US 3886759 A
Description (OCR text may contain errors)
McNamee June 3, 1975 METHOD FOR RECOVERY OF HYDROCARBON VAPORS Gerald'P. McNamee, 2320 Riverside Dr., Santa Ana, Calif. 92706 Filed: Jan. 26, 1973 Appl. N0.: 326,606
US. Cl 62/54; 55/88; 220/85 VR Int. Cl. Fl7c 13/00 Field of Search 62/54, 402; 220/85 VR,
References Cited UNITED STATES PATENTS Primary Examiner-Meyer Perlin Assistant Examiner-Ronald C. Capossela Attorney, Agent, or Firm-Joseph E. Kieninger 5 7 1 ABSTRACT A method for the recovery of hydrocarbon vapors from a vessel while vapors are being expelled from the vessel during the filling thereof is disclosed. The composition of the expelled hydrocarbon vapor is controlled to a given composition by either dillution with air or by enrichment with hydrocarbons. A controlled volume of this hydrocarbon composition at a controlled temperature is passed'into a compressor and compressed. The compressed gas is cooled. This compressed gas is passed through an absorber at a controlled temperature where more than 90% of the hydrocarbons are removed. A nonflammable gas mixture having less than 2% hydrocarbon can be obtained, if desired.
8 Claims, 2 Drawing Figures METHOD FOR RECOVERY OF HYDROCARBON VAPORS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for the recovery of liquefiable components of hydrocarbon vapors, and more particularly to the recovery of gasoline components from vapors expelled from tanks during the filling thereof. a
2. Prior Art In the loading of car, truck, railroad, ships and other tanks with volatile hydrocarbons liquids such as gasoline and the like, substantial quantities of the lighter components will vaporize and escape into the atmosphere. The quantity of gasoline lost as a vapor in such situations is quite substantial, being dependent upon the vapor pressure of the initial liquid and the temperature at which the filling of the tanks occur. This substantial vapor loss is undesirable primarily for two reasons. One reason is the economic loss and the second reason is the air pollution, that is the vapor loss is a contributory factor in the formation of smog.
This problem has been extensively studied and is the subject of numerous patents. The following US. Pat. Nos. are examples of inventions related to solving this problem: 2,037,679; 2,853,149; 3,266,262; 3,648,436; 3,672,180 and 2,765,872.
The methods described in the aforementioned patents have both saved money and have reduced pollution. The method described in the patent to Hartman US Pat. No. 2,765,872 has been particularly widely used by the major petroleum companies and has been a success particularly in the recovery of the gasoline vapor for economic reasons. While the primary beneficial effect by the use of this process has been for economic purposes in recovering the vapors, the need to further reduce amount of hydrocarbon vapors that still escape using these existing prior art methods for pollution reasons has increased. As the people and the government regulatory agencies become more conscious and aware of this pollution, new governmental standards are set which limit the amount of gasoline vapor which can be vented into the atmosphere to levels lower than the existing levels. For example one of the governmental air pollution control agencies has recently required that 90 percent or more of the hydrocarbon vapors coming from tanks be recovered.
Some of the prior art methods referred to earlier vent a purified mixture which is flammable. These methods will not yield a nonflammable mixture.
Another problem is that some of these prior art methods are only suitable on hydrocarbon liquids having a high vapor pressure, for example a Reid vapor pressure of 14.5 psi. These methods do not work satisfactorily when the Reid vapor pressure is of the order of l to 6 psi. Still another problem is that some of the prior art methods only work well at moderate ambient temperatures. that is around 75 to 80F. While these process may remove 90 percent of the hydrocarbons when the ambient temperature is 75 to 80F, they remove considerable less when the ambient temperature is 90 to 1 10 F. Hence, none of the existing prior art methods are entirely satisfactory.
SUMMARY OF THE INVENTION It is an object of this invention to provide an improved method for recovering substantially all of the hydrocarbons vaporized and given off during the loading of liquid hydrocarbon mixtures into tanks.
It is another object of this invention to provide an improved method of removing hydrocarbons from a mixture which yields a nonflamable gas.
It is yet another object of this invention to provide a method of removing hydrocarbons from hydrocarbon containing gaseous mixtures which enables the treated vapor-air mixture to pass existing pollution control regulations.
It is a further object of this invention to provide a method suitable of removing substantially all of the hydrocarbons from a gaseous mixture under different ambient conditions.
These and other objects are accomplished by a method which removes a substantial portion of the hydrocarbon vapors from the hydrocarbon-air mixture which is expelled from the vessel during the filling thereof. The method includes controlling the composition of the expelled hydrocarbon vapor. A controlled volume of this known vapor composition is passed into a compressor at a controlled temperature. The gas is compressed and cooled. The compressed gas is passed through an absorber at a controlled temperature where more than percent of the hydrocarbons are removed.
Other objects and advantages of the invention will be apparent from the following detailed description, reference being made to the accompanying drawings wherein a preferred embodiment of this invention is shown.
IN THE DRAWINGS FIG. 1 shows a flow diagram of the system employed in carrying out the method of the present invention in loading of a gasoline tank truck at a refinery, terminal or bulk plant station.
FIG. 2 shows a modification of FIG. 1.
DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS As shown in FIG. 1, gasoline from a stock tank 2 is pumped by means of pump 3 into a tank truck 4. The stock tank 2 may be either a floating roof stock tank or a cone roof stock tank. The type of tank is not critical in the practice of this invention. During the pumping of the gasoline into the tank truck 4 some of the lighter components of the gasoline are vaporized and/or entrained into the air contained in the tank truck. This gasoline vapor-air mixture is passed through line 5 into a gas holder 6. The gas holder 6 is preferably a vessel which takes in flucuating amount of gas but discharges only a constant controlled amount. An example of such a holder is a spherical vessel which has a hemispherical flexible membrane on the inside, the outer edge of the membrane being fastened to the shell of the sphere at the equator. The air vapor mixture is stored between the membrane and the shell.
The composition of the gasoline vapor-air mixture is monitored after it leaves the exit 7 of the gas holder 6 for either oxygen content or hydrocarbon content by any conventional analyzer 13 before it passes into the compressor 14. If the composition is rich in hydrocarbon or gasoline component composition, air is introduced at point 8 by means of valves which are common in the art until the desired composition is obtained.
While this method may be used for any vapor mixture composition containing to 100 percent hydrocarbon. the preferred range is 4 to 75 percent hydrocarbon. The gasoline vapor-air mixture is then passed to heat exchanger in order to obtain the required temperature. If the air had been added at point 8 to obtain the desired composition, the mixture passes through line 11A into the top of the gasoline contactor 12. Since the gasoline air mixture enters the contactor 12 above the contact zone, the composition of the mixture is not changed.
If on the other hand. the gasoline vapor-air mixture coming from the .exit 7 of the gas holder 6 is deficient in its hydrocarbon or gasoline content in the composition, a controlled portion of the mixture goes through line 118 into the bottom of the contactor 12. The gas mixture then goes up through the liquid gasoline or bydrocarbon in the contactor 12 and the composition of the gasoline or hydrocarbon component in the gasoline vapor-air mixture is increased to the desired level. Pump 17 moves hydrocarbon liquid from the bottom of contactor 12 to tank 2. Pump 17A moves the hydrocarbon liquid from inside the bottom of contactor 12 to the top of contactor 12. The gasoline vapor-air mixture passes from the top of the contactor 12 through line 13A into the compressor 14. An analyzer 13 associated with line 13A measures the oxygen or the hydrocarbon content of the vapor mixture which goes into the compressor 14.
The thermodynamic and transport properties of the gasoline vapor-air mixture are regulated by controlling the composition of the mixture. The flow rate of the mixture into the compressor 14 is regulated by controlling the temperature and pressure of the mixture. At this point the temperature of the gas mixture is between 50 to 150 F. The preferred temperature range is 35 to 120 F. The pressure of the gasoline-vapor mixture is from O to 300 psia. The preferred pressure range is 10 to 25 psia. An example is a gasoline vapor-air mixture containing 20 percent hydrocarbon at a temperature of 100 F. and under a pressure of 14.7 psia. The intake volume of the compressor 14 is essentially constant so that the control of the temperature and pressure at the inlet of the compressor 14 insures a constant molal flow rate into the system (PV nRT). Control of the gasoline vapor-air mixture composition by the addition of either air at point 8 or hydrocarbons in the saturator 12 will determine the compression characteristics of the gasoline vapor-air mixture as well as its other thermodynamic and transport properties.
The compressor 14 compresses the gasoline vaporair mixture to a pressure and temperature below the auto ignition limits of the particular mixture. The temperature of the gasoline vapor-air mixture after it leaves the compressor 14 into lines 15A or 158 is preferably below 300 F. The pressure at the outlet of the compressor is preferably between 14.7 and 165 psia. An example is a compressed mixture at a temperature of 280 F. under a pressure of 65 psia. The compressed gasoline vapor-air mixture may be passed through the line 158 to the heat exchanger 10 for the purpose of heating the gasoline vapor-air mixture as discussed previously. The gasoline vapor-air mixture from the compressor 14 goes by path 15A or the path 158 as described before into the cooler 16 where the temperature is cooled down to a temperature in the range of 32 4 to 300 F. A preferred temperature range is to 130 F.
The mixture than proceeds to separator 18 where a small amount, if any, of the condensed hydrocarbon is removed. The separator 18 serves more or less as a surge tank in addition to collecting the condensed hydrocarbon vapors. The mixture than goes to the second stage compressor 20 where the gasoline vapor-air mixture is compressed to a pressure in the range of 14.7 to 1000 psia. A preferred pressure range is to 600 psia. The temperature range of the gas mixture at the outlet on the compressor 20 is the same as for the outlet of compressor 14. The mixture from compressor 20 goes to the heat exchanger 22 where the temperature is lowered depending on the composition and the cooling capacity of the absorber 24. The mixture is then passed from the heat exchanger 22 into the absorber 24. As the gasoline vapor-air mixture passes through the absorber 24 the major portion of the hydrocarbons are removed. The temperature of the gasoline or other liquid in the absorber 24 used to absorb the hydrocarbons from the gasoline vapor-air mixture is between 296 to 170 with the preferred temperature range being 40 to F. The temperature of the gas as it leaves the absorber 24 is substantially the same as the temperature of the liquid used in the absorber 24. Gasoline from tank 2 is cooled by coolers 29 and 27 as well as heat exchanger 26. Pump 25 recirculates the hydrocarbon liquid through heat exchanger 26, cooler 27 and absorber 24. The hydrocarbon liquid flows substantially counter current to the air flow in absorber 24. Pump 31 moves gasoline through heat exchanger 29, and cooler 27 into absorber 24. The pressure in absorber 24 effects the flow of hydrocarbon liquid through heater exchanger 29 to contactor 12, where degassing of the hydrocarbon liquid occurs. The flow of liquid in contactor 12 is counter current to the air flow therein as it is in the system in general.
By the proper control of the composition and the temperature as well as the flow rate of the gas going into compressor 14, close control of the cooling and compressing conditions and by controling the temperature of the absorber 24 liquid over 90 percent of the hydrocarbons can be removed by the time the gas exits from the absorber 24. Preferably, the conditions are controlled to produce a nonflammable mixture containing less than 2 percent hydrocarbon.
The substantially hydrocarbon free air leaves the absorber 24 and passes through the heat exchanger 22 and than passes to the expander 28. 1n the expander 28 the pressure is decreased to between 0 and 1000 psia with preferred range being between 14.7 and 600 psia. The temperature of the hydrocarbon free gas at this point is between -3l0 and F. with a preferred temperature range being between -200 and 130 F. The gas from the expander 28 goes to point 30 and then passes through either heat exchanger 26 or heat exchanger 10 to provide cooling in the system and is then vented into the atmosphere. The energy released by expansion in expander 28 is recovered and may be used for compression.
A preferred embodiment of this invention is shown in FIG. 2. in this embodiment the only difference in the system is in the absorber train which comes after the cooling unit 22. As shown in FIG. 2, the vapor-air mixture goes to absorber 24A. The absorber 24A is cooled by gasoline or another hydrocarbon which is pumped by A. The gasoline is cooled by heat exchanger 27A. 1n absorber 24A most of the hydrocarbons are removed. The resultant vapor-air mixture is passed through line 40 through cooler 42 into absorber 44. In absorber 44 gasoline or another suitable hydrocarbon is passed through the system in order to remove additional hydrocarbon. The temperature of the hydrocar bon in absorber 44 is lower than the temperature of the hydrocarbon in absorber 24A. The gasoline is pumped by pump 46 through cooler 48 into the absorber 44. After the hydrocarbon has been removed from the air in absorber 44, the substantially hydrocarbon-free air is passed through head exchanger 22A and into the expander 28A in a manner similar to that described in FIG. I.
An example of a method using the system shown in FIG. 2 would involve a hydrocarbon vapor-air mixture passing through 24A in which over 90 percent of the hydrocarbons are removed while the temperature of absorber 24A is at about 100 F. The mixture than goes through the absorber 44 where additional hydrocarbon is removed so that the gas leaving the absorber 44 contains on the order of l percent hydrocarbon.
It is to be noted that the embodiment in FIG. 1 as well as the embodiment shown in FlG. 2 can be used to obtain substantially hydrocarbon free gas mixtures containing on the order of 1 percent hydrocarbon. FIG. 2 describes an embodiment that lends itself conventional cooling equipment to obtain temperatures on the order of F. in the absorber 44 with a minimum of difficulties.
EXAMPLE NO. 1
A gasoline-air mixture was expelled froma tank as it was being loaded with gasoline (having a Reid vapor pressure of 6.5 psi) contained 24.1 percent hydrocarbon and 75.9 percent air. (Hydrocarbon Mixture No. l in table). This gasoline vapor-air mixture was heated to a temperature of 100 F. and diluted with sufficient air to form a vapor mixture containing 20 percent gasoline vapor (Hydrocarbon Mixture No. 2 in table). The pressure of the gasoline-air mixture was 14.7 psia. This gasoline-air mixture was passed into the compressor in which the pressure of the mixture as it left the compressor was 65 psia and had a temperature of 280 F. The gaseous-air mixture was then cooled and passed through an absorber containing gasoline (Hydrocarbon Mixture No. 3 having a Reid vapor pressure of 6.5psi) therein as the liquid to absorb the gasoline vapor. The temperature of the gasoline-air mixture at the absorber outlet was 100 F. The gasoline vapor-air mixture as it left the absorber outlet had a composition of 2.8 percent hydrocarbons and 97.2 percent air (Hydrocarbon Mixture No. 4). The flamability limits of this mixture are 1.7 percent hydrocarbon and 8.9 percent hydrocarbon.
EXAMPLE NO. 2
The same composition was passed under the same temperature and pressure conditions as setforth in Example No. 1 with the exception at the absorber where the temperature of the gasoline was kept at 35 F. so that the temperature of the gasoline vapor-air mixture leaving the outlet was also 35 F. The composition of the gasoline-air mixture as it left the absorber outlet was 0.9 percent hydrocarbon and 99.1 percent air (Hy drocarbon Mixture No. 5). The flamability limits of this mixture are 1.98 percent and 9.82 percent hydrocarbon. The gasoline-air mixture obtained in this example was non-flamable. L y
The composition of the gasoline (No. 3 )fgasoline vapor air mixtures (No. 1 and No. 2) and the gasoline-air mixtures thatwere obtained in examples 1 and '2 (No. 4 and No. 5) are set forth in the following table.
Hydrocarbon Mixtures COM- PONENT No. 1 No. 2 No. 3 No. 4 No. 5 vol 71 vol '7! mol '7! vol 71 vol "/1 C 4.83 4.00 .022 .401 .26 C 1.00 .83 .033 .093 .04 Q, .46 .38 .066 .047 .02 i-C 1.89 1.57 .786 .208 .07 n-C 4.81 3.99 2.823 .555 .16 i-C 5.99 4.97 9.49 .730 .18 nC 1.32 1.10 2.96 .178 .04 C; 3.23 2.68 24.60 .455 .09 C .23 .19 5.53 .034 .01 C .21 .18 18.15 .036 .01 C, l3 11 33.70 .024 C 1.94
total Hydro- 24.10 20.00 100. 2.763 .88 carbon Air 75.90 80.00 97.237 99.12
The method set forth in accordance with this invention has a number of advantages not obtainable with the present prior art methods. This method enables the government air pollution control standards to be met. that is over 90 percent or more of the hydrocarbon vapors coming from the tanks can be recovered. The purified mixtures can also be controlled so that they are no longer flamable. While flamable mixtures may be obtained with this invention, by proper adjustment of the composition and/or temperatures throughout the method a non-flamable mixture can be obtained. Another advantage found with this method is that this method is suitable for use on hydrocarbon liquids having a high vapor pressure, for example Reid vapor pressure of 14.5 psi, as well as with hydrocarbons having a low Reid vapor pressure, that is 1-6 psi. Still another advantage in this system is that it will work as high ambient temperatures, that is temperatures as high as 90130 F.. whereas the existing prior art methods are usually suitable only at lower temperatures. that is around -80 F.
While the invention has been described in terms of a preferred embodiment, the scope of the invention is defined in the following claims:
1. A method of recovering hydrocarbon vapors from a hydrocarbon vapor-air mixture which is being expelled from a vessel during the filling of said vessel with hydrocarbons comprising the steps of controlling the hydrocarbon content of said vapor/air mixture by controlling the temperature thereof, controlling the volume of said vapor/air mixture and passing said vapor/air mixture into a compressor. compressing said vapor/air mixture to form a compressed mixture. cooling said compressed mixture. and passing said compressed mixture into a first absorber to form and absorbed mixture having a liquid hydrocarbon at a controlled temperature whereby said liquid hydrocarbon removes a substantial protion of said hydrocarbon vapor from said compressed mixture.
2. A method as described in claim 1 whereby the hydrocarbon content of said vapor/air mixture passing into said compressor is between 4 to 75 percent hydrocarbon and the temperature of said mixture is between 35 to 120 F.
3. A method as described in claim 1 whereby said vapor/air mixture is compressed up to 600 psia.
4. A method as described in claim 1 whereby said liquid hydrocarbon in said first absorber is at a temperature between -40 to 130 F.
5. A method as described in claim 1 including the step of passing the absorbed mixture into a second ab- PI'CSSOI'.