Patent Grant
7721557
The present invention relates generally to methods and systems for removing gaseous materials from storage vessels. More particularly, the invention relates to methods and systems for removing propane-containing vapors, such as Liquefied Petroleum Gas, from storage tanks and condensing the vapors to form a propane-containing liquid product. The process preferably proceeds entirely in the absence of combustion of LP-gas and thus not only conserves valuable natural resources, but contributes to the environmental quality as well.
Liquefied Petroleum Gas (LP-gas or LPG) is a relatively clean-burning hydrocarbon fuel of mixed composition. It is used primarily as a heating fuel in the United States, but also has found widespread utility as a cooking and vehicle fuel and as a refrigerant.
In North America, propane is the major constituent of LP-gas, with minor amounts of propylene, butane, and/or butylene depending on the particular grade. Because these components are gases at ambient temperature and pressure, LP-gas is stored under pressure in steel tanks of varying size depending on their place in the distribution chain.
Retail LP-gas is distributed from bulk plants where it is stored in large spherical or cylindrical tanks, often having capacities from 10,000 up to 30,000 gallons. The bulk plants are supplied by large multi-axel transport trucks carrying 18,000 gallon tanks which have been filled with LP-gas at the refinery or pipeline. From the bulk plant, LP-gas is shipped to retail customers in single-axel delivery truck called bobtails which have varying capacities, but are usually from 1,800 gallons up to 3,500 gallons. The bobtails are used to fill larger stationary tanks called “pigs,” installed on the customer's property.
The Liquefied Petroleum Gas Code, also known as “Pamphlet 58,” issued by the National Fire Protection Association (NFPA), requires that LP-gas tanks be retrofit to add or modify internal valves by Jul. 1, 2011. To accomplish the retrofit, the propane containers will have to be emptied of both liquid and gaseous propane. Liquid propane, which fills about 80-85% of the volume in a full container, must first be drained. Removal of the remaining propane vapor is conventionally accomplished by a controlled burn-off, which can take several days for a large stationary 30,000 gallon tank.
According to the National Propane Gas Association, there are an estimated 7,000 LP-gas transport trucks, 35,500 bobtails, and 17,000 railcars servicing over 14 million residential LP-gas customers and over one million commercial LPG customers. To bring all of these tanks into compliance with NFPA Pamphlet 58, a massive volume of propane must be burned off if the conventional practice is maintained. Controlled burns are not only time-consuming and potentially dangerous, but wasteful of an increasingly expensive commodity.
It is an object of the present invention to provide a method and system for extracting propane-containing vapors from pressurized storage tanks, without the need for a controlled burn-off, whereby the vapors are condensed to provide a propane-containing liquid.
In accordance with the foregoing objective and others, the present invention provides a system and method for extracting Liquefied Petroleum Gas (LP-gas) from a tank without combustion. The extracted vapors are condensed to form a useable (and saleable) liquid LP-gas product. The process preferably proceeds entirely in the absence of combustion of propane and thus not only conserves valuable natural resources, but contributes to the environmental quality as well.
In one aspect, a mobile system for propane recovery is provided comprising, in fluid communication, a liquid trap; a vapor line for permitting passage of vapors from a storage tank to the liquid trap; a compressor which receives vapors from said liquid trap, one or more liquid condensing tanks downstream from said compressor, a vapor line for passing effluent from said compressor to said one or more liquid condensing tanks, and a refrigeration unit for cooling said vapor line at a point downstream from said compressor and upstream from said one or more liquid condensing tanks, the system being mounted on a mobile platform. In one embodiment, the refrigeration unit employs as a refrigerant liquefied propane drawn from said one or more liquid condensing tanks and operates by forcing the liquefied propane refrigerant through a thermal expansion valve to convert it into a vapor state. The vaporized propane refrigerant may be recycled by combining it with propane vapors drawn from said storage tank at a point in the vapor line upstream from said liquid trap. In another embodiment, the refrigeration unit may comprise a gas absorption refrigerator. The system may comprising a truck, suitable for driving to the site of a storage tank, and a trailer connected to the truck, the bed of the trailer securely supporting the liquid trap, compressor, one or more liquid condensing tanks, and, if present, the refrigeration unit disposed in thermal contact with the vapor line between the compressor and the liquid condensing tank. An engine for driving the compressor will also typically be secured to the bed of the trailer and may ideally be a propane engine which operates on propane fuel drawn from the received stream of propane.
In another aspect, a method for removing propane from a storage tank is provided comprising: (i) providing a mobile system for propane recovery and positioning said mobile system in proximity to the tank; (ii) establishing fluid communication through a vapor line between the mobile system and the storage tank to be evacuated, typically by connecting the vapor line to a suitable valve on the tank and to a suction manifold feeding the liquid trap; (iii) draining any liquefied propane from said storage tank into a receiving tank, for example by drawing the liquid through the vapor line and into the liquid trap and then pumping liquid from the bottom of the trap into the receiving tank, to achieve an internal volume of liquid in the storage tank of less than about 3% (v/v); (iv) using suction created by a compressor, drawing a stream of propane vapors from the storage tank through the liquid trap, to remove any liquefied propane entrained in the vapor stream, and passing the stream into and through the compressor, which pressurizes and heats said stream of propane vapors; (v) cooling the stream of pressurized propane vapors exiting the compressor by passing the vapors through a refrigeration unit at a point in the vapor line downstream from the compressor and upstream from the one or more liquid condensing tanks; (vi) bubbling the cooled and pressurized stream of propane vapors through liquefied propane in the one or more liquid condensing tanks to condense the propane vapors into a liquid state, about 60% to about 85% (v/v) of the internal volume of said liquid condensing tanks comprising liquefied propane; and (vii) pumping the recovered liquefied propane from the one or more liquid condensing tanks, optionally through a meter, into a receiving tank.
The refrigeration unit may employ liquefied propane drawn from the one or more liquid condensing tanks as a refrigerant. In this embodiment, a stream of liquefied propane, for example, from one or more of the condensing tanks, is forced through a thermal expansion valve (TXV) to reduce the pressure of the liquid and then through an evaporator to convert the liquefied propane refrigerant into cooled vapor. The cooled refrigerant vapor may be passed through coils or the like in a first stage cooler and the vaporized refrigerant exiting the first stage cooler may be recycled by combining it with propane vapors drawn from the storage tank at a point in the vapor line upstream from the liquid trap. The vapor line containing the propane effluent from the compressor may be passed, for example, through coils in an air-to-air cooler having one or more hydraulic fans to provide airflow, the air-to-air cooler being in heat exchange contact with the first stage cooler.
In use, the system is capable of reducing the pressure in an propane storage tank having a capacity of about 30,000-33,000 gallons and an initial pressure of about 110 psi at about 80° F. to a substantially evacuated state in about 5 hours or less under continuous operation. Preferably, substantially all of the propane from the storage tank is recovered, except for any amount combusted in connection with operation of the compressor.
In another implementation, cooling coils may be disposed in the liquid condensing tanks in heat exchange relationship with the liquefied propane in order to keep the temperature of the liquefied propane at a suitably low temperature. The cooling coils may be part of a refrigerant system and may have a refrigerant gas flowing through them. The refrigerant gas may be cooled propane vapors and may come from propane that has been drawn from the liquid condensing tank and passed through a TXV valve and an evaporator. The effluent from the cooling coils may optionally be returned to the liquid trap.
It is yet another object of the invention to provide a method and system for extracting propane from a first propane storage tank comprising the steps of: (i) extracting liquid propane from said propane storage tank, if present, to achieve a final internal volume of liquid propane less than about 3% (v/v); (ii) extracting propane vapors from said propane storage tank by drawing a stream of propane vapor through a liquid trap, for removing liquid propane from said stream of propane vapor, and into a compressor for pressurizing said stream of propane vapor, (iii) bubbling said pressurized stream of propane vapor through a liquid condensing tank for converting said pressurized stream of propane vapor into a liquid state; wherein about 60% to about 85% (v/v) of the internal volume of the liquid condensing tank comprises liquid propane and wherein cooling coils containing circulating refrigerant are disposed within the liquid condensing tank in thermal contact with said liquid propane; to thereby condense said pressurized stream of propane vapor into liquid propane; and (iv) discharging said liquid propane from said liquid condensing tank, optionally through a meter, into a second storage tank.
The methods of the invention may further comprise the steps of (i) purging the emptied first storage tank with air, (ii) modifying the first storage tank, in compliance with NFPA Pamphlet 58, to include internal valves; and (iii) optionally transferring the liquid propane from said second storage tank back into said modified first storage tank.
These and other aspects of the invention will become more apparent from a reading of the following detailed description and accompanying drawings.
As used herein, the term “propane” is intended to have its ordinary meaning in the art and includes without limitation the grades of propane known in the art as commercial grade, HD-5, HD-10, R-290 and Special Duty propane. Unless the context implies otherwise, the term “propane” is meant to include gas mixtures composed primarily of propane, such as LP-gas.
Referring now to
Liquid trap 30 is disposed in the vapor line between the compressor 20 and the customer's tank 70 to remove any liquid propane from the incoming vapor stream in hose 3 and prevent it from entering the compressor. The liquid trap 30 is generally a vertical cylinder having one or more angular internal baffles 33 and/or one or more perforated baffles 34, as illustrated in
There is essentially no constraint on the selection of compressor, aside from its compatibility with the particular substance to be pumped and condensed and its suitability for the capacity of the tank and the desired rate of vapor extraction. The compressor may be without limitation a horizontal or vertical compressor, of single-stage, two-stage, three-stage, or four-stage design, and may be oil-free, non-lubricated, or lubricated. Vertical, oil-free, single-stage compressors covering a range of capacities from 24 to 361 gpm (5.5 to 82 m3/hr) in liquid transfer are available from Corken® (Oklahoma City, Okla.) as Models 91, 291, 491 & 691) and a double-acting single-stage vertical gas compressor capable of capacities from 337 to 757 gpm (76.5 to 171.9 m3/hr) is available from Corken® as Model D891. The compressor may be, for example, a reciprocating compressor, a centrifugal compressor, or the like.
The compressor 20 takes vapor having a first pressure and first temperature at the compressor inlet and produces a vapor stream having a second pressure and second temperature at the outlet of the compressor. The second pressure is typically greater than the first pressure by a factor of 1.5, 2, 3, 4, 5, 6, or even 7 and the second temperature is typically greater than the first temperature.
The pressurized, warm vapors exiting the compressor 20 are transmitted to the liquid condensing tank 40, for example through hose or pipe 7. Liquid condensing tank 40 may be a single tank or may be two or more tanks. Liquid condensing tank 40 may be any size, but a tank of about 150 gallons has been found to fit well on the trailer 14, particularly where two such tanks are used in parallel. In other embodiments, the liquid condensing tank 40 may be up to 2,500 gallons or larger. In a currently preferred implementation, the warm vapors exiting the compressor 20 are fed into two liquid condensing tanks, each having a capacity of 2,500 gallons and each being mounted on bed 14. The liquid condensing tank or tanks are typically kept about 60-80% full with liquid propane. The volume of liquid propane in the tank 40 may be monitored by level control means 43 which may be any suitable device for determining the liquid level, including internal floats and controls or the like.
In one embodiment, the liquid propane in the liquid condensing tank 40 is cooled by a refrigerant circulating through internal cooling coils 210 disposed in the internal volume of the tank, as illustrated in
In one embodiment, a 150 gallon liquid condensing tank is cooled by about 100 feet of internal cooling coils 210. The propane itself may be used as the refrigerant in the cooling coils by continuously drawing an amount of liquid propane from the condensing tank and affecting a phase change (boiling) to the vapor state. Of course, it is not required that the cooling coils be fed from the propane line and any external source of suitable coolant can be used. The phase change may be brought about, for example, by forcing the liquid propane through a thermal expansion valve and then through an evaporator. The vapors exiting the cooling coils may then be recycled by combining them with another vapor line in the system, preferably by returning the vapors to the liquid trap 30A, as shown in
When all liquid is drained from tank 70, the pressure of propane gas in the tank will be about 110 pounds per square inch (psi) at about 80° F. As propane vapors are withdrawn from the tank through line 3 and compressed by compressor 20, the pressure in tank 70 will fall, eventually reaching about 0 psi in the substantially evacuated state. By “substantially evacuated state” is meant that the amount of propane vapor remaining in the storage tank is less than about 1% of the original amount, preferably less than about 0.5% of the original amount, and more preferably less than about 0.1% of the original amount on a weight basis. With each unit pressure (psi) decrease in tank 70, the temperature of the remaining vapor may increase by about 2° F., such that a temperature increase from start to finish in a 30,000 gallon tank may be about 220° F. This substantial increase in temperature during the pumping cycle could make it progressively more difficult to condense the vapors in the liquid condensing tank 40 and to maintain the liquid condensing tank 40 at a suitably low temperature to continue condensing vapors without the use of an external coolant. Thus, in some embodiments, it may be necessary to pause the pumping in order to maintain the liquid condensing tank 40 at an appropriate temperature to condense propane vapors.
However, it has surprisingly been found that by cooling the propane gas before it enters the condensing tank 40, rather than by solely cooling the liquid in the condensing tank 40, the system is capable of continuous operation and can pump vapor from a tank of about 30,000 gallons without interruption. To accomplish continuous evacuation of vapors from a 30,000 gallon tank, it may be desirable to include a cooling means (e.g., a refrigeration system) 110 in the line between compressor 20 and liquid condensing tank 40 to lower the temperature of the vapor entering the liquid condensing tank(s). Thus, in some embodiments, a suitable means of cooling the line downstream from compressor 20 is employed in place of, or in addition to, the cooling coils 210 disposed in the internal volume of the liquid condensing tank 40.
Thus, as shown in
The pressurized propane vapors exiting the compressor 20 may be cooled, as discussed above, in refrigeration unit 110 and then bubbled through the liquid propane in the liquid condensing tank 40 to further cool and condense the vapors. It has been found to be particularly advantageous to force the compressed propane gas through a stem pipe, shown as 140 in
The liquid propane may be withdrawn from the liquid condensing tank 40 by a hose or pipe 9 and transferred into the customer's storage tank. In
The system for extracting and condensing propane gas according to the invention will find use wherever it is necessary or desirable to empty a propane tank. For example, a tank which must be modified in conformance with NFPA Pamphlet 58 will have to be completely emptied of propane before the internal tank modifications can be made. Any amount of liquid propane in the tank will first be drained according to customary practice. The remaining propane vapors and any small amount of liquid propane heel remaining (e.g., up to about 3% by volume) can then be extracted using the methods of the invention. The invention offers complete recovery of the propane from the customer's tank and a return of the recaptured propane to the customer in useable form. Unlike the conventional practice of conducting a controlled burn-off, the method according to the invention allows complete recovery of the propane from the customer's tank in a matter of hours, and does not require regulatory approvals.
The recovered propane may be returned to the customer without cost as part of the service, or may be sold back to the customer at or below the prevailing market value of the particular propane product.
The invention is not limited, of course, to the evacuation of 30,000 gallon storage tanks. It is contemplated that any tank size, including without limitation, about 60,000 gallons, about 45,000 gallons, about 30,000 gallons, about 18,000 gallons, and about 12,000 gallon tanks can be evacuated and the gaseous product recondensed using the methods and apparatus of the invention.
Because the system is self-contained and mobile, the operator can conveniently reach any tank in need of servicing. This is contemplated to be of particular advantage in the case of propane tanks which are at risk of BLEVE (boiling liquid expanding vapor explosion) due to fire and which therefore require rapid response. For example, a propane tanker which has been involved in a highway accident has the potential to undergo BLEVE if the contents are heated by flames beyond the capacity of the relief valves to dissipate pressure. In responding to such situations, fire personnel will typically cool the tanker with a constant stream of water. The ability to rapidly remove the propane from the compromised tank in concert with efforts to cool the tank could prevent a catastrophe. Thus, in one embodiment, the system may be used to rapidly extract propane from a tank at risk of BLEVE due to fire or which has otherwise been compromised. In some circumstances, it may not be possible to connect a vapor line directly to a valve on the tank because, for example, a railcar or transport tank has rolled over and the valves have become inaccessible. It may be possible in such situations to apply a “hot tap” on the shell of the tank by welding a threadolet to the tank and fitting a valve onto the threadolet. The drill of a hot tap machine is then fit through the open valve and used to drill a hole in the tank shell. After the hole is drilled, the coupon is retracted, preferably by pressure from within the tank, and the drill is removed and the valve closed. The vapor line to the propane extraction system may then be connected to this hot tap valve and the contents extracted as discussed above.
It is contemplated that the methods and systems of the present invention will find equal applicability to the extraction and reclamation from storage tanks of gases other than propane, including without limitation, natural gas, butane, ammonia, and the like. The foregoing description of various exemplary embodiments of the invention, therefore apply equally to the case where the gas to be evacuate is ammonia, natural gas, LNG, butane, etc.
The invention having been described by the foregoing description of the preferred embodiments, it will be understood that the skilled artisan may make modifications and variations of these embodiments without departing from the spirit or scope of the invention as set forth in the following claims.
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