Review pubs.acs.org/IECR
Problems in Supercritical Water Oxidation Process and Proposed Solutions Violeta Vadillo,* Jezabel Sánchez-Oneto, Juan Ramón Portela, and Enrique J. Martínez de la Ossa Department of Chemical Engineering and Food Technology, Faculty of Sciences, Agro-food International Excellence Campus CeiA3, University of Cádiz, 11510 Puerto Real, Spain ABSTRACT: Supercritical water oxidation (SCWO) is a promising green technology to completely convert hazardous wastewaters to innocuous products, allowing energy recovery. This process has been extensively applied to many model compounds and real wastewaters at laboratory scale. However SCWO treatments at the pilot plant scale of real wastewaters are much less extensive in literature. Furthermore, the application of this technology to industrial wastewaters has the two main drawbacks of corrosion and salt deposition, and some other problems to be solved related to management of biphasic wastes, presence of suspended solids, high costs, etc., so currently the industrial scale-up and commercialization of the process is still subject to difficulties. This work reviews the main technical solutions studied by numerous authors to avoid the drawbacks mentioned. Besides, since the economic feasibility of the process will depend on the energy recovery of the reactor effluent, this aspect is also presented in this review.
1. INTRODUCTION Supercritical water oxidation (SCWO) is a promising technology to treat a wide variety of industrial wastewaters. The main advantage of SCWO over other treatment methods such as landfill is that it is a destruction method. Destruction methods based on oxidation of organic matter include activated carbon treatment, biological treatment, incineration, wet air oxidation, and supercritical water oxidation. Choosing the method to be used depends on the organic content of wastewater. For organic contents up to 1%, biological and activated carbon treatments are suitable. On the other hand, incineration is suitable to highly concentrated wastewaters, but in the range of 1−20% organic matter, SCWO is a better option, due to the toxic gases produced and the high cost of incineration. SCWO is an oxidation process that takes place above the critical point of water, that is, 374 °C and 22.1 MPa. Water polarity is a function of temperature and pressure. At supercritical water conditions water is a nonpolar solvent completely miscible with organics and gases like oxygen.1 Under these conditions there is a homogeneous reaction medium where there are no mass transfer limitations. Furthermore, as the reaction takes place at high velocity due to the high temperature used, the residence times necessary to achieve high destruction levels (99%) are lower than 1 min.2 Furthermore, the reaction products are not toxic. Production of NOx, SOx, and dioxins are negligible because the temperature is too low for these compounds to be produced.3 In the region close to the critical point of water, density is highly dependent on pressure due to the fluid being very compressible. So in the supercritical range, the water density is around 0.1 g/cm3, which is similar to gas densities.4 The dielectric constant has a value of 80 at 25 °C and 1 atm and decreases down to 1−2 at 450 °C so supercritical water behaves as a nonpolar solvent.5 On the other hand, the low value of the dielectric constant reduces the solubility of inorganic salts in supercritical water. In relation to viscosity and diffusivity, the © XXXX American Chemical Society
former is lower and the latter is higher in supercritical conditions so reactions take place at very high velocities and without mass transfer limitations.6
2. CURRENT SCWO PROBLEMS The SCWO process has some disadvantages that can be classified in technical drawbacks, high investment, and high operational costs. The combination of high temperature and harsh chemical environment inherent to this application causes the main technical problems of SCWO, corrosion and salt precipitation.7 Corrosion is influenced by the dissociation of acid, salts and bases, solubility of gases, solubility of corrosion products, and the stability of the protecting oxide layer. However, it is difficult to find a material that can stand all conditions, so it is necessary to choose the material as function of the operation conditions.8 The huge relevance of the corrosion problem is because it has limited the SCWO development. For this reason, several authors have focused their work on the detailed study of this phenomenon.3,8−13 There are several ways to manage corrosion7 including a cooling strategy to avoid the conditions of high temperature and density which are the conditions of high corrosion rates,3 and new reactor concepts such as Transpiring Wall or Film-Cooled Reactors.14−23 Owing to the low solubility of inorganic compounds in supercritical water, the precipitation of salts occurs in the SCWO reactors and can lead to equipment plugging. Solid salts form conglomerations that cover the walls of the equipment, reduce the heat transfer in heat exchangers, and produce plugging in pipes and reactor. Besides, between the salt layer Received: January 15, 2013 Revised: May 14, 2013 Accepted: May 14, 2013
A
dx.doi.org/10.1021/ie400156c | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Review
Figure 1. Main problems that can occur in different steps of the conventional SCWO process.
and the wall of the reactor there is a dead place where heavy corrosion can occur.3 As a summary, Figure 1 shows the main problems that can occur in different steps of the conventional SCWO process. The solution to these problems is necessary to advance in the scale up of the SCWO process. Corrosion. In relation to corrosion, several authors have studied different ways to solve this problem; specifically, Marrone and Hong7 carried out a detailed study on corrosion control methods in supercritical water oxidation. Their main points are highlighted as follows: Use of high corrosion resistance materials (Inconel 625 and Hastelloy 600), use of liners, use of coating, design SCWO systems including transpiring wall/film-cooled wall reactors, assisted hydrothermal oxidation,17 use of a base to preneutralize the feed stream, cold (ambient temperature) feed injection, addition of quench water, optimization of process operating conditions (such as temperature, pH, electrochemical potential, etc.) as is the case shown by Kritzer and Dinjus,3 who used a cool down strategy to minimize corrosion. Another easy solution is to avoid corrosive feeds or the pretreatment of the feed to remove corrosive species as in the case of Hong et al.18 Salt Precipitation. In relation to salt precipitation, Marrone et al.19 studied salt precipitation and scale control in supercritical water oxidation focused on commercial scale applications. Marrone et al.19 summarized the commercially designed approaches developed in the last two decades. Those methods are specific reactor designs (such as reverse flow, tank reactor with brine pool, transpiring wall reactor, adsorption/ reaction on fluidized solid phase, reversible flow in tubular reactor, and centrifuge reactor) and specific techniques (such as high velocity flow, mechanical brushing, rotating scraper, reactor flushing, additives, low turbulence, homogeneous precipitation, crossflow filtration, density separation, and extreme pressure operation). To avoid corrosion and salt precipitation problems, industrial wastewaters to be treated by means of SCWO in tubular reactors need to satisfy certain requirements, as can be seen in Table 1; this kind of wastewater has to be toxic, non-
Table 1. Wastewater Ideal Requirements To Be Treated by SCWO in a Tubular Reactor21 parameter
value
type of wastewater
toxic nonbiodegradable
flow rate
>100 kg/h 50 g O2/L
salt content
NaCl < 200 ppm Na2SO4 < 1 ppm CaCl2 < 3 ppm Na2CO3 < 1 ppm Mg(OH)2 < 0.003 ppm 2 < pH < 11 22.1 MPa), postponed or inhibited the industrial scale up and commercialization of SCWO technology. For this reason, the SCWO process has to be considered not only as a process in which a residue is completely removed, but also as a possible source of energy production due to the possibility of exploiting the exothermicity of the oxidation reactions. Because of interest in energy production, various researchers have addressed the issue through theoretical studies and simulation. Cocero et al.23 developed an energy study for a 2 m3/h SCWO plant using the software Aspen Plus. In this study, the reactor effluent was used to preheat the feed and to produce the energy necessary to the air compression. They claimed that to achieve it, a fraction of 26.5% of the effluent should be expanded in a turbine. Later, Bermejo et al.35 studied SCWO of coal including energy recovery as an alternative to carbon power plants. They proposed a solid separator downstream reactor where solids were removed of the effluent. This stream that has 650 °C and a pressure of 30 MPa was expanded in a turbine achieving 37% efficiency. If expansion was conducted in two steps with intermediate reheating, efficiency would be increased up to 40%. However, Marias et al.36 claimed that energy recovery proposed by Bermejo et al.35 was not feasible because it was based on particle separation at 650 °C and 30 MPa and a separator and turbine with these properties cannot be presently built using current technology. From a theoretical point of view, they used Pro Sim Plus software to simulate energy recovery, obtaining 1.75% of efficiency in the Rankine cycle, which is not economically feasible. As an alternative, they proposed energy recovery in SCWO using an auxiliary fluid that receives the energy produced in the SCWO process. Specifically, they claimed that it is possible to recover 627 kW from a stream at 650 °C and 30 MPa resulting from SCWO of 900 kg/h of wastewater. On the other hand, Svanström et al.,37 simulated energy recovery from 10% weight sewage sludge SCWO using Aspen Plus software. Results of the simulation showed that the reactor effluent with 4.9 kg/h and 359 °C can be used to heat a municipal water stream from 40 to 90 °C at a rate of 54.8 kg/h. Recently, Jiménez-Espadafor et al.38 proposed that it is possible to decrease SCWO treatment costs by recovering energy at low temperature and high fluid pressure, such as water heating and steam generation. For example, for a supercritical flow of 1000 kg h−1 (water and air), the recovered energy ranges from 118 kW (1700 m3 h−1 of hot water at 65 °C) to 75 kW (100 kg h−1 of steam flow at 1.1 bar and 170 °C). E
dx.doi.org/10.1021/ie400156c | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Review
Table 2. Summary of Studies of Hydrothermal Flame Formation (n/a: Not Availale) fuel
concentration
oxidant
ignition temperature (°C)
reactor
refs
methanol isopropyl alcohol methanol methanol methanol isopropyl alcohol
30% mol 2% volume 15% weight 16% weight 16% weight 4% weight
air air oxygen oxygen oxygen air
410 476 460 472 452 450
n/a tubular transpiring wall reactor transpiring wall reactor WCHB-3 tubular 1/4″
51 49 45 46 50 53
SCWO plant so the objective is to design it with a small volume. In this way, Abeln et al.10 estimated that the tubular reactor costs represent 10% of the overall equipment costs in a SCWO plant able to treat 100 kg/h of wastewater. In a recent study, Marulanda and Bolaños54 estimated that reactor cost represent 7% of the overall equipment costs in a mobile SCWO plant able to treat 1 L/h of organic wastewater. From the point of view of operational cost the choice of the oxidant is a key point, Bermejo and Cocero39 claimed that it is more economical to use pure oxygen instead of air because at industrial scale the compression cost is very high. On the other hand, Savage et al.55 suggested that a catalytic SCWO process is a more competitive alternative because with the use of a catalyst the temperature necessary to reach removal efficiencies higher than 99% is reduced significantly decreasing the energy demand. At present, economical studies on SCWO at the industrial scale are scarce in literature. Gidner and Stenmark27 estimated operational costs of a sewage sludge SCWO plant based on a flow rate of 7 m3/h of sewage sludge being 137 €/t of dried sludge. Svanström et al.2 estimated total cost for a 1 t/day plant being 243 $/t dried sludge. O’Regan et al.33 claimed that treatment cost of sewage sludge SCWO is in the range 36.6− 73.15 €/t. Abeln et al.10 first, estimated treatment cost of an ideal wastewater made of a mixture of ethanol 10% weight and water using air as oxidant in a plant of 100 kg/h with two different reactors: tubular and transpiring wall reactor, being the treatment costs 406 €/t and 660 €/t, respectively. Later, they estimated the treatment cost for a 1 t/h plant being 330−430 €/t for the transpiring wall reactor plant and 203−264 €/t for the tubular reactor plant. Recently, Marulanda and Bolaños54 estimated that treatment cost for a mobile SCWO unit able to treat 1 L/h of oils contaminated with PCBs amounts to 75 $/L. Finally, Vadillo et al.21 estimated the treatment cost for a real wastewater SCWO in a 1 t/h plant that amounts to 230 €/t. Economic results showed that although SCWO technology was initially shown as a technology suitable for all kinds of wastes, research conducted over the last three decades showed that this technology can only be applied at the industrial scale using a tubular reactor and to treat wastewaters that satisfy the requirement shown in Table 1.
alcohol. Besides, concentration higher than 5% volume produced continuous flames and that would be the minimum concentration needed to get spontaneous ignition of a stable flame in the semicontinuous reactor shown by Pohsner and Franck.48 They studied the effect of injection temperature and proposed to start the process at 380 °C, and, due to the exothermic character of the reaction, the temperature increased up to 470 °C when hydrothermal flames appeared. However, this way of operation has the disadvantage of the formation of CO and a low removal efficiency (
