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Vapor recovery

AQT is very proud to introduce you to the world of VRUs…

In this section we’ll provide basic explanation of VRUs process based on activated carbonand of AQT’s state-of-the-art implementations. This documentation is not intended to cover any detailed aspect of the process, his purpose is to clarify as simply as possible the basic concept itself.

Basic Process Description:

AQT Vapor Recovery Unit is based on the technology of aDsorption-aBsorption;

The Vapor Recovery Unit is intended to process an incoming pipelined flux consisting of hydrocarbon vapor mixed with air.

The AQT process when compared to other vapor control technologies, is not only very efficient but also reliable and relatively simple.

We use to define it a two-step process:

The first processing step (aDsorption) consists of passing the incoming flux through a bed full of activated carbon (vessel) which captures the hydrocarbon vapor by adsorption onto its surface while allowing the hydrocarbon free air to pass through and vent to the atmosphere. The step endures until the carbon bed got completely saturated. The adsorbed hydrocarbon vapor is then desorbed (removed) from the activated carbon using a vacuum system.

This desorbed vapor, coming from the saturated bed and passed through the vacuum system, is then subjected to a second step (aBsorption) in which it is recovered as a liquid product by absorption into a stream of circulating liquid absorbent and re-processed in the adsorbing beds for the small amount of remaining mixed vapors.

ADSORPTION PROCESS: activated carbon has the ability to selectively attract and capture (adsorb) onto its surface the hydrocarbon vapor fraction from the hydrocarbon vapor-air mix. Unfortunately, because the carbon has a definite and limited ability to adsorb hydrocarbon molecules, it must be periodically regenerated, otherwise it would become saturated and would not adsorb further vapors.

Several factors concur to favour adsorption, include higher hydrocarbon concentrations in the inlet vapor stream and higher operational pressures. The AQT’s VRUs technology implements and enhances these factors reversing the events that are favourable to adsorption process. While the carbon bed regeneration cycle is running, desorption (removal) of hydrocarbon vapor from the twin carbon bed is obtained by generating a high vacuum (low absolute pressure) in the adsorber equipment.
During the last part of this phase a small amount of purge air (stripping) is allowed into the adsorber to modulate the highest vacuum level, this creates a condition favourable for desorption and provides the most functional way to regenerate the activated carbon for optimal reuse in the further adsorption cycles.

The AQT VRU is equipped with two identical beds filled with activated carbon. One vessel is on-line receiving vapors in adsorption mode, while the other is off-line in regeneration mode. Actuated switching valves are provided to automatically alternate the two beds between adsorption and regeneration, this way an adsorbing cycle is always on-stream, this assures uninterrupted vapor processing capability.

While the inlet valve is open, hydrocarbon vapor-air mixture to be processed flows up through the on-line vessel. Into the bed the activated carbon adsorbs the hydrocarbon vapor and allows clean air to vent with only minimal hydrocarbon content.
At the same time, the other carbon bed regeneration is accomplishing as described previously.

REGENERATION PROCESS

LAST UPDATED TECHNOLOGY: dry screw vacuum pumps are used as the source of vacuum for carbon regeneration. The vacuum pumps extract concentrated hydrocarbon vapors from the carbon bed being processed. Some large systems may have two or more vacuum equipment’s operating in parallel.

During the carbon bed regeneration cycle, the vacuum system reduces the adsorber pressure from atmospheric pressure to 50 mmbarA. Once reached the highest vacuum level a small amount of purge air (stripping) is introduced into the adsorber to assist in the regeneration process. Regeneration process of the carbon bed results in previously adsorbed hydrocarbon vapors being removed extracted from the carbon itself and the reestablishment of its adsorption capacity.
This extracted rich hydrocarbon vapor stream contains only a minimal amount of air, and the flow coming from the adsorber to the vacuum system is discharged into the absorber tower.

The hydrocarbon vapor, with a small residual air component, will flow into the base of a vertical aBsorber column that functions as the recovery device.
In this absorber column the hydrocarbon vapor flows up through packing where it is liquefied and subsequently recovered by absorption into a hydrocarbon liquid absorbent.
Fresh product from the customer’s loading line (gasoline) is utilized as the absorbent. A centrifugal pump circulates the product from the loading line to the absorber column. In the absorber column, the absorbent absorbs the rich hydrocarbon vapors, dropping the fresh product right over the up going stream.
The recovered liquid and absorbent, together plummeted, are then pumped back to the loading line. A small stream of air and residual vapors exit the top of the absorber column due to the positive pressure generated from the out bounding flux of the vacuum pumps and are recycled to the on-line carbon bed where the residual hydrocarbon vapor is re-processed.

When loading has been completed, it is necessary to leave the unit running for at least two (but sometime more) full regenerating cycles to ensure that both carbon beds are completely regenerated and ready for the first next day loading operation.
During this “postload” period, absorbent could be circulated from a dead loading line. In this case, since fresh product is not continuously supplied to the system, the absorbent will increase in temperature due to the heat of compression and heat of absorption in the system, thus an evaporative cooler is supplied to cool the absorbent where is needed.

The vacuum pumps, vacuum booster blowers (if included) packaged on a single module (regeneration skid).

The six adsorber switching valves, their associated piping and other adsorber control items are packaged as a separate module (adsorber valve skid).

The two adsorber vessels (carbon beds), the absorber column, and the product return pump are shipped separately for installation directly on the VRU foundation adjacent to the regeneration and adsorber valve skids.

All interconnecting piping between the regeneration skid, the adsorber valve skid, the carbon beds, the absorber column, and the product return pump are included.
The product supply pump could be shipped separately for installation close to the absorbent supply tank, this is necessary when the post loading process is served with a dead-line piping condition (the longer is the piping the lower is the rising exercise product temperature), or could be joined to adsorber column skid if the stream of fresh product is available while the post loading phase is on.

Unless differently specified, the control panel is also shipped separately for installation away from the VRU in a safe area. All motors and other electrical are designed for compliance with the hazardous area specifications.
Where needed, the control panel also could reside close to the VRU, in this case it’s supplied within special anti-deflagrate

OLD EXISTING UNITS: liquid ring vacuum pumps are used as the source of vacuum for carbon regeneration. The vacuum pumps extract concentrated hydrocarbon vapors from the carbon bed being processed. Some large systems may have two or more vacuum equipments operating in parallel.
The vacuum pumps require a sealing fluid to operate. This seal fluid is a specially blended ethylene glycol-based fluid.

During the carbon bed regeneration cycle, the vacuum system reduces the adsorber pressure from atmospheric pressure to 3” HgA. Once reached the highest vacuum level a small amount of purge air (stripping) is introduced into the adsorber to assist in the regeneration process. Regeneration process of the carbon bed results in previously adsorbed hydrocarbon vapors being removed extracted from the carbon itself and the reestablishment of its adsorption capacity.
This extracted rich hydrocarbon vapor stream contains only a minimal amount of air, and the flow coming from the adsorber to the vacuum system mixes with seal fluid in the vacuum pump and then is discharged into a multi-phase separator.

This separator is provided to allow efficient separation between the vacuum pump’s ethylene glycol based seal fluid, any hydrocarbon liquid that may have condensed in the vacuum pump, and the non-condensed hydrocarbon vapors.
The seal fluid is heavier than the hydrocarbon condensed liquid (and is insoluble with), therefore, it settles to the bottom of the separator where it is removed and sent back to the vacuum pump. A seal fluid cooler is provided to low the heat of compression from the seal fluid before this is returned to the vacuum pumps.
Non-condensed hydrocarbon vapor, with a small residual air component, disengages from the liquid in the separator and will flow from the separator ceiling into the base of a vertical aBsorber column.
Also, if there is any condensed hydrocarbon liquid, it will float on top of the seal fluid in the separator as a second liquid phase. It will eventually rise high enough for being flown, via apposite piping, to the absorber column.

Non-condensed hydrocarbon vapors, along with hydrocarbon condensate flow from the separator to a vertical packed absorber column that functions as the recovery device.
In this absorber column the hydrocarbon vapor flows up through packing where it is liquefied and subsequently recovered by absorption into a hydrocarbon liquid absorbent.
Fresh product from the customer’s loading line (gasoline) is utilized as the absorbent. A centrifugal pump circulates the product from the loading line to the absorber column. In the absorber column, the absorbent absorbs the rich hydrocarbon vapors from the separator, dropping the fresh product right over the up going stream.
The recovered liquid and absorbent, together plummeted, are then pumped back to the loading line. A small stream of air and residual vapors exit the top of the absorber column due to the positive pressure generated from the out bounding flux of the vacuum pumps, and are recycled to the on-line carbon bed where the residual hydrocarbon vapor is re-processed.

When loading has been completed, it is necessary to leave the unit running for at least two (but sometime more) full regenerating cycles to ensure that both carbon beds are completely regenerated and ready for the first next day loading operation.
During this “postload” period, absorbent could be circulated from a dead loading line. In this case, since fresh product is not continuously supplied to the system, the absorbent will increase in temperature due to the heat of compression and heat of absorption in the system, thus an evaporative cooler is supplied to cool the absorbent where is needed.

AQT’s VRUs are provided in modularized packages (skids) allowing for convenient and flexible field installation, future upgradability, and to provide adequate spacing for ease of equipment maintenance.
The vacuum pumps, vacuum booster blowers (if included), separator, seal fluid pump, and seal fluid cooler are packaged on a single module (regeneration skid).

The six adsorber switching valves, their associated piping and other adsorber control items are packaged as a separate module (adsorber valve skid).
The two adsorber vessels (carbon beds), the absorber column, and the product return pump are shipped separately for installation directly on the VRU foundation adjacent to the regeneration and adsorber valve skids.

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