇ maintained the TOC of the effluent at <1500 ppm, which was ~3000 and 5500 ppm for Na₂S₂O₃ and NaNO₃, respectively. Thus, it can be reasonably concluded that the addition of an oxidant could effectively increase the oxidation capacity of the original SCWO system. Further, compared with KClO₄ and Na₂S₂O₃, NaNO₃ and K₂Cr₂O₇ exerted a better effect on SCWO at high feed concentrations (10%).

Fig. 3Full-screen View

TOC at different feed concentrations a. TOC of the effluent with the feed concentration b. TOC Removal of the influent with the feed concentration

It is inferrable that the addition of a small amount of an ionic oxidant might affect the SCWO system in the following aspects: 1) promote the generation of free radicals in the system, 2) interact with the free radicals in the system to extend their existence, 3) avail other oxidation fragments while participating in the reaction, 4) and increase the ion reaction share in the SCWO system. Thus far, there has been no direct evidence regarding the role of ionic oxidants in it; thus, further research is still required.

Analyses of the gas products

In the above experiments, the generated gas was directly introduced into the gas chromatograph for online detection. The gas products of each SCWO system are shown in Fig. 4 with the exclusion of O₂.

Fig. 4Full-screen View

Effects of the reaction temperatures on the gas products

Under the research conditions of this work (residence time = 45s, pressure = 25 MPa, and the organic feed concentration = 4% (V/V), the results demonstrated that the main gas products included CO₂, CO, CH₄, H₂, and small amounts of other organic gases. The high temperature and strong oxidizing properties of SCWO availed a favorable environment for the production of these gases. The organic matter underwent different reactions in SCWO, including free-radical, pyrolysis, hydrolysis, and ionic reactions [31]. Water is a reaction medium, which accelerates reaction processes; simultaneously, it also participates in the reaction, supplying a certain amount of H· free radicals and producing a small amount of hydrogen[32]. Notably, a small part of TBP would be carbonized at the feed inlet, and the generated tar/jar/coke would produce CO and H₂ via the water–gas shift reaction at a high temperature. The reactions that significantly affect the gaseous products mainly include steam reforming, water–gas shift, methanation, and minor hydrogenation reactions (see Eqs. (8)–(13))[33-35]. ${\text{CH}}_x{\text{O}}_y\text{+ }\left(\text{2-}y\right){\text{H}}_{\text{2}}\text{O}\to \left(\text{2-}y\text{+}x\text{/2}\right){\text{H}}_{\text{2}}{\text{+ CO}}_{\text{2}}$ (8) ${\text{CH}}_{\text{x}}{\text{O}}_y+\left(1-y\right){\text{H}}_{\text{2}}\text{O}\to \left(1-y+x/2\right){\text{H}}_{\text{2}}\text{+ CO}$ (9) ${\text{H}}_{\text{2}}\text{O + CO}\to {\text{CO}}_{\text{2}}{\text{+ H}}_{\text{2}}$ (10) ${\text{4H}}_{\text{2}}{\text{+ CO}}_{\text{2}}\to {\text{CH}}_{\text{4}}{\text{+ 2H}}_{\text{2}}\text{O}$ (11) ${\text{3H}}_{\text{2}}\text{+ CO }\to {\text{CH}}_{\text{4}}{\text{+ H}}_{\text{2}}\text{O}$ (12) ${\text{2CO + 4H}}_{\text{2}}\to {\text{2CH}}_{\text{4}}{\text{+ O}}_{\text{2}}$ (13)

Figure 4 reveals that the reaction temperature significantly affected the composition of the gas-phase products, especially at 500 °C. As the temperature increased from 450 to 500 °C, the CO₂ content of the product also increased significantly along with other incompletely oxidized gases, such as CO, CH₄, H₂, and the other organic gases, which gradually decreased. Namely, as the temperature increased, these non-CO₂ gases (incompletely oxidized gases) were further oxidized. When the temperature reached 500 °C, CO₂ was the main product in the gas, and its content reached >99%; the remaining component of the gas was CO, which was <1%. In addition to CO₂, most of the remaining gaseous products were CO. At 450 °C, the proportion exceeded 30%. Similarly, as the proportion decreased rapidly, the temperature increased. When the temperature increased to 500 °C, the proportion of CO decreased sharply to <1% probably because of the following two reasons: 1). at high temperatures, organic matters tend to be completely oxidized and converted into CO₂ rather than other incomplete gas products; 2. CO was further oxidized into CO₂, thereby reducing the CO content. Compared with CO₂ and CO, the contents of CH₄, H₂, and other organic gases were relatively low under all the conditions employed in this study. The proportions of CH₄, H₂, and the total amount of the other gases were <1%, <5%, and <2%. As the temperature changed, their changes were largely the same, i.e., they decreased with the increasing temperature until they became undetectable. Notably, relative to the temperature dependence of the production of the other gases, that of CH₄ production did not exhibit much dependence on the temperature probably because the temperature was related to the low content of CH₄ and the generation process of CH₄, as well as the subsequent oxidation process.

The additions of the oxidants (NaNO₃, Na₂S₂O₃, KClO₄, and K₂Cr₂O₇) to the SCWO system also exerted a certain effect on the gas products. At <500 °C, the addition of the oxidants increased the non-CO₂ components significantly. Compared with the TOC in the liquid effluent (previous chapter), the SCWO system increased the degradation rate of the organic matters via the addition of the oxidants. Therefore, more gases were produced under the same conditions; these gas products were not further oxidized, and this increased their proportion. When the temperature increased to ≥500 °C, the proportion of CO₂ increased rapidly to >90%, and those of the other gases decrease sharply until they were below the detection limit, indicating that temperature significantly affected the gas product at ≥500 °C under the conditions of this study.

Figure 5 shows the effects of the feed concentration on the gas products. This part of the study was conducted at a reaction temperature of 550 °C, the residence time of 45 s, and pressure of 25 MPa. As observed, CO₂ was still the main gas product, and its proportion was >50%. As expected, the non-CO₂ components increased with the increasing feed concentration. During the degradation process, an increasing number of non-CO₂ gaseous products were generated as the organic matters in the SCWO system increased. Figure 5 shows that the proportion of CO increased rapidly and the TBP feed in the system increased. The highest proportion of CO was >38.25%. However, the proportion of CH₄ also increased slightly. The CH₄ concentration was <2%. In the SCWO process, the carbon in the organic matter was more likely to be converted into CO₂ and CO than into CH₄. Moreover, increasing the feed concentration benefitted the production of CO. Under the experimental conditions employed in this study, the highest proportions of CH₄, H₂, and the other organic gases were 1.99%, 5.26%, and 6.54%, respectively.

Fig. 5Full-screen View

Effects of the feed concentration on the gas products

Notably, the addition of ionic oxidants, especially KClO₄, exerted an inhibiting effect on the gas products, although its impact on the gas products was almost negligible. As the concentrations of the organic matter in the system increased, their influence on the gas products became increasingly negligible. Meanwhile, the additions of NaNO₃, Na₂S₂O₃, and K₂Cr₂O₇ exerted a certain inhibitory effect on the productions of CH₄ and H₂. Particularly, when the concentration of organic matter in the system was <10%, the additions of the oxidants would effectively reduce the productions of H₂ and CH₄.