a new system design for supercritical water oxidation, chem eng j 2015

7/23/2019 A New System Design for Supercritical Water Oxidation, Chem Eng J 2015

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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 Xu a,c,

a Chongqing Institute of Green and Intelligent Technology (CIGIT), Chinese Academy of Sciences, Chongqing 400714, PR Chinab Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of China, Tsinghua University, Beijing 100084, PR Chinac Key Laboratory of Reservoir Aquatic Environment, Chinese Academy of Sciences, Chongqing 400714, PR China

h i g h l i g h t s

A lab-scale SCWO system based onDGSWR was described in detail.

Gas seal of DGSWR was verified under

supercritical conditions.

Sewage sludge with 2.6211.78% DS

was safely treated.

Gravity sedimentation of solids

partially achieved.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:

Received 6 December 2014

Received in revised form 29 January 2015

Accepted 3 February 2015

Available online 11 February 2015

Keywords:

Anti-corrosion

Anti-plugging

System design

Dynamic gas seal wall reactor

Supercritical water oxidation

a b s t r a c t

As the main obstacles for the industrialization of supercritical water oxidation (SCWO) technology, cor-

rosion and plugging are mostly occurring in the high pressure high temperature (HPHT) sections, includ-

ing 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 2829 MPa and around 400 C. Sewage sludge with

2.6211.78% dry solid has been degraded and the COD removal efficiency can reach up to 99.15%. How-

ever, 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 down-

ward solids areresponsible. Future improvements for the SCWO systemwere also discussed at the end of

this article.

2015 Elsevier B.V. All rights reserved.

1. Introduction

For environmental awareness, water is undoubtedly the opti-

mal 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 prop-

erty is overturned in supercritical water (SCW, Tc= 373.946 C,

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 oxida-

tion (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

http://dx.doi.org/10.1016/j.cej.2015.02.005

1385-8947/2015 Elsevier B.V. All rights reserved.

Corresponding author at: 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 400714, PR China. Tel.: +86 23 65935819.

E-mail address:[email protected](Y. Xu).

Chemical Engineering Journal 269 (2015) 343351

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nearly completely degraded by SCWO in less than 1 min[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 tech-

nology 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 plug-

ging are the main obstacles[27]. 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:

1.1. 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 aque-

ous waste preheater [810]. 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 poly-

merize in the absence of oxidant [3,10]. Both of salt precipitation

and polymerization may lead to plugging problems in waste

preheater.

1.2. 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 environ-

ment of presence of acids, high concentration of oxidant, high tem-

perature and high pressure, should undergo severe corrosions[8

10]. The inorganic salts for several reasons [7] would like to precip-

itate from SCW to scale on the surface of reactor and leads to

severe plugging problems.

1.3. 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 [810]. 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 con-

ditions 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 2829 MPa around 400 C.

2. A new SCWO system design

A new SCWO system with a maximum treatment capacity of

2 k g h1 at 15% dry solids (DS) has been designed in CIGIT basedon the preliminary researches[11,12]. The system was particularly

descripted in the following four parts as illustrated inFig. 2.

2.1. Reactor design

As the heart of SCWO, reactor always suffers from both corro-

sion and plugging. Several types of reactors have been invented

to handle these problems. The related reviews can be found in lit-

eratures [2,79,13]. The DGSWR was adapted in the new system.

The basic structure of DGSWR is the same as that of a TWR: a dou-

ble 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 flowsthrough 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 fea-

sibility of DGSWR has been proved in previous research[11].

Fig. 1. Typical SCWO process. Adapted from literatures[8,9].

344 Z. Chen et al. / Chemical Engineering Journal 269 (2015) 343351


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Two of the same units named as Reaction Vessel 1 and Reaction

Vessel 2 (seeFig. 2) were connected by Flange (seeFig. 3) to con-

stitute the reaction area of the new system. Both of them are the

type of DGSWR. Each unit with height of 500 mm consists of pres-

sure 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 50 MPa. 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 2 105 m

and inner 5 floors nets with pore size of 5 106 m. Its inner diam-

eter 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 por-

ous wall and mix with wastes in reaction area, and then the reac-

tion 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 characteris-

tic 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 mea-

suring range from 0.00 to 8.33 104 m3 s1 (standard condi-

tions). 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

m

3

s

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 con-

sideration 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 is not easy to pump mul-

tiphase wastewater over a pressure of 22.1 MPa, such as oilwatermixture and oilwatersolid 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 C 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 [1621].

Oxygen multi-injection developed in CNRS of France is another

effective way[22]. Waste water was fed at subcritical or near-crit-

ical temperature and oxygen was injected through three inletsalong the tubular reactor [2325]. The temperature of mixtureFig. 3. Picture of the Flange connecting Reaction Vessel 1 and Reaction Vessel 2.

Fig. 2. Schematic diagram of the new SCWO system.

Z. Chen et al. / Chemical Engineering Journal 269 (2015) 343351 345
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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 agi-

tation easy[3]. And by coincidence, both of preheating at low tem-

perature (less than 350 C[26]) and presence of oxidant avoid the

polymerization of organics in waste water. The corrosion of sub-

critical water and the polymerization of organics are the main rea-

sons of operation problems in waste preheater and may be avoided

by this design.

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 C 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 tem-

perature of Stream 1 (T2in Fig. 2) is over 500 C 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 (T3inFig. 2) ranges from room temperature to 350 C depended

on the type of waste. Due to gravity, Stream2 witha higher density

flows down and mixes with upward Stream 1 in the Reaction Ves-

sel 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 50 MPa and

rated flow of 2 L h1 was purchased from Hangzhou Zhejiang Pet-rochemical 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 inFig. 3, the

inlet and distributor with the same inner diameter of 8 mm 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 h1. The two preheaters for

Stream 1 are both the type of coil electric heater. The maximum

power is 4 kW for Preheater B and 3 kW for Preheater C. Each hea-

ter was made of a single tube with inner diameter of 8 mm andlength of 6.5 m.

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 oxi-

dation), 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], 555%[30]and even much lower [31]. It must benoted that the waste water in these researches[2731]is artificial

and the salts is the type of sticky salts that has good water-solubil-

ity. 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 cannot 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

(seeFig. 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 inFig. 2, a Condenser was set up to quench the efflu-

ents 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 9.11 103 m2 for Heat

Exchanger and 1.53 102 m2 for Cooler.

Eleven K-type thermocouples with range from 0 to 800 C were

adapted to measure the temperatures and the exact positions wereshown inFig. 2. In order to protect thermocouple from corrosion, a

tube with inner diameter of 2 mm was equipped for each thermo-

couple (T1T4,T10T11, see Figs.1and2), and one single tube with

inner diameter of 4 mm was equipped for thermocouples ofT5T9.

They are evenly spaced for T5T9 and the distance of two neighbor-

ing positions is 100 mm.

The system pressure was controlled by a back pressure valve as

usual designs. In order to avoid operation problems caused by sol-

ids in effluent[3], a Filter with pore size of 5 105 m was placed

between Cooler and Pressure Control (see Fig. 2). The solid parti-

cles 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 inTable 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 2.6211.78% 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 showninTable 2.



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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 systemwas adjusted timely by switching on and off the Pre-

heating A, B, C and heating jackets of reactors. Table 3exhibits the

pressures and temperatures under steady state of each operationconditions. In addition, the operation processes of Exp. 1, Exp. 3

and Exp. 5 were recorded in details, and the temperatures variation

curves are shown inFigs. 46, respectively.

To preheat sewage sludge directly from room temperature to

supercritical condition will lead to plugging problems. Yin et al.

[33]suggest the preheating temperature should be no more than

200 C, under which the sewage sludge maintains good fluidity.

Hence, the inlet temperature of Stream 2 (T3) in this study was con-

trolled below 200 C (seeTable 3). According to operation steps,

the SCWO system would be preheated to supercritical conditions

before pumping Stream 2, which leads to the location ofT3 was

also warmed up to over 300 C during heating step. As shown in

Figs. 46, 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 ofT3 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 h1 is necessary in

the new SCWO system.

As shown inFigs. 46,T1,T4andT5T11(exceptT2andT3) 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 (seeTable 3). Such temperature

distribution was caused by feeds inlet temperatures, heating jack-

ets and heat dissipation. The temperature of Stream 1 dropped

from 572597 C (T2) to 385454 C (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

385454 C mixes with Air 1 of room temperature and Stream 2

of 149188 C, which leads to T1> T4. The exothermic reaction of

sewage sludge leads to the T1 increased about 50 C at the moment

of about 80 min after feeding Stream 2, as shown inFigs. 46. 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, whichcausesT5 T6< T7 T8> T9.

There is an interesting phenomenon in Exp. 1 that T3 sharply

rose about 100 C and the T1, T4T11 decreased slightly about

27 C when switching Stream 2 from sewage sludge (2.62% DS)

to pure water at same flow rate, as shown inFig. 4. This indicates

the pyrolysis of sewage sludge (preheating) is an endothermic

reaction and the SCWO of sewage sludge is exothermic.

Table 3

Pressures and temperatures of SCWO system under steady state of each operation conditions.

Exp. P (MPa) T1(C) T2 (C) T3(C) T4 (C) T5(C) T6 (C) T7(C) T8 (C) T9(C) T10(C) T11(C)

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

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.

Table 2

Operation conditions.

Exp. Stream 1 (sewage sludge) Stream 2 (water and isopropanol) Air Stoichiometric

oxygen excess

COD removal

efficiency (%)Solid

content

(% DS)

Mass flow rate

(kg h1)

Isopropanol

concentration

(wt.%)

Mass flow rate

(kg h1)

Mass flow rate of Air

1 (kg h1)

Mass flow rate of Air

2 (kg h1)

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 1

Properties of the sewage sludge.

Properties Values standard deviation

of three replicates

Dry solids (DS, 105 C, wt.% wet mass) 19.73 0.12

Ash content (815 C, 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 kg1 dry mass) 241 2

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During the study, it was found that the heat dissipation of Con-

denser 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 284333C (T10) was recovered by

the Heat Exchanger, the Stream 1 can be preheated from room

temperature to 154275 C (T11).

Sewage sludge was degraded to almost transparent liquid prod-

uct and brick-red solid product by the new SCWO system, as

shown inFig. 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 appear-

ance of No. 2 is nearly the same as that of de-ionized water. The

chemical oxygen demand (COD) removal efficiency range of

84.0299.15% was listed inTable 2, which is lower than that of lit-

erature reports[3,5]. Two factors are responsible. One is the low

reaction temperature due to heat dissipation and low inlet temper-

atures of air and sewage sludge. Temperature significantly impacts

the COD removal efficiency in SCWO. For most SCWO, operation

temperature is around 500 C which is much higher than 400 C

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 01

[34,35]. M. Goto et al. [36]proposed the reaction order for oxygen

is 0.51 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. How-

ever, 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 0.731.68 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

0.61kg h1 under 28.529.2 MPa. Besides, adding isopropanol as

co-fuel aggravated the lack of oxidant. As shown in Tables 2and

3, isopropanol did not significantly improve the reaction tempera-

tures, 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 Stream2. So pure water with inlet temperature of over

550 C, by contrast with isopropanol aqueous, is more economic

and practical in the new SCWO system.


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. 7. Images of sewage sludge and effluents in Exp. 5.


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. 8. Images of liquid products. No. 15 correspond to Exp. 15, respectively, and

No. 6 is the de-ionized water.



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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 con-

ditions 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 super-

critical 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 Sol-

ids 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 mea-

sured by Rise-2008 laser particle size analyzer and the results are

shown in Fig. 10. The particle size range from 0.10 106 m to

4.00 106 m and the average diameters of Solid A, Solid B and

Solid C are 0.93 106 m, 0.90 106 m and 0.77 106 m,

respectively.

When Rep< 2, the sedimentation of solid particle can be

described by Stokes Law[37]:

utd2

pqp qg

18l1

where, the l and q are the viscosity (Pa s) and density (kg m3) 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 qp is the true density of solid particles, 2537 kg m3.

The g is the constant of gravitational acceleration, 9.81 m s2. The

Rep is the particle Reynolds number and expressed by Eq. (2):

Rep dpupq

l2

As shown in Fig. 2, the reaction medium in reaction vessels

flows upward with a velocity ofu (m s1), and the solid particles

moves down by gravity with a settling velocity ofut(m s1). Thus,

the apparent velocity (up, m s1) of particles defined as Eq. (3):

up uut 3

When up > 0, the particles will be carried out by the reaction

medium; on the contrary, the particles will move downward when

up< 0. Theu in Eq.(3)is defined as Eq.(4):

u Q=q

3600A 4

where, theQis mass flow rate of reaction medium, kg h1. TheA is

cross sectional area of the porous wall, 6.8 104 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.

Thel andq of reaction medium were calculated by Eq. (5) [38]:

YXn

i

viYi 5

Fig. 9. Images of pressure bearing walls and porous walls after Exp. 5, without anycleaning. Fig. 10. Particle size distribution of solids in Exp. 5.

http://-/?-http://-/?-http://-/?-


8/9

where, the transport properties of the mainstream (Y) are the massaverage of corresponding pure component properties (Yi). The vis-

cosity 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 1061.50 105 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 inFig. 11, theup< 0 when the particle size is over

the critical diameter (1.36 105 m), which signify only the solid

particle with a diameter of over 1.36 105 m can be settled by

gravity under Exp. 5 operation conditions.

The calculation result is not fully accordant with the experi-

mental 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

(seeFig. 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 Eqs. (1), (3) and (4), both

lower Qand bigger dp will promote the gravity sedimentation of

solid particles. Hence, the counter-effect of upward u and down-

ward 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. Accord-

ing to Stokes Law (Eq.(1)),ut is proportional to the square ofdp,

which suggests solid with smaller diameter was easier to be

brought to move upward by reaction medium (combiningEqs.(1)and(3)). This can explain the average diameter is the big-

gest 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 sys-

tem, has been achieved under 2829 MPa and around 400 C.

Through the multi-feed injection technologies, sewage sludge with

a solid content up to 11.78% DS can be safely pumped and pre-

heated, and mixed with high temperature water to form supercrit-ical 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 sta-

bility 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 Tech-

nology R&D Programs of Chongqing (Grant number: cstc2012gg-

sfgc20001 and cstc2011ggC20014), the National Natural Science

Foundation of China (Grant number: 41203047) and the Key Labo-

ratory for Solid Waste Management and Environment Safety Open

Fund (Grant number: SWMES 2013-06).

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a new system design for supercritical water oxidation, chem eng j 2015

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