Process for vapor emission control

A vapor emission control system for recovering hydrocarbon vapors displaced as vessels are loading consisting of two stages of carbon adsorption vapor recovery units. The first stage with two or more parallel carbon beds recovers the heavier C4-C6+ hydrocarbons on a first carbon bed which are then removed as a gas via vacuum and then converted into a liquid product via a vapor-to-liquid conversion unit. Lighter C2-C3 hydrocarbon vapor discharged from the first stage is recovered on two or more parallel carbon beds of the second stage. The vapor is then removed via vacuum as a concentrated gas for use as fuel or sent to a flare. The hydrocarbon lean first portion of the off-gas from each vacuum desorption is recycled to the other in-parallel carbon bed. The load and regeneration cycles alternate for the two carbon beds in each of the two stages based on an optimized time cycle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process and system for treating crude oil vapors such as those produced from crude oil loading or any hydrocarbon vapor comprised of light hydrocarbons in the range of C2-C3, i.e. ethane and propane and derivative hydrocarbons, and heavy hydrocarbons in the range of C4-C6 and greater, i.e. butane, butane derivative hydrocarbons, and heavier hydrocarbons.

2. Description of the Related Art

When loading crude oil or other hydrocarbons containing light hydrocarbons in the range of C2-C3 and heavy hydrocarbons in the range of C4-C6 and greater onto a tank truck or tanker vessel, some of the light and heavy hydrocarbons vaporize. These vapors may be initially treated to remove such things as sulfur and those vapors that will condense to a liquid when cooled are recovered as a liquid. From that point, the remaining vapor which contains a large amount of light and heavy hydrocarbons must be treated before venting.

For safety and environmental reasons, venting of hydrocarbons is often accomplished through a flare or other combustion device which burns the hydrocarbons. However, burning of hydrocarbons produces carbon dioxide, i.e. a greenhouse gas, and other pollutants. Thus the practice of burning the hydrocarbons through a flare or other combustion device is environmentally undesirable. Also, burning of the hydrocarbons wastes valuable resources which might otherwise be recovered for use as fuel or as a salable product.

Currently hydrocarbon vapor recovery systems consist of passing the vapors through an activated carbon bed. Activated carbon attracts hydrocarbon material on its surface, with a higher preference for adsorbing the heavy hydrocarbons in the range of C4-C6 and greater. Thus, the activated carbon bed will selectively adsorb most of the heavy hydrocarbons in the range of C4-C6 and greater. The light hydrocarbons in the range of C2-C3 will generally pass through the activated carbon bed which has already adsorbed on it heavy hydrocarbons, and those light hydrocarbons will be vented to atmosphere or be vented to a flare to be burned.

Once the carbon bed is loaded with hydrocarbons during the adsorption phase, it is then taken off line and regenerated by subjecting it to a vacuum. The hydrocarbons that were adsorbed onto the carbon bed will be drawn off by the vacuum and the discharge from the vacuum pump will next be transferred to the inlet of a liquid contact absorption unit. Within the liquid contact absorption unit, the vapor will pass through a liquid hydrocarbon shower, such as for example gasoline, where the gaseous hydrocarbons will be absorbed in the liquid hydrocarbons, thereby increasing the amount of liquid hydrocarbons exiting the unit. This recovers much of the heavy hydrocarbons in the range of C4-C6 and greater.

However, even though this type of treatment does recover much of the heavy hydrocarbons in the range of C4-C6 and greater, most of the light hydrocarbons in the range of C2-C3 pass through the carbon bed without being adsorbed and are either vented to atmosphere or are vented to a flare where they are burned. Both options provide no benefit or profit to the operation and both venting and flaring create environmentally undesirable situations.

The present invention addresses this problem by providing a two stage carbon bed adsorption system consisting of a first carbon bed adsorption unit and a second carbon bed adsorption unit in series with the first one. The first stage carbon bed adsorption unit removes the heavy hydrocarbons in the range of C4-C6 and greater, and the vapor discharged from the first stage carbon bed adsorption unit is then passed through a second stage carbon bed adsorption unit where the light hydrocarbons in the range of C2-C3 are removed from the remaining gaseous components of the vapor before the vapor is then discharged to atmosphere. The second stage vapor contains minimal heavy hydrocarbons which improves the working capacity for the light hydrocarbons.

During regeneration of the first stage carbon bed, the heavy hydrocarbons in the range of C4-C6 and greater are drawn off in the discharge from a vacuum pump and are subjected to further treatment to liquefy them so that they can be recovered as a liquid product for storage.

During regeneration of the second stage carbon bed, the light hydrocarbons in the range of C2-C3 are drawn off in the discharge from a second vacuum pump as a gaseous vapor containing a rich concentration of hydrocarbons which can be used as fuel gas for facility operations.

SUMMARY OF THE INVENTION

This vapor emission control system is designed to recover vapors produced from crude loading or loading of any hydrocarbon vapor comprised of light hydrocarbons in the range of C2-C3 and heavy hydrocarbons in the range of C4-C6 and greater. Vapors are displaced from loading liquids into vessels and brought through a vapor line and recovered in two stages of carbon adsorption vapor recovery units. The first stage recovers the heavier hydrocarbons on a first stage carbon bed which are then removed via vacuum from the first stage carbon bed as a gas and then the gas is converted into a liquid using compression and cooling or alternately using absorption or refrigeration. The second stage recovers the light ends on a second stage carbon bed which are then removed via vacuum from the second stage carbon bed as a concentrated gas for use as fuel in a boiler or other device.

The vacuum regenerated hydrocarbons from the first stage are sent to a vapor-to liquid conversion unit, which could be a compressor and then a cooler, to convert them into a liquid form for storage. Alternately, the vapor-to liquid conversion unit may be a refrigeration system or a liquid absorption unit. The liquid is then available for fuel or blending into other hydrocarbon streams which are compatible. The recovered hydrocarbons from the second stage remain as vapors and are sent directly as a gaseous fuel as they as produced. The load and regeneration cycles alternate for the two carbon beds in each of the stages and these beds switch based on an optimized time cycle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and initially toFIG. 2, there is illustrated a typical tanker vessel loading operation10that is employing a vapor emissions control system20constructed in accordance with a preferred embodiment of the present invention. As shown, the system20receives vapors produced when crude oil or other similar hydrocarbon is being loaded onto a vessel12.

The hydrocarbon vapor contains light hydrocarbons in the range of C2-C3 and heavy hydrocarbons in the range of C4-C6 and greater. The vapor is typically pushed by blowers14through initial sulfur pre-treatment units16to remove sulfur and through cooling pretreatment units18for cooling the vapor to a desired temperature for treatment in the first and second stage carbon beds22and24that are a part of the vapor emissions control system20as will be more fully described hereafter.

Although the drawings illustrate a typical installation, it should be noted that the invention is not so limited and may be employed to treat any vapor where it is desirable to extract and recover from the vapor the lighter hydrocarbons in the range of C2-C3 and the higher hydrocarbons in the range of C4-C6 and greater.

Referring now toFIG. 1, the vapor emissions control system20is shown in more detail. Vapors displaced from loading hydrocarbon liquids into vessels12are brought through a vapor inlet line26and recovered in two stages of carbon adsorption vapor recovery units which are depicted inFIG. 1as being those items located within Box A and within Box B, respectively. The first stage, as illustrated in Box A, recovers the heavier hydrocarbons on one of two first stage carbon beds22that are arranged in parallel. Although only two first stage carbon beds22are illustrated and described, it is understood that more than two first stage carbon beds22may be employed. The heavier hydrocarbons are then removed from the first stage carbon bed22via a first stage vacuum pump or vacuum system28as a gas. The gas is then converted into a liquid in a liquid conversion unit30.

The liquid conversion unit30may be any type of equipment that will convert the gaseous hydrocarbons to a liquid form, such as using compression and cooling, refrigeration, using absorption, or any other effective means of converting the gaseous hydrocarbons to a liquid form.FIG. 1shows two first stage carbon beds22that are installed in parallel so that one first stage bed22is being regenerated while the other first stage bed22is in adsorptive service.

The second stage, as illustrated in Box B, receives vapor discharged from the first stage carbon beds22via a second stage vapor inlet line31. The second stage vapor inlet line31supplies vapor to both of the second stage carbon beds24. Again, although only two second carbon beds24are illustrated and described, it is to be understood that more than two second stage carbon beds24may be employed. The light ends contained in the vapor are recovered on one of two second stage carbon beds24. The light ends are then removed from the second stage carbon bed24via a second vacuum pump or vacuum system32as a concentrated gas for use as fuel in a boiler or flare.FIG. 1shows two second stage carbon beds24that are installed in parallel as one second stage carbon bed24is being regenerate while the other second stage carbon bed24is in service.

Although only one first stage vacuum pump28is described and illustrated and only one second vacuum pump32is described and illustrated, the invention is not so limited and more than one first stage vacuum pump28and more than one second stage vacuum pump32may be employed.

The vacuum regenerated hydrocarbons from the first stage carbon beds22are sent to a vapor-to-liquid conversion unit30, which is normally a compressor and a cooler, to convert them into a liquid form for storage. Alternately, the vapor-to-liquid conversion unit may be a refrigeration unit, a liquid absorption unit, or any type of unit that is capable of converting to a liquid the hydrocarbon vapors generated by the first stage carbon beds22and first stage vacuum pump system28. The liquid is then available for fuel or blending into other hydrocarbon streams which are compatible.

The recovered hydrocarbons from the second stage remain as vapors and are sent directly to an industrial application within the facility as a gaseous fuel as they are produced. The load and regeneration cycles alternate for the two carbon beds22in the first stage and also for the two carbon beds24in the second stage, with each set of beds22and24being switch based on an optimized time cycle.

In operation, vapors flow into the first stage vapor recovery unit (which is shown in Box A) and are sent to one of the in-parallel first stage carbon beds22. One of the first stage beds22can be adsorbing vapors while the other first stage bed22undergoes vacuum regeneration. Inlet valve34and outlet valve36are open to the bed22that is in the adsorption phase and a vacuum regeneration valve38is closed for that bed22. The other bed22is vacuum regenerated by closing the inlet and outlet valves34and36for that bed22and opening the vacuum regeneration valve38to the first stage vacuum pump28.

The heavier hydrocarbons are captured in the first stage beds22and the lighter hydrocarbons pass through with the air and or other inert gasses onto the second stage beds24. Although not illustrated, when a regenerated bed22or24is under deep vacuum, a purge valve opens to improve the regeneration. When regeneration is complete, the carbon beds22or24are slowly re-pressurized back to atmospheric pressure using outlet vapor from the bed22or24that is undergoing the adsorption phase. Re-pressurization can be accomplished with outlet valves36or other separate valves, not illustrated.

The first stage vacuum pump system28pulls the regenerated hydrocarbon vapors from the first carbon bed22undergoing regeneration and the second stage vacuum pump system32pulls the regenerated hydrocarbon vapors from the second stage carbon bed24undergoing regeneration. Hydrocarbon vapors which exit the first stage vacuum pump are sent to a vapor-to-liquid conversion unit30which could be a compressor and a cooler where the rich vapors are compressed to high pressure and cooled to condense them to a liquid. The compressor discharge is cooled in a cooler and any liquid condensed is removed and collected in a separator, as shown on the chart as “Liquid Products”40. Liquid condensate light ends can be used for many purposes or may be sold.

The majority of the heavier hydrocarbons is removed in the first stage vapor recovery unit and can be sent to a storage tank as a liquid. There still are appreciable light ends, i.e. ethane and propane, which exit the first stage but still need to be removed. The second stage is designed to capture the remaining light ends, concentrate them up to a richer stream while allowing the non-hydrocarbon air and or inert gases to exit the second stage beds24.

The second stage carbon beds24operate similar to the first stage carbon beds22with adsorption of light hydrocarbons while the non-hydrocarbons exit the beds and are vented out to atmosphere.

One of the second stage beds24can be adsorbing vapors while the other second stage bed24undergoes vacuum regeneration. Inlet valves34′ and outlet valve36′ are open to the bed24that is in the adsorption phase and a vacuum regeneration valve38′ is closed for that bed24. The other bed24is vacuum regenerated by closing the inlet and outlet valves34′ and36′ for that bed24and opening the vacuum regeneration valve38′ to the second stage vacuum pump system32.

Regeneration of the second stage carbon beds24is done with a second stage vacuum pump system32for removal of the hydrocarbons from the second stage carbon beds24in a similar manner to the functioning of the first stage vacuum pump28. This produces a rich gaseous hydrocarbon stream or fuel gas45. In order to produce a richer hydrocarbon stream, a first or initial portion of the gas produced by the second stage vacuum pump32when regenerating a second stage carbon bed24is recycled to the other second carbon bed24which is on line and is taking vapors. A second or final portion of vacuum regenerated gas produced by the second stage vacuum pump system32is produced upon reaching deep vacuum and is rich, containing little oxygen or nitrogen. This final portion of hydrocarbon rich gaseous stream45can be used to fire boilers or any other fuel-consuming device at the facility. There may be times when the fuel gas45generated is excessive, and must be disposed of in a standby flare. This flare could also be used when fuel quality of the fuel gas45is unacceptable or when other problems occur.

For safety reasons and for cooling, liquid ring vacuum pumps are preferably used for the first and second stage vacuum pump systems28and32, but the invention is not so limited. Although not illustrated, the liquid coolant from the vacuum pump systems28and32is recovered in a separator and the vapors are treated downstream. Liquid ring vacuum pumps are employed in the pump systems28and32and those pumps preferably use ethylene glycol as a coolant and an evaporative cooler is used to remove the heat generated by the recycled glycol. The recycled glycol is cooled by an evaporative cooler prior to re-use.

One key to the process is the proper handling of the initial portion of vacuum pull down material produced by the first and second stage vacuum pump systems28and32from both the first and the second stages which may not be rich enough to operate processes. The first portion of the vapor removed from either stage will contain more air and/or other inert gases than can be processed effectively by the recovery equipment downstream. The first portion of vapor is directly recycled back to the other bed in that stage during its adsorption step. In the first stage, the first portion of vapor is recycled to the other bed22via recirculation line42, and in the second stage, the first portion of vapor is recycled to the other bed24via recirculation line44.

When the vacuum level is deep enough to produce a rich hydrocarbon vapor, those richer vapors are sent into the recovery conversion portion of the system20. Richer vapor for the first stage are sent to the vapor-to-liquid conversion unit30such as a compression/cooling loop for liquefaction of the hydrocarbons or alternately to a unit for absorption by a suitable liquid. The second stage richer vapors in a fuel gas45that are rich enough to burn in a fuel consuming device such as a boiler.

By bypassing the first portion of the vacuum regenerated vapor in both the first and second stages, the downstream processing portion is smaller with lower capital and operating costs. By processing only the rich vapor portion of the regenerated vapor, more efficient operations are obtained.

Although the invention has been described and illustrated as being used in association with tanker ship loading, the invention is not so limited. The invention may be used in any application where a vapor contains light and heavy hydrocarbons that are to be removed and recovered from the vapor.