CN102269327B

CN102269327B - Recovery system and recovery method for vapor in storage tank

Info

Publication number: CN102269327B
Application number: CN 201110214125
Authority: CN
Inventors: 白改玲; 王红; 贾明; 蔡国勇; 陈令海
Original assignee: China Huanqiu Engineering Co Ltd
Current assignee: China National Petroleum Corp; China Huanqiu Contracting and Engineering Corp
Priority date: 2011-07-28
Filing date: 2011-07-28
Publication date: 2013-02-13
Anticipated expiration: 2031-07-28
Also published as: CN102269327A

Abstract

The invention relates to a recovery system and a recovery method for vapor in a storage tank. The recovery system comprises a compressor, a BOG (boil-off gas) output tube, a liquefier, a gathering tank, a recovery tube, a pressure-reducing valve, an external pump and an LPG (liquefied petroleum gas) output tube, wherein the compressor is used to compress a normal pressure BOG into a high pressure BOG; the BOG output tube is used to transport the normal pressure BOG to the compressor; the liquefier is used to liquefy the high pressure BOG into the high pressure LPG; the gathering tank is used to temporarily store the high pressure LPG; the recovery tube is used to transport the high pressure LPG in the gathering tank to a storage tank; the pressure-reducing valve with an adjustable opening is arranged on the high pressure LPG recovery tube and is used to reduce the pressure of the high pressure LPG; the external pump is used to send the normal pressure LPG to the liquefier so as to provide liquefaction cold capacity; and the LPG output tube is used to transport the normal pressure LPG in the storage tank to the external pump, wherein one end of the BOG output tube is communicated with a gas-phase space, the other end is connected with a BOG input end of the compressor, one end of the LPG recovery tube is connected with the gathering tank and the other end is arranged in the storage tank. The system and the method provided by the invention can reduce the energy consumption and the maintenance cost of BOG recycling and improve the working stability.

Description

Recovery system and recovery method for evaporation gas in storage tank

Technical Field

The invention relates to the technical field of storage and transportation of low-temperature Liquefied Petroleum Gas (LPG), in particular to a recovery system and a recovery method of boil-off gas in a storage tank.

Background

Liquefied Petroleum Gas (LPG) is typically stored in large storage tanks 101 as shown in figure 1. During the storage process, a part of the LPG1011 in the tank 101 absorbs the heat leaked from the environment and is gasified into boil-off gas (BOG) and the boil-off gas (BOG) is collected in the upper space of the tank 101, and the space formed by the BOG in the tank 101 is referred to as a gas phase space 1012 of the tank 101. Concurrent with the gasification of LPG is an increase in its volume (1 m)³The volume of BOG formed after LPG gasification becomes 200-230m³Left and right), which is a test for the storage tank 101 with the original atmospheric pressure, if the internal atmospheric pressure is too large, the storage tank 101 will be damaged, which may cause leakage of LPG and flammable BOG, and pose a great threat to the environment and safety. However, if the BOG is directly discharged into the atmosphere or transported to a flare system to be burnt, resource waste and environmental pollution are caused.

The prior art uses the BOG recovery system shown in fig. 1 to solve the problem of excessive air pressure in the storage tank 101 (mainly in the vapor space 1012). As shown in fig. 1, the prior art supplies the atmospheric BOG in the gas phase space 1012 to the compressor 102 for compression, and the generated high pressure BOG enters the air cooler 103, where the heat is taken away by the air entering the air cooler 103, so that the LPG, which is converted into liquid state by the high pressure BOG, is returned to the storage tank 101.

The prior art meets the BOG recycling requirement, but has the following problems: to achieve liquefaction of high pressure BOG as efficiently as possible, the compressor 102 typically compresses the atmospheric BOG to a very high pressure (above 1.7 MPaG), which places high demands on the power of the compressor, often requiring two or even more compressors to achieve this, and which also adversely affects the service life of the compressor 102. However, since the pressure of the high-pressure BOG is very high, the LPG returned to the storage tank 101 after being liquefied is in a normal pressure environment, such a large pressure difference change causes a large flash evaporation amount of the LPG returned from the air cooler 103 to the storage tank 101, that is, the BOG re-volatilized from the LPG due to a sudden pressure drop is large, and needs to be compressed again by the compressor 102, so in the prior art, the circulating workload of the compressor 102 is also large, which causes the capacity of the compressor 102 to be too large, and the energy consumption, the manufacturing cost and the service life to be short. In addition, the prior art liquefies the high-pressure BOG using the air cooler 103, which takes heat of the high-pressure BOG with flowing air, and thus has a very large size, a large floor space, and unstable operation of the compressor 102 due to the influence of ambient temperature. In summary, the BOG recovery technology provided by the prior art has high energy consumption and maintenance cost, and is unstable in operation.

Disclosure of Invention

The invention aims to provide a recovery system and a recovery method for boil-off gas in a storage tank, which can reduce the energy consumption and maintenance cost of BOG recovery and utilization and improve the working stability.

The technical scheme for solving the technical problems is as follows: a recovery system of boil-off gas BOG in a storage tank, wherein the storage tank is stored with liquefied petroleum gas LPG with normal pressure and low temperature, and the upper space in the storage tank is a gas phase space formed by the BOG with normal pressure; the system comprises: a compressor for compressing the BOG at normal pressure into a high-pressure BOG; a BOG output pipe for conveying the atmospheric BOG in the gas phase space to the compressor; a liquefier for liquefying the high-pressure BOG supplied from the compressor into high-pressure LPG; a collection tank temporarily storing the high pressure LPG from the liquefier; sending the high pressure LPG in the collection tank to an LPG recovery line in the storage tank; a pressure reducing valve with an adjustable opening degree, which is positioned on the LPG recovery pipe and changes the high-pressure LPG into normal-pressure LPG; an export pump for providing power to the atmospheric LPG for delivery to the liquefier to provide the refrigeration required for liquefaction to the liquefier; an LPG output pipe for conveying the normal pressure LPG in the storage tank to the outward conveying pump; wherein,

one end of the BOG output pipe is communicated with the gas phase space, and the other end of the BOG output pipe is connected with the BOG input end of the compressor; one end of the LPG recovery pipe is connected with the collection tank, and the other end of the LPG recovery pipe is positioned in the storage tank.

The invention has the beneficial effects that: the invention liquefies BOG by utilizing the cold energy of LPG which needs to be output in the storage tank, and the cold energy which can be provided by the LPG is more than that of air, so that the liquefier used by the invention has small volume, small floor area, higher liquefaction efficiency than that of an air cooler and stable work, thereby greatly reducing the power of a BOG compressor.

On the basis of the technical scheme, the invention can be further improved as follows:

further, still include: a liquid level detector for detecting a liquid level of the high-pressure LPG temporarily stored in the collection tank; and the first controller is respectively connected with the liquid level detector and the pressure reducing valve and controls the opening degree of the pressure reducing valve according to the liquid level height of the high-pressure LPG sent by the liquid level detector.

The LPG recovery device further comprises a switch valve which is positioned on the LPG recovery pipe and controls the on-off of the LPG recovery pipe through the switch state of the switch valve.

Further, still include: a booster pump for powering said high pressure LPG from said collection tank; a heater for heating the LPG in output pressure state supplied by the booster pump and outputting the LPG meeting the output condition; a booster output pipe for conveying the LPG in the output pressure state powered by the booster pump to the heater; and the LPG output flow control valve is positioned on the pressurizing output pipe and has adjustable opening.

Further, still include: a liquid level detector for detecting a liquid level of the high-pressure LPG temporarily stored in the collection tank; and a first controller connected to the liquid level detector and the LPG output flow control valve, respectively, for controlling the opening of the LPG output flow control valve according to the height of the high pressure LPG supplied from the liquid level detector.

And the liquefied petroleum gas output pipe is respectively connected with the pressurization output pipe and the liquefied petroleum gas output end of the liquefier so as to convey the LPG which loses the cold energy in the liquefier and is in the external pressure output state to the pressurization output pipe.

Further, the heater includes: a housing; a liquefied petroleum gas pipe passing through the housing; a steam pipe communicated with the casing and used for conveying steam to the inside of the casing; a condensate pipe communicated with the shell and used for discharging condensate in the shell; wherein,

one end of the liquefied petroleum gas pipe is connected with the pressurization output pipe, and the other end of the liquefied petroleum gas pipe outputs the LPG which meets the output condition;

the part of the liquefied petroleum gas pipe inside the shell is U-shaped, wherein the LPG in the output pressure state absorbs the heat of the steam inside the shell, and becomes the LPG which meets the output condition, and the steam inside the shell becomes the condensate.

Further, still include: a temperature detector for detecting a temperature of the LPG outputted from the LPG tube; the flow regulating valve is positioned on a pipeline connected with the liquefied petroleum gas pipe and the pressurizing output pipe, and the opening degree of the flow regulating valve is adjustable; and a second controller connected to the temperature detector and the flow regulating valve, respectively, for controlling the opening of the flow regulating valve according to the temperature of the LPG supplied from the temperature detector.

In addition, the invention also provides a method for recovering the boil-off gas BOG in the storage tank, which is based on the recovery system; the method comprises the following steps:

step 1: conveying the BOG with normal pressure in the storage tank to the compressor for compression, and conveying the obtained high-pressure BOG to the liquefier; the external delivery pump is used for providing power to deliver LPG in the storage tank to the liquefier;

step 2: the liquefier liquefies the high-pressure BOG by using the cold energy of the LPG, and transmits the obtained high-pressure LPG to the collecting tank for temporary storage;

and step 3: adjusting the opening degree of the pressure reducing valve on the LPG recovery pipe, reducing the pressure of the high-pressure LPG in the collection tank into normal-pressure LPG, and enabling the LPG to enter the storage tank along the LPG recovery pipe.

Further, the recovery system also comprises a switch valve which is positioned on the LPG recovery pipe and controls the on-off of the LPG recovery pipe through the on-off state of the switch valve; the method for adjusting the opening degree of the pressure reducing valve on the LPG recovery pipe in step 3 is as follows: detecting the liquid level of the high-pressure LPG temporarily stored in the collecting tank, judging whether the liquid level reaches a preset recovery height, if so, opening the switch valve, and adjusting the opening degree of the reducing valve according to the liquid level of the high-pressure LPG, otherwise, not opening the switch valve.

Further, the recycling system further includes: a booster pump for powering said high pressure LPG from said collection tank; a heater for heating the LPG in output pressure state supplied by the booster pump and outputting the LPG meeting the output condition; a booster output pipe for conveying the LPG in the output pressure state powered by the booster pump to the heater; the LPG output flow control valve is positioned on the pressurizing output pipe and has adjustable opening; characterized in that, after the step 2, the method further comprises the following steps:

step 31-1: opening the LPG output flow control valve on the pressurization output pipe;

step 31-2: the high-pressure LPG in the collecting tank is conveyed to the heater along the pressurization output pipe under the power of the pressurization pump;

step 31-3: the heater heats the high-pressure LPG and outputs the LPG which is obtained after heating and meets the output condition.

Further, the recovery system also comprises a liquefied petroleum gas output pipe which is respectively connected with the pressure boosting output pipe and the liquefied petroleum gas output end of the liquefier; characterized in that, the step 2 further comprises: when the LPG is required to be output, the liquefier conveys the LPG in the output pressure state which loses cold energy due to liquefaction into the pressurization output pipe, the LPG reaches the heater along the pressurization output pipe, the heater heats the LPG, and the heated LPG which meets the output condition is output.

Further, the method for executing the step 31-1 comprises the following steps: when the LPG is required to be output, the LPG output flow control valve on the pressurization output pipe is opened;

after said step 31-1, before said step 31-2, the method further comprises: step 31-1-1: and detecting and adjusting the opening of the LPG output flow control valve according to the liquid level of the high-pressure LPG which is temporarily stored in the collecting tank so as to ensure that the flow of the high-pressure LPG in the pressurization output pipe meets the requirement of the output flow of the LPG.

Further, the method for outputting the LPG meeting the output condition after heating in step 31-3 includes: outputting the heated LPG, detecting the temperature of the LPG output by the heater, judging whether the LPG is in the output temperature range, and if not, adjusting the flow of the LPG in the output pressure state entering the heater along the pressurization output pipe to enable the temperature of the LPG output by the heater to be in the output temperature range.

Drawings

FIG. 1 is a block diagram of a prior art BOG recovery system in a storage tank;

FIG. 2 is a block diagram of a BOG recovery system in a storage tank according to the present invention;

FIG. 3 is a flow chart of the BOG recovery method in the storage tank according to the present invention.

Detailed Description

The principles and features of this invention are described below in conjunction with the following drawings, which are set forth by way of illustration only and are not intended to limit the scope of the invention.

FIG. 2 is a block diagram of a BOG recovery system in a storage tank according to the present invention. As shown in fig. 2, the storage tank 201 stores Liquefied Petroleum Gas (LPG) at normal pressure and low temperature, and the upper space in the storage tank 201 is a gas phase space 2012 made of boil-off gas (BOG) at normal pressure, and since LPG is a low-temperature liquid, the temperature of the BOG that is gasified is much lower than the ambient temperature, and also belongs to a low-temperature substance. The recycling system shown in fig. 2 includes: a compressor 202 for compressing the atmospheric BOG into a high-pressure BOG; a BOG outlet line 221 for delivering atmospheric BOG from the vapor space 2012 to the compressor 202; a liquefier 203 for liquefying the high-pressure BOG supplied from the compressor 202 into high-pressure LPG; a collection tank 204 for temporarily storing the high-pressure LPG supplied from the liquefier 203; an LPG recovery pipe 205 for sending the high-pressure LPG in the collection tank 204 to the storage tank 201; a pressure reducing valve 206 with an adjustable opening degree, which is located on the LPG recovery pipe 205 and changes high-pressure LPG into normal-pressure LPG; an export pump 220 for providing power to the atmospheric LPG for delivery to the liquefier 203 to provide the refrigeration required for liquefaction to the liquefier 203, wherein the export pump 220 provides power to the atmospheric LPG by increasing the pressure of the atmospheric LPG to an export pressure (the export pressure is not a constant value but is determined according to the demand of the LPG user, but is typically around 0.1 MPaG); the normal pressure LPG in the storage tank 201 is delivered to the LPG output pipe 222 of the external delivery pump 220.

To meet the BOG and LPG delivery requirements described above, the BOG output line 221 is connected at one end to the vapor space 2012 and at the other end to the BOG input of the compressor 202, as shown in FIG. 2. An LPG recovery pipe 205 is connected at one end to the collection tank 204 and at the other end is located in the storage tank 201.

One end of the LPG output pipe 222 is connected to the input end of the external pump 220, and the other end is located below the level of the LPG stored in the storage tank 201. The output of the output pump 220 is connected via a line (not numbered in fig. 2) to the cold source (LPG) input of the liquefier. In the present invention, the position of the external transportation pump 220 can be located outside the storage tank 201 as shown in fig. 2, and in other embodiments, it can also be located below the liquid level of the LPG in the storage tank 201, so that the LPG can be transported to the liquefier 203 in the storage tank 201 by using the pipeline connecting the external transportation pump 220 and the liquefier 203, thereby reducing the influence of the LPG cold quantity received by the LPG output pipe 222 after receiving external heat, but this structure also increases the requirement for low temperature resistance of the external transportation pump 220.

In fig. 2, a line intersecting the storage tank 201 (e.g., a line connecting the LPG recovery pipe 205, the BOG output pipe 221, the LPG output pipe 222, or the external transfer pump 220 to the liquefier 203) may be disposed at the top of the storage tank 201, which is advantageous in preventing stress concentration of the storage tank 201 and improving safety of the storage tank 201.

In the present invention, the collection tank 204 is a device for collecting the high-pressure LPG liquefied by the liquefier 203, and has a certain space therein, so that a certain amount of high-pressure LPG can be temporarily stored. Here, the pressure of the high pressure LPG is generally lower than the delivery pressure. The collecting tank 204 is communicated with the storage tank 201 through an LPG recovery pipe 205, and the storage tank 201 stores LPG under normal pressure, so the collecting tank 204 in the present invention also serves as a buffer device between high-pressure LPG and normal-pressure LPG, as shown in fig. 2, an on-off valve 224 is further provided on the LPG recovery pipe 205 for controlling the on-off of the LPG recovery pipe 205 according to its own on-off state, and the switching of the on-off state can be controlled manually when there is an external LPG delivery demand, or can be controlled by a controller (e.g. the first controller 208 in fig. 2). The high pressure LPG in the collection tank 204 is isolated from the normal pressure LPG in the storage tank 201 when the on-off valve 224 is in the closed state, and since the pressure reducing valve 206 is always in the open state, only the opening degree is adjustable, when the on-off valve 224 is in the open state, the pressure of the high pressure LPG in the collection tank 204 is reduced to the normal pressure and enters the storage tank 201 along the LPG recovery pipe 205, and the opening degree of the pressure reducing valve 206 determines the flow rate of LPG in the LPG recovery pipe 205. The present invention utilizes the accumulator tank 204 and the pressure reducing valve 206 as buffering to solve the pressure releasing problem of LPG effectively.

Because the source of the cold energy for liquefying the high-pressure BOG by the liquefier 203 in the present invention is LPG in the storage tank 201 sent by the external pump 220, and the cold energy of the cold source is more, the liquefaction efficiency of the liquefier 203 is high and stable, and the volume and the floor area are smaller, which greatly reduces the power of the compressor 202 of the present invention, as shown in fig. 1 in the prior art, the compressor 102 needs to compress the atmospheric BOG to the high pressure of 1.7MPaG to meet the liquefaction requirement of the air cooler 103, while in the present invention shown in fig. 2, the compressor 202 only needs to compress the atmospheric BOG to the high pressure of 0.4MPaG to meet the liquefaction requirement of the liquefier 203, so the number, capacity, power and energy consumption of the compressor 202 in the present invention are much smaller than those in the prior art.

Therefore, the invention liquefies BOG by utilizing the cold energy of LPG which needs to be output in the storage tank, the provided cold energy is more than that of air, so the liquefier used by the invention has small volume, small floor area, higher liquefaction efficiency than that of an air cooler and stable work, thereby greatly reducing the power of a BOG compressor.

As shown in fig. 2, the collection tank 204 is connected to a booster pump 209 via an unnumbered line delivering high pressure LPG, and the recovery system further comprises: a booster pump 209 for supplying power to the high-pressure LPG supplied from the collection tank 204, wherein the booster pump 209 supplies power to the high-pressure LPG in order to increase the pressure thereof, that is, the booster pump 209 is a device for increasing the pressure of a liquid substance (in the present invention, the high-pressure LPG), and the pressure of the LPG is increased to an output pressure after the high-pressure LPG is pressurized; a heater (not numbered in the figure) for heating the LPG in the output pressure state sent by the booster pump 209 after being powered and obtaining the LPG meeting the output condition; an output pressure pipe 210 for delivering the LPG in an output pressure state obtained by powering the booster pump 209 to the heater; an LPG output flow control valve 211 with adjustable opening degree is positioned on the pressurizing output pipe 210. This configuration provides another way to utilize the high-pressure LPG stored in the collection tank 204, i.e. when the external user has a demand for export LPG (such as propane, butane, etc.) (the demand includes flow demand and temperature demand), the high-pressure LPG stored in the collection tank 204 can be preferentially heated to meet the export of LPG, so as to reduce the amount of BOG that needs to be recompressed by the compressor 202 and is volatilized again (called flash evaporation) after entering the storage tank 201, thereby reducing the energy consumption of the present invention.

In the present invention, the boost pump 209 is a device for supplying power to the high pressure LPG, since the heater is an LPG output that heats the LPG in the export pressure state to meet the export demand (including the export pressure demand and the temperature demand), which results in the lack of power of the export LPG in the collection tank 204, and thus the boost pump 209 is required to supply power to the high pressure LPG, so that the high pressure LPG meets the export pressure demand and continuously enters the heater.

There is also a gas phase space above the liquid level of the high pressure LPG stored in the collection tank 204, which gas phase space is composed of the high pressure BOG flashed off from the high pressure LPG, since the pressure of the high pressure BOG in the vapor space of collection tank 204 is different from the pressure of the atmospheric BOG in the vapor space 2012 of storage tank 201, if there is no high pressure LPG in the collection tank 204, then because both the on-off valve 224 and the pressure reducing valve 206 are liquid phase valves, without limitation, the gas, and thus the high pressure BOG in the vapor space of the collection tank 204 will enter the storage tank 201 along the LPG recovery line 205, causing the vapor space of the storage tank 201 to be over pressurized, damage to the tank 201 is done so that the level of high pressure LPG in the collection tank 204 cannot be too low, and, of course, due to the limited volume of the collection tank 204, the level of the high pressure LPG also has a maximum value, i.e., the level of high pressure LPG in the collection tank 204, needs to be limited to a range of values.

The booster pump 209 can supply power to the high pressure LPG, and in case of sufficient amount of high pressure LPG stored in the collecting tank 204, the booster pump 209 can continuously supply the high pressure LPG, but cannot control the output flow rate of the high pressure LPG, and in particular cannot stop the output in case of insufficient amount of high pressure LPG stored in the collecting tank 204, so the present invention provides an LPG output flow control valve 211 on the booster output pipe 210, and when the LPG output flow control valve 211 is in an open state, the booster pump 209 can pressurize and deliver the high pressure LPG stored in the collecting tank 204 to the heater, and when the LPG output flow control valve 211 is in a closed state, the high pressure LPG in the collecting tank 204 cannot enter the heater, and in addition, the opening degree of the LPG output flow control valve 211 can also control the flow rate of LPG delivered in the booster output pipe 210 to meet the output demand.

Under the condition that the high-pressure LPG temporarily stored in the collecting tank 204 cannot meet the output requirement, the invention can also output the normal-pressure LPG stored in the storage tank 201 in a pressurizing way, the pressurizing work is completed by the output pump 220, the pressure of the pressurized LPG reaches the output pressure, the LPG in the output pressure state enters the pressurizing output pipe 210 along the output pipeline (the output pipeline is not shown in figure 2) between the output pump and the pressurizing output pipe 210, is mixed with the LPG in the output pressure state pressurized by the pressurizing pump 209, and finally enters the heater for heating and output. Of course, in the case that the amount of the high-pressure LPG stored in the collection tank 204 temporarily can meet the demand of export, the present invention utilizes the booster pump 209 to preferentially output the high-pressure LPG stored in the collection tank 204, so as to save the energy required by recompression and liquefaction of the BOG flashed off after entering the storage tank 201. In both cases, the export pump 220 either pressurizes (to export pressure) the atmospheric LPG in the storage tank 201 for export to the pressurization export line 210, or pressurizes (to export pressure) the atmospheric LPG in the storage tank 201 for export to the liquefier, providing the refrigeration required for liquefaction, and thus is always in operation.

For the purpose of maintaining the level of the high-pressure LPG in the collection tank 204, the recovery system proposed by the present invention further comprises: a liquid level detector 207 for detecting the liquid level of the high-pressure LPG temporarily stored in the collection tank 204; a first controller 208.

As shown in fig. 2, the first controller 208 is connected to the liquid level detector 207 and receives the level of high pressure LPG in the collection tank 204. The first controller 208 is also connected to the pressure reducing valve 206, so that the opening degree of the pressure reducing valve 206 can be automatically controlled according to the liquid level of the high-pressure LPG supplied from the liquid level detector 207, thereby controlling the pressure reduction of the high-pressure LPG in the collection tank 204 and the flow rate of the high-pressure LPG into the storage tank 201.

Further, the first controller 208 is connected to an output flow control valve 211 for LPG, so that the first controller 208 can control the opening, closing and opening of the output flow control valve 211 for LPG according to the level of the high-pressure LPG supplied from the level detector 207, thereby controlling the flow of the high-pressure LPG supplied to the heater through the booster output pipe 210.

In fig. 2, the liquefier 203 may increase the temperature of LPG in the export pressure state (the temperature is much lower than the ambient temperature although the refrigeration is released) in which the refrigeration is lost by the participation in liquefaction after the high-pressure BOG is liquefied by using the refrigeration of the LPG in the storage tank 201, and may deliver the LPG to a heater for heating and export the LPG after the export temperature demand is reached. Therefore, the recycling system of the present invention may further include a LPG gas output pipe 223 in fig. 2, which is respectively connected to the pressurized output pipe 210 and the LPG gas output end of the liquefier 203, so as to deliver the LPG in the external pressure state without cooling capacity in the liquefier 203 to the pressurized output pipe 210, and then enter the heater through the pressurized output pipe 210 to be heated, and become the LPG output with the temperature according to the external demand.

As shown in fig. 2, the connection position of the LPG gas output pipe 223 and the pressure-increasing output pipe 210 is out of the limit range of the LPG output flow control valve 211 to the pressure-increasing output pipe 210, which means that the LPG in the output pressure state output by the liquefier 203 has a higher priority than the LPG in the high-pressure LPG in the collection tank 204 in the level of heating by the heater for output.

Thus, when the output is needed, three schemes can be used for realizing the output of the LPG: the first scheme is that LPG in the output pressure state which is in the liquefaction work of the liquefier 203 and loses cold energy is conveyed to a heater through a pressurization output pipe 210 for heating output, and the priority level of the scheme is higher than that of the other two schemes; the second scheme is that an LPG output flow control valve 211 is opened, a booster pump 209 provides power, high-pressure LPG temporarily stored in a collecting tank 204 is pressurized and conveyed to a heater for heating output, the priority level of the scheme is higher than that of the third scheme and lower than that of the first scheme, therefore, when the scheme is used for outputting the LPG, the LPG conveyed by a booster output pipe 210 can be all LPG from the collecting tank 204 (the liquefier 203 does not work at this time, and therefore LPG in an output pressure state losing cold energy is not output), and can also be a mixture of the LPG from the collecting tank 204 and the LPG losing cold energy from the liquefier 203 (the liquefier 203 works at this time, and LPG losing cold energy can be output); in the third scheme, the normal pressure LPG in the storage tank 201 is pressurized by the external delivery pump 220, then is directly conveyed to the pressurization output pipe 210 through a conveying pipeline (the conveying pipeline is not shown in fig. 2), and is further conveyed to the heater through the pressurization output pipe 210 for heating and outputting, wherein one end of the conveying pipeline in the scheme is connected with the outlet of the external delivery pump 220, and the other end of the conveying pipeline is connected with the pressurization output pipe 210.

The heater in the present invention may adopt a structure as shown in fig. 2, which includes: a housing 219; a liquefied petroleum gas pipe passing through the housing 219; a steam pipe 217 communicating with the casing 219 and supplying steam to the inside of the casing 219; a condensate pipe 218 communicating with the casing 219 and discharging condensate inside the casing 219; the steam inside the housing 219 heats the LPG in the liquefied petroleum gas pipe to raise its temperature to the export temperature requirement, but loses its heat and turns into a condensate, thus the heater is a shell-and-tube heater. In the case of the heater, it is preferable that,

one end (one end marked by 213) of the liquefied petroleum gas pipe is connected with the pressure-increasing output pipe 210, and the other end (one end marked by 214) outputs LPG meeting the output condition;

the portion of the lpg pipe inside the housing 219 has a U-shape, and is in contact with the vapor inside the housing 219 to perform heat exchange, and in order to improve the heat exchange efficiency, the portion may have a plurality of U-shapes connected in series. The LPG in the export pressure state flowing in the LPG pipe (for example, the LPG from the collection tank 204, or the mixture of the LPG from the collection tank 204 and the LPG losing cold energy from the liquefier 203, or the LPG from the storage tank 201) absorbs the heat of the steam, becomes the LPG output according with the export conditions (including the flow export condition and the temperature export condition), and makes the steam become the condensate due to losing heat; as shown in fig. 2, the steam pipe 217 may be positioned at an upper side of the casing 219, wherein steam (e.g., water vapor) flowing therein enters the casing 219 along the steam pipe 217 and releases heat to LPG in the LPG pipe, and after becoming condensed (e.g., liquid water) by itself, may be output from a condensate pipe 218 connected to a lower portion of the casing.

As shown in fig. 2, the recycling system further includes: a temperature detector 215, a flow regulating valve 212, and a second controller 216. Wherein the temperature detector 215 for detecting the temperature of LPG outputted from the liquefied petroleum gas pipe (line indicated by 214) is connected to the second controller 216, and the detected temperature of LPG is transmitted to the second controller 216. The flow regulating valve 212 is positioned on a pipeline (a pipeline at one end marked by 213) connecting the liquefied petroleum gas pipe and the pressurizing output pipe 210, and the opening degree is adjustable; the second controller 216 is connected to the flow control valve 212 in addition to the temperature detector 215, so that the second controller 216 can control the opening degree of the flow control valve 212 according to the temperature of the LPG supplied from the temperature detector 215, thereby controlling the flow rate of the LPG in the output pressure state from the pressurized output pipe 210 to the LPG pipe (line at one end indicated by 213) to ensure that the temperature of the LPG in the output meets the output requirement. This is because the LPG entering the LPG pipe (line at one end indicated by 213) from the pressurized output pipe 210 has a low temperature and is not in the temperature range of the external transportation, and thus needs to be heated, if the flow rate of the LPG entering the LPG pipe is too high, the temperature of the LPG output from the heater cannot reach the external transportation standard because the heating efficiency of the heater cannot follow up, and at this time, the flow rate of the LPG entering the LPG pipe from the pressurized output pipe 210 is limited, so as to reduce the input of low-temperature substances, thereby increasing the temperature of the LPG output and making it meet the external transportation temperature requirement.

In the present invention, all the devices (such as the external delivery pump 220, the compressor 202, the liquefier 203, the collection tank 204, the storage tank 201, the booster pump 209, etc.) and the devices (such as the pressure reducing valve 206, the LPG output flow control valve 211, the flow control valve 212, the switch valve 224, etc.) and the pipelines (such as the LPG output pipe 222, the BOG output pipe 221, the LPG recovery pipe 205, the booster output pipe 210, the pipeline for delivering LPG and BOG mixture from the petroleum gas pipe 213, and other pipelines which are not numbered in fig. 2 but deliver LPG or low-temperature BOG, and the external delivery pipelines which are not shown in fig. 2 but can be located between the external delivery pump 220 and the booster output pipe 210) which are in contact with the low-temperature substances such as LPG and BOG which is not heated by the heater are made of low-temperature resistant materials (such as low-temperature carbon steel, stainless steel, etc.) or special. The devices or pipelines (such as the shell 219 of the heater, the steam pipe 217, and the part of the LPG pipe from which heated LPG is output) contacting the high-temperature substance are made of high-temperature resistant materials.

The pressure reducing valve 206, the LPG output flow control valve 211 and the flow control valve 212 according to the present invention may be implemented by pneumatic or electric valves, and the on-off valve 224 may be implemented by manual valves in addition to pneumatic or electric valves. The temperature detector 215 may be implemented by a temperature sensor, the liquid level detector 207 may be implemented by a height sensor, and the first controller 208 and the second controller 216 may be implemented by circuits and devices having calculation and judgment functions, such as a microprocessor, a CPU, a programmable logic device, and the like.

Based on the recovery system provided by the invention, the invention also provides a recovery method of the evaporation gas in the storage tank, and fig. 3 is a flow chart of the method. As shown in fig. 3, the method includes:

step 1: conveying the BOG at normal pressure in the storage tank to a compressor for compression, and conveying the obtained high-pressure BOG to a liquefier; the LPG in the storage tank is conveyed to the liquefier by the power of the outward conveying pump.

Here, the storage tank is a device for storing low-temperature and normal-pressure LPG, and since the low-temperature LPG absorbs the volatilization of ambient heat, a gas phase space composed of BOG is formed in the upper space in the storage tank, and the pressure of the gas phase space must be kept at normal pressure, which may be slightly greater than atmospheric pressure, to ensure that the external air does not enter the storage tank, thereby preventing the flammable BOG from being burned and exploded. Therefore, the present invention feeds atmospheric BOG in the gas phase space of the storage tank to a compressor for compression and feeds the resulting high pressure BOG (about 0.4MPaG in pressure) to a liquefier for liquefaction.

The invention liquefies high-pressure BOG by using the cold energy of the low-temperature LPG stored in the storage tank, and the cold energy storage amount of the LPG is large, and the liquefaction efficiency is high and stable, thereby being a high-quality cold source. The invention uses LPG to liquefy BOG, which can greatly reduce the requirements for power, capacity and energy consumption of BOG compressor, thus being beneficial to improving the service life of the compressor and reducing the maintenance cost of the invention.

The external transfer pump of the present invention is a device for transferring LPG in the storage tank to the liquefier, and provides power to the LPG so that the LPG can be transferred to the liquefier at a stable flow rate.

Step 2: the liquefier liquefies the high-pressure BOG by using the cold energy of the LPG, and conveys the obtained high-pressure LPG to the collecting tank for temporary storage.

The liquefier is a device that performs heat (and, correspondingly, cold) exchange, and inside the liquefier, the cold of the LPG is released to the high-pressure BOG, or the heat of the high-pressure BOG is conducted to the LPG, so that the LPG is heated to a temperature close to the output temperature, and the high-pressure BOG is liquefied into the high-pressure LPG.

The collecting tank is a device for temporarily storing the high-pressure LPG, and meanwhile, the high-pressure LPG in the collecting tank is conveyed to the LPG which becomes the normal pressure in the storage tank, so that the collecting tank can play a role in buffering pressure change of the LPG.

And step 3: and adjusting the opening degree of a pressure reducing valve on the LPG recovery pipe, reducing the pressure of the high-pressure LPG in the collecting tank into normal-pressure LPG, and enabling the LPG to enter the storage tank along the LPG recovery pipe.

Here, the LPG recovery pipe is a line for transferring the high-pressure LPG in the collection tank into the storage tank, on which a pressure reducing valve is provided, which separates the high-pressure LPG in the collection tank from the normal-pressure LPG in the storage tank. The opening of the pressure reducing valve means the conduction of an LPG recovery pipe, and since one end of the LPG recovery pipe is connected with the collecting tank (storing high-pressure LPG and high-pressure BOG) and the other end is connected with the inside of the storage tank (storing normal-pressure LPG and normal-pressure BOG), the conduction means that the LPG is changed from the high-pressure state in the collecting tank to the normal-pressure state in the storage tank, and meanwhile the LPG changed into the normal-pressure state enters the storage tank through the delivery of the LPG recovery pipe.

The recovery system shown in fig. 2 further includes an on-off valve 224 on the LPG recovery pipe 205 for controlling the on-off of the LPG recovery pipe 205 according to its on-off state, so that step 3 herein is performed when the on-off valve 224 is in an on state, and in step 3, the method for adjusting the opening degree of the pressure reducing valve on the LPG recovery pipe may be automatic opening to improve the degree of automation of the present invention and reduce the manual usage rate, and the automatic opening mode is: the liquid level detector 207 in fig. 2 is used for detecting the liquid level of the high-pressure LPG temporarily stored in the collection tank, and sending the detection result to the first controller 208, the first controller 208 judges whether the liquid level of the high-pressure LPG in the collection tank reaches the preset recovery height, if so, the liquid level of the high-pressure LPG in the collection tank is higher than the minimum value, the internal high-pressure BOG can be ensured not to enter the storage tank along the LPG recovery pipe, so the switch valve is opened, and the opening degree of the reducing valve on the LPG recovery pipe is controlled and adjusted by the first controller 208 according to the liquid level of the high-pressure LPG in the collection tank 204, so that the high-pressure LPG in the collection tank is reduced to the normal-pressure LPG and enters the storage tank along the LPG recovery pipe at the set flow rate, otherwise, the switch valve on the LPG recovery pipe is not opened.

The recycling system shown in fig. 2 further includes: a booster pump 209 for powering the high pressure LPG supplied from the collection tank 204; a heater for heating the LPG in the output pressure state sent by the booster pump 209 after being powered and outputting the obtained LPG meeting the output condition; an output pressure boosting pipe 210 for delivering the LPG in the output pressure state after being powered by the booster pump 209 to the heater; an LPG output flow control valve 211 with adjustable opening degree is positioned on the pressurizing output pipe 210. Thus, after step 2 above, the method further comprises:

step 31-1: the LPG output flow control valve 211 on the booster output line 210 is opened.

This step means that the booster output line 210 in figure 2 is routed from the booster pump 209 to the heater, so that the booster pump 209 can power the high pressure LPG in the holding tank 204 for delivery to the heater.

Step 31-2: the high pressure LPG in the holding tank 204 is supplied to the heater along a pressurized output line 210 powered by a booster pump 209.

The second option of heating the export petroleum gas described above can be implemented using steps 31-1 through 31-3 herein.

Here, the steps 31-1 to 31-3 are performed after the step 2, and may be performed simultaneously with the step 3, or may be performed first and then, that is, the steps 31-1 to 31-3 may be performed before or after the step 3.

The recovery system further comprises a liquefied petroleum gas output pipe 223 which is respectively connected with the pressure-increasing output pipe 210 and the liquefied petroleum gas output end of the liquefier 203; thus, the step 2 may further include: when there is a demand for external transportation of LPG, the liquefier 203 delivers the LPG in the external transportation pressure state that loses cold energy due to liquefaction to the pressure boost output pipe 210 through the LPG output pipe 223, so that the LPG is delivered to the heater along the pressure boost output pipe 210, the heater heats the LPG, and the heated LPG conforming to the external transportation condition is output. Thus, the first scheme of realizing the highest priority for outputting LPG can be realized.

Of course, the present invention can also utilize an output line (not shown in fig. 2) connecting the output pump 220 and the pressure-increasing output pipe 210 to pressurize and convey the LPG stored in the storage tank 201 to the heater for heating, so as to obtain the LPG meeting the output requirement to be conveyed to the user, thereby implementing the third scheme of petroleum gas output.

The heater of the present invention can adopt the structure shown in fig. 2, and thus the method for heating to obtain LPG is as follows: LPG flowing in the LPG pipe in the pressure state is heated by the steam inside the casing 219 to be converted into LPG meeting the temperature and flow output conditions, and the LPG is output from one end of the LPG pipe indicated by 214.

The method for executing the step 31-1 comprises the following steps: when the output of LPG is needed, the LPG output flow control valve 211 on the pressurization output pipe 210 is opened.

The LPG output demand in the invention means that a user has a demand for LPG, and the demand includes a demand in terms of flow rate and a demand in terms of temperature, so that the flow rate of LPG output by the heater needs to meet the demand in terms of flow rate, the temperature of LPG also needs to meet the demand in terms of temperature, and whichever demand is not met, the output product is unqualified.

Thus, after step 31-1, before step 31-2, the method further comprises: step 31-1-1: the liquid level detector 207 in fig. 2 is used to detect the liquid level of the high-pressure LPG temporarily stored in the collection tank 204, and the detected result is sent to the first controller 208, and the first controller 208 controls and adjusts the opening degree of the LPG output flow control valve 211, so as to control the flow rate of the LPG in the pressurized output pipe 210 in the output pressure state, and ensure that the flow rate of the LPG in the pressurized output pipe 210 meets the above requirements on the output flow rate of the LPG.

In order to meet the temperature requirement of the output LPG, the invention also provides a temperature detector 215 and a second controller 216 in the recovery system shown in FIG. 2, and the method for outputting the heated LPG meeting the output condition in step 31-3 is as follows: the LPG obtained by heating the heater by the liquefied petroleum gas pipe (the pipeline at one end indicated by 214 in fig. 2) is output, the temperature of the LPG output by the heater is detected by the temperature detector 215, the detection result is sent to the second controller 216, the second controller 216 determines whether the temperature is in the output temperature range (i.e. whether the temperature meets the requirement of the user on the temperature of the output LPG), if not, the second controller 216 controls the flow of the LPG in the output pressure state to enter the heater (i.e. the pipeline at one end indicated by 213 on the petroleum gas pipe) along the pressurization output pipe 210, so that the temperature of the LPG output by the heater is in the output temperature range, thereby meeting the requirement of the output LPG in the temperature.

It can be seen that the present invention has the following advantages:

(1) the invention liquefies BOG by utilizing the cold energy of LPG which needs to be output in the storage tank, and the cold energy which can be provided by the LPG is more than that of air, so that the liquefier used by the invention has small volume, small floor area, higher liquefaction efficiency than that of an air cooler and stable work, thereby greatly reducing the power of a BOG compressor.

(2) The invention fully utilizes the LPG in the output pressure state output by the liquefier and the LPG from the collecting tank for output, reduces the steam consumption and the load of the heater and further reduces the energy consumption of the invention.

(3) In the invention, the first controller is used for automatically controlling the opening degrees of the pressure reducing valve and the LPG output flow control valve according to the liquid level of the high-pressure LPG in the collecting tank, so that the LPG is recovered when the liquid level of the high-pressure LPG in the collecting tank reaches the preset output liquid level, the pressure buffering problem of the LPG is solved, the comprehensive utilization of the high-pressure LPG in the collecting tank is realized, the energy consumption of the invention is reduced, and the output petroleum gas meets the requirements of users in the flow aspect; meanwhile, the second controller is used for controlling the opening degree of the flow regulating valve according to the temperature of the output LPG, so that the output LPG temperature control system enables the output LPG to constantly meet the requirements of users on both temperature and flow, and ensures the quality of output products.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.

Claims

1. A recovery system of boil-off gas BOG in a storage tank, wherein the storage tank is stored with liquefied petroleum gas LPG with normal pressure and low temperature, and the upper space in the storage tank is a gas phase space formed by the BOG with normal pressure; characterized in that the system comprises: a compressor for compressing the BOG at normal pressure into a high-pressure BOG; a BOG output pipe for conveying the atmospheric BOG in the gas phase space to the compressor; a liquefier for liquefying the high-pressure BOG supplied from the compressor into high-pressure LPG; a collection tank temporarily storing the high pressure LPG from the liquefier; sending the high pressure LPG in the collection tank to an LPG recovery line in the storage tank; a pressure reducing valve with an adjustable opening degree, which is positioned on the LPG recovery pipe and changes the high-pressure LPG into normal-pressure LPG; an export pump for providing power to the atmospheric LPG for delivery to the liquefier to provide the refrigeration required for liquefaction to the liquefier; an LPG output pipe for conveying the normal pressure LPG in the storage tank to the outward conveying pump; wherein,

2. The system of claim 1, further comprising: a liquid level detector for detecting a liquid level of the high-pressure LPG temporarily stored in the collection tank; and the first controller is respectively connected with the liquid level detector and the pressure reducing valve and controls the opening degree of the pressure reducing valve according to the liquid level height of the high-pressure LPG sent by the liquid level detector.

3. The system of claim 1 or 2, further comprising an on-off valve on the LPG recovery pipe to control the on-off of the LPG recovery pipe by its on-off state.

4. The system of claim 1, further comprising: a booster pump for powering said high pressure LPG from said collection tank; a heater for heating the LPG in output pressure state supplied by the booster pump and outputting the LPG meeting the output condition; a booster output pipe for conveying the LPG in the output pressure state powered by the booster pump to the heater; and the LPG output flow control valve is positioned on the pressurizing output pipe and has adjustable opening.

5. The system of claim 4, further comprising: a liquid level detector for detecting a liquid level of the high-pressure LPG temporarily stored in the collection tank; and a first controller connected to the liquid level detector and the LPG output flow control valve, respectively, for controlling the opening of the LPG output flow control valve according to the height of the high pressure LPG supplied from the liquid level detector.

6. The system according to claim 4 or 5, further comprising a liquefied petroleum gas output pipe connected to the pressurization output pipe and the liquefied petroleum gas output terminal of the liquefier, respectively, to deliver the LPG in the cold-depleted output pressure state in the liquefier to the pressurization output pipe.

7. The system of claim 6, wherein the heater comprises: a housing; a liquefied petroleum gas pipe passing through the housing; a steam pipe communicated with the casing and used for conveying steam to the inside of the casing; a condensate pipe communicated with the shell and used for discharging condensate in the shell; wherein,

8. The system of claim 7, further comprising: a temperature detector for detecting a temperature of the LPG outputted from the LPG tube; the flow regulating valve is positioned on a pipeline connected with the liquefied petroleum gas pipe and the pressurizing output pipe, and the opening degree of the flow regulating valve is adjustable; and a second controller connected to the temperature detector and the flow regulating valve, respectively, for controlling the opening of the flow regulating valve according to the temperature of the LPG supplied from the temperature detector.

9. A method for recovering boil-off gas BOG in a storage tank, which is based on the recovery system according to claim 1; the method is characterized by comprising the following steps:

10. The method of claim 9, wherein the recovery system further comprises an on-off valve on the LPG recovery pipe to control the on-off of the LPG recovery pipe by its on-off state; the method for adjusting the opening degree of the pressure reducing valve on the LPG recovery pipe in step 3 is as follows: detecting the liquid level of the high-pressure LPG temporarily stored in the collecting tank, judging whether the liquid level reaches a preset recovery height, if so, opening the switch valve, and adjusting the opening degree of the reducing valve according to the liquid level of the high-pressure LPG, otherwise, not opening the switch valve.

11. The method of claim 9, the recovery system further comprising: a booster pump for powering said high pressure LPG from said collection tank; a heater for heating the LPG in output pressure state supplied by the booster pump and outputting the LPG meeting the output condition; a booster output pipe for conveying the LPG in the output pressure state powered by the booster pump to the heater; the LPG output flow control valve is positioned on the pressurizing output pipe and has adjustable opening; characterized in that, after the step 2, the method further comprises the following steps:

12. The method of claim 11, the recovery system further comprising a liquefied petroleum gas output line connected to the pressurized output line and the liquefied petroleum gas output of the liquefier, respectively; characterized in that, the step 2 further comprises: when the LPG is required to be output, the liquefier conveys the LPG in the output pressure state which loses cold energy due to liquefaction into the pressurization output pipe, the LPG reaches the heater along the pressurization output pipe, the heater heats the LPG, and the heated LPG which meets the output condition is output.

13. The method of claim 11, wherein the method of performing step 31-1 is: when the LPG is required to be output, the LPG output flow control valve on the pressurization output pipe is opened;

14. The method as claimed in claim 11, wherein the output of the heated LPG according to the export condition in step 31-3 is as follows: outputting the heated LPG, detecting the temperature of the LPG output by the heater, judging whether the LPG is in the output temperature range, and if not, adjusting the flow of the LPG in the output pressure state entering the heater along the pressurization output pipe to enable the temperature of the LPG output by the heater to be in the output temperature range.