DISTILLATION | Packed Columns: High Capacity Internals☆

DISTILLATION | Packed Columns: High Capacity Internals☆

DISTILLATION | Packed Columns: High Capacity Internals☆ Zˇ Olujic´, Delft University of Technology, Delft, The Netherlands ã 2014 Elsevier Inc. All ri...

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DISTILLATION | Packed Columns: High Capacity Internals☆ Zˇ Olujic´, Delft University of Technology, Delft, The Netherlands ã 2014 Elsevier Inc. All rights reserved.

Introduction Capacity Bottleneck Performance Characteristics Newest Structured Packing Developments Liquid and Vapor Distributors and Liquid Collectors Closing Thoughts and Outlook References

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Introduction Distillation is a most widely used and most capital and energy-intensive separation method in process industries. It uses heat as separating agent, in a thermodynamically inefficient way, and, due to the scale of applications, it belongs to the largest users of energy in industrially developed countries. After first big energy crisis, in mid-1970s, much effort has been undertaken to find the way to improve it. On the equipment development side, a real technology breakthrough occurred in early 1980s, shortly upon the introduction of Sulzer corrugated metal sheet structured packing Mellapak in the market. Indeed, this packing, characterized by lowest pressure drop per separation stage, has revolutionized demanding vacuum distillations and soon other packing manufacturers followed. In the meantime thousands of tray columns have been revamped by structured packings, mainly those employed in vacuum and at near atmospheric pressures. Also thousands of new columns have been built with the structured packings. Indeed, thanks to their most favorable combination of efficiency and pressure drop, structured packings proved to be a nearly ideal vapor–liquid contactor for vacuum applications. First generation of corrugated sheet metal structured packings was provided with a corrugation inclination angle of 45 , which appeared to be a natural choice, i.e., a suitable combination of pressure drop and efficiency. At the same time packing manufacturers came with the 60 corrugation angle, with respect to horizontal, which was utilized previously only with wire-gauze packings to limit the pressure drop in high-vacuum applications. Indeed, as shown in greater detail elsewhere,1 with such a steep corrugation angle used in conjunction with corrugated sheet metal packings, the capacity increased considerably, however this was at the expense of a roughly equivalent loss in the efficiency. Practically, this means that existing columns equipped with a conventional, 45 packing cannot be revamped using the same type and size of the packing but with a corrugation inclination angle of 60 . With thousands of structured packing columns in operation, the ever growing need for more capacity has generated another push for further improvements in this respect. Indeed, this occurred recently. So, today existing columns equipped with conventional structured packings can be revamped using new generation of high-performance structured packings that provide for the same specific geometric area significantly more capacity at the same or even increased efficiency.

Capacity Bottleneck Regarding the capacity limitation of conventional metal sheet structured packings with corrugation inclination angle of 45 , the actual wrongdoer is a pronounced interaction of ascending vapor and the draining liquid at transitions between packing elements or layers upon reaching a high enough vapor load. As illustrated in the photograph shown in Figure 1, a packed bed consists of a number of relatively short elements/layers, with the subsequent elements/layers rotated usually by 90 to provide for large-scale mixing of both phases, which proved to be essential for mass transfer performance. However, this configuration with thin edges of corrugated sheets oriented perpendicularly to each other at the interface between packing layers represents a discontinuity for liquid flow, which mainly drops off and falls through ascending vapor leaving the element below. The vapor from the element below approaches the transition plane under an angle effectively equal to that of the corrugations and is forced to make a sharp bend to reach the channels of the element above. This is a critical place where the draining liquid interferes with ascending vapor, and at a high enough vapor loads, i.e. upon reaching so-called loading point, the liquid starts to buildup and gradually floods the bottom zone of the element above. As well documented in an experimental study (see 2), where gamma-ray technique was used to measure liquid hold-up along a packed bed at gradually increasing gas loads, this appears to be the capacity bottleneck of the conventional corrugated sheet packing operation. ☆

Change History: October 2013 Zˇ Olujic´ updated the changes on figures, references, and the text.

Reference Module in Chemistry, Molecular Sciences and Chemical Engineering

http://dx.doi.org/10.1016/B978-0-12-409547-2.10939-4

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Figure 1 Photograph of a 1.4-m-diameter packed bed consisting of the high-capacity packing Montzpak B1-250M. Olujic´, Zˇ.; van Baak, R.; Haaring, J.; Kaibel, B.; Jansen, H. Liquid Distribution Properties of Conventional and High Capacity Structured Packings. Chem. Eng. Res. Des. 2006, 84, 867.

Figure 2 Schematic illustration of corrugation geometry of conventional and two high-capacity packings.

With this in mind, the new challenge for packing manufacturers was to arrive at a structure that will combine the efficiency of a 45 packing with the capacity of a 60 packing. Effort along this line of development is summarized in a recent paper.3 Indeed, it appeared to be possible to reach this goal in practice by providing a larger hydraulic diameter at the transition between two packing elements (see4). According to this and other experimental studies,3,5 the bottom part of a packing element has been identified as the critical place, because it is here that the liquid starts to build up when the gas (vapor) reaches certain load. After some experimental performance evaluation and optimization efforts, two configurations have been adopted for commercial applications. Figure 2 shows a schematic illustration of the geometry of vapor flow channels (corrugations) of conventional packing and two proprietary configurations with two bends on both ends of corrugations (Sulzer and Koch-Glitsch) and that with one smooth but longer bend on the bottom end of corrugations (Montz). Main distinction between former two is a more (MellapakPlus) or less (FlexipacHC) smooth bend at both ends of corrugations. The packed bed shown in Figure 1 is comprised of Montzpak B1-250M high capacity packing.

Performance Characteristics Developments along this line will be illustrated on the example of Montz packings. Figure 3 shows the efficiency curve of the highcapacity Montzpak B1-250M compared with that of its conventional counterparts with the specific area of 250 m2 m3, one with corrugation inclination angle of 45 (B1-250), and the other one with 60 (B1-250.60). Figure 3 shows that in the preloading region the efficiency of high-capacity packing is somewhat lower and in the region of application interest is equal to that of the conventional B1-250 packing. The maximum efficient capacity is equal to that of B1-250.60 packing, at similar pressure drop, slightly lower than that of the conventional packing at the same point. This means that this size of corrugated sheet structured packings can operate efficiently up to a pressure drop of nearly 8 mbar m1. Operators can push that hard

DISTILLATION | Packed Columns: High Capacity Internals

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Figure 3 Total reflux performance curves of conventional (B1-250 and B1-250.60) and high-capacity (B1-250M) packings. Olujic´, Zˇ.; Seibert, A.F.; Kaibel, B.; Jansen, H.; Rietfort, T.; Zich, E. Performance Characteristics of a New High Capacity Structured Packing. Chem. Eng. Process. 2003, 42, 55.

only if such a pressure drop can be afforded in given application, i.e. if it will not cause a reduction in relative volatility that could affect separation adversely. To maximize capacity gain the columns equipped with high performance structured packings are usually designed to operate at higher pressure drop than conventional packings (say at 5 mbar m1 instead of 3 mbar m1). The measured pressure drop curve of B1-250M lies in between those of B1-250 and B1-250.60, approaching that of B1-250 at low- and that of B1-250.60 at high vapor loads, respectively. This indicates a change in the nature of the pressure drop with increasing vapor and liquid loads (at total reflux: L/G ¼ 1) imposed by specific corrugation geometry of B1-250M. Striking is the difference in absolute values of the pressure drop of B1-250 and B1-250.60 in the preloading region. According to Figure 3, at F-factor ¼ 1.3 Pa0.5, the pressure drop of 45 packing is three times larger than that of 60 packings, while it is only 1.3 times larger than that of B1-250M. The corrugations of the later are inclined under 45 , and via a long bend in the bottom part of the sheet end under an angle of 90 . In order to understand what occurs hydraulically during normal operation of a packed bed consisting of conventional B1-250 packing, we will follow gas or vapor on its passage through an irrigated bed. Since a packed bed is assembled by stacking packing elements/layers of certain height (0.20 m in present case), with each subsequent element or layer rotated by 90 (common case) to previous one, the ascending gas follows a pronounced zigzag flow pattern. This means that a sharp change in flow direction occurs five times per meter bed height and this is accompanied by a significant pressure loss, which, at higher gas loads (within loading region) gets strongly enhanced due to flow resistances associated with the build up and accumulation of the liquid (creation of additional form drag as well as gas acceleration related losses because the free area available for gas flow gets significantly reduced by the accumulated liquid). This, in turn, ensures a high degree of lateral transport and mixing of phases that facilitates achieving uniformity of both velocity and concentration profiles. Upon a direction change at the element/layer interface, gas flows from inlet to the outlet through a triangular, i.e. V-shaped flow channel with one side open, which creates an open interface where crossing gas flows interact and exchange momentum. The intensity of the contact depends strongly on the corrugation inclination angle, and in case of common 45 this is responsible for nearly half of the total pressure drop. However, such interference with other gas flows contributes to intensive small scale mixing of gas phase, which is favorable for achieving and maintaining the concentration distribution uniformity, i.e. a constant driving force on gas side within a packing element or layer. On the other hand the crossing flows superimpose on each other a rotational flow component, which means that gas advances along a V-shaped channel as a swirling flow, i.e. at a higher effective velocity than anticipated based on straight axial flow. This is generally favorable, i.e. increases the intensity of the contact of gas and liquid at the interface, provided all available packing surface area is wetted by a thin flowing film. On the other hand, the liquid phase, driven by gravity, tends to partly flow over the corrugation ridges which results in a flow angle that is steeper than the corrugation inclination angle. For liquid flowing over corrugation ridges the intersections of the alternately oriented corrugated sheets provide opportunity for mixing with the liquid from neighboring channels, and the extent of this (micro or small scale mixing) depends strongly on the shape and the pattern of surface texture. In summary, while ascending through an irrigated bed of corrugated sheet structured packing, the gas phase experiences various flow resistances, which influence both the capacity and mass transfer performance to a certain extent. There are three major components contributing to total pressure, i.e. that associated with respectively the gas–liquid interaction at the interface, gas–gas interaction at the crossings of the open sides of oppositely oriented V-shaped flow channels of adjacent corrugated sheets, and the pressure loss associated with the more or less sharp change in gas flow direction at transitions of packing elements. The latter two appeared to be strongly dependent on the corrugation inclination angle, and, in addition, the direction change loss becomes increasingly dependent on the liquid load in the loading region.

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In conjunction with common 45 corrugation inclination angle, the bend (to vertical) in corrugations as employed in B1-250M packing eliminates abrupt change in flow direction of the gas phase at the transitions among packing elements or layers. This in combination with a largely increased free space in bottom part of each packing element/layer reduces the gas velocity at the inlet to the layer above to a level that allows a smooth liquid drainage. Namely, with the bottom part of corrugations bend to vertical, the draining liquid uses both sides of the sheet, and a much higher vapor velocity is needed to detach the liquid from the surface of a vertically oriented sheet than in case of conventional packing with sheets inclined under 45 . From this reason the point of onset of loading, i.e. beginning of build up of liquid immediately above the layer interface of packings, has been effectively shifted to higher gas loads, which enables significant increase of capacity at the same efficiency and the same pressure drop compared to conventional packing with the same specific geometric area. As shown in Figure 3, the B1-250M achieves highest efficiency within loading region. The enhancement in mass transfer with respect to operation of B1-250 and B1-250.60 can be attributed to a fluidized bed like situation, with entrained droplets providing additional interfacial area, which is created under high vapor loads in large empty space in bottom part of corrugations. Since the point of maximum efficient capacity of B1-250M packing approaches closely that of a 60 packing, we may conclude that B1-250M packings practically combines the efficiency of 45 packing (B1-250) with the capacity of 60 packing (B1-250.60). This has been proven in many applications in practice. Similar performance can be expected from Koch-Glitsch Flexipac HC and Sulzer MellapakPlus series packings.

Newest Structured Packing Developments Certain loss of efficiency in the preloading region (see B1-250M vs B1-250 curve in Figure 3) was attributed to the reduction of the gas-gas interaction related pressure drop, because a long bend as employed in case of Montzpak B1-250M, eliminates a number of crossings of gas flow channels in the lower part of the corrugations. The strong gas-gas momentum exchange at crossings of gas flow channels contributes to efficiency in two ways. First is the effect of small scale mixing of gas streams at crossing of flow channels, which enables local equalization and maintenance of a uniform concentration profile, i.e. driving force for mass transfer. Second effect is purely kinetic, i.e. related to the fact that due to a strong momentum exchange at the crossings a tangential component is superimposed so that the vapor flow through V-shaped flow channels within each packing element is not axial but swirling. This implies that contact at the interface (surface of liquid film) between the phases occurs at a relatively higher relative velocity than that based on effective axial velocity. Since both effects are beneficial for separation efficiency, it became apparent that shortening the length of the bend accordingly should result in an increase in efficiency. Further increase in gas-gas interaction related pressure drop and consequently mass transfer efficiency could be achieved by reducing the corrugation inclination angle below common 45 . This has been implemented recently, and the new commercially available packing Montzpak B1-250MN performed very well in total reflux distillation experiments conducted using large scale test facilities at Bayer Technology Services at Leverkusen in Germany6 and Fractionation Research Inc., in Stillwater, OK, USA.7 Interestingly, at vapor loads of practical interest this packing exhibits excellent efficiency, which however is even significantly better in the preloading region. This makes this packing very suitable for application in dividing wall columns where certain sections in the partitioned part of the column operate at rather low vapor loads. A further gain in the capacity could be expected if the discontinuity in liquid and vapor flows at transition between packing layers could be avoided. This is a specific feature of a recent, highly promising development in the field of structured packings, i.e. RASCHIG Super-Pak.8,9 This packing relies on a patent issued to Raschig AG in 1990, but further refinements and thorough testing of prototypes occurred during last few years. Figure 4 shows a piece of this packing, which originally was made of smooth metal and in most recent time with a textured surface, which, according to Raschig, further improved overall performance. Unlike corrugated sheet structured packings this is a highly open structure, with reversely arranged lamellae, which resembles to some extent that of high performance random packings, particularly that of Raschig Super Ring, which represents the state of the art in this respect.10 However the surface of RASCHIG Super-Pak is continuous which means that a hold is provided for liquid to flow from top to the bottom of the element without strong interference with ascending vapor. The later ascends nearly vertically and on its way to the top of the element is in frequent contact with film covered lamellas that induce turbulence and local mixing. Transition from element to element (layer to layer) is straightforward for both phases and since there is no direction change in vapor flow there is no additional pressure drop generated and liquid drains freely. With this, the onset of loading and consequently flooding is shifted to high vapor loads, which, according to pilot scale experiments, could be beyond that achievable with high capacity corrugated sheet structured packings at the same pressure drop and same efficiency. Indeed a very promising development and we have to wait to see whether similar performance will be achieved in industrial applications.

Liquid and Vapor Distributors and Liquid Collectors A prerequisite for normal operation of packed columns is a uniform initial distribution of the liquid and vapor. One should note that distillation columns in mega plants have often diameters larger than 5 m (going up to 16 m), and arranging uniform initial distribution for both the liquid and the vapor is a major design and construction concern. All liquid distributors are assembled and water tested prior installation, which provides a proof on expected quality as well as required confidence regarding functionality

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Figure 4 Structure of the Raschig Super-Pak. Olujic´, Zˇ.; Jo¨decke, M.; Schilkin, A.; Schuch, G.; Kaibel, B. Equipment Improvement Trends in Distillation. Chem. Eng. Process. 2009, 48, 1089.

during operation. Testing prior installation is not possible in case of initial vapor distribution. Fortunately, with the time and experience gained, it appeared that designers can fully rely on outcomes of computational fluid dynamics (CFD) modeling, which allows prediction of flow filed imposed by geometry of inlet devices with sufficient accuracy. Indeed, in this and similar, homogeneous fluid flow applications, the CFD proved to be a valuable engineering tool.11,12 Based on practical experience, single bed length is kept limited to prevent the liquid maldistribution to develop to the extent that would affect performance adversely. A rule of thumb suggests that maximally 20 theoretical stages should be placed in a bed. Highcapacity packings behave in this respect similar to their conventional counterparts. A detailed account on gas and liquid distribution properties of Montz conventional (B1-250) and high capacity (B1-250M) structured packings, as observed during large scale distribution experiments conducted at Delft University of Technology in mid 2000s using air/water system at ambient conditions, can be found elsewhere, e.g. Refs. 13 and 14. In case of a high stage requirement (demanding separations) this means having several, column height increasing liquid redistribution sections installed in a packed column. As shown schematically in Figure 5, a liquid redistribution section consists of a liquid collecting device placed above a liquid distributor. The primary function of a liquid collector is to ensure that liquid leaving the bed is collected and thoroughly mixed, prior to being evenly distributed to the bed below. The pressure drop of narrow trough liquid distributors is rather low compared to that associated with operation of liquid collectors, because up to 50% free area is available for ascending vapor flow. Due to much lower free area (up to 30%) the liquid collectors, particularly those of the chimney-tray type, cause a significant pressure drop. Details on the experimental evaluation of the pressure drop of chevron (vane) type- and chimney type liquid collectors, and narrow trough liquid distributors as well as a method for estimation of their pressure drop can be found in a paper by Rix and Olujic´.15 One should realize that modern packings, operated at lower than usual vapor loads, for instance in situations where there is no other possibility to reduce total pressure drop, or in under-loaded sections of dividing wall columns, generate such a low pressure drop that a packed bed loses capacity to smooth out initial vapor maldistribution. This implies a deeper penetration of maldistributed vapor or gas profile into the bed, which can affect adversely the packing efficiency. It may appear surprising, but even the chimney type liquid collectors, which are generally considered as a means for suppressing vapor maldistribution, generate severe initial vapor maldistribution. This, however, being symmetrical, is smoothed out within two or three structured packing elements/layers (see16). Certainly, at lower end of vacuum applications of multistage distillation each millibar of pressure drop counts. Therefore the liquid collecting devices should be designed with minimum pressure drop without compromising the quality of vapor distribution. As demonstrated in a recent paper,11 CFD tools could be useful in this respect. Figure 6 compares the gas flow patterns produced by a conventional chevron (vane)-type liquid collector and the one made smoother, i.e. more streamlined using CFD. The top views of the simulated profiles, with the dark zones (blue color) corresponding with ‘zero’ velocity regions, indicate that with the more streamlined design a significant improvement is possible in the vapor distribution profile which enters the bed above. Most strikingly, this improvement in distribution was not achieved as usual, at the expense of the increased pressure drop. In contrary; it was accompanied by a 30% reduction in the pressure drop. Upon optimizing the design of the liquid collecting troughs (keeping liquid out of sight of vapor flow) the liquid collectors with improved vane geometry have been adopted as standard design for pressure drop sensitive applications. This kind of technology breakthroughs shows that there is still space for significant equipment performance improvements in a traditional field.

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Figure 5 Drawing of a typical liquid redistribution section in a packed column. Rix, A.; Olujic´, Zˇ. Pressure Drop of Internals for Packed Columns. Chem. Eng. Process. 2008, 47, 1520.

Closing Thoughts and Outlook The corrugated sheet flow channel geometries as employed in modern structured packings allow efficient operation at vapor loads higher than that of conventional structured packings, by 10–30%, depending on the operating pressure. Although such packings are basically low pressure drop devices, one should keep in mind that maximum throughputs are accompanied by a rather high pressure drop, up to 7 mbar m1. In multistage applications, where such a high unit pressure drop can not be afforded, lower but still significant capacity gains can be achieved. On pressure drop reduction and capacity increase side, the limits seem to be closely approached. To improve overall performance of packed columns equipped with modern structured packings, a further increase in efficiency at high throughputs is desired. A long bend in the bottom section of a corrugated sheet, as employed in case of Montz-pak B1-250M, appears to be favorable in this respect, as well as open structure with textured lamellas as employed in Raschig Super-Pak. In former case the key to success appeared to be to be an effective modification of macro geometry, while in the latter case a significant improvement in efficiency has been achieved by modifying the micro geometry of the packing. A further, incremental, but highly rewarding improvement in efficiency could eventually be expected from a structure that will allow better utilization of both, micro and macro geometry imposed effects, leading to creation of a relatively larger interface with intensified phase contacting under cost effective loads.

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Figure 6 CFD snap shots of a side cut of the gas flow through liquid redistribution section equipped with a conventional (left-hand side) and an improved geometry liquid collector, respectively, with top view of respective cross sectional gas velocity (ms1) distribution profiles. Adapted from Ali, A. M.; Olujic´, Zˇ.; Jansens, P. J. Experimental Characterisation and Computational Fluid Dynamics Simulation of Gas Distribution Performance of Liquid (re)Distributors and Collectors in Packed Columns. Chem. Eng. Res. Des. 2003, 81, 108.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Olujic´, Zˇ.; Seibert, A. F.; Fair, J. R. Influence of Corrugation Geometry on the Performance of Structured Packings: An Experimental Study. Chem. Eng. Process. 2000, 39, 335. Suess, P.; Spiegel, L. Hold-up of Mellapak Structured Packings. Chem. Eng. Process. 1992, 31, 119. Olujic´, Zˇ.; Jansen, H.; Kaibel, B.; Rietfort, T.; Zich, E. Stretching the Capacity of Structured Packings. Ind. Eng. Chem. Res. 2001, 40, 6172. Billingham, J. F.; Lockett, M. Development of a New Generation of Structured Packings for Distillation. Trans. of the Inst. of Chem. Eng. , Part A, Chem. Eng. Res. Des. 1999, 77, 583. Bender, P.; Moll, A. Modifications to Structured Packings to Increase Their Capacity. Trans. of the Inst. of Chem. Eng., Part A, Chem. Eng. Res. Des. 2003, 81, 58. Olujic´, Zˇ.; Rietfort, T.; Jansen, H.; Kaibel, B.; Zich, E.; Frey, G.; Ruffert, G.; Zielke, T. Experimental Characterization and Modelling of High Performance Structured Packings. Ind. Eng. Chem. Res. 2012, 51, 4414. Olujic´, Zˇ.; Kaibel, B.; Jansen, H.; Rietfort, T.; Zich, E. Fractionation Research Inc. Test Data and Modelling of a High-Performance Structured Packing. Ind. Eng. Chem. Res. 2013, 52, 4888. Schultes, M.; Chambers, S. How to Surpass Conventional and High Capacity Structured Packings with Raschig Super-Pak. Chem. Eng. Res. Des. 2007, 85, 118. Schultes, M. Raschig Super-Pak – Eine neue Packungstruktur mit innovativen Vorteilen im Vergleich. Chemie Ingenieur Technik 2008, 80, 927. Schultes, M. Raschig Super-Ring – A New Fourth Generation Packing Offers New Advantages. Chem. Eng. Res. Des. 2003, 81, 48. Ali, A. M.; Olujic´, Zˇ.; Jansens, P. J. Experimental Characterisation and Computational Fluid Dynamics Simulation of Gas Distribution Performance of Liquid (re)Distributors and Collectors in Packed Columns. Chem. Eng. Res. Des. 2003, 81, 108. Stemich, K.; Spiegel, L. Characterisation and Quantification of the Quality of Gas Flow Distributions. Chem. Eng. Res. Des. 2011, 89, 1392. Olujic´, Zˇ.; van Baak, R.; Haaring, J.; Kaibel, B.; Jansen, H. Liquid Distribution Properties of Conventional and High Capacity Structured Packings. Chem. Eng. Res. Des. 2006, 84, 867. Olujic´, Zˇ. Comparison of Gas Distribution Properties of Conventional and High Capacity Structured Packings. Chin. J. Chem. Eng. 2011, 19, 726. Rix, A.; Olujic´, Zˇ. Pressure Drop of Internals for Packed Columns. Chem. Eng. Process. 2008, 47, 1520. Olujic´, Zˇ.; Ali, A. M.; Jansens, P. J. Effect of Initial Maldistribution on the Pressure Drop of Structured Packings. Chem. Eng. Process. 2004, 43, 465.