Majari Magazine

Heuristics Rules for Process Equipment

by Marthin Winner on 18/03/08 at 1:55 am | 9 Comments | |

Filtration

• A convenient way to classify the ease of a given filtration task is to measure the rate of cake buildup on a laboratory vacuum leaf filter: 0.1 to 10 cm/s indicates rapid filtration; 0.1 to 10 cm/min, medium-speed filtration; and 0.1 to 10 cm/h, slow filtration
• Selection of the filtration method for a given task depends partly on whether the liquid phase or the solid phase is the one of value. Among the suitable methods if the liquid phase is desired are filter presses, sand filters and pressure filters. If the solid phase is desired, consider rotary vacuum filters

Drying of solids

• Continuous tray and belt dryers for natural or pelletized 3—15-mm granular material commonly have drying times in the range of 10-200 min
• Drum dryers that handle pastes and slurries operate with contact times of 3-12 seconds, generating flakes that are 1-3 mm thick; common evaporation rates are 15 to 30 kh/(m2)(h). Commonly found diameters are 1.5 to 5.0 ft; common rotation rates are 2-10 rev/min; evaporative capacities as high as about 3,000 lb/h are feasible in full-sale installations
• Fluidized-bed dryers work best on very small particles, with diameters of a few tenths of a millimeter; but this technique has also been used successfully with particles of up to 4 mm diameter. A suitable velocity for the fluidization gas is twice the minimum required for fluidization. In many continuous operations, drying times of 1-2 min are sufficient, but some products (including some pharmaceuticals) require much longer. Most spray dryers complete their task in less than one minute; in fact, the surface moisture is usually removed within the first 5 s. The wet feed and the drying air are most commonly fed in parallel. The atomizing nozzles typically operate at pressures of 300 to 400 psi, and have openings that measure 0.012 to 0.15 in.

Size reduction

• Ball or roller mills are commonly set up to operate in a closed circuit which includes size classification and the return of the oversize to the mill • Roll crushers come in two versions: either smooth or with teeth. A 24- in., toothed crusher can accommodate pieces of feed as large as 7 in. in radius. Smooth rolls achieve reduction ratios of about fourfold. Rotation speeds are typically 50 to 900 rev/min
• To produce particularly small particles, consider hammer mills. Large units operate at 900 rev/min; small ones can reach 16,000 rev/min
• Rod mills usually feed on particles of about 50-mm size. The material is usually reduced to about 8–65 mesh, but reduction to 300 mesh is achievable For fine grinding, consider ball mills, tube mills or pebble mills. The lastnamed are the choice when metal contaminations must be avoided
• For jaw crushers, the feed is usually under 4 in. diameter; typically, about eight to ten strokes are needed to achieve the required size reduction. Gyratory crushers can produce particles that are more rounded

Mixing and agitation

• Solids with a low settling velocity (such as 0.03 ft/s) can be successfully suspended with either turbine or propeller agitators; suspension of solids with settling velocities greater than 0.15 ft/s requires vigorous agitation with a propeller
• When only brief contact time (such as 1 or 2 s) is required between two components, inline blending should prove adequate. Typical power inputs are 0.1 to 0.2 hp/gal
• Small propellers are usually run at about 1,500 to 1,750 rev/min, large ones at about 400 to 800 rev/min
• Paddle agitators typically employ paddles that extend to about 50 to 80% of the inside diameter of the vessel. Typically, the rate of rotation lies between 20 and 150 rev/min

Agglomeration

• Some major methods of particle size enlargement are: compression into a mold; extrusion through a die followed by cutting or breaking to size; globule formation from molten material followed by solidification; and agglomeration under tumbling or other conditions of agitation without a binding agent
• For rotating-drum granulators typical length-to-diameter ratios are 2 to 3, rotational speeds are 10 to 20 rev/min, and the pitch as great as 10 deg. The size of the produced granules depends in part on the rotational speed, the residence time and the amount of binder. Commonly, the produced granules are 2 to 5 mm in diameter
• If uniformity of product size is important, rotary disk granulators are preferable to drum granulators, other things being equal
• For roll compacting and briquetting, typical rolls measure from 130-mm diameter by 30 mm wide to about 900-mm diameter by 550 mm wide. The material extruded by the rolls, typically about 1 mm thick, can be broken into any size desired
• Rotary compression equipment, feeding upon powders or granules and typically operating at around 100 rev/min, is suitable for producing tablets of uniform size, at rates as high as 10,000 tablets/min
• Fluidized-bed granulation is typically carried out in beds having a depth of 12 to 24 in. Typical air velocities are 0.1 to 2.5 m/s, which is three to ten times the minimum velocity needed for fluidization

Author: Alejandro Anaya Durand, Josseline Alarid Miguel, Gabriel Gallegos Diez Barroso, Marco Alejandro Leon Garcia, and Juan Pablo Sierra Angeles – National Autonomous University of Mexico