Supercritical Water Technology Applied to the Purification of Waters Contaminated by Toxic Micro-Polluting Organic Compounds

doi:10.4236/jwarp.2012.47053

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Journal of Water Resource and Protection, 2012, 4, 460-463

http://dx.doi.org/10.4236/jwarp.2012.47053 Published Online July 2012 (http://www.SciRP.org/journal/jwarp)

Super critical Water Technology Applied to the

Purification of Waters Contaminated by Toxic

Micro-Polluting Organic Compounds

Gabriele Di Giacomo, Luca Taglieri

Department of Chemistry, Chemical Engineering and Materials, University of L’Aquila, L’Aquila, Italy

Email: gabriele.dig[email protected]

Received April 12, 2012; revised May 15, 2012; accepted June 3, 2012

ABSTRACT

Purification of water contaminated by toxic organic compounds at low and very low concentration is a quite in teresting

challenge from both the technical and the economical point of view. In fact, the direct destruction of organic com-

pounds dissolved in very diluted aqueous solution is very costly and hardly achievable. To overcome this problems it

was studied and developed a new water purification process which is made of three step s: 1) removal of the diluted and

toxic polluting compounds by adsorption on activated carbon beds operating at ambient P ant T; 2) regeneration of the

exhausted carbon bed with supercritical water in order to obtain a mixture of water and polluting compounds signifi-

cantly more concentrated than the contaminated liquid water; 3) destruction of the toxic compounds in a continuous

Supercritical Water Oxidation Reactor. Step 1) was studied at laboratory scale in order to obtain all the required infor-

mation for modeling the adsorption operation; step 2) was modeled by using literature experimental data and, step 3)

was validated at pilot plant scale. In all the above mentioned steps, phenol was used as representative o f polluting com-

pounds.

Keywords: Supercritical Water Oxidation; Water Purification Pr ocess; S ol i d-Liquid Adsorpt i o n; Gas- Sol i d

Regeneration

1. Introduction

Millions of tons of organic compounds are manufactured

each year. Most of these compounds are toxic, poorly

biodegradable, and persistent, sometimes give biological

magnification. An important class among these chemi-

cals is aromatic chlorinated hydrocarbons such as chlo-

robenzene, chloronitrobenzene, poly-chlorinated biphen-

yls (PCB), polycyclic aromatic hydrocarbons (PAH) and

dioxins. These chemicals contaminate very large amount

of wastewater (often originated from chemical process

industry both using and producing organic compounds)

and drinking water (when spills or soil pollutants reach

the groundwater lev el). A process co mmonly us ed for the

purification of waste waters contaminated by low con-

centrations of organic compounds is the adsorption on

activated carbon beds [1,2]. Two main problems arise

from this process: the regeneration of activated carbon

and the ultimate disposal of polluting organic compounds.

It has been demonstrated [3,4] that the Supercritical Wa-

ter (SCW) can be efficiently used for regenerating the

spent activated carbon bed; coherently with the fact that

SCW behaves as an organic liquid solvent. In addition,

many researchers have shown the effectiveness and the

wide range of applicability o f Supercritical Water Oxida-

tion (SCWO) in destroying toxic organic compounds

[5-9]. The purpose of this paper is to develop and to de-

scribe a new process (obtained by properly combining an

adsorption section with a SCWO section) characterized

by high efficiency in both water purification and destruc-

tion of polluting materials without producing any haz-

ardous by-product.

2. Materials and Methods

Activated carbon and phenol, as representative polluting

compound, were purchased from SICAV (Chieti, Italy)

and Aldrich, respectively. All the thermodynamic and

kinetic details related to the adsorption section were

studied at laboratory scale and reported in previous pa-

pers [1,2], while all the details related to the regeneration

of the spent activated carbon bed with SCW were re-

ported by [3] and [4].

The SCWO of the gaseous stream containing the pol-

lutants removed from the activated carbon bed was vali-

dated by using a pilot plant which can operate at pressure

up to 80 MPa and at temperature up to 1000 K. The con-

tinuous SCWO plant was made by a 0.9 m, 9/16” OD,

G. DI GIACOMO, L. TAGLIERI 461

3/16” ID AISI 316 tube. Pre-heaters and SCWO reactor

are equipped with 3 PID controllers in order to maintain

the reaction temperature at a fixed value. The gaseous

stream obtained by cooling to room temperature the su-

percritical solution coming out from the SCWO reactor is

measured with an accuracy of ±0.1% by a mass flow

meter which also acts as totalizer (MICRO MOTION

D6).

3. Results and Discussion

By using the results on both equilibrium and kinetics ad-

sorption of compounds polluting liquid aqueous solutions

[1,2], alo ng with the results of [3] and [4], on th e regene-

ration of activated carbon with SCW, it was developed a

new process which enables one to remove almost com-

pletely the polluting compounds from the liquid water

which, in turn, are completely destroyed in the SCWO

section of the process shown in Figure 1.

For this process, the material balance along with the

values of P and T of each stream are reported in Table 1.

As can be seen the SCW required to regenerate the

spent activated carbon bed is about 20% of the treated

contaminated water. As a consequence the quantity of th e

water processed in the SCWO section is 5 times lower,

while the concentration of the polluting compound is 5

times higher. Since the SCWO section is characterized

Figure 1. Flow sheet of the proposed process.

G. DI GIACOMO, L. TAGLIERI

462

Table 1. Material balance of the process shown on Figure 1, based on 100 m3/h of micro-polluted water.

Stream n. Description Temperature (˚C) Pressure (bar)

Mass flow rate

(kg/h)

1 Air 25 1 522

2 Demineralized water 25 1 171,422

3 Contaminated water 25 1 100,000

4 Air - 350 522

5 Air 440 350 522

6 Contaminated water 25 1.75 100,000

7 Contaminated water 25 1.75 100,000

8 SCW 25 1 19,828

9 Purified water 25 1 80,070

10 SCW 25 350 19,828

11 SCW 78 350 19,828

12 SCW 450 350 19,828

13 SCW and desorbed contaminants 450 350 19,854

14 Gaseous mixtures from SCWO reactor 450 350 20,374

15 Gaseous mixtures from SCWO reactor 470 350 20,374

16 Gaseous mixtures from SCWO reactor 374 350 20,374

17 Water and gas 300 350 20,374

18 Water and gas 300 1.2 20,374

19 Steam 120 1.2 171,422

20 Water and gas 40 1.2 20,374

21 Recovered steam 120 1.2 1807

22 Gas 40 1.2 532

23 Purified water 40 1.2 19,843

24 Steam 120 1.2 169,615

25 Condensed water 99 1.2 1807

26 SCW 179 350 19,828

27 Purified water 25 1 80,070

28 Flue gas 800 2

7118

29 Flue gas 460 2 7118

30 Flue gas 426 2 7118

31 Purified water 40 1.2 99,913

by high investment and operating costs in comparison

with the adsorption section, the proposed process allows

one to purify the water and to destroy the toxic comput-

ing compounds in an optimal way from both the techni-

cal and economical point of view.

In Table 2 are reported the list of symbols used in the

process flow sheet along with the corresponding signifi-

cance.

4. Conclusion

It has been found that by properly combining an adsorp-

tion section with SCWO section it is possible to effi-

ciently purify polluted and micro-polluted water and to

G. DI GIACOMO, L. TAGLIERI 463

Table 2. Description of the symbols used in Figure 1.

Symbol Description

W01 Overheater

W02 Preheater

W03 Overheater

W04 Condenser

W05 Vaporizer

W06 Overheater

C01 Compressor

B01 Feed tank

B02 Gas-liquid separetor

B03 Tank

P01/06 Pumps

R01 SCWO reactor

K1 Fixed bed adsorber

K2 Fixed bed adsorber

V01/27 Valves

VR01/03 Ropture disk

M01/02 Mixer

efficiently destroy the toxic polluting organic compounds

in an optimal way. In fact, the process developed in this

study allows one to take advantage of the best characteri-

stics of the combined techn ologies. In addition, since the

regeneration of the spent activated carbon bed in done by

using SCW, it is possible to avoid the production of any

kind of polluting stream and the consumption of any kind

of chemicals.

5. Acknowledgements

This work has been financially supported by the Italian

Ministry of the Research, RIA Program.

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