FR2863907A1 - Separator comprising a decanting basin with the inlet and outlet levels above the internal level of the fixed roof, in particular for separating hydrocarbons from water without evaporation to air - Google Patents

Abstract

The decanting basin (1) has an inlet (3) and an outlet (4) allowing the liquid to circulate. The levels of the inlet and outlet orifices are above the internal level (20) of the fixed roof (2). The inlet orifice has an inlet compartment (32) with an inlet pipe (31) placed above the internal level of the fixed roof, through which the loaded liquid is brought, and an opening (33) disgorging into the decanting basin, below the internal level of the fixed roof. The inlet compartment has an inlet overflow to limit the flow of liquid in the decanting basin. The outlet orifice is a siphon partition, formed by two vertical walls. The upstream wall (41a) leaves a passage at its low point and the downstream wall (41b) leaves a passage at its high point. The passage at the high point of the downstream wall forms an outlet overflow with a level above the internal level of the fixed roof. Vents (23) in the fixed roof are at atmospheric pressure, and are connected to a vent collector (24) opening to free air via a pipe. The diameters of the vents, the collector and the pipe are dimensioned to maintain the liquid surface at a saturated vapour pressure. The outlet orifice has means (47) of limiting the flow of liquid circulating in the decanting basin, in particular a retention container (47) for automatic overflow of the loaded liquid.

Description

The present invention relates to a fixed roof separator for

  the purification of a liquid loaded with product, especially hydrocarbons.

  It particularly concerns the separation of hydrocarbons mixed with water and their storage without evaporation in air. It is preferably adapted to separation and deoiling tanks generating very toxic aromatic hydrocarbon vapors such as benzene, toluene or xylene.

  The invention more generally relates to the separation of a product mixed with a liquid, immiscible with this liquid, having a density lower than that of said liquid and which one wants to avoid venting after separation for environmental reasons (ex : emission of polluting vapors), technical (eg oxidation of the separated product) or economic (ex: loss of a quantity of the separated product by evaporation).

  The separation tanks, also called settling ponds, de-oiling ponds or API ponds (in reference to American Petroleum Institute standards), are in principle open pit. As a result, the evaporation of hydrocarbons, such as benzene, which decant on the surface is very important. A settling tank separating annually 20,000 tons of benzene from the water can thus evaporate 3 to 5% or 600 to 1000 tons per year.

  Volatile Organic Compound (VOC) emission regulations, such as benzene, impose a very significant reduction in these emissions or even the total absence of emissions. Subsidies can be granted to industrialists to help them set up their facilities.

  Different solutions exist to reduce or eliminate VOC vapors. The petrochemical sites include floating roof tanks and fixed roof tanks.

  The tanks with floating roof make it possible, as a priority, the storage of the hydrocarbons while avoiding their evaporation in the air and incidentally their separation. These tanks generally consist of a vertical cylindrical envelope inside which slides a roof that floats on the surface of the stored product.

  The roof rests on the surface of the stored product and moves with it when filling or emptying the tank. There is no longer a layer of air between the surface of the stored product and the floating roof, VOC vapors can then no longer be formed.

  However, there are losses by evaporation of the stored product, because the seal is difficult to achieve between the edges of the floating roof and the cylindrical shell of the tank. This solution is therefore poorly suited for storing products with a high potential to emit VOC vapors.

  Fixed roof tanks are an alternative to floating tanks. In this case, a roof is fixed permanently above the tank.

  A gaseous sky, including VOC vapors emitted by evaporation, therefore exists continuously above the stored product.

  The solution regularly used to remove these VOC vapors is nitrogen inerting. The inerting is monitored regularly due to the migration of dissolved oxygen in the water of the liquid mixture to be treated to the gaseous sky. Indeed, in case of too much oxygen, the risk of explosion being increased, nitrogen is injected. The average flow rate of injected nitrogen is of the order of 10 m 3 / h. In addition and for safety, a slight overpressure is maintained in the gaseous sky to prevent any air coming from outside. Nitrogen, which is then charged with VOC, is expelled during purge injections and sent to a treatment unit that removes VOCs before being released into the atmosphere.

  There are also fixed-roof tanks with a floating roof to minimize evaporation of the stored product. But this solution does not completely solve the technical problem caused by the poor seal mentioned above, at the floating roof.

  At the petrochemical sites, one or more surface skimmers continuously pump the hydrocarbons present in the settling ponds to convey them to a water / hydrocarbon separator. These hydrocarbons are then stored to a tank which must also be equipped with a VOC vapor elimination device.

  The solutions described above thus have a certain number of disadvantages: the obligation to provide a surface skimmer to avoid the dangerous storage of too much hydrocarbons; obligation to provide a water / hydrocarbon separator for not unnecessarily storing water and a storage tank for hydrocarbons; emission of VOC vapors in the case of a fixed-roof storage tank requiring a VOC treatment unit; high nitrogen consumption; measurement and regulation of the gaseous sky pressure; monitoring the amount of oxygen in the gaseous sky; danger of explosion in case of lack of inerting; The purpose of the present invention is to mitigate this state of affairs, in particular because it makes it possible to treat a continuous flow for separation and storage in a specific settling basin, hydrocarbons mixed with water or any other mixed product. a liquid, immiscible with the liquid, having a density less than that of said liquid and which is to avoid venting after separation.

  Its purpose is also to prevent any air evaporation of the trapped products and thus to suppress the formation of VOC vapors in the case of hydrocarbons or the risk of oxidation, for example, in the case of other products, by eliminating the presence of a gaseous sky inside said settling tank.

  Another aim is to no longer resort to gaseous inerting or the treatment of VOC vapors in the case of the separation of hydrocarbons.

  These aims are achieved by a separator intended for the purification of a fluid loaded with product, in particular hydrocarbons, comprising a settling basin covered by a fixed roof, said settling basin being provided with an inlet and a an outlet port for the circulation of the charged liquid, characterized in that the levels of said inlet port and said outlet port are above the internal level of said fixed roof.

  This particular configuration allows a permanent hydraulic loading which prevents any entry of air into the settling tank may form a gaseous sky above the separate product.

  More particularly, this permanent hydraulic loading is obtained through an inlet port having an inlet compartment whose level can never go below the internal level of the fixed roof, even at zero inlet flow and an overflow the level of which is located above the internal level of the fixed roof.

  The objects of the invention are also achieved by the establishment of a siphoid partition in the settling basin for separating the product from the charged liquid and to store it.

  Vents can also be put in place to avoid any risk of formation of a gaseous sky in the settling basin.

  Other characteristics and advantages of the invention will emerge more clearly on reading the following description, given by way of non-limiting indication, with reference to the appended drawings, in which: FIG. 1 is a vertical sectional view along AA of the separator object of the invention showing its different constituent elements; Figure 2 is a vertical sectional view along B-B of the inlet port of the separator according to the invention.

  The description relates to the separation of hydrocarbons mixed with water. But the invention, more generally, applies to the separation of a product mixed with a liquid, immiscible with the liquid, having a density lower than that of said liquid and which is to avoid venting after separation .

  Referring to Figure 1, the separator object of the invention is constituted by a settling tank 1 which allows the water to be purified gradually and which allows to store the hydrocarbons which we want to avoid the setting air after separation. A fixed roof 2 covers the upper part of this settling basin.

  The charged liquid is fed to the settling basin via an inlet orifice 3. This inlet orifice is constituted by an inlet duct 31 whose level 34 is situated above the internal level 20 of the fixed roof 2. This inlet conduit leads to an inlet compartment 32 that can be integrated into the structure of the settling tank. This compartment has in its lower part an opening 33 opening into the settling basin 1, below the internal level of the fixed roof 2.

  As shown in FIG. 1, the arrangement of these different elements is such that, after filling of the settling basin, the minimum level of charged liquid contained in the inlet compartment 32, corresponding to the level 34 of the inlet duct, can never descend, even at zero input flow, below the internal level of the fixed roof.

  The maximum flow rate of charged liquid entering the settling basin is regulated by a valve 35 placed at the opening 33. This valve also makes it possible to isolate the settling basin when intervention is necessary.

  The inlet compartment 32 is equipped in its upper part with an inlet overflow 36 defining the maximum level of charged liquid admitted into this compartment. As long as the incoming flow rate is less than or equal to the maximum flow rate set by the valve 35, the charged liquid is completely diverted to the settling basin 1. When the inflow is greater than the set flow (in the event of a storm, for example), the excess passes over the inlet overflow to be directed, through an emergency line 37, to another separator or a spare pool (Figure 2).

  A deflector 11 is placed in the settling tank 1, facing the opening 32, to change the direction of the liquid flow loaded into the settling tank 1, for good settling. The dimensions of the settling basin allow to respect the ratios of horizontal and vertical speeds ensuring a good decantation.

  These ratios are calculated according to the type of products to be separated and make it possible to define the adequate dimensions of the settling basin.

  The heavy suspended matter settled in a settling cone 12 located on the lower part of the settling tank 1 (Figure 1). The sediments or oily sludge thus stored can then be evacuated via a conduit 14 provided for this purpose.

  The settling basin is equipped with a siphoid partition at the outlet orifice 4. This siphoid partition is formed by two substantially vertical walls, one of which, upstream 41a, leaves a passage at its low point and the the other, situated downstream 41b, leaves a passage at its high point (FIG. 1).

  The passage at the high point of the downstream wall 41b forms an outlet overflow 42 whose level is located above the internal level of the fixed roof 2.

  The level of the overflow 42 corresponds to the level 34 of the inlet duct, this in order to avoid any return in case of zero flow.

  The hydrocarbons, having a density lower than the density of the water, will migrate towards the upper part of the settling basin and will be trapped at the upstream vertical wall 41a, between the fixed roof 2 and the water in the course of decantation (Figure 1). The purified water 43, of higher density than that of the hydrocarbons, will settle towards the lower part of the settling basin and after passing through the low point of the siphoid partition, will pass over the outflow overflow 42 and pour into the a lifting station 44 for storage. The purified water is then discharged through an outlet pipe 45 placed in the lower part of the lifting station. The purified water level in the lifting station 44 can be kept constant, regardless of the treated liquid flow rate treated, by means of a slave discharge pump placed at the outlet duct 45.

  The levels on the inlet and the outlet of the settling basin 1 are therefore located above the internal level of the fixed roof 2. This arrangement allows a permanent hydraulic loading of the settling basin, the separator object of the invention is having in fact like a big U-shaped pipe filled with liquid. There is therefore no more air intake possible in the settling basin during the circulation of the charged liquid, the hydrocarbons or any other separated product remaining in permanent contact with the fixed roof 2. The risks of evaporation, VOC formation, ignition and explosion are thus completely eliminated.

  Referring to Figure 1, the upstream wall 41a of the siphoid wall and the downstream wall of the lifting station serve to form a retention chamber 47 placing the separator object of the invention in automatic overflow: if the slave pump placed in the Exhaust duct level 45 fails, the containment enclosure 47 fills with clean water until there is no further circulation downstream of the settling basin. In this case, upstream of the settling basin, the charged liquid overflows from the inlet compartment 32 by the inlet overflow 36 to be directed to another separator or to a relief basin.

  Automatic overtopping occurs in an equivalent manner if the possible discharge rate on the slave pump placed at the outlet duct 45 becomes less than the flow rate set by the valve 35.

  In the presented configuration, a downstream lifting of the liquid to be treated is performed, but this lifting could be upstream with downstream discharge in a gravity outlet channel.

  Similarly, the installation could be completely gravity and have no upstream lift or downstream lift, but simply be equipped with an inlet duct and an outlet duct connected to the settling basin to allow the flow of the liquid loaded , the levels of these two ducts being located above the internal level of the fixed roof 2.

  Visits hatches 22 located on the fixed roof 2 make it possible to communicate with the interior of the settling basin 1 during an intervention.

  In some cases and in particular for the treatment of water loaded with hydrocarbons, microbubbles of air or gas may be trapped or form and rise towards the fixed roof 2 during separation. To avoid these risks of forming pockets of gas or gaseous, it is expected to put the pressure vessel atmospheric through vents 23, located at the hatches of visits, which let escape the gases of the settling tank. These vents are connected to a vent collector 24 opening into the open air. The vent collector 24 is connected to a flame shield 26 to prevent any risk of backfire or explosion.

  Vents at atmospheric pressure are partially filled with hydrocarbons. The hydrocarbon height in these vents (reference 25, FIG. 1) is then that of the level of the outlet overflow 42. The gas pockets or the gaseous sky can not therefore be formed inside the settling basin.

  The vent collector 24 opens in the open air through a small section duct.

  The diameters of the vents, the vent collector and the open-air duct will be sized to minimize the evaporation surface and to keep the hydrocarbon liquid surface at a saturating vapor pressure, since evaporation can no longer occur under these conditions. The diameter of all these ducts will preferably be around 25 millimeters.

  The vents 23 are also used during the filling of the settling basin 1: as the liquid level increases in this settling basin, the gas pocket between the charged liquid and the fixed roof is driven by these vents. The same result could be obtained by leaving the trap doors open until the settling basin is completely filled.

  The vents are used only in the case where there is a risk of forming a gaseous sky resulting from the imprisonment or the formation of gas in the charged liquid to be treated.

  The very good conditions of conservation of the product in the upper part of the settling basin allow its storage medium or long term for its future recovery. With extraction ducts 27 located at the fixed roof 2, the product can be directly dumped by truck or pumped to units where it will be used.

  These good storage conditions concern in particular the safety aspect and the protection against oxidation of the product. The storage conditions are close to that of a floating roof pond, but in the liquid separator according to the invention, the water floor is mobile.

  Monitoring and management of this storage is achieved through the implementation of an interface measurement evaluating the thickness of the product stored in the settling basin. This measurement is performed by a probe 28 of the radar type where a wave is reflected at the interface of two different dielectrics, any other equivalent measuring means can be used.

  The measurement of this thickness, correlated with the geometry of the settling basin, allows precise knowledge of the volume of stored product. The maximum storable volume is in particular a function of the geometry of the siphoid partition and the vertical position of the opening 33 at the settling basin 1 which can be inconveniently well below the internal level of the fixed roof 2. The volume of the storage can reach more than half the volume of the settling basin 1.

  According to a preferred embodiment, the interface measurement is associated with alarm or fault thresholds respectively when the maximum storage volume is reached or when, after removal, the entire product has been extracted.

  The invention finds particular application in the construction of settling tanks and separation of highly toxic aromatic hydrocarbons such as benzene, toluene, or xylene in oily sewers, ponds predecantation of light oils, separators Hydrocarbon storage facilities, hydrocarbon separation units for ships' ballast water and for the storage of residual hydrocarbons, barges for the recovery of oil polluted water.

  It relates more generally to the separation of a product mixed with a liquid, immiscible with this liquid, having a density lower than that of said liquid and which it is desired to avoid venting after separation for environmental, technical or economic reasons .

  The presence and positioning of the various constituent elements give the object of the invention a maximum of useful effects that had not been obtained so far by similar devices.

2863907 -

Claims (10)

  1. Separator for purifying a liquid loaded with product, in particular hydrocarbons, comprising a settling tank (1) covered by a fixed roof (2), said settling basin being provided with an inlet orifice (3) and an outlet (4) for the circulation of the charged liquid, characterized in that the levels of said inlet port and said outlet port are above the internal level (20) of said fixed roof. .   Separator according to Claim 1, characterized in that the inlet orifice (3) comprises an inlet compartment (32) provided with an inlet duct (31) situated above the internal level ( 20) of the fixed roof (2) through which is fed the charged liquid, and an opening (33) opening into the settling tank (1), located below said internal level of the fixed roof.   3. Separator according to claim 2, characterized in that the inlet compartment (32) comprises an inlet overflow (36) for limiting the flow of charged liquid circulating in the settling tank (1).   4. Separator according to one of the preceding claims, characterized in that the outlet (4) is formed by a siphoid partition.   5. Separator according to claim 3, characterized in that the siphoid wall is formed by two substantially vertical walls, the upstream wall (41a) leaving a passage at its low point and the downstream wall (41b) leaving a passage to its high point.   6. Separator according to claim 4, characterized in that the passage at the high point of the downstream wall (41b) forms an outlet overflow - 12 - (42) whose level is located above the internal level (20). fixed roof (2).   7. Separator according to one of the preceding claims, characterized in that vents (23), located on the fixed roof (2), are placed at atmospheric pressure.   8. Separator according to claim 6, characterized in that the vents (23) are connected to a vent collector (24) opening into the air through a conduit, the diameters of said vents, said collector and said duct being dimensioned to maintain the liquid surface at a saturating vapor pressure.   9. Separator according to one of the preceding claims, characterized in that the outlet (4) comprises a means (47) for limiting the flow of charged liquid circulating in the settling tank (1).   10. Separator according to claims 3 and 9, characterized in that the means for limiting the flow of charged liquid circulating in the settling tank (1) is a retention chamber (47) for the automatic overflow of the liquid. charge..