Countercurrent flow rotary

The System 64MT low-temperature thermal desorption (LTTD) system is a commercially available ex situ thermal desorption technology. This system uses a countercurrent flow rotary drier to heat soils contaminated with volatile organic compounds (VOCs) to temperatures sufficient to cause contaminants to volatilize and physically separate from the soU. Filter bags remove particulate matter and afterbumers/oxidizers are used to destroy organic constituents that remain in the filtered airstream. 

A program on a computer has been prepared by Pimentel (P3) for both parallel and countercurrent flow rotary driers. 

Figure 1.1 Schematic diagram of countercurrent flow rotary kiln configuration.

The UOP rotary valve has been used in hundreds of Sorbex units across a variety of applications. The purpose of the rotary valve is to simply move the inlet and outlet ports of the net streams (feed, desorbent, raffinate, extract) around the 24 beds in stepwise fashion, creating a semi-continuous countercurrent flow of adsorbent relative to the entry and exit points of the net streams to and from the adsorbent chambers [25]. The rotary valve consists of a rotor plate pressed against a stator plate inside a pressure vessel that indexes the net desorbent, feed, extract and raffinate streams around the adsorbent chambers [26]. An alternative method of moving the inlet and outlet streams around the adsorbent chambers is used in other technologies where multiple automatic on-off valves at each distributor grid inlet are employed [5]. 

Solid-State Reactions of Iron Compounds. Black iron oxides obtained from the Laux process (see below) or other processes may be calcined in rotary kilns with an oxidizing atmosphere under countercurrent flow to produce a wide range of different red colors, depending on the starting material. The pigments are ground to the desired particle size in pendular mills, pin mills, or jet mills, depending on their hardness and intended use. 

Rotary cyhndrical dryers operate with superficial air velocities of 5-10 ft/sec, sometimes up to 35 ft/sec when the material is coarse. Residence times are 5-90 min. Holdup of solid is 7-8%. An 85% free cross section is taken for design purposes. In countercurrent flow, the exit gas is 10-20°C above the soUd in parallel flow, the temperature of the exit solid is 100°C. 

Dispersed or dilute. This is a fully expanded condition in which the solids particles are so widely separated that they exert essentially no influence upon one another. Specifically, the solids phase is so fully dispersed in the gas that the density of the suspension is essentially that of the gas phase alone (Fig. 12-29). Commonly, this situation exists when the gas velocity at all points in the system exceeds the terminal settling v ocity of the solids and the particles can be lifted and continuously conveyed by the gas however, this is not always true. Cascading rotary dryers, countercurrent-flow spray dryers, and gravity settling chambers such as prilling towers are three exceptions in which gas velocity is insufficient to entrain the solids completely. 

As an example a dried alumina carrier is used, which should be calcined to the highest possible surface area, but the temperature must reach at least 500°C and the use of a rotary kiln with countercurrent flow of air is considered. The kiln is heated indirectly. 

A third type of extractor is the belt-type (Fig. 11.15), such as one manufactured by Desmet Ballestra, but this extractor is more commonly used on oilseeds other than soybeans that are prepressed. In this type of extractor, the flakes are conveyed through a series of solvent sprays by means of a belt. Fresh solvent is introduced at the discharge end and is circulated countercurrent to the flow of flakes by a series of stage pumps. No dividers are in the belt, but the belt is inclined to assure countercurrent flow of solvent to solids. The drained marc discharges the extraction belt by means of rotary paddles. 

Revolving extractor, countercurrent gravity percolation extractor, solvent-proof body with slowly rotating vanes (rotating individual extraction sections). The rotary-vane passes the continuously fed conditioned solids from the feed shaft over a fixed sieve tray in one rotation to the solids discharge shaft. Sieve tray slots are concentric and expanded at the bottom. At the same time solvent is trickled over the solids. Solvent percolates through the solids and the sieve tray and collected in underneath extract chambers. Countercurrent flow of solids and solvent. 

Heat Transfer in Rotary Kilns. Heat transfer in rotary kilns occurs by conduction, convection, and radiation. In a highly simplified model, the treatment of radiation can be explained by applying a one-dimensional furnace approximation (19). The gas is assumed to be in plug flow the absorptivity, a, and emissivity, S, of the gas are assumed equal (a = e ) and the presence of water in the soHds is taken into account. Energy balances are performed on both the gas and soHd streams. Parallel or countercurrent kilns can be specified. 

The common types of dryers are rotary, hearth, flash (spray), and fluidized beds (10). Hot gases are used invariably to remove moisture. The gas flow can be either cocurrent or countercurrent to the flow of soHds, the former tends to be more efficient. In the hearths, the gas flow is countercurrent as the soHds are raked down from one hearth to the next below. Flash dryers are very rapid because the soHds are exposed only briefly to the hot gases. Fluidized-bed dryers, which use hot gases to suspend the soHds, are rapid and efficient, but require elaborate dust coHection systems. These are preferred when fine soHds are involved, and are used commonly for drying fine coal. Indirect-fired dryers are used when the soHds are heat sensitive or combustible. 

A potential problem for rotary valve usage is that they tend to pull material preferentially from the upside of the valve, which can affect the mass flow pattern. Another problem is that once soHd drops from the vane, the air or gas that replaces it is often pumped back up into the bin. In addition, air can leak around the valve rotor. Such air flows can decrease the soflds flow rates and/or cause flooding problems. A vertical section shown in Figure 13 can alleviate the preferential flow problem because the flow channel expands in this area, usually opening up to the full outlet. To rectify the countercurrent air flow problem, a vent line helps to take the air away to a dust collector or at least back into the top of the bin. 

Fig. 23. Flow sequence for countercurrent rotary brown-stock washers (29).

Gas flow in these rotary dryers may be cocurrent or countercurrent. Cocurrent operation is preferred for heat-sensitive materials because gas and product leave at the same temperature. Countercurrent operation allows a product temperature higher than the exit gas temperature and dryer efficiency may be as high as 70%. Some dryers have enlarged cylinder sections at the material exit end to increase material holdup, reduce gas velocity, and minimize dusting. Indirectly heated tubes are installed in some dryers for additional heating capacity. To prevent dust and vapor escape at the cylinder seals, most rotary dryers operate at a negative internal pressure of 50—100 Pa (0.5—1.0 cm of water). 

Extraction can be performed in stirred tanks if the process proceeds fast and separation of phases is ea.sy, but column extractors are most commonly used. The column can be filled with a particulate material. The liquids flow countercurrently whereby the flow can be uniform or pulsed. Reciprocated and rotary agitators are often used to enhance mass transfer. An example of the latter type is shown in Fig. 7.2-13 (asymmetric rotating disk (ARD) extractor). 

---