weeping in distillation column

Several problems are found during the testing of the column: bottlenecking of liquid flow at trays, weeping and poor separation of the distillate. For trays of the distillation tower, the incipient flood point corresponds to the best efficiency point. Deflected flow, missing contactors (Figure 10) and a liquid gradient across bubble-cap trays can cause vapor maldistribution. The chances of entrainment are more if the droplet is small, if the gas velocity is large, or if the tray spacing is small. Install a liquid-level controller or probe to monitor the liquid level of the column. less than optimal separation). 1B) will also back up and flood. This flow can hinder the flow of liquid into a downcomer or cause excessive aeration. Determine the stage and reflux requirements: the . The value of the hydraulic gradient on a tray should not be more than a fraction of an inch. Your email address will not be published. Vapor flow is bounded on the low side by that required for efficient liquid-vapor contacting and weeping prevention. This allows the vapor to flow upwards to downcomer B. lines Ensure that the process stream is free from contaminants that can cause fouling or corrosion. This indicates that the percentage of propane in the bottoms product (butane) is held constant. Unexpected foaming will excessively aerate ! This helps to reduce the disengagement load of the downcomer and often lessens the tendency of the outlet row of contactors to blow liquid over the outlet weir into the downcomer. separation zones are used instead of plates or trays. This mixture Already a Chemical Engineering member? The feed was cool and far from its bubble point, so flashing in the sparger could be ruled out. , Reorinted from CHEMICAL ENGINEERING Mau 10S0 mnuhi ioao hu i ,.., *m, Continue reading here: Vapor Blanketing Reboiler Corrective Actions. 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. This will increase the tray pressure drop. The AIChE online library includes articles, journals, books, blog posts, and more on a variety of topics. Suggest three (3) possible steps that can improve the efficiency and solve the problems . In such a case, jet flooding can begin on a few trays and develop into column flooding without a detectable loss of separation. Save my name and email in this browser for the next time I comment. Disclosure: Some of the links above are affiliate links. The IDEAL path to equity, diversity, & inclusion. 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. "distillation". If any of the foregoing contributors level on the j becomes excessive (for instance, too tray be- > much head loss under the downcomer or aeration due to foaming), liquid will back up in the downcomer and spill onto the tray. Install a reboiler cooler: A reboiler cooler can help reduce or control weeping by cooling the liquid going into the reboiler. An excessive liquid gradient causes severe malfunctioning of the tray as most of the gas flows through the holes near the middle of the tray and at the outlet weir section (where the effective liquid depth on the tray is low) and only a small part of flows through the holes at the liquid inlet side of the tray. Several factors contribute to where the operating liquid level is in a downcomer. This turndown ratio can be increased by reducing the fractional hole area. A distillation column may have to be raised off the ground in order to accommodate the reboiler. It is occasionally used in vacuum columns to iower the tray pressure drop by reducing the height of the liquid crest over the outlet weir. It usually adjusts the weir height to between 2 and 3. Preferably, it should be kept within inch. 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. Fig. The onset of distillation column flooding is associated with a change in the flow regimes of the gas and liquids flowing inside the column. The tray internals is selected & designed keeping in view the complexity of the flow and problems that arise out of it. ratio, when an infinite number of trays will be required This description is somewhat oversimplified because the liquid actually flows across the tray, creating a concentration gradient. Limiting the hole area to 13-14% prevents weeping in distillation columns. Weeping happens in distillation when a descending liquid reaches a particular limit and then misses the contact zone, which makes the process less effective . <<4e6228ae77f767429ef5f679323dfefe>]>> the liquid in the downcomer, causing j excessive downcomer backup. Numerical results are presented for all three Lewis cases over the range of variables of interest to tray designers. is easy to verify using the McCabe-Thiele method. By comparing design or demonstrated liquid flowrates with actual flowrates, the troubleshooter can evaluate whether downcomer design is limiting column capacity. 0 Reduce the amount of liquid in the sump and on the trays to avoid flooding the column. Model your distillation column in Aspen Plus and use Aspen Rate-Based Distillation for rigorous design, providing more accurate simulations for maximum reliability. to 1 in. Special valve designs provide additional turndown. p. 116. The top temperature of the distillation tower decreases. 0000002804 00000 n Corrective action: One solution might have been to i plug some of the orifices to raise the discharge velocity to j several ft/s. Three- and four-pass trays are used less frequently because of their tendency to distribute vapor and liquid poorly. Feed to the column could be hiked if the total reflux was left unchanged. xb```b``AXf&f$A/lp~O &1zIoc`fc``dcoqmEOnN}3hpk]DEq,3"pHa5~: 0000002427 00000 n Distillation columns are used to separate mixed feed streams into their own distinct products. 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. High liquid gradients across bubble-cap trays will distribute vapor flow disproportionately toward the outlet side. 0000004860 00000 n So 50 percent is often used for many hydrocarbon services. The column was operating at about only ! Engineered distributors are designed for a specific range of liquid flows. Weeping is indicated by a sharp pressure drop in the column and reduced separation efficiency. The pinch between operating and equilibrium The most common tray liquid-flow arrangement is the single-pass cross-flow. 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. in diameter, are punched in tray panels to provide a hole area of from 5% to 15% of a tray's total surface area (Figure 5). At each turnaround, the trays had to be cleaned, a difficult and time-consuming task. .Marc.". If the downcomer clearance (the distance between the bottom edge of the downcomer and the tray below) is too large, the downcomer becomes unsealed. 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). Distillation Column Modeling for Cost-Effective Designs and More Reliable Operations. The liquid starts overflowing the outlet weir and drains through the downcomer to the tray below. A noticeable loss of separation at a high throughput rate is often a precursor to jet flooding. Weeping determines the minimum vapour flow required while flooding determines the maximum vapour flow allowed, hence column capacity. In the first article of this series, Jim discussed control relevant issues associated with distillation columns.The second article presented the major disturbances affecting composition control and the importance of properly functioning regulatory controls. Most common in vacuum and low-pressure columns (in which the vapor flowrate is much higher than the liquid flowrate), jet flooding can also occur on trays having a low percentage sieve-hole area or a small number of valves or bubble caps per tray transfer area. Columns distilling hydrocarbons often operate at Murphree efficiencies near to, or above, 100%. dideo Tennessee Eastman Co., and John J. France, Glitsch. Sequence of jet and downcomer flooding in a trayed column. This dimension should be carefully checked by process personnel when a tower is opened for inspection. Distillation and Unit Operation Control & Troubleshooting (8) zWhen flooding starts on a tray, all the trays above that point will also flood, but trays below that point will go dry . Inc.*. ' Computers and Chemical Engineering Supplement (1999) 5831-5834 (;) 1999 Elsevier Science Ltd. 0000002056 00000 n When flooding commences on a tray, all the trays above the flooding point will also be flooded, but trays below that point will get dried up. Affecting Operation, Other Figure 7. This course is designed to provide you with a complete understanding of construction details and functioning of distilling columns. A plait point is the point on the solubility curve, where the tie line reduces to a point. To eliminate potential problems at higher rates, the reboiler return was deflected away from the seal pan, and the reboiler overflow weir was lowered. The liquid actually is froth or foam in nature which is called aerated liquid. Along with a review of vapor/liquid equilibrium, this course covers column operation, process design and simulation issues. Weeping in a distillation column. 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. Then vapor flows up the downcomer, and flooding occurs. Distillation columns of various designs and applications are encountered nowadays throughout petroleum refining, petrochemical, chemical, and process industries. In a distillation column tray, vapor passes upward through liquid that is flowing across a horizontal perforated plate. Low vapor flowrates and missing contactors will also cause weeping . These transitions are often mechanically complex, and their installation must be carefully evaluated for possible hindrance to liquid or vapor disengagement. Modeling. Because of this, the turndown of a sieve tray is often limited to about 50% of its design rate. Tray flooding is often differentiated into jet flooding and downcomer flooding. As an engineer, you must have heard the term fillet welding. In a trayed column, these flow areas are fixed by how the design engineer allocates downcomer and transfer area. Trays can operate in a continuous vapor regime (with small droplets of liquid dispersed in the vapor above the trays) or in a continuous liquid regime (with bubbles dispersed in the liquid). Foaming Liquids that foam require longer than usual residence times in downcomers. Because the pressure drops in the stripping and ! As in a packed column, gravity is the driving force for liquid downflow. Weeping usually subsides with higher flowrates. trying to separate have large difference in boiling points. Flooding Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the vapour up the column. %%EOF a tray efficiency of 70% and a 21-in. In many process services, tray efficiencies of between 40% and 80% are realized. 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. In distillation column, liquid flows in downward direction by gravitational force but it flows against the pressure. 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. where crest height = inches of the level of clear liquid overflowing the outlet weir; GPM = gallons (U.S.) per minute of liquid that leaves from the tray. comers. ____. Fouled trays Any restriction to vapor flow accelerates the velocity of the vapor as it enters the liquid (see box, p. 130). For normal distillation column operation the valid limit of weir loading is 1.49 - 32.29 m2/s. Define Turn down Ratio and its Significance in Distillation column. Vapor leakage Poor assembly or corrosion damage can let vapor leak into a downcomer. Weeping is indicated by a sharp pressure drop in the column and reduced separation efficiency. Weeping is when liquid flows downward through the holes in a Because of the empirical character of flooding correlations, a predicted value of 80% flooding may correspond to actual flooding flowrates. Highly complex system. This adversely affects the mass transfer which reduces the tray efficiency. 377 0 obj<>stream Manage Settings Periodically inspect the column to check for any signs of mechanical damage or leaks. Although not as ! The driving force that causes this entrainment is the vapor flow through the distillation tower. resulting in less than optimal vapor/liquid contact (and therefore 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. The model does not include effects due to column internals (e.g., pressure drops and flooding/weeping). Make sure the condenser is operating efficiently and the condensate is being removed quickly. contacts the liquid on the tray. A short bar is sometimes installed to deflect this horizontal vapor flow from the contactor. 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. Numerical results are presented for all three Lewis cases over the range of variables of interest to tray designers. operating costs (more reflux means higher reboiler duty). Flooding detected without such signs of lost efficiency or a sudden steep rise in pressure drop indicates downcomer flooding (Figure 8). Vapor fed into a downcomer or liquid feed flashing in a downcomer will reduce downcomer capacity. 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. This . Conduct a feed quality analysis: An analysis of the feed stock may suggest ways to reduce weeping, such as using a less volatile feedstock or changing the feed rate. Very useful concepts for interview purposes as a chemical engineerThanks for sharing, You have to cite Norman Lieberman book since this is just a copy paste, Your email address will not be published. For information about the authors, see p. 123 of the first article o: this series. 375 20 We have derived the following formula: On the basis of hundreds of field measurements, we have observed. Vapor jetting under the downcomer from closely located contactors (especially valves or bubble caps), or vapor entering downcomers that have been bowed outward into the tray transfer area, represent additional routes by which vapor can leak into a downcomer. This rapidly increases the tray pressure drop and the liquid recycle across the tray. The two effects normally cancel each other, and efficiency is practically independent of load changes (assuming no excessive entrainment or weeping). Plot the reflux rate and pressure drop against the feedrate. volatality. This indicated flooding in the rectification j section. 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). Check the condenser and reboiler systems for any potential fouling. As the liquid height on a tray increases, the height of liquid present in the downcomer that is fed to this tray will increase by the same amount. Reactive distillation is potentially attractive whenever a liquid phase reaction must be carried out . The distillation process is used to separate components based on their boiling points. So lets say the vapor rises in the column from the bottom to the top while the liquid mixture trickles down to the bottom. It is therefore desirable that the trays should have some degree of flexibility to accommodate variable throughput. Corrective action: The material was found to be readily soluble in an acid solution, so the trays were washed with acid. Principles, Vapour rate rising nonlinearly before signs of flooding appear (indicating a higher rate is required for efficient separation) suggests a loss of efficiency from liquid entrainment prior to jet flooding. The algorithm for developing a dynamic distillation column model is as follows: Step 1) Write component material balance for each stage in the column. 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). Continue doing this until the signs of flooding are detected, then reduce the feedrate to avoid flooding. ), 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. The flow patterns on a tray of a distillation column with gas rising continuously through the downflowing liquid are pretty complex. 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. Raising the reflux only a little made the column unstable. is rarely used because of low efficiencies and limited operating flexibility-. Excessive liquid fiov: The cross-sectional area of a downcomer and the clearance under it limits its capacity. Common causes of downcomer flooding include: excessive liquid flow, restrictions. ratio because this is approximately the region of minimum pan to minimize the chance of the downcomer panel being incorrectly installed or dislocated. Problem: The carbon-steel valve trays of a naphtha splitter were being severely scaled, often limiting capacity. Proper liquid distribution is key to packed column performance. Learn essential distillation concepts likely to be faced by process support, operations and process design engineering personnel. Other restrictions to vapor flow mav include the fol. The flow regime associated with flooding generates more high frequency white noise, which can be detected in the DP signal across the column. 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 Information, self cleaning NextGenaro -The Team with International Industry Expertise. Restrictions Blockage of the flow area under a downcomer will back up liquid. No weeping at low vapour flow or high liquid rates. The pressure drop of the vapor flowing through the tray above the downcomer. . Analytical equations are given for Lewis' cases 2 and 3 for plug flow of liquid on the tray. A higher level raises the tray pressure drop and boosts the tendency of liquid weeping through the vapor flow openings. Select the operating conditions: batch or continuous; operating pressure. On Improper feed introduction Vapor or flashing feeds that enter into or above downcomers can promote flooding. The design of the distillation column is critical to unit performance, size, and cost. So be with me for the next couple of years! order to separate them by evaluating the boiling point, simple distillation is separation of material depending upon The upgoing vapor drives away the downflowing liquid. Fractional distillation is needed when the two products have very K = 0.35 to 0.40: Entrainment occursan increase in reflux ratio significantly reduces tray efficiency. Holes or orifices, typically lA in. 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. with its large openings, the more-troublefree handling of dirty and fouling liquids. To learn more about weeping, the trays that can prevent it, and other types of tray designs, please visit AIChEs Distillation Technology for Operators course at www.aiche.org/ela113. Stability of the column may become an issue. Only an inspection disclosed the problem: The case of the top-flooding column. In some cases, commonly with pipe downcomers, the outlet weir may be located some distance away, to provide a dead, or quieting, zone between the weir and the pipe, which disengages considerable vapor from the liquid and vapor before the frothy mixture overflows into the downcomer. Because of this flexibility, valve trays are often chosen for services involving variable liquid and vapor flowrates. Flooding of a distillation column is a phenomenon when liquid flows across a tray and goes toward the outlet weir. In addition, you will also learn about specification and selection of the appropriate distillation hardware. In addition to that, the excessive liquid present in the downcomer or froth levels causes flooding and loss of efficiency of the tray of the tower. 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". The outlet zone extends from the last row of contactors to the outlet weir. This increases both pressure drop and entrainment. The overall pressure drop was erratic and high, ; and the column did not properly separate the key components. a column's vapor capacity rises as a function of the square of its diameter, whereas the liquid flowrate across any chord length only increases proportionally to. headroom. The trouble-shooter decided to inspect the tower. Overall, trayed columns operate via countercurrent liquid and vapor flow, with staged contacting for heat and mass transfer. There are two things to be considered i) weep point and ii)weep rate. Maximum bed height is dependent on thetype and size of the packing. Designed trays were tested using CFD simulations to observe its hydrodynamic performance during operation of the column. 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. The course features 5 major items : 1- VAPOR LIQUID EQUILIBRIA : (thermodynamics of pure components and complex mixtures) Volatility of . Make sure the reflux ratio is appropriate for chemical and physical properties of the compounds being separated. Copyright American Institute of Chemical Engineers. It is from this area that jet flooding and liquid weeping originate. Most columns are The total height of clear liquid in the downcomer weir is the summation of four factors stated below: But in the actual scenario, there is no clear liquid exists either in the downcomer, on the tray itself, or outlet weir. Since about the 1950s, sieve and valve trays have virtually displaced bubble-cap trays, which are more expensive and offer lower capacity. Make sure the tower is properly vented, that the vapors do not cause excessive back pressure and condensation. The control of the distillation tower is such that both the pressure and bottom temperature are kept constant.