Analytical equations are given for Lewis' cases 2 and 3 for plug flow of liquid on the tray. Dual-flow trays do not have down-comers. A troubleshooter who suspects a design limitation should check the columns hydraulic performance. 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. Inspect the column for any visible signs of damage, such as leaks, corrosion, or other physical abnormalities. The liquid entering the downcomer is aerated, however. 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. ), 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. Increases tray efficiency. process. The height of the crest is similar to the height of water overflowing a dam or a river. Equipment and Operation, Column The reflux. 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. This sensitivity to liquid rate suggested a liquid flow-path problem, such as downcomer flooding. 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. So lets say the vapor rises in the column from the bottom to the top while the liquid mixture trickles down to the bottom. Check the condenser and reboiler systems for any potential fouling. Excessive liquid fiov: The cross-sectional area of a downcomer and the clearance under it limits its capacity. The algorithm for developing a dynamic distillation column model is as follows: Step 1) Write component material balance for each stage in the column. 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. Excessive weeping will lead to dumping. Adjust the reflux ratio and/or other operating conditions to optimize performance. That is the liquid on all trays will crash (dump) through to the base of the column (via a domino effect) and the column will have to be re-started. Install a reboiler cooler: A reboiler cooler can help reduce or control weeping by cooling the liquid going into the reboiler. is rarely used because of low efficiencies and limited operating flexibility-. 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. The higher vapor velocity resulting from the reduced contacting area can initiate jet flooding. , Reorinted from CHEMICAL ENGINEERING Mau 10S0 mnuhi ioao hu i ,.., *m, Continue reading here: Vapor Blanketing Reboiler Corrective Actions. For trays of the distillation tower, the incipient flood point corresponds to the best efficiency point. Maintain a clean environment and check for any signs of fouling. Loss of liquid level in the bottom of the column is an early indication of flooding in a distillation column. 0000000711 00000 n
0000002804 00000 n
One can call this as optimum reflux ratio which would be an alternative description of the incipient flood point. is easy to verify using the McCabe-Thiele method. Pipeline Stress Analysis is a process that involves evaluating the mechanical behavior of a pipeline under various operating conditions, including pressure, temperature, and external loads such as What is Fillet Welding? Entrainment Back-mixing via liquid entrainment reduces efficiency. The term "weeping" is usually used in tray columns where the liquid leaks through the perforations or valves because of insufficient counter current gas flow. You must be logged in to view this page. The course features 5 major items : 1- VAPOR LIQUID EQUILIBRIA : (thermodynamics of pure components and complex mixtures) Volatility of . becomes more pronounced and more and more trays are required. <<4e6228ae77f767429ef5f679323dfefe>]>>
An inadequate liquid seal also allows vapor to flow into a downcomer. Because of the empirical character of flooding correlations, a predicted value of 80% flooding may correspond to actual flooding flowrates. the liquid in the downcomer, causing j excessive downcomer backup. Weeping causes some reduction of the tray efficiency because the liquid dripping down to the tray below through the perforation has not been in full contact with the gas or vapor. Most other tray types represent a modification of the sieve, valve or bubble-cap tray. Weeping is indicated by a sharp pressure drop in the column and reduced separation efficiency. If placed higher, the vapor expanding into the tray above could initiate flooding. Excess loading As with a packed column, the troubleshooter should have some expectation of the maximum capacity or pressure drop of the column, as designed or previously operated. The switch to sieve trays reduced the frequency of tray cleaning to remove scale. Each stage is often modeled as a mixer-separator, with liquid-vapor contacting followed by the separation of liquid and vapor for transport to, respectively, the stages below and above (Figure 1). Modeling. Analytical equations are given for Lewis' cases 2 and 3 for plug flow of liquid on the tray. This is one of the most common terms related to welding for joining two metal pieces together to get a permanent joint. The function of a tray is to mix the vapor and liquid together to form foam. 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". 0000001598 00000 n
In extreme cases, the vapor flow may be reduced to nothing on the inlet edge of the tray, allowing liquid to flow back through the cap riser. The closest analogous concepts for packed columns would be mal-distribution (as you have suggested) or wetting rate. The primary objective of the distillation column is to create high surface area with the . The section where the vapour leaves the column to the K = 0.35 to 0.40: Entrainment occursan increase in reflux ratio significantly reduces tray efficiency. weir height and clearance under the downcomer had been j lowered to provide greater turndown (calculations indicated the 1-in. Internals, Distillation To minimize scaling and facilitate cleaning, the decision was made to replace the valve trays with stainless-steel sieve trays. The next problem may lead to the flooding of the tower. The downcomer typically takes up a segment of a column's diameter but can be a pipe, box or other shape. The most common tray-column vapor-and-liquid contactors are sieve, valve and bubble-cap trays. This turndown ratio can be increased by reducing the fractional hole area. C. Results due to very high gas velocity. The distillation column is a key component of ammonia-water absorption units including advanced generator-absorber heat exchange (GAX) cycle heat pumps. xb```b``AXf&f$A/lp~O
&1zIoc`fc``dcoqmEOnN}3hpk]DEq,3"pHa5~: 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. of Columns, Basic 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. Distillation columns are the primary equipment used in the chemical process industry to carry out the separation of organic chemicals. The remainder of the tray is active in contacting vapor and liquid. Because of this, the turndown of a sieve tray is often limited to about 50% of its design rate. Low Flow in Pipes- posted in Ankur's blog. A high liquid level in the column base will cause liquid to back up into this downcomer. distillation tray. Implement regular maintenance on the column and its components, including the trays, valves, and piping. laboratory work most of time the distillation column will work with a total reflux, i.e., the entire vapor at the top of the column is condensed. Resources, Copyright Weeping is undesirable and the liquid should flow across the tray and down the downcomer. ratio, when an infinite number of trays will be required Outcome: The hydraulic hammer was eliminated. Maximum bed height is dependent on thetype and size of the packing. 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). The weep point is defined as the velocity of vapor becoming significantly low which reduces the tray efficiency. One useful test is to increase the column feedrate while monitoring the column pressure drop and the reflux rate required to maintain the separation. The droplet may descend back into the liquid on the tray or may be carried into the tray above based on the size of a droplet, its projected velocity, and the drag force acting on it due to the gas velocity. The column was operating at about only ! The bubble cap's more-positive liquid trapping confers good operating-rate. Outcome: Restarted, the column operated properly at design capacity. Unexpected foaming will excessively aerate ! Flooding detected without such signs of lost efficiency or a sudden steep rise in pressure drop indicates downcomer flooding (Figure 8). to 1 in. The drawings revealed the following: the flow from reboiler jetted directly into the ; seal pan; an inlet weir had been placed on the feed tray; the ! a tray efficiency of 70% and a 21-in. The sparger discharge-orifice velocity was calculated to be less than 1 ft/s. Therefore, liquid starts to leak through perforations. 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. low tray pressure drop can result in liquid weeping, whereas operation at a pressure drop higher than 8 to 9 mm Hg per tray will often bring on flooding. Periodically inspect the column to check for any signs of mechanical damage or leaks. However. 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. A higher level raises the tray pressure drop and boosts the tendency of liquid weeping through the vapor flow openings. FIGURE 1. Principles, Vapour vapour pressure is related to boiling. A trayed distillation column with reflux has been designed to separate a liquid mixture of acetone and water. Other restrictions to vapor flow mav include the fol. to ensure that vapor will not enter the downcomer.