WO1985001674A1 - Gas distributor for fluidised beds - Google Patents

Abstract

Hot gases from a combustion system are passed through a plurality of horizontal or slightly inclined tubes (3) into which a large number of distribution jets (4) are fitted that regulate the hot gas flow into a fluid bed of solids. The tubes are mounted at one end allowing them to expand freely without any leakage of hot gases. The gaps between the tubes allow solids to pass between them and form a static layer to protect the casing of the fluid bed chamber. These solids can be either the material being treated or an alternative material more suited for insulation. The tube distributor provides a simple reliable system for hot gas distribution into a fluid bed and eliminates many of the problems associated with steel plate, refractory arch and air cooled multi plate types of distributors.

Description

Gas distributor for fluidised beds INTRODUCTION Most fluid bed dryers are faced with complex design problems in introducing hot gases uniformly into a fluidising zone of solids. The concept used is to form a barrier between the hot gas and fluidising zones, using a horizontal or slightly inclined plate into which gas passages are incorporated that control the uniform distribution of hot gases to the fluidising material. This plate is sometimes constructed from wholly refractory material, or heat resisting steel or a combination of carbon steel and heat resisting steel and in each case allowance has to be made for thermal expansion of the component parts of the plate whilst maintaining a gas pressure seal between the hot gas and fluidising zones.

It is also necessary to install this distributor plate into a plenum chamber that has to be constructed to withstand hot gases of up to 1000ºC, which requires costly refractory linings and insulating materials. It is the objectiveof this invention to provide a simpler construction of distributor plate and plenum chamber that eliminates complex expansion allowances refractories and/or air cooling systems whilst maintaining the -same performance.

DESIGN CONCEPT Hot gas for the drying or heat treatment process is provided by a combustion system 2 mounted horizontally or vertically on one end of the treatment chamber arranged to pass the hot gases into a mixing chamber 1. The hot gases from the combuster are mixed with cool dilution air in the mixing chamber to achieve the desired inlet hot gas temperature to the treatment process.

Typically this temperature could be between 400° C and 1000°C. The mixing chamber is constructed preferably with a double steel shelled arrangement with the inner shell being refractory lined. The cavity between the two shells is used to introduce the dilution; air so that the heat losses through the casing are minimised and the dilution air is preheated.

The hot gas distributor 3 is formed by a plurality of horizontal or slightly inclined tubes of typically 300mm to 400mm in diameter mounted on flanges in the lower half of the mixing chambers. The tubes may be other than of circular cross-section, for example, they may be square or flat-topped. These tubes are constructed in heat resisting steel that will withstand the hot gas mixture without additional insulation or cooling, "Incaloy" being a typical material of construction. In each tube are welded a number of distributor jets 4 that are capped to prevent the ingress of solids into the tube, whilst at the same time allowing a uniform distribution of hot gases over the whole length of the tube. The jets are typically pitched at 100mm but different pitches can be selected to suit the need of the process. It is also possible to use a plurality of small holes instead of the capped jets again depending on the process requirements. The jets or holes in the tubes create an even distribution of the hot gases exiting along the tube length to provide uniform fluidising conditions.

The fixture of the tubes at the mixing chamber end is by flanges on the tubes being bolted to mounting spigots at the hot air end of the casing and because this is the only mounting, the tubes are free to expand towards the discharge end of the dryer without further restriction. The tube weight is taken on simple cross support members 5 .

The lower casing 6 of the dryer is constructed in carbon steel and is a simple enclosure without refractory or insulating material, a necessary requirement in most other plenum chambers.

The dryer top casing 7 is again a simple carbon steel construction into which the material feed chute and dryer gas outlet are mounted.

METHOD OF OPERATION The material to be processed enters into the feed chute in the top casing and initially fills the voids in the lower casing below and between the distributor tubes. This static layer of material forms an adequate insulation effect between the tubes and casing. For application where different materials are to be processed and minimal contamination is required, a prepacked loose fill of an inert material can be used to pre-load the lower casing. Simple keep plates between the tubes can be added to reduce the contamination between the inert charge and the material being processed.

After the initial charge is established the incoming material is fluidised in the lower chamber immediately above the tubes at a controlled depth to suit the treatment process. The fluidised material passed uniformly along the length of the vessel and is discharged through an opening at end remote from the mixing chamber. The method of controlled discharge may be a weir or plug flowsystem, both of which are well known on dryers using the fluid bed principle. The gases leave the dryer through the off-take duct in the upper casing and pass to dust collection equipment.

TYPICAL OPERATING PARAMETER The hot gases leaving the combustion chamber are between 1400°C and 1600°C and after dilution In the mixing chamber the inlet temperature would be 850°C or less, typically 800°C. The pressure of the gas in the mixing chamber would be approximately 25 to 30" W.G. The hot gases pass through the tube distributor system which has a pressure drop of about 10" W.G. at the operating temperature and the fluid bed solids will have a temperature of between 60°C and 140°C depending on the degree of drying required. The bed depth of fluidised material is between 6" and 12".

ADVANTAGES OF THE INVENTION 1. Simpler, troublefree distributor construction which requires no refactory or air cooling and no expansion allowance.

2. Simple construction of lower chamber that requires no special form of refactory insulation and can adopt the material being processed as insulation protection.

3. Alternative loose fill insulation of inert material can be used instead of the material described in paragraph 2.

4. No limitation in the size of the drying chamber as the tube distributor can be used in large numbers or longer depending on the proportion of the drying chamber.

Claims

1. A gas distributor for a fluid bed comprising a plurality of transverse (e.g. horizontal or slightly inclined to horizontal) tube members arranged as a grid and provided with distributor jets for the gas.

2. A gas distributor as claimed in Claim 1 so arranged to allow the fluid bed treatment material to form a static layer in the casing below the tubes for protection of the casing.

3. A gas distributor as claimed in Claim 1 so arranged as to allow a material different from that being fluid bed treated to be pre-fed to form a static layer in the casing below the tubes for protection of the casing.

4. A gas distributor for a fluid bed constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.

5. A fluid bed dryer, including a combustion system and a mixing chamber leading to a casing for the reception of material to be dried, wherein hot gases fed from the mixing chamber pass Into a gas distributor as claimed in any one of the preceding claims.