Certain industrial manufacturing processes are carried out in the presence of a volatile solvent which vaporizes as it is used. Merely by way of example, such volatile solvents are used during the manufacture or pharmaceuticals. Typically, such manufacturing processes are "ventilated" using an inert gas such as nitrogen to carry away the vaporized solvent. In some instances, it had been the practice to exhaust the gas with its entrained, vaporized solvent through a conduit such as an exhaust stack and release it to the ambient atmosphere.
More recently, environmental studies indicate (or at least strongly suggest) that vaporized solvents which are so released have an adverse effect upon the upper atmosphere. As a consequence, refrigeration systems including multistage systems of the cascade type have been developed to recover vaporized solvent.
One such system uses a pre-cooler such as a water-ethylene glycol chiller to reduce the temperature or the inert gas-vaporized solvent mixture flowing in the stack. Such temperature is reduced from a nominal ambient temperature of 70.degree.-80.degree. F. to a level slightly above tne freezing point of water, e.g., to about 35.degree. F., to remove moisture from such mixture. For further temperature reduction, a plurality of heat transfer units is arranged downstream of the pre-cooler. Such transfer units are connected to the refrigeration system and disposed in a heat transfer relationship to the mixture. These transfer units result in the direct transfer of heat from the mixture of vaporized solvent and gas to the units. As used herein, "direct transfer" and like phrases means transfer of heat between a medium (such as the gaseous mixture) and a refrigerant without using other heat-carrying media.
As a result of such heat transfer, the temperature of the vaporized solvent decreases to levels well below 0.degree. F. and such solvent is thereby condensed to a liquid. It is to be appreciated that solvent recovery systems may involve very low temperatures. For example, the temperature of the gas mixture adjacent the "upstream" transfer unit may be about -5.degree. F. while that adjacent the second or downstream transfer unit may be about -85.degree. F. which generally corresponds to the temperature of the condensed solvent. Once the solvent is "stripped" from the mixture by condensing it, the inert gas (now substantially free of vaporized solvent) is vented to atmosphere through the exhaust end of the stack.
The solvent recovery system shown in U.S. Pat. No. 4,506,515 (Bedolo) uses a single compressor and the system evaporator does not extract heat directly from the gaseous mixture flowing in the tube. Rather, the evaporator cools an intervening media, i.e., a volume of glycol which, in turn, is circulated through a pair of heat exchangers which envelope the conduit carrying the vaporized solvent. These heat exchangers extract heat from and cool the gaseous effluents, thereby condensing the solvent.
Condensation apparently occurs at or perhaps above -20.degree. C. (about -4.degree. F.) since this is the lowest temperature in either heat exchanger. The condenser coil is used to reheat the gas flowing in the conduit to a temperature of about 10.degree. C. (about 50.degree. F.). If the refrigerant in the condenser coil is not sufficiently cooled by rejection of heat from its heat exchanger, a fan-and-coil cooling means, perhaps thermostatically controlled, is used. Frost formation is apparently a problem in the Bedolo system since the condenser coil is periodically de-activated (under control of a timer) and defrost coils are thereupon activated to get rid of condensed, frozen moisture.
Another solvent recovery system is shown in U.S. Pat. No. 3,232,029 (Evans, Jr.) and uses a two-section coil to perform cooling and heating functions. The first section cools gas to condense solvent and the second section (in series with the first section) reheats the outgoing gas. The Evans, Jr. system is similar to that shown in the Bedolo patent at least to the extent that both systems use a refrigerated coolant (as opposed to a refrigerant) to extract heat from the gas stream.
Known systems for recovering vaporized solvent tend to be characterized by certain disadvantages. In such systems using multistage cascade refrigeration, electric energy consumption is relatively high because refrigerant compression requires high input energy to the compressor electric drive motors.
Another disadvantage of low temperature recovery systems is that the inert gas leaving the exhaust stack is very cold, perhaps -85.degree. F. a example. In other words, it is well below ambient temperature. Often, a visible plume of water vapor forms near the exhaust end of the stack as the extremely cold gas contacts warmer ambient air. Such visible plumes create unnecessary concern on the part of uninformed observers and, perhaps, unwarranted complaints to the system operator. Yet another disadvantage is that the system may exhibit "backstreaming," i.e., the migration of moisture and other infiltrating substances into the exhaust end of the stack.
An improved vapor recovery system which sharply reduces the amount of required input energy to the refrigeration system, warms the exiting gas stream to a temperature above that of ambient and helps prevent moisture and other infiltrating substances from migrating backward into the exhaust stack would be an important advance in the art.