GB2098874A - Condensation method and apparatus - Google Patents

Abstract

A storage tank 2, containing a volatile liquid such as petrol, has a vapour recovery pipeline 32 with a valve 34 in it. The pipeline 32 terminates at the top of a condenser 38 fitted with a spray head 40 for spraying liquid nitrogen into the condenser 38. The flow of liquid nitrogen to the spray head 40 is controlled by valve 44 which opens upon the pressure in the ullage space 6 of the vessel 2, as sensed by pressure sensor 30, reaching a first chosen value. Thus, the condenser is pre-cooled. On the pressure in the ullage space 6 reaching a second value, higher than the first valve 34 opens and vapour flows through the pipeline 32 into the condenser 38 where it is condensed by the liquid nitrogen. The resulting condensate is returned to the tank 2 via a phase separator 52. <IMAGE>

Description

SPECIFICATION Condensation method and apparatus This invention relates to a method and apparatus for condensing vapour evolved from a volatile liquid into a gas space at the head of a storage vessel containing the volatile liquid.

It is common practice in the chemical and petroleum industries to store volatile liquids, typically organic, in storage tanks fitted with a pressure relief valve and a vacuum relief valve. When liquid is discharged from the tank the pressure in the ullage space of the tank falls. The vacuum relief valve (or demand) is set so as to open the ullage space to the atmosphere should the pressure in the ullage space approach ambient pressure. Air is thus drawn into the ullage space through the vacuum relief valve and thus a negative pressure is not created in the head space.

Once a discharging operation has been finished in-leak of heat into the storage tank will cause some of the volatile liquid to vaporise.

The formed vapour enters the ullage space and the pressure therein rises. In order to prevent the creation in the ullage space of a pressure so great as to present a hazard to the fabric of the tank, the pressure relief valve opens before the pressure in the tank reaches a hazardous level. Thus, the air-vapour mixture in the ullage space is vented from the vessel. The pressure relief valve will also open during an operation in which the tank is filled with volatile liquid as this will result in a diminution of the ullage space with a concomittant pressure rise. Owing to diurnal variations in ambient temperature and repeated filling and discharging operations, a large volume of the vapour of the volatile liquid stored in such tanks is discharged to the atmosphere every day, particularly in the summer. This is undesirable for economic reasons.Also, many organic liquids have varying degrees of toxicity and it is undesirable to cause unnecessary quantities of the vapour of such liquids to enter the atmosphere.

Our UK patent specification No. 1 582 955 discloses a method of condensing the vapour of a volatile liquid contained in a process gas stream.

It is an aim of the present invention to provide a method of recovering vapour evolved from a volatile liquid into a gas space at the head of a storage vessel (ie the ullage space thereof) containing the volatile liquid.

According to the present invention there is provided a method of condensing vapour evolved from a volatile liquid into the ullage space of a storage vessel cotaining the volatile liquid, which method comprises the steps of starting to precool a condenser by introducing a liquefied gas into the condenser upon the pressure in the ullage space of the storage vessel rising to a first chosen value and thereby reducing the temperature in the condenser to a chosen value at which the vapour condenses; allowing vaporised liquefied gas to pass out of the condenser so as to prevent the pressure therein from rising substantially above atmospheric pressure; controlling the admission of liquefied gas to the condenser so as to keep the temperature therein at or close to the chosen value; causing the gas-vapour mixture in the ullage space of the storage vessel to flow into the condenser and therein contact liquefied gas or its cold vapour upon the pressure in the ullage space reaching a second chosen value greater than said first chosen pressure value, condensed droplets of vapour in the gas-vapour mixture thereby being formed; separating the condensed liquid from the gas, venting the gas, and returning the liquid to the storage vessel.

The invention also provides apparatus for condensing vapour evolved from a volatile liquid into the ullage space of a storage vessel containing the volatile liquid, which apparatus comprises means for sensing the pressure in the ullage space; an automatically operable valve for controlling the flow of liquefied gas into a condenser, said valve being adapted to open upon the pressure in the ullage space rising to a first chosen value; means for sensing the temperature in the condenser (or an outlet thereof), said means operatively associated with the said flow control valve so as to maintain a temperature in the condenser at or close to a chosen value (or in a chosen range), said vapour condensing at said temperature; a conduit, when open, placing the condenser in communication with the ullage space of the storage vessel; an outlet from the condenser, whereby vaporised liquefied gas can, in use, flow out of the condenser thereby preventing the pressure in the condenser from rising substantially above ambient pressure; an automatically operable valve in the said conduit, adapted to open upon the pressure in the head space rising to a second chosen value greater than said first chosen pressure value, whereby gas-vapour mixture from the ullage space is able to flow therefrom into the condenser and be condensed so as to form a gas laden with droplets of condensed vapour of said volatile liquid; a separator communicating with said outlet separating the droplets from the gas, means for venting the gas from the separator and means for returning the condensed liquid from the separator to the storage vessel.

Typically, gas-vapour mixture flows from the head space into the condenser so long as the pressure in said ullage space is at or above said chosen value.

The liquefied gas is preferably liquid nitrogen. It is possible, however, to use other liquefied gases. For example, liquid argon could be used.

The condensation temperature may be cho sen in accordance with the desired maximum content of vapour in the gas vented after separation of the condensed liquid. For example, if the volatile liquid is petrol (gasolene) and if it is desired to keep the proportion of petrol vapour in the vented gas below 1.5% by volume, a temperature of - 40"C will be suitable.

Typically, though not necessarily, the volatile liquid is liquid at ambient temperature.

The volatile liquid may, for example, be petrol, propylene oxide, or any other organic liquid stored in bulk storage vessels, eg having a capacity of more than 1,000 litres. The method and apparatus according to the invention may be employed to condense the vapour of volatile liquids other than organic liquids.

For example, they may be used in the storage of bromine.

Apparatus according to the invention may be fitted to an existing storage tank. For reasons of wishing to reduce the risk of fire or explosion, to exclude oxygen, or to keep the volatile liquid dry, it may be desired to blanket the liquid in the storage vessel with a gas which is neither combustible nor supports combustion and contains negligible amounts of oxygen and water (if any). Preferably, nitrogen' is used as such gas and the nitrogen for this purpose may be taken from the same storage tank as liquid nitrogen for condensing the vapour of the volatile liquid.In such an example, the vacuum relief or demand valve of the storage vessel may be placed in communication with a source of the blanketing gas or may be replaced with a valve adapted to open upon the pressure in the ullage space falling to a third chosen value, said third chosen value being less than the first chosen pressure value.

The method and apparatus according to the present invention will now be described by way of example, with reference to the accompanying drawing which is a schematic flow diagram illustrating a storage tank fitted with a condenser for performing the method according to the invention.

Referring to the drawing, a storage tank 2 cotains a volume 4 of volatile liquid, say petrol, and above the surface of the liquid 4 is the ullage space (or head space) 6 of the storage tank. The storage tank 2 has a horizontal pipe 8 extending from outside the tank 2 to a region inside the tank, about 1 metre above the bottom thereof. The pipe 8 has a valve 10 disposed therein outside the tank 2.

The pipe 8 may be used to charge the tank with petrol or in the discharge of petrol therefrom. Storage tank 2 has vent pipes 12 and 14 communicating with the head space 6. A pressure relief valve 16 is located in the vent pipe 12 and a similar pressure relief valve 18 is located in the vent pipe 14. The vent valves are set to open at a pressure of, say, at least 7 inches water gauge. The purpose of having two vent valves is so as to enable the tank to be vented should just one valve fail.

The tank 2 is provided with a blanketing pipeline 20 terminating in its roof and communicating with the ullage space 6. The inlet end of the pipeline 20 communicates with the gas line of a vacuum-insulated storage tank 22 adapted to supply gaseous nitrogen and liquid nitrogen through separate outlet pipes.

Such tanks are well known to cryogenic engineers and shall not be described in further detail herein. In the pipeline 20 is a pressure reducing valve 24 which is typically set to reduce the pressure of the nitrogen upstream of the ullage space 6 from the supply pressure of, say, 35 psig to a pressure of, say, 10 inches water gauge. Downstream of the pressure reducing valve 24 is an automatically operable valve 26 controlled by a valve controller 28 adapted to receive signals from a pressure sensor 30 adapted to sense the pressure in the head space of the storage tank 2.

The arrangement is typically such that whenever the sensed pressure is 4 inches water gauge or less the valve 26 is open.

The storage tank 2 has a vapour recovery pipeline 32 whose inlet communicates with the ullage space 6 of the tank 2 and whose outlet communicates with a thermally-insulated condenser 38. An automatically operable on-off valve 34 is located in the vapour recovery pipeline 32. Operation of the valve 34 is controlled by valve controller 36 operatively associated with the pressure sensor 30 such that the valve 34 is open whenever the pressure in the head space 6 of the storage tank 2 is, say, 6 inches water gauge or more.

The condenser 38 is provided with a spray head 40 adapted to spray liquid nitrogen into the interior of the chamber 38. The spray head 40 is fitted to the outlet end of a liquid nitrogen supply pipeline 42. This supply pipeline is typically thermally insulated. Its inlet is connected to the liquid nitrogen outlet of the vacuum-insulated liquid nitrogen storage vessel 22. A liquid nitrogen flow control valve 44 is disposed in the pipeline 42. Valve 44 is automatically operable by a controller 46 which is responsive to the pressure sensed in the head space 6 by the sensor 30 such that, on the pressure in the head space rising to a chosen pressure (less than that at which the valve 34 opens, say, 5 inches water gauge, the valve 44 opens to allow liquid nitrogen to pass along the pipeline 42 to the spray head 40. By this means the temperature of the gas space in the condenser 38 is rapidly reduced. The valve controller 46 is also responsive to signals generated by a temperature sensor 48 located in the interior of the condenser 38 (or an outlet thereof).

The arrangement is typically such that the controller 46 will adjust the position of the valve 44 so as to reduce or stop the flow of liquid nitrogen to the spray head 40 on the temperature sensed by the sensor 48 falling to a chosen value of, say, - 40"C and there after to keep the sensed temperature within, say, plus or minus 0.2 C of - 40"C while the pressure sensed by the sensor 30 is 5 inches water gauge or more. The arrangement is thus that with a rising pressure in the head space 6 the valve 44 opens in advance of the valve 34 and thus the condenser is pre-cooled before any nitrogen-petrol vapour mixture flows into the condenser 38 through the vapour recovery pipe line 32.

The condenser 38 has an outlet pipe 50 at its bottom communicating with a separator 52 at a level below layers 54 of knitted wire mesh or other material capable of acting as a demister to remove droplets of condensed petrol from vapour-gas mixture. The separator 52 is typically thermally insulated and has at its top a pipe 54 having a union with an outlet or vent pipe 56. The vent pipe 56 has a valve 58 therein set to open when the pressure in the separator 52 is greater than atmospheric pressure. Thus, liquid nitrogen sprayed into the condenser 38 will evaporate and resulting vapour will pass through the pipeline 50 into the separator 52 from where it will pass upwardly into the pipe 54 and be vented to the atmosphere through the vent pipe 56.Thus, a pressure in the condenser 38 is prevented from rising substantially above atmospheric pressure and is always less than the pressure at which valve 34 opens. Thus, when the valve 34 is open there is always flow of vapour-gas mixture from the ullage space 6 into the condenser 38.

With the pressure in the ullage space at the said value of 6 inches water gauge, or a greater value, nitrogen-petrol vapour mixture flows through the pipeline 32 into the condenser 40 and is condensed by contact with cold nitrogen vapour therein or with liquid nitrogen being sprayed for the time being into the condenser 38. As a result, nitrogen laden with droplets or condensed petrol leaves the condenser 38 through the pipe 50 and enters the separator 52. It passes upwardly through the demister 54 and thus droplets are separated therefrom. The droplets collect at the bottom of the separator 52. Typically, nitrogen containing less than 2% by volume of petrol vapour leaves the separator 52 through the outlet 55 and is vented to the atmosphere through the vent pipe 56. Liquid condensate collects at the bottom of the separator 52.

When the level of the condensate reaches that of an upper level sensor 60 located in the separator 52, a pump 64 is actuated and the condensate is withdrawn from the separator 52 through a pipeline 66 and returned to the storage tank 2. When the level of the condensate in the separator 52 falls to that of a level sensor 62 the pump 64 is stopped and does not start working again until the level of condensate in the separator 52 rises to the level sensor 60. The pipeline 66 has in it stop valves 68 and 70 located upstream and downstream respectively of the pump 64. If it is required to perform a routine maintenance operation on the pump 64, it may be isolated from the separator 52 and the storage tank 2 by closing the valves 68 and 70, through in normal operation of the apparatus shown in the drawing, these valves remain open.Located downstream of the pump 64 but upstream of the valve 70 is a non-return valve 72 adapted to prevent return of liquid after it has passed through the pump 64. A flow meter 74 is located downstream of the valve 70 and is able to record the volume of liquid condensate returned to the storage tank 2.

If desired, samples of the gas-vapour mixture entering the condenser 38 may be taken from immediately upstream of the condenser 38 through a sampling pipe 76 terminating in the pipeline 32 by opening a valve 78 in the pipe 76. Analogously, a sample of the gas leaving the separator 52 may be taken from a pipe 80 which has a union with the pipes 55 and 56. The sample may be taken by opening the valve 82 in the pipe 80.

The apparatus shown in the drawing is also provided with a pipeline 84 whose inlet communicates with the blanketing pipeline 20 and whose outlet terminates in the union of the pipes 55, 56 and 80. Located in the pipeline 84 is a bleed valve 86. In operation, a relatively small flow of nitrogen gas is bled into the separator 52 via the pipeline 84 in order to ensure that a positive pressure is always maintained in the separator 52.

Suppose petrol is being discharged from the storage tank 2 through the pipe 8. The level of petrol in the tank 2 falls. This causes a reduction in the pressure in the ullage space 6 of the tank 2 and the pressure falls to 4 inches water gauge. The sensor 30 sends a signal to the blanketing valve 26 to open the blanketing pipeline 20. Nitrogen then passes from the vacuum-insulated nitrogen storage tank 22 into the head space 6 via the pipeline 20. This maintains a positive pressure in the head space 6 and forms a blanket of nonflammable gas which does not support combustion of the liquid 4 in the tank 2. When the discharging operation is complete the valve 10 is closed and the pressure in the storage tank 2 will tend to rise owing to the evaporation of petrol therein as a result of heat in-leak into the tank 2.Accordingly, the pressure in the ullage space 6 soon rises above 4 inches water gauge and the valve 26 closes. When the pressure reaches 5 inches water gauge the senor 30 passes a signal to the valve controller 46 and the valve 44 is opened thereby causing liquid nitrogen to flow from the vacuum-insulated storage tank 22 to the spray nozzle 40 and then to be sprayed into the condenser 38 so as to lower its temperature. The temperature sensor 48 generates signals which the controller 46 uses to control the position of the valve 44 so as to maintain the temperature in the condenser 38 at - 40"C (once the temperature has been reduced to this valve).

Petrol continues to vaporise in the tank 2 and as a result the pressure in the ullage space gradually rises. When it reaches 6 inches water gauge as sensed by the pressure sensor 30 the valve controller 36 generates a signal to open the valve 34 and thereby cause nitrogen-petrol vapour mixture to flow from the ullage space 6 through the pipeline 32 into the condenser 38. Contact of this gasvapour mixture with liquid nitrogen or cold nitrogen vapour in the condenser 38 causes droplets of petrol to condense in the gas and nitrogen laden with such droplets leaves the condenser 38 through the pipe 50 and enters the separator 52 in which the liquid phase is separated from the gaseous phase. The nitrogen typically containing less than 2% by volume of petrol vapour is vented from the separator 52 through the vent pipe 56.Liquid petrol collects at the bottom of the separator 52 and when its level has reached that of the level sensor 60 pump 64 is started and the liquid is withdrawn from the separator 52 through the pipeline 66 and returned to the storage tank 2. Operation of the pump 64 continues until the level of petrol in the separator 52 has fallen to that of the lower level sensor 62.

Should the valve 34 fail to open at the chosen pressure of 6 inches water gauge, an alarm may sound and when the pressure reaches 7 inches water gauge, the pressure relief valve 16 and 18 will open thereby venting the head space 6 to the atmosphere.

Suppose petrol is discharged from the storage tank 2 through the pipe 8. The pressure in the head space 6 will fall as a result of the expansion of the gas-vapour mixture therein.

Accordingly, when the pressure falls to below 6 inches water gauge the valve 34 closes thereby stopping passage of nitrogen-petrol vapour mixture from the head space 6 through the pipeline 32 to the condenser 38.

When the pressure in the head space 6 falls to 5 inches water gauge (as sensed by the sensor 30) the valve controller 46 operates to close the valve 44 thus stopping the spraying of liquid nitrogen into the condenser 38.

Should the pressure in the head space 6 fall to 4 inches water gauge then the valve controller 28 opens the valve 26 and nitrogen is admitted to the head space 6 so as tomaintain a positive pressure therein.

Should the storage tank 2 be filled through the pipe 8 the volume of the gas-vapour mixture in the ullage space 6 will be reduced and accordingly the pressure will rise therein.

Should the pressure rise to 5 inches water gauge as sensed by the pressure sensor 30 the valve controller 46 generates a signal effective to reopen the valve 44 and thus restart spraying of the liquid nitrogen into the condenser 38 so as to lower the temperature of the gas space therein to - 40"C. When the pressure in the ullage space 6 rises to 6 inches water gauge as sensed by the pressure sensor 30 then the valve controller 36 generates a signal to open the valve 34 and there by cause nitrogen-petrol vapour mixture to flow from the ullage space 6 through the pipeline 32 into the condenser 38 wherein petrol vapour is condensed and out of which nitrogen laden with condensed droplets of petrol vapour passes into the separator 52.

Petrol is then returned to the tank 4 from the separator 52 in the manner hereinabove described.

Claims (13)

1. A method of condensing vapour evolved from a volatile liquid in the ullage space of a storage vessel containing the volatile liquid, which method comprises the setps of starting to precool a condenser by introducing a liquefied gas into the condenser upon the pressure in the ullage space of the storage vessel rising to a first chosen value and thereby reducing the temperature in the condenser to a chosen value at which the vapour condenses; allowing vaporised liquefied gas to pass out of the condenser so as to prevent the pressure therein from rising substantially above atmospheric pressure; controlling the admission of liquefied gas to the condenser so as to keep the temperature therein at or close to the chosen value; causing the gas-vapour mixture in the ullage space of the storage vessel to flow into the condenser and therein contact liquefied gas or its cold vapour upon the pressure in the ullage space reaching a second chosen value greater than said first chosen pressure value, condensed droplets of vapour in the gas-capour mixture thereby being formed; separating the condensed liquid from the gas, venting the gas, and returning the liquid to the storage vessel.

2. A method as claimed in claim 1, in which vapour mixture flows from the ullage space into the condenser for so long as the pressure in said ullage space is at or above said chosen value.

3. A method as claimed in claim 1 or claim 2, in which the liquefied gas is liquid nitrogen.

4. A method as claimed in any one of the preceding claims, in which the liquid in the storage vessel is blanketed with a gas which is neither combustible nor supports combustion, and which contains negligible amounts of oxygen and water (if any).

5. A method as claimed in claim 4, in which said gas is nitrogen.

6. A method as claimed in claim 4 or claim 5, in which the storage vessel has a demand valve placed in communication with a source of the blanketing gas.

7. A method as claimed in claim 4 or claim 5, in which the storage vessel has associated therewith a valve adapted to open upon the pressure in the ullage space falling to a third chosen value, said third chosen value being less than the first chosen pressure value.

8. A method of condensing vapour evolved from a volatile liquid, substantially as described herein with reference to the accompanying drawing.

9. Apparatus for condensing vapour evolved from a volatile liquid into the ullage space of a storage vessel containing the volatile liquid, which apparatus comprises means for sensing the pressure in the ullage space; an automatically operable valve for controlling the flow of liquefied gas into a condenser, said valve being adapted to open upon the pressure in the ullage space rising to a first chosen value; means for sensing the temperature in the condenser (or an outlet thereof), said means operatively associated with the said flow control valve so as to maintain a temperature in the condenser to or close to a chosen value (or in a chosen range), ssaid vapour condensing at said temperature; a conduit, when open, placing the condenser in communication with the ullage space of the storage vessel; an outlet from the condenser, whereby vaporised liquefied gas can, in use, flow out of the condenser thereby preventing the pressure in the condenser from rising substantially above ambient pressure; an automatically-operable valve in the said conduit, adapted to open upon the pressure in the head space rising to a second chosen value greater than said first chosen pressure value, whereby gas-vapour mixture from the ullage space is able to flow therefrom into the condenser and be condensed so as to form a gas laden with droplets of condensed vapour of said volatile liquid; a separator communicating with said outlet separating the droplets from the tas, means for venting the gas from the separator and means for returning the condensed liquid from the separator to the storage vessel.

10. Apparatus as claimed in claim 8, arranged such that, in use, gas-vapour mixture flows from the head space into the condenser for so long at the pressure in said ullage space is at or above said chosen value.

11. Apparatus as claimed in claim 9 or claim 10, additionally including means for supplying to the ullage space of the storage vessel a blanketing gas which is neither combustion nor supports combustion and which contains negligible amounts of oxygen and water (if any).

12. Apparatus as claimed in claim 11, in which said means includes a valve adapted to open upon the pressure in the ullage space falling to a third chosen value, said third chosen value being less than the first chosen pressure value.

13. Apparatus for condensing the vapour of a volatile liquid, substantially as described herein with reference to the accompanying drawing.