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Abstract
If left to conventional treatment processes, the destruction of toxic organic wastewaters and associated sludge volume reduction clearly becomes an overwhelming problem. Two options, incineration
and supercritical water oxidation (SCWO), exist for the complete destruction of toxic wastewaters and organic sludges. Incineration has associated problems such as very high cost and public resentment; on the other hand SCWO has proved to be a very promising method for the treatment of many different wastewaters and sludges. In early 1998 Chematur Engineering inaugurated its 250 kg/h SCWO demonstration facility and since then several successful treatability tests have been performed. Among these are tests with different complex wastes, from a SCWO process point of view. They have been performed in two different ways:
1.Making the waste treatable in an “ordinary” SCWO unit, i.e. remove halogens and dissolved salts.
2.Making the SCWO unit able to handle the difficulties in the waste, e.g. halogens.
Another waste, de-inking sludge, has been treated several times. The appealing results from these tests are that, besides the complete destruction of the organic material, the remaining paper filler has shown to be good enough for reuse. This exemplifies the general possibility to destroy the organic material in the waste by SCWO and leave a “clean” valuable inorganic material in the effluent, which could easily be recovered. The extremely clean inorganic material recovered ought to increase the interest in SCWO treatment. This type of waste may very well be the breakthrough of SCWO.
Introduction
Oxidation of organic wastes to carbon dioxide, water, and other small molecules can effectively minimise waste volume and detoxify many hazardous compounds. Incineration in air at atmospheric pressure is the most common oxidation technique currently practised. However, incineration meets an increasing opposition from the public and furthermore the cost for incineration of waste has a tendency to rise constantly because of the increasing demand on the flue gas cleaning. A supercritical water oxidation (SCWO) system can handle aqueous streams containing organic material in relatively low concentrations
and offers inherent control over emissions and coupling to energy recovery systems. Under normal conditions, water is seen in either of its three states: atmospheric steam, liquid water, or ice. If water is heated and compressed to sufficiently high temperature and pressure a new fluid state of water emerges. Water at high temperatures and pressures, above 374 °C and 221 bar, is a fluid that is neither a gas nor a liquid, it is in its supercritical state. The reason for the efficiency of SCWO in destroying organic compounds is the unique properties of water above its critical point. The change in magnitude of different properties is very rapid at entrance into the supercritical region. Any substance above its critical point has properties that are in between those of a gas and a liquid. The density of supercritical water (SCW) is comparable with liquid water densities, and high enough for reasonable throughputs in a process. On the other hand the viscosity and diffusivity in the supercritical region are more like that of a gas. Water gets a high solvating power, because of the low dielectric constant, obtained when becoming supercritical and most organic compounds and gases are completely miscible in all proportions in SCW.
In 1995 Chematur Engineering (CEAB) started its SCWO activities by a license agreement for SCWO signed with Eco Waste Technologies (EWT) and early 1999 Chematur acquired the exclusive world-wide rights to EWT’s SCWO technology. The Chematur SCWO process is marketed under the trade name AquaCritox®. In early 1998 CEAB inaugurated its 250 kg/h SCWO demonstration facility based on the EWT process and since then several successful and extensive treatability tests, with real waste have been done [1]. CEAB has a Japanese licensee, Shinko Pantec, they have built a pilot/small full-scale SCWO unit with a capacity of about 1100 kg/h. The unit was commissioned and started up about half a year ago. Shinko Pantec foresees the treatment of sewage sludge to be the main market for SCWO within Japan. The size of their unit corresponds to the production of sewage sludge from a sewage works for more than 50,000 inhabitants.
The Pilot Plant
The feed tank is equipped with a stirrer designed for viscous sludge. The bottom outlet from the feed tank, Fig 1, is connected to a mono pump and a macerator in order to eliminate big particles entering
the high-pressure pump. The plant is equipped with two different high pressure feed pumps, one for clear liquids and one for dispersions and sludges. The high pressure feed pumps raise the feed pressure to
about 250 bar and pump the feed through the SCWO system. The feed enters the tube side of a doublepipe economiser where it is preheated by the reactor effluent. After leaving the economiser, the feed
enters the heater. At start up or if the organic concentration is lower than about 3%, the feed has to be heated further, before reaching the reactor, e.g. in a gas heater.
From the heater outlet, the hot feed enters the reactor. In the reactor, oxygen is injected to start the oxidation reaction. The oxidation reaction generates heat and, as a result, the reactor temperature
increases. The inlet feed concentration may be too high for complete oxidation of the organic material to occur in one step without exceeding the reactor’s design temperature, 600 ºC. As a result, the waste may be oxidised in two stages. At the beginning of the second stage quench water is added with the oxygen.
The water cools the effluent from the previous stage enough to allow the additional oxygen to continue the oxidation reaction without exceeding the temperature limit. After passing through the reactor, the effluent flows through the economiser, outer tube, where it preheats the incoming feed. The effluent is cooled to its final exit temperature in the effluent cooler prior to passing through the pressure control valve or the pressure reduction coils. This proprietary pressure reduction system is used for pressure control when the effluent contains solid inorganic material. Adding choke water before the coils controls the pressure drop in the coils. The pressure is reduced from about 250 bar a to slightly above atmospheric pressure. The effluent then enters a gas/liquid separator where the carbon dioxide generated in the process is separated from the effluent.
Fig. 1: Simplified flow sheet of the pilot plant.
De-inking Sludge
When recycling paper, de-inking sludge is received as a by-product. This sludge contains about 3% organic material, mainly fibres, and 3% inorganic material, mainly paper filler. The ability of SCWO to
destroy the organic material in this sludge completely has been proven elsewhere [2,3]. In addition to the destruction of organic matter, the industry has an interest in recycling the paper filler after the SCWO treatment.
Initial experiments showed fouling problems in the heat exchanger due to excessive amount of dissolved salts in the sludge. Hence, further experiments were performed with a partially dewatered
sludge which was dispersed in fresh water to give a COD value of about 70 000 ppm before SCWO treatment.
Tests with this sludge were carried out without any problems and were performed at 540 and 580oC. No significant difference in destruction efficiency was obtained between the two different reaction
temperatures. After fine-tuning the reaction conditions very bright paper filler was recovered from the process, close to the value of virgin material, see Fig. 2.
Furthermore, later experiments showed that more concentrated sludge may be treated and that the fibres are the limiting material with regards to treatability. Test performed with sludge containing more
than 6 % fibres showed the same excellent result as the earlier work. About 3 ton (dry substance) paper filler was produced during the last test period. The filler was used in a full-scale paper machine, producing paper for a major Swedish newspaper. The paper produced showed normal quality.
Fig. 2: The appearance of recovered paper filler, wet and dry, together with untreated de-inking sludge.
Wastewater from Fine Chemical Production
A wastewater from a fine chemical producer has been treated with SCWO. The original wastewater was pre-treated in a forced circulation evaporator. The purpose of the evaporation was to remove the considerable amount of dissolved salt in the original wastewater in order to prevent scaling of the SCWO unit. The salt free condensate was then fed to the SCWO plant, Fig 3. The condensate contains the light organic compounds and water. The light organic compounds were mainly methanol, ethanol and acetone, see Table I, but also traces of toxic and persistent substances. The remaining solution, containing salt and heavy organic compounds, from the evaporation was filtered and the solids would be sent for destruction by incineration.
Fig. 3: Concept for treatment of a waste stream containing substantial amount of dissolved salt.
SCWO treatment of the produced condensate from the evaporator was completed in a satisfying way. The experiments were carried out at 570oC and the conversion of the organic material to carbon dioxide and water was above 99.99%.
Table I. Evaporation results
By using evaporation and SCWO the amount of waste that needs to be incinerated is reduced to about 20% of the original waste stream. Even though this solution includes several steps, it is economic
compared to direct incineration of the entire stream.
A WAY OF HANDLING CORROSIVE MATERIAL IN A SCWO UNIT
The invention [4] uses the fact that no corrosion (almost/generally) appears in the supercritical region and that the corrosion is most severe between 270 – 390oC [5-9]. Besides the nature of the
corrosives, the lower limit depends of course on concentration and the higher on pressure. Consequently, the invention makes it possible to avoid this temperature range in the traditional SCWO system. The
invention is described below as well as two different examples of its use.
Corrosive Feed
The pipe, feeding the cold corrosive waste, is led into a pre-heated stream, after the economiser and heater, see Fig. 4. A certain distance before this pipe ends and the two streams are mixed begins a
pipe (mixing pipe) which is placed in the main pipe. This mixing pipe should have enough length for the mixing to be completed before it ends and the mixed stream temperature should be above 390oC. The mixing pipe, as well as the injection pipe, should be of a corrosion resistant material, e.g. titanium [10], and easy to change if corrosion does occur. The mixed stream is finally fed to the reactor or continuing its way through the reactor.
Fig. 4: Arrangement to avoid corrosion during heating.
Cooling of Corrosive Effluent
In the same manner as above, the cooling of a corrosive effluent may be done by injection of cooling water. The effluent from the reactor is cooled to a temperature just above the critical point and
enters into a bigger pipe, where the cooling water flows, Fig. 5. To ensure that no liquid elements are in contact with the main pipe before the mix is homogenous (considering temperature); a mixing pipe is
installed. Just as for the feed system above, the mixing and injection pipe should be of a corrosion resistant material. The cooled effluent temperature is controlled by the amount of cooling water, e.g. to
270 ºC, depending on the concentration and nature of the corrosive compound. Finally the cooled effluent is fed for further cooling.
Fig. 5: Arrangement to avoid corrosion during cooling.
Case 1: Wastewater from Amine Manufacturing
The invention has been used on a wastewater which was produced in a process for amine manufacturing. The waste contains ammonia and short chain amines and gives a nitrogen rich water
containing almost as much total nitrogen (Tot-N) as total organic carbon (TOC), about 15 000 to 20 000 mg/l. Although SCWO has been demonstrated to be effective for the destruction of most organic compounds, little success has been achieved in the complete destruction of ammonia or nitrogen in highly nitrogen containing wastes. However it is known that it is possible to destroy ammonia if the ratio of total organic carbon (TOC) and total nitrogen (Tot-N) in the wastewater is high [11,12]. If this ratio is low it is not possible to destroy all ammonia with oxygen but it is shown elsewhere [13,14,15] that it is feasible to destroy the ammonia using nitric acid for the oxidation. The authors have described elsewhere [1], how the ammonia was destroyed using nitric acid. The experiment was carried out by injection of oxygen and after the oxidation by the oxygen was completed, nitric acid was injected into the reactor, as in Fig 4. The mixing pipe as well as the injection pipe was made of titanium, to ithstand nitric acid with a concentration of 65%. At the end of the mixing pipe the mixed flow was well above supercritical temperature. Establishing the method for destroying of ammonia in a continuous process, the pilot unit was operating using the invention for about 100 hours. No damage could be seen on the titanium parts by visual inspection, at the end of the experiments. Even if titanium do not withstand concentrated nitric acid for an extended period, e.g. 5 years, it seems to cope with the conditions described above for several hundred hours and if needed the titanium parts could easily be replaced and the cost would be insignificant.
Case 2: Different Wastes Containing Chlorine, Bromine and Iodine
Several tests have been accomplished with wastes containing, chlorine, bromine, iodine or a combination of those. Both wastes with the elements present as ions and in organic molecules have been
treated. If the corrosive element is a part of an organic compound it is generally not a problem before the oxidation, unless hydrolysis occur. However in one waste the halogen was present as methyl iodide and that molecule hydrolyses very easily. At the destruction of methyl iodide HI is formed. However, HI is reported to be oxidised to I2 very rapidly [9]. The invention was used to insure that no HI, if any, could come in contact, at subcritical temperatures, with the material of construction (a high nickel alloy) used in the pilot plant. The methyl iodide containing waste was injected, cold, directly into the reactor, and mixed with a hot stream of water and oxygen, in the same way as for the nitric acid above. The concentration of methyl iodide based on the mixed stream varied between 1000 – 1500 ppm. Since almost all iodide was oxidised to I2 the effluent was not corroding the nickel alloy downstream the reactor.
Another rather concentrated organic waste containing chloride and bromide was injected into the reactor using the invention as for the methyl iodide. The concentration of chloride and bromide based on
the mixed stream was about 400 ppm each. In order to minimise the corrosion downstream the reactor, sodium hydroxide was added to adjust the pH to normal levels, pH=3,5-4, for SCWO effluent, without
halogen ions. After the pH adjustment the Ni levels in the effluent decreased from about 5 ppm to about 0.3 ppm and the content of Cr was unchanged around 0.5 ppm. However, the Mo content in the effluent was significantly higher after pH adjustment in some of the samples, whereas in other samples it was basically unchanged.
Both experiments were carried out for about 100 hours. No damage could be seen on the titanium parts. However, for a continuous processing of the tested material containing the bromide and chloride, or a similar waste containing the same or higher amount of halogens it would probably be preferred to use the quench cooling in a titanium mixing pipe, Fig 5.
CONCLUSION
The presented studies performed on a state of the art pilot plant, shows that the SCWO technology is ready for commercialisation. Furthermore for wastes like de-inking sludge, containing valuable inorganics, SCWO treatment offer, besides an extremely clean effluent, the possibility to recover these compounds. The extremely clean inorganic material recovered ought to increase the interest in SCWO treatment. This type of waste may very well be the breakthrough of SCWO.
SCWO has two major limitations with respect to substances present in the water, dissolved salts and acids formed of chlorine, sulphur etc. There are simply two ways of treating such wastes: 1. Making the waste treatable in an “ordinary” SCWO unit, i.e. remove halogens and dissolved salts. 2. Making the SCWO unit able to handle the difficulties in the waste, e.g. halogens. It is not probable that there will be a general SCWO system suitable for all such waste from both economical and technical point of view. It has to be reviewed from case to case which solution should be preferred.
Abstract
Supercritical water oxidation (SCWO) is an innovative, economic and effective destruction method for organic wastewater and sludge and is a realistic alternative to conventional methods. From 1998 to 2007 extensive evaluations of the destruction of sewage sludge by SCWO were performed by Chematur Engineering AB on two state of the art pilot plants. These units had capacities of 250 kg/h and 1100 kg/h, respectively. The results achieved showed that the technology easily gave 99.9% destruction of the organic material in the sludge and the inorganic material left in the effluent was non leachable and is very easily settled. The very encouraging results indicated that the technology was ready to be commercialised for treatment of sewage sludge. In 2007 SCFI Group, acquired the patented super critical water oxidation technology (AquaCritox®) from Chematur Engineering AB of Sweden. Further work on design capacity and energy recovery by SCFI Group has led to a reduction in over all build cost, a
significant increase in processing capacity and increased energy recovery in the form of electricity generation. SCFI Group in 2008 commissioned its 250 litre per hour demonstration AquaCritox® super critical water oxidation demonstration plant in Cork, Ireland. Since May 2008, the plant has been subjected to continuous operation on sewage sludge to document the long term reliability of the AquaCritox® SCWO technology.
Introduction
Oxidation of organic wastes to carbon dioxide, water, and other small molecules can effectively minimise waste volume and detoxify many hazardous compounds. Incineration in air at atmospheric pressure is the most common oxidation technique currently practised. However, incineration meets increasing opposition from the public concerned about dioxin production and toxic ash. The ash from incineration is usually treated as hazardous and therefore requires disposal at hazardous landfill. As landfill costs are increasing year on year, this places increased pressure on costs of incineration.
A Super Critical Water Oxidation System (SCWO) will oxidise aqueous streams containing organic material in relatively low concentrations. SCWO is an exothermic process and is autothermal at just three percent organic content in the waste stream. When the organic content within the waste stream is in excess of three percent, the excess energy may be utilised to generate electricity and heat. The heat can be utilised to generate steam and hot water which can find application in sludge thermal hydrolysis and or anaerobic digester heating requirements. Waste streams do not require drying before treatment and the SCWO process produces no hazardous by-products and the inert residues are sterile, stable, non-toxic and suitable for reuse or recovery. The first commercial SCWO plant was built in 1994 in the USA, for the Huntsman Chemical Corporation by Eco Waste Treatment (EWT). The SCWO plant design was based on the extensive research and development work EWT had carried out in conjunction with University of Texas at Austin Texas. This plant operated successfully from 1994 until 2000 when it was decommissioned. In 1995 Chematur Engineering AB licensed the SCWO technology from EWT and in 1998 acquired EWT. Chematur built a 250 litre per hour demonstration SCWO plant in Karlskoga, Sweden in 1998. During the period 1998 to 2007 Chematur performed extensive research and development on the treatment of sewage sludge using scwo technology leading to further patented improvements in design that over came the issues relating to pumping, blockages, and fouling that had previously been seen by industry as impediments to the adoption of SCWO technology. In all this facility accumulated over eight thousand hours of operation.
Supercritical Water
Under normal conditions, water is seen in any of its three states: steam, liquid water, or ice. If water is heated and compressed to sufficiently high temperature and pressure, water enters a 4th state known as the super critical state. In the case of water this occurs above 374oC and 221 bar, (figure 1). The reason for the efficiency of SCWO in destroying organic compounds is the unique properties of water above its critical point. Above the critical point, water has properties between those in its liquid and gas state.
Figure 1: A simplified phase diagram of water
The density of supercritical water (SCW) is comparable with liquid water densities, and high enough for reasonable throughputs in a process. On the other hand the viscosity and diffusivity in the supercritical region are more like that of a gas. Due to the low dielectric constant of water in the super critical state, the solubility of organic compounds and gases is high. This together with the high diffusivity means there is an insignificant mass transfer resistance thus enabling very fast reaction rates.
Figure 2: Physical properties of Super Critical Water.
Destruction of Organic Molecules in SCWO
SCWO destroys all organic wastes containing any combination of elements. Low biodegradability or high toxicity has no effect on suitability for treatment by SCWO. Higher molecular weight organic compound are destroyed or transformed almost immediately, smaller molecules such as acetic acid and are generally slower and are typically the rate controlling compounds in the process. Never
the less reaction time is extremely fast, at typically less than 60 seconds. Nitrogen containing compounds will revert to elemental nitrogen with out production of NOx. The process does not produce dioxins.
Process Description
Following some limited pre-treatment (agitation and heating for viscosity control and milling to control particle size) a high pressure pump is used to raise the pressure of the stream to 250 bars. Feed enters the economiser where it is heated to supercritical temperature by the reactor effluent. After leaving the economiser the feed enters the reactor. At start-up, or if the organic concentration is less than 3%, the feed is heated at the reactor by external booster heatersg
In the reactor oxygen is injected to start the oxidation reaction. The reaction is exothermic and temperatures increase as the reaction progresses. After passing through the reactor the effluent flows through the economiser where it heats the incoming stream. In larger plants the heat of reaction can be recovered via a steam generator for power generation. District heating is also an option depending on the plants location. Following any heat recovery the effluent is cooled to its exit temperature by a cooler prior to passing through the pressure reduction system where the pressure is lowered to <10 barg. The effluent then passes to a gas /liquid separator where the CO2, N2 and residual O2 are separated from the liquid stream. The inorganic residue is removed from the liquid stream. This stream has a COD of <5mg/ltr. The inorganics are inert and suitable for use a building material, non-hazardous landfill, or, depending on the components can be further treated for recovery of the phosphorous and other valuable compounds.
Design Considerations
In order to ensure an efficient, economical and reliable process the properties of the sludge to be treated must be understood at the start of the design process. SCFI therefore carry out full chemical and physical analysis and pilot assessment utilising our demonstration Aqua Critox® plant before proceeding to detailed engineering. The SCFI SCWO process, marketed as AquaCritox® has been designed to maximise efficiency and throughput while minimising downtime for maintenance and cleaning. Maintenance of a constant feed to the high pressure pumps is essential. The AquaCritox® process includes, where required, milling of the incoming feed to ensure consistent feed which also helps prevent blockages. Agitated and heated holding tanks prevent sedimentation and thus control viscosity and the solids concentration. The waste is recirculated around these tanks and the high pressure pump is fed from this recirculation line thus maintaining constant feed pressure. A high pressure pump designed for the handling of sludge is important. Extensive work has been carried out in the selection of suitable pumps. This work was carried out to allow for consistent pumping of particulate containing sewage sludge over extended periods. Certain materials which commonly cause difficulty in high pressure pumping of sewage sludge are hair and plastic fragments from items such as Q-tips. Generally for sewage sludge a piston diaphragm pump is used. This type of pump is particularly suitable as the sludge does not come into contact with moving parts which may be plugged. The pumps are sized to give sufficient velocity in the system to minimise fouling and blockages.
The Economiser and Reactor design are protected by patents. The economiser is a tube in tube type heat exchanger to minimise any risk of blockages. The reactor is a plug flow reactor designed to give the highest efficiency at minimum volume. The design of the pressure reduction system is also critical and protected by patents. Although possible, the use of a single valve for pressure reduction is not recommended. A single valve would result in extremely high velocities, severe erosion and noise issues. The AquaCritox® demonstration plant uses a patented capillary system, where pressure drop is achieved by distributing flow over a number of long capillaries. Accurate adjustment of pressure is achieved by selection of the number of tubes to be used together with the controlled addition of choke water. Larger plants may employ this system or a series of nozzles, depending on the design. In order to maintain efficient use of oxygen and to maintain process efficiencies and plant safety, the AquaCritox®plant is supplied with a sophisticated control system which limits the amount of manual intervention required by plant operators. The control system monitors and controls plant temperatures and pressures, oxygen supply and residual oxygen in the off gasses. Off gas make up is constantly monitored as is effluent water quality. The system allows for automatic switchover to standby exchangers and cleaning preventing blockages and down time.
In larger plants the configuration of the system, and the exothermic nature of the reaction, means that there is significant waste heat available for recovery. The AquaCritox® process includes a provision for recovery of this heat in the form of electricity generation. Depending on the size of the plant, sufficient power can be generated to power the plant and have residual power for export to the national grid. There is also an option for recovery of the low grade heat from smaller plants either for heating options earlier in the waste treatment process or for district heating. The residual inorganic fraction of the sludge exits the process as a very fine inert material. The material contains phosphorous and iron which may be recovered as phosphoric acid and an iron coagulant if required.
Alternatively the material is attractive as a construction or filler material. SCFI are currently researching further uses of this material that exploits its particle size. Evaluation of scwo for the complete oxidation of sewage sludge Chematur Engineering and more recently SCFI have performed a series of evaluations with undigested and digested primary and secondary sewage sludges. The tests were performed at the Chematur and SCFI demonstration plants, which were designed as described above, excluding the steam boiler. Both dewatered sludge and sludge “as received” have been used.
The influence of temperature and concentration on the efficiency of the process has been monitored. The evaluations confirmed that all organic material was easily destroyed but that a minimum temperature was required to ensure all nitrogen containing compounds were completely broken down. This temperature of 540 oC is easily achieved with in the SCWO process.
Variation on feed concentration led to no significant variation in final COD; however a minimum concentration is required to eliminate the need for booster heating of the process. SCFI have determined that organic concentrations of 3 to 10% with a total solid content not greater than 20% is the most efficient range for treatment.
This in effect means that SCWO can be utilised to process liquid or dewatered sludges The destruction efficiency of the organic material using SCWO is significantly higher than that achieved with wet air
oxidation and other process operating below the critical point. A number of such plants are used to treat sewage sludge and they typically reduce the organic load by approximately 70%. A long residence time is required for this. This compares to a 60 second residence time in the reactor for a 99.99% reduction using SCWO. During the test work CEAB and SCFI have shown that the off gasses contain no NOx or SOx were detected. These results form the basis of a long scale demonstration currently in progress at the SCFI pilot plant facility in Cork,Ireland Economics SCFI Group are confident that the process is economically competitive, not just with incineration, but also with land spreading, when the costs of pre-treatment and transport are taken into account. The most significant operating
cost for SCWO is oxygen. For large scale Aqua Critox® plants liquid oxygen (LOX) is the chosen oxidant. Where bulk LOX supply is not available at competitive costs we provide on site generation.
For smaller applications we can operate the Aqua Critox® system using air as the oxidant. Every sludge producer’s sludge will have many variables such as volume, percentage dry solids, percentage
inorganic, calorific content and salt content. SCFI evaluate each enquiry by performing an initial desktop study and then making a business case to the sludge producer. It is our objective to provide a secure long term sludge solution for our customers.
Cost of sludge treatment will normally be in the region of £30- £60 GBP per tonne of sludge cake. This cost is based on SCFI designing, financing , building, owning and operating an AquaCritox® SCWO plant to treat sewage sludge at the sewage works.
Conclusions
The results obtained in the original pilot trials and subsequent evaluation studies by SCFI Group show that SCWO is a viable alternative for the treatment of swage sludge. The reaction is exothermic giving an opportunity to recover the heat gained, either as electricity, in larger plants, or as low grade process heating in smaller plants. Waste heat can also be recovered for district heating and other uses depending on the site set-up. The process offers complete mineralisation of sewage sludge, the potential for renewable energy generation, and has the potential to significantly reduce the carbon foot print of the STW.
AquaCritox® offers a further benefit in that there are no further toxic/hazardous residues requiring disposal and offers the option of recovery of phosphorous and coagulant from the inert residue.
The process is robust. There are few moving parts and the equipment has been designed specifically for the variation in feed experienced in a standard sewage treatment plant.
Incineration is the traditional method used to recover precious metals from spent catalysts, but it has a number of drawbacks, particularly from an environmental point of view. Now a British-Swedish joint venture, involving Chematur Engineering AB and Johnson Matthey, has developed a process that uses supercritical water oxidation to recover the precious metals. The technology, called AqauCat®, offers organic destruction efficiencies of close to 100%, but without any of the problems associated with incineration.
Precious metals are used extensively as catalysts in a wide range of industrial chemical processes. They can be used in a homogeneous form, but more commonly they are heterogeneous, i.e. they are fixed to a solid support for ease of handling. In many applications only a precious metal catalyst can provide the necessary speed or selectivity to the reaction, while in others these attributes, together with a long catalyst life make the overall system the most cost effective choice. However, precious metals, such as platinum, palladium or rhodium, constitute a huge investment, so a key factor in the economics of these processes is the ability to recover the precious metal content from the catalyst once it is spent. Quick and efficient retrieval of the metals is therefore very important. However, spent process catalysts are typically contaminated with organic materials that were present in the reaction mixture. These organics, which are often hazardous, as well as any carbon present in the support, have to be removed before the complex process of metal recovery can begin.
What is Supercritical Water Oxidation?
The precious metal recovery process has almost exclusively used incineration to destroy the organic content of the catalyst material, followed by a chemical treatment of the remaining metal oxides. However, incineration as a process suffers from a number of drawbacks, including incomplete combustion and the subsequent need to scrub the stack gases to remove environmentally hazardous substances such as dioxins. Now through a joint venture between Johnson Matthey (UK), and Chematur Engineering AB (Sweden) a new recovery process, called AquaCat®, has been developed (Figure 1). The process utilizes supercritical water oxidation (SCWO); a technique previously patented by Chematur Engineering. When water’s temperature and pressure are above 374 °C and 221 bar, respectively, it enters a supercritical condition or ‘fourth phase’, i.e. an additional phase to its more familiar solid, liquid and gaseous phases. Under these conditions the physical properties of water change. For example:
• its density is less than that of the liquid;
• its viscosity is the same as the gas; and
• its diffusivity is mid-way between the liquid and the gas.
Most importantly the solubility of gases and organic compounds are increased to almost 100%, while inorganic compounds become insoluble. When oxygen is added to organic contaminants under these conditions, a very rapid reaction occurs, resulting in the almost complete destruction of the organics. Unlike incineration, the only gaseous emissions from this process are carbon dioxide (CO2) and nitrogen (N2) at room temperature.
Figure 1: Schematic of the AquaCat® process developed for the recovery of precious metal catalysts.
The AquaCat® Process in Detail
Figure 2: View of the AquaCat® reactor.
The AquaCat® process, which took three years to develop, is based on Chematur’s AquaCritox® process, where water is heated up to approximately 400 °C and pressurized to 250 bar. It then enters the supercritical condition, where it can ‘burn’ organic materials extremely quickly and efficiently through the addition of an oxidant, leaving no harmful residues. In the AquaCat® process hydrocarbons are converted to CO2 and water, with a guaranteed extent of conversion of at least 99.99%. Early experiments confirmed the AquaCat® process has an extremely rapid reaction rate because the presence of the catalyst assists the oxidation process. The majority of materials that come for catalyst recovery are heterogeneous in form. In the AquaCat® recovery process, these heterogeneous materials are first made into a slurry in water. After sampling to determine precious metal content the mixture is pumped to supercritical pressure and heated to supercritical temperature and transferred to the reactor (Figure 2). Oxygen is then added, and the organic backbone of the spent catalyst is immediately burnt off, leaving behind almost clean water containing a fine particulate phase of precious metal oxides (Figure 3). The solid precious metal oxides are then separated from the water, and are ready for refining and use in the preparation of fresh catalysts.
Benefits Over Incineration
The AquaCat® process solves many of the problems associated with traditional incineration. From the environmental and economic points of view it is more energy-efficient because it does not require an external source of energy. It also eliminates the need for complex and expensive exhaust gas treatment and reduces the amount of physical handling of the catalyst materials, which again makes ontainment easier. From a customer’s point of view, the biggest advantage of AquaCat® over incineration is that it allows pre-treatment sampling of the material. This is important because precious metal catalyst materials are very expensive. Therefore, it’s important to know exactly how much is being processed and to be able to move it through as quickly as possible, i.e. speed can save money.
On-Site Recovery Plants
Johnson Matthey, which specialises in the field of catalyst recovery, can use the AquaCat® method to retrieve the metals and make new catalysts for their customers. In addition to providing this as a service carried out in its own plants, AquaCat now makes it possible for the company to offer the processing equipment itself to certain customers. In some cases, for example in pharmaceutical anufacture, the residues are strongly bioactive and must be handled with extreme care. In other situations, such as in a petrochemical plant, large volumes of organic residue are generated, which are
both costly and difficult to transport. For customers like these, Johnson Matthey and Chematur are now be able to provide an on-site SCWO plant, so that these hazardous materials can be treated at the point of generation, avoiding additional handling or transport of the residue. According to Johnson Matthey, the delicate control required by classical incineration techniques meant that on-site units using that technique were never a viable option.
Is this the End for Incineration?
With all the obvious benefits of AquaCat®, does it mean the end of incineration in this field? Johnson Matthey does not believe so. While it believes that the AquaCat® process will be the benchmark for precious metal recovery from spent catalysts there is still a role for classical incineration. Certain materials, particularly those contaminated with extraneous material or plant debris, will not be suitable for the AquaCat® process, and will continue to be burnt in the traditional way. Furthermore, classical incineration remains the most suitable technique for treating other forms of material, which contain precious metals, such as scrap electronic components. Other Applications for AquaCritox® In addition to the AquaCat® application, Chematur Engineering are optimistic about further developments for its AquaCritox® technology. One area is in the treatment of sludge from municipal treatment plants. The process can be easily incorporated into the sewage treatment plant operation. In pilot studies both raw and digested sludge can be treated at 15-20% dry solids, which means simplified dewatering. The effluent chemical oxygen demand (COD) level is less than 5 parts per million (ppm), so no further treatment is necessary. The inorganics settle as an inert, non-leachable residue. The high purity and fine grain appearance of the residue make it a viable starting material for the recovery of coagulants and/or phosphorous.
Chematur has also been involved in a number of paper recycling projects, looking at oxidising de-inking sludge to recover valuable inorganic materials, such as paper filler. The de-inking sludge contains organic material (ink and fibres), as well as inorganic matter (mainly paper filler). When the sludge is treated with the AquaCritox process, the organics have been destroyed, while the white filler is left behind to be recycled.
Benefits of the AquaCritox® Technology
Waste to Energy
AquaCritox® unlocks the energy present in organic wastes. The process releases energy and generates steam at 500oC which is used to generate heat and electricity.
Resource Recovery
AquaCritox® provides a complete resource recovery solution for organic wastes. AquaCritox® can be used to recover materials such as phosphorus and precious metals.
Complete Organics Destruction
AquaCritox® results in complete and rapid destruction of organic wastes with no hazardous byproducts or emissions. Conversion of organic material to carbon dioxide is achieved in supercritical water at 221bar and 374oC.
Supercritical water oxidation
AquaCritox® uses the phenomenon of supercritical water oxidation to break down organic wastes to CO2 in supercritical water in 60 seconds.