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. Poor tray blanking (such as installing wide blanking strips parallel to the liquid flow) can create liquid paths unagitated by vapor contact. lowing: mats left from the previous entry into the column, fallen panels from upper trays, dislocated baffles, or forgotten blanking strips installed to prevent weeping during a previous low-rate operation. operating costs (more reflux means higher reboiler duty). The bottom temperature of the tower starts to decrease. distillation tray. <<4e6228ae77f767429ef5f679323dfefe>]>> 0000002427 00000 n Feed to the column could be hiked if the total reflux was left unchanged. Sieve trays are the least expensive because they are simple to make. | Its Parts, Types, Applications, Symbols, and Differences with Butt Welding (PDF). The feed was cool and far from its bubble point, so flashing in the sparger could be ruled out. 0000002056 00000 n FIQURE 2. 1 The ease of the noisy feedpipe i Problem: A water-hammer-type pounding at the col-| umn feedpoint was violently shaking column and the connecting piping. Manage Settings The design of a distillation column can be divided into the following steps: Specify the degree of separation required: set product specifications. As an engineer, you must have heard the term fillet welding. Improper installations Vapor or flashing feed (the result of volumetric expansion, flowpath deflection or velocity gradient) may interfere with the normal liquid and vapor traffic in a column. Check the condenser and reboiler systems for any potential fouling. 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. The reduction in distillation tray efficiency caused by uniform weeping of liquid from the tray has been calculated. Excessive weeping will lead to dumping. 4. Monitor the pressure in the column and keep it under the maximum operating range. distillation is the process of repeated evaporation and 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. Among the most common cause of inefficient distribution are outlet weirs that are not level. In such a case, jet flooding can begin on a few trays and develop into column flooding without a detectable loss of separation. 2023 Copyright COSTELLO. On Problem: The carbon-steel valve trays of a naphtha splitter were being severely scaled, often limiting capacity. 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. Plot the reflux rate and pressure drop against the feedrate. ), typical I trays will operate at a pressure drop of i about 4 to 11 mm Hg per stage, com- i pared with 1 to 4 mm Hg per stage for a I 30-in. All rights reserved. The most common tray-column vapor-and-liquid contactors are sieve, valve and bubble-cap trays. A distillation column or chamber is required to separate the volatile liquids and gases from the fluid. Troubleshooting: The column pressure drop indicated flooding. Weeping determines the minimum vapour flow required while flooding determines the maximum vapour flow allowed, hence column capacity. Preferably, it should be kept within inch. This turndown ratio can be increased by reducing the fractional hole area. Dual-flow trays do not have down-comers. In petroleum refining, the word "fractionation", not "fractional To minimize scaling and facilitate cleaning, the decision was made to replace the valve trays with stainless-steel sieve trays. Jet flooding occurs when the downcomers and trays consist of froth or foam, there is a quantity of entrained liquid that is lifted above the froth level on the trays of the tower. Such variations in diameter or flow require a specially designed transition tray. This adversely affects the mass transfer which reduces the tray efficiency. Online Pipeline Stress Analysis Course using Caesar II for Beginners. Valve trays offer about the same capacity and efficiency as sieve trays, but i tend to weep less. liquid may be able to drop to the next stage through the holes, Your question is not Similarly, liquid and vapor returning to th reboiler and impinging on the exit of the bottom seal pan will restrict liquid flow from the downcomer. Use a overflash alarm to detect any potential flooding. Distillation columns weeping This means both vapor and liquid loadB are raised and lowered simultaneously. Good mixing is particularly important in columns having few trays. Consequently, sieve trays are most of ten used when turndown is not critical, i Valve trays represent a compromise I between bubble-cap ana sieve trays, i providing greater operating flexibility i at a cost only slightly higher (Figure 6). to effect separation. As vapor velocity is not enough, vapors will not pass through tray holes, moreover vapors will pass through gap between tray and column. One can call this as optimum reflux ratio which would be an alternative description of the incipient flood point. The inlet weir on the feed tray was also left out. A liquid feed that contains low-boiling components can flash in the downcomer after mixing with the liquid flowing off the tray. The modeling of a steadystate distillation column is based mainly on mass and energy balances; in this way, it is needed to understand the equipment layout to obtain such mathematical equations [8, 12, 13]. Only reducing the boUup rate to a fraction of design rate kept the tower stable. With their larger holes, sieve trays resist fouling better than do bubble-cap and valve trays. Description Hydrocarbon feed is preheated with hot circulating solvent and fed at a midpoint into the extractive distillation column (EDC). Monitor the temperature of the column and make sure it is maintained within the specified range. The algorithm for developing a dynamic distillation column model is as follows: Step 1) Write component material balance for each stage in the column. being designed so that upflow-ing vapor and downflowing liquid pass through the same orifices. %%EOF Sieve tray performance at various relative liquid and vapor flowrates, FIQURE 3. Equipment and Operation, Column Valve trays (such aa the four-pass one shown upper left) represent a compromise between the bubble-cap and sieve trays. ' Distillation does involve evaporation. Because the tower did not show signs of corrosion, it was concluded that the material was being carried into the plant by the natural gas stream. To guarantee a proper downcomer seal, the bottom edge of a downcomer should be about 0.5 inches below the top edge of the outlet weir. The reduction in distillation tray efficiency caused by uniform weeping of liquid from the tray has been calculated. This . This video demonstrates how you can use Aspen HYSYS to see what's going on inside distillation columns to quickly troubleshoot column issues, optimize operations, and minimize capital used in revamps. stacking up liquid on the upper tray. and a liquid-outlet zone. Tray efficiency depends on tray geometry, liquid and vapor properties and operating conditions. Check that the cooling and heating systems are operating correctly. Increasing vapor rate reduces efficiency, while increasing liquid rates raises efficiency. ____. Jet flooding results from more energy being transferred from the entering vapor into the liquid on a tray than can be dissipated in the space above the tray. As one of the most expensive and energy-consuming units in a plant, the fluid dynamics of the . Thus, tray improper installation restricted downcomer clearance, which limited the liquid throughput and caught the larger pieces of scale and loosened tray gasketing. Weeping is a distillation column phenomena that is not easy detectable by scanning and needs skill and experience. The next reason for flooding is inadequate clearance of the downcomer and tray deck which is shown in the figure below (Fig-2). Another sign of. Hilton Americas - Houston, George R. Brown Convention Center, Houston, TX, Crowne Plaza Princeton Conference Center, Plainsboro, NJ, 2023 ACS Spring Hybrid National Meeting, Indianapolis, IN, USA, Texas A&M University, College Station, TX, Institute for Learning & Innovation (ILI), Disability & Outreach and Inclusion Community (DORIC), CO2 Leaks From Geological Storage: Geomechanics, Fluid Flow and Phase Transitions, Convection Battery - Elimination of Dendrite Failure. spacing (a HETP" of 30 in. Contact Us Design your distillation scheme with Aspen Distillation Synthesis. Problem: After several months of operation, an amine j stripper in a natural gas treating plant designed to remove i C02 from a rich amine solution became hydraulically unstable, surging reflux into the overhead accumulator. Because its violent method of generating mass-transfer area (compared with ; the packed column) occurs at the expense of pressure drop energy, a trayed column operates at a higher pressure drop per stage than does a packed columns. A check of the design drawings indicated that the feedpipe and feed sparger might be somewhat oversized, especially at the 30% feed rate. 375 20 Increase the reflux ratio: Increasing the reflux ratio can allow the top section of the column to be cooler, helping to reduce weeping. Analytical equations are given for Lewis' cases 2 and 3 for plug flow of liquid on the tray. Liquid exit velocity from the downcomer onto the below tray. How weeping avoid in distillation column? of Columns, Basic 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. Liquid and vapor flows are not altogether countercurrent in a trayed distillation column. Although operating rate is also a factor, the former condition is characteristic of vacuum columns and the latter of high-pressure and high liquid flowrate columns. 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. Therefore, liquid starts to leak through perforations. Continue doing this until the signs of flooding are detected, then reduce the feedrate to avoid flooding. 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. By Jim Riggs 1. 375 0 obj<> endobj 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. reaction-followed-by-distillation processes [2,12,13,17,54]. K = 0.18 to 0.25: Tray operation is close to its best efficiency point. High liquid gradients across bubble-cap trays will distribute vapor flow disproportionately toward the outlet side. So 50 percent is often used for many hydrocarbon services. Modify the columns pressure or temperature settings: Adjusting the feed pressure or the steam temperature can help reduce or eliminate weeping. Reduce the amount of liquid in the sump and on the trays to avoid flooding the column. The main drawback to sieve trays is the reduced turndown, because the holes can weep liquid at low vapor flowrates. In the Hi There! The weeping phenomenon increases with. distillation" is used, often interchangeably with The pinch between operating and equilibrium As per the figure depicted in Fig.1A, it has been clearly shown that downcomer B is flooding. For information about the authors, see p. 123 of the first article o: this series. This will ensure that the flooding point of the column is not reached. 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. 0000002463 00000 n The weight flow of vapor and the velocity of the vapor through the tray increase. The height of the outlet weir is less than the bottom edge of the downcomer from the upper tray. The distillation column is a key component of ammonia-water absorption units including advanced generator-absorber heat exchange (GAX) cycle heat pumps. Implement regular maintenance on the column and its components, including the trays, valves, and piping. The pressure drop of the vapor flowing through the tray above the downcomer. This is known as "weeping" and . Ensure that the process stream is free from contaminants that can cause fouling or corrosion. The weep point is defined as the velocity of vapor becoming significantly low which reduces the tray efficiency. separated simple distillation, then fractional distillation is Maximum bed height is dependent on thetype and size of the packing. However, accumulated liquid upon collapsed tray panels can give an opposite indication. Distillation Column Modeling for Cost-Effective Designs and More Reliable Operations. Inc.*. ' 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. A vapor or a flashing feed should never be introduced into, or near to, the entrance of a downcomer. Several factors contribute to where the operating liquid level is in a downcomer. Additionally, we make customized vessels (10 mL to 100 L in capacity), distillation apparatus, multiple jacketed condensers, manifolds, silver jacketed columns, and special adapters that are best suited for your requirements. Inspect the column for any visible signs of damage, such as leaks, corrosion, or other physical abnormalities. During designing, the trays were tested to avoid both flooding and weeping. One useful test is to increase the column feedrate while monitoring the column pressure drop and the reflux rate required to maintain the separation. Corrective action: One solution might have been to i plug some of the orifices to raise the discharge velocity to j several ft/s. This can often be seen as low pressure drop through a section of a column, because the vapor flow is meeting little resistance. We have derived the following formula: On the basis of hundreds of field measurements, we have observed. This allows the vapor to flow upwards to downcomer B. The relatively small pressure-drop rises at the time of surging reinforced this conclusion. The liquid starts overflowing the outlet weir and drains through the downcomer to the tray below. Make sure the condenser is operating efficiently and the condensate is being removed quickly. Column Design, Effects In distillation column, liquid flows in downward direction by gravitational force but it flows against the pressure. This indicated flooding in the rectification j section. Data from similar columns (including pilot-plant units) afford the best basis for estimating tray efficiencies. Gaining detailed knowledge of column internals is a high priority for engineers, especially regarding the behavior of equipment and processes. This flow can hinder the flow of liquid into a downcomer or cause excessive aeration. In Packed column you have channelling (liquid flows alongthe wall of the column after traveling certain height of packed bed)and in tray column you have weeping (vapour flow rate becomes insufficient to hold the liquid on tray). . At each turnaround, the trays had to be cleaned, a difficult and time-consuming task. Operation at very ! (: Distillation columns) . Two-pass trays alternate between a center downcomer and two side-down-, Bubble-cap trays offer a high turndown ratio but the capa take up a lot of space, PKHJRK 5. The height of the crest is similar to the height of water overflowing a dam or a river. Ensure that the column trays and packing are installed correctly. Install an internal or external condenser: Internal or external condensers can help reduce weeping by cooling the overhead vapors in the column. This signifies that if we calculate a level of clear liquid of 12 inches in the downcomer, then the actual level of foam in the downcomer is 12 inches/(0.50) = 24 inches of foam. Distillation column analysis is one of the key areas of focus for chemical engineers. K = 0.5: Tray is fully engulfed with floodopening a vent on the overhead vapor line will blow out liquid with the vapor. The liquid distributors may have a more tightly constrained operating range than the theoretical operating range for the packing. "distillation". Excess loading, fouled trays, restricted transfer area and poor vapor distribution, improper feedpipe and tray installation. Similarly, downcomers downstream of partial sidedraws may not be sized to handle the liquid flow if the sidedraw is stopped or restricted. The understanding of these effects is very important for both retrofit design and grass-roots design. The model does not include effects due to column internals (e.g., pressure drops and flooding/weeping). K = 0.35 to 0.40: Entrainment occursan increase in reflux ratio significantly reduces tray efficiency. Liquid bypass Short-circuiting liquid flow across a tray bypassing vapor contact diminishes tray efficiency. Maintain a clean environment and check for any signs of fouling. This will increase the tray pressure drop. 30% of design rate. Thus, improved separation at higher throughput may indicate a weeping problem. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. Liquid flows under the downcomer and into the inlet zone, where a few inches of the tray area is intentionally void of contactors, so that vapor will not blow into the downcomer. Reactive distillation is potentially attractive whenever a liquid phase reaction must be carried out . Check the control system to ensure all components are functioning correctly. Weeping is undesirable and the liquid should flow across the tray and down the downcomer. The height of the foam resides on the tray, The velocity of the vapor through the tray, DP = pressure drop across a tray section, psi, s.g. = specific gravity of the clear liquid, at flowing temperatures. Whether flooding starts in the transfer area or in the downcomer will depend on which area is limiting. Bubble-cap trays often can operate at vapor flowrates as low as one tenth of their maximum capacity without significant hydraulic or efficiency problems (Figure 4). 19S9. 0000004860 00000 n The intermediate equipment load that results in the most efficient operation is called the best efficiency point. In the packed column, however, the liquid contribution is not significant, except at high liquid or vapor flowrates i.e.. with increased liquid holdup on the packing. As in a packed column, gravity is the driving force for liquid downflow.