When handling volatile liquids such as hydrocarbons including gasoline and kerosene, air-volatile liquid vapor mixtures are readily produced. The venting of such air-vapor mixtures directly into the atmosphere results in significant pollution of the environment and a fire or explosion hazard. Accordingly, existing environmental regulations require the control of such emissions.
As a consequence, a number of processes and apparatus have been developed and utilized to recover volatile liquids from air-volatile liquid vapor mixtures. Generally, the recovered volatile liquids are liquefied and recombined with the volatile liquid from which they were vaporized thereby making the recovery process more economical. The initial vapor recovery systems utilized in the United States in the late 1920's and early 1930's incorporated a process combining compression and condensation. Such systems were originally only utilized on gasoline storage tanks. It wasn't until the 1950's that local air pollution regulations began to be adopted forcing the installation of vapor recovery systems at truck loading terminals. Shortly thereafter, the "clean air" legislation activity of the 1960's, which culminated in the Clean Air Act of 1968, further focused nationwide attention on the gasoline vapor recovery problem. As a result a lean oil/absorption system was developed. This system dominated the marketplace for a short time.
Subsequently, in the late 1960's and early 1970's cryogenic refrigeration systems began gaining market acceptance (note, for example, U.S. Pat. No. 3,266,262 to Moragne). While reliable, cryogenic systems suffer from a number of shortcomings including high horsepower requirements. Further, such systems require relatively rigorous and expensive maintenance to function properly. Mechanical refrigeration systems also have practical limits with respect to the amount of cold that may be delivered, accordingly, the efficiency and capacity of such systems is limited. In contrast, liquid nitrogen cooling systems provide more cooling than is required and are prohibitively expensive to operate for this type of application.
As a result of these shortcomings, alternative technology was sought and adsorption/absorption vapor recovery systems were more recently developed. Such a system is disclosed in a number of U.S. patents including, for example, U.S. Pat. No. 4,276,058 to Dinsmore, the disclosure of which is fully incorporated herein by reference. Such systems utilize a bed of solid adsorbent selected, for example, from silica gel, certain forms of porous mineral such as alumina and magnesia, and most preferably activated carbon or charcoal. These adsorbents have an affinity for volatile hydrocarbon liquids. Thus, as the air-hydrocarbon vapor mixture is passed through the bed, a major portion of the hydrocarbons contained in the mixture is adsorbed on the bed. The resulting residue gas stream comprising substantially hydrocarbon-free air is well within regulated allowable emission levels and is exhausted into the environment.
It should be appreciated, however, that the bed of adsorbent used in these systems is only capable of adsorbing a certain amount of hydrocarbons before reaching capacity and becoming ineffective. Accordingly, the bed must be periodically regenerated to restore the carbon to a level where it will effectively adsorb hydrocarbons again. This regeneration of the adsorbent is a two step process.
The first step requires a reduction in the total pressure by pulling a vacuum on the bed to remove the largest amount of hydrocarbons. The second step is the addition of a purge air stream that passes through the bed. The purge air polishes the bed so as to remove substantially all of the remaining adsorbed hydrocarbons. These hydrocarbons are then pumped to an absorber tower wherein lean oil or other nonvolatile liquid solvent is provided in a countercurrent flow relative to the hydrocarbon rich air-hydrocarbon vapor mixture being pumped from the bed. The liquid solvent condenses and removes the vast majority of the hydrocarbons from that mixture and the residue gas stream from the absorber tower is recycled to a second bed of adsorbent while the first bed completes regeneration.
It should be appreciated that for the vapor recovery system to operate at maximum efficiency, the adsorption and regeneration cycles must be initiated at the appropriate time. Further, the change over between cycles must be smooth and precise. More specifically, the pressure in the vapor recovery system and particularly the reaction vessels holding the beds of adsorbent may be evacuated to a pressure as low as, for example, 27 inches of mercury vacuum during regeneration. In contrast, during the adsorption cycle the beds are subjected to a pressure of atmospheric pressure or slightly greater depending on the system for delivering air-volatile liquid vapor to the beds. Thus, it should be appreciated that the beds are subjected to very significant pressure changes during each complete operating cycle.
Improper operation of the valves controlling the flow through the vapor recovery system may in some instances lead to very sudden pressure changes. Such sudden changes have a tendency to fracture/crumble the adsorbent in the beds thereby detrimentally affecting the pore structure of the adsorbent and, consequently, its adsorbtivity. Hence, the effective service life of the adsorbent may be significantly reduced inadvertently through improper operation of the valves. Similarly, sudden pressure changes in the vapor recovery system due to the too rapid opening and closing of valves separating portions of the system at significantly different pressure levels may result in heavy loads being placed upon the vacuum pump. These loads increase the stress and strain on the pump components possibly causing excessive wear and thereby significantly reducing the operational life of the pump. Unfortunately, as manual valve operation is by necessity subject to the learning curve of new operators and the ever present problem of human error, these sudden pressure change induced problems are prevalent in state of the art vapor recovery systems.
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