PRACTICAL GUIDE ON THE REDUCTION OF DISCHARGES TO ATMOSPHERE OF VOLATILE ORGANIC COMPOUNDS (VOCs)

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Process Engineering Guide: GBHE-PEG-015

PRACTICAL GUIDE ON THE REDUCTION OF DISCHARGES TO ATMOSPHERE OF VOLATILE ORGANIC COMPOUNDS (VOCs)

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PRACTICAL GUIDE ON THE REDUCTION OF DISCHARGES TO ATMOSPHERE OF VOLATILE ORGANIC COMPOUNDS (VOCs) FOREWORD CONTENTS 1 INTRODUCTION 2 THE NEED FOR VOC CONTROL 3 CONTROL AT SOURCE 3.1 Choice or Solvent 3.2 Venting Arrangements 3.3 Nitrogen Blanketing 3.4 Pump Versus Pneumatic Transfer 3.5 Batch Charging 3.6 Reduction of Volumetric Flow 3.7 Stock Tank Design 4 DISCHARGE MEASUREMENT 4.1 By Inference or Calculation 4.2 Flow Monitoring Equipment 4.3 Analytical Instruments 4.4 Vent Emissions Database 5 ABATEMENT TECHNOLOGY

5.1 Available Options 5.2 Selection of Preferred Option 5.3 Condensation 5.4 Adsorption 5.5 Absorption 5.6 Thermal Incineration 5.7 Catalytic Oxidation 5.8 Biological Filtration 5.9 Combinations of Process technologies 5.10 Processes Under Development



6 GLOSSARY OF TERMS 7 REFERENCES Appendix 1. Photochemical Ozone Creation Potentials Appendix 2. Examples of Adsorption Preliminary Calculations Appendix 3. Example of Thermal Incineration Heat and Mass Balance Appendix 4. Cost Correlations



FOREWORD This guide has been prepared on behalf of RENALT ENERGY and is issued as one of a series of Process Safety Guides by GBH ENTERPRISES (GBHE). Its purpose is to provide process engineers, and others, who are faced with the problem of handling and treating VOC emissions from process plant, with advice on the methods available to minimize SHE impact. It recommends best practice based on current knowledge and experience from both inside and outside of GBHE. Hopefully it will make a useful contribution to achieving environmental objectives. 1 INTRODUCTION The purpose of this Practical Guide is to assist the selection of the appropriate process route to reduce the discharge to atmosphere of Volatile Organic Compounds (VOCs). It must not be used as a process engineering design guide to actually design equipment; advice should always be sought for this purpose, and Hazard Studies should be carried out for all modifications of equipment. 2 THE NEED FOR VOC CONTROL Photochemical oxidants, notably ozone, are formed indirectly by the action of sunlight on NO2. The chemical reactions involved are complex and multi-stage. The presence of hydroxyl radicals and VOCs in the atmosphere causes a shift in the atmospheric equilibrium towards much higher concentrations of ozone. VOCs can be classified according to their Photochemical Ozone Creation Potential (POCP) [2] [3]. The POCPs for a number of VOCs are shown in Appendix 1. Photochemical oxidants can damage crops, trees and other vegetation as well as affect the human respiratory function. Ozone is strongly implicated in widespread forest damage across Europe. Ozone and most VOCs are "greenhouse" gases, in some cases with "greenhouse potentials" more than a million times that of CO2 on a w/w basis.



In particular, it has been estimated that ozone is responsible for up to 10% of potential global warming. Therefore, there are concerns on the rising trends of concentrations of ozone and VOCs in the troposphere. The World Health Organization has developed air quality guidelines for ozone of 150 - 200 μg/m3 (1-hr exposure time) and 100 - 120 μg/m3 (8-hr exposure time). Legislative discharge limits for VOCs should always be determined from the local Environmental Agency. The limit for small volumetric emissions is likely to be of the order of 50 mg/m3 whereas that for large volumetric discharges could be based on the mass discharge rate as well as the VOC concentration. This practical guide is aimed at the discharge of VOCs from discrete vents. Also of great importance is the discharge directly to atmosphere from diffuse and fugitive sources and, in addition, the discharge of VOCs dissolved in aqueous effluents. However, these discharges are not covered in this practical guide. 3 CONTROL AT SOURCE It is important to eliminate or, where this is not possible, to reduce the size of the end-of-pipe abatement problem by controlling the VOC discharge at source. Some examples of techniques are given below. 3.1 Choice or Solvent Where a solvent is being used in a process, the problem of VOC discharge can sometimes be avoided or reduced by using no solvent, using a water-based solvent or by using a solvent with lower volatility. 3.2 Venting Arrangements Subject to considerations of safety, cross-contamination and plant layout, a number of stock tanks can sometimes be connected to a common venting system to reduce the overall volumetric vent flow rate. This is particularly effective when transfers are made between the stock tanks in question.



Similarly, the vent on a road tanker or other transportable container that is being loaded or off-loaded to a stock tank should, wherever possible, be connected (i.e. back-balanced) to the stock tank vent system. 3.3 Nitrogen Blanketing Where there is a need to inert the vapor space in a stock tank by using nitrogen in order to keep out oxygen or moisture, this should be achieved by means of a pressure-controlled nitrogen supply to the tank and a pressure-controlled relief on the tank rather than a continuous nitrogen sweep. 3.4 Pump Versus Pneumatic Transfer Transfer by pump, rather than pneumatically, reduces significantly the emission of vapor after the transfer and often of mist at the end of the transfer. 3.5 Batch Charging The charging of material through an open lid or charge port into a reactor or other vessel containing VOCs usually results in VOC losses to atmosphere. If the vessel is at or above atmospheric pressure these losses occur locally, if the vessel is under some vacuum there will be an ingress of air which could result in a discharge of VOC to atmosphere at a point remote from the charge point. If the material to be charged is liquid or can be dissolved in a liquid, a closed charging system should be used. Where this is not possible, a charge hopper should be considered with a narrow entry point from the hopper to the vessel in which there is a rotary, baIl or slide valve. Where an open lid or charge port does have to be used, hardware systems or procedures should be applied to ensure that the vessel is open only during the charging operation. Also, if the vessel is under any pressure, the pressure should be let down prior to charging through a vent abatement or recovery system and not through the charge port. If a draught ventilation system is used for occupational hygiene purposes, it should be switched off either automatically or manually when the charge port or vessel lid is closed.



3.6 Reduction of Volumetric Flow The flow rate of inerts must be minimized. The following examples are all common sources of flows often containing VOCs at saturation level:- Unnecessary purging, draughting, poorly designed or faulty pneumocators, valves on nitrogen blowing or blanketing systems that are passing or left open. 3.7 Stock Tank Design The liquid inlet should, wherever possible, be below the liquid level in the stock tank so as to minimize the disturbance of the vapor space. This reduces the chance of the out breathing vapor being saturated with VOC. 4 DISCHARGE MEASUREMENT 4.1 By Inference or Calculation Often the concentration and quantity of VOC(s) discharged can be calculated or inferred indirectly from other data. For example volumetric flow-rates can sometimes be determined from nitrogen usage or from fan characteristics, average VOC discharge rates can sometimes be determined from solvent usage rates, and VOC concentrations can sometimes be calculated from a knowledge of operating temperatures and vapor pressure data. However, caution is strongly advised especially where batch operations are concerned. Experience has shown that initial estimates of VOC losses based on calculated rather than measured data can be as much as an order of magnitude too low. 4.2 Flow Monitoring Equipment The main types of portable equipment suitable for measuring the flow of gases in vent pipes and ducts are shown in the following table. Prices are typically in the range $1,650 to $3,300.



Care should be taken as only some hand-held instruments are intrinsically safe but many of the others could be used providing that adequate precautions are adopted and local site regulations are observed. Some manufacturers offer an instrument and data logger that can be used with a plug-in pitot tube, vane anemometer, hot-wire anemometer, temperature and humidity probe or other probes in a single unit. Whilst more expensive than any of these individual items, the added flexibility of low cost probes plus a data logging facility in a single unit can offer advantages. 4.3 Analytical Instruments The main types of equipment suitable for analyzing VOC concentrations are shown below. It should be noted that there are wide differences in features, measuring ranges, complexity and costs. (i) Mass Spectrometers These are moveable rather than truly mobile versions of laboratory Mass Spectrometers but require fairly large vacuum pumps for their operation.



They could be permanently housed in a laboratory or in a purpose- designed mobile van with long sample lines connecting to the vents. They are expensive analyzers but are capable of analyzing most gas mixtures in concentrations from ppm(s) up to 100% subject, in general, to the number of components not exceeding about 5. (ii) Gas Chromatography Equipment has become available recently that is mobile and more compact than Mass Spectrometer units. Maintenance requirements associated with this type of equipment has traditionally been high but self diagnostic features are now becoming standard. These units can measure a similar range of gases and concentrations to Mass Spectrometers and give better quantitative results with organic mixtures but are not as good for identifying individual components. Gas sampling techniques for vent or area monitoring can be simplified by using adsorption tubes or syringes full of the gas which can be taken to the Gas Chromatograph located away from the plant. (iii) Infra-Red Analyzers These are semi-portable units. Their operation is based on the different absorbance of infra-red energy by different compounds. They are capable of detecting a wide range of compounds but only up to about four components in a mixture. The maximum concentration limit is about 3000 ppm for a limited range of materials but is much less for most components. (iv) Photo Ionization These are portable units using ultra-violet light which ionizes the VOCs and the unabsorbed UV light is detected and measured and compared with a similar beam passing through a reference gas. They are limited to concentrations in the range 0.1 to 2000 ppm and UV lamps of different energy levels are required to cover the range of materials that can be detected.



(v) Flame Ionization These are portable units that use a hydrogen flame to cause ionization of the gas phase and then measure the current generated as the positive ions reach an electrode. They give an output reading regardless of the VOC(s) detected which is correct if the only VOC present is that which was used for calibration purposes. Nevertheless, they are useful, if used with care, for providing indicative concentrations of VOCs not used in the calibration or for mixtures of VOCs. This method is particularly suitable for measuring hydrocarbons (as methane equivalents) but is less sensitive to other organics. The units require the use of fuel cylinders as well as calibration gases all of which makes the units more difficult to use than would first appear. Intrinsically safe versions are available which may make these units worth considering for difficult access areas despite the above disadvantages. 4.4 Vent Emissions Database The database is a system which allows a plant or site to store information from vent surveys and rapidly produce documentary evidence for total annual emissions. This is particularly useful when year on year comparative data is required. Emissions are totaled by site, area, plant, unit or material and emission data sheets can be printed for individual plant items.



5 ABATEMENT TECHNOLOGY

5.1 Available Options The following options are available (i) Condensation (ii) Adsorption (iii) Absorption (iv) Thermal incineration (v) Catalytic oxidation (vi) Biological filters (vii) Membranes (viii) Combinations of (i) to (vii) Section 5.2 discusses the main factors to be considered when choosing the preferred option and some details of the options are presented in Sections 5.3 to 5.9 below. Some process technologies that are still at the development stage are outlined in Section 5.10. Further background reading on available technical options can be found in the BASF technical information guide "Recovery of organic solvents from off-gas streams" [7].



5.2 Selection of Preferred Option 5.2.1 Main Issues and Factors When determining the optimum VOC abatement system for a plant or site, the following inter-related issues should be considered (1) What modifications can be made to plant and/or procedures to reduce the VOC emission at source. (2) Which technology to use for abatement. (3) Whether to treat individual vents locally or whether they should be collected together for a common treatment on a plant or site basis. In some cases it may be appropriate to use a combination of these with concentration or bulk removal being done locally and final abatement being done on a central basis. Whilst there are economies of scale for common treatment plant, this may be outweighed by the need for extra costs for fans, blowers and piping. If the various streams contain similar VOCs but at different concentrations, mixing them could prove more expensive than treating them separately. Also, the mixing of streams containing different VOCs can require complex treatment plant. For example, if there is a number of vents to be treated that comprise some containing substantial flows of VOC hydrocarbons and others containing small flows of halogenated or nitrated organics, it is possible that it would be better to segregate the halogenated and nitrated organics for separate treatment and to collect the hydrocarbons for common treatment in a simple incinerator. (4) Whether to recover VOCs for recycle back into the process or to destroy them. (5) Whether new site services such as steam or refrigeration are required.



Information on the following factors is generally required to enable the preferred treatment option to be selected :- (a) Volumetric flow of inerts (b) VOC concentration(s) (c) Calorific value of the VOC(s) (d) Commercial value of the VOC(s) (e) Removal efficiency required (f) Water solubility of the VOC(s) (g) VOC volatility (h) Biodegradability of the VOC(s) (i) VOC freezing point (j) Chemical reactivity of VOC(s) e.g. hydrolysis, polymerization, flammability, susceptibility to oxidation (e.g. aldehydes and ketones), peroxide formation (from ethers). (k) Number of VOCs present (l) Presence of dust, moisture, metals, heavy organics (m) Configuration of existing plant (n) Availability of site services, e.g. steam, aqueous effluent treatment plant (0) Site policy (p) Statutory requirements 5.2.2 Feasibility and Characteristics of Different Technologies Table 1 indicates the feasibility (also see Section 5.2.3 below) of different abatement technologies and Table 2 shows some of their characteristics.



TABLE 1 FEASIBLE APPLICATION OF DIFFERENT TECHNOLOGIES



NOTES (1) Boiling point > 35 o C (2) Boiling point < 200 o C (3) Distillation needed so unlikely to be cost effective (4) Molecular weight < 100 (5) High incineration temperature followed by flue gas cleaning required (6) Must be biodegradable (7) Not ethers (form peroxides) (8) Approximate guide only. Also see Figures 1, 2, 3 and 4. The VOC mass flow rate is sometimes more important than the VOC concentration.



TABLE 2 CHARACTERISTICS OF DIFFERENT TECHNOLOGIES



NOTES FOR TABLE 2 WASTE PROBLEMS (1) Disposal of spent adsorbent (2) Disposal of contaminated water (3) Disposal of contaminated water if flue gas scrubbing installed (4) Disposal of bio sludge (5) Production of NOx OPERATING PROBLEMS (6) Hydrolysis of halogenated VOCs (7) Polymerization of some unsaturated VOCs (8) Concern over dioxin formation from chlorinated hydrocarbons (CHCs) (9) Risk of catalyst poisoning and de-activation (10) Possible hydrate or ice formation OTHER NOTES (11) Concentrates VOC, not a recovery process per se (12) Cooling is required if flue gas cleaning is required (13) Connection to steam services may be required if steam is raised for export (14) dcost/dconc becomes +ve for VOC concentrations so high that dilution is required (15) dcost/dconc = gradient of graph of cost versus inlet VOC concentration (16) Post-recovery treatment, such as azeotropic distillation, drying and/or re-stabilization may be required



5.2.3 Economic Applicability of Different Technologies Figures 1, 2, 3 and 4 indicate the typical economic regimes for which each technique is applicable for the main classes of VOCs :hydrocarbon, oxygenated hydrocarbons and chlorinated hydrocarbons. The derivation of Figures 1, 2, 3 and 4 takes into account factors (a) to (h) in Section 5.2.1. For some of the technologies available, there is not yet sufficient cost information to assess their region of applicability and these will be added later. As these figures are, necessarily, based on a large number of assumptions, they must only be used as a guide. The assumptions are listed in Section 5.2.3.1, use of the figures is explained in Section 5.2.3.2, notes on the figures are given in Section 5.2.3.3 and cost correlations used to generate the figures. 5.2.3.1 Assumptions Implicit in Figures 1, 2, 3 and 4 (1) The total cost for each technology is calculated as installed capital plus three times the annual operating costs. A credit is made against operating costs for any VOC recovered. (2) The capital cost is the budget installed cost of the technology package plus 10% for civil. It does not include costs for services and piping up to the VOC abatement unit boundary or costs to raise the standards of design or construction over and above those of the equipment supplier. (3) All general services, including power, gas, cooling water and steam import/export, are assumed to be available. (4) Other facilities such as an existing biological treatment unit or spare refrigeration capacity are assumed not to be available. (5) The voc is assumed to be in an air stream. (6) Air regenerative adsorption is evaluated in combination with thermal incineration. (7) If adsorption, catalytic combustion or thermal incineration is used and the VOC concentration > 25% LEL, the vent stream to be treated is diluted with air to achieve < 25% LEL.



(8) The volumetric flow rate of the stream to be treated and the VOC concentration are steady. (9) The values of other variables are those given. 5.2.3.2 Use of Figures 1, 2, 3 and 4 In general, water-cooled condensation should be considered for the pre-treatment of streams with high dew points; the VOC concentration that can be achieved can be calculated from vapor pressure data. Such typical concentrations for the common classes of VOCs are shown in Figure 5 for a saturation temperature of 32 o C.



For all the technologies considered except refrigerated condensation, the capital and operating costs are mainly functions of the flow rate and the VOC inlet concentration. Hence it is possible to display singularly defined economic boundaries around these technology regimes as shown by solid lines in Figures 1, 2, 3 and 4. However, costs for refrigerated condensation are mainly functions of the flow rate and the VOC exit concentration. Hence there is no upper concentration boundary for refrigerated condensation and the lower concentration boundary varies according to the VOC exit temperature. These boundaries have been drawn for an economic refrigerant temperature and are shown as dotted lines in Figures 1, 2, 3 and 4. When using refrigerated condensation to recover VOCs, it is usually necessary to use another technology downstream in order to achieve an acceptably low VOC concentration. The dotted lines on Figures 1, 2, 3 and 4 show the economically optimum VOC concentration leaving the refrigerated condenser and entering the downstream treatment unit. This optimum VOC concentration depends on the VOC boiling point, the recovery value of the VOC and the abatement technology used. Hence there is a number of dotted lines on each figure. The recovery value is either the fuel value or the product value whichever is the higher. (See Note (1) in Section 5.2.3.3) If the VOC inlet concentration is only 2 or 3 times higher than that shown by the appropriate dotted line, it is unlikely that refrigerated condensation would be worth installing. 5.2.3.3 Examples for use of figures 1,2,3,4: (1) VOC Inlet Stream: Hydrocarbon concentration = 0.001 kg/m3 Flow rate = 1000 Nm3/hr Which technology should be used? Plot the given co-ordinates on Figure 1. The point lies in the region for Steam Regenerative Adsorption.



(2) VOC Inlet Stream: Hydrocarbon concentration = 0.1 kg/m3 Flow rate = 1000 Nm3/hr Boiling point = 80 o C Which technology should be used? Plot the given concentration and flow rate on Figure 1. The plotted point lies in the refrigeration region. Draw a horizontal line from the point until it reaches the dotted line for a boiling point of 80 o C. Drop a vertical line to the concentration axis and read off the value. It should be approximately 0.012. This means that the exit concentration from the refrigerated condenser will be 0.012 kg/m3. Therefore, another technology should be used downstream to remove the rest of the hydrocarbons. 5.2.3.4 Notes on Figures 1, 2, 3 and 4 (1) The economics of steam regenerative adsorption and thermal incineration depend very much on the value placed on the recovered VOC which, in turn, depends on whether the recovered VOC is burned as a fuel or is recycled as product. Therefore two typical cases for hydrocarbon VOCs are presented, one at $100/te value in Figure 1 (fuel) and the other at $200/te in Figure 2 (product). (2) The economics of steam regenerative adsorption for water soluble oxygenates varies very much from case to case because of distillation costs. If these are low then steam regenerative costs could be economic in the area of Figure 3 shown for catalytic combustion. (3) The difference in costs between catalytic combustion and thermal incineration is always quite small for non-halogenated VOCs. Therefore, the costs of both options should be assessed. At pressures above atmospheric, catalytic combustion could show a cost advantage.



(4) Thermal incineration using regenerative heat recovery and air regenerative adsorption followed by incineration tend to be more costly than thermal incineration using recuperative heat recovery when treating non-chlorinated VOCs. However, the economics of the first two technologies improve when support fuel costs are higher than assumed or when steam export has no value or is expensive to implement. (5) Air regenerative adsorption can be attractive in concentrating VOCs before passing to a central abatement system. Also, it is particularly useful as a primary treatment process to smooth out mass flow rates of VOCs especially in batch operations. (6) Thermal incineration tends to be much more expensive than steam regenerative adsorption for treating chlorinated hydrocarbon (CHC) VOCs and so does not appear on Figure 4. Therefore, whilst there may be some problems in using steam regenerative adsorption, it may be worth significant effort to resolve them. It should be noted that the above cost difference can be reduced if a value can be put on the Hel produced. (7) Air regenerative adsorption followed by thermal incineration tends to be significantly cheaper than thermal incineration on its own for chlorinated hydrocarbon VOCs. 5.3 Condensation 5.3.1 Indirect Condensation 5.3.1.1 Process Technology Conventional condensation using cooling water at, say, 20 o C or even refrigeration at - l5 o C is unlikely to reduce VOC concentrations low enough for direct discharge to atmosphere. However, it can sometimes be used advantageously as a pre-treatment upstream of adsorption or thermal incineration. Where the dew point of the gas stream is well above the coolant temperature, this technique can often enable a large proportion of the VOC to be recovered for recycling and, in so doing, render the gas stream suitable for viable adsorption or thermal incineration.



The presence of moisture does not favor cryogenic condensation, particularly where ice or hydrates could form and foul the heat transfer surfaces. For the same reason, the freezing point of the VOC must be considered. It should also be noted that condensation of VOCs from dilute streams at low temperatures can give rise to 'fogs' or aerosols due to premature cooling. Liquid nitrogen can be used as a once-through refrigerant for very low loadings and can be a very attractive option if liquid nitrogen is vaporized continuously on the site for other purposes. Conventional design procedures for heat exchangers with partial condensation should be used as the basis of design. Air Products offer their CRYO-CONDAP packaged unit designed to recover VOCs from inert gas (nitrogen) drying systems such as film coating and laminating machines as shown in Figure 6. The nitrogen stream laden with VOC from the processing machine is pre-cooled in a recuperator and then passes through a refrigerated condenser and thence to a separator before returning through the recuperator shell to the processing machine. A proportion of gas stream from the separator is drawn through a cryogenic condenser by means of an injector driven by nitrogen evaporated from stock. The ejector efflux is used to provide sealant gas to the labyrinth seals on the processing machine. Cryo-Condap® technology for VOC recovery

A 150 SCFM (standard cubic foot per minute) Cryo-Condap installation with redundant low temperature condensers.



5.3.2 Direct Condensation 5.3.2.1 Process Technology Direct cryogenic condensation can avoid problems of icing up of condenser tubes where the gas stream contains moisture. The gas stream is bubbled through a vessel containing the same liquid VOC as is to be condensed. The liquid VOC is refrigerated. Means must be provided to remove the ice crystals from the condenser vessel and either separate them from the liquid VOC or melt them and separate the water and VOC. If the ice crystals are more dense than the liquid voc they can be removed via a conical base to the vessel as shown in Figure 7. 5.3.3 Combination of Direct and Indirect Condensation 5.3.3.1 Process Technology An example of this type of system is the Sulzer 'APOVAC' as shown in Figure 8. In this unit a liquid ring vacuum pump is used to draw the VOC-laden vent stream into a two-stage condensation train. The pump is primed with the VOC to be condensed and the discharge from the pump comprises a two-phase mixture so that the first stage of condensation is essentially direct contact. Exhaust gas from the gas-liquid separator under the primary condenser feeds to the second stage indirect condenser. A coolant supply is required which is usually refrigerated and is fed through the second stage then the first stage in series (i.e. counter- currently).



5.3.3.2 Applications On account of the integral vacuum pump, the APOVAC system is particularly suited to duties where the exhaust gas from the indirect condenser can be re-circulated to the process and pick up more VOC such as in pressure filter drying applications. Re-circulating nitrogen containing high VOC loadings without risks of fire or explosions and without a vent to atmosphere can be used. 5.4 Adsorption 5.4.1 Process Technology (see Figure 9) 5.4.1.1 Introduction Adsorption is the most widely adopted technique for solvent recovery and granulated active carbon (GAC) is the predominant adsorbent. For very low duties, say 5 kg/day VOCs, such as deodorizing vents, small disposable cartridges of activated carbon may suffice. For higher duties it is usually only viable to install beds that can be regenerated. For duties up to about 20 kg/day VOCs, it may be more economic to utilize off- site re-activation through a merchant operator rather than regenerate in situ. For higher duties, regeneration in-situ might be more appropriate. This enables VOC recovery although post treatment of the recovered VOCs is usually needed to dry them, to separate mixtures of recovered VOCs or to re-formulate solvent mixtures including the need to add fresh stabilizer. In-situ regeneration is generally suitable for VOCs that can be recovered as liquids at ambient conditions. It can be costly to recover VOCs with boiling points below about O·C and, for these materials, off-site re-activation or thermal incineration are usually the best options as compared with recovery into pressurized and/or refrigerated storage.



A system based on in-situ regeneration typically comprises three carbon beds with two beds operating in series whilst the third bed is regenerated or is on standby. When breakthrough is detected between the two operating beds, the first is taken off line for regeneration, the second is transferred to the duty of first operating bed and the third bed is put on line as the second-in-line operating bed. The three-bed system with two beds operating in series has the added benefit of coping well with wide fluctuations in VOC loading. It is also helpful where there is a mixture of VOCs present with widely differing adsorption behavior likely to lead to preferential adsorption with possible elution from the first bed of less adsorbable materials. Before a bed is put on line for the first time or after regeneration it should be conditioned by cooling, if necessary, and drying in order to release the active sites on the particles. In addition, the gas may need pre-treatment before adsorption. The sequence, then, is as follows:- Gas pre-treatment Adsorption as primary bed Breakthrough detection Desorption Bed conditioning (drying and cooling) Stand by Adsorption as secondary bed 5.4.1.2 Gas Pre-treatment (i) Pre-filtration Particulate matter must be removed from the gas stream by, say, filtration to prevent clogging of the carbon bed. (ii) De-humidification Although water vapor is not actually adsorbed, condensation in the carbon pores acts as a barrier and reduces adsorption capacity, This problem of humidity can occur in vents from aqueous scrubbers. It is normally solved by slightly cooling the vent to condense some of the water followed by re-heating to, say, 5°C above the dew point.



(iii) Dilution VOC adsorption is an exothermic process and so it may be necessary to dilute the VOC in the gas stream, usually to about 40g/m3, in order to reduce the adiabatic temperature rise across the bed which might otherwise severely limit the bed's adsorptive capacity. Occasionally internal cooling within the bed is used instead of dilution. It should be noted that it might be necessary to dilute the VOC content to 25% LEL regardless of the adsorption process in order to avoid flammability risks in the gas stream upstream of the adsorber. 5.4.1.3 Adsorption Typically, a bed of granular activated carbon is held in a cylindrical vessel that is orientated either vertically or horizontally. The gas stream containing VOCs is passed through the adsorbent at a superficial velocity of about 12-20 m/min normally downwards if in a vertical bed in order to avoid elutriation of fines. VOCs diffuse through the carbon pore structure and are subsequently adsorbed at sites, appropriate to the nature of the VOC molecule, normally in the adsorbent micro pores. As with all diffusion processes, a concentration profile is developed in the carbon bed and this moves down stream through the bed as a concentration 'wave-front' until breakthrough occurs. In practice, the bed is usually taken off-line after a set operating time pre-determined in order to avoid breakthrough. The VOC-laden gas stream is then directed to a second adsorption bed whilst the saturated carbon in the first bed is either regenerated in situ or removed for contract recovery or disposal. Good gas distribution is necessary in order to avoid hot spots during adsorption and also, often more importantly, to achieve effective regeneration. Adsorption can sometimes be inappropriate if the VOC is likely to undergo chemical change during adsorption or desorption. In particular, the polymerization of monomers can cause bed blockages and the thermal breakdown of chlorinated hydrocarbons can release HCl which can result in corrosion.



Varying concentrations of VOCs can result in partial desorption or movement of the concentration profile down-stream in the bed during periods of high inerts flow rates and low VOC concentrations. This can be problematical with odorous VOCs. However, the installation of a back- up adsorption bed in series with the main adsorber usually solves the problem. 5.4.1.4 Regeneration, Re-activation and Disposal (i) In-situ Regeneration by Steam Desorption This is an attractive option for VOCs immiscible with water. Steam, normally low-pressure, is blown counter-currently through the carbon bed in order to strip the VOC from the carbon surface. Steam and VOG vapors are then condensed and, for VOCs immiscible with water, the recovered VOC separated by decanting. For low boiling point VOCs or if inerts are present, it may be necessary to pass the exhaust steam from the condenser through the adsorber on line. Also, the aqueous condensate will be saturated with VOC and may require treatment before discharge. Steam regeneration is usually for about 30 minutes which typically strips 50% to 60% of the adsorbed VOC. The bed then requires drying with, say, hot air and then cooling with, say, cold air before being made available for adsorption. Drying and cooling normally takes 20 to 30 minutes. Another 20 to 30 minutes should be allowed in the cycle for change over and contingency. Therefore, the total time required for regeneration is about 30+30+30 = 90 minutes. It is important to check that the estimated adsorption time for a regenerated bed is longer than the regeneration time. If this not the case, then it may be necessary to install several beds in series. Typically, 2 kg to 4 kg steam are required for every kg of VOC stripped. About 1/3 of this is required to warm the bed, about 1/3 to volatilize the VOC and about 1/3 acts as carrier to transport the VOC from the bed.



When selecting materials of construction, consideration must be given to the possibility of hydrolysis of VOCs, e.g. chlorinated hydrocarbons, to acids. (ii) In-situ Hot Gas Regeneration This technique may be required for VOCs miscible with water to avoid the need for costly separation, say by distillation, from water following steam regeneration. It is also applicable where it is necessary to recover the VOC(s) in a dry form or where adsorption is used to concentrate the VOC(s) before thermal incineration or catalytic oxidation. (Also see Section 5.9.2.) The VOC is stripped from the carbon bed by a stream of hot gas which is usually air or nitrogen although hot process gas could used if it is available. However, air or process gas containing oxygen should not be used for flammable VOGs on fixed adsorption beds. The stripping gas is then passed through one or more condensers to recover the bulk of the VOC. The condenser vent is then recycled back to the adsorber on line. Hot gas regeneration can be used for low volatility VOCs only if a suitable grade of carbon can be identified. It is not a widely used technique. (iii) Pressure Swing Adsorption takes place at the gas stream pressure and desorption is achieved by volatilization at a lower pressure or under vacuum. Pressure swing adsorption (PSA) is used widely for bulk process gas separations, e.g. hydrogen recovery and air purification. However, it can also be used for reducing high concentrations of light VOCs in vent gases. Because the VOCs are stripped off the adsorbent into a gas stream, PSA is always combined with a downstream recovery process such as condensation. In general, for PSA to be considered suitable, the VOC molecular weight should be below about 100 and VOC concentration should be between about 1% and 10% v/v.



PSA does not normally achieve very low exit VOC concentrations; 3000 ppm is not untypical, but energy consumption is very low. PSA for VOC removal from vent streams requires the use of special grades of carbon. The initial market demand that generated a large need for PSA carbons came from the control of hydrocarbon emissions from automobile fuel tanks. Applications in major processing plant came via gasoline vapor recovery in loading and storage, then vent processing in polymer plants and has now evolved to include several applications in petroleum production. Figure 10 shows a simplified flowsheet for gasoline recovery on a refining or terminal installation and Figure 11 shows hydrocarbon recovery from a stripping nitrogen stream in a polyethylene plant. PSA systems for recovery of VOCs in emissions are marketed in Europe by BOC. The BOC system uses a process feed gas e.g. oxygen or nitrogen to regenerate the adsorbent by partial pressure swing. (iv) Off-site Regeneration by Contract Re-activation Off-site contract re-activation should be considered where in-situ regeneration is technically difficult or expensive to provide, e.g. if the adsorbent has suffered significant loss of activity through VOC polymerization or in-situ regeneration is required only, say, four times per year. Carbon is 're-activated' typically with steam at about 800 o C in a multiple hearth or rotary furnace. The VOCs are essentially pyrolyzed and the flue gases are thermally oxidized. This service is offered by some carbon suppliers. Re-activated carbon is delivered by tanker in quantities up to 20 m3 and is discharged pneumatically to a holding vessel whilst spent carbon is transferred pneumatically to the empty tanker. The adsorber vessel is then re-filled with the re-activated charge and put back into service.



(v) Disposal to Landfill and Thermal Incineration Where the adsorbed material is acceptable, landfill is often the cheapest disposal option. Current landfill prices are about $85/te in some areas of the world but this figure is rising rapidly. Thermal incineration costs about $825/te but can vary considerably according to the nature of adsorbed material, e.g. a halogenated hydrocarbon could cost more than $825/te but a simple hydrocarbon would likely cost much less. 5.4.1.5 Bed Conditioning Following steam regeneration, the adsorbent bed must be dried and cooled in order to achieve efficient adsorption. Similarly, an adsorbent bed must be cooled after hot gas regeneration. Drying is usually achieved by passing warm air through the bed. It is important to ensure adequate steam desorption of flammable VOCs before conditioning with air otherwise there could be a risk of fire. Drying is usually followed by a period of blowing cold" air to reduce the bed temperature. Typically drying and cooling takes about 20 minutes with a volumetric air flow approximately equal to the flow through the bed during the absorption phase. The early part of the drying stage normally produces a plume of steam containing some VOC. It is recommended that this be recycled for about 2 minutes through the condenser to the adsorber inlet. 5.4.2 Adsorbents Although there is a miscellany of commercially-available adsorbents, for general, non-selective organics recovery, activated carbon is the most common in industrial use. The main reasons for this are cost, which is typically $4 - 6/kg compared with about $25/kg for an equivalent resin, and the ability of carbon to separate organics from water and water vapor. In order to achieve an acceptable pressure drop through the bed and to reduce the elutriation of fine particles, the carbon is normally supplied in pellet form. Carbon fiber woven into a fabric or formed into a "paper" can be a convenient adsorbent for some applications, 'e.g. the Durr Epocure- KPR rotor system for concentrating VOCs in air streams (see Figure 12).



(Also see Section 5.9.2.) Fluidized bed systems are offered by some suppliers including Lurgi who supply the Kontisorbon process. The most commonly used carbons are based on coal or wood charcoal although coconut charcoal is sometimes used. Resins and polymers have been developed for carbon-type applications but they are generally tailored for more selective adsorption. Amberlite, supplied by Rohm and Haas, and Bono Pore, supplied by Chematur, are two of the more popular resins. For preliminary design purposes, the bulk density of activated carbon can usually be taken to be 450 kg/m3. Molecular sieves are used commercially for some VOC applications but are normally restricted to simple hydrocarbons such as aliphatic chlorinated hydrocarbons. 5.4.3 Basis of Design The process engineering design of an adsorption unit is based largely on equilibrium data and the bed 'breakthrough' characteristics. Equilibrium data are represented in the form of adsorption isotherms which are correlations between the adsorptive capacity (weight of VOC adsorbed per unit weight of adsorbent) and the concentration or partial pressure of the VOC in the gas phase at constant temperature. The effect of temperature is shown by plotting separate isotherms as shown in Figure 13. Generally, adsorptive capacity increases with increased molecular weight of the VOC, increased polarity, increased degree of cyclization (i.e. aromatic VOCs are more readily adsorbed than aliphatic VOCs) and decreased vapor pressure. It is emphasized that an isotherm determined for one proprietary carbon is likely to be different for a different grade of carbon or ostensibly the same grade from a different supplier. Furthermore, the effects of humidity must be recognized.



Adsorption isotherms are drawn for freshly supplied carbon. The actual working capacity of a bed regenerated in situ is significantly lower than that determined from the adsorption isotherm because the regeneration step does not restore the virgin bed conditions. The degree of desorption must be balanced against the steam consumption and the time allowed in the operating cycle for desorption. Adsorption beds are generally designed for superficial linear velocities of 12 to 20 m/min and, typically, this results in a wave-front of only about 5 to 8 cm which is usually insignificant as compared to the total bed height. The volume of adsorbent bed required can be estimated from a knowledge of the adsorption capacity, the VOC feed rate and the cycle time. (See example in Appendix 4) Some typical design data are shown below :- Maximum VOC inlet concentration 40 g/m3 Pressure drop across bed 12" wg Superficial linear velocity in bed 12 to 20 m/min Cyclic capacity 10 to 20 % w/w Once-through capacity up to 50% w/w Regeneration steam usage - 3 kg/kg VOC Minimum cycle time 1.5 hours Maximum Relative Humidity 50%



FIGURE 13: TYPICAL ADSORPTION ISOTHERMS



5.4.4 Costs Typical costs for activated carbon are $4/kg to $6/kg as compared to about $25/kg for an equivalent resin. Re-activated carbon (see 5.4.1.4 (vi) above) costs about $3/kg. Operating costs cover: Carbon replacement Steam @ 3 kg/kg VOC Cooling water Air to condition bed @ 50·C and a flow rate - adsorbing flow rate for 20 mins. Other services, e.g. instrument air Manpower Maintenance, say 4% of capital 5.4.5 Suppliers (i) Adsorbent manufacturers who also provide an engineering service (ii) Engineering contractors who build units and supply suitable adsorbents (a) Steam Regeneration (b) Hot Gas Regeneration (c) Pressure Swing Adsorption



(iii) Choice The choice is very often dictated by the technical features or uncertainties of the process. For a relatively simple application, e.g. a single solvent for which adsorption capacity, bed characteristics, etc., are well established, e.g. ketones, chlorinated hydrocarbons, etc" no experimental work would be necessary and one of the engineering contractors would likely be able to design and supply the plant. For a complicated system and sophisticated materials of construction, design data would likely have to be generated and this could be done by one of the adsorbent suppliers on their test facilities. Individual suppliers have built up experience with specific solvents, e.g. some European companies have considerable experience with acetone, including the potential fire and explosion hazards whereas the German companies have more experience of dealing with chlorinated hydrocarbons. 5.5 Absorption 5.5.1 Introduction The only known information available within GBH Enterprises is that supplied by Corning Process Systems. Their process was developed in Germany in 1985 by their sister company 'QVF Glass'. The absorbent used is a polyethylene ether which is miscible with water. The process is based on a conventional gas scrubbing tower to absorb VOCs followed by a conventional steam stripper to regenerate the absorbent liquor and recover the absorbed VOCs (see Figure 14). It is claimed that the process can deal with a wide range of VOCs up to 50-100 g/m3 Corning have activity coefficients for design purposes for 30 to 50 compounds. It has been suggested that several absorbers could be sited local to individual VOC discharge sources with a central desorber unit. This idea has not yet been tried in practice.



5.5.2 Absorption (See Figure 14) Ambient inlet temperature is preferred, although higher temperatures can be accommodated increasing the duty on the cooler in the absorbent circulation system. The absorber operates at atmospheric pressure. Ideally, the absorption fluid temperature should be less than 20·C to 25°C in order to maintain a sufficiently low VOC vapor pressure. Consequently, a chilled cooler may be required. Suitable materials of construction for the column and the internals must obviously be used but these need not necessarily be glass. High performance structured packing (e.g. Sulzer) are usually used. 5.5.3 Desorption Desorption of the VOC from the absorbent is effected by steam stripping plus indirect heating. Sometimes water plus indirect heating is used. The condensing temperature required is, of course, dependent on the volatility. The stripping column operates at -70 torr with the vacuum being provided by a once-through oil vacuum pump or other means. The means of separating the recovered VOC(s) in the stripping column overhead product from the aqueous condensate necessarily depends on the solubility of the VOC(s) in water and vice versa, the relative densities of the VOC(s) and water and the need for the recovered VOC(s) to be dry. The main service usage is steam on the desorber. Corning recommend using 80 kg steam per 1000 m3 air (STP) for preliminary estimates.



5.5.4 General Matters The design should include heat recovery from the desorber bottoms stream to the desorber feed. The only storage capacity in the system for polyethylene ether is the bottoms of the two columns which should be level controlled. The polyethylene ether is stabilized with an amine to prevent peroxide formation. This limits applications to non-acidic gases. Start-up can be achieved from cold in about 20 minutes. This could be reduced, if necessary, by adding a pre-heater in the desorber feed line. There is some doubt as to whether polymerization problems could be encountered with VOC monomers; this has yet to be tested.



5.6 Thermal Incineration 5.6.1 Process Technology Oxidation of hydrocarbon VOCs produces H20 and CO2, If the VOCs are halogenated, hydrogen halides, e.g. HCl, are also produced provided there is sufficient hydrogen available from the VOC(s), water vapor or a support fuel such as methane. VOCs containing sulfur produce SOx; organically bound nitrogen produces NOx or free nitrogen. Heat can be recovered from the flue gas in recuperative or regenerative systems. (See Section 5.6.1.5 below.) Flue gas treatment may be required to remove acid gases such as HCl. 5.6.1.1 Excess Oxygen In order to achieve complete oxidation, preliminary calculations should be based on about 25% excess oxygen in the feed stream over and above the stoichiometric requirement; this will usually result in about 4% v/v oxygen in the flue gas. This excess oxygen must be added as combustion air if it is not already available as free or combined oxygen in the gas stream to be treated. (Also see NOx abatement in Section 5.6.1.2 below.) If support fuel is required, about 25% excess combustion air for that support fuel should also be used. 5.6.1.2 Combustion Temperature A minimum combustion temperature of about 750°C is necessary to achieve adequate destruction of most VOC(s) comprising carbon and hydrogen or carbon, hydrogen and oxygen. However, 850·C is more typical and is more likely to be required by the Authorities. 1100 o C to 1200 o C is required for halogenated VOCs (see Section 5.6.1.7 below). If the VOC(s) contain nitrogen, measures must be taken to avoid unacceptable NOx levels. Short-flame low-NOx burner nozzles are available for this purpose.



In addition, in may be necessary to have a primary combustion chamber operating under starved air conditions, say just below stoichiometric requirements, followed by a secondary combustion chamber operating with, say, 4% v/v oxygen in the flue gas. This configuration enables combined nitrogen to be converted to elemental nitrogen in the primary chamber followed by completion of the oxidation of carbon monoxide and residual hydrocarbon species in the secondary chamber. 5.6.1.3 Residence Time Residence time within the incineration chamber is typically 0.5 secs but can be as high as 2 secs for halogenated VOCs. Linear velocity within the incineration chamber is typically 12 to 15 m/s. 5.6.1.4 Turbulence Turbulence is necessary to ensure a high degree of mixing between the VOC(s), the combustion air and, if used, the support fuel. Radial mixing should be high but axial mixing should be low otherwise a proportion of the VOCs would experience a low residence time. This is usually achieved by the use of radial-swirl burners sometimes together with longitudinal baffles. 5.6.1.5 Heat Recovery Systems Support fuel is generally required at start-up and during some up-set conditions. In order to reduce or eliminate support fuel during normal operation, the vent stream containing VOC(s) and/or the additional combustion air (if required) is often pre-heated by recovering heat from the flue gas leaving the incinerator using either recuperative or regenerative heat exchange. If the oxidation adiabatic temperature rise is greater than about 170°C, it is often worth installing a recuperative heat exchanger to raise steam in addition to any heat recovery into the inlet vent stream.



(i) Recuperative Heat Recovery Heat is recovered by indirect transfer from the hot flue gas leaving the incinerator to the in-coming VOC stream/combustion air. Standard shell-and-tube heat exchangers can be used but sometimes an integral unit is used with the heat exchanger tubes built around the combustion chamber (see Figure 15). Thermal efficiencies (see Section 9) up to 0.7 or 0.8 can be achieved. If the adiabatic temperature rise is greater than about 150°C, recuperative heat recovery usually enables operation without support fuel. (ii) Regenerative Heat Recovery These units operate batch-wise by heating up a matrix, e.g. ceramic honey-comb or sand bed, by direct contact with hot flue gas and then using the hot matrix to pre-heat the VOC stream/combustion air. It is claimed that thermal efficiencies (see Section 9) up to 0.95 can be achieved. Such high efficiencies enable operation without support fuel at quite low adiabatic temperature rises. Above adiabatic temperature rises of about 150°C, recuperative units tend to give lower overall costs than regenerative units on account of much lower capital costs without the need for support fuel. For adiabatic temperature rises less than about 150°C, regenerative units tend to show a lower overall cost because support fuel can be avoided. However, the resulting low flue gas temperature usually precludes economic waste heat recovery. There are two main variations of design of regenerative heat recovery units:-



The first and more common type of regenerative unit is illustrated in Figure 16. The incoming gas stream is heated by direct contact with a pre-heated matrix CA) upstream of a combustion chamber. Burner nozzles supplied with fuel are positioned in the combustion chamber for start-up purposes and to provide support fuel if required during normal operation. Oxidation of the VOC(s) begins in the hot matrix and is completed in the combustion chamber. The flue gas from the combustion chamber passes through and heats up a second matrix (B) before discharging to atmosphere. After a pre-determined time, the direction of flow is reversed so that the in-coming stream is pre-heated in matrix B whilst matrix A is heated by the flue gas from the combustion chamber. Matrix A contains some untreated VOCs at the time of change-over. In order to avoid these VOCs being discharged to atmosphere when matrix B is put onto the upstream duty, a third matrix (C) is often installed which then becomes the down-stream matrix to enable matrix A to be purged within the sequence shown below.

Parallel streams can be used for large gas flows. However, it is possible to design the sequence of matrix change-over's such that only one is being purged or is on standby. Thus there is always an odd number of matrices. In the second type of regenerative unit a single bed is used in which the combustion takes place at the centre and the flow of gas is reversed about once every two minutes on a time switch. (See Figure 17). The bed downstream of the combustion zone is heated by the hot flue gas whilst the in- coming VOC stream/combustion air is pre-heated as it approaches the combustion zone. During flow reversal, the VOC(s) in the upstream part of the unit are purged to atmosphere and this limits the thermal efficiency (see Section 9) to a maximum of about 90%. This type of regenerative bed is often started up from cold using an electrical heater in the centre of the bed to establish the temperature profile. Typically, the exhaust gas is about 25 o C hotter than the in-coming vent stream and the electricity consumption for the heater and for blowers is about 0.01 kWh/m3 of vent gas.



5.6.1.6 Heat Balance Standard thermodynamic calculation procedures should be used to determine the adiabatic temperature rise resulting from the exothermic oxidation process. Heats of combustion and specific heats are available in the literature (12). However, as a first approximation, the heat of combustion of most VOCs can be taken as 42 kJ/g and that for methane support fuel as 50 kJ/g. Also as a first approximation, the specific heat of the flue gas can be taken as 1.3 kJ/Nm3·C. If support fuel is required, any extra combustion air required must necessarily be included in the heat balance. An example of a heat and mass balance is given in Appendix 5. 5.6.1.7 Halogenated VOCs If the VOC(s) are halogenated, a combustion temperature of 1l00 o C to 1200 o C is necessary in order to avoid the formation of halogenated dioxins and furans at unacceptable levels. As temperatures as high as 1200 o C are not generally suitable for the materials of construction of regenerative or recuperative gas-gas heat recovery units, support fuel is invariably required. However, the temperature of the flue gas must be reduced before acid gas scrubbing and waste heat recovery to raise steam is often used for this purpose. The thermal incineration of halogenated VOCs requires flue gas scrubbing to remove acid gases. This usually comprises a first-stage acid scrubbing unit followed by sn alkaline back-up unit. If the VOC(s) contain fluorine, it is usual to install a down-fired combustion chamber that feeds directly into a water quench underneath because of the highly-corrosive nature of HF. (See Figure 18.) 5.6.1.8 Plume Visibility Flue gas that has been through an aqueous scrubber will produce a visible steam plume unless it is further treated. In order to satisfy statutory Authorities and/or site policy, it may be necessary to avoid a visible plume by re-heating the flue gas. This is usually best achieved by adding a pre- heated air stream to the flue gas immediately up-stream of the stack to raise the discharge temperature to a minimum of about 80 o C. If possible, hest for this duty should be taken from the hot flue gas up-stream of the aqueous scrubber.



5.7 Catalytic Oxidation 5.7.1 Process Technology The following key design factors are discussed below: Feed pre-treatment - filtration Operating temperature Catalyst volume Heat balance Heat Recovery Start-up 5.7.1.1 Feed Pre-treatment - Filtration It may be necessary to filter the feed gas stream to avoid blinding the catalyst surface. 5.7.1.2 Operating Temperature Catalytic oxidation reactions require a minimum temperature to be reached before any significant degree of oxidation occurs. The temperature required for essentially complete reaction is dependent on this ignition temperature, the contact time and the type of catalyst. Examples of minimum pre-heat temperatures are shown below:-

Streams containing halogenated VOCs are not normally fed to catalytic oxidation units because the operating temperatures are not high enough to avoid the formation of chlorinated dioxins and because chloride tends to sinter the catalyst.



5.7.1.3 Catalyst Volume The space velocity (m3 of gas per hour per m3 of catalyst) is normally about 50000 hr-1 for clean streams but lower space velocities of around 20000 hr-1are needed when there is likely to be catalyst poisoning. However, it is not usually cost-effective to carry out pilot plant trials for every individual new application so, if previous operating data are not available, a space velocity of 50000 hr-1will invariably be quoted by the catalyst supplier. The bed dimensions are determined by pressure drop considerations or by the cross-sectional area of the duct if that is determined by process considerations. 5.7.1.4 Heat Balance See Section 5.6.1.6 above. 5.7.1.5 Pre-Heat & Heat Recovery The feed gas to the catalytic oxidizer must be pre-heated to the catalytic ignition temperature. This can be done either by a continuous pre-heat burner or by heat exchange with the exhaust gas from the catalytic oxidizer. The appropriate heat exchange system depends on the VOC concentration, the volumetric flow rate, the VOC heat of combustion and the temperature of the gas stream fed to the unit. In most cases heat is recycled from the exhaust gas to the feed stream by means of counter-current heat exchange as shown in Figure 19. It is not normally economic to design the inter-changer with a thermal efficiency greater than about 0.7. (See Section 9.) In some designs, the heat exchanger may be of a regenerative type where cold feed gas and hot exhaust gas are alternately switched between two beds of hot ceramic packing. CJB claim this to be more economic than conventional heat exchangers and also it enables Autothermal operation to be more readily achievable. It may be possible to get similar benefits by using printed circuit heat exchangers. If the VOC concentration in the gas stream to be treated is so low that this heat exchanger size does make the system Autothermal, it would be necessary to use support fuel or to pre-heat the feed stream.



If the adiabatic temperature rise is greater than 170°C it is often economic to recover heat for external use as shown in Figure 20. If the adiabatic temperature rise is greater than about 350°C, there are two potential problems :- (i) The temperature in the reactor may give rise to short catalyst life or high equipment design temperatures. (ii) The concentration may be such as to get close to the lower explosive limit (LEL). System suppliers do not recommend exceeding 25% LEL but this should be reviewed depending on the controllability of the upstream process. The first problem can be overcome by using a multi-stage cold shot reactor as shown in Figure 21. Both problems can be overcome by recycling part of the exhaust gas to the reactor as shown in Figure 22. If the adiabatic temperature rise is much greater than 400°C, it would normally be more economic to use thermal incineration (see Section 7.6) rather than catalytic oxidation. 5.7.1.6 Start-up Means must be provided to establish the oxidation temperature in the reactor. This may necessitate by-passing the economizer, installing a start-up heater and/or adding hydrogen to enable the reactor to strike at low temperature. 5.7.2 Catalyst & Catalyst Support The catalysts used usually are precious metal impregnated on to a support, but also non-precious metal catalysts both impregnated and homogeneous types are used. These are usually oxides of Cr, Mn, or Co and are essential if high concentrations of sulfur derivatives are present in the vent stream.



In order to obtain a low pressure drop and to avoid pressure build-up by particulates, ceramic monolith honeycomb type catalyst supports are preferred. The monolith material is normally a cordierite or mullite alumina- silicate with a surface area of about 2000 m2/m3 and an open cross- sectional area of up to 67%. The monolith is coated with a high surface area wash-coat, typically gamma alumina, and this is often impregnated with platinum to give a loading of about 3 kg Pt per m3 of catalyst support. Catalysts gradually de-activate over time and their life is generally 2 to 4 years provided the operating temperature does not exceed 500·C - 600·C. Catalysts are not normally regenerated but, in the case of precious metal catalysts, are sold back to the supplier for metal recovery. However, as the life of regenerated catalyst is normally shorter than that of fresh catalyst, it is only worthwhile doing 2 or 3 regenerations before the catalyst is changed. The optimum choice of catalyst involves a good knowledge of all likely compositions of the vent stream to be treated including trace quantities of potential catalyst poisons. Some catalyst poisons, such as organic silicone and organic metal compounds, deposit solids on the catalyst and are permanent whereas other poisons can be removed by regeneration or by installing a catalyst pre-heater to burn off the poisons intermittently at high temperature. A list of typical poisons and counter-measures is given in the table below. Precious metal catalysts are more prone to sintering and poisoning than are base-metal oxide catalysts. A fluidized bed catalytic combustion system can be used for dirty gas streams containing high levels of poisons. LIST OF CATALYST POISONS



5.7.3 Catalytic Oxidation Of Chlorinated Hydrocarbons Some companies, particularly in North America, have recently been developing catalysts for catalytic oxidation of chlorinated hydrocarbons. The development has been directed at the avoidance of catalyst sintering by chloride. However, there is a potential problem of chlorinated dioxin formation. Whilst the catalyst supplier will maintain that this is not a problem, it may cost an inordinate amount of money to prove that this is not the case on each application. The catalyst manufacturers have not addressed this issue. One North American catalyst supplier offers two options for catalytic combustion of gases containing a mixture of halogenated and non- halogenated hydrocarbons: (i) High temperature catalytic combustion to destroy all the combustible species. (ii) Low temperature catalytic combustion with a special catalyst that will combust only the non-halogenated hydrocarbons. The halogenated hydrocarbons can be removed downstream e.g. by adsorption. 5.7.4 Suppliers The catalyst is supplied to the equipment vendor by a number of catalyst manufacturers; a few of which supply the total package.



5.8 Biological Filtration 5.8.1 Principles of Operation Low volumetric flow-rates of vent gas containing low concentrations of biodegradable VOC(s) are fed through beds of peat, heather, compost or a mixture of these which serve as carrier material for micro organisms and also provide some nourishment to the biomass. Operation of biological filters is based on absorption~ biological break-down and, to a lesser degree, adsorption. It has been postulated [13] that the reaction kinetics are zero order. As the process is essentially microbial and aerobic the vent stream must contain sufficient oxygen and the beds must be kept moist in order to sustain the micro organisms living in the pores of the carrier. CO2 and H20 are the principal products of degradation. Non-volatile oxidation products remain on the carrier. If acid metabolites are produced (e.g. from the degradation of halogenated hydrocarbons, ammonia, hydrogen sulfide), bio-filtration can be sustained only if the pH of the bed is maintained by, for example, the admixture of insoluble alkali such as limestone. Easily biodegradable VOCs such as alcohols, esters (e.g. ethyl acetate, butyl acetate) and ketones (e.g. acetone, MIBK) generally degrade on natural packing materials. The maximum concentration of these VOCs that can be handled by biological filtration is about 1500 mg/m. VOCs that are less easily biodegradable, such as aromatics (e.g. benzene, toluene, styrene, xylenes) and chlorinated hydrocarbons (e.g. dichloromethane, vinyl chloride) generally require inoculation of the bed with specially cultivated organisms. The maximum concentration of these more difficult non-chlorinated VOCs that can be handled is about 500 mg/m3 and for chlorinated VOCs is about 20 mg/m3. The life of a carrier bed is usually between 2 and 4 years.



5.8.2 Bed Structure and Bed Conditioning The bed should be very porous but the structure should not be so open as to result in poor contact between the gas stream and the bed. This can be satisfied by having alternate layers of, say, peat and heather fiber. (One European company uses expanded polystyrene beads in some of their units.) There will generally be several types of micro organism in an un-treated carrier and these will often multiply and adapt when exposed to a given vent stream containing VOCs. It is also possible to add specific micro organisms to the carrier material and it may be necessary to add nutrients, such as N03, P04, Ca, K or Fe, to the bed. Usually it takes 2 to 4 weeks for a bed to condition naturally. The bed must be loosely packed in order to provide high porosity and good distribution of oxygen to the micro organisms. 5.8.3 Operating Conditions High humidity must always be maintained within the bed to keep the bed moisture content between about 40% and 60%. This can be achieved by means of water spray nozzles located above the bed or, preferably, by passing the vent stream through a simple wetting column up-stream of the bed. In one filter, the moisture content of the bed is monitored by the continuous measurement of bed weight using load cells and by the pressure drop across the bed. Standard micro organisms are active generally between 15 o C and 35 o C. Therefore, it may be necessary to pre-heat or cool the vent stream fed to the bed. Other important conditions include pH, presence of nutrients, concentration of VOC(s), and the concentration of pollutants poisonous to the micro organisms. The optimum superficial velocity is usually determined by the allowable pressure drop which is proportional to the bed height. Most reported duties involve low pressure feeds restricting the bed height to about 1 m and an associated maximum superficial velocity of about 100 m/hr.



J A Don [14] reports pressure drops of 500 Pa for conventional filter compost and 100 Pa for a mixture of fine compost and coarse bark for a superficial velocity of 100 m/hr through a bed 1 m high. 5.9 Combinations of Process Technologies There are three classes of systems that employ combinations of process technologies :- (i) Bulk removal/recovery units followed by a polishing unit. (ii) Concentration or buffering unit followed by a recovery or a destruction unit. (iii) Mixtures of dissimilar VOCs where different components are treated in different ways. 5.9.1 Bulk Removal Or Recovery Unit Followed By A Polishing Unit This technique is generally used only when the bulk removal or recovery unit is condensation. The polishing unit can be adsorption, catalytic oxidation or thermal incineration. If solvent adsorption is used to remove or recover high VOC concentrations it is generally more economic to make the adsorber taller to achieve lower VOC exit concentrations rather than to install separate technology for polishing. 5.9.2 Concentration / Buffering Unit Followed By A Recovery / Destruction Unit Adsorption with hot gas or pressure swing regeneration is often used to concentrate a dilute VOC stream which can then be recovered or destroyed. Concentration factors are typically 10 to 15. The concentrated regeneration stream can be sent either to a condenser or absorber system for recovery or to a thermal incineration or catalytic oxidation unit for destruction. In the case of pressure swing adsorption, the VOC is always recovered from the regeneration stream. If the regeneration stream from a thermal swing adsorption system is sent to a condenser for recovery of the VOC, there is scope for operating the regeneration unit in an optimum way to get high peak concentrations in the regeneration gas that can be condensed out at higher temperatures than with a standard regeneration cycle.



The combination of regenerative adsorption and thermal incineration can be used to advantage where there is a large site thermal incinerator. The regeneration gas can be passed to the thermal incinerator along much smaller pipe-lines than those used for the dilute feed gas. Alternatively, when an adsorbent bed is due for regeneration it can be replaced with a fresh bed and physically transported to a central regeneration unit next to the thermal incinerator. This combination is particularly beneficial for small intermittent emissions from, say, stock tank breathing. The combination of regenerative adsorption and catalytic or thermal incineration is also applicable for continuous emissions with very low concentrations of VOC say < 100 ppm, in high volumetric gas flow rates, say> 30000 Nm3/hr. This technique enables the size and hence capital cost of the Oxidizer to be reduced significantly and for the support fuel consumption to be greatly reduced or even eliminated. Rotary adsorption beds are particularly suitable for this type of application (see Section 5.4.2 and Figure 12) where the regeneration air temperature is typically 120°C for activated carbon and 200°C for zeolites. Carbon is used in the form of a fiber which is fabricated into a block with longitudinal flow channels through which the gas passes. This technology is licensed from Toyobo in Japan. Some companies, supply a complete package including carbon adsorption beds with hot air regeneration into a thermal incinerator. Other companies, claim to have developed integrated units that perform both adsorption and oxidation over a catalyzed adsorbent, but these are not yet commercially available. 5.9.3 Mixtures Of Dissimilar VOCs When a vent contains a mixture of a chlorinated hydrocarbon at low concentration and non-chlorinated VOC at a higher concentration, it can be advantageous if the chlorinated hydrocarbon can be removed by once-through adsorption so that the non-chlorinated VOC can then be destroyed by catalytic or thermal incineration. Obviously, great care must be exercised to ensure no break-through of the chlorinated hydrocarbon in order to avoid possible damage to the oxidizer or likely emission of, for example, HCl.



5.9.4 Membrane Technology The use of membranes for VOC abatement falls into the category of concentration followed by a recovery or a destruction unit, i.e. 7.9(ii). Pervaporation (permeation and evaporation) and reverse osmosis are two methods where membranes can be used to concentrate VOC streams (typically in the range 100-5000 ppm). Followed by a technology such as condensation to recover VOCs, membranes can be the more economic proposition, especially for less volatile compounds. 5.10 Processes Under Development 5.10.1 Combined Absorption/Chemical Oxidation This process involves absorbing water-soluble VOCs in water containing an oxidizing agent. The two oxidizing systems of primary interest are ozone and hydrogen peroxide. Ozone is the most effective but is disadvantaged by the capital cost of ozone generation. Peroxide is relatively cheap to provide but it usually requires a catalyst to promote the oxidation; this is typically Fenton's reagent (iron salts) or UV radiation. 5.10.2 Catalytically Stabilized Thermal Oxidation (CST Oxidation) CST oxidation reactors use catalytic surface oxidation reactions to help ignite and sustain homogeneous gas-phase reactions. They are high- throughput, high temperature (900 o C to 1300 o C), low residence time devices whereas (surface) catalytic oxidation reactors (see Section 5.7) have lower throughputs and operate at lower temperatures with higher residence times. CST oxidation reactors can be designed to approximate a plug-flow reactor with combustion stability maintained without back-mixing. In a tubular geometry, the average residence time distribution of the flow is much narrower than in a back-mix burner and this is especially important where even low concentrations of primary VOC or partial combustion products formed may be toxic. This property is particularly beneficial in the oxidation of chlorinated hydrocarbon VOCs.



Work has been done at Yale University using methyl chloride and methylene chloride at the laboratory scale. In addition, one European company have carried out work on 1.2-dichloropropane with a ceramic fiber type reactor also at the laboratory scale. 6 GLOSSARY OF TERMS CST Catalytically Stabilized Thermal (Oxidation) CHC Chlorinated hydrocarbon GAC Granulated active carbon LEL Lower Explosive Limit N Cubic meters at Normal conditions, i.e. 15 o C and 1 atmos Oxygenates VOCs containing oxygen, e.g. alcohols, ketones POCP Photochemical Ozone Creation Potential THERMAL The temperature rise of a stream within a heat EFFICIENCY exchanger divided by the difference in inlet temperatures of the two streams UNECE United Nations Economic Commission for Europe VOC Volatile Organic Compound



7 REFERENCES [1] "Draft Technical Annex on Control Measures for Emissions of Volatile Organic Compounds (VOCs) From Stationary Sources", UNECE, Executive Body for the Convention on Long-range Trans boundary Air Pollution, Working Group on Volatile Organic Compounds Fourth Session (Geneva, 16-20 July 1990). EWB.AIR/WG.4/R.9. [2] I T Marlow & M J Woodfield, "UK Industry and Ozone Pollution From VOC Emissions", Warren Spring Laboratory, 28 Aug 1990, LR 775(PA). [3] RG Derwent and M E Jenkin, "Hydrocarbon Involvement In Photochemical Ozone Formation In Europe", AERE Report R13736, May 1990 [6] A Thompson, "Environmental Technology : Removal Of Volatile Organic Compounds (VOCs) From Gaseous Effluent, FCMO PTD Technical Memorandum, Dept Ref No RDl1226A, January 1991 (Prepared as a technical paper for inclusion in the Institution of Chemical Engineers Environmental Protection Bulletin). [7] "Recovery of organic solvents from off-gas streams", BASF Technical Information TI-CIW/ES 014a, July 1989 (JWF) [12] R H Perry & D Green, "Perry's Chemical Engineers' Handbook", McGraw Hill Book Company [13] S P P Ottengraf and R Disks, "Biological Purification Of Waste Gases", Chim. Oggi, 1990, V8, N5, p41-45. [14] J A Don, "The Rapid Development Of biofiltration For The Purification Of Diversified Waste Gas Streams", VDI-Berichte, 1986, V561, p63-73.



APPENDIX 1. PHOTOCHEMICAL OZONE CREATION POTENTIALS (POCP)



APPENDIX 2. EXAMPLES OF ADSORPTION PRELIMINARY CALCULATIONS A2.1 INTERMITTENT VENT CONTAINING NITROBENZENE A2.1.1 The Problem Maximum venting rate = 40m3/hr nitrogen saturated with nitrobenzene Maximum temperature = 25°C Venting period = 8 hrs/day, 5 days/week and 48 weeks/year Estimate the size and cost of a carbon adsorption system to reduce the nitrobenzene concentration to ppm levels. A2.l.2 Adsorption Bed Size

A2.1.3 Costs The capital cost should be only a few hundred pounds at the most. If the carbon is purchased in ready-to-use cartridges, the cost of carbon is likely to be about $5/kg, i.e. 90 x 5 = $450 per cartridge. The cartridge case would cost about $80 and so the cost of each new full cartridge would be about $450 + $80 = $530. Therefore the annual cost for cartridges would be about 12 x $530 = $6,360/year.



The disposal cost to merchant incineration would be about $1,650/te = 1.65/kg, i.e. 90 x 12 x 1 = $1,080/year. Therefore, the total annual cost would be about $6,360 + $1,080 = 7,440/year plus, say, $660/year labor costs for cartridge changing and maintenance = $8,100/year. A2.2 CONTINUOUS VENT CONTAINING TOLUENE A2.2.1 The Problem Venting rate = ~ 200m3/hr nitrogen saturated with toluene Temperature = 5 o C Venting period = continuous Estimate the size and cost of a carbon adsorption system to reduce the toluene concentration to ppm levels. A2.2.2 Adsorption Bed Size

This is clearly a case for in-situ regeneration with a freshly regenerated bed being kept on-line for, say, 2.5 hours before being taken off-line for regeneration. It is important to have checked that the estimated adsorption time for a regenerated bed, 2.5 hours in this example, is longer than the typical 90 minutes regeneration time (see Section 5.4.1.4).



A2.1.3 Costs Three beds of carbon, each containing 225 kg, are required. Therefore, the cost of the initial charge of carbon, @ say $5/kg, = $5/kg x 3 x 225 = $3,375. The gross installed battery limits capital cost of the adsorption plant would be about $66000. The operating costs would cover : carbon replacement steam @ 3 kg/kg toluene 3 x 8 = 24 kg/hr cooling water @ 20 o C rise = 1m3/hr air to condition bed other services, e.g. instrument air manpower maintenance, say 4% of capital It should be noted that operating costs can sometimes be off-set by a credit for the recovered VOCs.



APPENDIX 3. EXAMPLE OF THERMAL INCINERATION HEAT AND MASS BALANCE A3.1 THE PROBLEM Incineration at 850 o C of a 1000 m3/hr vent stream comprising 0.3% v/v benzene in air at atmospheric pressure and 15°C with at least 25% excess combustion air over and above stoichiometric requirements. A3.2 COMBUSTION AIR REQUIREMENTS AND PRODUCTS OF COMBUSTION



A3.3 HEAT BALANCE A3.3.1 COMBUSTION OF BENZENE

A3.3.2 COMBUSTION OF METHANESUPPORT FUEL

In practice, it would likely be beneficial to recover heat from the hot flue gas in order to raise the temperature of the in-coming vent stream and thereby reduce or eliminate the support fuel requirements. A3.4 SUMMARY



A4 COST CORRELATIONS This appendix contains the cost correlations used to generate Figures 1, 2, 3 and 4. The correlations can also be used directly to provide crude cost estimates when comparing options for VOC abatement technologies. It is stressed that they should only be used as guides and must not be used for stand-alone estimates.

PRACTICAL GUIDE ON THE REDUCTION OF DISCHARGES TO ATMOSPHERE OF VOLATILE ORGANIC COMPOUNDS (VOCs)

Technology

Transcript of PRACTICAL GUIDE ON THE REDUCTION OF DISCHARGES TO ATMOSPHERE OF VOLATILE ORGANIC COMPOUNDS (VOCs)