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A new system design for supercritical water oxidation
Accepted Manuscript A New System Design for Supercritical Water Oxidation Zhong Chen, Guangwei Wang, Fengjun Yin, Hongzhen Chen, Yuanjian Xu PII: DOI: Reference:
S1385-8947(15)00177-1 http://dx.doi.org/10.1016/j.cej.2015.02.005 CEJ 13257
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
6 December 2014 29 January 2015 3 February 2015
Please cite this article as: Z. Chen, G. Wang, F. Yin, H. Chen, Y. Xu, A New System Design for Supercritical Water Oxidation, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.02.005
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Research Article A New System Design for Supercritical Water Oxidation Zhong Chen a,b,c, , Guangwei Wang a,b,c, Fengjun Yin a,c, Hongzhen Chen a,c, Yuanjian Xua,c, *.
a
Chongqing Institute of Green and Intelligent Technology (CIGIT), Chinese Academy of Sciences,
Chongqing, P. R. China, 400714.
b
Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of
China, Tsinghua University, Beijing, P. R. China, 100084.
c
Key Laboratory of Reservoir Aquatic Environment, Chinese Academy of Sciences, Chongqing, P.
R. China, 400714.
Corresponding author: Prof. Yuanjian Xu.
Phone number: +86 23 65935819.
E-mail: [email protected].
Address: Environmentally-Benign Chemical Process Research Center, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, No.266 Fangzheng Avenue, Shuitu Hi-tech Industrial Park, Shuitu Town, Beibei District, Chongqing, P. R. China, 400714.
Abstract As the main obstacles for the industrialization of supercritical water oxidation (SCWO) technology, corrosion and plugging are mostly occurring in the high pressure high temperature (HPHT) sections, including preheater, reactor, heat exchanger and cooler. In this paper, a lab-scale SCWO system based on dynamic gas seal wall reactor (DGSWR) has been described, tested and discussed in detail. The results showed that the preheating problems of waste with high solid content has been solved and the “gas seal” of DGSWR has been successfully verified under 28-29MPa and around 400℃. Sewage sludge with 2.62-11.78% dry solid has been degraded and the COD removal efficiency can reach up to 99.15%. However, the solid particle sedimentation was only partly achieved. According to the results analysis, based on the Stokes’ Law, both small particle size and counter-current of upward reaction medium and downward solids are responsible. Future improvements for the SCWO system were also discussed at the end of this article.
Keywords:
Anti-corrosion; Anti-plugging; System design; Dynamic gas seal wall
reactor; Supercritical water oxidation
1. Introduction For environmental awareness, water is undoubtedly the optimal reaction medium. But at room temperature, the reaction is too slow for most redox reactions, especially for the destruction of organic wastes. One of main reasons is the solubility in water for both organic waste (mostly nonpolar) and oxidant (mostly oxygen) are
very low at room temperature. Interestingly, this property is overturned in supercritical water (SCW, Tc=373.946 ℃ , Pc=22.064 MPa [1]), which can be completely miscible with organic compounds and oxygen [2]. Besides, SCW also possess other unique properties [2, 3], such as high diffusivity and density, low viscosity and inorganic solubility. Supercritical water oxidation (SCWO) is a redox reaction to destroy organic compounds in SCW with the participation of oxidant (such as air, oxygen and hydrogen peroxide etc.) [2]. In general, most of organics can be nearly completely degraded by SCWO in less than 1 minute [3], and the products are the environmentally acceptable effluents, such as H2O, CO2, and N2 etc. [4]. As the excellent
merits,
the
SCWO
has
been
hailed
as
an
emerging
and
environmentally-benign technology for the treatment of various hazardous wastes in the last three decades [4, 5]. The SCWO, as such a great potential technology, its commercial development is actually far lag behind the expectation. Most of the full-scale commercial plants have been shut down and only two of them are in operation as of January 2012 [6]. Corrosion and plugging are the main obstacles [2-7]. To overview the typical SCWO process (Fig.1), it can be found that both corrosion and plugging are mainly occurring in the high pressure and high temperature (HPHT) sections, and the details are discussed as fellow: ·Preheaters. The oxidant preheater can be slightly corroded in the presence of water, such as the wet air without dehydration. The ion product of water will maximize under subcritical conditions, which will lead to corrosion problems in both pure water
preheater and aqueous waste preheater [8-10]. Some salts dissolved in the waste stream will precipitate out with the increasing of temperature [10] and some organic compounds in the aqueous waste will polymerize in the absence of oxidant [3, 10]. Both of salt precipitation and polymerization may lead to plugging problems in waste preheater. ·Reactor. The heteroatoms (S, Cl, P, N, etc.) contained in the organic waste will be dissociated to form corresponding acids in reactor. The SCWO reactor, where is a harsh chemical and physical environment of presence of acids, high concentration of oxidant, high temperature and high pressure, should undergo severe corrosions [8-10]. The inorganic salts for several reasons [7] would like to precipitate from SCW to scale on the surface of reactor and leads to severe plugging problems. ·Cooler and Heat Exchanger. Same as in the process of preheating, the cooling and heat exchanging should undergo the subcritical conditions, under which the corrosion of water is much higher [8]. The effluents containing various acids, if without neutralization, will lead to severe corrosion in cooler and heat exchanger, which is more severe than that in reactor [8-10]. Part of salts failed to separate in reactor will also result in plugging problems in cooler and heat exchanger. So far, a “super material” that can withstand all corrosion conditions in SCWO has not yet been reported [8]. If any, the plugging and others problems will also hinder the development of SCWO. Therefore an appropriate system design for SCWO is necessary. In this paper, the focus of anti-corrosion and anti-plugging has been expanded
from reactor to the whole HPHT sections (see Fig.1). A novel reactor concepts named as “Dynamic Gas Seal Wall Reactor” (DGSWR) [11] was adopted, which was optimized from “Transpiring Wall Reactor” (TWR) and was designed to handle the reactor corrosion and plugging problems. A technology of multi-feed injection was designed to handle the waste preheating problems. A lab-scale SCWO device based on this novel design was manufactured and tested under 28-29MPa around 400℃.
2. A new SCWO system design A new SCWO system with a maximum treatment capacity of 2 kg h-1 at 15% dry solids (DS) has been designed in CIGIT based on the preliminary researches [11, 12]. The system was particularly descripted in the following four parts as illustrated in Fig. 2.
2.1. Reactor design As the heart of SCWO, reactor always suffers from both corrosion and plugging. Several types of reactors have been invented to handle these problems. The related reviews can be found in literatures [2, 7-9, 13]. The DGSWR was adapted in the new system. The basic structure of DGSWR is the same as that of a TWR: a double wall reactor consists of outer pressure bearing wall and inner transpiring wall (also named as porous wall). Transpiring fluid fills the annulus between the two walls and continuously flows through the transpiring wall to form a protective film on the inner surface of transpiring wall. The protective film is a mobile surface and can protect transpiring wall from corrosion and salt deposition [2]. Pure water was used as the
transpiring fluid in TWR and was replaced by air in DGSWR, which is the essential difference between these two types of reactors. Based on the special physical properties of air, DGSWR can enhance the anti-corrosion and anti-plugging of TWRs and to avoid the demerits of TWRs. The feasibility of DGSWR has been proved in previous research [11]. Two of the same units named as Reaction Vessel 1 and Reaction Vessel 2 (see Fig.2) were connected by Flange (see Fig.3) to constitute the reaction area of the new system. Both of them are the type of DGSWR. Each unit with height of 500 mm consists of pressure bearing wall and porous wall. The pressure bearing wall with inner diameter of 45 mm was made of 316L stainless steel and can withhold a pressures up to 50MPa. The porous wall is the type of sintered wire netting [14] and made from 316L stainless steel. It was made of outer 32 floors nets with pore size of 2×10-5 m and inner 5 floors nets with pore size of 5×10 -6 m. Its inner diameter and thickness are 29.5 mm and 5 mm respectively. In the new system design, air is not only the oxidant but also the transpiring fluid of DGSWR. The air must flow through the porous wall and mix with wastes in reaction area, and then the reaction would happen. Therefore, the sooner they mix the better. The preliminary research [11] indicates that the transpiring fluid tends to flow through the porous wall around the transpiring fluid inlet section and upper section of porous wall. Based on the characteristic of DGSWR and the transpiring fluid dynamics aforementioned, a pair of symmetrical air inlets with inner diameter of 2 mm was set at the lower section of each reaction vessel (see Fig.2). The distances
from the bottom to inlets are 50 mm for Air 1 and 110 mm for Air 2, respectively. Air was supplied by Air Compressor B (Atlas, GX4FF-10) and was divided into two branches, Air 1 and Air 2. Air 1 was delivered into Reaction Vessel 1 by Booster Pump B and the flow rate was controlled by Flow meter B from SEVENSTAR (D07-9E) with measuring range from 0.00 to 8.33×10 −4 m3s−1 (standard conditions). Air 2 was delivered into Reaction Vessel 2 by Booster Pump A and the flow rate was controlled by Flow meter A from SEVENSTAR (D07-7B) with measuring range from 0.00 to 1.67×10 −4 m3s−1 (standard conditions). Both booster pumps (STT60AL, Shineeast) were driven by Air Compressor A (Atlas, GXe11FF-10). The feasibility analysis of DGSWR suggests that air in low temperature has a better performance on the “gas seal” and the heat capacity of air is much lower than that of water [11], therefore the air was not preheated in the new system. In consideration of the high energy consumption in start-up and the heat dissipation during operation, each reaction vessel was equipped with a heating jacket with maximum power of 3 kW.
2.2. Multi-feed injection Besides the reactor, waste preheater is also vulnerable to attack by corrosion and plugging. In addition, it’s not easy to pump multiphase wastewater over a pressure of 22.1 MPa, such as oil-water mixture and oil-water-solid mixture. Several full scale SCWO plants have been shut down due to these reasons [6]. Hydrothermal flame, which was first reported in 1986 by E.U. Franck from University of Karlsruhe [15], is one of the effective methods to solve the preheating problems. Auxiliary fuel (such as isopropyl alcohol, methanol, etc.) is injected into
SCWO reactor through a special device and “burn” with oxidant. The flame with temperature of typically over 1000℃ is a heat source and then the waste water can be fed at subcritical temperature or even room temperature [3]. This technology has been applied in several types of SCWO reactors in the last two decades [16-21]. Oxygen multi-injection developed in CNRS of France is another effective way [22]. Waste water was fed at subcritical or near-critical temperature and oxygen was injected through three inlets along the tubular reactor [23-25]. The temperature of mixture increases along the length of reactor as the reaction is exothermic, and the critical point of water is overcome in reactor rather than in preheater (like most designs). This design can reduce the energy consumption of preheating and makes the control of thermal agitation easy [3]. And by coincidence, both of preheating at low temperature (less than 350℃[26]) and presence of oxidant avoid the polymerization of organics in waste water. The corrosion of subcritical water and the polymerization of organics are the main reasons of operation problems in waste preheater and may be avoided by this design. V. Vadillo and coworkers from University of Cadiz in Spain reported a new feed system for multiphase wastewater in SCWO [26]. A non-aqueous waste with no preheating and an aqueous waste over 400℃ ware pumped independently and mix at the beginning of reactor to form a supercritical homogeneous phase. The new feed system, which can avoid the problems of pumping, preheating and thermal control, is another candidate. Based on the advantages of above technologies (hydrothermal flames [16],
oxygen multi-injection [22] and new feed system [26]), a new methods named as multi-feed injection was designed in the new SCWO system, as shown in Fig.2. Stream 1 (Aqueous waste or Water with Fuel) is pressurized by Pump C and preheated by Heat Exchanger, Preheater C and Preheater B in turn, and finally introduced into the Reaction Vessel 1 in the bottom. The inlet temperature of Stream 1 (T2 in Fig.2) is over 500℃ typically. Stream 2 (e.g., sewage sludge, oil waste, etc.) was stored in Tank A and Tank B and one operates after the other, alternatively. Water is delivered by Pump A to drive the piston in Tank A or Tank B and then the waste is extruded out into Preheater A, and eventually introduced into Reaction Vessel 1 on the top. The inlet temperature of Stream 2 (T3 in Fig.2) ranges from room temperature to 350℃ depended on the type of waste. Due to gravity, stream 2 with a higher density flows down and mixes with upward Stream 1 in the Reaction Vessel 1. Part of the waste is oxidized in Reaction Vessel 1 and the rest is further degraded in Reaction Vessel 2. Pump A (2J-X 2/50) with maximum pressure of 50MPa and rated flow of 2 L h-1 was purchased from Hangzhou Zhejiang Petrochemical Equipment CO., LTD, China. The effective volume of each tank (Tank A and Tank B) with inner diameter of 95 mm is 2.5 L. Preheater A with maximum power of 4 kW was designed as a single straight tube heater to avoid some blind spaces and to reduce the flow resistance. The length and inner diameter of the tube are 750 mm and 8 mm, respectively. As shown in Fig.3, the inlet and distributor with the same inner diameter of 8mm are set on the flange and the Stream 2 would be dispersed by the distributor.
Pump C (2J-X 4/50) was supplied by the same manufactory as Pump A and the rated flow is 4 L h-1. The two preheaters for Stream 1 are both the type of coil electric heater. The maximum power is 4KW for Preheater B and 3KW for Preheater C. Each heater was made of a single tube with inner diameter of 8 mm and length of 6.5m.
2.3. Gravity sedimentation To avoid plugging, one of the dominant technologies is the reverse flow reactor with a brine pool [7]. Salts precipitated in supercritical zone (upper section of reactor) will fall down to the subcritical zone (lower section of reactor) and re-dissolve in quench water [13, 14], and then the salts can be removed by brine effluent. Only for the salt separation purposes (without waste oxidation), the recovery of this design can reach up to 97% for type 1 salts [27], 95% for type 2 salts and mixtures of two salts [28]. But for degradation of salt containing waste water, the efficiency is only 65% [29], 5~55% [30] and even much lower [31]. It must be noted that the waste water in these researches [27-31] is artificial and the salts is the type of sticky salts that has good water-solubility. But many potential SCWO applications (such as sewage sludge, oily sludge and so on) contents not only sticky salts, but also non-sticky solid [2]. The non-sticky solid can’t be dissolved by the quench water and will lead to plugging problems in following units, such as heat exchanger, cooler and so on. The new design in this paper intends to separate both sticky salts and non-sticky solid. The mixture of salts and solids are designed to be settled by gravity and stored in the Solid Collector (see Fig.2), batched emptied. The Solid Collector with a length of 300 mm has the same inner diameter as pressure bearing wall, and the effective
volume is 0.45 L. As shown in Fig.2, a Condenser was set up to quench the effluents from Reaction Vessel 2. The basic structure of the Condenser is as same as the Reaction Vessel units. There is only one inlet with inner diameter of 2 mm in the middle of Condenser. Quench Water was delivered by Pump B that is exactly the same as Pump A. Besides cooling, the Condenser can separate salts and solids that failed to settle in Solid Collector. On one hand, the sticky salts will re-dissolve in the quench water; on the other hand, the non-sticky solid particles tend to be captured by the subcritical water and stored in the Condenser. Such design tries to enhance the effect of gravity sedimentation and to avoid the plugging problems in heat exchanger and cooler.
2.4. Heat recovery and pressure control In order to achieve a self-sustaining running, heat exchanger was always used to recovery the heat of effluents. This method was also adapted in the new SCWO system. As shown in Fig.2, the Effluent was cooled in Heat Exchanger and Cooler, in turn. Both Heat Exchanger and Cooler are type of single double-pipe. The effective heat-exchanging surface is 9.11×10-3 m2 for Heat Exchanger and 1.53×10 -2 m2 for Cooler. Eleven K-type thermocouples with range from 0℃ to 800℃ were adapted to measure the temperatures and the exact positions were shown in Fig.2. In order to protect thermocouple from corrosion, a tube with inner diameter of 2mm was equipped for each thermocouple (T1-T4, T10-T11, see Fig.1 and Fig.2), and one single tube with inner diameter of 4mm was equipped for thermocouples of T5-T9. They are
evenly spaced for T5-T9 and the distance of two neighboring positions is 100mm. The system pressure was controlled by a back pressure valve as usual designs. In order to avoid operation problems caused by solids in effluent [3], a Filter with pore size of 5×10 -5 m was placed between Cooler and Pressure Control (see Fig.2). The solid particles failed to separate will be captured by the Filter. So the other function of the Filter is to check the separation efficiency of gravity sedimentation.
3. Experimental tests and results Sewage sludge from Xiaojia River wastewater treatment plant in Chongqing was selected as the typical waste in the tests. The properties of the sewage sludge are shown in Table 1. The ash of sewage sludge is 45.53% of its dry mass. In order to increase the fluidity, the sewage sludge with an initial solid content of 19.73% DS was diluted to 2.62 - 11.78% DS. The diluted sewage sludge is the Stream 2 in the tests of the new SCWO system. As most methods, isopropanol was added as the co-fuel [32] in this paper. The aqueous of isopropanol or de-ionized water was used as feedstock of Stream 1. The concentrations of isopropanol in the aqueous ranged from 0.0 wt. % to 3.0 wt. %. Five experiments were carried out in the study and operation conditions were shown in Table 2.
3.1 Operation results In general, a complete operation process includes the following steps: pressurizing, heating, pumping Stream 1, pumping Stream 2, sampling, depressurizing
and cooling. The temperatures of the SCWO system was adjusted timely by switching on and off the Preheating A, B, C and heating jackets of reactors. Table 3 exhibits the pressures and temperatures under steady state of each operation conditions. In addition, the operation processes of Exp.1, Exp.3 and Exp.5 were recorded in details, and the temperatures variation curves are shown in Fig. 4, Fig.5 and Fig.6, respectively. To preheat sewage sludge directly from room temperature to supercritical condition will lead to plugging problems. F.J. Yin et al. [33] suggest the preheating temperature should be no more than 200℃, under which the sewage sludge maintains good fluidity. Hence, the inlet temperature of Stream 2 (T3) in this study was controlled below 200℃ (see Table 3). According to operation steps, the SCWO system would be preheated to supercritical conditions before pumping Stream 2, which leads to the location of T3 was also warmed up to over 300℃ during heating step. As shown in Fig. 4-6, the T3 will decrease to appropriate temperature ranges while the sewage sludge was pumped into reactor. But it must be pointed out that an interrupt or a low flow rate of Stream 2 will increase the risk of plugging at the position of T3 because both of them will result in temperature rise of T3. Therefore, a steady Stream 2 with a mass flow rate of over 0.5 kg h-1 is necessary in the new SCWO system. As shown in Figs.4-6, T1, T4 and T5-T11 (except T2 and T3) have gone up after pumping Stream 2. While reaching steady state, the temperature of low inlet section is higher than that of upper outlet section in Reaction Vessel 1, and the temperature of
middle section is the highest in Reaction Vessel 2 (see Table 3). Such temperature distribution was caused by feeds inlet temperatures, heating jackets and heat dissipation. The temperature of Stream 1 dropped from 572-597℃ (T2) to 385-454℃ (T1) by the cooling of Air 1 (which was not preheated) and heat dissipation of Solid Collector (no heating jacket installed). In Reaction Vessel 1, Stream 1 of 385-454℃ mixes with Air 1 of room temperature and Stream 2 of 149-188℃, which leads to T1>T4. The exothermic reaction of sewage sludge leads to the T1 increased about 50℃ at the moment of about 80 min after feeding Stream 2, as shown in Figs.4-6. In Reaction Vessel 2, the mixture from Reaction Vessel 1 was cooled by Air 2 and the top area was cooled due to heat dissipation, which causes T5≈T6T9. There is an interesting phenomenon in Exp.1 that T3 sharply rose about 100℃ and the T1, T4-T11 decreased slightly about 2-7℃ when switching Stream 2 from sewage sludge (2.62% DS) to pure water at same flow rate, as shown in Fig.4. This indicates the pyrolysis of sewage sludge (preheating) is an endothermic reaction and the SCWO of sewage sludge is exothermic. During the study, it was found that the heat dissipation of Condenser is very significant because there was no heat preservation equipment installed. So the quench water was not necessary and not delivered for all experiments. Part of waste heat of effluents with temperature range of 284-333℃ (T10) was recovered by the Heat Exchanger, the Stream 1 can be preheated from room temperature to 154-275℃ (T11). Sewage sludge was degraded to almost transparent liquid product and brick-red
solid product by the new SCWO system, as shown in Fig.7. Almost all of the solids is the type of non-sticky solid and come from sewage sludge (Stream 2). The liquid products of the five experiments are exhibited in Fig.8, where the appearance of No.2 is nearly the same as that of de-ionized water. The chemical oxygen demand (COD) removal efficiency range of 84.02-99.15% was listed in Table 2, which is lower than that of literature reports [3, 5]. Two factors are responsible. One is the low reaction temperature due to heat dissipation and low inlet temperatures of air and sewage sludge. Temperature significantly impacts the COD removal efficiency in SCWO. For most SCWO, operation temperature is around 500℃ which is much higher than 400℃ of this study. The other one is the low oxygen excess, as listed in Table 2. According to the kinetic model of waste oxidation under hydrothermal conditions, the reaction order for oxygen is 0-1 [34, 35]. M. Goto et al. [36] proposed the reaction order for oxygen is 0.5-1 under subcritical condition because of the existence of gas and liquid phases, and is zero under supercritical condition as the single phase. In addition, the oxygen is in excess for most SCWO. Therefore, the oxygen was considered as an independent factor on the COD removal efficiency by most SCWO researches. However, the influence of oxygen cannot be ignored in the new SCWO system. Phases interface occurred in the reaction area because the air was delivered at room temperature. Besides, the oxygen excesses are only 0.73-1.68 except Exp.2 as shown in Table 2. These can explain the COD removal efficiency increases with the air excess, as shown in Table 2. As shown in Fig.2, both of Booster Pump A and B were drove by Air
Compressor A. The insufficient force results in that the total mass flow rate of Air 1 and Air 2 is only in the range of 0.56-0.61 kg.h-1 under 28.5-29.2MPa. Besides, adding isopropanol as co-fuel aggravated the lack of oxidant. As shown in Table 2 and Table 3, isopropanol did not significantly improve the reaction temperatures, but consumed significant amount of oxidant. Results of Exp.4 indicate that Stream 1 (pure water without isopropanol) can achieve appropriate reaction temperatures at a mass flow ratio of 1.6 times of Stream 2. So pure water with inlet temperature of over 550℃, by contrast with isopropanol aqueous, is more economic and practical in the new SCWO system.
3.2 Gas seal performance The most important design point of the new SCWO system was the “gas seal”. The preliminary research [11] demonstrated that this design was feasible, so one of the purposes for this study was testing the gas seal performance of DGSWR under real SCWO conditions. After experiments were finished, imagines of pressure bearing walls and porous walls were captured, as shown in Fig.9. It can be seen that no solids particles deposit on the surface (both inner surface of pressure bearing wall and outer surface of porous wall) except the locations around air inlets, and the amount of particles is much more in Reaction Vessel 2 than in Reaction Vessel 1. Such phenomena can be explained by the pumping conditions of air streams. As the pulse nature of flow rate of booster pumps, the reaction mixtures around the air inlets were perturbed, which results in part of solids was rushed out of the porous wall. And higher the air mass flow rate, more violent the turbulence. The flow rate of Air 2 is
1.36 times of that of Air 1 (see Table 1), which is the reason of more solids particles seen in Reaction Vessel 2. Look it the other way, if “gas seal” is not working under supercritical condition, there should be solid deposits seen all around the reaction vessel, especially at the end away from the air inlet because the air flow is slower at that end as compared to the air inlet end. Now only slight solid deposits can be seen at the air inlet end in this study. So the “gas seal” should be working. The slight solid deposits around the air inlet is most likely been “blown out” by the air pulse. To sum up, the effect of “gas seal” has been verified under real SCWO conditions and a serious pulsating air stream will break this function at the area around air inlet. So a smoother and isotropic-dispersed air stream is necessary.
3.3 Gravity sedimentation performance The new design intended to settle solid particles by gravity and store in Solid Collector, but the results indicated such design has only achieved partial success. The result of Exp.5 showed that only about 30% of solid particles was captured in Solid Collector (named as Solids A), about 23% was captured by Condenser (named as Solids B), about 12% was carried out by effluents stream (named as Solids C), and remaining about 35% scaled on the inner surface of porous walls, Flange and somewhere else. The particle sizes (dp) of Solid A, Solid B and Solid C were measured by Rise-2008 laser particle size analyzer and the results are shown in Fig.10. The particle size range from 0.10×10-6 m to 4.00×10-6 m and the average diameters of Solid A, Solid B and Solid C are 0.93×10-6 m, 0.90×10-6 m and 0.77×10 -6 m, respectively. c
When Rep<2, the sedimentation of solid particle can be described by Stokes’ Law [37]: 2
d ( ρ − ρ )g ut = p p 18µ
(1)
Where, the µ and ρ are the viscosity (Pa s) and density (kg m-3) of reaction medium, respectively. In this paper, the reaction medium can be considered as the mixture of Stream 1, Stream 2, Air 1 and Air 2. The ρp is the true density of solid particles, 2537 kg m-3. The g is the constant of gravitational acceleration, 9.81 m s-2. The Rep is the particle Reynolds number and expressed by Eq.(2).
Re p =
d pu p ρ µ
(2)
As shown in Fig.2, the reaction medium in reaction vessels flows upward with a velocity of u (m s-1), and the solid particles moves down by gravity with a settling velocity of ut (m s-1). Thus, the apparent velocity (up, m s-1) of particles defined as Eq.(3): u p = u − ut
(3)
When u p>0, the particles will be carried out by the reaction medium; on the contrary, the particles will move downward when u p<0. The u in Eq.(3) is defined as Eq.(4).
u =
Q /ρ 3600A
(4)
Where, the Q is mass flow rate of reaction medium, kg h-1. The A is cross sectional area of the porous wall, 6.8×10 -4 m2. In order to simplify the calculations, both Stream 1 and Stream 2 are considered
as pure water, and all the feeds (Stream 1, Stream 2, Air 1 and Air 2) are injected at once in the bottom of Reaction Vessel 1. The µ and ρ of reaction medium were calculated by Eq.(5) [38]: n
Y =
∑i χiYi
(5)
Where, the transport properties of the mainstream (Y) are the mass average of corresponding pure component properties (Yi). The viscosity and density of pure fluids (water, oxygen and nitrogen) were obtained from the NIST database [39]. The apparent velocity (up) of solid particle with a diameter (dp) range of 0.10×10 -6 -1.50×10-5 m has been calculated under the operation conditions of Exp.5. All the calculated results satisfied Rep < 2, which suggest the settling velocity of particles is a creeping flow. As shown in Fig.11, the u p < 0 when the particle size is over the critical diameter (1.36 ×10 -5m), which signify only the solid particle with a diameter of over 1.36 ×10 -5m can be settled by gravity under Exp.5 operation conditions. The calculation result is not fully accordant with the experimental result, which is due to the simplification. Under the real conditions, on one hand, the feeds of Stream 1, Stream 2, Air 1 and Air 2 were injected at multiple points in the new SCWO system (see Fig.2). The actual flow rate (Q) was less than the total mass flow rate of reaction medium and the fluid state in reactor was complex and probably a turbulence flow, rather than a creeping flow. On the other hand, some of solid particles would aggregate to form larger particles. According to Eq.(1), (3) and (4), both lower Q and bigger d p will promote the gravity sedimentation of solid particles. Hence, the
counter-effect of upward “u” and downward “ut” on the solid particles movement (Eq.(3)) causes the experiment results described above: some solids settle in the Solid Collector, some scaled on reaction vessels, some was captured by Condenser and some was carried out by effluents stream. According to Stokes’ Law (Eq.(1)), ut is proportional to the square of dp, which suggests solid with smaller diameter was easier to be brought to move upward by reaction medium (combining Eq.(1) and Eq.(3)). This can explain the average diameter is the biggest for Solid A, second for Solid B and the smallest for Solid C.
4. Conclusions and future work A new SCWO system based on DGSWR has been designed and described in detail. The preliminary experimental results indicate the “gas seal” of DGSWR, which is the kernel of the new SCWO system, has been achieved under 28-29MPa and around 400 ℃. Through the multi-feed injection technologies, sewage sludge with a solid content up to 11.78% DS can be safely pumped and preheated, and mixed with high temperature water to form supercritical medium. The COD removal efficiency can reach up to 99.15%. However, there are also some shortcomings in the new SCWO system. The most obvious one is solid precipitation due to the counter-current of upward reaction medium and downward solid particles. The structure of the SCWO system should be adjusted to insure that the reaction medium and solid particles will form as a co-current flow rather than a counter-current flow. Both stability and flow rate of air
stream are insufficient, which leads to lose efficacy of gas seal and low oxygen excess, respectively. The air mass flow rate should be increased and air stream should be pumped smoothly and isotropic-dispersed. Besides, enhancing heat preservation is also necessary. All the aforementioned improvements will be investigated in future.
Acknowledgments This work was financially supported by the 100-talent program of Chinese Academia of Sciences (Y33Z050M10), the two Key Technology R&D Programs of Chongqing (grant number: cstc2012gg-sfgc20001 and cstc2011ggC20014), the National Natural Science Foundation of China (grant number: 41203047) and the Key Laboratory for Solid Waste Management and Environment Safety Open Fund (grant number: SWMES 2013-06).
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Fluids. 88 (2014) 117-125. [12] S. Yang, G. Wang, Y. Xu, New design of supercritical water oxidation reactor for sewage sludge treatment, Adv. Mater. Res. 774–776 (2013) 212–215. [13] A. Martin, M.D. Bermejo, M.J. Cocero, Recent developments of supercritical water oxidation: A patents review, Recent Pat. Chem. Eng. 4 (2011) 219-230. [14] D.H. Xu, S.Z. Wang, Y.M. Gong, Y. Guo, X.Y. Tang, H.H. Ma, A novel concept reactor design for preventing salt deposition in supercritical water, Chem. Eng. Res. Des. 88 (2010) 1515-1522. [15] E.U. Franck, Dense fluids – New aspects and results, Phys. B C. 139 (1986) 21-30. [16] B. Wellig, Transpiring wall reactor for supercritical water oxidation, Doctoral Thesis, No. 15038, ETH Zurich, Switzerland, 2003. [17] B. Wellig, K. Lieball, P.R. von Rohr, Operating characteristics of a transpiring-wall SCWO reactor with a hydrothermal flame as internal heat source, J. Supercrit. Fluids. 34 (2005) 35-50. [18] R.M. Serikawa, T. Usui, T. Nishimura, H. Sato, S. Hamada, H. Sekino, Hydrothermal flames in supercritical water oxidation: investigation in a pilot scale continuous reactor, Fuel, 81 (2002) 1147-1159. [19] P.R. Von Rohr, K. Prikopsky, T. Rothenfluh, Flames in supercritical water and their applications, Strojnicky Casopis, 59 (2008) 91-103. [20] P. Cabeza, M. Dolores Bermejo, C. Jimenez, M. Jose Cocero, Experimental study of the supercritical water oxidation of recalcitrant compounds under hydrothermal
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[28] M. Schubert, J.W. Regler, F. Vogel, Continuous salt precipitation and separation from supercritical water. Part 2. Type 2 salts and mixtures of two salts, J. Supercrit. Fluids. 52 (2010) 113-124. [29] K. Prikopsky, B. Wellig, P.R. von Rohr, SCWO of salt containing artificial wastewater using a transpiring-wall reactor: Experimental results, J. Supercrit. Fluids. 40 (2007) 246-257. [30] M.D. Bermejo, F. Fdez-Polanco, M.J. Cocero, Effect of the transpiring wall on the behavior of a supercritical water oxidation reactor: Modeling and experimental results, Ind. Eng. Chem. Res. 45 (2006) 3438-3446. [31] E. Fauvel, C. Joussot-Dubien, V. Tanneur, S. Moussiere, P. Guichardon, G. Charbit, F. Charbit, A porous reactor for supercritical water oxidation: Experimental results on salty compounds and corrosive solvents oxidation, Ind. Eng. Chem. Res. 44 (2005) 8968-8971. [32] J. Abelleira, J. Sánchez-Oneto, J.R. Portela, E.J. Martínez de la Ossa, Kinetics of supercritical water oxidation of isopropanol as an auxiliary fuel and co-fuel, Fuel, 111 (2013) 574-583. [33] F.J. Yin, Y.J. Xu, G.W. Wang, S. Yang, H.Z. Chen, Z. Chen, Macromolecular products from bio-solids and its morphologic change employing hydrothermal treatment, Adv. Mater. Res. 864-867 (2014) 1976-1980. [34] J.R. Portela, E. Nebot, E.M. de la Ossa, Kinetic comparison between subcritical and supercritical water oxidation of phenol, Chem. Eng. J. 81 (2001) 287-299. [35] J. Sanchez-Oneto, F. Mancini, J.R. Portela, E. Nebot, F. Cansell, E.J. Martinez de
la Ossa, Kinetic model for oxygen concentration dependence in the supercritical water oxidation of an industrial wastewater, Chem. Eng. J. 144 (2008) 361-367. [36] M. Goto, T. Nada, A. Kodama, T. Hirose, Kinetic analysis for destruction of municipal sewage sludge and alcohol distillery wastewater by supercritical water oxidation, Ind. Eng. Chem. Res. 38 (1999) 1863-1865. [37] Jiahua. Zhu, Shichao. Ye, Sulan. Xia, Principles of Chemical Engineering, second ed., Science Press of China, Beijing, 2005, p.206-207. [38] M. Dolores Bermejo, A. Martin, J.P.S. Qu-iroz, I. Bielsa, V. Rios, M. Jose Cocero, Computational fluid dynamics simulation of a transpiring wall reactor for supercritical water oxidation, Chem. Eng. J. 158 (2010) 431-440. [39] NIST, Thermo physical properties of fluid systems. Available from: http:// webbook. nist. Gov/chemistry /fluid/.
Table 1 Properties of the sewage sludge. Properties
Values ± Standard deviation of three replicates
Dry solids (DS, 105℃, wt.% wet mass)
19.73±0.12
Ash content (815℃, wt.% dry mass)
45.53±0.10
Elemental analysis (wt.% dry mass) C
28.05±0.13
H
4.77 ±0.11
N
4.57 ±0.01
S
0.80 ±0.06
Others (rest to 100%)
61.81
TOC (g C kg-1 dry mass)
241 ±2
Table 2 Operation conditions. Exp. Stream 1 (Sewage sludge) Solid content (% DS)
Stream 2 (Water and isopropanol)
Air
Isopropanol
Mass flow
Mass flow
Mass flow
flow rate
concentration
rate (kg h-1)
rate of Air 1
rate of Air 2
efficiency
(kg h-1)
(wt. %)
(kg h-1)
(kg h-1)
(%)
Mass
Stoichiometric
COD
oxygen excess
removal
1
2.62
2.10
3.0
1.81
0.26
0.33
0.73
84.02 c
2
2.90
0.65
1.0
0.56
0.27
0.34
3.97
99.15
3
3.18
1.48
2.0
1.38
0.25
0.33
1.14
90.83
4
8.55
0.84
0.0
1.34
0.25
0.31
1.68
97.70
5
11.78
0.78
0.0
2.81
0.25
0.34
1.49
95.76
Table 3 Pressures and temperatures of SCWO system under steady state of each operation conditions.
Exp.
P (MPa)
T1 (℃)
T2 (℃)
T3 (℃)
T4 (℃)
T5 (℃)
T6 (℃)
T7 (℃)
T8 (℃)
T10
T11
(℃)
(℃)
T9 (℃)
1
28.9
454
588
149
383
371
370
377
380
365
333
275
2
28.5
385
572
172
373
362
375
387
381
354
284
154
3
28.8
445
596
188
387
376
374
382
381
357
316
235
4
29.2
414
597
176
394
379
378
394
397
383
311
205
5
28.3
394
576
185
400
386
387
396
394
372
329
236
Figure Captions (PS: all the figures are only color on the Web.) Fig.1 Typical SCWO process. Adapted from literatures [8, 9]. Fig.2 Schematic diagram of the new SCWO system. Fig.3 Picture of the Flange connecting Reaction vessel 1 and Reaction vessel 2. Fig.4 Temperature variation curves of Exp.1. Where, (a) Start to pressurize; (b) Start to heat system; (c) Start to pump Stream 1 (3.0 wt. % isopropanol); (d) Start to pump Stream 2 (sewage sludge at 2.62% DS); (e) Switch Stream 1 from isopropanol aqueous to pure water. Fig.5 Temperature variation curves of Exp.3. Where, (a) Start to pressurize; (b) Start to heat system; (c) Start to pump Stream 1 (2.0 wt. % isopropanol); (d) Start to pump Stream 2 (sewage sludge at 3.18% DS). Fig.6 Temperature variation curves of Exp.5. Where, (a) Start to pressurize; (b) Start to heat system; (c) Start to pump Stream 1 (pure water); (d) Start to pump Stream 2 (sewage sludge at 11.78% DS). Fig.7 Images of sewage sludge and effluents in Exp.5. Fig.8 Images of liquid products. No.1-5 correspond to Exp.1-5, respectively, and No.6 is the de-ionized water. Fig.9 Images of pressure bearing walls and porous walls after Exp.5, without any cleaning. Fig.10 Particle size distribution of solids in Exp.5. Fig.11 Calculation results of solids apparent velocity versus solids diameter under Exp.5 conditions.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Highlights A lab-scale SCWO system based on DGSWR was described in detail. Gas seal of DGSWR was verified under supercritical conditions. Sewage sludge with 2.62-11.78% DS was safely treated.
Gravity sedimentation of solids partially achieved.
S1385-8947(15)00177-1 http://dx.doi.org/10.1016/j.cej.2015.02.005 CEJ 13257
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
6 December 2014 29 January 2015 3 February 2015
Please cite this article as: Z. Chen, G. Wang, F. Yin, H. Chen, Y. Xu, A New System Design for Supercritical Water Oxidation, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.02.005
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Research Article A New System Design for Supercritical Water Oxidation Zhong Chen a,b,c, , Guangwei Wang a,b,c, Fengjun Yin a,c, Hongzhen Chen a,c, Yuanjian Xua,c, *.
a
Chongqing Institute of Green and Intelligent Technology (CIGIT), Chinese Academy of Sciences,
Chongqing, P. R. China, 400714.
b
Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of
China, Tsinghua University, Beijing, P. R. China, 100084.
c
Key Laboratory of Reservoir Aquatic Environment, Chinese Academy of Sciences, Chongqing, P.
R. China, 400714.
Corresponding author: Prof. Yuanjian Xu.
Phone number: +86 23 65935819.
E-mail: [email protected].
Address: Environmentally-Benign Chemical Process Research Center, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, No.266 Fangzheng Avenue, Shuitu Hi-tech Industrial Park, Shuitu Town, Beibei District, Chongqing, P. R. China, 400714.
Abstract As the main obstacles for the industrialization of supercritical water oxidation (SCWO) technology, corrosion and plugging are mostly occurring in the high pressure high temperature (HPHT) sections, including preheater, reactor, heat exchanger and cooler. In this paper, a lab-scale SCWO system based on dynamic gas seal wall reactor (DGSWR) has been described, tested and discussed in detail. The results showed that the preheating problems of waste with high solid content has been solved and the “gas seal” of DGSWR has been successfully verified under 28-29MPa and around 400℃. Sewage sludge with 2.62-11.78% dry solid has been degraded and the COD removal efficiency can reach up to 99.15%. However, the solid particle sedimentation was only partly achieved. According to the results analysis, based on the Stokes’ Law, both small particle size and counter-current of upward reaction medium and downward solids are responsible. Future improvements for the SCWO system were also discussed at the end of this article.
Keywords:
Anti-corrosion; Anti-plugging; System design; Dynamic gas seal wall
reactor; Supercritical water oxidation
1. Introduction For environmental awareness, water is undoubtedly the optimal reaction medium. But at room temperature, the reaction is too slow for most redox reactions, especially for the destruction of organic wastes. One of main reasons is the solubility in water for both organic waste (mostly nonpolar) and oxidant (mostly oxygen) are
very low at room temperature. Interestingly, this property is overturned in supercritical water (SCW, Tc=373.946 ℃ , Pc=22.064 MPa [1]), which can be completely miscible with organic compounds and oxygen [2]. Besides, SCW also possess other unique properties [2, 3], such as high diffusivity and density, low viscosity and inorganic solubility. Supercritical water oxidation (SCWO) is a redox reaction to destroy organic compounds in SCW with the participation of oxidant (such as air, oxygen and hydrogen peroxide etc.) [2]. In general, most of organics can be nearly completely degraded by SCWO in less than 1 minute [3], and the products are the environmentally acceptable effluents, such as H2O, CO2, and N2 etc. [4]. As the excellent
merits,
the
SCWO
has
been
hailed
as
an
emerging
and
environmentally-benign technology for the treatment of various hazardous wastes in the last three decades [4, 5]. The SCWO, as such a great potential technology, its commercial development is actually far lag behind the expectation. Most of the full-scale commercial plants have been shut down and only two of them are in operation as of January 2012 [6]. Corrosion and plugging are the main obstacles [2-7]. To overview the typical SCWO process (Fig.1), it can be found that both corrosion and plugging are mainly occurring in the high pressure and high temperature (HPHT) sections, and the details are discussed as fellow: ·Preheaters. The oxidant preheater can be slightly corroded in the presence of water, such as the wet air without dehydration. The ion product of water will maximize under subcritical conditions, which will lead to corrosion problems in both pure water
preheater and aqueous waste preheater [8-10]. Some salts dissolved in the waste stream will precipitate out with the increasing of temperature [10] and some organic compounds in the aqueous waste will polymerize in the absence of oxidant [3, 10]. Both of salt precipitation and polymerization may lead to plugging problems in waste preheater. ·Reactor. The heteroatoms (S, Cl, P, N, etc.) contained in the organic waste will be dissociated to form corresponding acids in reactor. The SCWO reactor, where is a harsh chemical and physical environment of presence of acids, high concentration of oxidant, high temperature and high pressure, should undergo severe corrosions [8-10]. The inorganic salts for several reasons [7] would like to precipitate from SCW to scale on the surface of reactor and leads to severe plugging problems. ·Cooler and Heat Exchanger. Same as in the process of preheating, the cooling and heat exchanging should undergo the subcritical conditions, under which the corrosion of water is much higher [8]. The effluents containing various acids, if without neutralization, will lead to severe corrosion in cooler and heat exchanger, which is more severe than that in reactor [8-10]. Part of salts failed to separate in reactor will also result in plugging problems in cooler and heat exchanger. So far, a “super material” that can withstand all corrosion conditions in SCWO has not yet been reported [8]. If any, the plugging and others problems will also hinder the development of SCWO. Therefore an appropriate system design for SCWO is necessary. In this paper, the focus of anti-corrosion and anti-plugging has been expanded
from reactor to the whole HPHT sections (see Fig.1). A novel reactor concepts named as “Dynamic Gas Seal Wall Reactor” (DGSWR) [11] was adopted, which was optimized from “Transpiring Wall Reactor” (TWR) and was designed to handle the reactor corrosion and plugging problems. A technology of multi-feed injection was designed to handle the waste preheating problems. A lab-scale SCWO device based on this novel design was manufactured and tested under 28-29MPa around 400℃.
2. A new SCWO system design A new SCWO system with a maximum treatment capacity of 2 kg h-1 at 15% dry solids (DS) has been designed in CIGIT based on the preliminary researches [11, 12]. The system was particularly descripted in the following four parts as illustrated in Fig. 2.
2.1. Reactor design As the heart of SCWO, reactor always suffers from both corrosion and plugging. Several types of reactors have been invented to handle these problems. The related reviews can be found in literatures [2, 7-9, 13]. The DGSWR was adapted in the new system. The basic structure of DGSWR is the same as that of a TWR: a double wall reactor consists of outer pressure bearing wall and inner transpiring wall (also named as porous wall). Transpiring fluid fills the annulus between the two walls and continuously flows through the transpiring wall to form a protective film on the inner surface of transpiring wall. The protective film is a mobile surface and can protect transpiring wall from corrosion and salt deposition [2]. Pure water was used as the
transpiring fluid in TWR and was replaced by air in DGSWR, which is the essential difference between these two types of reactors. Based on the special physical properties of air, DGSWR can enhance the anti-corrosion and anti-plugging of TWRs and to avoid the demerits of TWRs. The feasibility of DGSWR has been proved in previous research [11]. Two of the same units named as Reaction Vessel 1 and Reaction Vessel 2 (see Fig.2) were connected by Flange (see Fig.3) to constitute the reaction area of the new system. Both of them are the type of DGSWR. Each unit with height of 500 mm consists of pressure bearing wall and porous wall. The pressure bearing wall with inner diameter of 45 mm was made of 316L stainless steel and can withhold a pressures up to 50MPa. The porous wall is the type of sintered wire netting [14] and made from 316L stainless steel. It was made of outer 32 floors nets with pore size of 2×10-5 m and inner 5 floors nets with pore size of 5×10 -6 m. Its inner diameter and thickness are 29.5 mm and 5 mm respectively. In the new system design, air is not only the oxidant but also the transpiring fluid of DGSWR. The air must flow through the porous wall and mix with wastes in reaction area, and then the reaction would happen. Therefore, the sooner they mix the better. The preliminary research [11] indicates that the transpiring fluid tends to flow through the porous wall around the transpiring fluid inlet section and upper section of porous wall. Based on the characteristic of DGSWR and the transpiring fluid dynamics aforementioned, a pair of symmetrical air inlets with inner diameter of 2 mm was set at the lower section of each reaction vessel (see Fig.2). The distances
from the bottom to inlets are 50 mm for Air 1 and 110 mm for Air 2, respectively. Air was supplied by Air Compressor B (Atlas, GX4FF-10) and was divided into two branches, Air 1 and Air 2. Air 1 was delivered into Reaction Vessel 1 by Booster Pump B and the flow rate was controlled by Flow meter B from SEVENSTAR (D07-9E) with measuring range from 0.00 to 8.33×10 −4 m3s−1 (standard conditions). Air 2 was delivered into Reaction Vessel 2 by Booster Pump A and the flow rate was controlled by Flow meter A from SEVENSTAR (D07-7B) with measuring range from 0.00 to 1.67×10 −4 m3s−1 (standard conditions). Both booster pumps (STT60AL, Shineeast) were driven by Air Compressor A (Atlas, GXe11FF-10). The feasibility analysis of DGSWR suggests that air in low temperature has a better performance on the “gas seal” and the heat capacity of air is much lower than that of water [11], therefore the air was not preheated in the new system. In consideration of the high energy consumption in start-up and the heat dissipation during operation, each reaction vessel was equipped with a heating jacket with maximum power of 3 kW.
2.2. Multi-feed injection Besides the reactor, waste preheater is also vulnerable to attack by corrosion and plugging. In addition, it’s not easy to pump multiphase wastewater over a pressure of 22.1 MPa, such as oil-water mixture and oil-water-solid mixture. Several full scale SCWO plants have been shut down due to these reasons [6]. Hydrothermal flame, which was first reported in 1986 by E.U. Franck from University of Karlsruhe [15], is one of the effective methods to solve the preheating problems. Auxiliary fuel (such as isopropyl alcohol, methanol, etc.) is injected into
SCWO reactor through a special device and “burn” with oxidant. The flame with temperature of typically over 1000℃ is a heat source and then the waste water can be fed at subcritical temperature or even room temperature [3]. This technology has been applied in several types of SCWO reactors in the last two decades [16-21]. Oxygen multi-injection developed in CNRS of France is another effective way [22]. Waste water was fed at subcritical or near-critical temperature and oxygen was injected through three inlets along the tubular reactor [23-25]. The temperature of mixture increases along the length of reactor as the reaction is exothermic, and the critical point of water is overcome in reactor rather than in preheater (like most designs). This design can reduce the energy consumption of preheating and makes the control of thermal agitation easy [3]. And by coincidence, both of preheating at low temperature (less than 350℃[26]) and presence of oxidant avoid the polymerization of organics in waste water. The corrosion of subcritical water and the polymerization of organics are the main reasons of operation problems in waste preheater and may be avoided by this design. V. Vadillo and coworkers from University of Cadiz in Spain reported a new feed system for multiphase wastewater in SCWO [26]. A non-aqueous waste with no preheating and an aqueous waste over 400℃ ware pumped independently and mix at the beginning of reactor to form a supercritical homogeneous phase. The new feed system, which can avoid the problems of pumping, preheating and thermal control, is another candidate. Based on the advantages of above technologies (hydrothermal flames [16],
oxygen multi-injection [22] and new feed system [26]), a new methods named as multi-feed injection was designed in the new SCWO system, as shown in Fig.2. Stream 1 (Aqueous waste or Water with Fuel) is pressurized by Pump C and preheated by Heat Exchanger, Preheater C and Preheater B in turn, and finally introduced into the Reaction Vessel 1 in the bottom. The inlet temperature of Stream 1 (T2 in Fig.2) is over 500℃ typically. Stream 2 (e.g., sewage sludge, oil waste, etc.) was stored in Tank A and Tank B and one operates after the other, alternatively. Water is delivered by Pump A to drive the piston in Tank A or Tank B and then the waste is extruded out into Preheater A, and eventually introduced into Reaction Vessel 1 on the top. The inlet temperature of Stream 2 (T3 in Fig.2) ranges from room temperature to 350℃ depended on the type of waste. Due to gravity, stream 2 with a higher density flows down and mixes with upward Stream 1 in the Reaction Vessel 1. Part of the waste is oxidized in Reaction Vessel 1 and the rest is further degraded in Reaction Vessel 2. Pump A (2J-X 2/50) with maximum pressure of 50MPa and rated flow of 2 L h-1 was purchased from Hangzhou Zhejiang Petrochemical Equipment CO., LTD, China. The effective volume of each tank (Tank A and Tank B) with inner diameter of 95 mm is 2.5 L. Preheater A with maximum power of 4 kW was designed as a single straight tube heater to avoid some blind spaces and to reduce the flow resistance. The length and inner diameter of the tube are 750 mm and 8 mm, respectively. As shown in Fig.3, the inlet and distributor with the same inner diameter of 8mm are set on the flange and the Stream 2 would be dispersed by the distributor.
Pump C (2J-X 4/50) was supplied by the same manufactory as Pump A and the rated flow is 4 L h-1. The two preheaters for Stream 1 are both the type of coil electric heater. The maximum power is 4KW for Preheater B and 3KW for Preheater C. Each heater was made of a single tube with inner diameter of 8 mm and length of 6.5m.
2.3. Gravity sedimentation To avoid plugging, one of the dominant technologies is the reverse flow reactor with a brine pool [7]. Salts precipitated in supercritical zone (upper section of reactor) will fall down to the subcritical zone (lower section of reactor) and re-dissolve in quench water [13, 14], and then the salts can be removed by brine effluent. Only for the salt separation purposes (without waste oxidation), the recovery of this design can reach up to 97% for type 1 salts [27], 95% for type 2 salts and mixtures of two salts [28]. But for degradation of salt containing waste water, the efficiency is only 65% [29], 5~55% [30] and even much lower [31]. It must be noted that the waste water in these researches [27-31] is artificial and the salts is the type of sticky salts that has good water-solubility. But many potential SCWO applications (such as sewage sludge, oily sludge and so on) contents not only sticky salts, but also non-sticky solid [2]. The non-sticky solid can’t be dissolved by the quench water and will lead to plugging problems in following units, such as heat exchanger, cooler and so on. The new design in this paper intends to separate both sticky salts and non-sticky solid. The mixture of salts and solids are designed to be settled by gravity and stored in the Solid Collector (see Fig.2), batched emptied. The Solid Collector with a length of 300 mm has the same inner diameter as pressure bearing wall, and the effective
volume is 0.45 L. As shown in Fig.2, a Condenser was set up to quench the effluents from Reaction Vessel 2. The basic structure of the Condenser is as same as the Reaction Vessel units. There is only one inlet with inner diameter of 2 mm in the middle of Condenser. Quench Water was delivered by Pump B that is exactly the same as Pump A. Besides cooling, the Condenser can separate salts and solids that failed to settle in Solid Collector. On one hand, the sticky salts will re-dissolve in the quench water; on the other hand, the non-sticky solid particles tend to be captured by the subcritical water and stored in the Condenser. Such design tries to enhance the effect of gravity sedimentation and to avoid the plugging problems in heat exchanger and cooler.
2.4. Heat recovery and pressure control In order to achieve a self-sustaining running, heat exchanger was always used to recovery the heat of effluents. This method was also adapted in the new SCWO system. As shown in Fig.2, the Effluent was cooled in Heat Exchanger and Cooler, in turn. Both Heat Exchanger and Cooler are type of single double-pipe. The effective heat-exchanging surface is 9.11×10-3 m2 for Heat Exchanger and 1.53×10 -2 m2 for Cooler. Eleven K-type thermocouples with range from 0℃ to 800℃ were adapted to measure the temperatures and the exact positions were shown in Fig.2. In order to protect thermocouple from corrosion, a tube with inner diameter of 2mm was equipped for each thermocouple (T1-T4, T10-T11, see Fig.1 and Fig.2), and one single tube with inner diameter of 4mm was equipped for thermocouples of T5-T9. They are
evenly spaced for T5-T9 and the distance of two neighboring positions is 100mm. The system pressure was controlled by a back pressure valve as usual designs. In order to avoid operation problems caused by solids in effluent [3], a Filter with pore size of 5×10 -5 m was placed between Cooler and Pressure Control (see Fig.2). The solid particles failed to separate will be captured by the Filter. So the other function of the Filter is to check the separation efficiency of gravity sedimentation.
3. Experimental tests and results Sewage sludge from Xiaojia River wastewater treatment plant in Chongqing was selected as the typical waste in the tests. The properties of the sewage sludge are shown in Table 1. The ash of sewage sludge is 45.53% of its dry mass. In order to increase the fluidity, the sewage sludge with an initial solid content of 19.73% DS was diluted to 2.62 - 11.78% DS. The diluted sewage sludge is the Stream 2 in the tests of the new SCWO system. As most methods, isopropanol was added as the co-fuel [32] in this paper. The aqueous of isopropanol or de-ionized water was used as feedstock of Stream 1. The concentrations of isopropanol in the aqueous ranged from 0.0 wt. % to 3.0 wt. %. Five experiments were carried out in the study and operation conditions were shown in Table 2.
3.1 Operation results In general, a complete operation process includes the following steps: pressurizing, heating, pumping Stream 1, pumping Stream 2, sampling, depressurizing
and cooling. The temperatures of the SCWO system was adjusted timely by switching on and off the Preheating A, B, C and heating jackets of reactors. Table 3 exhibits the pressures and temperatures under steady state of each operation conditions. In addition, the operation processes of Exp.1, Exp.3 and Exp.5 were recorded in details, and the temperatures variation curves are shown in Fig. 4, Fig.5 and Fig.6, respectively. To preheat sewage sludge directly from room temperature to supercritical condition will lead to plugging problems. F.J. Yin et al. [33] suggest the preheating temperature should be no more than 200℃, under which the sewage sludge maintains good fluidity. Hence, the inlet temperature of Stream 2 (T3) in this study was controlled below 200℃ (see Table 3). According to operation steps, the SCWO system would be preheated to supercritical conditions before pumping Stream 2, which leads to the location of T3 was also warmed up to over 300℃ during heating step. As shown in Fig. 4-6, the T3 will decrease to appropriate temperature ranges while the sewage sludge was pumped into reactor. But it must be pointed out that an interrupt or a low flow rate of Stream 2 will increase the risk of plugging at the position of T3 because both of them will result in temperature rise of T3. Therefore, a steady Stream 2 with a mass flow rate of over 0.5 kg h-1 is necessary in the new SCWO system. As shown in Figs.4-6, T1, T4 and T5-T11 (except T2 and T3) have gone up after pumping Stream 2. While reaching steady state, the temperature of low inlet section is higher than that of upper outlet section in Reaction Vessel 1, and the temperature of
middle section is the highest in Reaction Vessel 2 (see Table 3). Such temperature distribution was caused by feeds inlet temperatures, heating jackets and heat dissipation. The temperature of Stream 1 dropped from 572-597℃ (T2) to 385-454℃ (T1) by the cooling of Air 1 (which was not preheated) and heat dissipation of Solid Collector (no heating jacket installed). In Reaction Vessel 1, Stream 1 of 385-454℃ mixes with Air 1 of room temperature and Stream 2 of 149-188℃, which leads to T1>T4. The exothermic reaction of sewage sludge leads to the T1 increased about 50℃ at the moment of about 80 min after feeding Stream 2, as shown in Figs.4-6. In Reaction Vessel 2, the mixture from Reaction Vessel 1 was cooled by Air 2 and the top area was cooled due to heat dissipation, which causes T5≈T6
solid product by the new SCWO system, as shown in Fig.7. Almost all of the solids is the type of non-sticky solid and come from sewage sludge (Stream 2). The liquid products of the five experiments are exhibited in Fig.8, where the appearance of No.2 is nearly the same as that of de-ionized water. The chemical oxygen demand (COD) removal efficiency range of 84.02-99.15% was listed in Table 2, which is lower than that of literature reports [3, 5]. Two factors are responsible. One is the low reaction temperature due to heat dissipation and low inlet temperatures of air and sewage sludge. Temperature significantly impacts the COD removal efficiency in SCWO. For most SCWO, operation temperature is around 500℃ which is much higher than 400℃ of this study. The other one is the low oxygen excess, as listed in Table 2. According to the kinetic model of waste oxidation under hydrothermal conditions, the reaction order for oxygen is 0-1 [34, 35]. M. Goto et al. [36] proposed the reaction order for oxygen is 0.5-1 under subcritical condition because of the existence of gas and liquid phases, and is zero under supercritical condition as the single phase. In addition, the oxygen is in excess for most SCWO. Therefore, the oxygen was considered as an independent factor on the COD removal efficiency by most SCWO researches. However, the influence of oxygen cannot be ignored in the new SCWO system. Phases interface occurred in the reaction area because the air was delivered at room temperature. Besides, the oxygen excesses are only 0.73-1.68 except Exp.2 as shown in Table 2. These can explain the COD removal efficiency increases with the air excess, as shown in Table 2. As shown in Fig.2, both of Booster Pump A and B were drove by Air
Compressor A. The insufficient force results in that the total mass flow rate of Air 1 and Air 2 is only in the range of 0.56-0.61 kg.h-1 under 28.5-29.2MPa. Besides, adding isopropanol as co-fuel aggravated the lack of oxidant. As shown in Table 2 and Table 3, isopropanol did not significantly improve the reaction temperatures, but consumed significant amount of oxidant. Results of Exp.4 indicate that Stream 1 (pure water without isopropanol) can achieve appropriate reaction temperatures at a mass flow ratio of 1.6 times of Stream 2. So pure water with inlet temperature of over 550℃, by contrast with isopropanol aqueous, is more economic and practical in the new SCWO system.
3.2 Gas seal performance The most important design point of the new SCWO system was the “gas seal”. The preliminary research [11] demonstrated that this design was feasible, so one of the purposes for this study was testing the gas seal performance of DGSWR under real SCWO conditions. After experiments were finished, imagines of pressure bearing walls and porous walls were captured, as shown in Fig.9. It can be seen that no solids particles deposit on the surface (both inner surface of pressure bearing wall and outer surface of porous wall) except the locations around air inlets, and the amount of particles is much more in Reaction Vessel 2 than in Reaction Vessel 1. Such phenomena can be explained by the pumping conditions of air streams. As the pulse nature of flow rate of booster pumps, the reaction mixtures around the air inlets were perturbed, which results in part of solids was rushed out of the porous wall. And higher the air mass flow rate, more violent the turbulence. The flow rate of Air 2 is
1.36 times of that of Air 1 (see Table 1), which is the reason of more solids particles seen in Reaction Vessel 2. Look it the other way, if “gas seal” is not working under supercritical condition, there should be solid deposits seen all around the reaction vessel, especially at the end away from the air inlet because the air flow is slower at that end as compared to the air inlet end. Now only slight solid deposits can be seen at the air inlet end in this study. So the “gas seal” should be working. The slight solid deposits around the air inlet is most likely been “blown out” by the air pulse. To sum up, the effect of “gas seal” has been verified under real SCWO conditions and a serious pulsating air stream will break this function at the area around air inlet. So a smoother and isotropic-dispersed air stream is necessary.
3.3 Gravity sedimentation performance The new design intended to settle solid particles by gravity and store in Solid Collector, but the results indicated such design has only achieved partial success. The result of Exp.5 showed that only about 30% of solid particles was captured in Solid Collector (named as Solids A), about 23% was captured by Condenser (named as Solids B), about 12% was carried out by effluents stream (named as Solids C), and remaining about 35% scaled on the inner surface of porous walls, Flange and somewhere else. The particle sizes (dp) of Solid A, Solid B and Solid C were measured by Rise-2008 laser particle size analyzer and the results are shown in Fig.10. The particle size range from 0.10×10-6 m to 4.00×10-6 m and the average diameters of Solid A, Solid B and Solid C are 0.93×10-6 m, 0.90×10-6 m and 0.77×10 -6 m, respectively. c
When Rep<2, the sedimentation of solid particle can be described by Stokes’ Law [37]: 2
d ( ρ − ρ )g ut = p p 18µ
(1)
Where, the µ and ρ are the viscosity (Pa s) and density (kg m-3) of reaction medium, respectively. In this paper, the reaction medium can be considered as the mixture of Stream 1, Stream 2, Air 1 and Air 2. The ρp is the true density of solid particles, 2537 kg m-3. The g is the constant of gravitational acceleration, 9.81 m s-2. The Rep is the particle Reynolds number and expressed by Eq.(2).
Re p =
d pu p ρ µ
(2)
As shown in Fig.2, the reaction medium in reaction vessels flows upward with a velocity of u (m s-1), and the solid particles moves down by gravity with a settling velocity of ut (m s-1). Thus, the apparent velocity (up, m s-1) of particles defined as Eq.(3): u p = u − ut
(3)
When u p>0, the particles will be carried out by the reaction medium; on the contrary, the particles will move downward when u p<0. The u in Eq.(3) is defined as Eq.(4).
u =
Q /ρ 3600A
(4)
Where, the Q is mass flow rate of reaction medium, kg h-1. The A is cross sectional area of the porous wall, 6.8×10 -4 m2. In order to simplify the calculations, both Stream 1 and Stream 2 are considered
as pure water, and all the feeds (Stream 1, Stream 2, Air 1 and Air 2) are injected at once in the bottom of Reaction Vessel 1. The µ and ρ of reaction medium were calculated by Eq.(5) [38]: n
Y =
∑i χiYi
(5)
Where, the transport properties of the mainstream (Y) are the mass average of corresponding pure component properties (Yi). The viscosity and density of pure fluids (water, oxygen and nitrogen) were obtained from the NIST database [39]. The apparent velocity (up) of solid particle with a diameter (dp) range of 0.10×10 -6 -1.50×10-5 m has been calculated under the operation conditions of Exp.5. All the calculated results satisfied Rep < 2, which suggest the settling velocity of particles is a creeping flow. As shown in Fig.11, the u p < 0 when the particle size is over the critical diameter (1.36 ×10 -5m), which signify only the solid particle with a diameter of over 1.36 ×10 -5m can be settled by gravity under Exp.5 operation conditions. The calculation result is not fully accordant with the experimental result, which is due to the simplification. Under the real conditions, on one hand, the feeds of Stream 1, Stream 2, Air 1 and Air 2 were injected at multiple points in the new SCWO system (see Fig.2). The actual flow rate (Q) was less than the total mass flow rate of reaction medium and the fluid state in reactor was complex and probably a turbulence flow, rather than a creeping flow. On the other hand, some of solid particles would aggregate to form larger particles. According to Eq.(1), (3) and (4), both lower Q and bigger d p will promote the gravity sedimentation of solid particles. Hence, the
counter-effect of upward “u” and downward “ut” on the solid particles movement (Eq.(3)) causes the experiment results described above: some solids settle in the Solid Collector, some scaled on reaction vessels, some was captured by Condenser and some was carried out by effluents stream. According to Stokes’ Law (Eq.(1)), ut is proportional to the square of dp, which suggests solid with smaller diameter was easier to be brought to move upward by reaction medium (combining Eq.(1) and Eq.(3)). This can explain the average diameter is the biggest for Solid A, second for Solid B and the smallest for Solid C.
4. Conclusions and future work A new SCWO system based on DGSWR has been designed and described in detail. The preliminary experimental results indicate the “gas seal” of DGSWR, which is the kernel of the new SCWO system, has been achieved under 28-29MPa and around 400 ℃. Through the multi-feed injection technologies, sewage sludge with a solid content up to 11.78% DS can be safely pumped and preheated, and mixed with high temperature water to form supercritical medium. The COD removal efficiency can reach up to 99.15%. However, there are also some shortcomings in the new SCWO system. The most obvious one is solid precipitation due to the counter-current of upward reaction medium and downward solid particles. The structure of the SCWO system should be adjusted to insure that the reaction medium and solid particles will form as a co-current flow rather than a counter-current flow. Both stability and flow rate of air
stream are insufficient, which leads to lose efficacy of gas seal and low oxygen excess, respectively. The air mass flow rate should be increased and air stream should be pumped smoothly and isotropic-dispersed. Besides, enhancing heat preservation is also necessary. All the aforementioned improvements will be investigated in future.
Acknowledgments This work was financially supported by the 100-talent program of Chinese Academia of Sciences (Y33Z050M10), the two Key Technology R&D Programs of Chongqing (grant number: cstc2012gg-sfgc20001 and cstc2011ggC20014), the National Natural Science Foundation of China (grant number: 41203047) and the Key Laboratory for Solid Waste Management and Environment Safety Open Fund (grant number: SWMES 2013-06).
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Table 1 Properties of the sewage sludge. Properties
Values ± Standard deviation of three replicates
Dry solids (DS, 105℃, wt.% wet mass)
19.73±0.12
Ash content (815℃, wt.% dry mass)
45.53±0.10
Elemental analysis (wt.% dry mass) C
28.05±0.13
H
4.77 ±0.11
N
4.57 ±0.01
S
0.80 ±0.06
Others (rest to 100%)
61.81
TOC (g C kg-1 dry mass)
241 ±2
Table 2 Operation conditions. Exp. Stream 1 (Sewage sludge) Solid content (% DS)
Stream 2 (Water and isopropanol)
Air
Isopropanol
Mass flow
Mass flow
Mass flow
flow rate
concentration
rate (kg h-1)
rate of Air 1
rate of Air 2
efficiency
(kg h-1)
(wt. %)
(kg h-1)
(kg h-1)
(%)
Mass
Stoichiometric
COD
oxygen excess
removal
1
2.62
2.10
3.0
1.81
0.26
0.33
0.73
84.02 c
2
2.90
0.65
1.0
0.56
0.27
0.34
3.97
99.15
3
3.18
1.48
2.0
1.38
0.25
0.33
1.14
90.83
4
8.55
0.84
0.0
1.34
0.25
0.31
1.68
97.70
5
11.78
0.78
0.0
2.81
0.25
0.34
1.49
95.76
Table 3 Pressures and temperatures of SCWO system under steady state of each operation conditions.
Exp.
P (MPa)
T1 (℃)
T2 (℃)
T3 (℃)
T4 (℃)
T5 (℃)
T6 (℃)
T7 (℃)
T8 (℃)
T10
T11
(℃)
(℃)
T9 (℃)
1
28.9
454
588
149
383
371
370
377
380
365
333
275
2
28.5
385
572
172
373
362
375
387
381
354
284
154
3
28.8
445
596
188
387
376
374
382
381
357
316
235
4
29.2
414
597
176
394
379
378
394
397
383
311
205
5
28.3
394
576
185
400
386
387
396
394
372
329
236
Figure Captions (PS: all the figures are only color on the Web.) Fig.1 Typical SCWO process. Adapted from literatures [8, 9]. Fig.2 Schematic diagram of the new SCWO system. Fig.3 Picture of the Flange connecting Reaction vessel 1 and Reaction vessel 2. Fig.4 Temperature variation curves of Exp.1. Where, (a) Start to pressurize; (b) Start to heat system; (c) Start to pump Stream 1 (3.0 wt. % isopropanol); (d) Start to pump Stream 2 (sewage sludge at 2.62% DS); (e) Switch Stream 1 from isopropanol aqueous to pure water. Fig.5 Temperature variation curves of Exp.3. Where, (a) Start to pressurize; (b) Start to heat system; (c) Start to pump Stream 1 (2.0 wt. % isopropanol); (d) Start to pump Stream 2 (sewage sludge at 3.18% DS). Fig.6 Temperature variation curves of Exp.5. Where, (a) Start to pressurize; (b) Start to heat system; (c) Start to pump Stream 1 (pure water); (d) Start to pump Stream 2 (sewage sludge at 11.78% DS). Fig.7 Images of sewage sludge and effluents in Exp.5. Fig.8 Images of liquid products. No.1-5 correspond to Exp.1-5, respectively, and No.6 is the de-ionized water. Fig.9 Images of pressure bearing walls and porous walls after Exp.5, without any cleaning. Fig.10 Particle size distribution of solids in Exp.5. Fig.11 Calculation results of solids apparent velocity versus solids diameter under Exp.5 conditions.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Highlights A lab-scale SCWO system based on DGSWR was described in detail. Gas seal of DGSWR was verified under supercritical conditions. Sewage sludge with 2.62-11.78% DS was safely treated.
Gravity sedimentation of solids partially achieved.