APPARATUS AND METHOD FOR EVAPORATING LIQUIDS CONTAINING POTENTIALLY EXPLOSIVE IMPURITIES

Abstract

The present invention relates to an apparatus and a method for evaporating liquids containing potentially explosive impurities of lower volatility than the actual liquid compound. The set-up of the evaporator according to the invention allows its operation with complete evaporation of a liquid without formation of a liquid sump of not yet evaporated liquid.

Claims

1. An Evaporation apparatus (100) comprising: (i) at least one inlet (2) for a liquid (1) to be evaporated, the inlet(s) (2) being located at the top of the evaporation apparatus (2.1) and/or on a side of the evaporation apparatus (2.2); (ii) optionally a seal pot (3) into which any inlet (2.1) immerses; (iii) a liquid distributor (4), which is located (a) underneath any inlet (2.1) or, if present, underneath the seal pot (3), and/or (b) above any inlet (2.2), with the liquid distributor (4) being connected to the inlet (2.2); (iv) an upper heating unit (5.1) arranged horizontally in the evaporation apparatus underneath the liquid distributor (4); (v) a lower heating unit (5.2) arranged horizontally or with a downward slope in the evaporation apparatus underneath the upper heating unit (5.1); (vi) a heatable flat plate (10) arranged horizontally in the bottom of the evaporation apparatus underneath the lower heating unit (5.2); and (vii) an outlet (15) for the evaporated liquid (i.e. the desired gas stream) (14).

2. The evaporation apparatus of claim 1, in which the upper heating unit (5.1) and the lower heating unit (5.2) both comprise a bundle of tubes.

3. The evaporation apparatus of claim 2, in which the upper tube bundle (5.1) and the lower tube bundle (5.2) are connected to each other with a bended U-shaped connecting piece thereby forming a U-shaped tube bundle (5) with an upper part (5.1) and a lower part (5.2).

4. The evaporation apparatus of claim 2, in which the lower tube bundle (5.2) has a downward slope of from >0.7°.

5. The evaporation apparatus of claim 2, in which the layers of heatable tubes which form the tube bundles are arranged such that the gaps between individual tubes of one layer of tubes are covered by tubes of the layer of tubes above and/or underneath it.

6. The evaporation apparatus of claim 1, in which a baffle plate (18) having slots for the heating units (5.1) and (5.2) is arranged in the evaporator vertically above the heatable flat plate (10) in a position between the inlet (1) and the outlet (15) such that the heating units (5.1) and (5.2) run through the slots and the baffle plate's upper end extends to the inner top shell of the evaporator.

7. The evaporation apparatus of claim 1, in which a guiding device is arranged in the evaporator so as to direct the gaseous stream of evaporated liquid (1) compulsorily over the surface of the horizontally-aligned heatable flat plate (10).

8. The evaporation apparatus of claim 7, in which the guiding device is a baffle plate (18) having slots for the heating units (5.1) and (5.2), which is arranged in the evaporator vertically above the heatable flat plate (10) in a position between the inlet (1) and the outlet (15) such that the heating units (5.1) and (5,2) run through the slots and the baffle plate's upper end extends to the inner top shell of the evaporator and the baffle plate's lower end extends o the section of the evaporator which is underneath the lower heating unit (5.2).

9. The evaporation apparatus of claim 1, in which the liquid distributor (4) is equipped with guiding vanes (4.1).

10. The evaporation apparatus of claim 1, in which the flat plate (10) is equipped with a visible circumferential edge (10.1).

11. The evaporation apparatus of claim 1, in which the liquid (1) is selected from the group consisting of chlorine, dinitro toluene and ethers.

12. A method for operating an evaporation apparatus according to claim 1, comprising: (I) introducing a liquid (1) to be evaporated through (a) an inlet (2.1), if present via the seal pot (3), and/or (b) through an inlet (2.2) onto the liquid distributor (4) and from there onto the upper heating unit (5.1), which is heated, whereby the mass flow of the liquid (1) is chosen such that the design evaporation capacity provided by the upper heating unit (5.1) is not exceeded; (II) guiding any not evaporated droplets onto the heated flat plate (10); and (III) discharging the evaporated liquid (14) via the outlet (15).

13. The method according to claim 12, in which the heating units (5.1) and (5,2) are steam-heated.

14. The method according to claim 12, in which the flat plate (10) is steam-heated.

15. The method according to claim 12, in which the liquid (1) is selected from the group consisting of chlorine, dinitro toluene and ethers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Various embodiments of the invention are described hereinafter. Different embodiments can be combined with one another as desired, unless the context suggests otherwise.

[0033] FIG. 1 shows a preferred embodiment of an evaporating apparatus (100) according to the invention.

[0034] FIG. 2a shows a schematic cross-sectional view of an evaporating apparatus (100) according to the invention.

[0035] FIG. 2b shows an enhanced plan view on the top surface of the heatable flat plate (10) of the evaporator (100) shown in FIG. 2a.

DETAILED DESCRIPTION OF THE INVENTION

[0036] Suitable inlets (2) are known to the skilled person, for example feed pipes, which are preferably equipped with shut-off devices. In one embodiment of the invention, the inlet (2) is located at the top of the evaporator (inlet type (2.1)). In a preferred embodiment, the inlet (2.1) is dipped into a seal pot (3) which acts as a hydraulic lock and thus prevents a back-flow of evaporated liquid into the inlet (2).

[0037] In another embodiment of the invention, the evaporator comprises an inlet (2.2) which is located at a side of the evaporation apparatus, in which case the liquid distributor (4) is located above said inlet (2.2) and is connected with said inlet (2.2). In this context, the term “connected” means that said inlet (2.2) is arranged with respect to the liquid distributor (4) such that any liquid (1) which is introduced into the inlet (2.2) can flow through the inlet (2.2) upwards onto the liquid distributor (4) and from there downwards onto the upper heating unit (5.1). This embodiment is especially useful in cases with varying liquid flow (1), because in such cases a seal pot (3) might not be sufficient to safely avoid the back-flow of evaporated liquid (1) into the feed piping system connected to the evaporator. In all cases where this effect might occur and disturb the process operation, the liquid inlet (2.2) can be used to feed the liquid (1). As a result of the connection between the inlet (2.2) and the liquid distributor (4), a liquid leg is formed in the inlet (2.2) and the piping system which connects the inlet (2.2) to the reservoir of the liquid (1). This arrangement will safely avoid any disturbance of the process by the backwards movement of gas bubbles formed by evaporated liquid (1).

[0038] It is also possible to construct the evaporator such that it comprises both kinds of inlets, (2.1) and (2.2). Preferably, only one kind of inlet is used at a time, the other being shut-off. The choice which kind of inlet is actually used depends on the operational circumstances. For example, if no great reservoir of liquid (1) is available, it is preferred to feed the liquid (1) via inlet (2.1) into the evaporator. Thereby pressure is created without an additional pump. If, on the other hand, large amounts of liquid (1) are to be evaporated, e.g. from a buffer tank, it is preferred to feed these through an inlet (2.2) into the evaporator.

[0039] The liquid distributor (4) ensures even distribution of the liquid (1) over the upper heating unit (5.1). Suitable liquid distributors are known in the art and are for example described in Perry's Chemical Engineers' Handbook, Perry's chemical engineers' handbook, Don W. Green, Robert H. Perry, eighth edition 2008, McGraw-Hill Professional, ISBN 9780071422949, Chapter 14.4.5, “Distributors”.

[0040] In a preferred embodiment, the liquid distributor (4) is a distribution tray. In a further alternative design, the liquid distributor (4) is equipped with downwardly directed guiding vanes (4.1) which avoid that liquid leaving the liquid distributor may be spread directly to the shell of the evaporator where it might by-pass the heating area.

[0041] The heating units (5.1) and (5.2) are designed such that they can be heated sufficiently to evaporate and overheat the liquid (1), respectively. Heating can be accomplished, for example, electrically, or by passing a suitable heating medium such as steam, a salt melt, hot water, hot oil or hot combustion gases through the inside of the heating units (5.1) and (5.2). Steam-heating, however, is preferred.

[0042] Suitable embodiments of heating units (5.1) and (5.2) are, for example, heater coils or heatable tubes. In the case of heatable tubes, these have either a plain surface or a structured surface (fins, ribs, grooves, etc.) to improve the heat transfer values. In a preferred embodiment of the evaporation apparatus, the heating units (5.1) and (5.2) are bundles of heatable tubes, each bundle comprising of from 10 to 2,000 tubes, preferably of from 100 to 1,000 tubes, more preferably of from 200 to 500 tubes. It is preferred that the layers of heatable tubes which form the tube bundles are arranged such that the gaps between individual tubes of one layer of tubes are covered by tubes of the layer of tubes above and/or underneath it as shown schematically in FIG. 2a.

[0043] In a particularly preferred variant of the embodiment with heatable tubes as heating units, the upper tube bundle (5.1) and the lower tube bundle (5.2) are connected to each other with a bended U-shaped connecting piece, i.e., the respective upper and lower tubes constitute two parts of on piece of equipment, a U-shaped tube bundle (5) with an upper part (5.1) and a lower part (5.2). This design renders the installation of an expansion joint in the heat exchanger shell for compensating thermal stresses unnecessary. Such a compensator is usually a weak point in the mechanical design and would furthermore bear the risk of formation of a sump of not evaporated liquid (1).

[0044] In a further alternative design of this embodiment, the upper part (5.1) of the U-shaped tube bundle is aligned horizontally, which avoids a deflection of liquid chlorine droplets alongside the tubes, whereas the lower part (5.2) has a downward slope of from >0.7°, preferably of from 0.8° to 5°, more preferably of from 1° to 3°. If the tube bundle (5) is heated by steam, which is the most preferred heating mode, the drainage of steam condensate from the tubes is improved thereby.

[0045] The horizontally-arranged upper heating unit (5.1) acts as heating area to evaporate the liquid (1), whilst the lower heating unit (5.2) serves to overheat the gaseous stream of evaporated liquid. It is preferred that the lower heating unit (5.2) has the same theoretical heating capacity as the upper heating unit (5.2).

[0046] In a preferred embodiment of the invention, a baffle plate (18) having slots for the heating units (5.1) and (5.2) is arranged in the evaporator vertically above the heatable flat plate (10) in a position between the inlet (1) and the outlet (15) such that the heating units (5.1) and (5.2) run through the slots and the baffle plate's upper end extends to the inner top shell of the evaporator. The lower end of the baffle plate may extend to a position immediately underneath the lower end of the lower heating unit (5.1) so as to just enclose the lowest part of the lower heating unit (5.1). It may, in a preferred embodiment described in more detail below, also significantly extend into the section of the evaporator which is underneath the lower heating unit (5.2) as shown in FIG. 1. However, in neither case does the lower end of the baffle plate extend completely to the heatable flat plate (10) itself. By this arrangement, the evaporator is separated into two areas:

[0047] A first area which is dedicated to the evaporation of the introduced liquid (1) and in which the gaseous stream of evaporated liquid flows downwards co-currently with not-yet evaporated liquid droplets (i.e. the area on the side of the baffle plate (18) which faces the inlet (2)) and a second area where the evaporated liquid is directed upwards towards the outlet (15) on top of the evaporator (i.e. the area on the side of the baffle plate (18) which faces the outlet (15)). In the second area, the evaporated liquid is overheated before it leaves the evaporator via the outlet (15).

[0048] When the baffle plate (18) is used in combination with a U-shaped tube bundle (5), the baffle plate (18) is preferably located in the position which separates the straight parts (5.1 and 5.2) of the tube bundle (5) from the bended connecting piece as shown in FIG. 1.

[0049] The horizontally-aligned heatable flat plate (10) serves as a safety measure which ensures evaporation of any non-evaporated liquid droplets which should pass the two heating units (5.1) and (5.2). Under ordinary operating conditions, it is not to be expected that any droplets of liquid (1) should reach the bottom of the evaporator. However, in case of irregular operating conditions like, for example, a failure or shortage of the heat supply to the heating units (5.1) and (5.2), it might happen that some droplets of liquid (1) pass the heating units (5.1) and (5.2). Single droplets of liquid (1) which pass the heating units of the evaporator will immediately evaporate when touching the surface of the plate (10), without a possibility for accumulating dangerous amounts of hazardous substances. To this end, the flat plate (10) is heated, preferably from below with steam, most preferably with saturated steam. A larger amount of liquid (1), will, due to the horizontally-levelled alignment of the flat plate (10), evenly distribute over the surface of the plate (10). By the horizontal alignment it is avoided that liquid (1) will collect in one corner of the heated plate. A possible exceeding of the area-related acceptable threshold concentration of possibly explosive substances in said liquid (1) is thereby safely avoided. (The area-related threshold concentration refers to the amount of the possibly explosive compound per area. In case of nitrogen trichloride in chlorine, a value of 1.5 g/cm.sup.2, preferably of 0.3 g/cm.sup.2, should not be exceeded; see Euro Chlor GEST 76/55. More reference values for acceptable concentrations of explosive impurities can be found in the respective technical literature.) The liquid (1) which is collected on the horizontally-aligned flat plate (10) will then be evaporated again and leave the evaporator together with the main flow (14) of evaporated liquid through the outlet (15). In order to ensure that the flat plate (10) is aligned as near an ideal horizontal orientation as possible, it is preferred that the flat plate (10) be equipped with a visible circumferential edge 10.1 as shown in FIG. 1.

[0050] It is preferred to construct the flat plate (10) with sufficient mechanical strength in order to avoid any damage in case hazardous impurities (such as, for example, NCl.sub.3 in chlorine), although only present in an amount not yet sufficient to cause an explosion, should decompose in an exothermic reaction. The impact of such an accelerated decomposition can be furthermore limited by separating the surface area of the horizontally-aligned flat plate (10) into smaller sub areas by means of installation of curbs (11) on the surface of the plate (10). These curbs will stop the propagation of a starting decomposition and thus mitigate the hazardous effects any decomposition inevitably has. It is preferred that the curbs are of a relatively low height such as from 1 mm to 5 mm.

[0051] In a preferred embodiment, the gaseous stream of evaporated liquid (1) is directed to flow compulsorily over the surface of the horizontally-aligned heatable flat plate (10) by means of a suitable guiding device known to the skilled person, such as a guide plate, a guide tube, a baffle plate and the like. In doing so, the evaporating liquid (1) on the surface of the flat plate (10) is kept in a thermodynamic equilibrium with the gaseous stream of already evaporated liquid (1), thereby avoiding that the flat plate (10) might act as a second distillation stage, which would lead to a furthermore increased concentration of hazardous substances in the remaining liquid (1). In a preferred design, the desired direction of the gas flow over the surface of the horizontally-aligned heatable flat plate (10) is obtained by a direct extension of the above mentioned baffle plate (18) which separates the pipe section of the evaporator into the section of the evaporator which is underneath the lower heating unit (5.2) as shown in FIG. 1.

[0052] The method for operating the evaporation apparatus according to the invention is primarily characterized in that the mass flow of the liquid (1) to be evaporated is chosen such that the design theoretical evaporation capacity provided by the upper heating unit (5.1) is not exceeded. In doing so, the probability of the formation of a liquid sump of non-evaporated liquid in the bottom of the evaporator is greatly reduced and normally not to be expected at all. In the preferred embodiment of a U-shaped tube bundle (5) with an upper part (5.1) and a lower part (5.1), the upper heating unit (5.1) is considered to be encompassed only by the straight part of the tube bundle (5.1), i.e. the bended connecting piece combining the upper and lower part to form one piece of equipment is, for the purpose of determining the theoretical design evaporation capacity, not considered to be part of the upper heating unit (5.1).

[0053] To this end, the mass flow of the liquid (1) to be evaporated must be adjusted to a given theoretical evaporation capacity of the upper heating unit (5.1). The theoretical evaporation capacity of the heating unit depends on various factors, such as the surface area and surface shape of the heating unit, the amount of heat supplied per hour to the heating unit, temperature of the heating medium, physical data of the liquid to be evaporated etc. All these factors are known for a given design of an evaporator, so that the skilled person can easily calculate the theoretical evaporation capacity of the upper heating unit (5.1).

[0054] The supply of heat to the heating units (5.1) and (5.2) is preferably chosen such that no decomposition reactions or any other undesired reactions (such as, in case of a potentially corrosive liquid (1), corrosion of the evaporator material) are to be expected. For example, in the case of the evaporation of liquid chlorine using steam as heat source, this means that the absolute pressure of the steam used is preferably not higher than 1.98 bar, more preferably not higher than 1.43 bar, even more preferably not more than 1.10 bar, at which pressure the chlorine can be safely handled within an operating temperature equal to or below 120° C. or, respectively, equal to or below 110° C., or, respectively, equal to or below 102.5° C.

[0055] A particularly preferred embodiment of an evaporator (100) according to the invention is described hereinafter with reference to the drawings:

[0056] The evaporator is equipped with a seal pot into (3) which the inlet (2.1) immerses. The liquid distributor (4) is equipped with guiding vanes (4.1) (cf. FIG. 2a) to avoid that liquid droplets can bypass the tube bundle by being sprinkled to the gap between tube bundle and evaporator shell.

[0057] The heating units (5.1) and (5.2) are combined in one U-shaped tube bundle with an upper part (5.1) and a lower part (5.2). For the sake of clarity, only one upper part and one lower part are shown in FIG. 1. These represent in fact a plurality of tubes as shown in FIG. 2a. The liquid distributor (4) connects a tube sheet (17) holding the tubes and a baffle plate (18) guiding the flow of gaseous stream of evaporated liquid (1) and any not-yet evaporated liquid (1) onto the top surface of the flat plate (10). The latter is equipped with curbs (11) as can be seen more clearly in FIG. 2b and a circumferential edge (10.1) visible from the outside which allows easy horizontal levelling during installation of the evaporator.

[0058] The tube bundles (5.1) and (5.2) are heated with steam (6) produced in steam generation pot (23). The steam (6) is introduced via steam inlet (7) into the upper chamber (8.1), from which it flows through the upper tube bundle (5.1) and after that through the lower tube bundle (5.2), before it enters the lower chamber (8.2), both chambers being separated by a Leidig: separating plate (8.3). The heating chamber (12) below the flat plate (10) is fed directly with steam and condensate out of the lower chamber (8.2) via connecting pipe (9) which ensures always an uninterrupted heat supply. In the heating chamber (12), the residual steam vapour provides additional heating energy through the flat plate (10) to the shell side of the evaporator in order to evaporate any liquid chlorine with NCl.sub.3 which might have accumulated on the top surface of the flat plate (10). The vaporized chlorine stream (16) sweeps above the flat plate (10). Stream (16) is further superheated by the U-shaped part of the tube bundle and then leaves the evaporator through nozzle (15).

[0059] The condensate stream (13) flows back freely to the low pressure steam generation pot (23). The condensate flows over into condensate pot (29) via overflow pipe (24) which is dipped under the condensate level to prevent steam losses. The vent nozzle (21) is set to release any inert gas via a time-control valve (22).

EXAMPLES

Example 1

Simulation: Evaporation of Liquid Chlorine Containing Nitrogen Trichloride

[0060] In the evaporator shown in FIG. 1, stream (1), liquid chlorine containing of from 10 ppm to 20 ppm of nitrogen trichloride (NCl.sub.3) is fed at an absolute pressure of 5,000 mbar to inlet (2.1). The inlet (2.2) is not used and is shut-off. The upper part of the tube bundle (5.1) is designed sufficiently big to provide enough heat to completely evaporate the chlorine which is supplied with a feed rate of about 5,000 kg/h. Stream (1) out of seal pot (3) and is evenly distributed via liquid distributor (4) onto the upper part (5.1) of the U-shaped tube bundle. Guiding vanes (4.1) (cf. FIG. 2a) at the side of the liquid distributor (4) avoid that liquid chlorine droplets can bypass the tube bundle by being sprinkled to the gap between tube bundle and evaporator shell. Steam is introduced via line (28) and control valve (27) into the low pressure steam generation pot (23), where the steam is adjusted to the desired pressure and de-overheated (i.e. cooled to its saturation temperature). The saturated steam vapour (6) having an absolute pressure of about 1.1 bar coming from the low pressure steam generation pot (23) is introduced into the evaporator (100) via inlet (7). Steam condensate from the heating process leaves the evaporator (100) via outlet pipe (13) back to the steam generation pot (23), where it is partially used to de-overheat the introduced overheated steam. The valve (22) can be used to vent the steam system to the atmosphere to avoid the accumulation of inert gases in the steam system. Excess condensate is discharged via discharge pipe (24) into the condensate pot (29). The liquid level in the condensate pot (29) in combination with the length of the discharge pipe (24) ensure that the saturated steam pressure and thus the evaporation temperature is not higher than the chlorine process design temperature. From here it is discharged via free overflow (26). The condensate pot is vented to the atmosphere by the vent pipe (25). The “overdesigned” evaporator tends to evaporate all the liquid chlorine (1) upon contact with the upper part of U-shaped tube bundle (5.1).

[0061] The partially condensed steam (6) is carried out by steam vapour flow to the lower part of the U-shaped tube bundle (5.2), in which the condensate can flow by both bundle (5.2) via lower channel head chamber (8.2) and connection pipe (9) to the additional heating chamber (12). In the latter, the residual steam vapour provides additional heating energy through the flat plate (10) to the shell side of the evaporator in order to evaporate any liquid chlorine with NCl.sub.3 which might have accumulated on the top surface of the flat plate (10).

[0062] The vaporized chlorine stream (16) sweeps above the flat plate (10). Stream (16) is further superheated by the U-shaped part of the tube bundle and then leaves the evaporator through nozzle (15).