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Europem n.v.Europem n.v.Proces, Energie & Milieu

TreatmentTreatment of of vocvoc fromfrom storagestorage tankstanksNorbert Vandendriessche

November 29th 2006

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Treatment of VOC loaded waste air/N2 streams

General

VOC loaded waste vapours originate from:chemical processesindustrial painting linescoating processes , like in textile plantsloading terminalsstorage tank facilities

ATEX Most of the VOC loaded vapour streams are based on air , or have a high O2 content .Explosive mixtures must be considered , and all required precautions have to be taken to reduce the risk of an explosion to zero.Please contact TUV –SUD for more information and guidance (Mr. Peter Szalata)

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Absorption / scrubbing

VOC loaded vapour is flowing - and being distributed - from bottom to top through a packed or tray column.

A liquid absorbent hydro-carbon flows from top to bottom.Based on solubility levels, the VOC ‘s are transferred to the liquid via direct contact ( scrubbing).

The scrubbing is effective for low concentrations (a few vol. %)

Scrubbers are mostly used in chemical plants , because of the presence of additionally required equipment, like distillation towers, etc.

Scrubbers are not very often used for vapour treatment from storage tanks.(exceptions are chlorinated hydrocarbons)

Wide range of flow ratesVOC load: limited concentrations

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Activated carbon adsorption

Mobile or fixed modules, filled with porous activated carbon.

The VOC loaded vapours are flowing through the carbon bed , and the hydrocarbons are adsorbed by the carbon on to the surface and onto the surface of the pores.At some point the carbon becomes saturated and must be regenerated or replaced by virgin carbon.

Regeneration can be done with steam , hot nitrogen or hot air .

Attrition: permanent bounding of small quantities of VOC . Requires the periodic replacement by virgin carbon , after a certain time of regeneration.

Advantages:Activated carbon absorbers are suitable for discontinuous processes with high concentrations.For continuous processes , VOC concentrations up to 1 gr / Nm3 are advised.Eliminates odour problems , like from S2 – containing hydrocarbons (mercaptanes, H2S, …).

Wide range of flow ratesVOC load: high concentrations , if discontinuous process. (up to 600 g / Nm3)

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Vapour recovery unitsActivated carbon adsorption with automatic regeneration mode.

Fixed installation.

Two beds of activated carbon absorbers are recycling periodically on an automatic cycle .Attrition requires periodical change of the carbon.

The recovery of hydrocarbons is possible , and is by preference limited to one single hydrocarbon component (for 95 % gasoline) .

VOC concentrations in the waste vapour stream : up to 40 vol %

Guaranteed emissions:normal: 35 g / Nm3 since 2006 10 g / nm3“TUV - VRU“ 150 mg / Nm3 (this VRU includes a scrubber).

Wide range of flow ratesVOC load: wide range of concentrations

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CondensationThe VOC loaded vapours are cooled below the dew point to condense the hydrocarbons.Cooling mediums are usually cooling water , chilled water and refrigerants.The cooling medium is recycled , re-cooled and reused for additional VOC condensation.

The efficiency improves as the VOC concentration at the inlet increases.Extensive cooling is required for low concentrations.

Cryogenic temperatures may be necessary.

Good for low volatility VOC’s (high boiling point).

Moderate flow rates VOC load: high concentrations preferred.

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Membrane separationVOC gaseous emissions are concentrated using organic selective VOC permeable membranes

Membranes are 10 to 100 times more permeable for VOC’s than air.The exit membrane stream VOC concentration can be increased 5 to 50 times as compared with the inlet concentration.

Air and VOC ‘s permeate through the membrane at rates determined by their relative permeability's, and the pressure difference across the membrane.

High pressure difference may require high energy consumption.

High efficiencies are achieved when combined with condensation.

Can handle high volatility VOC’s ( low boiling point) .

Can handle low VOC concentrations.

REMARK: membrane technology is new, with little industrial experience.

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Catalytic recuperative oxidiserThe VOC’s are oxidised with the assistance of a catalyst.Oxidation temperatures are in the range of 300 to 400 °C .The catalyst exit temperature can be as high as 450 - 500°C, depending on the VOC concentrations.

Because of the oxidation process , a minimum of O2 content is required in the waste vapour stream.

The waste vapour stream is preheated by the exit flow of the catalyst.A supporting fuel gas burner adds the extra required heat to obtain the correct temperature for catalyst assisted oxidation.

Attention must be paid to the explosion levels of the mixtures at the operating temperatures.

Wide range of flow ratesLimited VOC concentrations

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Incineration at high temperatureIncineration of VOC loaded vapour streams is used only when:

1. Heavy odour producing VOC’s are involved.2. There is a need for extra energy carriers (steam, thermal oil, hot water, …)

Oxidation at high temperature results in 100 % destruction of the hydrocarbons.

Temperatures from 850 °C up to 1100 °C are used.The higher temperatures ranges are required if mercaptanes , acrylates , benzene , …are involved.

Specially designed mixing techniques leads to lower fuel consumptions (like “high intensity“ combustion = HI).

In case no energy recovery is required, the waste vapour stream will be pre heated by the exit flue gas stream (with attention to the explosive levels).

Advantage: Can handle any concentration, any mixture of hydrocarbons, any component .

Wide range of flow ratesAll concentrations

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RTO (regenerative thermal oxidiser)RTO is a thermal incineration process at 800 – 850 °C

The thermal energy is recovered at high efficiency by heating up a heat exchanger which consist of a bed of ceramic material. (honey cumb, or saddles).

The waste vapour stream is introduced in to the hot bed ( from bottom to top ) , and pre heated up to +/- 800 °CThe VOC ‘s brings the temperature to 825 -850 °C in the combustion chamber on top of the “ heat exchangers “

The waste vapour stream should contain a minimum of O2 ( 10 -12 % ) .

While one bed is heated by the purified vapour stream ( from top to bottom), the VOC containing stream flows through the hot bed which is heated in a previous cycle.

The cycling is done by a number of automatically operated dampers .

The third cycle is purging the bed where the waste stream was heated to lower the stack emissions

AUTOTHERM : an RTO can operate auto thermally from 1 .2 – 1.5 g / Nm3 VOC load.

Wide range of flow rateLimited VOC concentration

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ROTOR concentratorA rotating wheel consisting of zeolithe, activated carbon or an other adsorbing medium adsorbs the VOC’s from a certain waste stream flow.

By rotating, a section of the wheel meets a lower flow rate of hot air which adsorbs the VOC ‘s carried by the wheel.

By doing so, a lower flow rate with a higher VOC concentration is obtained.

This stream can be treated by any type of treatment system.

The concentrator is mostly used for high flow rates with low VOCconcentrations, so that the lower flow rates which are to be purified are usually treated by an RTO.

High flow ratesVery low VOC concentrations

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Open flareFlare stacks are mostly used to burn hydrocarbon vapours released by safety valves.

The combustion with an open flare is not complete, and emission guarantees can not be obtained.

Mobile, transportable “ open flares” can be used to purify a storage tank , prior to star maintenance- and cleaning work.

Attention is to be paid to:the explosive characteristics of the vapour streams.heat radiation.

Wide range of flow rateVOC load: no limitation

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Enclosed flareEnclosed flares are also called “ground flares“

The combustion of the vapours takes place in a combustion chamber.By doing so, the flame becomes invisible and there will be no heat radiation.

The combustion chamber will be internally lined with ceramic fibre type of heat resistant insulation.Ceramic fibre allows for sudden introduction ( fast heating up ) of the waste vapours, so that pre heating is not required.

When the ground flare is equipped with temperature control , 100 % destruction of the VOC’s can be achieved

An enclosed flare can be mobile for limited flow rates

Limited range of flow ratesVOC load: no limitation

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Metal fibre burner (MFB)In the listing of flare systems of the EU commission , the MFB is selected as best available technology

The MFB is burning the hydrocarbons on the surface of a metal fibre carpet, and achieves 100 % destruction and very low NOx levels.

The MFB burns with invisible flame.

The advantages are :no heat radiationinvisible flameguaranteed emission levels

The MFB is designed on a modular base , and can be easily used as mobile flare.

High flow rates ( and / or high VOC concentrations ) require high number of modules ( or big size MFB ) , leading to big systems.

Wide range of flow ratesVOC load : no limitations

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