What is meant by vapor and liquid loading? What is its significance?

Mass Transfer Answers

24/04/2011 A: Frank Rukovena Jr, Rukovena Consulting: FRI-Retired, Rukovenaengr@gmail.com Building upon Ralph’s answer, as he says, loading refers to the vapor and liquid traffic passing through a packed bed or tray. When considering the effect of them on the capacity of either of these devices it is necessary to consider their ration to one another as well as their independent flow rates. As a general statement for counter-current devices the higher the liquid load the lower the allowable vapor loads and vice versa. For trays, it is possible to blow all of the liquid off of the tray deck if the liquid rate is low and the vapor velocity high. This is known as tray blowing. At higher liquid rates with high velocity vapor, the liquid plus froth will build up on the tray until its level will fill the tray space and reach the tray above. This is typically called flooding. Also at high pressure drop across tray it is possible to back the liquid level in the downcomer and flood the tray above. This is called downcomer backup flooding. At high enough liquid rates and a small enough downcomer inlet open area not all of the liquid leaving a tray will not have downcomer open area to get into the top of the downcomer, this is called downcomer choking. This will also flood the tray which it is suppose to be leaving. In all of these types of flooding the separation efficiency will decline and the pressure drop vary widely.For packing operating in counter current mode similar phenomena happen. At high enough vapor velocities (loading) the liquid flowing down the packing will backup and not be able to leave the packed bed. This is also known as flooding. For low pressure drop packing this can happen with pressure drops per foot of packing in the range of 0.75 in to 1 in of fluid per foot of packing. The liquid holdup in a packing is mostly independent of vapor rated until the loading region is reached at this point the liquid hold up in the packing will start to increase until the packing is filled with liquid and is in “flood”. When the flood point is reached the column separation efficiency will decline and the pressured drop start to fluctuate widely. For packing and trays, flooding does not always start first at the top of the column but it starts in the section of the column that has the highest vapor and liquid traffic or some combination thereof and the most restrictive equipment design. You may want to consult with “Perry’s Chemical Engineering Handbook” or book’s by Henry Kister “Distillation Design” and “Distillation Operation”.Other Considerations: For trays how the liquid is feed on to a tray does effect the operation of that tray and maybe even the tray below it if it is done very poorly. Vapor entering a tray tower if done incorrectly can upset the tray above but it a tray tower is not as sensitive to this is as a packed tower because the higher pressured drop of the tray compared to a packed tower. Also for a tray tower make sure that the liquid leaving the downcomer is not interfered with by the entering vapor. For large towers with multiple flow paths, vapor and

liquid misdistribution per pass can occur and reduce the towers performance.For packed towers, the specific attention needs to be paid to the design of the liquid distributors. A poorly designed liquid distributor will lower the efficiency of the packed bed below it. Because of the low pressured of packing one must pay more attention to the vapor velocities entering the tower and in some cases a vapor distributor may be required to make the vapor enter the packed bed evenly and/or to prevent liquid being carried back up the tower by the entering vapor.

03/01/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ragsdales@juno.com It means the actual flow quantities up and down through the equipment. The "loadings" are then compared with the maximum allowable quantities as determined by the physical size of the equipment as well as the operating P and T and properties of the flowing fluids. For example, through a section of fractionating trays, that comparison would be expressed as "percent of flood". Typical design is 80 to 85 percent of flood for a fractionator.

Structured Packing

INTRODUCTION:In chemical processing, a “packed bed” is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like “Raschig rings” or else it can be a specifically designed “structured packing”.The purpose of a packed bed is typically to improve contact between two phases in a chemical or similar process. Packed beds can be used in a chemical reactor, a distillation process, or a scrubber.The term structured packing refers to a range of specially designed materials for use in absorption and distillation columns and chemical reactors. Structured packings typically consist of thin corrugated metal plates or gauzes arranged in a way that they force fluids to take complicated paths through the column, thereby creating a large surface area for contact between different phases. MECHANISM:The principle of structured packing operation is based on having a large number of obstacles intercepting entrained droplets in a vapour flow. Structured columns packing consist of metal wire mesh with corrugated sheets which are aligned and placed vertically. The wire structure slopes are reversed in other sheets stacked with alternating 90 degree orientation of each layer resulting in inclined flow passages and a relatively high surface area. This arrangement allows thin liquid films to be continuously spread across the tower as liquid flows through the packing. Capillary action and high surface areas result in maximum exposure to high mass transfer efficiency. MATERIAL OF CONSTRUCTION:

The structured packing is made up of metallic wires and corrugated metallic sheets. The material selection depends on the vapour-liquid mixture to be transferred, their corrosive properties, acid-alkali nature, working temperature, etc. Some of the commonly used materials are:

SS 304 – SS 304L – SS 316 – SS 316L

Monel 400 – Monel 600 – Monel 800

Inconel 625 – Inconel 825

 APPLICATION:A properly designed structured packing is employed for one or more of the following reasons:

Mass transfer of temperature sensitive chemical products

Proper lateral distribution of liquids and gases

Solvent recovery

Batch and continuous distillation

Low liquid loading systems

Providing highest theoretical stages per meter of packing height

Reducing liquid hold up

Economizing operating loads of vessels

 TYPES:We manufacture the following types of structured packings in our facilities:

Wire Mesh Structured Packing

Sheet Metal Structured Packing

Liquid Vapor Separator

Introduction:Vapor flowing at high velocity carries liquid particles with it. This phenomenon is called “entrainment”. A demister pad is an entrainment eliminator. In simple words, a demister pad is a “liquid-vapour separator”. A properly installed demister can achieve up to 99.9% separation efficiency, with a nominal pressure drop.Demisters can reduce the residence time required to separate a given liquid droplet size thereby reducing the volume and associated cost of separator equipment. Demisters are often used where vapor quality is important in regard to entrained liquids; particularly where separator equipment costs are high (e.g., high pressure systems) or where space or weight savings are advantageous.Mechanism:The principle of demister operation is based on having a large number of obstacles intercepting entrained droplets in a vapor flow. This large number of obstacles must provide minimum drop in pressure over the removal system.The demister is employed where the liquid droplet size is below 10 microns. The pad is made up of knitted mesh of metallic or synthetic material. The vapor is made to pass through this mesh, causing the vapor to flow freely but the liquid droplets being entangled due to inertia.The droplets stick to the mesh surface for a brief period of time. As more and more droplets collect, they coalesce into a larger droplet, gaining weight and eventually falling off due to gravity. The vapour thus separated is almost free of liquid. Material Of Construction:Some of the commonly used materials are:

SS 304 – SS 304L – SS 316 – SS 316L

Monel 400 – Monel 600 – Monel 800

Teflon

Hostaflon

Poly-propylene

Inconel 625 – Inconel 825

Design:The demister pad is designed depending on factors like vapour-velocity, liquid particulate size, fluid viscosity, surface tension, temperature, pressure and pressure drop required, nature of fluid passing through, man-hole size, ease of handling, etc.The demister pads come in various sizes and forms depending on the shape of the vessels they are to be fitted on. Small sized demisters are directly knitted while larger sized are cut into segments and held together by grids and supporting rods. These accessories help in easy handling of the pad. The segment size of the demister usually depends up on the man-hole size provided on the vessel.

Application:The formation of mist leads to process inefficiencies, downstream damages to vessels as well as product loss. A properly designed demister pad is employed for one or more of the following reasons:

Reducing loss of valuable chemical products.

Increasing throughput capacity.

Improving product purity.

Eliminating contamination.

Providing protection to equipment.

Eliminating air pollution.

Types:We manufacture the following types of mist eliminators in our facilities:

Mesh-Pad Type

Vane-Pack Type

We use SS, MS, GI, Copper, Nickel, Inconel, Monel and Aluminum wires, procured from vendors like Tata SSL, Viraj Steel, Usha Martin, Etc.