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

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Process Engineering Guide:
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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
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
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 down-
stream 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.

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

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)