WO1999065799A1

WO1999065799A1 - Method and device for suppressing escape of volatile compounds

Info

Publication number: WO1999065799A1
Application number: PCT/SE1999/000962
Authority: WO
Inventors: Bertil Berggren
Original assignee: Bertil Berggren
Priority date: 1998-06-05
Filing date: 1999-06-04
Publication date: 1999-12-23
Also published as: SE9801999L; SE9801999D0; AU4668199A; SE512536C2

Abstract

A method and a device for diminishing the amount of volatile compounds that is exhausted from a non-pressurized vessel provided with a ventilation tube (2) and containing said volatile compound of liquid and gaseous phases in equilibrium with each other, said volatile compound being carried out to the environment by the created vapor pressure through a tube (4) provided with bends and connected to said ventilation tube (2). The volatile compound of gaseous phase is led through multiple helically wound tubes (4) arrayed in parallel.

Description

METHOD AND DEVICE FOR SUPPRESSING ESCAPE OF VOLATILE COMPOUNDS

The present invention concerns a method and a device for diminishing the amount of volatile compounds that are exhausted from a non-pressurized vessel provided with a ventilation tube that contains a compound.

Hundreds of millions of SEK (Swedish Kronor) evaporate annually from refineries and gasoline stations in Sweden. The loss at a single refinery can amount to SEK 40 million. These losses involve not only considerable expense but also significant pollution of the environment. Light and very carcinogenic fractions are formed in the process of cracking that even in small amounts can have an injurious effect on people and animals. Examples of such compounds are benzene, toluene, and light fractions that are obtained when cracking or dissociating heavy hydrocarbons in petroleum to lighter hydrocarbons. Gas is used in various ways in connection with volatile compounds. GB

153 020, for example, shows a way to forestall explosions in tanks with very flammable liquids, whereby air can be prevented from entering the tank. This is accomplished through a gas being introduced into the tank that is prevented from exiting from it by means of an arrangement of tubes with shielded inlets and outlets.

Gasoline tanks can, however, tolerate an overpressure of only 0.3-0.4 bar, and they are also very sensitive to being subjected to less than atmospheric pressure; this negative pressure can be at about 0.2 bar. At a negative pressure of 0.3 bar there is an implosion. These tanks must thus be provided with a ventilation tube that relieves almost all pressure.

In a tank of this type a solution containing a volatile compound with a low boiling point, for example benzene, is heated by its surroundings, whereby the vapor pressure rises. The energy level of the gas phase in equilibrium with the volatile compound in the liquid phase then increases. The vapor pressure formed in a ventilation tube in a non-pressurized tank then expels the volatile molecules.

The problem with released volatile materials has been solved for gasoline vapors by, for example, collecting, condensing, and recovering them in the form of liquid gasoline. For other applications there are various types of cooling systems.

Various devices have been suggested as ventilation tubes for the non- pressurized tank. In GB 2 203 229 a jacketed ventilation valve, i.e. a tube, with cooling for a storage tank for fuel is arrayed vertically, whereby the evaporated fuel is condensed in this ventilation valve. In order to maximize the condensation of liquid this valve/tube contains a helical-formed insert of thermally conducting material, which is cooled.

A complicated condensing device is similarly shown in SU 1 839 162 for a ventilation tube for a storage tank with volatile liquid. The device encom- passes an insulated vessel with a vertical helical form that is connected with the outlet of a expansion valve with two inlets, the one connected with the gas phase in the tank and the other to the liquid side of a Dewar-vessel.

SU 1 293 079 shows a cap for reduction of emitted products of coking, whereby a mixture of vapor and gas is prevented from being released into the atmosphere. The cap is designed as an inverted vessel that is coaxial with a vertical ventilation tube.

US-A-1566825 shows and describes a device for preventing evaporation from tanks. A ventilation tube from the tank is connected with a lower part of a closed container. At an upper part of the container an outlet is provided. In the container a number of horizontal impingement baffles are arrayed that to a high degree hinder diffusion of volatile substances from the tank. The volume of the container is greater than the volume of gas that can be forced from the tank with, for example, variations in temperature.

Available devices in connection with non-pressurized vessels are thus complicated just because the vessel must be non-pressurized. In order to pre- vent volatile compounds from escaping from these vessels drastic methods have been used up to now that cause turbulence in the vessel and disturb the equilibrium between the volatile compound in its liquid phase and gaseous phase. Since the equilibrium is disturbed, the vapor pressure increases, which in turn causes an increased amount of volatile material that has to be prevented from passing through the ventilation tube into the environment. With serious disturbance of the equilibrium an overpressure can even occur in the vessel.

An object of the invention is to provide a method of hindering or reducing the amount of volatile material that is lost into the environment from a non- pressurized vessel via its ventilation tube. By means of a simple and inexpensive device that can be used with different sizes of non-pressurized vessels, equilibrium can quickly be reestablished in the system after filling or tapping from the vessel.

To this end the method according to the invention include those distinc- tive features that are described in Claim 1 , and its use has incorporated the features that are described in Claim 4.

The invention will now be described in more detail using exemplary embodiments with reference to the accompanying drawings on which

FIG 1 schematically shows an embodiment of a device according to the invention, FIG 2 is a partially sectional side view of a practical embodiment of the device in FIG 1 , FIG 3 is a plane view of the device in FIG 2, FIG 4 is a partly sectional side view of an alternative practical embodiment in FIG 1 , and FIG 5 is a plane view of the device in FIG 4.

FIG 1 schematically shows an embodiment of the device according to the invention that is attached to the ventilation tube on a vessel of considerable size, for example a tank, that contains volatile material in the form of hydrocarbons or similar.

A devicel according to the invention is connected here to the tank's ventilation tube 2 via an inlet expansion space 3. A number of horizontally, par- allel-running tubes 4 with bends is in one end connected with an inlet expansion space 3. A second end of the tubes 4 is connected with an outlet expansion space 5. This is, moreover, in its upper part provided with a filling unit 8, preferably formed as a funnel. The expansion spaces 3 and 5 can be provided in a lower part with tubes incorporating bleeder valves 7. In the embodiment according to FIG 1 all tubes 4 with bends are arranged horizontally. The number of tubes 4 is dependent on the diameter of ventilation tube 2, which in turn is dependent on the size of the tank. For a small, simple gasoline tank only a single tube is required, and this does not need to be provided with expansion spaces 3 and 5. Retardation of a volatile compound in its gas phase occurs mechanically by the tube with bends. Through the fact that the tube is arranged horizontally, turbulence is reduced in the material that passes through the tube, which would be able to cause interference in the system of equilibrium.

Tube 4 is preferably made with bends of a helical-formed tube, but it can naturally also be bent or twisted in another way with a circular or hexagonal cross-section, see FIG 3 and FIG 5.

Carrying out experiments with a gasoline-filled vessel that according to the invention was fitted with a tube with bends simulated the conditions in non- pressurized tanks. This was shaped as a helical form. The weight loss in this vessel was continually compared on a scale that was capable of weighing to an accuracy of 10^~5 g with a reference vessel - an identical vessel, which was equipped with a traditionally bent ventilation tube. The diameter of all tubes was 5 mm.

A marked lessening of the amount of evaporated gasoline was obtained already with a tube with 4-5 turns in a helical form that was operated in accor- dance with the method according to the invention. An increase of the number of turns in the helical form to 10 brought about a great lessening in that the weight loss of volatile substance was reduced to 30 % of that in the reference vessel. A good effect was also obtained with15 turns. The extension or length of the tube between the expansion spaces, given as L in FIG 1 , is approximately 1 m. Greater values of L can promote the risk of rapid pressure changes in the tank, since the flow resistance can become too large with too long a tube.

The results show that a tube provided with bends in helical form has a braking effect in itself on volatile material in the gas phase expelled by vapor pressure. It should be noted here that a tube with bends according to the invention does not function as a cooler in the normal sense in that no drastic measures were taken to bring about as efficient condensation as possible of the volatile compound, such as cooling with water or cold air.

In another comparative experiment a funnel directed upwards was ar- ranged at the outlet end of the horizontal tube with a helical form of 10 turns. An inert gas with a density greater than that of the volatile material in its gaseous phase, in this case carbon dioxide, was carefully poured into the funnel. The result was that the vessel, which was designed in accordance with this embodiment of the invention, showed an 80 % lower rate of evaporation of volatile material than the reference vessel. The effect obtained was stable for a long time.

The heavier, inert gas thus forms an "upper boundary" above the volatile material in a course of events that can be compared to an inversion. It should be noted that since a heavy gas with its higher density is pressed in to- wards the tank through a tube with bends from the outlet end of the tube, the tube has a braking effect on this gas.

Again with reference to FIG 1 , where a device for larger non- pressurized vessels according to the invention is referred to, an inert, nonflammable gas fills the device via a funnel-shaped part 6. After every emptying and filling of the tank new gas is filled in. The gas is filled carefully so that the given inert gas is layered on and then floats over the given volatile compound in its gaseous phase that attempts to stream out of the tank. The supply must thus occur with minimal pressure in order to avoid turbulence and accompanying overpressure in the tank. The pressure head of the applied gas is suited to the number of tubes 4 with bends in the device.

The type of inert gas that should be used naturally depends on the volatile compound(s) present in the tank. Besides carbon dioxide, dinitrogen tetroxide can be used, for example, provided that the inert gas has a higher density than the volatile compound in its gaseous phase. Carbon dioxide is preferably used. In the horizontal tubes with bends 4 arranged in parallel with each other both the volatile compound in its gaseous phase and the heavier gas are mechanically retarded before they come into contact with each other.

Tubes 4 with bends are preferably arranged in parallel in order to avoid interference in them that also could arise because of a "chimney effect." The volatile compound in its gaseous phase loses energy when slowed down in tubes with bends 4, the vapor pressure sinks, and the speed of the outflow diminishes. By means of counter-pressure from the heavy, nonflammable gas a continual process of slowing down occurs that brings about a further reduction of losses to the environment. The system thus still exists in balance. In order not to disturb this too much, the whole device should therefore be shielded from too much influence from sun and wind, etc., from the environment.

Through the fact that expansion spaces 3 and 5 are arranged on both sides of horizontal tubes 4, both the volatile compound in its gaseous phase and the inert gas are equally distributed. Expansion spaces 3 and 5 function also as a buffer for possible turbulence in the system that could disturb the balance in tubes 4.

The mechanical retardation of a volatile compound in its gaseous phase has the result that a certain portion condenses. The system equilibrium causes condensed liquid to return again to its gaseous phase, so the amount of liquid remains very small or even negligible.

In a configuration according to the invention that is foreseen for a larger non-pressurized vessel such as a gasoline tank, tube 4 with bends should have a diameter that is considerably smaller than ventilation tube 2. When the tank is emptied of its volatile liquid, the total area of retarding tubes 4 should be larger than the area of ventilation tube 2 so that air can be supplied to the tank from the surrounding environment during the emptying without bringing about a critical negative pressure. In order to ensure a rapid exchange of air during empty- ing and filling, this total area is preferably 1.5 times greater than the area of the ventilation tube. In this way the risk of implosion is avoided and that an excessively high overpressure arises when the tank is filled quickly.

In the practical layout according to FIG 2 and FIG 3 tubes 4 are designed as a helical winding with a circular form. The outer diameter D of the winding can be on the order of 30-70 mm. In experiments an especially good result was achieved with D = 45 mm. The diameter of the tube d = 5 mm. Even somewhat larger tubes can be used.

In the practical layout according to FIG 4 and FIG 5 tubes 4 are designed as a helical winding with a hexagonal form. The dimensions of the tube and winding correspond to the dimensions in the embodiment according to FIG 2 and FIG 3 and the same reference characters have been used for corresponding elements.

The horizontal extension that tubes 4 have in the layouts shown is suitable when a gas is to be used in order further to lessen leakage from the tank. If such a gas is not used, tubes 4 can be arrayed with a vertical extension.

Claims

CLAIMS:

1. A method to diminish the amount of volatile compound that is exhausted from a non-pressurized vessel provided with a ventilation tube (2) and containing said volatile compound having liquid and gaseous phases in equilibrium with each other, said volatile compound of gaseous phase being carried out to the environment by the created vapor pressure through a tube (4) provided with bends and connected to said ventilation tube (2), c h a r a c t e r i s e d by directing said volatile compound of gaseous phase, before it is being let out to the environment during minimal system disturbance by turbulence, through multiple helical tubes (4) connected in parallel.

2. A method according to Claim 1 , wherein said volatile compound is carried through tubes (4) arranged in a horizontal main direction.

3. A method according to Claim 2, wherein an inert gas having higher density than said volatile compound of gaseous phase is added at the outlet of said tube (4) with bends.

4. A method according to Claim 3, wherein said inert gas is carbon dioxide.

5. A device to diminish the amount of a volatile compound that is exhausted from a non-pressurized vessel provided with a ventilation tube (2) and containing said volatile compound having liquid and gaseous phases in equilibrium with each other, a tube (4) with bends being connected to said ventilation tube, c h a r a c t e r i s e d in that the ventilation tube (2) is connected to multiple helically wound tubes (4) arrayed in parallel.

6. A device according to Claim 5, wherein tubes (4) are arranged with horizontal extension.

7. A device according to Claim 6, wherein tubes (4) at their outlets are con- nected to a filling unit (8) for reception and retaining of an inert gas.

8. A device according to any one of Claims 5 - 7, wherein the total area of the bend-designed tubes (4) is greater than the area of the ventilation tube (2).

9. A device according to Claim 8, wherein the total area of the bend- designed tubes (4) is O S times greater than the area of the ventilation tube .

10. A device according to Claim 9, wherein tubes (4) are helically wound with 4 - 15 turns.

11. A device according to any one of Claims 5 - 10, wherein first expansion means is arranged at a first end of tubes (4) for distribution of the volatile compound of gaseous phase.

12. A device according to any one of Claims 7, wherein second expansion means (5) is arranged at a second end of tubes (4) for distribution of the inert gas.