Types and design of the towers trays
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The document outlines design guidelines for trayed towers used in distillation, focusing on tray hydraulics, design considerations, and types of trays including bubble cap, sieve, dual flow, and valve trays. It emphasizes the importance of tray performance in achieving effective separation through vapor-liquid contact and details specific parameters affecting tray design such as pressure drop and efficiency. The guideline provides theoretical foundations and practical examples to assist engineers in designing efficient trayed towers.
![6 typesanddesignof the towerstrays Mohamedsalah
B. Sieve Tray
Sieve tray is perforated plate with holes punched into the plate and usually
has holes 3/16 in to 1 in diameter. The standard size is 0.5 inch with the
perforation punched downward. Vapor comes out from the holes to give a
multi orifice effect. The vapor velocity keeps the liquid from flowing do through
the holes (weeping). Vapor flow through the tray deck to contact the liquid is
controlled by the number and size of the perforations.
The punching direction affects the dry pressure drop, a smaller hole
diameter result in lower pressure drop for the same open area. This due to the
ratio of hole diameter to the tray thickness. The number and hole size are
based on vapor flow up the tower. The liquid flow is transported down the
tower by down-comers, a dam and overflow device on the side on the plate.
A sieve tray has higher entrainment than valve tray at the same vapor
velocity. This is due to the spray of liquid directed upwards to the next tray. For
efficient operation, the hole velocity must be sufficient to balance the head of
liquid on the tray deck and thus prevent liquid from passing through the
perforations to the tray Sieve trays can be used in almost all services.
Sieve deck tray has a minimum capacity approximately 70%. Their capacity
and efficiency are at least as high as that of other standard trays used
commercially. Sieve trays may be used in moderately fouling services, provided
that large holes (3/4 to 1 in. [19 to 25 mm]) are used. Sieve trays are simple
and easy to fabricate and therefore relatively inexpensive.](https://image.slidesharecdn.com/typesanddesignofthetowerstrays-150225005112-conversion-gate02/85/Types-and-design-of-the-towers-trays-6-320.jpg)
Types and design of the towers trays
- 1. 1 typesanddesignof the towerstrays Mohamedsalah Suez University Faculty Of Petroleum & Mining Engineering Student/ Mohamed Salah abou El_hamed Department/ Petroleum Refining Year/ Fourth
- 2. 2 typesanddesignof the towerstrays Mohamedsalah Scope This design guideline covers the basic elements of tray hydraulics in sufficient detail to allow an engineer to design a trayed tower with the suitable size of tray spacing, weir height, holes, down-comer, and open area. These factors will influential flooding and weeping, and will effect the tray performance, capacity and efficiency. The design of trays depends on the density, the rate of vapor and liquid through the tray. Each tray in a distillation column is designed to promote contact between the vapor and liquid on the stage, so there is mass and heat transfer. Ideally, the vapor and liquid leaving the stage are in equilibrium. Equilibrium is a function of the rate of mass and heat transfer between liquid and vapor in distillation and it has an effect on the efficiency of tray. To increase efficiency and capacity of tray, a system is needed which will cover the whole spectrum of tray applications. One of the keys to make good quality products and high purities is tray hydraulics. Parameters of tray hydraulics beside the geometry of the tray as it mention in first paragraph, are vapor-liquid loading, the allowable pressure drop on the tray and in the downcomer, flooding, turndown and weir loading. The design of tray hydraulics may be influenced by many factors, including process requirements, economics and safety. In this section, there are tables that assist in making these factored calculations from the various reference sources. Generally, path flow liquid on tray is divided into single pass, two pass, three pass, and four pass. In this guideline, these differences will be discussed in detail for the proper engineering design for vapor-liquid loading and down comer design. The theories used in this guideline are commonly used in industries such as Bennet,Van Winkle, Fair and Eduljee. Their application to tray hydraulic theory with examples will help the engineer understand tray hydraulics and be ready to perform the actual design of the trayed tower. Included in this guideline is an example data sheet which is generally used in the industry and a calculation spreadsheet for the engineering design.
- 3. 3 typesanddesignof the towerstrays Mohamedsalah In order for good separation (or fractionation) to occur in distillation column, the vapors must have good contact with the liquid on each tray. When the large bubble is broken into several smaller bubbles, the smaller bubbles expose more surface area for vapor liquid contact. Types of Tray Distillation Column Distillation is the most widely used separation process in the chemical industries. It is normally used to separate a mixture of materials to obtain one or more desired products which is achieved by selection of conditions of temperature and pressure so that at least a vapor and a liquid phase coexist and a difference in relative concentration of the materials to be separated in the two phases is attained. Trays are the most commonly selected type of distillation column. Trayed Columns utilize a pressure and temperature differential to separate the products. For most tray columns, the weir holds a liquid level of each tray. The vapor must overcome this liquid head to move up the column. On the tray the vapor and liquid are contacted and then above the tray they are separated. Trayed column perform well in high liquid and vapor loading. At low flow parameters the capacity and efficiency of trays can be reduced. Tray have higher pressure drop than packing, and it may also have higher resistance tocorrosion. Some other items are to consider when to use trays in a tower. 1. Usually trays have downcomer capacity problems in heavy foaming service. 2. Trays have higher pressure drop than structured packing column 3. Trays have a high resistance to corrosion 4. Entrainment is an issue with trays. Trays usually have more entrainment than packing. Excessive entrainment can lead to efficiency loss.
- 4. 4 typesanddesignof the towerstrays Mohamedsalah 5. Excessive vapor and liquid maldistribution can lead to a loss of efficiency in a tray tower. In order for good separation (or fractionation) to occur in distillation column, the vapors must have good contact with the liquid on each tray. When the large bubble is broken into several smaller bubbles, the smaller bubbles expose more surface area for vapor liquid contact. There are five major types of tray column; bubble cap tray, sieve deck tray, dual flow tray, valve tray and baffle tray. Bellow is discussed each type.
- 5. 5 typesanddesignof the towerstrays Mohamedsalah A. Bubble Cap Tray The oldest widely used equilibrium-stage plate is the bubble cap tray. A bubble cap tray is perforated flat plate which has a riser (chimney) over the holes covered with a cap. They are usually equipped with slots to allow the passage of vapor to be mixed with the liquid flowing across the tray forming bubble where the mass transfer takes place. Each tray is provided with one or more downcomers which the liquid flowing across the tray is conducted to the tray below. A liquid head is maintained on the tray by a dam placed on outlet side of the tray near the downcomer, it called the outlet weir. Bubble cap tray is able to operate at low vapor and liquid rates (less than 2 gpm per foot of average of flow width) because liquid and foam is trapped on the tray to a depth at least equal to the weir height. Bubble cap trays work well in high turndown applications because the orifices in the bubble caps are in the form of risers whose top opening is elevated significantly above the tray deck. The size of the cap tends to create hydraulic gradient across the deck and a high vapor side pressure drop. The cost of bubble cap tray is by far the highest.
- 6. 6 typesanddesignof the towerstrays Mohamedsalah B. Sieve Tray Sieve tray is perforated plate with holes punched into the plate and usually has holes 3/16 in to 1 in diameter. The standard size is 0.5 inch with the perforation punched downward. Vapor comes out from the holes to give a multi orifice effect. The vapor velocity keeps the liquid from flowing do through the holes (weeping). Vapor flow through the tray deck to contact the liquid is controlled by the number and size of the perforations. The punching direction affects the dry pressure drop, a smaller hole diameter result in lower pressure drop for the same open area. This due to the ratio of hole diameter to the tray thickness. The number and hole size are based on vapor flow up the tower. The liquid flow is transported down the tower by down-comers, a dam and overflow device on the side on the plate. A sieve tray has higher entrainment than valve tray at the same vapor velocity. This is due to the spray of liquid directed upwards to the next tray. For efficient operation, the hole velocity must be sufficient to balance the head of liquid on the tray deck and thus prevent liquid from passing through the perforations to the tray Sieve trays can be used in almost all services. Sieve deck tray has a minimum capacity approximately 70%. Their capacity and efficiency are at least as high as that of other standard trays used commercially. Sieve trays may be used in moderately fouling services, provided that large holes (3/4 to 1 in. [19 to 25 mm]) are used. Sieve trays are simple and easy to fabricate and therefore relatively inexpensive.
- 7. 7 typesanddesignof the towerstrays Mohamedsalah C. Dual flow tray Dual flow tray is a sieve tray without a downcomer. Vapors move up to the tray above through the hole, while the liquid traveles down in the same hole that can result in mal-distribution and low efficiency. Dual flow trays are designed with enough open area on the tray deck to eliminate stagnation and promote back missing that makes it suited to handle highly fouling services, slurries, and corrosive services. Dual flow trays are well suited also for the fractionation of polymerizable compounds and give more bubbling area, therefore have a greater capacity than other tray types. Dual flow tray is also the least expensive to make and easiest to install and maintain. Dual flow tray performs best in the operating region of 60 to 85 % of flood and increases the efficiency with vapor rate. The challenge of dual flow tray is mal-distribution in larger diameter towers. The top of a column will move in a typical storm as much as six inches. This movement will cause the hydraulic load to migrate in the column. If hydraulic flow instability is developed it propagates down the column. Improper feed, reflux or vapor distribution can also create mal-distribution problems, Dual flow tray have poor turndown ratio resulting from the rapid fall off in efficiency as the vapor loading decreases. Therefore the operating vapor and liquid rate ranges must be kept small. Two types of dual flow trays are available; standard deckand rippled deck. The standard deck has a flat plate, and the rippled deck has sinusoidal waves.
- 8. 8 typesanddesignof the towerstrays Mohamedsalah D. Valve Tray Valve Tray uses valve (moveable disc) which almost closes off completely at low vapor rate, thus minimize tray open area. When lifted, as vapor rate increases, the open area increases for vapor flow between the valve disc and the tray deck. Valves can be round or rectangular, with or without caging structured. Most types of valves, the opening may be varied by the vapor flow, so that the trays can operate over a wide range of flow rates with high separation efficiency and large flexibility. Because of their flexibility and price (slightly more expensive than sieve tray), valve trays are tending to replace bubble-cap trays. Valve tray has minimum capacity of approximately 60%. The dry pressure drop of valve tray is lower than bubble cap, because the valve does not need a chimney for the vapor and it depends on weight of valve. The valve tray can used in condition where vapor rate change unpredictability over a given section of tower, a tower utilized in blocked operation at varying rate and feed compositions, a fluctuations in feed rate, and servicing of auxiliary equipment operating the entire unit at low rate.
- 9. 9 typesanddesignof the towerstrays Mohamedsalah Comparsion of Common Tray Types
- 10. 10 typesanddesignof the towerstrays Mohamedsalah Summary the Geometry and Layout of Common Tray Types at Different Operating Pressures