SOLVENT CAPTURE AND RECOVERY IN PRACTICE: INDUSTRY EXAMPLES

Transcription

1 GG100 GUIDE ENVIRONMENTAL TECHNOLOGY BEST PRACTICE PROGRAMME Be Solvent Wise SOLVENT CAPTURE AND RECOVERY IN PRACTICE: INDUSTRY EXAMPLES GOOD PRACTICE: Proven technology and techniques for profitable environmental improvement

2 SOLVENT CAPTURE AND RECOVERY IN PRACTICE: INDUSTRY EXAMPLES This Good Practice Guide was produced by the Environmental Technology Best Practice Programme Prepared with assistance from: McLellan and Partners Ltd Crown copyright. First printed November This material may be freely reproduced except for sale or advertising purposes. Printed on paper containing 75% post-consumer waste.

3 SUMMARY This Good Practice Guide consists of eight Industry Examples, each demonstrating the cost-saving and environmental benefits to companies that have resulted from the installation of solvent recovery equipment. This Guide should be read in conjunction with GG12, Solvent Capture for Recovery and Re-use from Solvent-laden Gas Streams, which is available free of charge through the Environmental Helpline on Adsorption, condensation and absorption are the three main solvent recovery technologies used by UK industry. The Industry Examples show how these techniques have been applied to advantage within various industrial situations. The message of the Industry Examples to other companies interested in the potential for solvent recovery is clear - the variety of approaches and techniques available make solvent recovery for re-use or emissions control a realistic, cost-effective option in many different manufacturing processes. The Guide s Industry Examples provide a wide range of applications in a variety of industries so that as many companies as possible can draw parallels with their own processes and size of operation. Where appropriate, each Example outlines the various alternative control options considered and shows the reasons for their acceptance or rejection by the host company concerned. The Industry Examples demonstrate that successful solvent recovery is not limited by the size of the organisation, the complexity of its processes, or the number of solvents used. They show that it has been possible in each case to: reduce annual expenditure on solvent purchases (in one case by 95%); reduce solvent emissions to atmosphere; minimise occupational exposure to solvents; improve workplace safety. Further benefits have included: payback in each case of under three years; increased quality of processes; improved competitiveness and market share; enhanced corporate image; inspiration to explore further savings opportunities from waste recovery. Of the three main solvent recovery options considered in this Guide, in every case at least some of the recovered solvent is re-used. This re-use is achieved either within the manufacturing process or through other associated operations such as cleaning.

4 CONTENTS Section Page 1 Introduction Purpose of the Guide Target audience Why solvent recovery? 2 2 Selecting the appropriate solvent recovery option Adsorption Condensation Absorption 5 3 Industry Examples 6 1 Adsorption reduces solvent emissions at Entek International Ltd 7 2 Adsorption increases efficiency of solvent capture at D H Greaves Ltd 10 3 Using adsorption to recover THF for re-use at 13 Smith & Nephew Medical Ltd 4 Low-cost reduction in emissions through condensation at Evode Ltd 16 5 Condensation reduces solvent consumption at Hexcel Composites Ltd 18 6 Condensation reduces process emissions at Dow Corning Ltd 21 7 Cryogenic condensation for solvent recovery at Buna Sow Leuna 23 Olefinverbund GmbH 8 Solvent emissions reduced by condensation and absorption techniques 26 at Pfizer Pharmaceuticals Production Corporation 4 Conclusion and action plan 30 Appendix Suppliers of solvent abatement equipment 31

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6 1 INTRODUCTION 1.1 PURPOSE OF THE GUIDE This Good Practice Guide provides examples of how the three main technologies for the capture, recovery and subsequent re-use of solvent have been successfully implemented in various industrial sectors. The Guide s purpose is to publicise the details of these Industry Examples, thereby providing guidance to other companies and encouraging them to adopt similar cost-effective technologies and techniques. section 1 The general operational principles of the three technologies - adsorption, condensation and absorption, and their associated techniques - are examined in an earlier Good Practice Guide (GG12) Solvent Capture for Recovery and Re-use from Solvent-laden Gas Streams, also published by the Environmental Technology Best Practice Programme. This Guide should be read in conjunction with GG12, which is available free of charge through the Environmental Helpline on The Industry Examples in this Guide cover a wide range of applications in a variety of industries and are summarised in Table 1. Where appropriate, each case discusses the various alternative control options considered and outlines the reasons for their acceptance or rejection by the host company concerned. Industry Industry Company and technique used Estimated Example net annual savings 1 Plastic Entek International Ltd, Newcastle upon Tyne processing Carbon adsorption 2 Printing D H Greaves Ltd, Scarborough Carbon adsorption 3 Healthcare Smith & Nephew Medical Ltd Carbon adsorption and inert gas desorption 4 Adhesives Evode Ltd, Stafford Condensation 5 Composite Hexcel Composites Ltd, Cambridge manufacture Closed cycle inert gas condensation 6 Chemicals Dow Corning Ltd * Cryogenic condensation 7 Chemicals Buna Sow Leuna Olefinverbund GmbH, Merseberg, Germany ** Cryogenic condensation (new technology) 8 Pharmaceuticals Pfizer Pharmaceuticals Production Corporation Condensation and absorption * Excluding operating costs ** While at the moment savings are relatively small, as capacity grows savings have the potential to be considerable Table 1 Summary of Industry Examples 1

7 1.2 TARGET AUDIENCE This Guide is intended for the site managers of all companies that use solvents. It is likely to be especially relevant to companies in the following industrial sectors: section 1 printing; coating processes; manufacture of coatings, varnishes, inks and adhesives; chemical, fine chemical and pharmaceutical manufacture; surface cleaning. The Guide should be particularly useful where companies are considering fitting new abatement equipment to existing plant, upgrading existing equipment or installing new process plant. 1.3 WHY SOLVENT RECOVERY? Increasingly, solvent-using companies are having to reduce their solvent emissions in order to meet current and future environmental legislation. When reducing solvent consumption is not possible, eg through solvent management, 1 then recovery and re-use can offer a return on capital investment. By capturing solvent emissions and re-using the recovered solvent, companies can save money and meet their environmental obligations. The recovery of solvent will: reduce your solvent purchase cost and therefore increase your profitability; reduce solvent emissions to the atmosphere; eliminate the need for pollution abatement equipment by reducing solvent consumption below the registration threshold for Integrated Pollution Control (IPC)/Local Air Pollution Control (LAPC); reduce the cost of pollution abatement equipment by process optimisation, eg reducing the size and thus cost of the equipment needed. 2 1 See also Good Practice Guide (GG13) Cost-effective Solvent Management, which is available free of charge through the Environmental Helpline on

8 2 SELECTING THE APPROPRIATE SOLVENT RECOVERY OPTION While the potential for solvent capture and recovery may be well known, the optimum recovery technology for a particular process application may not be immediately obvious. Relevant information on the alternatives is available from the Industry Examples that make up this Guide. In some cases it may be necessary to approach the whole issue of solvent recovery from first principles, and to consider each of the following factors: nature of the air stream; properties of the solvents used; process characteristics; airflow characteristics; potential for using the recovered solvent; solvent costs. section 2 Table 2 summarises the most appropriate solvent abatement options for different process conditions, and these can be used for further detailed discussion with equipment suppliers. Recovery and re-use techniques Destructive techniques Adsorption Conden- Absorption Thermal Catalytic Biosation oxidation oxidation technology Low flow/low A B A B D A concentration High flow/low A C C C A A concentration Low flow/high D A A A D B - D concentration High flow/high D A B A D E concentration Hydrocarbon gases D E B - D A A A - C Halogenated or D E A B D C - E sulphanated organic gases Aminated organic gases D E C - D C C B - C Hydrocarbon A A B - C A A A - C condensables* Halogenated or A A A - B B D C - E sulphanated organic condensables* Aminated organic A A B C C A - C condensables* Continuous A A A A A A Batch or variable A A A D D A Removal efficiency B C A B C A Pressure drop C B B A C A Volatile organic B A B E E E compound recovery * At the operating temperatures and pressures of the process, a gas will remain in the gaseous phase whereas a condensable will contain vapour. Key: A = Excellent B = Good C = Satisfactory D = Poor E = Unacceptable Table 2 Solvent abatement options for various process conditions 3

9 The following summaries, drawn from GG12, outline the main considerations for adsorption, condensation and absorption. This Guide does not cover destructive techniques, for more information on these please contact the Environmental Helpline free of charge on ADSORPTION section 2 Extensively used for solvent recovery, with granular activated carbon as the most common adsorbent. Particularly practical for intermittent solvent sources. Adsorption capacity increases with increasing molecular weight and boiling point of solvent, but decreases with increasing polarity. Activated carbon adsorption is impaired when humidity of gas stream exceeds 60%. Desorption (recovery) can be carried out using steam, a hot inert gas such as nitrogen, or under vacuum. Continuous adsorption-desorption process systems can be either fixed beds that operate in alternate modes, or systems where the adsorbent is continuously conveyed between the adsorber and the desorber. Adsorbent life is significantly reduced by contaminating particulates and by high boiling point solvents. Using an inert gas rather than air allows the airflow rate to be reduced and solvent concentrations to be increased without compromising safety. Reducing the airflow rate in this way will reduce operating costs and improve the cost-effectiveness of solvent capture and re-use. 2.2 CONDENSATION Coolant/refrigerant condensation is proven technology, traditionally used for preliminary solvent recovery, eg prior to adsorption. Conventional techniques are particularly suited to recovery of concentrated emissions of high vapour pressure solvents. Cryogenic systems are less widely used, but can be used for recovery of all solvents, irrespective of vapour pressure. Economic feasibility of condensation depends on the temperature reduction required for effective recovery. Condensation temperatures well below 0 C can involve higher capital and operating costs unless nitrogen is already used on-site. Differential freezing points are likely to occur in the air stream because of the presence of water vapour and/or other components. Depending on the condensation temperature, this may give rise to frozen material on the heat transfer surface and a subsequent reduction in condensation rate. Using an inert gas rather than air allows the airflow rate to be reduced and solvent concentrations to be increased without compromising safety. Reducing the airflow rate in this way will reduce operating costs and improve the cost-effectiveness of solvent capture and re-use. 4

10 The following summaries, drawn from GG12, outline the main considerations for adsorption, condensation and absorption. This Guide does not cover destructive techniques, for more information on these please contact the Environmental Helpline free of charge on ADSORPTION section 2 Extensively used for solvent recovery, with granular activated carbon as the most common adsorbent. Particularly practical for intermittent solvent sources. Adsorption capacity increases with increasing molecular weight and boiling point of solvent, but decreases with increasing polarity. Activated carbon adsorption is impaired when humidity of gas stream exceeds 60%. Desorption (recovery) can be carried out using steam, a hot inert gas such as nitrogen, or under vacuum. Continuous adsorption-desorption process systems can be either fixed beds that operate in alternate modes, or systems where the adsorbent is continuously conveyed between the adsorber and the desorber. Adsorbent life is significantly reduced by contaminating particulates and by high boiling point solvents. Using an inert gas rather than air allows the airflow rate to be reduced and solvent concentrations to be increased without compromising safety. Reducing the airflow rate in this way will reduce operating costs and improve the cost-effectiveness of solvent capture and re-use. 2.2 CONDENSATION Coolant/refrigerant condensation is proven technology, traditionally used for preliminary solvent recovery, eg prior to adsorption. Conventional techniques are particularly suited to recovery of concentrated emissions of high vapour pressure solvents. Cryogenic systems are less widely used, but can be used for recovery of all solvents, irrespective of vapour pressure. Economic feasibility of condensation depends on the temperature reduction required for effective recovery. Condensation temperatures well below 0 C can involve higher capital and operating costs unless nitrogen is already used on-site. Differential freezing points are likely to occur in the air stream because of the presence of water vapour and/or other components. Depending on the condensation temperature, this may give rise to frozen material on the heat transfer surface and a subsequent reduction in condensation rate. Using an inert gas rather than air allows the airflow rate to be reduced and solvent concentrations to be increased without compromising safety. Reducing the airflow rate in this way will reduce operating costs and improve the cost-effectiveness of solvent capture and re-use. 4

11 2.3 ABSORPTION Feasible solvent recovery option provided solvent is readily soluble in either water or an organic compound with a high boiling point. Can take place in packed, plate or spray columns and requires little floor space. Scrubbing liquids can be regenerated using either steam stripping (for organic liquids) or distillation (aqueous liquids). Degradation of scrubbing liquids may result from high desorption temperatures, chemical reactions or impurities in the feed. This increases treatment/disposal costs and necessitates space for the appropriate equipment. Using an inert gas rather than air allows the airflow rate to be reduced and solvent concentrations to be increased without compromising safety. Decreasing the airflow rate in this way will reduce operating costs and improve the cost-effectiveness of solvent capture and re-use. section 2 5

12 3 INDUSTRY EXAMPLES Adsorption Industry Example 1 Adsorption Reduces Solvent Emissions at Entek International Ltd Industry Example 2 Adsorption Increases Efficiency of Solvent Capture at D H Greaves Ltd section 3 Industry Example 3 Using Adsorption to Recover THF for Re-use at Smith & Nephew Medical Ltd Condensation Industry Example 4 Low-cost Reduction in Emissions Through Condensation at Evode Ltd Industry Example 5 Condensation Reduces Solvent Consumption at Hexcel Composites Ltd Industry Example 6 Condensation Reduces Process Emissions at Dow Corning Ltd Industry Example 7 Cryogenic Condensation for Solvent Recovery at Buna Sow Leuna Olefinverbund GmbH Condensation and absorption Industry Example 8 Solvent Emissions Reduced by Condensation and Absorption Techniques at Pfizer Pharmaceuticals Production Corporation 6

13 ADSORPTION REDUCES SOLVENT EMISSIONS AT ENTEK INTERNATIONAL LTD Entek International Ltd uses large quantities of solvent for product cleaning during its manufacturing process. When the factory was set up, the Company installed an adsorption-based extraction plant to recover solvent for re-use, reduce emissions to atmosphere and minimise occupational exposure to solvents. Estimates suggest that, by using this technique, Entek reduced its annual solvent expenditure by 95% and achieved net cost savings of more than 1.6 million/year. The estimated payback period on a capital expenditure of around 2 million is 1.25 years. Background Inspecting polyethylene film Entek International Ltd of Newcastle produces specialised polyethylene products for use in the manufacture of lead/acid batteries. The polyethylene film from which these products are made is produced by heating polymer granules, extruding them through a die and forcing the polymer between two rolls. Significant quantities of lubricating oil are added to the polymer during the extrusion process to reduce friction and prevent adhesion to the rolls. The resulting membrane may, typically, contain 60% by weight of oil immediately after forming. The oil is removed from the membrane surface after extrusion and before further processing. This involves passing the polymer film through a tank of warm trichloroethylene (TCE) solvent. As the membrane passes over guide rolls in the tank, it is washed by solvent circulating in the opposite direction. The solvent passes through a distillation unit for cleaning before being recirculated. The film emerging from the washing tank passes through an air drier where the solvent evaporates. The dry film is cut to the required width and rolled onto spools before being packaged for despatch to the customer. Because of the large quantities of solvent used in the washing process, Entek installed a solvent extraction and recovery plant to handle the solvent-laden air from the drier, recover solvent for reuse, and ensure that the solvent content of the workplace atmosphere remains within safe limits. This plant has been in place since 1989, when the factory started up, and the Company is currently implementing an ISO environmental management system. 7AdsorptionIndustry Example 1

14 Adsorption Industry Example 1 The Solvent Recovery Process Solvent extraction and recovery by adsorption is a batch process in which a solvent-laden gas stream is passed through a suitable adsorption bed to remove the solvents from the stream. After a period of time, the bed becomes saturated with solvent, its adsorption efficiency falls, and it must be taken out of commission for a period of regeneration. It is normal to operate at least two adsorption beds so that one is regenerating while the other is adsorbing. Prior to 1989, several other companies in the US organisation to which Entek International belongs had already successfully used this adsorption technique in plants producing similar products. It was therefore introduced to Entek as proven technology. The solvent-laden gas stream is extracted from the drier by large fans and is passed through activated carbon beds to remove the solvent (Fig 1). The plant has four such beds, each containing approximately ten tonnes of carbon. This ensures that at least two can be operated in adsorption mode at any one time, while the others are being regenerated. Exhaust to atmosphere Water in Water out Water in Water out Adsorption Desorption Steam Water cooled condenser Distillation column To main solvent tank Water in Water out Solvent store Granular activated carbon adsorption beds x 4 Fig 1 The solvent recovery process at Entek International Ltd Carbon regeneration and solvent recovery is carried out as follows: Water cooled condenser Decanter Solvent laden air 1 Steam is passed through the solvent-laden bed. 2 The resulting mixture of solvent and steam passes through a water-cooled condenser, producing cooled solvent and condensate. 3 The cooled solvent and condensate separate out in an adjacent decanter. The recovered solvent is pumped to a solvent storage tank. The water is returned to the cooling tower. 4 The condensate leaving the condenser is finally passed through a distillation column fitted with a second condenser. This recovers the last of the solvent and ensures that emissions to atmosphere are, typically, less than five parts per million. Fan 8

15 Cost Savings and Other Benefits Product leaving the wash tank Direct re-use of the recovered solvent in the product cleaning process significantly reduces the quantities of solvent that would otherwise have to be purchased each year. Estimates indicate that due to the solvent capture plant, the Company is achieving net annual cost savings of more than 1.6 million, with a payback period on the capital outlay of 1.25 years (Table 3). Other benefits include reduced solvent emissions and improved workplace safety. Annual value Estimated solvent purchases without solvent recovery Solvent purchases with solvent recovery Estimated reduction in solvent purchases Percentage saving 95% Estimated costs of plant operation* ( ) Estimated net savings Estimated capital expenditure on solvent recovery plant Estimated payback period 1.25 years * Conservative estimate based on experience at other plants Table 3 Economic analysis Our solvent recovery process enables us to recycle solvent, meet emission targets and improve health and safety. Mr R Howe, Operations Manager, Entek International Ltd 9AdsorptionIndustry Example 1

16 Adsorption Industry Example 2 ADSORPTION INCREASES EFFICIENCY OF SOLVENT CAPTURE AT D H GREAVES LTD The inks used for gravure printing at D H Greaves Ltd contain high levels of toluene. To prevent this solvent escaping to atmosphere as it evaporates, the Company practises solvent recovery, either re-using the recovered product or selling it to the ink manufacturer for re-use. In 1996, the existing adsorption plant was replaced with a more modern unit, improving the efficiency of solvent capture by 20%, enhancing health and safety and environmental performance, and increasing annual revenue/cost savings from to Background Computerised on-line monitoring D H Greaves Ltd of Scarborough is a subsidiary of Watmoughs (Holdings) plc, which owns printing operations at several sites in the UK and elsewhere in Europe. Approximately 340 people are employed in a gravure printing process at the Scarborough site, generating a turnover of about 45 million/year. Gravure printing is commonly used to produce coloured images in documents of the newspaper type such as colour supplements and magazines. The inks used typically contain approximately 60% toluene, although further additions of solvent may be made, as required, to control ink viscosity. These solvents evaporate as the inks dry and, unless captured and abated, pass directly into the atmosphere. Solvent recovery has taken place at the Scarborough site for nearly 20 years. The Company s first granular activated carbon adsorption system was installed in 1979 and replaced in By 1996, increases in production meant that this second plant was operating beyond its maximum design capacity, a situation that could potentially give rise to unauthorised solvent emissions. Recognising that new plant was required, the Company took the opportunity of installing a modern, improved recovery system that was capable not only of meeting expected future European Union emission limits but also of generating additional cost savings. Commissioning was completed in August

17 The Solvent Recovery Process A wide range of solvents can be removed and recovered by adsorption. During the process the solvents are readily adsorbed onto activated carbon or other porous, granular adsorbents. Solvents are removed by passing a solvent-laden gas stream through an adsorption bed where the contaminated molecules from the air stream are attracted to and retained by the solid surface of the bed. Over a period of time the adsorbent becomes exhausted and its adsorption efficiency diminishes. The bed then needs to be taken out of operation while it undergoes a period of regeneration. To allow for regeneration, it is usual to operate at least two adsorption beds so that one is in operation while the other is out of service. The solvent recovery system installed at D H Greaves Ltd has six adsorption beds containing granular activated carbon. Solvent-laden air is removed from the printing area using variable speed extraction fans and is passed through a filter to remove any dust before it enters those beds that are in adsorption mode (Fig 2). The porous structure of the beds attracts the solvent in the air to the carbon surfaces, leaving the exhaust air solvent-free. Solvent laden air Boiler Filter Adsorption Desorption Fan Granular activated carbon adsorption beds x 6 Heat exchanger Solvent storage Condenser Product cooler Decanter Liquid phase adsorption feed tank Fig 2 The solvent recovery process at D H Greaves Ltd Exhaust to atmosphere Liquid phase adsorption Boiler transfer holding tank Adsorption Industry Example 2 Inspecting the plume control ducting The adsorption beds 11

18 Adsorption Industry Example 2 When an adsorption bed is saturated with solvent, it is automatically taken off-line and regenerated. This involves injecting steam into the top of each bed and passing it downwards through the bed to remove the solvent adhering to the carbon surfaces. The resulting steam/solvent mixture passes first to a condenser and then to a decanter, where the solvent and water condensate separate out. The recovered solvent is then automatically transferred to a storage tank and is either re-used in the process as a thinners material or sold to the ink manufacturer for re-use. The water condensate, however, still retains small amounts of solvent which must be removed before the water can be returned to the hot well of the boiler. This is achieved using a liquid phase adsorber. Before the regenerated bed can be brought back on-line, the carbon must be dried. This is achieved by blowing warm air through the carbon layers. Because of improvements in process control procedures, the adsorption beds are now regenerated when necessary rather than at regular intervals. This has reduced the quantities of steam required for regeneration and the associated costs of boiler operation. The solvent recovery plant has sufficient capacity during periods of peak demand to allow two beds to be off-line awaiting service and regeneration at any one time. The Company has encountered no significant problems over the last decade with the general operation of its solvent recovery plants. Granular activated carbon adsorption has proved to be very reliable. Cost Savings and Other Benefits Analysis of Company data indicates that the new plant has improved the efficiency of solvent capture from 71% to 91% (an increase of 20%). This, combined with the reduction in boiler costs, means that, over a one-year period, cost savings have risen from approximately to (Table 4). For a plant currently operating without solvent capture equipment, the payback period on a capital investment of 2.5 million would be approximately three years. In addition to the financial benefits, the use of granular activated carbon adsorption techniques over a prolonged period has reduced the Company s solvent emissions to atmosphere and improved its health and safety and environmental performance. Annual value* Annual value* Change in annual Former system ( ) New system ( ) value ( ) Solvent re-used in process Solvent sold to ink manufacturer for re-use Revenue/savings Boiler costs (estimate) (93 569) (74 855) Net financial benefit Capital investment cost 2.5 million Payback period 3.1 years Efficiency of solvent capture 70.8 % 91.0 % +20 % * Extrapolated from 1996 data for the efficiency of solvent recovery Table 4 Economic analysis Our new recovery plant provides increased solvent recovery at reduced cost, enabling us to keep emissions below the limits set. Mr D Waller, Plant Manager, D H Greaves Ltd 12

19 USING ADSORPTION TO RECOVER THF FOR RE-USE AT SMITH & NEPHEW MEDICAL LTD Smith & Nephew Medical Ltd uses tetrahydrofuran (THF) in the manufacture of polyurethane film. Recognising the financial and environmental potential of capturing and re-using this solvent, the Company installed a recovery process that combines carbon bed adsorption with inert gas desorption. Recycling the recovered solvent reduces the costs of solvent purchase by about /year, generating net annual savings of around The Company also benefits from an improved public image, lower solvent emissions, improved health and safety performance and a reduced dependence on solvent suppliers. Background Smith & Nephew Medical Ltd is a leading manufacturer of healthcare products which are used in a wide range of medical applications. The plant at Hull employs approximately people in the production of dressings, plasters, bandaging and other wound care products. Various solvents are used at the Hull site in a range of applications, and the Company has more than 40 years experience of solvent recovery techniques, including the direct condensation of dichloromethane and the granular activated carbon adsorption of petroleum spirit. This Industry Example focuses on THF, a solvent used in the production of products based on polyurethane film. Polyurethane film is manufactured by dissolving polyurethane resins in THF and using the resulting liquid to produce a thin, oven-dried film. During the early 1980s, the Company recognised the costsavings and environmental potential of capturing and re-using the THF solvent it emitted. It considered several different recovery techniques, avoiding those options that use steam, as THF mixes readily with water. The most suitable dry technique identified was a carbon bed adsorption process incorporating inert gas desorption for solvent recovery. This was duly commissioned. The Solvent Recovery Process Packaging dressings at the Hull plant This adsorption-based technique is extensively used for solvent extraction and recovery. It is a batch process where a solvent-laden gas stream is passed through a suitable adsorption bed to remove the solvents from the stream. After a period of time, the effective working capacity of the adsorbent is reached and when exhausted it needs to be taken out of service for regeneration. It is normal to operate at least two adsorption beds so that one is regenerating while the other is adsorbing. Adsorption Industry Example 3 13

20 Adsorption Industry Example 3 Solvent recovery at Smith & Nephew uses two granular activated carbon beds, each containing approximately five tonnes of carbon. Solvent-laden air enters the top of the adsorber, and the solvent is adsorbed on to the carbon in the bed (Fig 3). Desorption/regeneration is triggered when either solvent emissions from the bed reach a predetermined level or the maximum adsorption period of four hours is exceeded. The main components of the desorption process are shown in Fig 3. Fig 4 summarises the main process stages. Solvent laden air To atmosphere Cooler Condenser Recovered solvent Adsorption Desorption Adsorber 1 6. Carbon bed returned to adsorption mode. Nitrogen tank 5. Carbon bed conditioned cooled and wetted to improve its adsorption efficiencies. 4. Condensers switched on. THF removed from carbon bed, recovered at condensers and stored for re-use. Adsorber 2 Molecular sieve Heat store Fig 3 The solvent recovery process at Smith & Nephew Medical Ltd Desorption Fig 4 A summary of the desorption process at Smith & Nephew Medical Ltd Heater Fan To atmosphere 1. System purged with nitrogen to render it inert. Oxygen level reduced to 5%. 2. Cooler switched on, allowing nitrogen to cool molecular sieve. Recovered heat retained in heat store. 3. Heater switched on. Water preferentially removed from adsorption bed and deposited on molecular sieve. 14

21 Cost Savings and Other Benefits Process data show that Smith & Nephew is recovering approximately 70-80% of the THF it uses. This is recycled directly to the manufacturing process, saving the Company an estimated /year in solvent purchases (Table 5). With bed replacement and operating costs accounting for approximately /year, the Company s net annual savings are currently around Annual value ( ) Solvent recovered Carbon bed replacement (66 000) Other annual operating costs (87 000) Net cost benefit (1996) Approximate installation costs (1984) Table 5 Economic analysis In addition to the financial advantages, the Company has identified other benefits of solvent recovery which allow it to meet customers expectations of a healthcare company: reduced solvent emissions and compliance with emission limits; improved health and safety performance in the workplace; reduced dependence on solvent suppliers, with lower delivery costs and less likelihood of spillage. The Company is continually looking for ways of improving the process and has identified two projects that will further reduce operating costs: the installation of a pre-treatment filter to remove impurities that currently poison the carbon beds and necessitate premature bed replacement; pipework modifications that will allow purging directly into the carbon beds, thereby reducing nitrogen consumption. Smith & Nephew is also exploring the use of alternative non-solvent-based manufacturing techniques that will eliminate the use of THF at the Hull plant, thereby further improving both health and safety performance and the environmental performance of the plant. The solvent recovery process has enabled us to significantly reduce our costs, improve our market competitiveness and reduce solvent emissions. Miss M Devine, Site Engineering Manager, Smith & Nephew Medical Ltd Adsorption Industry Example 3 15

22 Condensation Industry Example 4 LOW-COST REDUCTION IN EMISSIONS THROUGH CONDENSATION AT EVODE LTD Evode Ltd uses various solvents for manufacturing adhesives, mixing them with other raw materials in a range of mixing vessels. Company concern over solvent emissions during the mixing process has resulted in the in-house development of a simple heat exchanger for condensing the solvent in the loading vent and returning it to the vessel. Trials show that the units, as well as minimising emissions, reduce solvent consumption by about 4% and could reduce Company solvent costs by at least 3 000/year. Background Evode Ltd is based in Staffordshire and is a leading UK manufacturer of adhesives. The Company employs approximately 430 people. The manufacture of adhesives involves the accurate blending and mixing of a wide range of solvents, resins, polymers and fillers. Typical solvents used include toluene, acetone and methylene chloride. These are blended with other raw materials in a range of mixing vessels and the resulting products are transferred to suitable containers for sale to industrial and commercial retailers. The way in which additions are made to certain horizontal mixing vessels can, potentially, result in high concentrations of solvent vapour in the workplace. In particular, the loading of resins, polymers and fillers down vertical filling vents can cause displaced solvent-laden air to enter the working environment. Company concern over this issue resulted, initially, in the installation of fume extraction equipment which was operated during the loading process. However, once loading was complete, a lid was closed and the fume extraction equipment was switched off, despite the fact that small quantities of solvent (fugitive emissions) could, potentially, be emitted from the loading vents during the mixing cycle. To minimise these fugitive emissions and ensure that solvent is captured for process re-use, a lowcost, condensation-based solvent recovery system was designed and manufactured in-house by the Company s maintenance department. The Solvent Recovery Process Installing the condenser unit Solvent recovery from horizontal mixing vessels is based on a simple tube-and-fin heat exchanger, which is lowered into the filling vent once a vessel has been fully charged (Fig 5). Cold water is pumped through the tube arrangement, causing the surrounding solvent to condense and return to 16

23 the mixing vessel. The water, which is cooled using the plant s existing evaporative cooling system, is continually recirculated. The fact that the units are removable allows both mixing vessels and condensers to be cleaned between batches, thereby preventing product cross-contamination. Evode has found that, on some mixers, the design does not permit easy installation and removal. A second generation of condensers is now being considered. These may be fixed in position and operate using liquid nitrogen instead of water to increase solvent recovery rates. Cost Savings and Other Benefits As the equipment was manufactured inhouse, it is difficult to identify the true cost of producing and installing the condenser units. Standard estimating techniques suggest the cost of each unit is in the region of 350. Equipment operating costs are negligible as evaporative water coolers have already been installed on-site for other purposes. Monitored trials, based on mass balance techniques, indicate that the condensation units reduce solvent consumption by approximately 4% (Table 6). If the same level of reduction were to be achieved for all the appropriate mixing vessels, the Company could, potentially, reduce its solvent costs by 5 000/year. A more conservative estimate, based on a recovery rate of 2.5%, indicates an annual saving of more than Resin/hardener/filler Liquid solvent from storage tank As well as reducing solvent consumption and costs, Evode Ltd has been able, for only a modest outlay, to reduce solvent emissions to atmosphere and improve environmental performance. While acknowledging that the device lacks sophistication, the Company has clearly demonstrated that plant and process improvements can often be achieved quite simply in-house, without adopting sophisticated systems designed and installed by outside experts. Furthermore, encouraged by the success of small-scale trials, Evode believes that further savings are possible from solvent recovery in other areas of the plant. The Company is currently considering more sophisticated options for recovering toluene from another process, with potential savings of /year. This condensation technique has proven the benefits of solvent recovery and inspired new developments. Mr M Topley, Manufacturing Manager, Evode Ltd Cold water Condenser Filling vent Fig 5 The solvent recovery condenser unit at Evode Ltd Solvent loss (%) Test without condenser (Test 1) 6.7 Tests with condenser Test Test Test Mean 2.7 Mean reduction in solvent loss 4.0 Table 6 Trial results Condensation Industry Example 4 17

24 Condensation Industry Example 5 18 CONDENSATION REDUCES SOLVENT CONSUMPTION AT HEXCEL COMPOSITES LTD Hexcel Composites Ltd manufactures lightweight, high performance composite materials and structures for use in the commercial aerospace, space and defence, marine, sports and leisure and general industrial markets. The Company is now using best available technology to recover the large quantities of solvent used in the process, thereby reducing solvent emissions by 94%, improving Company safety levels and enhancing its corporate image. Re-use of the recovered solvent for cleaning has reduced the cost of solvent purchases by approximately /year. Annual net cost savings are around Background Discharging recovered solvent Hexcel Composites Ltd of Duxford, Cambridgeshire specialises in the manufacture of composite materials that are subsequently used in the production of a range of products for aerospace, industrial and sports applications. The manufacture of composite fabrics is a two-stage process. Epoxy or phenolic resins are first coated on to a siliconised release paper and oven-dried. The dried resins are then combined with carbon, glass or kevlar fibres. The two main solvents used in the process are methylene chloride and methyl ethyl ketone. The main operation (textile coating and finishing process) is categorised as a Part B process under the Environmental Protection Act 1990 and is authorised by the local authority. In 1989 Hexcel installed a larger coating machine and drier to allow the production of wider rolls of composite material. At the same time, anticipating forthcoming environmental legislation and general pressures to improve environmental performance, the Company decided to incorporate solvent recovery in the proposed development. Initially, two techniques were believed to be potentially suitable for solvent recovery: adsorption, using granular activated carbon, and condensation. Consideration was also given to thermal oxidation, which destroys the waste solvent. After a detailed review of the options, Hexcel decided to install a coating head and drier that operate in an inert atmosphere. Solvent recovery is achieved via a closed-cycle inert gas condensation process, using nitrogen as the carrier gas.

25 The choice of solvent recovery technique was influenced by several factors: the reduced risk of explosion associated with significantly reducing oxygen levels in the coating process; the technique s flexibility, allowing it to be used with a range of different solvents; concern over the thermal oxidation of chlorinated solvents; the presence of ketones in the recovered solvents (granular activated carbon adsorption is not generally regarded as suitable in these circumstances); the higher cost of adsorption for the volumes of gas involved. The Solvent Recovery Process Solvent-laden nitrogen is extracted from the gas drier and passed through a heat exchanger where it is cooled (Fig 6). The cooled gas is transferred to the base of a spray condenser, rising through the condenser s ceramic packed bed and coming into contact with cold liquid solvent. Some of the solvent vapour condenses out and is collected at the base of the condenser. It is pumped to storage containers and a proportion is re-used directly for process cleaning. The remainder is transported off-site for the removal of excess impurities and is then returned and used for cleaning pipes. Cleaned nitrogen to drying oven Recovered solvent Solvent laden nitrogen from drying oven Heat exchanger Ceramic bed Spray condenser N 2 + solvent Deep cool cryogenic system Nitrogen to atmosphere Most of the cleaned nitrogen is pre-heated in the heat exchanger and returned to the drying oven. However, because additional nitrogen is introduced into the system to maintain the required atmosphere by creating inert gas curtains, 1 an equivalent volume of used nitrogen must be removed from the process to maintain equilibrium. A proportion of the gas leaving the spray condenser is therefore diverted away from the system and, because it contains a solvent concentration that is higher than the authorised level for discharge to atmosphere, it is passed to a secondary solvent recovery system that uses a deep cool cryogenic process. It is then discharged to atmosphere via a stack. Only one operational problem has been experienced. Because the methylene chloride solvent contains trace quantities of water, some freezing occurred during the first cold period after the system was commissioned. This caused maintenance and production delays. The problem was quickly rectified by introducing antifreeze into the system. -1 C Fig 6 The solvent recovery process at Hexcel Composites Ltd Refrigerated solvent cooler Condensation Industry Example 5 1 For further details see GG12 Solvent Capture for Recovery and Re-use from Solvent-laden Gas Streams: Section

26 Condensation Industry Example 5 Cost Savings and Other Benefits Hexcel Composites Ltd re-uses the recovered solvent for process cleaning, eliminating the need to purchase low-grade solvents for this purpose and saving an estimated /year (Table 7). These savings more than offset the operating costs of the recovery system, the main component of which is the m 3 /year of nitrogen required. This costs approximately /year (Table 8). Annual net savings to the Company are therefore about Regular monitoring of the coating process Solvent Estimated Cost/tonne Saving recovered number of ( ) ( /year) (tonnes/year) recycles Table 7 Reduction in solvent consumption and costs Nitrogen Cost/100 m 3 Operating consumption ( ) cost (m 3 /year) ( /year) Table 8 Nitrogen consumption and cost The solvent recovery system has benefited the Company in several other ways: It has reduced solvent emissions by 94%. It ensures that the Company meets local authority environmental requirements using best available technology. This helps to enhance the Company s corporate image. It has improved Company safety levels. The installation is physically small and so does not occupy much space. More generally, the project has shown that upgrading or replacing production equipment can create opportunities for effective solvent recovery and reduced emissions. As a result of its experience of solvent recovery, the Company is now planning to install an activated carbon adsorption system to recover methylene chloride from another part of the manufacturing process. Implementing solvent recovery provided an opportunity to improve our process and quality whilst making significant cost savings. Mr J Tattersall, Head of Operations, Hexcel Composites Ltd 20

27 CONDENSATION REDUCES PROCESS EMISSIONS AT DOW CORNING LTD Dow Corning Ltd uses large quantities of methyl chloride in the production of speciality chemicals. The Company has practised solvent recovery and re-use for several years using condensation techniques. It has now added a cryogenic process to the plant to increase levels of solvent recovery, thereby reducing process emissions to levels that are considered insignificant. Although environmental improvement rather than cost savings was the main objective, the Company has reduced its solvent purchases by about 40 tonnes/year. The associated cost saving is about /year. Background Dow Corning is a manufacturer of speciality chemicals, producing silicon-based products mainly for industrial and consumer applications. The Company is based at Barry in South Wales and employs approximately 550 people. The manufacture of silicon products at the Barry site incorporates two fluidised bed reactions that use methyl chloride as the primary reactant with silicon powder. The resulting vapour is a mixture of chlorosilanes and unreacted methyl chloride. These two components are separated in a distillation system, with the methyl chloride entering a recovery unit and being recycled for re-use in the same reaction vessels. The process is authorised as a Part A process under the Environmental Protection Act 1990 and is regulated by the Environment Agency. The Solvent Recovery Process Methyl chloride has been recovered from the process for a number of years using a system of water-cooled condensers and compressors, with the vapour being compressed to allow condensation within the chilled condensers. In 1995, Dow Corning identified opportunities for recovering increased quantities of methyl chloride, thereby reducing process emissions still further and providing additional economic benefits. A cryogenic recovery process using liquid nitrogen was therefore added to the methyl chloride recovery plant (Fig 7). Silicon Methyl chloride Fluidised bed reactor Adsorption Desorption Product distillation Methylchlorosilane product Final vent Methyl chloride recovery Cryogenic recovery Fig 7 Solvent recovery at Dow Corning Ltd Condensation Industry Example 6 Inspecting the pipework Inspecting system pressure 21

28 Condensation Industry Example 6 Cryogenic recovery was chosen for several reasons: methyl chloride has a low boiling point (-23.7 C); nitrogen bulk storage and pipework was already available on site; a nitrogen vaporiser could readily be incorporated into the recovery system. Although much of the equipment for the cryogenic recovery system was already available on site, the capital investment required was still significant at approximately /tonne of additional solvent recovered. Cost Savings and Other Benefits By installing the cryogenic system, Dow Corning has significantly increased the quantities of methyl chloride recovered from the process for re-use. Estimates suggest that, in the first full year of operation, purchases of this solvent fell by approximately 40 tonnes (Fig 8). The associated costs have fallen by an estimated /year. Against this must be set the operating costs which, in this case, consist primarily of the costs of the liquid nitrogen. When compared with guidelines published by the Environment Agency, site emissions of volatile organic compounds (VOCs) have been reduced to levels that are considered insignificant. Dow Corning is currently implementing a major expansion at the Barry site which will increase its production capability by a factor of four or five. The new development will feature other solvent recovery techniques that use a combination of recently developed cooling techniques and other modern technology. Dow Corning is confident that these techniques will allow the reductions in VOC emissions to be as significant as those achieved by retrofitting the cryogenic system and will be achieved at lower cost. This solvent reduction project is just one example in our continuing effort to reduce waste and emissions at our site. Tonnes purchased Year Mr T Strange, Environmental Manager, Dow Corning Ltd Cryogenic unit installed 1996 Fig 8 Purchases of methyl chloride 22

29 CRYOGENIC CONDENSATION FOR SOLVENT RECOVERY AT BUNA SOW LEUNA OLEFINVERBUND GMBH Buna Sow Leuna Olefinverbund GmbH manufactures polyester resins from a range of chemicals and solvents in the eastern part of Germany. It has recently installed a cryogenic condensation plant for solvent recovery and now meets the stringent emission control limits that have been imposed since German reunification. The solvent recovered is re-used within the process, reducing solvent purchases and the associated costs. In addition, the nitrogen used for solvent recovery is subsequently re-used elsewhere in the manufacturing process. Background Buna Sow Leuna Olefinverbund GmbH (BSL) is a large chemical company located near Leipzig, Germany. It specialises in the production of polyester resins and was formed by the merger of independent chemical companies in the former Republic of East Germany. It is currently managed by the Dow Corporation. Since German reunification in 1990, East German companies have been required to comply with the stringent West German TA Luft emission control regulations. As a result, BSL and many other East German companies have had to review their manufacturing processes and make the appropriate modifications. The manufacture of polyester resin involves complex reactions between a range of chemicals and solvents (including methanol, styrene and acetone) and can result in the emission of volatile organic compounds (VOCs) to atmosphere. These emissions occur at a number of plant locations and can vary considerably, depending on air flow rates. Investigations showed that emissions reduction was necessary if the plant was to comply with the regulatory limit of 150 mg/m 3. BSL therefore identified the main emission sources and then reviewed several potentially suitable emission control options. Of these, three were considered to be particularly appropriate: thermal oxidation; gas washing and biological treatment; cryogenic solvent recovery. Nitrogen tank and solvent recovery system Condensation Industry Example 7 23

30 Condensation Industry Example 7 Detailed cost estimates for thermal oxidation indicated that installing and commissioning suitable equipment would be expensive, eg the pipeline carrying waste gases to a centralised thermal oxidation unit would require significant modification to ensure its successful integration with existing systems. Not only would total costs be four to five times the cost of an appropriate cryogenic unit, but the Company was concerned about additional operating costs and possible secondary pollution from the combustion of off-gases. Biological treatment techniques are becoming more widely used for the control of VOC emissions. However, in this case, such techniques were thought to be inappropriate because of the different decomposition rates of methanol, acetone and styrene. Trials also generated unpleasant odours, and these were viewed as a potential problem for residents in the immediate vicinity. Cryogenic recovery techniques were believed to be the most effective, not only reducing emissions but providing BSL with an opportunity for increasing cost savings by re-using the recovered solvent. Consequently, following successful trials, a cryogenic condensation plant was commissioned in December The Solvent Recovery Process The patented process installed by BSL consists of a column of recirculating stainless steel ball bearings on which the solvent vapours condense and freeze (Fig 9). The ball bearings pass from the top to the bottom of the column and, as they descend, they are indirectly cooled in the middle section by liquid nitrogen. This allows temperatures of -150 C to be achieved. The solvent-laden waste gas enters at the base of the column and passes upwards, in the opposite direction to the ball bearings. It is cooled by direct heat transfer. The solvent vapours freeze or condense onto the surface of the ball bearings and are transported in this state to the lower part of the column where they melt under the warmer conditions that exist there. The liquid solvents drip through a mesh floor and are recovered. The ball bearings are dried and returned to the top of the column by bucket conveyor. Liquid nitrogen Clean gas Gaseous nitrogen Liquid nitrogen Waste gas Condensate Fig 9 The solvent recovery process at BSL Bucket elevator Pre-cooler Spheres cooler Condenser/ desublimator Sieve Drier 24

31 One major advantage of this process over other cryogenic techniques is the fact that the frozen solvent does not cause excessive pressure drops or blockages. Furthermore, the nitrogen used for cooling is used directly in the manufacturing process. Because of the solvents used in this particular application and the high temperature of the waste gas, it is theoretically possible for the styrene to polymerise onto the surface of the cold ball bearings. This could give rise to a build-up of static charge on the spheres, creating the conditions for a potential explosion. The plant has therefore been fully earthed, and a device for monitoring the conductivity of the spheres has been developed and installed as a safeguard, to ensure polymerisation does not occur. For applications where polymerisation could not occur, modifications of this type would not be necessary. Extensive trials were conducted during plant commissioning to establish a correlation between the temperature and solvent concentration of exit gases. By writing a suitable algorithm, BSL is now able to monitor solvent concentrations in process emissions simply by recording the temperature. Such a technique is approved by the regulatory authority and provides a low-cost, indicative means of continuously monitoring process emissions. Cost Savings and Other Benefits The benefits of installing this continuous cryogenic solvent recovery process have been considerable. They include: emissions are now within regulatory limits; solvents are recovered for re-use, reducing solvent purchases and the associated costs; there is no secondary pollution. The double use of nitrogen, with the solvent recovery unit acting as a first-stage vaporiser, was also a significant component of the project s economic justification. This new technology enables us to meet higher emission targets at negligible cost. Herr D Riecke, Plant Manager, Buna Sow Leuna Olefinverbund GmbH Condensation Industry Example 7 25

32 Condensation and Absorption Industry Example 8 26 SOLVENT EMISSIONS REDUCED BY CONDENSATION AND ABSORPTION TECHNIQUES AT PFIZER PHARMACEUTICALS PRODUCTION CORPORATION Pfizer Pharmaceuticals Production Corporation has used a combination of condensation and absorption units to reduce overall solvent emissions to atmosphere by around 80%. This allows the Company to meet stringent emissions control legislation. Condenser units were installed initially to control peak emissions of a wide range of solvents. The new absorption units have effectively captured much of the remainder, particularly where solvent concentrations were low. Some of the recovered solvent is re-used, reducing solvent purchases by /year and generating net cost savings of around /year. Background Part of the solvent recovery system Pfizer Pharmaceuticals Production Corporation manufactures a range of bulk pharmaceuticals at a plant in Ringaskiddy, Cork, Ireland. Manufacturing takes place in three multi-purpose organic synthesis plants (OSP I, OSP II and OSP III). These were commissioned in 1972, 1984 and 1995 respectively. The manufacture of pharmaceuticals uses solvents to dissolve and transport reactants and products. Typical solvents used at the Ringaskiddy plant include ethanol, ethyl acetate, methanol, isopropanol, acetone, toluene and xylene. In 1987, the introduction of the Air Pollution Act in Ireland, which specified emission limits equivalent to those in the German TA Luft guidelines, required Pfizer to make a significant reduction in volatile organic compound (VOC) emissions to atmosphere from the OSP II plant, and subsequently from the other two plants. This was reinforced by the issuing of an Integrated Pollution Control Licence under the Environmental Protection Agency Act (1992). The first step was to reduce peak emissions from the OSP II process. Studies identified that these peak emissions coincided with the operation of certain items of equipment. Where practicable, condensers were installed on each such item.

33 However, it was not feasible to install condensers in all vents as, in many cases, the solvent concentrations involved were too low for effective capture. The Company therefore reviewed three other techniques for reducing the baseline solvent concentrations to below the authorised limit: thermal oxidation; adsorption; absorption. Thermal oxidation is suitable for applications such as this where the waste stream may contain various different solvents. It was rejected in this instance because of the potential presence of a halogenated solvent which would have required a more expensive flue gas control system to ensure that emissions remained within recognised limits. Adsorption is potentially the most widely used recovery technique for general applications, and either adsorption or absorption could have been used by Pfizer. The Company chose to add an absorption unit to the existing solvent recovery system for several reasons: it is more effective in situations where there are solvent mixtures and fluctuating solvent loads; it is able to cope with high solvent concentrations; it operates as a continuous process: this ensures that the solvent-laden air is always in contact with regenerated absorbent, maximising VOC removal. The Solvent Recovery Process Solvent recovery and emissions control on the OSP II plant is achieved using a combination of condensation and absorption techniques (Fig 10). Fan Acid scrubber Water in Water out Vapour phase Vent header Spiral condenser Solvent laden air Pump Liquid phase Scrubber Absorber Pump Heat exchanger VOC absorption plant Desorber Pump Continuous monitor Aqueous phase Exhaust to atmosphere Plant air Condenser Decanter Organic phase Condensation Industry Example 8 and Absorption Fig 10 The solvent recovery process at Pfizer Pharmaceuticals Production Corporation 27

34 Condensation and Absorption Industry Example 8 Staff training in solvent recovery system operation Condensation Condensation techniques are widely used for solvent recovery. They may be based on water-cooled heat exchangers or refrigerants and are typically used to control peak emissions. In many cases, a two-stage condensation system is used. This consists of a primary condenser serviced with cooling water and a secondary condenser serviced with a low-temperature coolant. In one example, installing a spiral condenser to limit peak emissions reduced the solvent stream to between 60% and 75% of its unabated level. The solvents collected are re-used in the primary manufacturing processes. Absorption Waste process air is fed upwards through an absorption column where it is washed by a liquid absorbent, polyethylene glycol dibutyl ether, which flows down through the column. This removes solvent from the air. The mixture of solvent and absorbent is collected at the base of the column and pumped to a desorption column, where the solvents are removed by steam stripping. The purified absorbent accumulates at the base of the desorption column and is returned, via a heat exchanger and cooler, to the top of the absorption column. The steam/solvent mixture is removed, condensed and the two components separated out. However, the solvent component generally consists of a complex mixture of solvents that is unsuitable for recycling. Depending on miscibility, it is either disposed of in a suitably designed thermal oxidation unit or via appropriate treatment systems. 28