060522 Ensure Proper Design

060522 Ensure Proper Design - Distillation Ensure Proper...

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Unformatted text preview: Distillation Ensure Proper Design and Operation of Multi-Pass Trays To function properly at optimum capacity and efficiency, multi-pass trays must be designed so that the liquid and vapor flows are properly balanced. This article provides guidance on how to achieve that. Mark Pilling Sulzer Chemtech USA, Inc. S tandard crossflow trays in distillation columns operate with gravity-driven liquid flowing downward in a serpentine pattern while contacting the vapors that are flowing upward. As the liquid flowrates increase, the frictional resistance and corresponding liquid head increase on the tray deck and in the downcomers. Once the liquid resistance becomes substantial, the tray configuration is split into multiple streams or passes. Two-pass trays are quite common and generally don’t pose significant design problems, since the flow paths and tray designs are symmetric. Tray designs with more than two liquid passes are considerably more challenging because of their lack of symmetry, and need to be designed with care. Three-pass trays are rare because of their inherent asymmetric design and difficulty to balance. Four-pass trays are more common. Trays with more than four passes are rare as well. Unless noted otherwise, the multi-pass trays discussed in this article are four-pass trays. Hydraulic considerations One of the most critical operational aspects is the balance of the vapor and liquid streams on the tray deck. It is imperative that the vapor and liquid streams contact each other and flow evenly across the tray. When the vapor flow is disproportional to the liquid flow in different areas of the tray, a compositional pinch can occur and limit the efficiency. From a hydraulic standpoint, when the flows are imbalanced, the side that is more heavily loaded will prematurely limit the tray’s capacity. The effect of an imbalance varies depending upon the tray 22 www.cepmagazine.org June 2005 CEP design and the application. For example, in applications that are heavily vapor-loaded, excessive vapor flow to one section will cause a flood in that section; in highly liquid-loaded applications, excessive liquid flow to a section may cause a localized flood there as well. A common term used to evaluate the vapor and liquid flows is V/L, the vapor/liquid ratio. When measured on a molar basis, this value is generally unity for a column operating in a total reflux mode. Correspondingly, the V/L ratio should also be unity for a perfectly balanced tray operating at total reflux. Thus, V/L is the defining measure of balance between passes. Since most columns do not operate at total reflux, the V/L ratio may vary widely from unity. The absolute value of the number is really not important, but the consistency of this number between the various passes of a multi-pass design is critical. Tray geometry and nomenclature In its simplest form, a standard crossflow tray consists of a tray deck and a downcomer. The tray deck is perforated with either sieve holes or valves that allow the vapor to travel vertically upward through the deck to contact the liquid layer that is flowing horizontally across it. After contact, the vapor disengages from the liquid and travels to the next tray immediately above. The mixing of the vapor and liquid generally forms a froth or spray on or above the deck. As the frothy liquid mixture leaves the deck, it travels into the top portion of the downcomer. The downcomer is designed to disengage any additional vapor that is in this mixture. The disengaged vapor leaves out the top of the Center Downcomer W1 Side Downcomer W2 Panel D Panel C W3 W4 Panel A C1 C2 Panel B OCDC Inboard Side OCDE Outboard Side ■ Figure 1. Four-pass tray features. downcomer, while the clarified liquid travels downward through the downcomer and is fed to the tray below. Other important mechanical features on the tray are the downcomer clearance (C) and the tray outlet weir (W). The downcomer clearance is the vertical gap formed between the lower edge of the downcomer wall and the tray deck. It serves as a flow orifice that meters the liquid onto the tray and also maintains a liquid seal on the downcomer. The outlet weir is a vertical metal piece that is placed at the interface between the end of the tray deck and the inlet of the downcomer. Typically, it extends 25–75 mm (1–3 in.) above the tray deck. It serves to maintain the desired liquid level on the tray deck, and in many designs is used to ensure a hydraulic seal above the downcomer clearance. These features will be discussed later in more detail, since they are important in balancing the vapor and liquid flows. Multi-pass trays have the following geometric features: Downcomers. All four-pass trays have three types of downcomers (Figure 1). Side downcomers are formed by the column wall and a downcomer panel. They are the most common type of downcomer, and are found in all single- and multi-pass tray designs. The center downcomer is located at the tower centerline, and is formed between two downcomer panels offset from the centerline. The center downcomer is symmetrical, and is also used in two-pass designs. Two side downcomers and one center downcomer are located on alternating trays, either all the odd-numbered or all the even-numbered trays. The third type of downcomer is an intermediate, or offcenter, downcomer (OCDC). The OCDC is located between the side and center downcomers on the alternating trays between those containing the side and center downcomers. They are used for multi-pass tray designs that have more than two passes. The OCDC itself is asymmetric. The side closer to the centerline of the tower (referred to as the inboard side) has a longer chordal length than the side farther away from the centerline (the outboard side). Active area panels. Four-pass trays consist of two mechanically different trays, each of which works in conjunction with the trays immediately above and below it. There are four different types of active area panels on four-pass trays, designated as A, B, C and D (Figure 1): • Panel A: between the OCDC outboard clearance and the side downcomer inlet • Panel B: between the OCDC inboard clearance and the center downcomer inlet • Panel C: between the side downcomer clearance and the OCDC outboard inlet • Panel D: between the center downcomer clearance and the OCDC inboard inlet. The tray containing side and center downcomers will have one A panel and one B panel on each half of the tray. The tray containing two OCDCs will have one C panel and one D panel on each half. These panels are described further below. Liquid flows For the sake of simplicity, the fluid flow through the downcomers and across the decks will be referred to as a liquid. This fluid is more likely to be a two-phase froth (especially in high-pressure applications), but the term liquid will suit the purposes of the discussion. Figure 2 shows the directions of the liquid flows with respect to the downcomers through a four-pass tray, which are as follows: Liquid leaving the OCDC is split into two portions. One side feeds panel A en route to the side downcomer (blue). The other portion feeds panel B en route to the center downcomer (green). If the tray is designed correctly, the liquid stream flowrates will be proportional to the active area to which they are being fed. Liquid leaving the center downcomer should always be split into equal parts. These feed the D panels immediately on either side of the center downcomer (violet). The side downcomers are rather isolated and can only transfer liquid to and from the OCDC outboard side. Liquid to the side downcomer travels from the OCDC outboard clearance across panel A and into the side downcomer (blue). Liquid leaving the side downcomer travels across panel C and then flows into the OCDC outboard side (red). Balancing the liquid flows Liquid flows are balanced by modifying the liquid restrictions in the downcomer and on the tray decks, namely, the downcomer clearance and the tray outlet weir. Downcomer clearances. All downcomer clearances need to be sized to obtain the appropriate pressure drop. This is CEP www.cepmagazine.org June 2005 23 Distillation done to ensure the proper capacity, sealing of the downcomer, and characteristics of the flow from the downcomer. However, since the side and center downcomers have only one possible destination, the OCDC, no balancing mechanism needs to be applied to them. The balancing of liquid with the downcomer clearances is necessary only with the OCDC. The downcomer clearance is calculated as a simple open area arranged in the vertical plane. The orifice coefficient for this opening is generally assumed to be constant. Therefore, to adjust the downcomer clearance, either the clearance height or the length (perpendicular to the flow) of the clearance needs to be decreased. Since the clearances are formed by the downcomer chordal length, they cannot be made wider, only narrower. It is important to note that the effect of increasing the height of a downcomer clearance varies depending upon the liquid head in the downcomer. Because of this, it is never recommended to balance the liquid distribution from the two sides of the OCDC by using different clearance heights. Changing the length of the clearance alone is the recommended method of adjustment. Otherwise, the partitioning of the liquid will vary with process rates. The effective downcomer clearance area is decreased by blocking off portions of the downcomer. This is typically done with bracing, which both blocks the desired amount of the clearance width and supports the downcomer wall. Care must be taken to make the blocked sections small enough and well enough dispersed to avoid flow maldistribution. Outlet weirs. Weir loading is measured as a volumetric flow across a unit length of weir. The crest of the liquid over a weir is essentially a function of the weir loading to the 2/3 power. As weir loading increases, the crest increases, the liq- uid height on the upstream tray panel increases, and finally the hydraulic resistance for the vapor increases. In order to properly balance the weir loadings on a multipass tray, the weir loading for the side and center downcomers needs to be constant, and the weir loadings on either side of the OCDC need to be constant. As with the clearances, changing the vertical height of different weirs to balance passes is not an acceptable method, since the crest over the weir is a function of both the weir height and the weir load. If this were to be done, the crest over the weir would vary under different process conditions and would not allow proper balancing of the tray under all conditions. Therefore, the only acceptable way to balance passes with the outlet weir is to make the effective weir length either longer (swept-back weirs) or shorter (picketing). Swept-back weirs. Side downcomers tend to have a relatively short weir length because the downcomer chord is far from the tower centerline. This is especially important in equal-bubbling-area designs, since the side panel A will have the same liquid flow as the center panel B, but the side downcomer weir is much shorter than the center downcomer weir. Figure 3 compares a swept-back weir design (on the right) with a standard weir design. The swept-back weir creates a longer effective side downcomer and makes it easier to balance the weir loading on the two panels. An effect of using a swept-back weir is that the weir occupies some additional bubbling area on the A panel. This reduction of area is generally minimal, but it must be accounted for in the tray balancing process. Picketed weirs. Picketed weirs (Figure 4) are used to block off a portion of the weir evenly over the entire weir length. They effectively shorten the flowing length of the weir. Swept-Back Weir Liquid Flow Liquid Flow Downcomer Area Area Behind Weir Standard Weir Layout ■ Figure 2. Four-pass tray liquid flow paths. 24 www.cepmagazine.org June 2005 CEP Swept-Back Weir Layout ■ Figure 3. Standard and swept-back weir layouts. 3 in. (75 mm) 3 in. (75 mm) Weir Height Weir Length Tray Deck ■ Figure 4. Picket fence weir with an effective weir length of 50%. When designing picketed weirs, it is important to place the pickets uniformly over the length so as to not block off large sections of the weir and create localized maldistribution. The height of the pickets is also important — they should be tall enough that the froth height on the tray deck will not exceed the height of the pickets at normal operating conditions. Pickets are typically designed for 50% of the tray spacing. Vapor flows Vapor flows upward through the trays following the path of least resistance. Assuming that no vapor tunnels are used, some vapor flows are trapped on one side of a downcomer and have no choice but to flow to the panel directly above. Other vapor flows are split and then can flow to either of two panels. The distribution of this split is determined by the hydraulic resistance from each of the panels. For instance, vapor leaving panel A is trapped on the outboard side of the intermediate downcomer, with no option but to flow upward through panel C. However, once it proceeds through panel C, it can rise above the OCDC and flow to either panel A or panel B, depending on the pressure balance. Vapor leaving panel B is trapped between the inboard sides of the two OCDCs. This vapor can travel to either of the D panels above, but since the tray is symmetrical, the flow between these two panels should be essentially even, unless there was some initial maldistribution. This specific transition allows the equalization of vapor between the two halves of the tower. Vapors leaving panels C and D are isolated to one half of the column by the center downcomer. These vapors are free to travel to either panel A or panel B, but cannot travel to the other half of the tower. The distribution of the vapor above the OCDC is critical to balancing irregularities in the tray design or operation. Layout and balancing of four-pass trays There are generally two schools of thought when it comes to balancing four-pass trays: equal bubbling area and equal flow-path length. Equal bubbling area means that the active area panels A–D will be essentially the same size with the same amount of orifice perforation (open) area. This design method assumes that the vapor will be split into quarters and will flow equally to each of the panels. In order to balance the tray, the liquid will then need to be split into quarters as well and proportioned equally to each of the panels. In an equal-flow-path-length design, the downcomers are arranged to create an equal flow-path length along each of the panels A–D. This design, by definition, cannot have equal bubbling area on the panels since the tower cross-section is round. Therefore, the side panels (A and C) will have considerably less bubbling area than the more central panels (B and D). The proper design technique here assumes that these panels will have a perforated area that is proportional to their active area and that the vapor will flow to these panels proportional to their active area. The liquid will need to be proportioned to the tray panels relative to their active area as well. Both of these design methods are quite popular and have their own advantages and disadvantages. With any tray design, the ultimate goal is to have the vapor and liquid contact evenly across the entire active area of the tray. The critical division of fluids on a four-pass tray is at the OCDC. This is because the liquid can move in either direction from the OCDC depending on the pressure balance, and the vapor will adjust above the OCDC as well. In order to balance the fluid flows, adjustments must be made to make sure the split is proportional to the bubbling areas that it is feeding. Step 1: Adjusting downcomer sizes and locations for capacity. Multi-pass trays should be designed so that the downcomer sizes match the process requirements. The desired result is that the downcomers have similar, or even identical, velocities. The location of the outside wall of the side downcomer is fixed, as is the centerline of the center downcomer. The other dimensions and locations are available for adjustment during tray layout. The side and center downcomer widths must be adjusted such that they will have the same velocities. With equal-bubbling-area designs, this is rather straightforward, since it is assumed that the liquid will be split into quarters. With equalflow-path-length designs, the proportioning of liquid between the side and center downcomers is an iterative procedure, since changes to any of the downcomer dimensions will affect the corresponding active areas and thus change the proportioning of the liquid flowing to the side and center downcomers. The size of the OCDC will be set to match the velocities in the side and center downcomers. However, the centerline of CEP www.cepmagazine.org June 2005 25 Distillation the OCDC will need to be adjusted to make the panels either equal bubbling area or equal flow-path length. The cross-sectional area of the OCDC varies as a function of the downcomer width and the location of the centerline. Therefore, to set the downcomer velocity in the OCDC, the designer must know its required location and then calculate its necessary width. This nested iterative process can be quite challenging. When the downcomers are sloped to improve the performance of the tray, the design process becomes even more challenging. To be successful at these layouts, the designer needs to be very patient or have a layout optimization program. Step 2a: Balancing flows for an equal-bubbling-area design. A balanced design for an equal-bubbling-area tray requires the following features: Deck orifices. Deck orifices must be allocated equally to all passes, with 25% of the orifices in each pass. Actual bubbling areas need to be approximately the same, but the number of orifices or slot areas should be exact. This can be difficult with larger valves or orifices on the tray deck, since the outer panels (A and C) have more curvature and can make it difficult to fit enough valves on these panels. Outlet weirs. The weir on the top of the center downcomer, W2, must be picketed in order to have the same effective length as the weir on the top of the side downcomer, W1. This assures that the crest over the weir is equal on both passes regardless of the total liquid rate. Picketing is the only acceptable way to achieve this. As discussed earlier, varying the weir height is unacceptable because this type of design produces even distribution for only one specified liquid rate. The weir on the inboard side of the OCDC, W4, should also be picketed to have the same effective length as the outboard side weir of the OCDC, W3. This maintains the same crest over the weir for both streams entering the intermediate downcomer and thus helps to keep the liquid level equal on both the C and D passes. Picketing of the OCDC weirs is mandatory if the vapor is equalized across the OCDC (e.g., by vapor tunnels or truncated downcomers) but is optional with convenitonal downcomers since the vapor has no alternative path. Downcomer clearances. The clearance on the inboard side of the OCDC, C2, should be blocked (or picketed) so its effective release length is equal to the clearance on the outboard side of the intermediate downcomer, C1. This ensures an equal split of the liquid to the passes on either side of the downcomer. Step 2b: Balancing flows for an equal-flow-path-length design. Balancing modifications for an equal-flow-pathlength design are basically similar to those used for equalbubbling-area designs, although there are some differences. Deck orifices. The deck orifices must be allocated to the individual passes so that their number is proportional to the bubbling area of that pass. 26 www.cepmagazine.org June 2005 CEP Weirs. The center downcomer weir, W2, should be picketed so that its length divided by the length of the side downcomer weir, W1, is proportional to the panel B bubbling area divided by the panel A bubbling area. One way to confirm that the design is balanced is to check that the weir loading is the same for the side and center weirs. Since picketing of the center downcomer tends to minimize the tray capacity, the amount of Vapor Tunnels Vapor tunnels are tubes or channels that pass through either center or off-center downcomers and allow vapor to travel from bubbling areas on either side of the downcomers in order to equalize pressures and flows on the tray (Figure 5). Tunnels through the center downcomer allow equalization between the opposite halves of the trays. Tunnels through the OCDC allow equalization between the outboard passes (A and C) and the inboard passes (B and D). If a column is fed and drawn properly, tunnels through the center downcomer are not needed, since the tray is symmetric. Small irregularities can be equalized above the center downcomer between the B passes on either side. Nor should vapor tunnels on the OCDC be needed if the trays are balanced properly and are fed and drawn properly. With equal-flow-path-length designs with sloped downcomers, the ratio of vapor going between passes A and C will not be the same as the ratio of vapor going between passes B and D. Therefore, if vapor tunnels were to be used, there would be flow through them. However, in order to balance the V/L ratio on these trays as closely as possible, we prefer to keep the C and D vapors isolated so that all of the vapor from pass C goes to pass A and all the vapor from pass D goes to pass B. We prefer to not use vapor tunnels on the OCDC. Assuming the feed and draw streams are properly designed, our preference is to not use vapor tunnels at all. A properly balanced tray should overcome small imbalances without vapor tunnels. Also, vapor tunnels may actually hurt performance when used in an OCDC with an equal-flow-path-length design. Finally, vapor tunnels add complexity and cost to the tray design. ■ Figure 5. Off-center downcomer (OCDC) vapor tunnel. picketing should be kept as low as possible. This can be done by making the side downcomer weir effectively longer by widening the side downcomer, by using a swept-back weir on the side downcomer, or by a combination of the two. For the equal-flow-path-length design, the liquid flowing from the side downcomer to the OCDC is significantly less than the liquid flowing from the center downcomer to the intermediate downcomer. Therefore, the weir on the outboard side of the OCDC, W3, should also be picketed to have a length equal to the length of the inboard weir, W4, multiplied by the ratio of the panel C area to the panel D area (i.e., W3 = W4 × [Area C/Area D]). This maintains a weir length proportional to the liquid flow to each pass and ensures the same crest over the weir for both streams entering the intermediate downcomer, and thus helps to keep the liquid level equal on both passes. Varying weir heights between balancing passes is not permitted. Downcomer clearances. The clearance on the outboard side of the OCDC, C1, should be blocked so its effective release length is equal to the clearance on the inboard side of the intermediate downcomer, C2, multiplied by the area ratio (C1 = C2 × [Area A/Area B]). This ensures a proportional split of the liquid to the passes on either side of the downcomer. (Note that the downcomer clearance is not varied for the inboard and outboard sides of the OCDC.) Final thoughts If a tray is properly sized, then a balanced design should perform as expected. However, if a tray is not properly balanced, the effects can cause a tray to fail. Proper flow balancing cannot make a poorly sized tray work, but improper flow balancing can certainly make a moderate or aggressively C EP sized tray fail miserably. MARK PILLING is the manager of technology for Sulzer Chemtech USA, Inc. (Tulsa, OK; Phone: (918) 447-7652; E-mail: mark.pilling@sulzer.com), where he specializes in high-performance trays and various process technologies. His responsibilities include new product development and testing, development of equipment design correlations and programs, as well as troubleshooting and conducting test runs on new equipment. He has written various papers on mass transfer issues and product developments and has two patents. He is a registered professional engineer and holds a BS in chemical engineering from Univ. Oklahoma. He is a member of the Technical Committee for Fractionation Research, Inc. www.cepmagazine.org or Circle No.121 CEP www.cepmagazine.org June 2005 27 ...
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This note was uploaded on 12/29/2011 for the course CHE 128 taught by Professor Scott,s during the Fall '08 term at UCSB.

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