A quantitative comparison of the three frequent trays used in respect of capacity, efficiency, flexibility, cost & other criteria is given in the table below:ParameterBubble cap traySieve trayValve trayCapacityModerateHighHigh to very highEfficiencyModerateHighHighEntrainmentHighModerateModeratePressure dropHighModerateModerateTurn downExcellentAbout 24-5Fouling tendencyHigh, tends to collect solidLowLow to moderateCostHighLowAbout 20% more than sieve traysApplicationRarely used in new columnsMost applications if turndown is not importantPreferred for high turndown is anticipatedShare of marketAbout 5%25%70%Table 1: Quantitative comparison of the three frequently used trays in the Distillation Column. The clearance problem was mainly attributed to scale left on the tray support rings during the installation of new panels that raised the height of the tray panels. 0000000016 00000 n Monitor temperature and pressure readings to ensure that the operating conditions remain within the specified range. rectification sections could not be gauged separately, it was i not possible to locate the floodpoint. It usually adjusts the weir height to between 2 and 3. Often, the downcomer seal pan is recessed to improve the disengagement of vapor from the liquid. very minor difference in their boiling points and cannot be As a result of that Tray-2 gets flooded. The next reason for flooding is inadequate clearance of the downcomer and tray deck which is shown in the figure below (Fig-2). Restricted transfer area Poor vapor distribution across a tray can result in local areas of high entrainment, and initiate jet flooding. Common causes of downcomer flooding include: excessive liquid flow, restrictions. Columns distilling hydrocarbons often operate at Murphree efficiencies near to, or above, 100%. One useful test is to increase the column feedrate while monitoring the column pressure drop and the reflux rate required to maintain the separation. If the operator increases the reflux flow to the distillation column then the following condition will occur (refer to Fig-2): As per figure (Fig-3A) Point A is called the incipient flood point, that point in the towers operation at which either an increase or a decrease in the reflux rate results in a loss of separation efficiency. Weeping is when liquid flows downward through the holes in a distillation tray.Normally vapor rises up through the holes and contacts the liquid on the tray.If the. 2 Horizontal Thermosyphon Reboiler This is perhaps the most common type of reboiler. The variables that primarily affect jet flooding are the tray vapor area, tray spacing, and vapor flowrate and density. The formula for the calculation of crest height is. A noticeable loss of separation at a high throughput rate is often a precursor to jet flooding. However, accumulated liquid upon collapsed tray panels can give an opposite indication. heat exchanger. You would use simple distillation when the two products you are comers. Check the condenser and reboiler systems for any potential fouling. the liquid in the downcomer, causing j excessive downcomer backup. Weeping is undesirable and the liquid should flow across the tray and down the downcomer. Poor tray blanking (such as installing wide blanking strips parallel to the liquid flow) can create liquid paths unagitated by vapor contact. Flooding Troubleshooting: The location of the noise suggested a problem with the feedpipe. Engaged in the engineering of Hydrogenation, Hydrotreating (Naphtha, Diesel & Gasoil), Naphtha reforming, FCC units, & Polymer process in Petrochemical units. .Marc.". Increase the columns tray spacing: Increasing the tray spacing on the column can reduce weeping by increasing the distance through which the vapor and liquid must pass, thus reducing the chance of weeps. Uneven vapor distribution can also occur when one area of a tray has a disproportionate amount of missing or degraded contactors, dislodged trays, or missing manwavs (Figure 9). If the total height of the downcomer along with the height of the weir is 24 inches, then the height of the foam in the downcomer is 24 inches resulting in downcomer flooding. Excessive weeping will lead to dumping. Learn Industrial Distillation from Industry ExpertComplete Understanding of Design, Operation, Troubleshooting and Optimization of industrial distillation column.Rating: 4.5 out of 515 reviews10 total hours55 lecturesAll LevelsCurrent price: $14.99Original price: $24.99. Valve trays (such aa the four-pass one shown upper left) represent a compromise between the bubble-cap and sieve trays. ' Corrective action: The travs were removed, the support ring cleaned, the trays reinstalled, and the gaskets omitted. ratio, when an infinite number of trays will be required The dominant one is the liquid backup that balances the pressure drop across the tray. startxref A plant is designed for a particular capacity range that may have to operate at an enhanced or reduced throughput depending upon the changes in the production rate or demands or various factors. Such maldistribution of the gas or the vapor called vapor channeling severely reduces tray efficiency. Further consequence of the excessive pressure drop created from the high weir loading could be downcomer back-up or choke flooding. The two effects normally cancel each other, and efficiency is practically independent of load changes (assuming no excessive entrainment or weeping). Citing Literature It remains pretty small for the sieve tray. If the air ingression into the system is undesirable, an inert purge is done at the vent. Proper liquid distribution is key to packed column performance. generally distributed onto a distillation tray via a weir. The troubleshooter can sometimes distinguish between the two types of flooding on the basis of a column's response to changes in liquid and vapor loading. Make sure the condenser is operating efficiently and the condensate is being removed quickly. K = 0.10 to 0.12: Low tray efficiency, due to tray deck leaking. If liquid and vapor flowrates are expected to differ (e.g., be higher above feed tray than below), the design engineer will often arrange different column diameter sections for differences in the flow pattern, such as changing from single-pass to two-pass trays at the feedpoint. Since about the 1950s, sieve and valve trays have virtually displaced bubble-cap trays, which are more expensive and offer lower capacity. High liquid gradients across bubble-cap trays will distribute vapor flow disproportionately toward the outlet side. Design of distillation columns Involves determination of number of stages required for a given separation Graphically by McCabe- Thiele method (uses operating lines and equilibrium curve) 0000000711 00000 n Flooding Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the vapour up the column. The relation between the pressure drop of a distillation tower and flooding is important to understand for the prediction and prevention of flooding. Then a further increment of vapor flow causes a significant increase in the foam height of the tray of the distillation tower, which increases the height of the spray section. written clearly. It is the least prone to vapor and liquid maldistribution (Figure 7). Distillation is one of the most common processes in industry, & is very energy intensive (heating and cooling), it can be up to 50% of costs. Similarly, downcomers downstream of partial sidedraws may not be sized to handle the liquid flow if the sidedraw is stopped or restricted. a tray efficiency of 70% and a 21-in. The most common tray-column vapor-and-liquid contactors are sieve, valve and bubble-cap trays. The driving force that causes this entrainment is the vapor flow through the distillation tower. No liquid flow under valves Minimized weeping due to lateral vapor release Improved turndown ratio compared to round valves Improved efficiency/capacity profile compared to conventional round valve trays Wide legs and lack of valve rotation reduce damage and wear Design allows for top side valve replacement Part 1 - Tray Pressure drop and Weeping in Distillation Column Sandeep Academy [IITR, GATE AIR 1] 39.7K subscribers 14K views 4 years ago Distillation Operation Part 2 - Flooding in. Most common type considers the vapor rate in relation to the weeping and entrainment points weeping entrainment Vapor Rate HETP This is an acronym, which stands for "Height Equivalent of a Theoretical Plate". Weeping in a Distillation Column Source: AIChE Duration: 2 minutes Skill Level: Basic Type: Academy Video Language: English 33/32 in the series Distillation in Practice When the vapor rate is low, liquid is allowed to flow through the tray holes before mixing with the vapor. Affecting Operation, Other This may cause a mechanical design problem with the column. Among the less influential variables are tray geometry, type of contacting medium, liquid flowrate and other liquid and vapor properties. Weeping usually subsides with higher flowrates. the feedpipe was turned so that the discharge orifices were on top of the pipe; this ensured that the sparger remained full of liquid at low feedrates; additionally, a deflector bar was installed above the orifices to keep feed from impinging on the tray above. The bottom temperature of the tower starts to decrease. j. 0000002056 00000 n As an engineer, you must have heard the term fillet welding. Low Flow in Pipes- posted in Ankur's blog. Liquid Equilibria, Distillation One postula-tion was that feed liquid was running out of the upstream ; orifices, allowing vapor to enter the feed sparger through ' open downstream orifices, and that the condensation of this vapor in the feed sparger was causing a hydraulic hammering. Download Free PDF Related Papers Choosing Trays and Packings for Distillation Rakesh Seelochan Download Free PDF View PDF This is called jet flooding because the flooding is initiated by the jetting of liquid from the tray transfer area (Figure 3). You must be logged in to view this page. Once tray-2 floods, downcomer C (shown in Fig. The tray internals is selected & designed keeping in view the complexity of the flow and problems that arise out of it. Sieve trays have a low turndown ratio of about 2. 0000001911 00000 n On the other hand, dumping is an extreme case of leakage through the tray deck if the vapor velocity is low and the vapor pressure drop across the tray is not sufficient to hold the liquid. 4. 377 0 obj<>stream If the bottom edge of the downcomer is too adjacent to the below tray then a higher pressure drop is needed for the liquid to escape from downcomer B onto tray-1 & which causes the liquid level in downcomer B to back up onto tray-2. i dont think there will be anycorrelation for calculating channelling in the packed bed but in order to avoid it you have to restrict the packing bedheight. 0000002463 00000 n Analytical equations are given for Lewis' cases 2 and 3 for plug flow of liquid on the tray. B. By varying the airflow in and vent gas out of the column through the column vent. Learn essential distillation concepts likely to be faced by process support, operations and process design engineering personnel. But the hydraulic gradients are really the difference of liquid heights. Deflected flow, missing contactors (Figure 10) and a liquid gradient across bubble-cap trays can cause vapor maldistribution. In a packed column, the apportioning of column cross-section for liquid and vapor flow can vary to suit the relative liquid and vapor loading. This foam should separate back into a vapor and a liquid in the downcomer & if it is not drained fast from a downcomer onto the below tray, then the foamy liquid or froth will back up onto the tray above. Required fields are marked *. If the vapor flows through a tray increases, the froth height in the downcomer draining the tray will increase as well. Troubleshooting: The column pressure drop indicated flooding. Manage Settings Disclosure: Some of the links above are affiliate links. So, the hydraulic gradient is a very important operational feature that needs to be checked during tray design. Raising the reflux only a little made the column unstable. The section where the vapour leaves the column to the However. The following book you can use as a reference and for further studies: A Chemical Engineering professional with extensive experience in the field of Process Engineering Design in Oil and Gas sector. The head loss caused bv the liquid flowing through the restricted space between the bottom of the downcomer panel and the seal pan also contributes to where the level is in a downcomer. reverse-flow trays having a single chord section divided into down-comer and seal-pan area are frequently installed to reduce the total downcomer area without resorting to extremely narrow ones (Figure 7. bottom). distillation tray. The design of the distillation column is critical to unit performance, size, and cost. Loss of liquid level in the bottom of the column is an early indication of flooding in a distillation column. The trouble-shooter decided to inspect the tower. 0000001598 00000 n and a liquid-outlet zone. designed to operate between 1.2 to 1.5 times the minimum reflux Thus, the total liquid head from downcomer backup is the sum of the tray pressure drop, the liquid and the head loss from the liquid flow under the downcomer. Differenciation of components of petroleum is done by this Adjust the reflux ratio and/or other operating conditions to optimize performance. Liquid flowrates higher than 12 gal/ min per inch of outlet weir increase the liquid crest over the weir, adding to the liquid level on the tray. Some common causes of efficiency problems include: collapsed trays, liquid entrainment, liquid bypass (caused by damaged or poorly assembled trays, weeping and dumping), and uneven liquid and vapor distribution. Although not as ! The inspection revealed that the clearance under the downcomers in the rectifying section ranged from % inch to % inch, vs. the 1 inch specified. High weir loading result into large crests and high froths thereby result into excessive pressure drop. This carryover of the suspended liquid droplet into the upper tray is termed entrainment. ing or hiking the tray pressure drop and backing up liquid into the next higher downcomer. Different packing styles and materials of construction require different minimum wetting rates. Tray pressure drops range from 2Vi to 8 mm Hg, depending considerably on I the liquid and vapor loadings, tray type, ; outlet weir height, and design pres- j sure-drop limitations. Continue doing this until the signs of flooding are detected, then reduce the feedrate to avoid flooding. Vapor bubbles through the holes of the sieve trays, or caps of the valve trays, on the tray deck, where the vapor comes into intimate contact with the liquid. In a packed column the liquid is intended to flow downwards across the entire cross section of the column, so weeping is not a relevant concept in a packed column. Limiting the hole area to 13-14% prevents weeping in distillation columns. Check the control system to ensure all components are functioning correctly. This dead zone is more needed for valve and bubble-cap trays than for sieve trays because vapor issues from the valves and caps with some horizontal velocity. Maintain the appropriate reflux ratio. The remainder of the tray is active in contacting vapor and liquid. Entrainment Back-mixing via liquid entrainment reduces efficiency. This will ensure that the flooding point of the column is not reached. The weep point is defined as the velocity of vapor becoming significantly low which reduces the tray efficiency. To calculate the height of liquid in the downcomer, due to liquid flowing through the downcomer clearance: where H = inches of clear liquid backup in the downcomer, due to the head loss under the downcomer V = horizontal component of liquid velocity, in ft/s, as the liquid exits from the downcomer.