FR2755873A1 - Recovery of volatile vapours from a gaseous mixture - Google Patents

Abstract

Installation for the recovery of volatile components present in the form of vapour in a gaseous mixture, using at least one purification column (1) enclosing a first condenser (2) in the bottom of the column and a second condenser (3) above the first. The two condensers are crossed by the same line (4) circulating cooling nitrogen, fed with liquid nitrogen upstream of the second condenser, in which the nitrogen vaporises and delivering the heated gaseous nitrogen downstream of the first condenser. The column has in its base means (8) of supplying the gaseous mixture and means (12) of extracting the condensate at a level below that of the first condenser. It also has means of interrupting the flow of liquid nitrogen and means (12) of feeding the two condensers with a heating fluid.

Description

 The present invention relates to an installation for the recovery of volatile compounds present in the form of vapor in a gas mixture.

It applies in particular to the recovery of hydrocarbon vapors, in particular of fuel such as petrol and diesel, in air, but also to the recovery of other compounds in a carrier gas, in particular inert .

 It is necessary in certain situations, such as purging the gas contained in a fuel tank which must be filled, to treat the gaseous effluent saturated with hydrocarbon vapors, on the one hand to limit the discharges of vapors d hydrocarbons in the environment, and on the other hand to avoid the losses of hydrocarbons implied by such discharges.

 To meet these requirements, the present invention proposes to provide an installation for recovering vapors of volatile compounds in a gaseous mixture, the constitution of which is such that the risks of explosion are limited, the implementation and maintenance of which are facilitated, in particular which is adaptable to any type of gas mixture, all of this with a relatively limited cost.

 These aims, as well as others which will appear subsequently, are achieved by the invention by means of a compact installation essentially comprising two condensers arranged one above the other in the same envelope.

 More particularly, the subject of the invention is an installation for the recovery of volatile compounds present in the form of vapor in a gaseous mixture, characterized in that it comprises at least one purification column containing a first condenser located in the lower part the column and a second condenser disposed above the first, the two condensers being crossed by the same cooling nitrogen circulation line, supplied with liquid nitrogen upstream of said second condenser, in which the nitrogen vaporizes, and delivering heated nitrogen gas downstream of said first condenser, said column being provided in its lower part with means for supplying said gas mixture and with condensate withdrawal means located at a level below the first condenser, and also being provided means for interrupting its supply of liquid nitrogen and means for supplying a fluid for heating the two cones densities.

The installation according to the invention may include one or more of the following characteristics
- the first condenser is a counter-current type heat exchanger
- the second condenser is a heat exchanger of the multi-pass type with baffles
the or each purification column also contains in its upper part a hydrocarbon adsorption device disposed above said second condenser and comprising a bed of adsorbent crossed by said upstream cooling nitrogen circulation line of the second condenser
the or each column is equipped with means for measuring the pressure difference of the treated gas between its upper part and its lower part
- It includes condensate storage means linked to said condensate withdrawal means
- It comprises a single column and means for circulating liquid alternately from the lower part of said column to said storage means, and from said storage means to the lower part of said column
the means for circulating the liquid are controlled by the means for controlling the pressure difference for the circulation of the liquid from said storage means towards the lower part of said column
- It includes means for isolating the adsorption device from the rest of the column when liquid is conveyed from said storage means to the lower part of the column
- It includes means of regeneration by evacuating the adsorption device
- It comprises two purification columns connected in parallel to means for supplying said gas mixture, and regeneration gas supply pipes connecting the outlet end of a cooling nitrogen circulation line to the inlet end of the other cooling nitrogen circulation line, the nitrogen serving as a heating fluid in this other line
- It comprises control means for supplying only one column at a time with said gas mixture and, simultaneously, closing the communication between the inlet end of the cooling nitrogen circulation line of this column and the outlet end of the cooling nitrogen circulation line of the other column
- A heater is installed on said regeneration gas supply pipe.

Examples of the invention will now be described with reference to the accompanying drawings, in which
- Figure 1 schematically shows an installation according to the invention, comprising a single purification column
- Figure 2 shows schematically another installation according to the invention, comprising a single purification column containing an adsorption stage
- Figure 3 schematically shows another installation according to the invention, comprising two purification columns in parallel.

 The installation shown in Figure 1 is intended to treat a gas comprising air, gasoline vapors and water, produced on a hydrocarbon storage site when filling a tank not shown. It essentially comprises a purification column 1, containing two superposed condensers 2 and 3 cooled by the same line 4 of coolant circulation, a buffer capacity 5 for the intermediate storage of condensate, and a tank 6 of liquid. The column is closed at its top by a removable cover 101.

 Column 1, made of stainless steel, thus contains in its lower part a first condenser 2, which is a heat exchanger of the counter-current type, and a second condenser 3, which is a heat exchanger of the multi-baffle type 103 horizontal, arranged above the first condenser 2. These two condensers are crossed by the line 4 for cooling fluid circulation, which is supplied with liquid nitrogen upstream from the second condenser 4 via a liquid supply pipe 7 connected to a tank 107.

 The installation works as follows.

 The gaseous mixture to be treated, consisting of air at about 70% humidity saturated with gasoline vapor, at room temperature (below about 300C) and at atmospheric pressure, is introduced into the lower part of column 1 via a gas supply pipe 8. This gas circulates in an ascending manner in column 1 against the flow of the direction of circulation of the cooling nitrogen.

 The coolant circulation line 4 initially crosses the second condenser 3 in the form of a tubular bundle 104 made up of tubes with vertical fins. Thus, the circulation of the ascending gas in the column takes place, essentially, perpendicular to the circulation of the cooling fluid, the latter following a sinuous ascending path defined by the chinanes 103.

The extraction of heat by liquid nitrogen in this condensation stage causes the complete vaporization of the liquid nitrogen, and the cooling fluid at the outlet of the second condenser 3 is therefore cold nitrogen gas.

 The coolant circulation line 4 then crosses the first condenser 2. In the variant shown in FIG. 1, this condenser comprises a filling body 9, around which circulates the coolant line 4 in the form of at least a helical coil. The extraction of heat by cold nitrogen gas, in this condensing stage, causes a relative heating of the nitrogen gas, which is then released into the atmosphere via a pipe 10 connected to the lower part of column 1, this pipe comprising a valve 110.

 The polluted gas introduced at the bottom of column 1 via line 8 therefore firstly crosses the first condenser 2 against the current, where it is cooled by the sensible heat of the stream of cold nitrogen gas flowing in line 4 The water can then start to condense, or even freeze. The condensate collects at the bottom of the column 1, while the possible ice crystals are partially deposited on the body of the first condenser 2. A large part of the crystals will be "washed" by the liquid condensates falling from the upper condenser and will therefore be mixed with the condensate bath in the bottom of the column.

 The air charged with gasoline vapor and substantially depleted in water then rises to the second condenser 3 and passes through the multi-pass exchanger. In this condensation stage, the liquid nitrogen circulating in line 4 is used to condense and / or crystallize the hydrocarbon vapors. For this, the nitrogen vaporizes and heats up to a predetermined temperature, for example -900C, in the exchanger 3.

 It is believed that the following equilibrium occurs: the first gasoline molecules cooled to the crystallization temperature, when they crystallize on the surface of the finned tubes of the multi-pass exchanger, create a thermal resistance which does not gradually allow to the next molecules than to condense and no longer to crystallize. There is therefore a flow of liquid hydrocarbon to the bottom of the column.

 The flow of liquid nitrogen introduced into the coolant circulation line 4 can be regulated by controlling the temperature of the gases at the head of the column 1, by means 111 which act on a regulating valve 11 disposed on the driving 7.

 The heat transfer in the second exchanger 3 is optimized through the use of finned tubes 104 for the circulation of nitrogen and baffles or baffles 103 for the circulation of condensing effluents.

 The gas arriving at the head of column 1 consists of air substantially free of hydrocarbon and moisture and is evacuated via a line 28 of purified gas. When the gas mixture to be treated consists of air saturated with petrol vapor or mixed vapor with petrol and diesel (in variable proportions, typically 60% diesel and 40% petrol), the purified air in column 1 typically has a hydrocarbon content of less than 35 g / Nm3.

 The condensed liquid collected at the foot of the column 1 can be conveyed to the buffer capacity 5 via a liquid supply line 12, equipped with a valve 112, this buffer capacity 5 communicating with a storage tank 6 on the site via a line 13 on which a reversible pump 14 is installed.

 The liquid contained in the storage tank 6, supplemented periodically by transferring the content of the capacity 5 by means of the pump 14, may be withdrawn by means not shown in the figure to be used on site as required.

 Alternatively, the capacity 5 can be deleted. The condensates are then stored in the column itself, provided that a sufficient retention height is provided in the column tank, or else sent directly to the main tank 6 or in pipes used for filling tank trucks.

 In operation, column 1 is the seat of phenomena not only of condensation, but also of crystallization, in particular of water and possibly of hydrocarbon, under the effect of intense cooling by liquid nitrogen. Care should be taken to ensure that the crystals formed which deposit on the condensers as well as the walls of the column do not occupy too large a volume, or even block the passage of gases inside the column 1.

 To this end, the installation shown in FIG. 1 includes means for regenerating the column, that is to say melting the crystals formed after a period of operation in purification mode.

 For this, means 15 are provided for measuring the difference between the pressure prevailing at the head of column 1 and the pressure prevailing at the foot of column 1, this pressure difference being indicative of the degree of blockage of the column. A valve 16, mounted in line 8, as well as valve 11 are slaved to these measurement means 15 to interrupt the arrival of the gaseous mixture to be treated via line 8 and the arrival of liquid nitrogen through line 7 when the level of fouling in the column no longer allows satisfactory purification.

 The pump 14 is then put into service to draw liquid from the reservoir 6, this liquid being at ambient temperature.

 Gradually, the buffer capacity 5, the pipe 12 and the column 1 are thus filled with liquid hydrocarbon at room temperature, allowing the crystals present to melt. When column 1 is filled with hydrocarbon, which can be controlled by the pressure difference measured by the means 15, the operating direction of the pump 14 is reversed and the liquid contained in column 1 and the buffer capacity 5 is sent to tank 6.

 A few minutes before restarting, the column is cooled by sending liquid nitrogen through the exchangers by reopening the valve 11, under the control of a temperature sensor 113 which measures the temperature of the nitrogen leaving the exchanger 3.

 This regeneration phase can take place as soon as the column has a quantity of crystals greater than a predefined quantity, or else, when the process is carried out discontinuously, before each period of shutdown of the installation.

 When the purification must provide ultra-pure air at the outlet of column 1, that is to say air which typically contains less than 150 mg / Nm3, the installation may further comprise an adsorption apparatus, downstream of the two condensation stages.

 This variant is illustrated in FIG. 2, in which the references already cited denote members identical to those described for FIG. 1.

 This installation further comprises an adsorption device disposed in the upper part of the column 1, above the second condenser 3, and comprising an adsorbent bed 18 separated from the rest of the column by a housing 19, and a pipe 20 for the gas supply which conveys the gas to be purified leaving the second condenser 3 through the support 19 as far as the adsorbent 18.

 The adsorbent bed 18 is crossed by the coolant circulation line 4, in which liquid nitrogen circulates.

 The adsorbent 18 can be any type of adsorbent capable of stopping vapors of hydrocarbons, such as in particular activated carbon.

 The housing 19 serves to isolate the adsorbent 18 from the rest of the column during the regeneration phase by immersion in the liquid hydrocarbon. To this end, the pipe 20 has an inverted U shape; a branch 120 of the U communicates with the top of the column, under the housing 19; the other branch 121 of the U opens into this box, under the adsorbent bed. At the top of the U, which is outside the column, the pipe 20 is equipped with a valve 21 intended to avoid contamination of the adsorbent 18 by the liquid hydrocarbon during regeneration.

 The adsorbent bed 18 is regenerated by evacuating the housing 19 by means of a pump 22: the gas desorbed by pumping is first of all returned to the bed via a pipe 23 equipped with a valve 123 to heat it , then, by closing the valve 123 and by opening a valve 124 of a pipe 24, the desorbed gas is injected into the lower part of the column 1 via this gas supply pipe 24, to undergo condensation there.

 As is clear from the above description, the installations shown in FIGS. 1 and 2 are suitable for the treatment of purification of the gaseous mixture in a discontinuous manner.

 The installation shown in FIG. 3, comprising two purification columns in parallel 1A and 1B, on the contrary allows the purification treatment to be carried out continuously. This installation works as follows.

 In FIG. 3, the elements bearing a numerical reference already cited are identical to those described previously under this reference. The elements designated A and B are identical to each other and correspond respectively to the two columns 1A and 1B.

 The installation comprises two purification columns each containing two condensation stages and an adsorption stage, of the type described relative to FIG. 2, the adsorbent beds 18A and 18B being this time simply carried by the supports 19A and 19B , without being isolated from the rest of column 1A, respectively 1B. Line 20 of Figure 2 is deleted.

 The two columns 1A and 1B are each connected, via lines 8A and 8B, to a common supply 8 of the gas mixture to be treated. The two lines 4A and 4B of coolant circulation are interconnected as follows. At the outlet of column 1A, line 4A is divided into a branch 10A opening into the atmosphere and a branch 25A of gas supply which opens into a common gas supply pipe 26 comprising a branch 26B, which opens into line 4B upstream of column 1B. Similarly, line 4B, at the outlet of column 1B, is divided into a branch 10B opening into the atmosphere and a branch 25B of gas supply opening into line 26, a second branch 26A of which is connected to line 4A upstream of column 1A. A heater 27 is installed at the point of convergence between the lines 25A, 25B and 26.

 In addition, a pipe 28A, 28B provided with a valve 29A, 29B starts from each column 1A, 1B, at the exchanger 2A, 2B, and is connected to the suction of a pump 30. The discharge of this pump is connected, via a line 31, to line 8B.

 In this installation, one column works in purification while the second works in regeneration, then the role of each of the columns is reversed.

 When column 1A operates in purification, that is to say in the condensation phase, all of the valves A are open, all of the valves B are closed.

 The polluted gas mixture introduced via line 8A into column 1A circulates in an ascending manner in the latter. The condensates flow downwards and collect at the foot of the column 1A before reaching the capacity 5 via the line 12A.

Liquid nitrogen, the flow rate of which can be regulated via the temperature control IIIA and the valve IIA, circulates in line 4A: first it cools the adsorbent bed 18A, then it vaporizes in the multi-pass exchanger 3A, and finally it is heated in the counter current exchanger 2A.

 The purified air escapes from column 1A via the purified gas evacuation pipe 28A, which, like line 28B, opens into a common evacuation pipe 28.

 Simultaneously, column 1B is regenerated, as follows.

 The nitrogen gas heated in the exchanger 2A is, at least in part, conveyed to the heater 27 via the line 25A, then sent to the column 1B via the lines 26 and 26B. It then circulates in line 4B as follows: first it crosses the adsorbent bed 18B while heating it to facilitate the desorption of the hydrocarbons, then it traverses the multi-pass exchanger where it melts any crystals, and finally it heats the counter-current exchanger 2B. At the outlet of column 1B, the gaseous nitrogen is sent into line lOB to be either discharged into the atmosphere, or directed to a subsequent application. Simultaneously, to promote desorption, gas is withdrawn from column 1B via line 28B and pump 30, which pumps this gas into column 1A via lines 31, 8B and 8A.

 When column 1A is blocked to a determined degree, which can be verified by measuring the pressure difference in the system by the means 15 already described, connected between lines 8 and 25 and common to the two columns, columns 1A and 1B swap roles. All the pipes A are closed, the pipes B are opened by means of the valves arranged on these pipes, and the column 1A enters into regeneration while the column 1B begins to condense.

 In this type of device, the adsorption device is optional and will be used only to produce ultra-pure air.

 By way of illustration, a device in which each column has a diameter of 900 mm and a height of 5,500 mm makes it possible to treat an amount of 1,750 Nm3 per day with 3.5 liters of liquid nitrogen per kilogram of gasoline recovered.

 It emerges from the description which has just been given that the devices according to the invention are very easy to use and also very safe since the hydrocarbon vapors come into contact only with immobile surfaces of the exchangers, which are typically made of stainless steel and copper, which minimizes the risk of explosion.

 The liquid nitrogen vaporized in the coolant circulation line can be reused in regeneration in a double column device, and also in a subsequent application.

 The absence of a rotating element and the reduced number of elements minimizes maintenance: only a limited number of parts such as solenoid valves and regulators are necessary for the proper functioning of the system.

 The use of liquid nitrogen makes it possible to very precisely produce the cold production just required by the treated gas mixture with variable hydrocarbon vapor content: the response capacity of the device is very high, whether the refrigeration power is high or low.

 For the above reasons (few exchangers, a single stream of nitrogen for the two condensers, and possibly for the regeneration of a first column during the operation of a second), the cost of such devices can be kept at a very low level.

 The invention also applies to the recovery, in analogous situations, of various volatile compounds, in particular organic, (acetone, methanol, etc.) contained in air or in another gas such as nitrogen d inerting.

Claims (13)

 1. Installation for the recovery of volatile compounds present in the form of vapor in a gaseous mixture, characterized in that it comprises at least one purification column (1; 1A, 1B) containing a first condenser (2; 2A, 2B ) located in the lower part of the column and a second condenser (3; 3A, 3B) arranged above the first, the two condensers being crossed by the same line (4; 4A, 4B) of cooling nitrogen circulation , supplied with liquid nitrogen upstream of said second condenser (3), in which the nitrogen vaporizes, and delivering heated nitrogen gas downstream of said first condenser, said column being provided in its lower part with means (8; 8A , 8B) for supplying said gas mixture and means (12 12A, 12B) for withdrawing condensate situated at a level below the first condenser, and also being provided with means for interrupting its supply of liquid nitrogen and means (12; 26A, 26B) of food tation in a heating fluid of the two condensers.  2. Installation according to claim 1, characterized in that said first condenser (2; 2A, 2B) is a heat exchanger of the counter-current type  3. Installation according to claim 1 or 2, characterized in that said second condenser (3 3A, 3B) is a heat exchanger of the multi-pass type with baffles (103).  4. Installation according to any one of the preceding claims, characterized in that the or each purification column (1; 1A, 1B) also contains, in its upper part, a hydrocarbon adsorption device (18, 19, 20; 18A, 19A) disposed above said second condenser (3; 3A, 3B) and comprising a bed of adsorbent (18; 18A, 18B) crossed by said line (4; 4A, 4B) of nitrogen circulation cooling upstream of the second condenser.  5. Installation according to any one of the preceding claims, characterized in that the or each column is equipped with means (15) for measuring the pressure difference of the treated gas between its upper part and its lower part.  6. Installation according to any one of the preceding claims, characterized in that it comprises means (5, 6) for storing said condensate connected to said means (12; 12A, 12B) for withdrawing condensate.  7. Installation according to claim 6, characterized in that it comprises a single column (1) and means (14) for circulating liquid alternately from the lower part of said column (1) to said means (5 , 6) for storage, and from said means (5, 6) for storage towards the lower part of said column (1).  8. Installation according to claims 5 and 7 taken together, characterized in that said means (14) for circulating the liquid are controlled by said means (15) for controlling the pressure difference for the circulation of the liquid from said means (5 , 6) for storage towards the lower part of said column (1).  9. Installation according to claims 4 and 7 taken together, characterized in that it comprises means (19, 20, 21) for isolating the adsorption device from the rest of the column when liquid is conveyed from said storage means (5, 6) towards the lower part of the column.  10. Installation according to claim 9, characterized in that it comprises means (22) for regeneration by evacuating the adsorption device (18, 19, 20).  11. Installation according to any one of claims 1 to 6, characterized in that it comprises two purification columns (1A, 1B) connected in parallel to means (8) for supplying said gaseous mixture, and in that regeneration gas supply lines (25A, 25B, 26, 26A, 26B) connect the outlet end of a line (4A, 4B) of cooling nitrogen circulation to the end of inlet of the other line (4B, 4A) for cooling nitrogen circulation, the nitrogen serving as heating fluid in this other line.  12. Installation according to claim 11, characterized in that it comprises control means for supplying only one column at a time with said gas mixture and, simultaneously, closing the communication between the inlet end of the cooling nitrogen circulation line of this column and the outlet end of the cooling nitrogen circulation line of the other column.  13. Installation according to claim 11 or 12, characterized in that a heater (27) is installed on said pipe (25A, 25B, 26, 26A, 26B) for supplying regeneration gas.