Falling film device and method of use

Abstract

Falling film devices have a tube insert located at the top end of the heat exchange tube. The tube insert has internal circumferential ribs. The ribs distribute the process flow into a uniform annular film within the tube insert and tube, eliminating dry spots even at low operating volumes. This provides a larger operating margin, which can improve operating efficiency and reduce fouling.

Description

[Technical field]

[0001] The present invention relates to a falling film apparatus, a tube insert for a falling film apparatus, and a method of using a falling film apparatus, including separating a crude mixture of isocyanates in a falling film evaporator. [Background technology]

[0002] Falling film devices are widely used industrially as heat exchangers. They are useful as evaporators, for performing separations, for performing crystallizations, as well as in other applications. Falling film devices have a number of vertical tubes supported at each end by tube sheets. The tubes are housed in a shell through which a heat exchange fluid circulates in contact with the vertical tubes to supply heat to or remove heat from the tubes. The liquid to be so treated passes downwardly through the interior of the tubes, where it is heated or cooled, as the case may be.

[0003] Optimal operation of a falling film device depends on forming a film of liquid on all interior surfaces of the tubes. The liquid must wet all interior surfaces of each tube and, in addition, wet those surfaces uniformly and continuously throughout the operation of the device. Uneven liquid distribution can cause a variety of problems, including reduced operating efficiency. The primary problems caused by improper liquid distribution in a falling film device are undesirable reactions and fouling.

[0004] For any liquid, there is a theoretical minimum flow rate per tube necessary to maintain a film on the inner surface of the tube. In practice, it is necessary to operate at several times this theoretical minimum flow rate per tube. This limits the range of usable operating rates. Using a higher flow rate promotes uniform wetting, but may produce a thicker film, which may result in less efficient operation. Fluids passing through the device often must be recirculated one or more times to achieve the desired results, thereby increasing operating costs. Residence times are also increased, which can be problematic, for example, when the liquid is sensitive to time-temperature effects. Design improvements that allow falling film devices to operate at flow rates closer to the theoretical minimum flow rate are desirable. Summary of the Invention

[0005] The present invention in one aspect provides a falling film device comprising: (a) an outer shell enclosing an interior volume; (b) upper and lower tube sheets dividing the interior volume into separate upper, middle, and lower chambers; (c) one or more vertically oriented hollow tubes having an open upper end and an open lower end, each hollow tube defining a fluid pathway from an upper chamber to a lower chamber; (d) for each at least one vertically oriented hollow tube, an associated tube insert located at an upper end of the associated hollow tube, the tube insert including a hollow member having (i) an upper section above the tube sheet, (ii) a lower section extending into the associated hollow tube, (iii) one or more process fluid openings in the upper section for admitting process fluid into the tube insert, (iv) an open lower end for transferring a thin film of process fluid from an inner surface of the tube insert onto an inner surface of the associated hollow tube, and (v) one or more circumferential ribs on the inner surface of the tube insert below the one or more process fluid openings. [Brief description of the drawings]

[0006] [Figure 1]1 is a cross-sectional front view of an embodiment of an upper tube sheet, hollow tubes, and tube inserts for use in a falling film device of the present invention; FIG. [Diagram 2] FIG. 1 is a schematic diagram of a separation apparatus including a falling film evaporator of the present invention. [Diagram 3] FIG. 2 is a front cross-sectional view of a pipe insert for use in the present invention. [Figure 4] 1 is a front cross-sectional view of a second embodiment of a pipe insert for use in the present invention. [Diagram 5] FIG. 2 is an enlarged cross-sectional view of a circumferential rib of a pipe insert for use in the present invention. [Figure 6] 4 is an enlarged cross-sectional view of a second embodiment of a circumferential rib of a tube insert for use in the present invention. FIG. [Figure 7] 11 is a front cross-sectional view of a third embodiment of a pipe insert for use in the present invention. FIG. [Figure 8] 13 is a front cross-sectional view of a fourth embodiment of a pipe insert for use in the present invention. FIG. [Figure 9] 13 is a front cross-sectional view of a fifth embodiment of a pipe insert for use in the present invention. FIG. [Figure 10] 13 is a front cross-sectional view of a sixth embodiment of a pipe insert for use in the present invention. FIG. [Figure 11] FIG. 1 is a front cross-sectional view of a precursor for machining into a tube insert for use in the present invention. [Figure 12] FIG. 13 is a front cross-sectional view of a seventh embodiment of a pipe insert for use in the present invention. [Figure 13] FIG. 13 is a front cross-sectional view of an eighth embodiment of a pipe insert for use in the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] With reference to FIG. 1, the present invention includes a tube sheet 5 and hollow tubes 4. The hollow tubes 4 have associated tube inserts 12. In the simplified cross-sectional view of the apparatus shown in FIG. 1, there is only one hollow tube 4. More typically, there will be multiple hollow tubes (and associated tube inserts 12). Each hollow tube 4 has an open upper end 36 and an open lower end 37. Each hollow tube 4 defines a fluid path from above the tube sheet 5 to below the tube sheet 5 (and below a lower tube sheet, such as lower tube sheet 6 as shown in FIG. 2) through an opening in the tube sheet 5. An opening in the tube sheet 5 receives each hollow tube 4. The tube sheet 5 is sealed around each hollow tube 4 such that fluid flowing from above to below the tube sheet 5 must pass through the hollow tube 4.

[0008] The hollow tube 4 is typically cylindrical with a circular cross-section, although the hollow tube 4 can have other cross-sectional shapes if desired.

[0009] The hollow tube 4 is oriented vertically, meaning that the hollow tube is oriented within 3 degrees of vertical, preferably within 1 degree of vertical.

[0010] Each tube insert 12 is located at the upper end of an associated hollow tube 4. As shown in Figure 1, the tube insert 12 has an upper section 25 that resides above the upper surface of the upper tubesheet 5 and a lower section 21 that extends into the associated hollow tube 4. A sidewall 40 defines an internal flow path through each tube insert 12.

[0011] The lower section 21 has an outer cross-sectional dimension that is less than or equal to the inner cross-sectional dimension of the associated hollow tube 4, such that the lower section 21 can extend into the associated hollow tube 4. The lower section 21 may or may not extend below the level of the tube sheet 5 (as shown in Figure 1). In Figures 3, 4 and 7-10, dotted lines 31 indicate various possible positions of the tube sheet 5 when the tube insert 12 is in place in the falling film apparatus.

[0012] Each tube insert 12 has one or more process fluid openings 24 in the upper section 25 for admitting process fluid into the tube insert. "Process fluid" means any fluid or mixture of fluids (which may further contain a dispersed solid phase) passing through the tube insert 12 and hollow tubes 4, including the starting feed fluid introduced into the tube insert 12 and hollow tubes 4, as well as any evaporated materials, reaction products or other products produced within the hollow tubes. The process fluid contains at least one component that is liquid under conditions present at the location of the circumferential ribs 22 of each tube insert 12. The composition of the process fluid may change as the process fluid travels through the tube insert 12 and / or hollow tubes 4.

[0013] In one embodiment, the process fluid opening is simply the open top of the tube insert 12. In other embodiments, a plurality of process fluid openings 24 are provided in the wall of the upper section 25 of the tube insert 12, as shown in Figures 1, 3, 4, 7-10, 12 and 13, respectively. The size and shape of the process fluid openings may vary. Figure 1 shows an elliptical process fluid opening disposed on the outer periphery of the upper section 25 of the tube insert 12 and longitudinally displaced from the upper edge of the tube insert 12. In the embodiment shown in Figures 3, 4, 7 and 8, the process fluid opening 24 is a rectangular slot oriented longitudinally on the outer periphery of the upper section 25 of the tube insert 12 and longitudinally displaced from the upper edge of the tube insert 12. In the embodiment shown in Figures 9 and 10, the process fluid opening 24 takes the form of a triangular (Figure 9) or rectangular (Figure 10) notch disposed on the outer periphery of the upper edge of the tube insert 12.

[0014] In the embodiment shown in FIG. 12, the process fluid opening 24 takes the form of a helical slot longitudinally displaced from the top edge of the tube insert 12 as shown. Although only one process fluid opening 24 is shown in FIG. 12, preferably there are multiple helical slots, arranged around the periphery of the top edge of the tube insert 12 to provide overlap between fluid streams entering the tube insert 12 from the multiple helical slots. The fluid supply opening 24 may have other shapes that may be convenient or beneficial, such as square, circular, diamond, oval, semicircular, trapezoidal, other polygonal, or any other shape. The number of process fluid openings 24 provided in the wall of the upper section 25 is not critical to the invention and may vary widely, such as from 2 to 20 or more. Similarly, the size of the multiple process fluid openings may vary widely as needed or desired, and the multiple process fluid openings may or may not all be the same size.

[0015] Additionally, each tube insert 12 has an open lower end 27 (FIGS. 1, 3, 4, 7-10, 12 and 13) in the lower section 21 for discharging process fluid from each tube insert 12 into the associated hollow tube 4. The tube inserts 12 may be chamfered at the open lower end 27 to provide a smooth transition from the tube insert 12 to the hollow tube 4, as shown in FIG.

[0016] The tube insert 12 has one or more circumferential ribs 22 present on the inner surface of the tube insert 12 below the process fluid openings. The circumferential ribs 22 may be present in the upper section 25 of the tube insert 12 (as in Figures 9 and 10, and as in circumferential ribs 22A, 22C, and 22D in Figures 7 and 8, respectively), in the lower section 21 (as in Figures 1, 3, 4, and as in circumferential ribs 22B and 22E in Figures 7 and 8, respectively), and / or at the boundary between the upper section 25 and the lower section 21 (i.e., at the level of the upper tube sheet 5). As shown in Figures 7 and 8, circumferential ribs may be present in both the upper section 25 and the lower section 21.

[0017] Any number of circumferential ribs may be provided, for example up to 10, up to 5, or up to 3, or there may be only one circumferential rib.

[0018] By "circumferential" it is meant that the rib extends completely around the inner wall of the tube insert 12 and forms a circle (when oriented perpendicular to the central longitudinal axis 60 of the tube insert 12 as shown in FIGS. 3, 4, 9, 10, 12 and 13) or an ellipse (when oriented non-perpendicular to the central longitudinal axis 60 of the tube insert 12 as shown by circumferential ribs 22A, 22C and 22D in FIGS. 7 and 8). As shown in FIGS. 7 and 8, multiple circumferential ribs may be oriented at different angles relative to the central longitudinal axis 60, if desired. In FIGS. 7 and 8, circumferential ribs 22A, 22C and 22D are oriented non-perpendicular to the central longitudinal axis 60, while circumferential ribs 22B and 22E are oriented perpendicular to the central longitudinal axis 60. All of the circumferential ribs 22 may be oriented at the same angle relative to the central longitudinal axis 60.

[0019] Each circumferential rib 22 projects radially inward (i.e., toward the central longitudinal axis) from the inner surface of the tube insert 12. The radial width W リブ (see FIG. 5) is in some embodiments at least 0.25 mm, and in some embodiments at least 1 mm or at least 2.5 mm. リブ is, for example, the inner diameter ID of the tube insert 12 TI 1 / 8 of (ID TI / 8), and in a particular embodiment, the maximum ID TI / 12 or ID TI / 16 may also be used.

[0020] Each circumferential rib 22 has a longitudinal width δ リブ (Figure 5). δ リブ In some embodiments, δ is at least 0.25 mm, and in some embodiments, δ is at least 1 mm or at least 2.5 mm. リブmay be, for example, up to 25 mm, up to 10 mm, up to 7.5 mm, or up to 5 mm.

[0021] The inwardly extending edge 42 of the circumferential rib 22 may be curved, and if curved, the inwardly extending edge 42 has a radius of curvature R リブ ≦δ リブ / 2, the radius of curvature R リブ R リブ = δ リブ When R is 0.25, the inward edge 42 has a semicircular cross section. リブ ≦δ リブ / 4, ≦δ リブ / 8, or ≦δ リブ / 12, in some embodiments, R リブ >δ リブ / 32, preferably >δ リブ / 24 or >δ リブ / 16.

[0022] The circumferential rib 22 may be integral with the sidewall 40 of the tube insert 12 or may be a separately manufactured member that is mounted in place within the tube insert 12. If separately manufactured, the circumferential rib 22 may be mechanically attached to the sidewall 40 by, for example, welding or gluing, such as by welding or adhesive 50 (FIG. 6), or through the use of various fasteners and hangers, such as hanger apparatus 30 and support 32 (FIG. 4). A circumferential slot may be provided in the sidewall 40 to receive the circumferential rib 22.

[0023] The integral circumferential ribs 22 may be created by machining the interior of the sidewall 40 of the precursor tube 12A to remove wall material above and below the location of each circumferential rib 22, as shown in Figure 11. The tube insert 12 is fabricated from a precursor tube 12A having a wall thickness Y that is greater than the sidewall thickness X of the tube insert 12 fabricated therefrom. The shaded portion 40A of the sidewall 40 in Figure 11 is machined away to create the tube insert 12 having the sidewall thickness X and the circumferential ribs 22. The combined thickness of the wall and ribs may be equal to the original wall thickness Y.

[0024] The inner surface of the sidewall 40 may be of uniform inner diameter (except for the ribs). In the embodiment shown in Figure 13, the inner surface of the sidewall 40 of the upper section 25 of the pipe insert 12 tapers outwardly in a region 25A starting below the fluid treatment openings 24 to above the ribs 22, thereby increasing the inner diameter of the pipe insert 12 through that region 25A of the upper section 25. Such a design further facilitates a smooth annular flow of the process fluid toward the circumferential ribs 22.

[0025] During operation, process fluid is introduced onto the upper surface of the tube sheet 5 and when it reaches a certain height (as indicated by liquid interface 28 in Figures 1 and 2), it spills through process fluid openings 24 into the upper section 25 of the tube insert 12. Upon entering the tube insert 12, the process fluid is driven downwardly within the associated tube insert 12 by gravity and / or differential pressure through the circumferential ribs 22, continues downwardly through the lower section 21 of the tube insert 12, and then exits the tube insert 12 through the open bottom end 27 and into the hollow tube 4. The circumferential ribs 22 capture the momentum of the falling process fluid and distribute the fluid evenly around the circumference of the tube insert 12, forming a uniform film without dry spots. The uniform film is maintained as the process fluid enters and passes through the open bottom end 27 into the associated hollow tube 4.

[0026] The tube insert 12 may further include one or more features, such as a flow deflector, that create a tangential flow of the process fluid entering the tube insert. The process fluid openings may be machined to create a tangential flow of the process fluid entering the tube insert.

[0027] Referring to Figure 2, the falling film device 1 comprises an outer shell 2 defining a vessel having an enclosed interior volume. An upper tube sheet 5 and a lower tube sheet 6 divide the enclosed interior volume into an upper portion 26 above the upper tube sheet 5, an intermediate portion 3 between the upper tube sheet 5 and the lower tube sheet 6, and a lower portion 7 below the lower tube sheet 6. The hollow tubes 4 define a flow path from the upper portion 26 through the intermediate portion 3 to the lower portion 7 of the enclosed interior volume. An opening in the upper tube sheet 5 and an opening in the lower tube sheet 6 receive each hollow tube 4. The upper tube sheet 5 and the lower tube sheet 6 are each sealed around each hollow tube 4 such that fluid flowing from the upper portion 26 down to the lower portion 7 must pass through the hollow tube 4.

[0028] The falling film heating apparatus 1 further comprises at least one process fluid inlet port 10 for introducing a process fluid into the upper part 26 of the interior space enclosed by the shell 2. Multiple process fluid inlet ports 10 may be provided. The falling film apparatus 1 further comprises at least one process fluid outlet port 11 for removing a process fluid from the lower part 7 of the interior space enclosed by the shell 2. Multiple process fluid outlet ports 11 may be provided. In the embodiment shown in Figure 1, a separate gas outlet port 13 is provided for removing steam from the lower part 7 of the space enclosed by the shell 2.

[0029] The falling film device 1 further comprises at least one heat exchange fluid inlet port 8 for introducing heat exchange fluid into the interior space 3 and at least one heat exchange fluid outlet port 9 for removing heat exchange fluid from the interior space 3.

[0030] In addition to the aforementioned mechanisms, the falling film device 1 may include various optional components. A distributor may be provided to distribute the process fluid onto the upper tube sheet 5. A wide variety of distribution systems are available to ensure uniform liquid levels on the upper tube sheet. One type of distributor is a flat-bottom vessel installed above the upper tube sheet 5. The vessel has holes that allow the process fluid to flow onto the upper tube sheet 5 between the tube inserts 12. A spray distribution system sprays droplets of the process fluid onto the upper tube sheet 5 and / or the flat-bottom vessel installed above the upper tube sheet 5. Other useful distributors include, for example, any of those described in U.S. Pat. Nos. 4,154,642, 4,199,537, and 9,101,852, and U.S. Patent Publication No. 2020 / 0030712. Other optional components include various valves, pumps, automatic process control devices, and the like.

[0031] During operation, process fluid is introduced into the upper portion 26 through one or more inlet ports 10. The process fluid remains on the upper surface of the upper tube sheet 5 and when it reaches a certain height (reference number 28 in Figs. 1 and 2), it spills through the process fluid openings 24 into the upper section 25 of the tube insert 12. Once the process fluid enters the tube insert 12, it falls downward under gravity and / or applied pressure, passing through the circumferential ribs 22, through the lower section 21 of the tube insert 12, then out of the tube insert 12, through the open lower end 27 into the hollow tubes 4, and through them, and finally into the lower portion 7 of the heat exchange device 1. As previously mentioned, the process fluid falling as it passes through the circumferential ribs 22 is evenly distributed around the circumference of the tube insert 12 to form a uniform film without dry spots. The uniform film is maintained as the process fluid enters and passes through the associated hollow tubes 4 through the open lower end 27.

[0032] In heat exchange operations, the hollow tubes 4 are maintained at a different temperature than the process fluid introduced through the feed inlet port 10, whereby heat is exchanged between the hollow tubes 4 and the process fluid. The hollow tube temperature may be higher or lower than the temperature of the incoming process fluid. The hollow tube temperature is typically higher for operations such as evaporation, pasteurization, and carrying out chemical reactions, and lower for operations such as crystallization.

[0033] Heat is supplied to or removed from the hollow tubes 4 via a heat exchange fluid introduced into the intermediate section 3 via a heat exchange fluid inlet port 8. The heat exchange fluid circulates within the interior space 3 between the upper tube sheet 5 and the lower tube sheet 6, contacting the hollow tubes 4 to heat or cool them (as the case may be) and is withdrawn through a heat exchange fluid outlet port 9. The heat exchange fluid may be liquid and / or gaseous. Some or all of the heat exchange fluid may undergo a phase change within the vessel, e.g., vapor may be partially or completely condensed within the vessel. The heat exchange fluid is selected at least in part based on the desired operating temperature. Examples of other heat exchange fluids include liquid water, air, nitrogen, argon, helium, liquid and / or gaseous halocarbons (including hydrohalocarbons), silicone fluids, ethylene glycol, propylene glycol and other alkylene and polyalkylene glycols, various alkylated aromatic compounds, various polyester compounds, and the like.

[0034] In some embodiments, the falling film device of the present invention is used to perform evaporation. In such embodiments, the process fluid is a single component liquid that is evaporated in the hollow tube 4, or more typically, a multicomponent fluid containing at least one component that is separated from at least one other component by fractional distillation in the hollow tube 4. The evaporation typically produces one or more gaseous products that represent the components of the process fluid that evaporate in the hollow tube 4, and one or more liquid products that are the components of the process fluid that pass through the hollow tube 4 without being evaporated. It is generally preferred to establish a downward gas flow through the hollow tube 4, such that the gaseous products are removed from the bottom of the hollow tube 4. However, it is within the scope of the present invention to remove the gaseous products from the top of the hollow tube 4.

[0035] The falling film device shown in Figure 2 is particularly suitable for carrying out evaporation: an annular film of process fluid flows downwardly into and passes through a hollow tube 4, where it is heated and at least a portion of one or more components of the process are volatilized to form a gas.

[0036] In the embodiment shown in Figure 2, both the volatile (gaseous) and non-volatile components of the process fluid flow into the lower section 7 of the vessel where they are separated. In the embodiment shown, the volatile components are removed from the lower section 7 via line 13, through which they are transferred to an optional gas-liquid separator 14 for removing entrained non-volatile material from the gaseous product. The gaseous product is then recovered via line 18.

[0037] The non-volatile components of the process fluid are removed from the lower portion 7 of the vessel via outlet port 11 and line 15. In any arrangement shown, the non-volatile components removed via line 15 are combined with additional amounts of non-volatile components removed from the gas-liquid separator 14 via line 16. The non-volatile product stream is recovered via recovery line 20.

[0038] All or a portion of the non-volatile components recovered from the falling film apparatus 1 may be recycled back to the falling film apparatus 1 as needed, for example in the case of incomplete evaporation of the volatile components of the process fluid. For example, in Figure 1, a recycle stream is removed via line 17 and recombined with fresh process fluid, and the mixture is fed to the upper portion 26 of the enclosed interior space 3 via line 19 and inlet port 10.

[0039] An important advantage of the present invention is that even at low liquid flow rates, an annular film of the liquid components of the process fluid can be formed on all of the interior surfaces of the tube insert 12 and hollow tube 4 without dry spots. Lower liquid flow rates produce thinner films. A thinner film allows the process fluid to be heated or cooled faster and more uniformly, and in the case of evaporation, allows for more complete removal of volatile components from the process fluid. As a result, less material needs to be recycled. Because recycled materials are exposed to high temperatures for a much longer period of time, they experience a more severe thermal history than materials processed in a single pass. When recycled materials contain heat sensitive components, the ability to reduce recycling and thereby reduce exposure time to high temperatures is often a major advantage.

[0040] The ability to produce an annular film without dry spots at low flow rates also expands the range of conditions under which a falling film device can be operated. For example, at certain times, such as during start-up or shut-down, it may be desirable or necessary to operate at relatively low flow rates without fouling the device or producing non-prime materials. The falling film device of the present invention can be operated over a wide range of flow rates.

[0041] The flow rate through the hollow tube device is determined by the minimum wetting velocity Γ min which is a function of the process fluid contact angle θ (between the incoming process fluid and the tube or insert), as well as the surface tension σ, viscosity μ, gravitational constant g, and density ρ, as follows:

[0042]

number

[0043] Conventional falling film devices with tube inserts typically have a γ factor of 5 or more. min The falling film device of the present invention operates at a flow rate of 1.5G and does not produce a uniform annular film at lower flow rates. min In some embodiments of the present invention, the falling film device of the present invention operates well at flow rates of about 1.5G or even less, and also operates well at flow rates much higher. min ~10Г min , 1.5g min ~5.0Г min、 Or 1.5~3Г min It operates at a flow rate of .

[0044] The falling film devices of the present invention are useful for performing many types of separations, including the production of concentrated fruit juices and concentrated food products such as concentrated milk and / or condensed milk, the production of alcoholic beverages such as whiskey, and many chemical and / or petrochemical processes.

[0045] Among the many chemical separations for which falling film units are useful is the separation of crude isocyanate mixtures produced by phosgenating mixtures of methylene dianiline and higher polymethylene polyaniline. In such separations, diphenylmethylene diisocyanate (MDI) is separated from higher polymethylene polyphenylene polyisocyanates (having three or more phenylisocyanate groups) by passing the crude mixture through a falling film unit operating at a tube temperature sufficient to volatilize the MDI but not the higher polymethylene polyphenylene polyisocyanates. This produces an MDI-rich vapor stream that may contain, for example, at least 98% MDI by weight, and a liquid stream of polymethylene polyphenylene polyisocyanates that is enriched in polymethylene polyphenylene polyisocyanates relative to the starting crude mixture and depleted in MDI.

[0046] Comparative sample A A hollow stainless steel plate having an inner diameter of 4.47 cm is fitted into the tube sheet located at the bottom of the vessel. A tube insert having an outer diameter of 4.47 cm and a wall thickness of 0.018 cm is inserted into the hollow tube such that the upper section is above the level of the tube sheet and the lower section is within the hollow tube below the level of the tube sheet. Eight process fluid openings in the form of longitudinal slots 3 cm wide are evenly spaced around the circumference of the upper section of the tube insert. A flow deflector is associated with each of the process fluid openings, located at an angle of approximately 20 degrees relative to the opening plane of the slot. The flow deflectors create a tangential flow of the process fluid into the tube insert. The inner surface of the tube insert below the process fluid openings is smooth and has a constant diameter.

[0047] Density (at experimental temperature) is 1106 kg / m 3 A fluid with a viscosity of 1.9 cP, an interfacial tension of 31 mN / m, and an advancing contact angle of 60 degrees with stainless steel is poured into the vessel at a flow rate of -82 kg / h and a wetting velocity Γ of 0.162 kg / ms. The minimum wetting velocity Γ of this fluid is min is calculated according to the above formula 1 to be 0.099kg / ms. min is 1.64. Under these conditions, the liquid forms a rivulet on the inner wall of the tube insert and does not wet the wall uniformly. A uniform annular film increases the flow rate and the operating ratio Γ / Γ min It is generated only if you set the value to a value greater than 4.

[0048] Example 1 Comparative Sample A is repeated, except this time the tube insert has two circumferential ribs on the inner wall directly below the process fluid opening. リブ is 0.762 mm, and W リブ is 2.03 mm, and δ リブ The diameter of the circumferential rib is 1.52 mm. When the process fluid passes downward through the circumferential rib, a uniform film is formed, and the operating ratio Γ / Γ min = 1.64 to wet all the inner surfaces of the tube insert.

[0049] Example 2 Comparative sample A is repeated again, except this time the tube insert has no flow deflector and has three circumferential ribs. The circumferential ribs have the same dimensions as described in Example 1. Run ratio Γ / Γ min = 1.64, a uniform film is formed as the process fluid passes down the circumferential ribs even without the benefit of tangential flow entry. All of the inner surfaces of the tube insert are wetted.

Claims

1. A falling film device, comprising: (a) an outer shell enclosing an interior volume; (b) upper and lower tube sheets that divide the interior volume into separate upper, middle, and lower chambers; (c) one or more vertically oriented hollow tubes having an open upper end and an open lower end, each hollow tube defining a fluid pathway from an upper chamber to a lower chamber; (d) for each at least one vertically oriented hollow tube, an associated tube insert located at an upper end of the associated hollow tube, the tube insert including: (i) an upper section residing above the tube sheet; (ii) a lower section extending into the associated hollow tube; (iii) one or more process fluid openings in the upper section for admitting process fluid into the tube insert; (iv) an open lower end for transferring a thin film of process fluid from an inner surface of the tube insert onto an inner surface of the associated hollow tube; and (v) a hollow member having one or more circumferential ribs residing on the inner surface of the tube insert below the one or more process fluid openings.

2. The at least one vertically oriented hollow tube and associated tube insert each have a circular cross section, and each circumferential rib has an ID of at least 0.25 mm and a maximum ID of at least 0.25 mm. Ｉ / 8 (ID Ｉ represents the inner diameter of the pipe insert) リブ and each circumferential rib has a radius of curvature R リブ <δ リブ / 2(δ リブ 10. The falling film device of claim 1, wherein the circumferential rib has a curved edge with a thickness of 0.5 mm.

3. The falling film device of claim 1 , wherein the upper section of the pipe insert is adapted to create a tangential flow of process fluid into the pipe insert.

4. 10. The falling film device of claim 1, wherein the one or more process fluid openings in the upper section of the tube sheet are spiral slots.

5. (e) at least one process fluid inlet port for introducing a process fluid into the upper chamber to contact the upper surface of the upper tube sheet; (f) at least one process fluid outlet port for removing at least the treated process fluid from the lower chamber; (g) at least one heat exchange fluid inlet port for introducing a heat exchange fluid into the intermediate chamber; (h) at least one heat exchange fluid outlet port for removing the heat exchange fluid from the intermediate chamber; 10. The falling film device of claim 1, further comprising:

6. 10. The falling film apparatus of claim 1 further comprising a process fluid distributor above said upper tube sheet.

7. 10. A process for separating components of a liquid mixture, comprising: introducing a process fluid containing the liquid mixture into a falling film device according to any one of claims 1 to 6; flowing the process fluid through the pipe insert of the falling film device to form an annular film of the process fluid; then transferring the film of the process fluid to a heated hollow tube of the falling film device; and flowing the film downwardly through the heated hollow tube, whereby at least one component of the liquid mixture at least partially evaporates from the film as it flows downwardly through the heated hollow tube, and at least one component of the liquid mixture does not volatilize.

8. 10. A process for separating diphenylmethane diisocyanate from polymethylene polyphenylene polyisocyanate, the process comprising: introducing a starting process fluid containing diphenylmethane diisocyanate and polymethylene polyphenylene polyisocyanate into a falling film apparatus according to any one of claims 1 to 6; flowing the starting process fluid through the tube insert of the falling film apparatus to form an annular film of the starting process fluid; then transferring the annular film of the starting process fluid to a heated hollow tube of the falling film apparatus; and flowing the film downwardly through the heated hollow tube, whereby diphenylmethane diisocyanate at least partially evaporates from the film as it flows downwardly through the heated hollow tube to produce a vapor stream containing diphenylmethane diisocyanate and a liquid stream enriched in polymethylene polyphenylene polyisocyanate and depleted in diphenylmethane diisocyanate relative to the starting process fluid.

9. A method for making a pipe insert having one or more internal circumferential ribs, the method comprising machining a precursor pipe to remove interior wall material above and below the location of each internal circumferential rib, thereby creating the internal circumferential rib.

Images