Patent Application
20140064017
The present invention relates to a process for mixing a liquid or mixture of a liquid and a fine solid present in an essentially self-contained vessel, with the proviso that the liquid or mixture fills only part of the internal volume of the vessel occupiable by a fluid phase, and the remaining occupiable internal volume of the vessel is filled by a gas phase, comprising supply of essentially the same liquid or essentially the same mixture into the vessel as a motive jet of a suction apparatus disposed in the liquid or in the mixture in the vessel.
The storage of liquids or of mixtures of a liquid and a fine solid in essentially self-contained vessels is common knowledge (for example for the purpose of storage). Typically, such vessels are also referred to as tanks. Normally, the vessels are not entirely self-contained, but rather generally have, for example, at least one withdrawal point through which it is possible to withdraw from the contents stored in the vessel as required, for example by means of a pump. Correspondingly, the vessel typically also has at least one feed point through which the contents to be stored can be supplied to the vessel. Shutoff members (for example valves or ballcocks) normally enable the liquid or the mixture to be let in and out, and simultaneously ensure that leaks are prevented when the vessel is inactive. In a similar manner, instruments for measuring temperature, fill level and pressure in the tank (vessel) can be introduced into the vessel.
Normally, the liquid or mixture of a liquid and a fine solid which is to be stored in the tank does not completely fill the internal volume occupiable by a fluid (gaseous or liquid) phase. Instead, for a wide variety of different reasons, some of this internal volume is occupied by a gas phase. When the liquid or mixture is stored at atmospheric pressure, the essentially self-contained vessel can in principle be open to the atmosphere on the gas phase side (for example via an offgas system leading through a flare (or another offgas purification system (for example gas scrubbing))). The opening cross section is normally such that it is firstly sufficiently small and secondly such that the gases balance in the course of filling and emptying of the vessel with significant pressure drop. Typically, the mean diameters of such opening cross sections are ≦25 cm (at fill volumes of typically ≧100 m3, frequently up to 10000 m3). Alternatively, devices for pressure release in the event of impermissible elevated pressure or reduced pressure, which seal tight to the response pressure (which may be at or above or below atmospheric pressure), are typically likewise installed into the relevant storage vessels (for example non-return valves). Frequently, the fill level in the storage tank is determined continuously at predefined heights in the gas and in the liquid phase by metering in a small amount (based on the volume of the gas phase in the vessel, generally <1% by volume/h) of a measurement gas. When the contents are known, the fill level is calculated directly from the difference of the metering pressure required for this purpose in each case.
In many cases, it is necessary that the contents of such a storage tank which are variable over time as a result of withdrawal and/or addition are mixed from time to time or constantly in order to increase or to ensure its homogeneity. The causes of this may be a wide variety of reasons. When the contents of the vessel are a mixture of a liquid and a fine solid (for example a slurry), there is frequently the risk that the fine solid settles out under the action of gravity during the storage in the tank, and the tank contents thus demix in the course of time. In the case of a withdrawal from the storage tank, it would then possibly, for example, no longer be the desired mixture which is withdrawn but rather only the liquid present therein. Examples of the aforementioned case include aqueous polymer suspensions. Depending on the specific weight of the liquid phase the fine solid present therein in disperse distribution can also cream and become enriched in the liquid/gaseous phase interface. One possible example of this is polymer dispersions (also aqueous polymer dispersions).
When only a liquid is stored in the tank (vessel), this may likewise be multiphasic (for example an emulsion; examples include oil-in-water emulsions and water-in-oil emulsions) and demix in the course of prolonged storage without intermediate homogenization, which is normally undesired.
However, a chemically homogeneous liquid too may form undesired physical inhomogeneities in the course of storage. These may consist, for example, of an inhomogeneous temperature distribution (for example caused by solar irradiation on one side of the tank). The consequence of this may, for example, be undesired crystal formation or unwanted decomposition of the liquid stored. Frequently, for the purpose of maintaining a desired storage temperature, a portion of the stored liquid may also be withdrawn continuously, conducted through a preferably indirect heat exchanger and then recycled into the storage tank. In this case, the storage vessel operator typically aims for very rapid temperature balancing between liquids still present in the storage vessel and liquid recycled into it via the heat exchanger by suitable rapid mixing. For the safe storage of free-radically polymerizable compounds (or solutions comprising them), for example acrolein, methacrolein, acrylic acid, methacrylic acid and/or esters thereof (especially the C1- to C8-alkyl esters), not only is careful temperature control of the liquid tank contents required. Instead, so-called inhibitors (free-radical scavengers) have to be added to the aforementioned, generally at least monoethylenically unsaturated, organic compounds (monomers), in order to avert and to prevent the occurrence of an accidentally initiated, undesired free-radical polymerization, in many cases, such inhibitors display their full effect only in the presence of molecular oxygen (which may in turn itself be an inhibitor). For this reason, such monomers are normally stored under a gas atmosphere comprising molecular oxygen (cf., for example, WO 2005/049543 and U.S. Pat. No. 6,910,511), and it should be ensured that the liquid monomer (or its solution) does not become depleted of the molecular oxygen dissolved therein. The latter can occur, for example, when the monomer temporarily crystallizes out locally and then goes back into solution. The resulting local depletion of molecular oxygen can equally be counteracted by appropriate mixing.
Should undesired free-radical polymerization of the tank contents be triggered in spite of the above-described precautionary measures, it can be counteracted by adding a medium for immediately ending the free-radical polymerization to the tank contents within a very short time and distributing it over the tank contents very rapidly (cf., for example, WO 00/64947, WO 99/21893, WO 99/24161, WO 99/59717). In this case too, very uniform and rapid mixing of the tank contents is required after the medium has been added.
In principle, the liquid contents of a tank can be mixed (cf.
Alternatively, the tank contents can be mixed by means of a stirrer. However, this requires a separate drive source and a drive shaft conducted through the vessel wall. However, the sealing of rotating elements conducted through a vessel wall is generally found to be particularly difficult. Moreover, in the case of large fill volumes of a tank (industrial scale fill volumes for storage tanks are typically from 100 m3 to 10000 m3, frequently from 200 to 1000 m3 or from 300 to 800 m3, characteristically 500 m3), the manufacture of a stirrer is already comparatively expensive.
Against this background, it has found to be appropriate to mix the liquid tank contents by withdrawing therefrom, with the pump available for tank withdrawal, a portion of the liquid or mixture of a liquid and a fine solid stored in the tank (vessel), and recycling at least some of the portion withdrawn through a motive nozzle which is disposed close to the bottom of the tank and is directed upward (in the simplest case a flow channel with cross section narrowing in flow direction, in which the pressure energy of a liquid flowing through is converted with low losses to additional kinetic energy, and the liquid stream is thus accelerated) as a (motive liquid) liquid jet (motive jet) into the tank.
In the course of this, the liquid jet directed upward, according to the laws of the free jet, along its path through the liquid present in the tank, is sucked in by the liquid, and the liquid media become mixed. Alternatively or additionally, for the purpose of mixing, the filling (refilling, but also first filling) of the vessel with the liquid or mixture can be effected in such a way that the liquid or mixture is supplied via an aforementioned motive jet.
However, a disadvantage of this method of mixing is that the mixing action of the free jet only captures a comparatively restricted space around it, so that the mixing action achieved is normally not entirely satisfactory (
A further disadvantage is that the liquid jet (especially in the case of falling fill level in the tank), owing to its comparatively high mea momentum density (and speed), leaves the liquid phase present in the tank comparatively easily (breaks through the phase interface between liquid and gaseous phase), and this leaving may be accompanied by intense droplet formation (spray formation) within the gas phase. This is disadvantageous especially when the tank contents comprise an organic liquid (for example acrolein, methacrolein, acrylic acid, methacrylic add, the esters of these acids or other organic monomers) whose gas phase may be explosive in the presence of molecular oxygen (cf., for example, DE-A 10 2004 034 515). Firstly, the finely distributed droplets in the gas phase increase their content of organic material, as a result of which a gas phase which may not have been explosive beforehand becomes an explosive gas phase, and the droplets formed regularly experience, in their flight through the gas phase, as a consequence of friction, electrical charging of their surface. Spark discharge which accrues as a consequence is capable of triggering ignition. When the droplets are those of an aqueous polymer dispersion, these may also, for example, film irreversibly in an undesired manner on their path through the gas phase and disrupt the polymer dispersion in later uses.
When the tank contents are the slurry of a fine solid in a liquid, the solid thrown onto the inner wall of the vessel by the jet which breaks through the phase interface may be capable of adhering to it, which removes it from the slurries stored in the vessel.
However, spray formation which is established as described above is also disadvantageous in the case of another liquid in that, inter alia, the small spray droplets have an elevated vapor pressure. This causes undesired evaporative cooling, which impairs the temperature constancy of the tank contents.
In order to intensify the mixing (cf. Chemie-Ing. Techn. 42, 1970, p. 474 to 479), in the prior art according to
The mixture which flows out of the momentum exchange tube has an already significantly weakened momentum of its elements (a reduced mea momentum density) in comparison to the motive jet, which lowers the above-described probability of exit with droplet formation (spray formation) (it will enter only at a comparatively lower level of the phase interface and with weakened mean exit momentum density; cf.
In view of this prior art, it was an object of the invention to provide an improved process for mixing liquid tank contents, which can be applied to all above-described problem cases and not least also enables more rapid mixing.
Accordingly a process is provided for mixing a liquid or mixture of a liquid and a fine solid present (stored) in an essentially self-contained vessel, with the proviso that the liquid or mixture fills only part of the internal volume of the vessel occupiable by a fluid phase, and the remaining occupiable internal volume of the vessel is filled by a gas phase, comprising supply of essentially the same liquid or essentially the same mixture into the vessel as a motive jet of a suction apparatus disposed in the liquid or in the mixture in the vessel, wherein the suction apparatus, with the aid of the motive jet, sucks in gas from the gas phase present in the vessel and releases the sucked-in gas together with the motive jet into the liquid or mixture present in the vessel.
Appropriately in accordance with the invention, the process according to the invention can be performed in a simple manner in such a way that it comprises the withdrawal of a portion of the liquid or of the mixture from the vessel and the recycling of at least some of the portion withdrawn as a constituent of the motive jet of the suction apparatus. In principle, the motive jet of the suction apparatus in the process according to the invention may also exclusively be at least some (or the entirety) of the liquid or mixture present in the vessel which has been withdrawn from the vessel beforehand.
If required, any part of the portion withdrawn which is not recycled as a motive jet can be sent to other uses.
It will be appreciated that the process according to the invention can also be performed without the liquid or mixture fed as a motive jet into the vessel comprising liquid or mixture withdrawn from the vessel, This is possible, for example, by virtue of the liquid or mixture to be conducted into the vessel for refilling being supplied to the vessel as a motive jet of the suction apparatus. It will be appreciated that the motive jet of the suction apparatus in the process according to the invention may also consist of a mixture of liquid or mixture to be conducted into the vessel for the purpose of refilling, and liquid or mixture withdrawn beforehand from the vessel.
Normally, the gas phase in the process according to the invention essentially does not experience any chemical conversion. In other words, the gas phase is essentially not consumed in the process according to the invention. In general, ≦1% by volume, preferably ≦0.75% by volume, more preferably ≦0.5% by volume, or ≦0.25% by volume, and most preferably ≦0.1% by volume, of the gas which is sucked out of the gas phase and is released into the liquid or mixture present in the vessel together with the motive jet is chemically changed as it is bubbled (ascends) once through the stored liquid or mixture.
In the simplest form, the process according to the invention can be implemented with the aid of an ejector (i.e. by the principle of the water-jet pump) as the suction apparatus. In this case, the motive jet is pumped through a motive nozzle which is fitted into the ejector in such a way that gas is sucked out of the gas phase as it passes through the nozzles, for example via a riser tube which projects into the gas phase of the vessel (it is generally held by fittings fastened to the vessel wall), and is released in the form of divided gas bubbles into the liquid contents of the storage vessel together with the motive jet. The basic structure and the labels of an ejector (also referred to in the literature and hereinafter as a jet compressor) are shown in
An ejector (cf., for example,
Motive nozzles whose nozzle openings generate liquid jets with increased turbulence are particularly advantageous, in the ejectors suitable in accordance with the invention, since motive jets leaving with increased turbulence entrain the gas from the suction chamber particularly effectively (the contact surface between gas and liquid phase is increased), which causes increased sucking action and increases the amount of gas sucked in per unit time, which improves the desired mixing. An additional improvement in the widening of the motive jet beyond its exit from the motive nozzle is achieved when a slight swirling motion is imparted to it before it passes through the motive nozzle. This is possible, for example, by installing suitable swirl bodies (9) just upstream of the motive nozzle. Useful such swirl bodies are, advantageously in accordance with the invention, for example, blade rings, as shown in
Alternatively and/or additionally to the swirling of the motive jet, it can be divided (into a plurality of individual jets), for example, by virtue of the exit cross section of the motive jet having a multitude of exit orifices (the cross section of the motive nozzle is provided with a motive jet divider). In the simplest manner, this is realizable by incorporating a screen (plate) which has a multitude of passage orifices (in the simplest case annular) into the exit cross section of the motive jet, as shown, for example, in
Instead of holes (in which case reference is made to screen or multihole nozzles), slot nozzles (for example concentric annular gaps), for example, are also useful.
By virtue of the fact that, in the inventive use of the ejector, the mixing action is effected not least by the gas injected into the stored liquid or into the stored mixture of liquid and fine solid, the ejector in the storage vessel need not, like the jet nozzle, be mounted obliquely upward, nor slightly elevated. Instead, the ejector can be mounted close to the bottom of the storage tank. In addition, the motive nozzle (in the ejector (and hence the ejector as such)) can also be incorporated parallel to the bottom of the storage tank (i.e. normally horizontally) essentially without loss of mixing efficiency. As a result of the horizontal incorporation, the phase interface (the liquid interface) in the storage vessel can be lowered to a significantly lower level before insufficient coverage with liquid is present. In the case of further lowering of the liquid interface down to below the diffusor of the horizontally installed ejector, the horizontal jet leaving the ejector, especially in the case of its previous swirling and/or division, is widened, and, when it hits the vessel wall, generates a reduced amount of spray compared to the jet nozzle. The design (which depends, for example, on the material data of the tank contents and on the tank geometry) of an ejector used to mix the liquid contents of a storage tank can be effected according to the statements made in the document cited. Useful manufacturing materials, adjusted to the properties of the stored liquid/mixture, include both stainless steels and plastics (for example fiber-reinforced plastic matrices, as recommended in EP-A 245844). When the storage contents are acrolein, methacrolein, acrylic acid, methacrylic acid, esters thereof or solutions thereof, a recommended ejector material is in particular stainless steel of DIN material numbers 1.4541 and 1.4547. In principle, the inventive use of an ejector is sufficient for the process according to the invention. Appropriately in accordance with the invention, it is positioned within the vessel such that the exit from the diffusor is in the middle of the vessel. It will be appreciated that it is also possible for a plurality of ejectors to be operated simultaneously in accordance with the invention in one and the same vessel. In this case, appropriately in accordance with the invention, ejectors of equal size will be used. The ejectors may in principle be arranged in any position relative to one another in the tank and, for example, form a star or ball star shape. It is favorable in accordance with the invention that the pump which delivers the motive jet can be identical to the pump to be used for withdrawing the liquid/mixture stored in the vessel (however, it is also possible to use two pumps for the two purposes). In the case of stored liquids comprising (meth)acrylic monomers (or other chemicals stored in liquid form), useful such delivery pumps are, for example, the delivery pumps with double sliding ring seal recommended in WO 2004/003389.
Useful alternative delivery pumps to these are, for example, those of U.S. Pat. No. 5,727,792, U.S. Pat. No. 4,168,936, EP-A 1 092 874 and U.S. Pat. No. 4,865,333.
The amount of gas sucked in from the gas phase per unit time by means of an ejector for the process according to the invention and (with it, the mixing action) can appropriately be enhanced in accordance with the invention (typically by a factor of from 2 to 3) by combining the advantageousness of the ejector for the process according to the invention with the advantageous features of the jet nozzle acknowledged at the outset of this document (and, for example, also described in DE-A 24 04 289) in a suitable manner to give a so-called ejector jet nozzle as a suction apparatus to be used in accordance with the invention alternatively to the ejector, which is depicted schematically in
In physical terms, the reason for this is that the gas in the ejector only comes into contact with the liquid of the motive jet, while a multiple of the amount of motive jet is additionally sucked in from the ambient liquid in the momentum exchange chamber of the jet nozzle. Expressed in simplified terms, an ejector jet nozzle is nothing other than a jet nozzle in which the motive jet used is the mixture of sucked-in gas and motive liquid pumped through the motive nozzle of the ejector which forms beyond the motive nozzle of an ejector.
For this purpose, the suction chamber of the ejector part of the ejector jet nozzle does not have a seamless transition into a mixing tube (a mixing chamber) as in the case of the ejector alone. Instead, the suction chamber here is designed to give a mixing nozzle (10) (the suction chamber opens out to a mixing nozzle), from which the mixture of motive liquid and sucked-in gas coming from the “ejector”, like the motive jet in the case of a jet nozzle, is ejected into a momentum exchange tube (generally a momentum exchange chamber (open at the inlet and outlet)). In the ejector jet nozzle, the suction chamber generally first has a constant cross section and then opens out in flow direction typically (but not necessarily) to a diffusor (the diffusor has a widening cross section in flow direction). The amount of liquid sucked in as a result from the environment of the mixing nozzle/momentum exchange tube transition in the storage tank per unit time is a multiple (generally from 1 or 2 to 10 times, frequently from 4 to 8 times) the motive liquid pumped into the ejector part per unit time.
The amount of liquid which entrains the sucked-in gas in an ejector jet nozzle overall in flow direction (always per unit time) is thus significantly greater than in the case of a pure ejector. This causes a significantly higher suction force and, for the purposes of the invention, an increased mixing action achievable as a result. Expressed in simplified terms, divided liquid droplets in the mixing tube of an ejector convey a continuous gas phase, while a liquid stream in the momentum exchange tube of the ejector jet nozzle conveys gas bubbles distributed therein.
Advantageously in accordance with the invention, the motive jet nozzle in the ejector part of the ejector jet nozzle also comprises elements which widen and/or divide the motive jet leaving the motive jet nozzle. As already detailed in the description of the jet compressor, useful such elements are, for example, swirl bodies and/or perforated or slotted screens (motive jet dividers). A further advantage of the ejector jet nozzle in comparison to the pure jet compressor is a finer gas distribution which is established, which equally has an advantageous effect on the desired mixing. In summary, in an ejector jet nozzle, the gas sucked in in the ejector part is conducted together with the sucking motive jet into a mixing nozzle and they are mixed together therein. The motive liquid-gas mixture thus obtained is introduced (injected) together into a momentum exchange chamber (at the narrowest cross section thereof) which is disposed in the stored liquid medium, extends in entry direction of the motive liquid-gas mixture and is very small in comparison to the vessel volume (generally, the volume of the momentum exchange chamber is from one hundredth to one hundred thousandth or one millionth of the maximum liquid capacity of the vessel). At the same time, on entry of the motive liquid-gas mixture flowing out of the mixing nozzle (the jet leaving (flowing out) (and leading into the momentum exchange chamber) through the center of the narrowest cross-sectional area of the mixing nozzle in the absence of the stored liquid medium will be referred to in this document as the central jet (see (11) in
The design of an ejector jet nozzle for a specific mixing problem can in turn be effected with reference to the document cited in this document in connection with the ejector jet nozzle (useful construction materials are those mentioned for the ejector).
The speed of the motive liquid on exit from the mixing nozzle will generally be from 10 to 100 m/s, preferably from 15 to 70 or to 30 m/s. The mean diameter of the entry orifice of the momentum exchange chamber will generally be from 1.1 to 4 times, preferably from 1.2 to 2 times, the mean diameter of the mixing nozzle, and the length of the momentum exchange chamber will typically be from 3 to 30 times, preferably from 3 to 10 times, its hydraulic diameter.
The mass flow rate which leaves the momentum exchange chamber typically has a mea momentum density of from 103 to 105 N/m2, preferably from 5·103 to 2·104 N/m2. In contrast, the mea momentum density of a motive jet in the process according to the invention is typically from 2.5·104 to 107 N/m2, frequently from 105 to 5·106 N/m2.
The mean diameter is understood to mean the diameter of a circle which has the same surface area as the cross section in question (which may also be polygonal) of the nozzle or of the entry orifice of the momentum exchange chamber, neither of which need necessarily be circular. The momentum exchange chamber normally has a constant cross section, and the diffusor generally a cross section which is enlarged in flow direction. In principle, the momentum exchange chamber can be constructed in various forms, this form appropriately being adjusted to the form of the mixing nozzle.
In general, the momentum exchange chamber used is generally cylindrical tubes, and the diffusor a frustocone. When the momentum exchange chamber is configured as a cylindrical tube, its length will generally be from 3 to 30 times, preferably from 3 to 10 times, its diameter, which in this case is simultaneously its hydraulic diameter. When the momentum exchange chamber does not have a circular cross section or a constant cross section over its length, its length will normally be from 2 to 30 times, preferably from 3 to 10 times, its hydraulic diameter. The hydraulic diameter is understood to mean the diameter of a cylindrical tube which, with the same throughputs and same length, exhibits the same pressure drop as the momentum exchange chamber in question.
In an ejector jet nozzle suitable in accordance with the invention, the narrowest cross-sectional area of the mixing nozzle will, appropriately from an application point of view, be at a distance from the motive nozzle of the ejector part which corresponds to from 1 to 10 times the narrowest hydraulic diameter of the mixing nozzle.
Moreover, the narrowest cross-sectional area of the mixing nozzle in an ejector jet nozzle suitable in accordance with the invention will, appropriately in accordance with the invention, not project deeper into the momentum exchange chamber (normally centered) than the extent which corresponds to from ≧0 to 3 or to 2 times the narrowest hydraulic diameter of the motive nozzle.
Advantageously in accordance with the invention, the mixing nozzle projects into the momentum exchange chamber. In principle, the narrowest cross-sectional area of the mixing nozzle to the entry into the momentum exchange chamber may also have a distance which may be, for example, up to 1 time or more times the narrowest hydraulic diameter of the motive nozzle.
Moreover, the narrowest cross-sectional area of the mixing nozzle of an ejector jet nozzle suitable in accordance with the invention, advantageously from an application point of view, will have from 1.5 to 15 times, preferably from 2 to 10 times, the narrowest motive nozzle cross-sectional area. The speed of the motive jet leaving the motive nozzle in the ejector part is, in a manner suitable in accordance with the invention in an ejector jet nozzle, generally from 20 to 50 m/s.
The statements made here on the possible dimensioning of the ejector part of the ejector jet nozzle also apply to an ejector alone.
Instead of only one ejector jet nozzle, it is also possible to use for the process according to the invention, as already mentioned in the case of use of pure ejectors, a plurality of (a bundle of) ejector jet nozzles in one and the same storage vessel. As in the case of the ejector too, it may be appropriate in accordance with the invention (especially to avoid deposits of fine solids in mixtures to be stored in accordance with the invention) to mount the ejector jet nozzle (or the ejector) in the middle of the vessel pointing vertically downward. It is also possible in accordance with the invention, in the case of one ejector jet nozzle, to combine a plurality of ejector parts comprising their mixing nozzle with a combined momentum exchange chamber, in which case its entry orifice cross section should correspond to the sum of the cross section required for the particular mixing nozzle in the case of its individual use.
In principle, the momentum exchange chamber and the ejector part, also comprising the mixing nozzle, of an ejector jet nozzle may be connected to one another via connecting elements (preferably via three connecting elements (enable completely satisfactory centering), of which in each case two enclose an angle of 120°). However, they may also be screwed into one another. In this case, slots mounted appropriately permit the sucking-in of the ambient liquid.
Typically, in the case that the process according to the invention is practised with an ejector jet nozzle, the ratio of total liquid volume conducted into the momentum exchange chamber to gas volume supplied may be in the range from 0.1 to 10.
Momentum exchange in the momentum exchange chamber and conversion of the kinetic energy to pressure energy in the diffusor results in static pressure buildup in the ejector jet nozzle. This compressing operation takes place owing to the greater amount of liquid with a better efficiency than in the case of ejectors. Another advantageous factor is that the flow losses as a result of wall friction in the momentum exchange chamber, which generally has a larger diameter compared to the mixing chamber of customary ejectors, under otherwise identical conditions are smaller owing to the lower flow rate.
In an embodiment particularly preferred in accordance with the invention, the suction apparatus used for the process according to the invention, according to
The volume of the gas phase in the vessel in the process according to the invention should be at least 5% by volume, or at least 10% by volume, of the liquid or mixture volume stored in the vessel. However, on the same basis, it may also be 30% by volume, 60% by volume, 90% by volume, 150% by volume, 250% by volume, 350% by volume and more.
Moreover, it is favorable in accordance with the invention when at least about 10−5 standard liter (gas volume at 0° C. and 1 atm in the unit of liters) of gas (but normally not more than one 10−1 standard liter) is injected per minute per liter of liquid contents of the storage vessel in the process according to the invention.
The vessel itself advantageously has cylindrical (for example with circular or square or rectangular cross section) structure which is concluded at the top by a conical roof or by a hemispherical or dome-shaped roof.
The process according to the invention is suitable in particular for the advantageous storage of all liquids mentioned at the outset of this document (but also, for example, of benzene, toluene, alcohols, other hydrocarbons) or mixtures of a liquid and a fine solid. These are generally shipped blanketed with a gas which is saturated with the vapor of the liquid (i.e. the gas phase typically does not consist only of evaporated liquid).
Useful such gases include, for example, inert gases such as N2, noble gases, for example Ar, and/or CO2.
It will be appreciated that such gases may also be air, or other mixtures of molecular oxygen and inert gas. The absolute pressure in the tank may, for example, be from atmospheric pressure to 50 bar; the temperature in the tank may, for example, be from 0 (or less) to 100 (or more)° C.
Neither of the two aforementioned parameters is subject to any restriction in the process according to the invention.
The process according to the invention is particularly advantageous when the stored liquid is at least one monoethylenically unsaturated organic compound (for example N-vinylformamide, vinyl acetate, esters of maleic acid, styrene and/or N-substituted acrylamides) or a solution comprising at least one such monoethylenically unsaturated organic compound, especially when it comprises an added polymerization inhibitor for the purpose of inhibiting undesired free-radical polymerizations.
Further examples of such at least monoethylenically unsaturated organic compounds include acrolein, methacrolein, acrylic acid, methacrylic acid and esters of acrylic acid and/or methacrylic acid and mono- or polyhydric alkanols. These esters include in particular those whose alcohol has from one to twenty carbon atoms, or from one to twelve carbon atoms, or from one to eight carbon atoms. Exemplary representatives of such esters include methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate and tert-butyl methacrylate. Useful inhibitors of free-radical polymerizations for the aforementioned monomers and their solutions in organic or aqueous solvents are, for example, the monomethyl ether of hydroquinone (MEHQ), hydrochinones, phenols (e.g. 2,4-dimethyl-6,6-butylphenol), quinones, butylpyrocatechol, phenothiazine, diphenylamine, p-phenylenediamines, nitroxyl radicals and/or nitroso compounds, for example nitrophenols (and also all other polymerization inhibitors mentioned in WO 00/64947). Based on the monomer content, the amount of polymerization inhibitors added for the purpose of storage may be from 0.5 to 1000 ppm by weight (frequently from 1 to 600 ppm by weight or from 2 to 500 ppm by weight).
In the case of glacial acrylic acid (acrylic acid content ≧99.5% by weight), generally 200±20 ppm by weight of MEHQ are added as a storage inhibitor (storage temperature recommendation: 15 to 25° C.). In the case of n-butyl acrylate (n-butyl acrylate content ≧99.5% by weight) and the other (meth)acrylic esters, generally 15±5 ppm by weight of MEHQ are added as a storage stabilizer (storage temperature recommendation: 20 to 35° C.). MEHQ is also the preferred storage stabilizer for the other (meth)acrylic monomers mentioned and solutions thereof.
As already mentioned, aforementioned polymerization inhibitors (especially MEHQ) display their full inhibiting action generally only in the presence of molecular oxygen. However, especially (meth)acrylic monomers are capable of forming explosive mixtures with molecular oxygen.
In order to rule out a corresponding explosion even in the case of spray (spray formation) in the storage tank, it has been necessary to date either to prevent such spray formation by means of a liquid level control which is widely available from a safety technology point of view, or to correspondingly restrict the oxygen content of the gas phase in the storage tank, as recommended in WO 2005/049543 in the context of U.S. Pat. No. 6,910,511.
The use of the inventive procedure, with whose aid it is possible to prevent spray formation even with a relatively low liquid level in the vessel, permits, in comparison, the comparatively simple and reliable blanketing of the tank contents with air saturated with the liquid stored in the tank. When, however, the stored acrylic acid (the stored acrolein) is acrylic acid (acrolein) which has been obtained by heterogeneously catalyzed partial gas phase oxidation of propylene in the presence of propane or by heterogeneously catalyzed partial gas phase oxidation of propane itself, the crude acrylic acid to be stored (the crude acrolein to be stored), after its removal from the product gas mixture, is generally obtained in a form saturated with propane. In this case, the gas mixture additionally comprises combustible propane. For safe storage, it is advisable in this case to comply with the lower limit in the limiting oxygen concentration by storage under lean air according to WO 2005/049543 in the gas phase.
In principle, with decreasing fill level in the storage vessel in the process according to the invention, the feed (recycle) rate to form the motive jet can be reduced.
Quite generally, the introduction of molecular oxygen into the liquid to be stored or into the mixture to be stored in the process according to the invention is made very simple.
The process according to the invention is suitable, for this reason too among others, especially for storage tanks with particularly large tank contents.
The present application thus comprises in particular the following inventive embodiments:
In an outdoor tank (wall thickness: 5 mm, manufacturing material: DIN 1.4541) stainless steel) according to
The withdrawal from the tank was effected by means of a CPK 50-200 centrifugal pump from KSB Aktiengesellschaft in D-67227 Frankenthal.
The barrier fluid present in the pump with double slip ring seal was a mixture of ethylene glycol and water. The glacial acrylic acid in the storage tank was covered by means of air. By means of an offgas system which was open to the atmosphere via a flare (orifice cross section in the conical roof=20 cm2), it was possible to release gas from the gas phase of the tank to a flare in the course of filling for pressure release.
In a corresponding manner, air was replenished via a pressure-retaining device for pressure equalization in the course of withdrawal of glacial acrylic acid from the tank. Close to the bottom, as can be seen in
The centrifugal pump was used to withdraw 40 m3/h of glacial acrylic acid continuously from the tank over a period of 1 week, and to recycle it as the motive jet into the ejector jet nozzle via the heat exchanger in
Finally, 1 l of a 0.1% by weight solution of phenothiazine in glacial acrylic acid was introduced all at once into the tank from the top (at maximum fill height). After 5 minutes, the concentration of equal distribution of the added phenothiazine had arrived within the range of ±10% about its theoretical value at the withdrawal point.
Subsequently, the recycle rate was retained, but the withdrawal rate was increased by 20 m3/h, i.e. the tank was emptied by 20 m3/h. It was possible without any problem to withdraw 99% of its liquid contents from the tank without spray being formed in the tank (in principle, it was also possible to withdraw glacial acrylic acid from the tank via an outlet which did not lead through the circulation pump).
In addition, the abbreviations in
In addition,
U.S. Provisional Patent application No. 60/846,095, filed on Sep. 21, 2006, is incorporated into the present application by literature reference. With regard to the above-mentioned teachings, numerous changes and deviations from the present invention are possible. It can therefore be assumed that the invention, within the scope of the appended claims, can be performed differently from the way specifically described herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102006045088.4 | Sep 2006 | DE | national |
This application is a divisional application of U.S. application Ser. No. 11/857,887 filed Sep. 19, 2007, which claims priority to U.S. provisional application 60/846,095 filed Sep. 21, 2006, and claims priority to German Patent 102006045088.4 filed Sep. 21, 2006.
| Number | Date | Country | |
|---|---|---|---|
| 60846095 | Sep 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11857887 | Sep 2007 | US |
| Child | 13972250 | US |
