Abstract:
A system for recovering and utilizing vapor from a source of vapor has a vapor holder for storing a quantity of vapor from the source of vapor. Also included is a condenser coupled to the vapor holder for receiving and condensing at least partially, vapor from the vapor holder. The system also has an engine and a generator driven by the engine for generating electrical power. The engine has an engine intake coupled to the condenser and an exhaust outlet. This engine is powered at least partially, by output from the condensing apparatus. The system also has a fuel adjustment apparatus and a fuel sensor apparatus. The fuel adjustment apparatus has a control input and is coupled between the engine and the condensing apparatus for adjusting fuel concentration into the engine intake in response to a signal on the control input. The fuel sensor apparatus is coupled to the engine intake (a) for sensing concentration of at least some constituents of vapor at the engine intake, and (b) for applying a signal to the control input of the fuel adjustment apparatus corresponding thereto. The system also includes an exhaust sensor apparatus coupled to the exhaust outlet for providing an exhaust signal signifying concentration of at least some constituents of the exhaust at the exhaust outlet.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to vapor recovery apparatus and, in particular, to using recovered vapor for power generation. 
     2. Description of Related Art 
     Since the passage of the Clean Air Act, the Congress of the United States has required all persons or organizations handling hydrocarbons or chemicals whose vapors may pollute the air to install means to recover and prevent the contamination of the air by such vapors. Such contaminants can include vapors of gasoline, methylene chloride and other organic compounds. 
     Such vapors are generated and displaced into the atmosphere when all types of tanks are filled with liquid hydrocarbons or liquid chemicals. Such tanks may be large storage tanks, railroad car tanks, truck tanks, underground storage tanks for gasoline stations and fuel tanks on trucks, buses and automobiles. When these various types of tanks are filled with liquid hydrocarbons or liquid chemicals, vapors escape into the atmosphere and, as is well known, such vapors become a source of smog, which under certain ambient conditions produce dangerous fog conditions and so pollute the atmosphere that they produce dangerous environmental health hazards for human beings. 
     Known vapor recovery systems have used closed refrigeration cycles to cool a medium that is then used to condense vapors. Condensate can be drained to a decanter to separate heavy and light constituents, such as gasoline and water. The condensing coils for such units are periodically warmed or defrosted to prevent a build up of ice and frost that may block the passage of vapors through the condensing unit. See for example, U.S. Pat. Nos. 4,027,495; 4,068,710; 4,077,789; and 5,291,738. 
     Such recovery units are typically designed to handle the peak flow of vapors that may be experienced during a course of a work day. To accommodate the peak load, the recovery units are engineered with a relatively high capacity, which still may not be sufficient condense highly volatile vapors. 
     A disadvantage with vapor recovery systems is the energy required to run these recovery systems. Moreover, certain highly volatile vapors can only be condensed after a high expenditure of energy. Accordingly, the environmental benefits of performing vapor recovery is partially offset by the additional energy consumed to run the recovery systems. 
     Many industries are economically dependent on inexpensive and abundant electrical power. Many utilities will charge a rate that depends upon the peak usage or the time when the peak usage occurs. For this reason, some industries have invested in cogeneration, wherein a modest private plant for generating electricity will supplement the power from a utility to reduce the peak demand and thereby reduce the rate charged for power. Depending upon the size of the plant, some cogeneration systems can actually return power to the utility lines to earn a credit. 
     While in principal, a cogeneration plant can be powered by the uncondensed vapor from a vapor recovery unit, the supply of vapor tends to be sporadic and will lack a constancy that will allow cogeneration to occur in a practical way. 
     Such a cogeneration system may employ a generator driven by an engine that is designed to be powered by a fossil fuel. When the engine is an internal combustion engine, regulating the air/fuel ratio can be difficult when the fuel source is the uncondensed vapor from a vapor recovery unit. The uncondensed vapor can include a variety of vapors whose constituent components cannot be known in advance. Therefore, regulating the speed and power of the engine can be difficult, when the nature of the fuel, and the fuel to air ratio may vary significantly. 
     Furthermore, one cannot be certain in advance that the combination of a vapor recovery unit and cogeneration system will succeed in providing a net environmental benefit. In particular, the engine exhaust may introduce significant pollutants that should not be exhausted to the atmosphere. 
     Accordingly, there is a need to recover vapors using a combination of effective techniques such as condensing vapors, as well as using those vapors that were not condensed, in a power generation system. 
     SUMMARY OF THE INVENTION 
     In accordance with the illustrative embodiments demonstrating features and advantages of the present invention, there is provided a system for recovering and utilizing vapor from a source of vapor. The system has a vapor holder for storing a quantity of vapor from the source of vapor. Also included is a condensing means coupled to the vapor holder for receiving and condensing at least partially, vapor from the vapor holder. The system also has an engine, and a generator driven by the engine for generating electrical power. The engine has an engine intake coupled to the condensing means and an exhaust outlet. This engine is powered at least partially, by output from the condensing means. 
     In accordance with another aspect of the invention, a system for recovering and utilizing vapor from a source of vapor includes a condensing means for receiving and condensing at least partially, vapor from the source of vapor. Also included is an engine having an engine intake coupled to the condensing means, as well as a generator driven by the engine for generating electrical power. The engine is powered at least partially, by output from the condensing means. The system also has a fuel adjustment means and a fuel sensor means. The fuel adjustment means has a control input and is coupled between the engine and the condensing means for adjusting fuel concentration into the engine intake in response to a signal on the control input. The fuel sensor means is coupled to the engine intake (a) for sensing concentration of at least some constituents of vapor at the engine intake, and (b) for applying a signal to the control input of the fuel adjustment means corresponding thereto. 
     In accordance with still another aspect of the invention, a system for recovering and utilizing vapor from a source of vapor, has a condensing means for receiving and condensing at least partially, vapor from the source of vapor. Also included is an engine coupled to the condensing means and having an exhaust outlet for conducting exhaust from the engine. The engine is powered at least partially by output from the condensing means and drives a generator for generating electrical power. The system also includes an exhaust sensor means coupled to the exhaust outlet for providing an exhaust signal signifying concentration of at least some constituents of the exhaust at the exhaust outlet. 
     By employing systems of the foregoing type, vapor can be effectively recovered and utilized. In a preferred embodiment, vapor can be stored in a vapor holder, which is a vessel fitted with a flexible membrane or bag that can accommodate the varying volume of vapor to be stored. Consequently, the vapor can then be delivered at a relatively constant rate. Preferably, any engine driven by the vapor can be started or stopped should the supply in the vapor holder become relatively high or low. 
     In any event, the vapor can be preferably passed through a pre-cooler and a finishing condenser, both containing coils that conduct a refrigerant. Vapors condensed in these two units can be delivered to a decanter that can separate water from other more volatile liquids. 
     Vapors that were not condensed either because of their high volatility or because of an inadequate capacity to condense, may in the preferred embodiment, be re-heated and passed through a flame arrester to an electrical power generation system. These incoming vapors can be blended with air by means of a preferred modulating valve that is controlled by a fuel sensor, to establish a proper air/fuel ratio. The preferred fuel sensor employs an infrared detector tuned to sense concentration of a particular hydrocarbon, such as butane. The engine can drive a preferred induction generator to return power to utility lines. 
     Preferably, the engine exhaust can be sampled, cooled by a radiator, and delivered to a continuous emissions monitor. This monitor can have an infrared sensor tuned to a specific hydrocarbon, such as propane. 
     The preferred system is integrated by linking the engine coolant system to a thermal transfer system employing a vessel filled with a heated medium. The medium heated by the engine coolant system can be circulated to perform a variety of tasks. For example, the medium can be used to defrost the pre-cooler and finishing condenser, as well as the decanter. Also, the medium can be used to re-heat the uncondensed vapors delivered from the finishing condenser to the engine. 
     Also in the preferred embodiment, various operating parameters can be measured and provided as inputs to a programmable logic controller. This controller can use some of the input signals as feedback for controlling the system components. In other cases, the controller will simply record the parameters to keep a record of system performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above brief description as well as other objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is an elevational view of a system in accordance with the principles of the present invention; 
     FIG. 2 is a schematic diagram of the vapor recovery portion of the system of FIG. 1; and 
     FIG. 3 is a schematic diagram of the engine/generator portion of the system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2, a vapor holder  10  is shown as a vessel fitted with a flexible membrane or bag  12  to contain a variable volume of vapor. In one embodiment, holder  10  had a volume of 6,000 cubic feet, but different volumes may be used in other embodiments. A source of vapor  14  shown flowing into conduit  16 , can fill holder  10 . As membrane  12  moves, its position is detected by four position sensors  18 ,  20 ,  22  and  24 , herein referred to as a storage sensor means. Sensor  18  indicates an excessively high volume of vapor in holder  10  and may alert an operator at a loading station to cease supplying vapor from source  14 . Sensor  24  indicates an unusually low level in holder  10  and may indicate a leak, and is therefore considered an alarm. Sensor  20  indicates a modestly high volume in holder  10  and may be used to call for consumption, by starting the engine to be described hereinafter. Sensor  22  indicates a modestly low volume in holder  10  and can signal the above noted engine to stop. 
     Sensors  18 - 24  are transducers that respond to a mechanical input by producing electrical outputs, shown applied to a control means  26 . Control means  26  is a signal processor that can respond to a variety of input signals to produce various output signals for controlling the system described herein. Preferably, control means  26  is a programmable logic controller, which is a microprocessor-based system having a variety of input and output cards to deal with a variety of analog and digital signals. To this end, control means  26  will include a number of analog to digital and digital to analog converters. Control means  26  may also include timers to produce a delayed response to various inputs. In other embodiments, control means  26  may be a different type of computer, or may be built from discrete logic circuits. Control means  26  is shown with a plurality of other inputs and outputs, indicated as dotted lines. These dotted lines indicate an electrical signal to/from the variety of transducers illustrated in this and other diagrams. 
     Conduit  16  a shown fitted with an analyzer  28  that can sense its hydrocarbon content. For example, analyzer  28  may include a infrared light source tuned to detect a specific substance such as propane, butane, etc. This analyzer is employed for the purpose of reporting the hydrocarbon content and is not part of a feedback loop. 
     Conduit  16  a shown feeding a precooler  30 , which is part of a condensing means. Precooler  30  is shown with an internal baffle  32  positioned to create a descending upstream path and an ascending downstream path. Refrigerant circulating through coil  34 , located in the ascending downstream path, can reduce the temperature at the outlet of precooler  30  to about 35° F., although other temperatures can be established instead. The temperature at the bottom and outlet of precooler  30  can be sensed by sensors  36  and  38 , respectively. These sensors  36  and  38  are coupled to the previously mentioned control means  26 . 
     The output of precooler  30  flows through conduit  40  to a finishing condenser  42 , which is also part of the condensing means. Condenser  42  also has a baffle  44  to again create a descending upstream path and an ascending downstream path. Refrigerant circulating through coil  46 , located in the ascending upstream path, can reduce the temperature at the outlet of condenser  42  to about −40° F., although other temperatures may be employed in alternate embodiments. Temperature at the bottom and outlet of condenser  42  can be sensed by sensors  48  and  50 , respectively. Sensors  48  and  50  are coupled to the previously mentioned control means  26 . The refrigeration system for providing refrigerant to coils  34  and  46  is conventional and is not specifically described in this diagram. 
     The condensate falling onto the floors of units  30  and  42  drain through conduits  52  and  54 , respectively, to a decanter  56 . This condensate initially flows to one side of a weir  58 , which allows water to descend and eventually overflow through pipe  60 . Lighter hydrocarbon condensate will flow over the weir to the right side of decanter  56 . Three sensors  61 ,  62  and  64  are used to detect whether the level of condensate in decanter  56  is too high, modestly high, or modestly low. Sensor  65  senses the temperature inside the decanter. Sensors  61 - 65  provide output signals to previously mentioned control means  26 . 
     Condensate from decanter  56  can be pumped by pump  66  to a storage tank (not shown), especially when sensor  61  indicates the condensate level in decanter  56  is excessively high. Pump  66  is controlled by transducer  68 , which receives its controlling input from previously mentioned control means  26 . 
     The line  76  has a valve  78  that can be opened to drain into decanter  56  liquid (for example, gasoline) that may have inadvertently spilled into holder  10 , Pump  66  can then be used to send this spillage to the storage tank. 
     Condensate in decanter  56  can also be withdrawn by pump  70 , which is controlled by a transducer  72  under the control of control means  26 . Pump  70  delivers condensate to spray heads  74 , which are also referred to herein as a saturating means. As explained further hereinafter, these spray heads can increase the concentration of fuel leaving finishing condenser  42 , when the vapor concentration is inadequate for the purposes to be described presently. 
     A thermal means is shown herein employing a vessel  80  filled with a medium such as a 60/40 mix of glycol and water. Vessel  80  is fitted with a temperature sensor  82  and a level sensor  84  to send monitoring signals to previously mentioned control means  26 . Vessel  80  can be heated by an electrical heater  86 , which is controlled by transducer  88 , under the influence of control means  26 . In one embodiment the medium in vessel  80  was regulated to a temperature of 130° F. At times, vessel  80  will need to be purged by nitrogen gas, which can be admitted through purge valve  90 . 
     The outlet of vessel  80  is drawn by pump  92 , which is under the control of transducer  94  and control means  26 . Pump  92  delivers heated liquid to heating coil  96  of reheater  98 , before returning through line  100  to vessel  80 . Reheater  98  is coupled to the outlet of condenser  42  in order to increase the temperature of vapor therefrom before delivery through conduit  99  to the engine to be described hereinafter. 
     The discharge from pump  92  can also circulate through condenser  42  when defrost valve  102  is opened by transducer  104 , under the influence of control means  26 . Valve  102  can be programmed to open periodically according to a schedule pre-programmed in control means  26 . For example, the defrost cycle can occur daily for about two hours. With valve  102  open, heated liquid can flow through defrost coil  106 , located in the descending upstream path of condenser  42 . Heated liquid can also flow through the defrost coils  108  mounted at the body of condenser  42  and at its outlet drain  54 . As before, coils  106  and  108  drain through line  100  to vessel  80 . 
     The discharge of pump  92  can also flow through defrost valve  110  under the control of transducer  112  and control means  26 . When valve  110  is open, heated liquid can flow through coils  114  and  116 . Coil  114  can defrost the liquid in decanter  56 , while coil  116  can heat the body of decanter  56 . Again, both coils  114  and  116  drain along line  100  to vessel  80 . 
     It will be noted by circuit  118  that the liquid of vessel  80  can circulate through another system, which will be described herein as a coolant system. 
     Referring to FIG. 3, previously mentioned conduit  99  is shown connected to detonation arrester  120 . Operating parameters of arrester  120  are monitored by transducer  122  which forwards its output to previously mentioned control means  26  (FIG.  2 ). The output of arrester  120  is coupled to a shut off valve  124 , which is operated by actuator  126 . Actuator  126  is operated by transducer  131 , under the control of the control means  26 . Valve  124  is used to shut down the system, not to regulate flow rates. 
     Shut off valve  124  feeds one inlet of modulating valve  132 , whose other inlet draws ambient air through filter  134 . The actuator  136  of modulating valve  132  is controlled by transducer  138 , under the influence of a control input from previously mentioned control means  26 . Actuator  136  is able to change the proportion of flow from the two inlets of modulating valve  132  to adjust the fuel/air ratio in engine intake  140 , and thereby act as a fuel adjustment means. 
     The fuel blend arriving in engine intake  140  is sensed by a fuel sensor means  141 . Means  141  employs an infrared sensor that is tuned to measure the butane in intake  140 , although other substances can be measured using different sensing means. Control means  26  (FIG. 2) senses the fuel concentration by means of transducer  144  and feeds back a control signal through transducer  138  to control the modulating valve  132 . Consequently, the control means can operate to maintain a relatively fixed concentration of fuel in engine intake  140 . In a preferred embodiment, the fuel concentration is compared to a target value to produce an error signal, which is integrated over time to produce a feedback signal for controlling the fuel adjustment means  136 ,  138 . 
     Engine intake  140  connects to the intake manifold of engine  142 . Engine  142  may be a naturally aspirated, internal combustion, piston engine, although other engine types may be employed in different embodiments. In one embodiment engine  142  was rated at about 230 horsepower and included a microprocessor-controlled throttle that kept engine speed at at least 1,825 rpm, although other speeds and horsepower ratings are contemplated, depending upon the desired capacity and throughput. 
     Engine  142  has a number of sensors such as sensor  158  for measuring various operating parameters of engine  142 , such as the pressure and temperature of oil and coolant, engine speed, etc. These parameters are sent as signals to previously mentioned control means  26 . Transducer  161  is a receiver that relays control signals sent from control means  26  in order to control operating parameters of engine  142 , such as engine speed. Sensor  163  connects to a vapor detector  164  to send an alarm to the control means  26  indicating the danger of a fire or explosion due to high vapor concentrations in the engine room. 
     Engine  142  has an exhaust outlet  146  that delivers its exhaust through spark arrester  148  and muffler  150  before being released through stack  152 . The stack temperature is measured by transducer  154 , which delivers an output to previously mentioned control means  26 . Blower  156  sends ambient air to mix with the exhaust arriving at the inlet to spark arrester  148 , and thereby act as a mixing means. Blower  156  will dilute the exhaust to reduce its temperature and thereby reduce the danger of ignition, which can be important in a division 1 explosion proof rated area. 
     Engine  142  has a coolant system with a coolant outlet that connects to a radiator means, shown herein as radiator  158 . Radiator  158  is cooled by an electric fan  160  controlled by transducer  162 , in response to signals from the previously mentioned control means  26 . Radiator  158  can be bypassed by valve  164  in response to temperature regulator  166 , in order to keep the output temperature on line  167  at a regulated value, for example 130° F. Line  167  is shown passing through circuit  118 . As previously mentioned, circuit  118  circulates through vessel  80  (FIG.  2 ). 
     Coolant returning from circuit  118  passes through one side of heat exchanger  168  before returning to the coolant inlet of engine  142 . The other side of exchanger  168  receives oil from an oil outlet of engine  142  in order to cool the oil. Oil leaving exchanger  168  passes through oil filter  170  before returning to an oil inlet of engine  142 . Engine oil temperature is measured by transducer  172 , which forwards its measurement to the previously mentioned control means  26 . 
     A sample of the exhaust from exhaust outlet  146  is drawn from sampling line  174  and cooled by cooler  176 , a radiator positioned next to radiator  158  so both can be cooled by the same fan  160 . The cooled exhaust sample from cooler  176  is delivered to an exhaust sensor means  178 . 
     Sensor means  178  can include an infrared sensor tuned to detect a specific hydrocarbon in the exhaust, for example, propane. Sensor means  178  can also include other types of sensors including an oxygen sensor. In one embodiment, sensor means  178  forwarded its measurement through transducer  180  to the previously mentioned control means  26 , which acts to shut down engine  142  if the propane level exceeds 10%. Sensor means  178  includes an inlet chamber with internal spiral corrugations to swirl the incoming exhaust and thereby centrifugally remove water droplets. 
     Sensor means  178  acts as a continuous emission monitoring system (CEMS) for recording emissions from the engine  142 . The exhaust signal from transducer  180  is monitored by control means  26  to perform a five-minute average. These five-minute averages are later converted to a one hour average. This data is kept as a record that can be printed out through the control means  26  for the purpose of documenting emissions compliance. 
     The output shaft  182  of engine  142  drives an induction generator  184 . Various operating parameters of the generator  184  (speed, voltage, current, temperature, etc.) can be monitored by transducers such as transducer  186 , whose output signal is sent to previously mentioned control means  26 . Also, transducer  187  is a receiver that relays control signals from previously mentioned control means  26 , in order to control operating parameters of the generator  184 . Generator  184  can be rated to deliver 125 kW, at 200 A and 60 Hz, although this rating can vary depending upon the system capacity or other requirements. It is highly desirable to keep the rating of generator  184  such that it will demand only approximately 30% to 40% of the rated horsepower of the engine  142 . A phase sequence relay  188  connected to generator  184  performs the conventional phase sequencing under the control of transducer  190  in accordance with signals from the previously mentioned control means  26 . 
     Three phase output power is provided by generator  184  on lines  192  to a switching means  194 . Switching means  194  can interrupt lines  192  when the output from generator  184  is low (when the motor is running slowly or stopped), or when a thermal overload is detected. Also, switching means  194  can be switched off under the influence of receiver  196 , which is controlled by control means  26 . Power transferred through switching means  194  can be measured by power meter  198 , whose measurement is sent through transmitter  199  to previously mentioned control means  26 . 
     Referring to FIG. 1, the present system is shown installed atop a pair of lifting beams  202 . A first compartment  204  and second compartment  206  are positively pressurized by a fan  208 , which draws remote air down through stack  210  for that purpose. Compartment  204  contains a compressor  212  and air cooled condenser  214  that provide refrigerant to the previously mentioned pre-cooler  30  and finishing condenser  42  (FIGS.  1  and  2 ). Compartment  204  also contains the previously mentioned control means and other electrical equipment. 
     Compartment  206  contains previously mentioned engine  142  and generator  184 . The previously mentioned spark arrester  148  and muffler  150  are shown projecting above compartment  206 . The previously mentioned fan/radiator combination  158 ,  160  is shown mounted between compartment  206  and a third compartment  214 . Vapor source inlet  14  is shown feeding into compartment  214 , which contains the previously mentioned precooler  30  and finishing condenser  42 . The output of finishing condenser  42  is shown connecting to reheater  98 . 
     It will be appreciated that different physical arrangements may be implemented. For example, the various components can be arranged in a different spatial order. Alternatively, the various components can be arranged as separate modules that may be interconnected by appropriate ducts, pipes, lines, etc. 
     To facilitate an understanding of the principles associated with the foregoing apparatus, its operation will be briefly described. The system illustrated in FIG. 1 can be installed near a source of vapor that feeds inlet  14 . This vapor source can be vapors that are displaced when a gasoline tanker truck is filled, or vapors from some other process. Vapors produced in quantity can be stored in vapor holder  10  (FIG.  2 ), whose membrane  12  rises as the volume of stored vapors increases. As the stored vapors increase, eventually sensor  20  signals control means  26 , which then attempts to start engine  142 . 
     First, valve  128  (FIG. 3) is opened to cause transducer  126  to open the shut off valve  124 , thus providing a fuel path to engine  142 . Engine  142  is cranked for a predetermined amount of time, while engine speed is monitored. If engine speed does not rise to level indicating a start, the cranking ceases and shut off valve  124  is closed again. Control means  26  will repeat this procedure two more times, if necessary. 
     If the engine does start, a partial vacuum is drawn through engine intake  140 , which is communicated through conduit  99 , units  30  and  42 , and conduit  16 , which connects to vapor holder  10 . Consequently, vapor is drawn from holder  10 , and possibly from vapor source  14  into precooler  30 . Refrigerant circulating through coil  34  essentially causes the water vapor present in the vapor stream to condense and drain through pipe  52  to a position behind weir  58  in decanter  56 . Since the temperature of vapor leaving through conduit  40  is only about 35° F., the more volatile vapors are not condensed and are instead delivered to finishing condenser  42 . 
     In finishing condenser  42 , refrigerant circulating through coil  46  reduces the temperature to about −40° F. to condense the more volatile vapors. These condensed vapors drain through pipe  54  to a position behind weir  58  in decanter  56 . Accordingly, decanter  56  has a combination of water and liquid hydrocarbons behind weir  58 . Since the water is heavier, it descends and discharges through overflow pipe  60 . The incoming liquid hydrocarbons eventually spill over weir  58 . 
     At times, vapor source  14  will result from the displacement caused by the loading of a truck that normally handles such distillates as diesel fuel, home heating oil, kerosene, jet fuel, and other less volatile liquids. In that case, vapor holder  10  will have insufficient combustible vapor for eventually running engine  142 . For this reason, pump  70  will normally be activated whenever engine  142  is running. This causes a return of condensed hydrocarbons back to the saturating spray heads  74  in finishing condenser  42 . Spray heads  74  will atomize the returning liquid into a fine mist that can be easily combusted by engine  142 . In some embodiments pump  70  can be started manually, or when sensor  28  detects a low hydrocarbon vapor content in conduit  16 . In cases where a more volatile vapor is being handled (e.g., gasoline vapors) the saturating spray heads  74  may not be used at all. 
     The liquid medium in vessel  80  has been warmed by regulated electric heater  86 , or by the engine coolant flowing through circuit  118 . Whenever engine  142  is running, pump  92  also runs to circulate the heated medium in vessel  80  through warming coil  96  of reheater  98 . This increases the temperature of the rather cold vapor that would otherwise come from finishing condenser  42 . This warming of the output of condenser  42  allows for easier combustion in engine  142 . 
     After passing through flame arrester  120  (FIG.  3 ), fuel is mixed with air at modulating valve  132 . Modulating valve  132  determines the balance between fuel and air, based on the hydrocarbon measurements performed by fuel adjustment sensor  141 . Preferably, an infrared sensor in sensor  141  detects the level of butane in conduit  140  and compares that measurement to a target value. The difference from this target value is time integrated in control means  26  (FIG. 2) to produce a feedback signal that is applied through transducer  138  to the actuator  136  of the modulating valve  132 . 
     As the engine warms up, electric fan  160  will eventually be turned on by control means  26  when the temperature of engine  142  rises sufficiently, as measured by one of the transducers, such as transducer  158 . Engine coolant is kept at a temperature of 130° F., under the regulation of bypass valve  164 . This coolant flow is used to cool oil by circulating through heat exchanger  168 . The coolant flow diverted through circuit  118  also works to bring the temperature of the medium in vessel  80  (FIG. 2) to a temperature of about 130° F., without the need for electric heater  86 . 
     The exhaust from engine  142  flows from outlet  146 , mixes with cooling air from blower  156 , and passes through spark arrester  148  and muffler  150  before being discharged through stack  152 . 
     As engine  142  reaches a speed of at least 1825 rpm, generator  184  may now be able to produce a power output that is sufficient to deliver power to the utility lines  200 . If the operating parameters of generator  184 , as measured by transducer  186 , are favorable, switch means  194  may be closed by means of transducer  196 , under the control of control means  26 . Once switch means  194  is closed, the power output is measured by power meter  198 . 
     Since generator  184  is an induction generator, its power output can be increased if needed by increasing its speed. When increased speed is needed, control means  26  sends appropriate signals through transducer  161 , requesting the microprocessor-controlled throttle of engine  142  to increase the engine speed appropriately. In the preferred design, the throughput of vapor is limited by the capacity of engine  142 . Typically, precooler  30  and finishing condenser  42  can deliver more fuel (either as uncondensed vapor or by spraying condensed liquid by means of spray head  74  in condenser  42 ) than engine  142  can handle without reaching an unacceptably high speed. 
     Control means  26  has a timer for periodically scheduling a defrost cycle. In some embodiments, defrosting may occur for two hours once every day. When a defrost cycle is initiated, valves  102  and  110  (FIG. 2) are opened by transducers  104  and  112 , respectively, under the influence of control means  26 . Consequently, pump  92  causes heated medium to flow through coils  106 ,  108 ,  114 , and  116 . As a result, ice that may have formed at condenser  42  or decanter  56  will be melted. At the end of the defrost cycle, valves  102  and  110  will be closed by control means  26 . 
     Since the delivery of vapors from source  14  can be sporadic, storage in vapor holder  10  will allow a more continuous delivery of vapor through conduit  16  into precooler  30 . At times however, vapor holder  10  will be depleted to the point that level sensor  22  signals a low condition to control means  26 . In response, control means  26  sends a command through a transducer, such as transducer  161 , to stop engine  142  (FIG.  3 ). Also, control means  26  will send a signal through transducer  131  to close valve  128 , which causes actuator  126  to close the shut off valve  124  in order to stop the delivery of fuel to engine  142 . Shut off valve  124  can also close under emergency conditions. For example, vapor detector  164 , or a smoke or fire detector, may produce an alarm signal that closes valve  124  under such conditions. 
     It is appreciated that various modifications may be implemented with respect to the above described, preferred embodiment. While an internal combustion, piston engine is illustrated, in other embodiments other types of engines may be used, including turbine engines. While vapor condensation is performed in two stages herein, in other embodiments the condensation can be performed in a fewer or greater number of stages. While a specific topology is shown for routing a coolant and heating medium, in other embodiments that topology can be arranged in a variety of ways to include different serial or parallel connections, or to include independent systems. Specifically, in some embodiments the defrosting system can be separate from the engine coolant system. While a weir is shown for separating water, in other embodiments different types of separation systems can be used, including centrifugal separation. A plurality of independent control systems, each using one of a variety of technologies, can be used instead of the single control means disclosed herein. Also, a variety of valves may be used that are controlled in a variety of ways including hydraulically, electrically, pneumatically, etc. Furthermore, a greater or lesser number of operating parameters can be measured in comparison to those measured in this disclosure. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.