Column for thermal treatment of fluid mixtures

Abstract

The present invention relates to a column (1) for thermal treatment of fluid mixtures, having a cylindrical, vertically aligned column body (2) which forms a column cavity (3), having a sequence of vertically spaced-apart dual-flow mass transfer trays (8) which are mounted in the column cavity (3) and which have orifices for passage of liquid and gas in countercurrent, and having at least one gas entry orifice (5) disposed below the lowermost of the sequence of dual-flow mass transfer trays (8). It is a characteristic feature of the column of the invention that a gas distribution tray (9) which is disposed between the lowermost of the sequence of dual-flow mass transfer trays (8) and the gas entry orifice (5) has orifices (32) for vertical passage of gas which can be introduced into the column cavity (3) via the gas entry orifice (5), the orifices (32) being formed so as to bring about equal gas distribution over the column cross section. The invention further relates to a process for thermal treatment of fluid mixtures in such a column (1).

Claims

1. A column for thermal treatment of fluid mixtures, comprising: a cylindrical, vertically aligned column body which forms a column cavity; a sequence of vertically spaced-apart dual-flow trickle sieve trays which are mounted in the column cavity and which have orifices for passage of liquid and gas in countercurrent; a gas distribution tray disposed below the lowermost of the sequence of dual-flow trickle sieve trays; and at least one gas entry orifice disposed below the gas distribution tray; wherein the gas distribution tray has orifices for vertical passage of gas introduced into the column cavity via the at least one gas entry orifice and a liquid draw for passage of liquid below the gas distribution tray, the orifices for vertical passage of gas are arranged in homogeneous distribution over the cross section of the gas distribution tray, a pressure drop obtained by the orifices of the gas distribution tray is 6 to 10 times the dynamic pressure of the gas at the at least one gas entry orifice, and a ratio of the orifice area of the orifices of the lowermost of the sequence of dual-flow trickle sieve trays relative to an inner cross-sectional area of the column is at least 1.13 times larger than a ratio of the orifice area of the orifices of the gas distribution tray to the inner cross-sectional area of the column, so as to obtain equal gas distribution over the column cross section of the gas introduced into the column through the at least one gas entry orifice.

2. The column according to claim 1, wherein the gas distribution tray has 0.2 to 1 orifice per square meter.

3. The column according to claim 1, wherein the proportion of the orifice area formed by the orifices of the gas distribution tray relative to the inner cross-sectional area of the column is within a range from 10% to 20%.

4. The column according to claim 1, wherein the pressure drop of the gas distribution tray is at least 20% of the pressure drop of the lowermost dual-flow trickle sieve tray of the sequence of dual-flow trickle sieve trays.

5. The column according to claim 1, wherein the proportion of the orifice area formed by the orifices of at least of the lowermost of the sequence of dual-flow trickle sieve trays relative to the inner cross-sectional area of the column is within a range from 14% to 20%.

6. The column according to claim 1, wherein the at least one gas entry orifice in the column is aligned such that gas entering the column cavity forms a horizontal vortex.

7. The column according to claim 1, wherein the liquid draw disposed above the gas distribution tray or in the gas distribution tray has an inlet for liquid from an upper collecting area of the gas distribution tray and an outlet in a region beneath the gas distribution tray.

8. The column according to claim 7, wherein a collecting tank for the liquid flowing through the liquid draw disposed between the inlet and the outlet of the liquid draw is arranged such that the liquid collecting in the collecting tank provides a hydraulic seal.

9. The column according to claim 8, wherein the liquid draw comprises a pipe in siphon form which forms the collecting tank.

10. The column according to claim 7, wherein the inlet of the liquid draw comprises an orifice in the upper collecting area of the gas distribution tray, from which a drainpipe extends downward, and wherein a collecting tank is a collecting cup disposed beneath the lower orifice of the drainpipe, the drainpipe passing through an area of the collecting cup which is formed by the upper edge of the collecting cup, and the upper edge of the collecting cup being disposed above the lower edge of the lower orifice of the drainpipe, such that liquid collecting in the collecting cup forms the hydraulic seal.

11. The column according to claim 1, wherein the gas distribution tray is a chimney tray comprising a chimney having a covering hood.

12. A process for thermal treatment of fluid mixtures in a column according to claim 1, comprising: introducing liquid into an upper region of the column wherein the liquid descends within the column and introducing gas into the column cavity through the at least one gas entry orifice wherein the gas flows upward through the orifices of the gas distribution tray, giving rise to a pressure drop, the orifices being formed so as to bring about equal gas distribution over the column cross section.

13. The process according to claim 12, wherein the ascending gas and/or the descending liquid comprises (meth)acrylic monomers.

Description

(1) There follows an elucidation of working examples of the column of the invention and working examples of the process of the invention with reference to the drawings.

(2) FIG. 1 shows a schematic view of a column in one working example of the invention,

(3) FIG. 2 shows a cross section of the column shown in FIG. 1 in the region of the gas inlet,

(4) FIG. 3 shows a cross section of the column shown in FIG. 1 in the region of the gas distribution tray,

(5) FIG. 4 shows a detail of a cross section of the gas distribution tray shown in FIG. 3 and

(6) FIG. 5 shows a schematic cross section of the lower region of a further working example of the column.

(7) The working example described hereinafter relates to a separation column 1 as used, for example, in a process for fractional condensation for separation of acrylic acid from a product gas mixture comprising acrylic acid from a heterogeneously catalyzed gas phase partial oxidation of a C3 precursor compound (especially propene and/or propane) of the acrylic acid with molecular oxygen to give acrylic acid.

(8) FIG. 1 shows the separation column 1 known per se in schematic form. It comprises a cylindrical column body 2, the axis of which is aligned vertically. The column body 2 is essentially a hollow cylinder. This means that the column body 2 forms a column cavity 3. The column body 2 is manufactured from stainless steel. On the outside, the separation column 1 is normally thermally insulated in a conventional manner. The height of the separation column 1 is 40 m. The internal diameter of the column body 2 is 7.4 m throughout.

(9) In the vertical direction, the separation column 1 is divided into three regions: the upper region A is referred to as the column head. At the column head is provided a feed 4 through which a liquid can be introduced into the column cavity 3. In addition, an offgas line 13 for withdrawal of the gaseous mixture is formed at the top.

(10) Beneath the column head, a region B is formed. In this region, the fractional condensation is conducted. In the region B, a sequence of vertically spaced-apart dual-flow trays 8 is secured in the column cavity 3. These dual-flow trays 8 serve for mass transfer. They are thus mass transfer trays. They have a multitude of orifices for passage of liquid and gas in countercurrent.

(11) The orifices of the dual-flow trays 8 are circular and have a uniform diameter of 14 mm, with the punching burr pointing downward in the separation column. The orifice ratio, i.e. the proportion of the orifice area formed by the orifices relative to the inner cross-sectional area of the column 1 or of the dual-flow tray 8, is 19.75%. The arrangement of the centers of the circular orifices follows a strict triangular pitch. The closest distance between the centers of two circles is 30 mm.

(12) In the case of such a geometry of the orifices, the pressure drop of the dual-flow trays 8 is so low that the inhomogeneous gas pressure distribution which arises when the gas flows in through the gas entry orifice 5 cannot be balanced out. The dry pressure drop of the dual-flow trays 8 of the present working example is 4 mbar for each of the trays 8.

(13) Also disposed within the region B is a withdrawal line 7 through which crude acrylic acid is withdrawn.

(14) Beneath the region B, the column bottom is formed in the region C. In the column bottom, there is a gas entry orifice 5 for tangential introduction of gas into the column cavity 3. The introduction of gas is shown in detail in FIG. 2. Through the gas entry orifice 5, the gas enters the column cavity 3 in a tangential manner and forms a horizontal vortex 14 therein. Since the diameter of the column 1 is relatively large, it may be the case here that the gas pressure in the outer region is greater than in the middle of the column cavity 3.

(15) Again with reference to FIG. 1, there is also an outlet 6 for the bottoms liquid in the column bottom. The liquid pumped away may be fed, for example, to a spray apparatus (quench). In the spray apparatus, the liquid sprayed is supplied with gas. Thereafter, the gas passes through the gas entry orifice 5 into the column 1.

(16) According to the invention, a gas distribution tray 9 in the form of a chimney tray is disposed beneath the lowermost dual-flow tray 8 but above the gas entry orifice 5, i.e. between the lowermost dual-flow tray 8 and the gas entry orifice 5. The gas distribution tray 9, in the case of the chimneys, has 11 orifices for vertical passage of gas which has been introduced into the column cavity 3 via the gas entry orifice 5. The size, geometry and number of these orifices are such that equal gas distribution over the column cross section beneath the lowermost dual-flow tray 8 is brought about.

(17) Equal gas distribution in this document is understood to mean that the dynamic pressure of the gas flowing into the column cavity 3 is to 1/10 of the pressure drop, especially the dry pressure drop, of the gas distribution tray 9. In this case, the pressure drop of the gas distribution tray 9 is sufficiently high to bring about equal gas distribution over the column cross section. If, for example, the backpressure at the gas entry orifice is about 2.4 mbar, the dry pressure drop of the gas distribution tray 9 is, for example, 17 mbar.

(18) In addition, the gas distribution tray 9 has a liquid draw 15. Through this liquid draw, the liquid which collects in the gas distribution tray 9 is guided into the column bottom.

(19) With reference to FIGS. 3 and 4, details of the gas distribution tray are elucidated:

(20) The gas distribution tray 9 has a diameter of 7.4 m, such that it can be secured horizontally in the column interior 3 on the column body 2. In addition, the gas distribution tray 9 has a total of twelve chimneys 12 which form orifices 23 for vertical passage of gas in the upward direction. The orifices 23 have a circular area and a diameter of 810 mm. The orifice ratio, i.e. the proportion of the area formed by the orifices 23 in the total area of the gas distribution tray 9 is thus 14.38%. The orifice ratio of the lowermost dual-flow mass transfer tray 8 is 19.75% which is 1.37 times larger than that of the gas distribution tray 9.

(21) The orifices 23 are distributed over the gas distribution tray 9 in such a way that the centers of eight orifices 23 are arranged in homogeneous distribution on a first outer circular ring which is concentric with respect to the column body 2, and the centers of four orifices 23 are arranged in homogeneous distribution on an inner circular ring which is also concentric with respect to the outer circular ring and to the column body 2. Two adjacent orifices 23 on the outer circular ring together with one orifice 23 on the inner circular ring form an equilateral triangle, and so a total of four equilateral triangles are formed. The orifices 23 are thus arranged in homogeneous distribution over the cross section of the gas distribution tray 9.

(22) For each of the orifices 23, a cylindrical body or chimney body 19 extends upward from a collecting area 18 of the gas distribution tray 9. Above the chimney body 19, spaced apart in the vertical direction, is a covering hood 20 which fully covers the orifice 23 and extends over the chimney body 19 in the horizontal direction. The covering hood 20 prevents the liquid droplets 22 trickling down from being able to pass through the orifice 23 through the gas distribution tray 9.

(23) In the middle of the gas distribution tray 9 is disposed a liquid draw 15. The liquid draw 15 comprises a circular orifice, from which a drainpipe 17 extends downward. Through the drainpipe 17, liquid which collects in the collecting area 18 of the gas distribution tray 9 can drain off downward. For this purpose, the collecting area 18 may be inclined in the direction of the drainpipe 17. Beneath the drainpipe 17 is a collecting cup 16 which forms a collecting tank for liquid. The lower edge 31 of the lower orifice of the drainpipe 17 is spaced apart vertically from the base 33 of the collecting cup 16. In addition, the upper edge 30 of the collecting cup 16 is disposed above the lower edge 31 of the drainpipe 17. Liquid which flows downward through the drainpipe 17 into the collecting cup 16 collects therein, such that the liquid level rises up to the upper edge 30 of the collecting cup 16. Thereafter, the liquid overflows out of the collecting cup 16 over the upper edge 30 and then passes into the column bottom. In this condition, the drainpipe 17 dips into the liquid present in the collecting cup 16. The drainpipe 17 thus passes through a theoretical area of the collecting cup 16 which is formed by the upper edge 30 of the collecting cup 16. In this way, a hydraulic seal is provided, which prevents gas ascending upward from being able to pass upward through the gas distribution tray 9 through the orifice of the drainpipe 17. It is thus possible to ensure that the gas 21 rising upward only rises upward through the orifices 23 of the chimneys 12.

(24) FIG. 5 shows, in schematic form, another example of the liquid draw 15 having the hydraulic seal:

(25) In this case, the gas distribution tray 9 does not have a liquid draw 15 in the middle. Instead, this liquid draw 15 is formed in the column body 2. For this purpose, an inlet 24 is provided in the column body 2 immediately above the collecting area 18 of the gas distribution tray 9. The inlet 24 is connected to a tube 25 which merges into a pipe 26 in siphon-like form. Thereafter, the pipe opens back into the column cavity 3 at an outlet 27 beneath the gas distribution tray 9. In this way, liquid 28 which collects in the collecting area 18 of the gas distribution tray 9 can be removed via the liquid draw 15 and fed to the column bottom. The inlet 24 is disposed beneath the upper edge of the chimney body 19, such that no liquid 28 can overflow the chimney body 19 and pass downward through the orifices 23.

(26) There follows a description of a working example of the process of the invention which is executed with the above-described separation column 1 of one of the working examples.

(27) The process is a thermal separation process between at least one gas ascending in the separation column 1 and at least one liquid descending in the separation column 1. The ascending gas and/or the descending liquid especially comprises (meth)acrylic monomers.

(28) In the separation process, a fractional condensation for separation of acrylic acid from a product gas mixture comprising acrylic acid from a heterogeneously catalyzed gas phase partial oxidation of a C3 precursor compound (especially propene and/or propane) of the acrylic acid with molecular oxygen to give acrylic acid is conducted in a separation column 1 comprising separating internals as described above. In the process, the dynamic pressure of the gas entering the column cavity 3 is to 1/10, preferably 1/7 to 1/10, of the pressure drop of the gas distribution tray 9. Otherwise, the process is conducted as described in documents DE 19924532 A1, DE 10243625 A1 and WO 2008/090190 A1.

(29) The term C3 precursor of acrylic acid encompasses those chemical compounds which are obtainable in a formal sense by reduction of acrylic acid. Known C3 precursors of acrylic acid are, for example, propane, propene and acrolein. However, compounds such as glycerol, propionaldehyde, propionic acid or 3-hydroxypropionic acid should also be counted among these C3 precursors. Proceeding from these, the heterogeneously catalyzed gas phase partial oxidation with molecular oxygen is at least partly an oxidative dehydrogenation. In the relevant heterogeneously catalyzed gas phase partial oxidations, the C3 precursors of acrylic acid mentioned, generally diluted with inert gases, for example molecular nitrogen, CO, CO2, inert hydrocarbons and/or water vapor, are passed in a mixture with molecular oxygen at elevated temperatures and optionally elevated pressure over transition metal mixed oxide catalysts, and converted oxidatively to a product gas mixture comprising acrylic acid.

(30) Typically, the product gas mixture comprising acrylic acid from a heterogeneously catalyzed gas phase partial oxidation of C3 precursors (e.g. propene) of the acrylic acid with molecular oxygen over catalysts in the solid state, based on the total amount of the specified constituents present (therein), has the following contents:

(31) 1% to 30% by wt. of acrylic acid,

(32) 0.05% to 10% by wt. of molecular oxygen,

(33) 1% to 30% by wt. of water,

(34) 0% to 5% by wt. of acetic acid,

(35) 0% to 3% by wt. of propionic acid,

(36) 0% to 1% by wt. of maleic acid and/or maleic anhydride,

(37) 0% to 2% by wt. of acrolein,

(38) 0% to 1% by wt. of formaldehyde,

(39) 0% to 1% by wt. of furfural,

(40) 0% to 0.5% by wt. of benzaldehyde,

(41) 0% to 1% by wt. of propene, and

(42) as the remainder, inert gases, for example nitrogen, carbon monoxide, carbon dioxide, methane and/or propane.

(43) The partial gas phase oxidation itself can be performed as described in the prior art. Proceeding from propene, the partial gas phase oxidation can be performed, for example, in two successive oxidation stages, as described, for example, in EP 700 714 A1 and in EP 700 893 A1. It will be appreciated, however, that it is also possible to employ the gas phase partial oxidations cited in DE 19740253 A1 and in DE 19740252 A1.

(44) In general, the temperature of the product gas mixture leaving the partial gas phase oxidation is 150 to 350 C., frequently 200 to 300 C.

(45) Direct cooling and/or indirect cooling cools the hot product gas mixture appropriately at first to a temperature of 100 to 180 C., before it is conducted, for the purpose of fractional condensation, into region C (the bottom) of the separation column 1. The operating pressure which exists in the separation column 1 is generally 0.5 to 5 bar, frequently 0.5 to 3 bar and in many cases 1 to 2 bar.

LIST OF REFERENCE NUMERALS

(46) 1 column, separation column 2 column body 3 column cavity 4 inlet 5 gas entry orifice 6 outlet 7 withdrawal line 8 mass transfer tray 9 gas distribution tray 11 liquid draw 12 chimneys 13 draw point 14 horizontal vortex 15 liquid draw 16 collecting cup 17 drainpipe 18 collecting area 19 cylindrical body; chimney body 20 covering hood 21 gas 22 liquid droplets 23 gas passage orifice 24 inlet 25 drainpipe 26 pipe in siphon-like form 27 outlet 28 liquid 29 liquid level 30 upper edge of collecting cup 31 lower edge of lower orifice of drainpipe 32 orifices 33 base of the collecting cup