IMPROVEMENTS IN DISTILLATION TRAYS

Abstract: A distillation tray 1 a of the chordal weir sieve type has vapour passages. The density of vapour passages in the area B immediately between the inlet 2 and the outlet 4 of the tray 1 a is higher than in the remaining area A of the tray so that more vapour flow will be directed into the more active main liquid flow area B of the tray. Thus, the vapour leaving the tray from area A has substantially the same composition as the vapour leaving the tray from the area B.

Description: The present invention relates to distillation trays for use in distillation columns. A conventional distillation column includes a plurality of horizontal distillation trays which are spaced apart vertically. Each tray has a plurality of evenly spaced vapour passages therethrough and is associated with downcomers. Each downcomer has an inlet which receives liquid from a tray above and an outlet from which the liquid discharges onto the adjacent lower tray. When a liquid mixture having two or more components is distilled, liquid descends the column tray by tray and, as it flows across the trays, it comes into intimate contact with vapour rising through the passages. Typically the column is provided with a condenser at its top to provide the downward passage of liquid and a boiler to promote the upward flow of vapour. Thus, as the vapour ascends the column it becomes progressively colder and as the liquid descends the column it becomes progressively warmer. The aforesaid intimate contact between the ascending vapour and the descending liquid causes the vapour to give up to the liquid more and more of the less volatile component or components of the mixture being distilled. Thus, the vapour as it ascends the column becomes progressively richer in the more volatile component or components of the mixture. As a result, the mixture separates into two fractions of different composition, one liquid and the other vapour. A conventional distillation tray is circular or part-circular in shape. In use, it may receive liquid along one chord and discharge liquid over an opposite chord. The flow path from liquid inlet to liquid outlet around the periphery of the tray is longer than that defined by a straight line extending perpendicularly from the chordal liquid inlet to the chordal liquid outlet. The liquid therefore tends to dwell at the periphery of the tray. As a result, a greater degree of contact takes place between the relatively cold liquid and the relatively warm vapour at the periphery of the tray than, say, at its centre. The liquid at the periphery of the tray becomes more depleted in its more volatile component or components than the liquid at the centre. This liquid is then less able to transfer the more volatile component or components to the vapour with which it comes into contact. The composition of the vapour ascending the column through peripheral tray areas therefore progressively diverges from that of the vapour ascending the column through more central areas, the former vapour being richer in the less volatile component or components of the mixture than the latter vapour. The peripheral areas of the trays are therefore less effective in fractionating the mixture than the more central areas, and accordingly more trays are employed in the distillation column than would be needed were the peripheral and central areas to be equally effective. Various means have been proposed to ameliorate this problem. For example, rather than relying on a hydraulic or liquid gradient to cause flow of liquid across a tray from its inlet to its outlet, US-A-3 417 975 proposes employing in addition to the conventional uniform pattern of fixed size openings or perforations with walls normal to the tray surface, a uniform pattern of obliquely inclined openings, the latter being oriented in the downstream direction of liquid flow. Such slotted sieve trays, as they have been called, are said to perform with improved efficiency owing to the elimination of longitudinal hydraulic gradient in the liquid on the tray. The neutralisation of the hydraulic gradient is stated to produce uniform resistance to vapour penetration through the liquid on the tray, and both vapour and liquid flows are distributed more evenly over the tray. In order to provide improved uniformity on the tray US-A-3 759 498 discloses employing bubble promoters near the liquid inlet to the tray with a view to preventing areas of inactivity occurring in the vicinity of the inlet. This has the effect of tending to overburden the tray with liquid in the vicinity of the liquid outlet. US-A-3 759 498 teaches employing a non-uniform distribution of the oblique slots (though it still employs the conventional uniform distribution of the conventional perforations) with there being a greater slot density in that half of the tray nearer the liquid outlet than in that half nearer the liquid inlet so as to counteract this problem. As a further measure specifically intended to enhance the liquid flow at the periphery of the tray, those slots in the outlet half of the tray towards its periphery are pointed in the direction of the liquid outlet so that the thrust imparted by the vapour to the liquid is in the direction of the outlet and thus the liquid at the periphery of the tray is urged towards the outlet and is prevented from dwelling too long at the periphery. US-A-3 759 498 further teaches trying to equalise over the whole active surface of the tray the product of the liquid or froth height and the volumetric fraction of liquid in the froth. We believe that the approach of using oblique slots to improve tray efficiency, as is for example described in the aforementioned US patent specifications, can achieve only partial success. It is an aim of the present invention to provide an alternative approach to improving the efficiency of distillation trays. More particularly, it is an aim of the present invention to provide a distillation tray of the sieve kind which tolerates different local flow rates of liquid thereacross, but which has a distribution of perforations such that the corresponding local flow rates of vapour through the perforations are matched more closely thereto (than in a conventional sieve tray). According to the present invention there is provided a distillation tray of the sieve kind being part-circular in shape and having vapour passages therethrough in the form of perforations, an inlet for liquid, an outlet for liquid spaced from and opposite said inlet whereby, in use, the liquid flow rate, F B , across a more central liquid-bearing surface of the tray between the said inlet and the liquid outlet is significantly greater than the liquid flow rate, F A , across the remaining, more peripheral, liquid bearing surfaces of the tray, wherein the average percentage perforate area, P B , in the said more central area of the liquid-bearing surface area of the tray is greater than the average percentage perforate area, P A , in the said more peripheral areas of the liquid-bearing surface of the tray, whereby, in use, the ratio of the flow rate F B to the flow rate V B of vapour through the perforations in the more central area of the liquid-bearing surface matches more closely the ratio of the flow rate F A to the flow rate V A of vapour through the perforations in the said more peripheral area of the liquid bearing surface. We therefore accept that there will be different liquid flow rates in different locations on the tray. Rather than trying to eliminate such differences we employ a smaller total cross-sectional area of vapour passages in regions of the tray where there is a relatively low liquid from rate than in regions of the tray where there is a higher liquid flow rate. Thus, we prefer, according to the invention, to employ a smaller cross-sectional area of vapour passages in peripheral areas of part-circular trays than in more central areas whereby the longer dwell time of liquid on such areas is compensated for by a reduced amount of liquid-vapour contact per unit time in such areas. In a preferred embodiment, the distillation tray is of the chordal weir or outlet type with the inlet and outlet diametrically opposed and the area immediately between the inlet and the outlet having a greater density of passages than the remaining are of the tray. The invention will now be described by way of example, reference being made to the Figures of the accompanying diagrammatic drawings, in which: Figure 1 is a schematic plan view of a known distillation tray of the chordal weir type and Figure 2 is a schematic plan view of a distillation tray according to the present invention. Figure 3 is a schematic plan view of another distillation tray according to the present invention. Figure 4 is an enlarged view of a fragment X of the tray shown in Figure 3, and Figure 5 is a schematic side elevation, partly in section, of a part of a distillation column showing the flow of liquid from tray to tray. The drawings are not to scale. As shown in Figure 1, the distillation tray is a sieve tray 1 of the chordal weir (or outlet) type and has an inlet 2 and diametrically spaced therefrom an outlet 4. If desired a weir may be situated along the chordal outlet. Although not shown, the base of the distillation tray is formed with a plurality of evenly spaced through passages throughout its area. The passages take the form of perforations whose axes extend perpendicularly to the tray surface. It is to be appreciated that the tray will have associated therewith a first downcomer at its inlet end and a second downcomer at its outlet end. The arrows indicate the general flow pattern to be expected when liquid enters the tray from the inlet 2 and proceeds across the tray to the outlet 4. The bulk of the liquid behaves reasonably ideally flowing from the inlet to the outlet weir. However, the areas indicated A have relatively low flows in and out and some recirculation may take place. The vapour flow is however generally uniform from area to area on the tray. The areas "A" effectively become stagnant and less efficient. The liquid in the areas "A" contacts a disporportionately high amount of vapour since it stays on the tray 1 longer. There is thus a mismatch between the liquid-vapour ratio associated with the areas A and the liquid/vapour ratio associated with the other areas of the tray. Referring now to Figure 2, the same parts of the tray 1a have the same reference numerals as tray 1, the areas A have a relatively low density of passages as compared to the area "B" immediately between the inlet 2 and the outlet 4 which has a relatively high density of passages. The passages are of the same kind as are employed in the tray shown in Figure 1. Thus, the tray 1a has a higher density of perforations in the area of highest liquid flow, which means that more vapour flow will be directed into the main liquid flow area "B" by increasing the perforation density in this area. It has been found that, by more closely matching the ratio of vapour and liquid flows in areas A with that in area B the better overall tray efficiency results. This matching is achieved by in effect reducing the flow of vapour through areas A while continuing to allow liquid to dwell in this area. Thus, if the average flow rate of liquid across areas A is Fa, the average flow rate of liquid across area B is Fb, the average flow rate of vapour through areas A is Va and the average flow rate of liquid across area B is Vb, Fb is significantly greater than Fa and Vb is significantly greater than Va but Fa/Va is approximately equal to Fb/Vb. Although in tray 1a the vapour passages each have the same cross-sectional area and there are more passages per unit area in area B than in area A, the passages in area B could have a larger cross-sectional area than the passages in area A with the number of passages per unit area in areas A and B being the same. It will be understood that a distillation column will have a plurality of trays 1a arranged spaced apart vertically as is known in the art. An alternative sieve tray according to the invention is shown in Figure 3 of the drawings. The tray 10 is supported by means (not shown) within the wall of column 12. The tray 10 is generally circular in shape and is disposed horizontally. The tray 10 has a chordal outlet in the form of a straight edge 14 or alternatively in the form of a weir (not shown). The space 16 defined between the outlet 14 and the wall of column 12 serves as the inlet to a downcomer 30 that in operation conducts liquid from the tray 10 to the next tray below in the column. Opposite the outlet 14 is an inlet 18. The inlet 18 comprises a central part-chordal straight edge 20 that extends parallel to the edge 14 and ends in wings 22 and 24 in the form of straight edges that extend from the edge 20 and which terminate in the curved sides of the tray. The wings 22 and 24 extend from the edge 20 towards the outlet 14 at angle a typically in the range of 10 to 20 degress. The inlet 18 is wider than the outlet 14 and thereby helps to reduce the relative area of the tray on which liquid tends to dwell in use of the tray. The liquid bearing surface of the tray 10 preferably has a narrow imperforate strip or region 26 adjacent and complementary to the inlet 18. The inlet 18 is also associated with a seal pan 28 of a complementary downcomer 30 that is effective to conduct liquid from the outlet of the tray above (not shown in Figure 3) to the tray 10. The imperforate strip 26 helps to calm the liquid flowing onto the tray 10 from the downcomer 30. In order to enable the necessary liquid-vapour contact to take place on the surface of the tray 10, the tray 10 has a multiplicity of perforations therethrough. For ease of illustration, the perforations are not shown in Figure 3. All the perforations have the same dimensions as one another. Each perforation has a vertical axis whereby, in use, the passage of the vapour through the perforations does not impart any horizontal component of velocity to the vapour. The tray 10 thus relies on a hydraulic gradient to enable liquid flow from the inlet 18 to the outlet 14 to take place. The perforations are distributed over the whole liquid-bearing surface area of the tray 10 save for the imperforate strip 26. The distribution of the perforations is such that over a wide range of liquid loadings the liquid/vapour (flow) ratio associated with peripheral areas of the tray typically matches that associated with the rest of the tray more closely than the corresponding liquid/vapour ratios are matched in use of the prior art tray shown in Figure 1. Thus, the density of perforations in the segments 32 and 34 of the tray 10 is less than the density of perforations in the remainder 36 of the perforate liquid bearing surface of the tray 10. The chords that notionally delineate the boundaries between the areas 32, 34 and 36 of the tray 10 are shown as dotted lines in Figure 3 of the drawings and are indicated by the references 38 and 40. Within each one of the areas 32, 34 and 36 the perforations are uniformly distributed, there typically being the same number of perforations per unit surface area of the tray 10 in the areas 32 and 34 while there is a greater number of perforations per unit surface area of the tray 10 in the area 36. Figure 4 illustrates the relative distribution of perforations in a fragment X (as shown in Figure 3) of the surface area of the tray 10 that straddles the chord 40. In use of the tray shown in Figure 3, a liquid mixture flows from the inlet 18 to the outlet 14 of the tray 10 and comes into intimate contact with vapour passing upwardly through the perforations in the tray. The liquid thus becomes richer in the less volatile component or components of the mixture and the vapour becomes richer in the more volatile component or components of the vapour. The liquid that flows over the relatively less perforate areas 32 and 34 of the tray 10 tends to dwell in comparison with the liquid that flows over the more perforate area 36 of the tray 10. However, there is less liquid-vapour contact on the areas 32 and 34 per unit time than there is on the more perforate area 36. Over a prolonged period of operation, the average composition of the vapour leaving the surface of liquid overlying the areas 32 and 34 does not deviate greatly from the average composition of the vapour leaving the surface of liquid overlying the area 36. This is because the liquid/vapour ratios over the respective areas also do not deviate much from one another, and such deviations as do occur will be less marked than when the prior art tray shown in Figure 1 is operated under comparable conditions. The flow paths of liquid and vapour through a section of the column 12 comprising three trays 10 is shown in Figure 5. Downcomers 30 effect transfer of liquid from one tray 10 to the next one below. The double-headed arrows 42 indicate liquid flow and the single-headed arrows 44 indicate vapour flow. It will be noted that the direction of the liqud flow on the middle tray (as shown) is opposed to the direction of flow on the other tray. Other flow arrangements are however possible, and the invention is not limited to trays for use in the kind of column shown in Figure 5. Thus, the invention may be applied to trays for use in columns that employ two sets of complementary trays arranged alongside one another. In designing a tray, say of the kind shown in Figure 2, a conventional tray of the kind shown in Figure 1 may be subjected to a dye test or the like to ascertain the relative flow rates in the periphreral and central areas. Say, it is found that the liquid dwells at the sides of the tray about 5 times longer than it does in the central areas directly between the liquid inlet and outlet. Then, a tray according to Figure 2 may be provided with the same overall perforated area but with a redistribution of the perforations. The area B in the tray shown in Figure 2 may thus be provided with a greater number of perforations than the corresponding area of the tray shown in Figure 1 and the areas A each provided with a significantly smaller number of perforations than the corresponding areas of the tray shown in Figure 1. The precise relative distribution of the perforations will depend on the size of the tray. The vapour and liquid flow rates may be calculated for each zone of the tray. The calculation may be based on experimental results or known tray design procedures. The active area, perforation diameter and pitch may then be chosen for each zone using standard hydraulic design methods for distillation trays, paying attention to point efficiency differences that may result from differing flow conditions. The predicted results for the tray may then be compared with results obtained experimentally (typically vapour-liquid dispersion heights) and in the event of the experimental results deviating from those desired, the design procedure may be repeated until a good match with experimental results is obtained.

Claims: A distillation tray (1a, 10) of the sieve kind being part-circular in shape and having vapour passages therethrough in the form of perforations, an inlet (2, 18) for liquid, an outlet (4, 14) for liquid spaced from and opposite to said inlet (2, 18) whereby, in use, the liquid flow rate, F B , across a more central liquid-bearing surface (B, 36) of the tray (1a, 10) between the said inlet (2, 18) and the liquid outlet (4, 14) is significantly greater than the liquid flow rate, F A , across the remaining, more peripheral, liquid bearing surfaces (A, 32, 34) of the tray (1a, 10), characterised in that the average percentage perforate area, P B , in the said more central area (B, 36) of the liquid-bearing surface area of the tray (1a, 10) is greater than the average percentage perforate area, P A , in the said more peripheral areas (A, 32,34) of the liquid-bearing surface of the tray (1a, 10), whereby, in use, the ratio of the flow rate F B to the flow rate V B of vapour through the perforations in the more central area (B,36) of the liquid-bearing surface matches more closely the ratio of the flow rate F A to the flow rate V A of vapour through the perforations in the said more peripheral area (A, 32, 34) of the liquid bearing surface. A distillation tray according to claim 1, wherein FA / VA equals FB / VB. A distillation tray according to claim 1 or claim 2, wherein there is a greater density of perforations in the said more central area (B, 36) than in the said more peripheral areas (A, 32, 34) of the liquid bearing surface. A distillation tray according to claim 1 or claim 2, wherein the density of perforations in the said more central area (B, 36) is the same as that in the more peripheral area (A, 32, 34) of the liquid bearing surface, but the area of each perforation in the former area is greater than that of each perforation in the latter area. A distillation tray according to any one of the preceding claims, wherein the said liquid inlet (18) has an edge longer than the edge of the said liquid outlet (14). A distillation tray according to claim 5, in which the said liquid inlet (18) has wings (22, 24) extending towards the said liquid outlet (14).

Inventors: LAVIN, JOHN TERENCE
Applicants: THE BOC GROUP PLC
Application year: 1985
Main classification: B01D3
IPC codes: B01D3