Continuous supercritical water oxidation treatment of oil-based drill cuttings using municipal sewage sludge as diluent

Journal of Hazardous Materials 384 (2020) 121225

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Continuous supercritical water oxidation treatment of oil-based drill cuttings using municipal sewage sludge as diluent Zhong Chena,d, Zhijian Zhengb, Dongyuan Lic, Hongzhen Chena, Yuanjian Xua,d,

T

⁎

a

Key Laboratory of Reservoir Aquatic Environment, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, 400714, China State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China University of Technology, Nanchang, 330013, China c School of Petroleum Engineering, Southwest Petroleum University, Chengdu, 610500, China d University of Chinese Academy of Sciences, Beijing, 100049, China b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Editor: Xiaohong Guan

Oil-based drill cuttings (OBDC) is a characteristic hazardous waste that is generated in oil and gas exploration. In this study, two typical OBDCs from shale gas fields were treated in a continuous supercritical water oxidation (SCWO) for the first time. Because both heat value and ash content (AC) in the OBDCs were well beyond the capacity of continuous operation, municipal sewage sludge (MSS) was innovatively adapted as the diluent. A mixed sludge with OBDC addition levels of 10%, 20%, and 30% was tested using a novel SCWO reactor. Mean residence times of reactants in different reaction zones were specifically calculated. Results indicated the organic carbon removal efficiency could reach up to 98.44%. Eight detected heavy metals were found to be almost completely removed into solid products, and the concentrations in liquid products were all below the discharge limits. It was also found that the SCWO reactor exhibited good anti-plugging and anti-corrosion performance. The AC in the feedstock was up to 28.58%. To the best of our knowledge, this has, hitherto, not been achieved in a continuous SCWO operation. This study provides a new approach for harmlessly and completely degrading OBDC, and is also helpful for the industrialization of SCWO technology.

Keywords: Hazardous waste Oily sludge Anti-plugging Residence time Shale gas

⁎ Corresponding authors at: Environmentally-Benign Chemical Process Research Center, Chongqing Institute of Green and Intelligent Technology (CIGIT), Chinese Academy of Sciences, No. 266 Fangzheng Avenue, Beibei District, Chongqing, 400714, China. E-mail addresses: [email protected], [email protected] (Z. Chen), [email protected] (Y. Xu).

https://doi.org/10.1016/j.jhazmat.2019.121225 Received 9 July 2019; Received in revised form 21 August 2019; Accepted 12 September 2019 Available online 12 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

Journal of Hazardous Materials 384 (2020) 121225

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1. Introduction

Pc = 22.064 MPa). SCWO has a similar treatment performance as that of incineration, but does not produce the noxious pollutants. Moreover, the aqueous wastes or sludge with high water content can be treated without dehydration because water is actually the reaction media in SCWO. Since its advent in 1980s, many studies have proved the SCWO is an excellent end-of-pipe technology for the treatment of a wide range of organic wastes, especially the ones with toxic, hazardous, persistent, and water-rich properties (Veriansyah and Kim, 2007; Bermejo and Cocero, 2006a; Vadillo et al., 2013; Qian et al., 2016; Qin et al., 2017). In our previous work (Chen et al., 2017), we investigated SCWO of OBDC in a batch reactor and the results confirmed the superiority of SCWO. In addition, an inclined plug-flow SCWO reactor was developed for continuous treatment of semi-solid organic waste (Chen et al., 2018a). In this study, the continuous treatment of OBDC using the aforementioned novel SCWO reactor was carried out for the first time. Two types of OBDC samples, (i) a diesel-based drill cuttings form a vertical well (henceforth termed “OBDC-A”) and (ii) a white oil-based drill cuttings form a horizontal well (henceforth termed “OBDC-B”), were tested. Considering the extremely high heat value and ash content (AC) of the OBDC samples, municipal sewage sludge (MSS) with 12% dry solid (DS) content was used as the diluent. A specific method for calculating the residence time of the mixed reactants in the SCWO reactor was demonstrated. Both the SCWO system performance and the product information (gas, liquid, and solid) were examined and discussed in detail.

Fossil oil plays an leading role in the national economic development. China has a vast reserve, but most of them consist of unconventional oil and natural gas resources, whose geological conditions are quite complex (Jia et al., 2016). Oil-based fluids have to be extensively used during the drilling operation as their indispensable drilling functions (Ball et al., 2012). Consequently, a large amount of drill waste, i.e., oil-based drill cuttings (OBDC), is produced in China. For example, the shale gas fields in Fuling District, Chongqing have generated more than 68,000 m3 of OBDC from 61 platforms as of 2015 (Xu et al., 2018). The OBDC, a mixture of oil-based fluids and rock cuttings, is a typical hazardous solid waste with toxic, mutagenic, and carcinogenic properties (Ma et al., 2016). Organic compounds, such as long-chain alkanes, polycyclic aromatic hydrocarbons, and benzenes, are the primary pollutants (Chen et al., 2018b). Current techniques focus on separating such oil organics from the OBDC as an oil content of no more than 1% is generally considered as the discharge limit (Chen et al., 2018b; 2019). The reported separation techniques include surfactant washing (Ma et al., 2016), biosurfactant washing (Yan et al., 2011), switchable solvents extraction (Wang et al., 2017a), supercritical CO2 extraction (Khanpour et al., 2014), pressurized hot water extraction (Chen et al., 2018b; 2019), and microwave heating pyrolysis (Santos et al., 2018; Robinson et al., 2009). These techniques can recycle most of the oil organics from the OBDC, but not all of them. Significant amounts of some complex organics with high molecular weight would still remain in the residuals (Chen et al., 2018b; 2019). Moreover, some other pollutants, namely heavy metals and salts, are ignored during the separating treatment. As a result, the OBDC residuals after separation are still considered as a hazardous waste in some regions. Solidification/stabilization (Kogbara et al., 2016; Leonard and Stegemann, 2010), burial (Ball et al., 2012), offshore discharge (de Almeida et al., 2017), and resource utilization (Ayati et al., 2019; Hejna et al., 2018; Wang et al., 2017b; Kogbara et al., 2017) are the current candidate techniques for the final disposal of such residuals. In addition to the above, decomposition is an essential technology because it provides the only way to degrade or destroy the organic pollutants in the OBDC, and some of the techniques include bioremediation (Ball et al., 2012), phytodegradation (Ji et al., 2004), and incineration (ASME, 2011). However, both bioremediation and phytodegradation possess low efficiency and cannot degrade some complex organics (Ball et al., 2012). Incineration is a highly efficient decomposition technology that can destroy all kinds of organic pollutants, but may result in the emission of secondary pollutants (e.g., dioxins, NOx). Fortunately, supercritical water oxidation (SCWO) provides an alternative way that can completely “burn” the organic compounds in an environment of supercritical water (SCW, Tc = 373.946 °C,

2. Experimental 2.1. Materials The two types of OBDC, mentioned earlier, were sampled from the shale gas wells in Fuling District, Chongqing, China. Their appearances were similar, and both samples were heterogeneous, dark (black) semisolid wastes, smelling strongly of heavy oil. To ensure their uniformity, the samples were ground in a ball mill (GMJ/B, Xianyang Jinhong General Machinery Co., Ltd., China) and then filtered through a 40mesh standard sieve. As shown in Fig. S1 (Supplementary material), the main organics are n-alkanes (n-C12-n-C25), branched-chain alkanes (C15-C20), and naphthalenes (NAPs) in OBDC-A; and n-C12-n-C20 and C15-C20 in OBDCB. More detailed information is listed in Table 1. It can be seen that the mean low heat value is up to 10,900 j g−1 for OBDC-A and 5329 j g−1 for OBDC-B. Both of them exceed the upper operating limit of the SCWO technology (4500 j g−1) (Bermejo and Cocero, 2006a). Moreover, the AC is in the range of 72.88%–81.34%, which is also exceedingly beyond the capacity of continuous operation. Thus, the OBDC should be diluted first. However, the semi-solid emulsion contains both light components (oil organics, ρ < 1gcm-3) and heavy components

Table 1 Properties of the OBDCs and the MSS. Information

OBDC-A

OBDC-B

MSS

Type of drill well Base fluids Water content (wt. %) Oil content (wt. %) Ash content, AC (wt.%) Total organic carbon, TOC0 (mg g−1) Chemical oxygen demand, COD (×104 mg L−1) Lower heat value (j·g−1) Elemental analysis (wt.%) C H N S Others (rest to 100%)

Vertical Diesel 1.01 ± 0.12 20.64 ± 0.26 72.88 ± 0.57 (wet mass) 200 ± 6 (wet mass) 65.5 ± 2.0 (wet mass) 10,900 ± 76 (wet mass) (wet mass) 21.64 ± 0.52 2.55 ± 0.02 0.14 ± 0.02 2.48 ± 0.18 73.20

Horizontal White oil 4.52 ± 0.23 13.18 ± 0.21 81.34 ± 1.59 (wet mass) 96 ± 4 (wet mass) 55.9 ± 2.3 (wet mass) 5329 ± 51 (wet mass) (wet mass) 10.14 ± 0.12 1.58 ± 0.01 0.04 ± 0.01 7.82 ± 0.13 80.42

– – 79.49 ± 0.52 – 49.50 ± 0.33 (dry mass) 211 ± 5 (dry mass) 18.9 ± 0.8 (wet mass) 9790 ± 67 (dry mass) (dry mass) 23.18 ± 0.21 3.62 ± 0.09 3.58 ± 0.15 0.60 ± 0.04 69.02

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16 ± 14 860 ± 125 676 ± 252 351 ± 12 638 ± 121 545 ± 85 700 ± 73 1.86 3.03 3.12 2.94 2.83 3.25 3.08

3.29 4.14 3.47 3.16 5.31 4.77 3.92 408 393 403 416 387 389 399

T' (℃)

510 409 435 490 416 419 444

1.24 2.03 1.67 1.37 1.92 1.86 1.68

t (min)

0.94 0.94 0.96 0.92 0.95 0.96 0.92

2.2. Continuous SCWO treatment system

0 10 20 30 10 20 30

(OBDC-A) (OBDC-A) (OBDC-A) (OBDC-B) (OBDC-B) (OBDC-B)

MSS+12.5%IPA 90 80 70 90 80 70

1.19 1.23 1.23 1.17 1.21 1.23 1.2

6.19 12.66 19.35 26.04 13.50 21.04 28.58

102968 46449 64112 79820 35724 44155 51848

2.96 3.01 3.05 2.98 2.99 3.08 2.95

0.71 0.35 0.43 0.47 0.32 0.37 0.41

As shown in Fig. 1, the SCWO system is comprised of four feed lines (waste, SCW, oxygen, and air) and two discharge lines (top and bottom effluents). The kernel is a “Y” shaped plug-flow reactor, which consists of an inclined left section and a vertical right section. The angle between the two sections is 70°, as displayed in Fig. 2. The left section, with a length of 1000 mm, has been adopted from a dynamic gas seal wall reactor (DGSWR) (Chen et al., 2014, 2015) that was designed to provide a dynamic gas film to minimize the stubborn problems of corrosion and salt deposition in the SCWO technology. It is a triple wall reactor consisting of an outer pressure bearing wall, a middle distribution wall and an inner porous wall. The pressure bearing wall with an inner diameter of 60 mm has been designed for a maximum operating pressure and temperature of 32 MPa and 600 °C, respectively. The distribution wall is a nonporous tube with an inner diameter of 46 mm. Four distributed holes are set on the underside of the distribution wall and their locations are marked Z1–Z4. The porous wall is essentially a sintered wire netting that is made of 20 layers of 316 L stainless steel nets with a pore size of 5 μm. Its inner and outer diameters are 35 mm and 40 mm, respectively. Except for the porous wall, all the walls are made of 316 L stainless steel. As shown in Fig. 2, the right section is basically a vertical tank reactor with an inner diameter of 80 mm and a height of 865 mm. A heat exchanger tube with a length of 10 m and an inner diameter of 4 mm is twined around its outside to recover part of the effluent heat, and to maintain subcritical temperature for the lower section. The left and right sections of the reactor are equipped with two independent electric heating furnaces with a maximum power of 6 KW. They are designed to supply the heat at the start-up and to maintain the reactor temperature during the operating. This novel reactor, which combines the advantages of MODAR reactor (Hong et al., 1989), horizontal stirred reactor (Calzavara et al., 2004), DGSWR (Chen et al., 2014, 2015) (or transpiring wall reactor (Mueggenburg et al., 1995)), and cool wall reactor (Cocero and Martinez, 2004), is specifically designed for continuous SCWO treatment of high solid content organic waste. The operating principle of this reactor is briefly described as follows. Waste at room temperature, SCW at high temperature, and O2 without any preheating are delivered individually and mixed together to be introduced into the reactor by the

0 1 2 3 4 5 6

OBDC (φ, %)

12%MSS (1-φ, %)

FW (L/h)

AC (%)

TOCin (mg/L)

FSCW (L/h)

FO2 (kg/h)

FAir (kg/h)

T (℃) Oxidant SCW Waste (OBDC + MSS) Exp.

Table 2 Operational conditions.

99.94 93.61 96.34 98.44 93.80 95.67 95.34

TOCout (mg/L) OC

t' (min)

Effluents

CRE (%)

± ± ± ± ± ± ±

0.05 0.93 1.36 0.05 1.18 0.67 0.49

(BaSO4, ρ = 4.3 g cm-3). Therefore, the emulsification system would be broken if only water is used as the diluent, which, in turn, would lead to feeding fluctuations or even jeopardize the continuous operation. After repeated investigations, we found that the MSS is an ideal diluent. The flocs in MSS can suspend both light and heavy components in the OBDC. The mixed sludge can maintain a steady state for days, which is a qualified feedstock for the continuous SCWO treatment. The MSS used in this study was procured from the Xiaojia River wastewater treatment plant in Chongqing, China. Its detailed information is also shown in Table 1. The MSS with an initial DS content of 20.51% was diluted to about 12% DS with de-ionized water, and then filtered through a net with pore size of 1 mm to isolate the particles that could cause blockages. The additional mass ratios of the OBDC (φ) were set at 10%, 20%, and 30%, respectively. As listed in Table 2, a total of six experiments were conducted, namely Exps.1–3 for OBDC-A and Exps.4–6 for OBDCB. An additional experiment, namely Exp.0 was also carried out as a controlled test, in which the feedstock is the MSS with a co-fuel of 12.5 wt.% isopropyl alcohol (IPA) (Chen et al., 2018a). Referring to the findings of the continuous SCWO treatment of MSS (Chen et al., 2018a), the flow rates of waste, SCW and air, namely FW, FSCW, and FAir were optimized as 1.2 L h−1, 3.0 L h−1, and 0.94 kg h−1, respectively for all the experiments. The oxygen coefficient (OC) of pure oxygen (FO2) was set at 2.5 according to the results of our previous batch study (Chen et al., 2017).

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Fig. 1. Schematic diagram of the continuous SCWO treatment system.

particles) and downward brine fluid (salts and solid particles with water) with the help of gravity. The remnant organics in the upward supercritical fluid are further degraded in the secondary reaction zone, and finally released out from the top outlet. Meanwhile, the downward brine fluid is collected and released out from bottom outlet. An image of the continuous SCWO system is shown in Fig. S2 (Supplementary material) and more detailed information can be found in a previous study (Chen et al., 2018a). The experimental procedure

mixer (inlet). Air is injected into the reactor by the single side inlet and distributed by the four holes of the distribution wall, and finally flows through the porous wall. It acts as both a transpiring fluid and an additional oxidant. All reactants flow forward along the inner space of porous wall, and the SCWO reaction takes place in the primary reaction zone at the same time. All products flow into the right section at the end of the porous wall. In the in-situ separation zone of the right section, the products are separated into upward supercritical fluid (no solid

Fig. 2. Schematic diagram and modeling of the inclined plug-flow reactor. 4

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(Section S1), product analysis (Section S2), and data expression and interpretation (Section S3) are also included in the supplementary material.

t2 (min) =

FA (L min−1) =

t3 (min) =

t(min) =

π×D2 ⎡ L1 L − L1 F L − L2 ⎤ × + 2 × ln ⎛1 + A ⎞ + 3 × 10−6 ⎢ F1 4 F F F1 + FA ⎥ A 1 ⎝ ⎠ ⎣ ⎦ (11) ⎜

⎟

As shown in Fig. 2, the secondary reaction zone is simplified as a plug flow reactor. The mean residence time (t') is written as:

t ' (min) =

π×D'2 H × × 10−6 4 F1 + FA

(12)

To sum up, the mean residence time of the “Y” shaped reactor is calculated in two distinct parts — t for the primary reaction zone and t' for secondary reaction zone. The density of the fluid can be obtained based on the actual operating pressure (P) and reaction temperature (T or T'). Table S1 in the supplementary material shows the related information of the SCWO system under steady state. Other parameters for the evaluation of Eqs. (2)–(12) can be found in Tables 1 and 2 and Fig. 2. The calculated results for each experiment are also displayed in Table 2. 3.2. Operating performance 3.2.1. Temperature profile Fig. 3 shows the temperature curves (T1–T11) of the six experiments. The exact locations of T1–T11 can be found in Figs. 1 and 2. Each of these curves includes three stages according to the experimental procedure, i.e., start-up (A–B), treatment (B–C), and flushing (after site C). At the end of the start-up (B), T1 and T4–T8 reached up to a steady value of approximately 386 °C for all the experiments, which indicates that the primary reaction zone has already been under supercritical conditions. Meanwhile, the temperatures in the secondary reaction zone (T9–T11) were near but below the supercritical point (Tc = 374 °C). Once the waste feed line was switched from water to mixed sludge, the SCWO treatment began functioning (location B in Fig. 3). Thereafter, barring T1, all the temperatures (i.e., T2–T11) more or less increased. T1 is the inlet temperature of the waste. As illustrated in Fig. 1, T1 is next to T2, which is the inlet temperature of the SCW. The feed of the waste line was also water during the start-up (i.e., A–B). T2 was as high as approximately 550 °C. As a result, the location of T1 was also heated up to the supercritical temperature due to the back-mixing effect of the SCW flow. After switching the feed from water to mixed sludge, the back-mixing was minimized owing to the slurry nature of waste. This was the reason for T1 decreasing dramatically to around 100 °C after location B, and getting restored again after location C, as shown in Fig. 3. Other temperatures also fluctuated obviously after site B, especially T4–T8 in the primary reaction zone. After approximately 60 min,

(3)

F2 (L min−1) = F1 1 L−L1 F × × Air 60 L 2 − L1 ρAir,0

(4)

analogously, the ρAir,0 and ρN2,0 are the density of air and nitrogen, respectively, under ambient temperature and pressure, and ρN2,T,P is the density of nitrogen under supercritical conditions. In an infinitesimal length of the gas film section (ΔL, mm), the related infinitesimal residence time (Δt) can be written as:

π×D2 × ΔL × 10−6 4F2

(10)

Eqs. (3)–(9) can be combined to give Eq. (10) as:

In L1–L2, it is assumed that the air flow is anisotropic with respect to the surface of the porous wall (Chen et al., 2014). With the addition of air flow, the flow rate increases along the length of the porous wall. At location L (Between L1 and L2, the volume flow rate (F2) increases to:

Δt(min) =

(9)

(1)

where ρH20,0 and ρO2,0 are the density of water and oxygen under ambient temperature and pressure, while ρH20,T,P and ρO2,T,P are their densities under supercritical conditions, respectively. These values can be obtained from NIST (National Institute of Standards and Technology, USA) based on the operating parameters, T and P (NIST, 2019). In L1, the residence time (t1) can be expressed as:

ρO2,0 ρN2,0 ⎤ ×⎡ + 0.79 × ⎢0.21 × ρ ρN2,T,P ⎥ O2,T,P ⎦ ⎣

π×D2 × (L3 − L 2) × 10−6 4 × (F1 + FA)

t(min) = t1 + t2 + t3

(2)

+

(8)

In summary, the total mean residence time in the primary reaction zone (t) is expressed as:

ρH2O,0 ρO2,0 1 × [(FW × (1−AC) + FSCW) × + FO2 × ] 60 ρH2O,T,P ρO2,T,P

π×D2 × L1 × 10−6 4F1

ρO2,0 ρN2,0 ⎤ 1 F × Air × ⎡ 0.21 × + 0.79 × ⎢ 60 ρAir,0 ⎣ ρO2,T,P ρN2,T,P ⎥ ⎦

Correspondingly, the residence time t3 in L2–L3 is expressed as:

However, the air is delivered by the side inlet individually. It flows into the primary reaction zone along the length of porous wall. Moreover, the density of both water and gas change dramatically under supercritical conditions. Therefore, a specific method for calculating the residence time has been developed first in this study. As shown in Fig. 2, the primary reaction zone is divided into three section according to the reactor structure, i.e., L1 (nonporous section), L1–L2 (porous or gas film section), and L2–L3 (nonporous section). The feedstock of the left inlet includes waste (Fw), SCW (FSCw), and oxygen (FO2). The volume flow rate of the mixture, F1 can be written as:

t1 (min) =

(7)

where FA is the volume flow rate of air under supercritical condition (T, P):

According to the previous study (Chen et al., 2017), OC, temperature (T) and residence time (t), all significantly affect the oxidation efficiency of OBDC in SCW. OC can be obtained by Eq. (S1). The temperatures in the primary and the secondary reaction zones (T and T', respectively) are defined as the average values of T5 – T7 and T9 – T10 (see Fig. 2), respectively, as listed in Table 2. While the calculation of the actual residence time for a continuous SCWO reactor is much more complicated (Bermejo and Cocero, 2006b; Fauvel et al., 2003; SierraPallares et al., 2016). Generally, the mean residence time is defined as the quotient of the reactor volume (V, L) and the feed volume flow rate (F, L h−1):

F1 (L min−1) =

(6)

FTotal (L min−1) = F1 + FA

3.1. Calculation of the mean residence time

V × 60 F

ΔtdL

In the last section, i.e., L2–L3, all of air is delivered into reaction zone. Then, the total volume flow rate (FTotal) can bewritten as:

3. Results and discussion

t (min) =

L= L2

∫L=L1

(5)

Then, the residence time (t2) in the gas film section (L1–L2) can be obtained through integration as: 5

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Fig. 3. Temperature variation during the continuous operation: (A) start of pressurization by delivering air and de-ionized water, (B) start of delivering waste and O2, (C) switching waste to de-ionized water.

the temperatures tended to become steady. Table S1 lists the average pressures and temperatures in the last 60 min before flushing (at location C). Fig. 3 and Table S1 indicate the difference between the treatment of OBDC-A and OBDC-B. The temperature fluctuation during the treatment of OBDC-A was much more obvious, especially at a high addition level (φ = 30%, Exp.3). The ranges of variation in Exp.3 (30% OBDCA) were 367–378 °C for T4, 450–500 °C for T5, 476–502 °C for T6, 489–508 °C for T7, and 481–507 °C for T8. These values, however, were 402–425 °C, 430–446 °C, 437–452 °C, 441–451 °C, and 441–448 °C, respectively in Exp.6 (30% OBDC-B). Moreover, at the φ, the average temperatures in both primaryand secondary reaction zones were also higher during the treatment of OBDC-A than those in the treatment of OBDC-B, as shown in Fig. 4. This behavior was owing to their different heat values. As listed in Table 1, the average low heat value of OBDC-A was nearly twice that of OBDC-B. Feedstock with a higher heat value meant that more reactive heat would be released out during the SCWO

Fig. 4. Temperature profile of the SCWO reactor under steady state.

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reaction. Besides, the maximum temperature in the primary reaction zone was T7 in Exps.1–3 (OBDC-A) and T6 in Exps.4–6 (OBDC-B), as shown in Fig. 4. The different organic compounds present in the OBDC samples were responsible for the above. OBDC-B is comprised white oil based drill cuttings, as shown in Table 1. The main organics are n-alkanes with carbon atom numbers ranging from 12 to 20 (see Fig.S1), while OBDC-A comprised diesel-based drill cuttings and its organics were much more complicated (see Table 1 and Fig. S1). Generally, an organic with lower molecular weight is more easily ignited under hydrothermal conditions (Zhang et al., 2019). Therefore, the feedstock mixed with OBDC-B was ignited faster than that mixed with OBDC-A. Therefore, the maximum temperature was observed much closer to the inlet during the treatment of OBDC-B, as illustrated in Figs. 1 and 2. Under the combined effects of low warm temperature (450 °C), less reactive heat, and cooling of the heat exchanger tube, the temperatures in the secondary reaction zone (T9–T11, T') were much lower than these in primarily reaction zone (T4–T8, T), as shown in Table 2 and Fig. 4. Hence, the residence times (t and t') were calculated individually in this study (see Section 3.1). Furthermore, an abnormal phenomenon could be noticed in Fig. 4, where the temperatures in the primary reaction zone (T4–T8) of Exp.0 were higher than these of Exps.1–6, but the tendency in the secondary reaction zone (T9–T11) was contrary to the former. T11 in Exp.0 even decreased to a minimum value of 343 °C, owing to a different operating procedure employed. The warming temperature of the secondary reaction zone was set as 450 °C for Exps.1–6, as described in Section S1 (supplementary material), while the warm furnace was shut down in Exp.0 (Chen et al., 2018a). Meanwhile, the temperature in the separation zone (T11) was always maintained at a subcritical temperature (around 360 °C), similar to the sub-zone in the MADAR reactor (Hong et al., 1989). This design was meant to re-dissolve the salts and collect the solid products.

Fig. 5. In-situ solid separation efficiency.

in Eq. (S4)) solid remained in the reactor after each run. Moreover, no visible solid particles were found in the top effluents. This suggested that the plug flow inthe left section and the in-situ solid separation in right section were achieved as intended. A certain weight of ARS would be retained in the primary reaction zone due to the inclined structure. But it did not accumulate further in the subsequent experiments. In other words, with the help of gravity, solids from the feedstock were continually pushed forward in the left inclined section and then separated into upward supercritical fluid and downward brine fluid in the right vertical section.

3.3. Degradation of organic pollutants As listed in Table 2, the TOC removal efficiency (CRE, defined as Eq. (S2)) ranged from 93.61% to 98.44% for OBDC-A, and 93.80–95.67% for OBDC-B, respectively. These values were much higher than those in the batch tests, where the maximum CRE was only 89.2% within 10 min at 500 °C (Chen et al., 2017). Such a difference proves the superiority of continuous operation. However, the TOC in the liquid products were still high, and both H2 and CO were found in gaseous products, as shown in Fig. 6. All the important parameters, namely the reaction temperature, residence time, and OC, were responsible for the aforementioned result. As shown in Table 2, the temperature appeared to depend mainly on the heat value (TOCin) of the waste. T in the primary reaction zone and T' in secondary reaction zone were 409 °C and 393 °C, respectively at 10% OBDC-A, and they increased up to 490 °C and 416 °C, respectively at

3.2.2. In-situ solid separation efficiency and pressure fluctuation One of the primary challenges for the continuous SCWO treatment of the OBDC is the high AC. As listed in Table 2, the AC was up to 26.04% in Exp.3 (30% OBDC-A) and 28.58% in Exp.6 (30% OBDC-B). It must be noted that AC was only the weight percentage of inorganic solids, and did not include the organic solids. Taking the MSS in Exp.0 as an example, the DS (dry at 105 °C) was 12.05%, while the AC (burnt at 800 °C) was only 6.19%, as listed in Table 2. Therefore, the real solid contents in the feedstock are higher than the AC values in Table 2. During the tests, it was found that the pressure fluctuations were negligible at the locations of P2, P3, and P4, but P1 was volatile, especially in Exp.3 and Exp.6. As illustrated in Fig. 1, P1 and P2 were the pressures of the waste feed line and the SCW feed line, respectively. The pressure fluctuation at only P1 suggested that plugging occurred somewhere in the waste feed line, but not inside the reactor. The inner diameter of the waste feed line was only 4 mm and there was a square elbow, shown as a marked yellow pentagon in Fig. 1 (next to V3). As a result, the waste feed line was prone to plugging, mainly at the location of the square elbow. It was observed during the experiment that P1 increased instantaneously when the plugging occurred. In most cases, the blockage could be extruded out under a maximum pressure of no more than 30 MPa, and then the pressure decreased back to normal. Several severe incidents took place in both Exp.3 and Exp.6. P1 continued to increase up to 33 MPa in less than 1 min. Then the waste feed line had to be shut down. A hammer was used to knock at the location of the square elbow repeatedly in order to shatter the blockages. Luckily, it worked and then the waste feeding continued. It must be emphasized that this is an unsafe procedure and should be avoided in normal course of operation. Hence, values of φ higher than 30% was excluded in this study. Surprisingly, there was no other plugging found during the operating. Fig. 5 corroborates the good anti-plugging performance of the SCWO reactor. The solid recovery (SR, defined in Eq. (S3)) efficiency was in the range of 83% to 105%, and approximately 65 g (ARS, defined

Fig. 6. Analysis result of gaseous products. 7

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Z. Chen, et al.

previous batch SCWO study (Chen et al., 2017), the residual solid was still with a gray or light brown color, while it completely changed to brick red color in this study. Its appearance was similar to nature soil. It is obvious in Fig. 7 that the solid products blended the features of both the OBDC and the MSS, which was also confirmed by the XRD results shown in Fig. S3. The characteristic peaks of BaSO4, which were only present in OBDC, became more and more obvious with the increase in φ. However, no new peak appeared in the XRD curve. This behavior suggested that the microstructure and the crystalline structure of the solids were not changed during the continuous SCWO treatment.

30% OBDC-A. Similar phenomena were also observed in treatment of OBDC-B. With the temperature increasing, the densities of water, air, and oxygen would increase, which further led to the reduction of the residence times (t and t'), according to Eqs. (11) and (12). Meanwhile the OC also decreased with the increase in TOCin. Therefore, T, t, and OC are all dependent variables of the feedstock, which is the main point of difference when compared to the batch operation (Chen et al., 2017). Comparing Exp.5 and Exp.6, it can be observed that T increased from 419 °C at 20% OBDC-B to 444 °C at 30% OBDC-B, while both of OC and t decreased. The counter-effect caused similar CRE values, as shown in Table 2. Besides, the temperature played a leading role in most of the experiments. With the increase in T, TOCout decreased and CRE increased linearly in Exps.1-3. The decrease in both H2 and CO contents in the gaseous products was also an evidence of the dominant effect of the temperature, as shown in Fig. 6. The previous batch SCWO treatments of OBDC (Chen et al., 2017) and oily sludge (Cui et al., 2009) confirmed that CO was the main stable intermediate. This could explain the presence of a considerable percentage of CO in the gas products. Furthermore, CO was an excellent reductant for the H2 production via water–gas shift reaction (CO + H2O = CO2 + H2) (Sato et al., 2015). As a result, small quantity of H2 was detected in the gas products. The presence of CO and H2 suggested that the SCWO reaction was not completed. As shown in Table 2, even though the best organic removal efficiency was up to 98.44%, it was not adequate. Results from the controlled experiment (Exp.0) suggested that a higher efficiency could be reached at a higher TOCin of the feedstock. It was found from Table 2 and Fig. 6 that the TOCout was only 16 mg L−1and no CO and H2 were produced in Exp.0. This result suggested that φ should be further increased to reach a higher TOCin. However, the feedstock with a φ value greater than 30% would lead to plugging problems in the waste feed line, as discussed in section 3.2.2. Thus, the design flaw in the waste feed line hindered further improvement of both treatment capacity and efficiency, and should be addressed in future. Nevertheless, the organic degradation performance was acceptable. The liquid effluents were colorless and transparent. According to the reaction pathway (Chen et al., 2017; Cui et al., 2009; Mylapilli and Reddy, 2019; Yao et al., 2018), the remaining organics were typically oxygen-containing organics with small-molecular weight, which mainly consisted of formic acid and acetic acid. They are biodegradable and harmless for the environment. The appearance of solid products shown in Fig. 7 demonstrated the excellent treatment efficiency. In the

3.4. Behavior of heavy metals The metal removal ratio, MRR, was chosen to represent the removal efficiency of the heavy metals. It indicates the prevention of heavy metal transfer from solid phase into liquid products, which would be harmful to the environment. A total of eight heavy metals in both solid and liquid products, i.e., mercury (Hg), nickel (Ni), chromium (Cr), copper (Cu), cadmium (Cd), lead (Pb), zinc (Zn), and iron (Fe), were quantified for all the experiments. Of these, Ni, Cr, and Fe were the main elements of the reactor materials (316 and 316 L stainless steel) (Yao et al., 2018), and others were the common heavy metals found in the OBDC (Xu et al., 2018). The results shown in Table 3 indicate that the concentrations of all the eight heavy metals were below the standard values (Chinese National Standard, 1996). Furthermore, Hg, Cd, and traces of Pb were not detected in the liquid products. The MRR values in Table 4 suggest the metals were efficiently removed into solid products. This was owing to the extremely low solubility of metal salts in supercritical water, and they could be efficiently separated in-situ with the help of gravity in left separation zone (see Fig. 2). These findings are also in agreement with previously published works (Yao et al., 2018; Zou et al., 2013). It was also observed in Table 4 that the MRR for Ni, Cr, and Fe exceeded 100%, which suggests the reactor corrosion occurred. Corrosion in SCWO is expected and has been widely reported (Bermejo and Cocero, 2006a; Vadillo et al., 2013; Qian et al., 2016; Yang et al., 2019). An effective way to alleviate this problem is to design a well-structure reactor, which was one of the main objectives of our previous study (Chen et al., 2018a). The MRR for Ni ranged from 1241% to 2535% in the previous batch study (Chen et al., 2017; Yao et al., 2018), and it sharply decreased to no more than 155% in this study, as listed in Table 4. This result indicated that the corrosion problem in our novel

Fig. 7. Images of feedstock and solid products. 8

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Table 3 Heavy metal concentrations in the liquid products (mg/L). Exp.

Hg

Ni

Cr

Cu

Cd

Pb

Zn

Fe

1 2 3 4 5 6 Limits (Chinese National Standard, 1996)

< 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 0.05

0.08 0.11 0.08 0.15 0.16 0.07 1.00

0.01 0.03 0.05 0.04 0.03 0.67 1.50

0.20 0.19 0.33 0.36 0.34 0.12 0.50

< 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 0.10

0.01 < 0.01 < 0.01 0.06 0.23 0.05 1.00

0.46 0.83 0.52 0.71 0.64 0.29 2.00

0.09 0.06 0.09 0.18 0.14 0.19 Not available

Appendix A. Supplementary data

Table 4 Metal removal ratio in the solid products (%). Exp.

Hg

Ni

Cr

Cu

Cd

Pb

Zn

Fe

1 2 3 4 5 6

94 101 94 90 82 72

138 100 101 151 154 155

168 96 95 132 123 144

79 87 101 82 96 98

98 97 104 106 95 100

90 96 96 87 89 95

103 98 104 101 100 98

103 116 151 100 128 146

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.121225. References ASME, 2011. Drilling Fluids Processing Handbook. Elsevier, Boston. Ayati, B., Molineux, C., Newport, D., Cheeseman, C., 2019. Manufacture and performance of lightweight aggregate from waste drill cuttings. J. Clean. Prod. 208, 252–260. Ball, A.S., Stewart, R.J., Schliephake, K., 2012. A review of the current options for the treatment and safe disposal of drill cuttings. Waste Manage. Res. 30, 457–473. Bermejo, M.D., Cocero, M.J., 2006a. Supercritical water oxidation: a technical review. AIChE J. 52, 3933–3951. Bermejo, M.D., Cocero, M.J., 2006b. Destruction of an industrial wastewater by supercritical water oxidation in a transpiring wall reactor. J. Hazard. Mater. 137, 965–971. Calzavara, Y., Joussot-Dubien, C., Turc, H., Fauvel, E., Sarrade, S., 2004. A new reactor concept for hydrothermal oxidation. J. Supercrit. Fluids 31, 195–206. Chen, Z., Wang, G., Mirza, Z.A., Yang, S., Xu, Y., 2014. Study of transpiring fluid dynamics in supercritical water oxidation using a transparent reactor. J. Supercrit. Fluids 88, 117–125. Chen, Z., Wang, G., Yin, F., Chen, H., Xu, Y., 2015. A new system design for supercritical water oxidation. Chem. Eng. J. 269, 343–351. Chen, Z., Chen, Z., Yin, F., Wang, G., Chen, H., He, C., Xu, Y., 2017. Supercritical water oxidation of oil-based drill cuttings. J. Hazard. Mater. 332, 205–213. Chen, Z., Chen, H., Liu, X., He, C., Yue, D., Xu, Y., 2018a. An inclined plug-flow reactor design for supercritical water oxidation. Chem. Eng. J. 343, 351–361. Chen, Z., Zhou, J., Chen, Z., Chen, H., Chen, Q., He, C., Liu, X., Xu, Y., 2018b. A laboratory evaluation of superheated steam extraction process for decontamination of oil-based drill cuttings. J. Environ. Chem. Eng. 6, 6691–6699. Chen, Z., Li, D., Tong, K., Chen, Z., Chen, H., Chen, Q., Xu, Y., 2019. Static decontamination of oil-based drill cuttings with pressurized hot water using response surface methodology. Environ. Sci. Pollut. Res. 26, 7216–7227. Chinese National Standard, 1996. GB 8978–1996, Integrated Wastewater Discharge Standard. Environmental Protection Agency of China, Beijing. Cocero, M.J., Martinez, J.L., 2004. Cool wall reactor for supercritical water oxidation: modelling and operation results. J. Supercrit. Fluids 31, 41–55. Cui, B., Cui, F., Jing, G., Xu, S., Huo, W., Liu, S., 2009. Oxidation of oily sludge in supercritical water. J. Hazard. Mater. 165, 511–517. de Almeida, P.C., Fernandes Araujo, Od.Q., de Medeiros, J.L., 2017. Managing offshore drill cuttings waste for improved sustainability. J. Clean. Prod. 165, 143–156. Fauvel, E., Joussot-Dubien, C., Pomier, E., Guichardon, P., Charbit, G., Charbit, F., Sarrade, S., 2003. Modeling of a porous reactor for supercritical water oxidation by a residence time distribution study. Ind. Eng. Chem. Res. 42, 2122–2130. Hejna, A., Piszcz-Karas, K., Filipowicz, N., Cieslinski, H., Namiesnik, J., Marc, M., Klein, M., Formela, K., 2018. Structure and performance properties of environmentallyfriendly biocomposites based on poly (ε-caprolactone) modified with copper slag and shale drill cuttings wastes. Sci. Total Environ. 640–641, 1320–1331. Hong, G.T., Killilea, W.R., Thomason, T.B. Method for solids separation in a wet oxidation type process, U.S. Patent, NO. 4822497, 1989. Hu, G., Li, J., Zeng, G., 2013. Recent development in the treatment of oily sludge from petroleum industry: a review. J. Hazard. Mater. 261, 470–490. Ji, G., Yang, Y., Zhou, Q., Sun, T., Ni, J., 2004. Phytodegradation of extra heavy oil-based drill cuttings using mature reed wetland: an in situ pilot study. Environ. Int. 30, 509–517. Jia, C., Pang, X., Jiang, F., 2016. Research status and development directions of hydrocarbon resources in China. Petrol. Sci. Bull. 01, 2–23 (in Chinese). Khanpour, R., Sheikhi-Kouhsar, M.R., Esmaeilzadeh, F., Mowla, D., 2014. Removal of contaminants from polluted drilling mud using supercritical carbon dioxide extraction. J. Supercrit. Fluids 88, 1–7. Kogbara, R.B., Ayotamuno, J.M., Onuomah, I., Ehio, V., Damka, T.D., 2016. Stabilisation/ solidification and bioaugmentation treatment of petroleum drill cuttings. Appl. Geochem. 71, 1–8. Kogbara, R.B., Dumkhana, B.B., Ayotamuno, J.M., Okparanma, R.N., 2017. Recycling stabilised/solidified drill cuttings for forage production in acidic soils. Chemosphere 184, 652–663. Leonard, S.A., Stegemann, J.A., 2010. Stabilization/solidification of petroleum drill cuttings. J. Hazard. Mater. 174, 463–472.

SCWO reactor was efficiently minimized. In other words, the idea of triple wall reactor with a dynamic gas film worked as intended during continuous SCWO operating.

4. Conclusion and outlook Two typical OBDC samples from a shale gas field were treated using a novel continuous SCWO reactor. MSS with 12% DS was innovatively adapted to dilute the OBDC samples as their over-limits of both heat value and AC. The mean residence times of the reactants in different reaction zones were calculated. The results indicated that the MSS with a maximum OBDC addition level of 30% could be safely and efficiently degraded. The reactor exhibited good response against both corrosion and plugging problems. The AC in the feedstock was up to 28.58%. To the best of our knowledge, this is a new achievement in the continuous SCWO technology. However, a design flaw was also found. The waste feed line was prone to plugging, which would prevent further improvement of both the treatment capacity and the efficiency. Nowadays, safe management of OBDC is a big challenge in the petroleum industry. This study provides an alternative approach for dealing with the above issue. In addition, the mixed sludge used in this study can be considered as a representative of real industrial wastes, which possess the properties of complex, toxic, hazardous, refractory, and highly water, organic and solid constituents. A large amount of industrial waste is being generated every day, especially the oily sludge (Hu et al., 2013). The technical superiority, as well as the sustainability with respect to the operating cost of the SCWO technology becomes obvious as it effectively treats the aforementioned waste. Based on the beneficial results of this study, pilot investigations are expected to be conducted in future. The outcome of this study will be helpful for the industrialization of the SCWO technology.

Acknowledgments This work was financially supported by Natural Science Foundation of Chongqing, China (cstc2019jcyj-msxmX0415), West Light (Young Scholar) Foundation of the Chinese Academy of Sciences (Y93A030M10), Open Project Program of the State Key Laboratory of Petroleum Pollution Control (PPC2016001), the CNPC Research Institute of Safety and Environmental Technology, and the National Natural Science Foundation of China (41763008). 9

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Z. Chen, et al.

52, 7617–7629. Veriansyah, B., Kim, J.-D., 2007. Supercritical water oxidation for the destruction of toxic organic wastewaters: a review. J. Environ. Sci. 19, 513–522. Wang, S., Zheng, C., Zhao, J., Li, X., Lu, H., 2017a. Extracting and recovering diesel from oil-based drill cuttings using switchable hydrophilic solvents. Chem. Eng. Res. Des. 128, 27–36. Wang, C., Lin, X., He, M., Wang, D., Zhang, S., 2017b. Environmental performance, mechanical and microstructure analysis of concrete containing oil-based drilling cuttings pyrolysis residues of shale gas. J. Hazard. Mater. 338, 410–427. Xu, T., Wang, L., Wang, X., Li, T., Zhan, X., 2018. Heavy metal pollution of oil-based drill cuttings at a shale gas drilling field in Chongqing, China: a human health risk assessment for the workers. Ecotoxicol. Environ. Saf. 165, 160–163. Yan, P., Lu, M., Guan, Y., Zhang, W., Zhang, Z., 2011. Remediation of oil-based drill cuttings through a biosurfactant-based washing followed by a biodegradation treatment. Bioresour. Technol. Rep. 102, 10252–10259. Yang, J., Wang, S., Li, Y., Zhang, Y., Xu, D., 2019. Novel design concept for a commercialscale plant for supercritical water oxidation of industrial and sewage sludge. J. Environ. Manage. 233, 131–140. Yao, G., Chen, Z., Chen, Q., Li, D., Xie, Z., Zhou, Y., Xiong, X., Xu, Y., 2018. Behaviors of organic and heavy metallic pollutants during supercritical water oxidation of oilbased drill cuttings. Water Air Soil Pollut. 229, 1–13. Zhang, J., Wang, S., Ren, M., Lu, J., Chen, S., Zhang, H., 2019. Effect mechanism of auxiliary fuel in supercritical water: a review. Ind. Eng. Chem. Res. 58, 1480–1494. Zou, D., Chi, Y., Dong, J., Fu, C., Wang, F., Ni, M., 2013. Supercritical water oxidation of tannery sludge: stabilization of chromium and destruction of organics. Chemosphere 93, 1413–1418.

Ma, J., Yang, Y., Dai, X., Chen, Y., Deng, H., Zhou, H., Guo, S., Yan, G., 2016. Effects of adding bulking agent, inorganic nutrient and microbial inocula on biopile treatment for oil-field drilling waste. Chemosphere 150, 17–23. Mueggenburg, H.H., Rousar, D.C., Young, M.F. Supercritical water oxidation reactor with wall on duits for boundary flow control, U.S. Patent, NO. 5387398, 1995. Mylapilli, S.V.P., Reddy, S.N., 2019. Sub and supercritical water oxidation of pharmaceutical wastewater. J. Environ. Chem. Eng. 7, 103165. NIST, 2019. Thermo Physical Properties of Fluid Systems. Available from: http:// webbook.nist.Gov/chemistry/fluid/. Qian, L., Wang, S., Xu, D., Guo, Y., Tang, X., Wang, L., 2016. Treatment of municipal sewage sludge in supercritical water: a review. Water Res. 89, 118–131. Qin, Q., Wang, S., Wang, H., Ma, H., Chen, K., Qiao, Y., He, L., Qian, Z., Liu, X., Li, Z., Xia, X., 2017. Treatment of radioactive spent extraction solvent by supercritical water oxidation. J. Radioanal. Nucl. Chem. 314, 1169–1176. Robinson, J.P., Kingman, S.W., Snape, C.E., Barranco, R., Shang, H., Bradley, M.S.A., Bradshaw, S.M., 2009. Remediation of oil-contaminated drill cuttings using continuous microwave heating. Chem. Eng. J. 152, 458–463. Santos, J.M., Petri, I.J., Mota, A.C.S., Morais, Ad.S., Ataíde, C.H., 2018. Optimization of the batch decontamination process of drill cuttings by microwave heating. J. Petrol. Sci. Eng. 163, 349–358. Sato, T., Sumita, T., Itoh, N., 2015. Effect of CO addition on upgrading bitumen in supercritical water. J. Supercrit. Fluids 104, 171–176. Sierra-Pallares, J., Huddle, T., Alonso, E., Mato, F.A., Garcia-Serna, J., Cocero, M.J., Lester, E., 2016. Prediction of residence time distributions in supercritical hydrothermal reactors working at low Reynolds numbers. Chem. Eng. J. 299, 373–385. Vadillo, V., Sanchez-Oneto, J., Portela, J.R., Martinez De La Ossa, E.J., 2013. Problems in supercritical water oxidation process and proposed solutions. Ind. Eng. Chem. Res.

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