Residence time rotary kilns

The key to efficient destruction of liquid hazardous wastes lies in minimizing unevaporated droplets and unrcacted vapors. Just as for the rotary kiln, temperature, residence time, and turbulence may be optimized to increase destruction efficiencies. Typical combustion chamber residence time and temperature ranges arc 0.5-2 s and 1300-3000°F. Liquid injection incinerators vary in dimensions and have feed rates up to 1500 gal/h of organic wastes and 4000 gal/h of aqueous waste. 

Cresols may be disposed of by landfill, land applications, biological waste water treatment, or incineration. In an activated sludge system, cresols exhibit a 96% reduction of the chemical oxygen demand and a biodegradation rate of 55 mg of oxygen/g-hour. Cresols may be disposed of in a rotary kiln incinerator with a temperature range of 820-C-1600-C and a residence time of seconds. Cresols may also be disposed of in a fluidized bed incinerator with a temperature range of 450 C-980 C and a residence time of seconds (HSDB 1989). 

The zinc salt and BaS solutions are mixed thoroughly under controlled conditions (vessel geometry, temperature, pH, salt concentration, and stirring speed, see (a) in Fig. 20). The precipitated raw lithopone does not possess pigment properties. It is filtered off (b2) and dried (c) ca. 2 cm lumps of the material are calcined in a rotary kiln (d) directly heated with natural gas at 650-700 °C. Crystal growth is controlled by adding 1-2 wt% NaCl, 2 wt % Na2S04 and traces of Mg2 + (ca. 2000 ppm), and K+ (ca. 100-200 ppm). The temperature profile and residence time in the kiln are controlled to obtain ZnS with an optimum particle size of ca. 300 nm. 

Figure 1730. Kilns and hearth furnaces Walas, 1959). (a) Temperature profiles in a rotary cement kiln, (b) Space velocities in rotary kilns, (c) Continuous lime kiln for production of approximately 55tons/24hr. (d) Stirred salt cake furnace operating at 1000°F, 11-18 ft dia, 6-10 tons salt/24 hr. (e) Multiple-hearth reactor one with 9 trays, 16 ft dia and 35 ft high roasts 1250 lb/hr iron pyrite. (f) Siements-Martin furnace and heat regenerators a hearth 13 ft wide and 40 ft long makes 10 tons/hr of steel with a residence time of 10 hr.

Fig. 2.4. Average conversion from residence time distribution—example of rotary kiln...

This paper presents the first experimental results for the solid motion inside a pilot-scale rotary kiln. Such data are useful to enable efficient pyrolysis reactions inside a rotary kiln to be carried out, through the prediction of residence time and material hold-up, and the evaluation of different surfaces and temperature profiles according to the operating conditions. In the first part, the pilot-scale rotary kiln and the principle of the experiments will be described. An original dynamic solid motion model will be presented in the second part, this dynamic model is derived from the original static model of Seaman [1], The static and dynamic experimental results are finally compared with the simulated results. 

P.S.T. Sai, G.D. Surender, A.D. Damodaran, V. Suresh, Z.G. Philip, K. Sankaran, Residence time distribution and material flow studies in a rotary kiln, Metall. Trans. B 21B (1990) 1005-1011. 

Use of rotary kilns for hazardous waste incineration is becoming more common for disposal of chlorinated hydrocarbons such as polychlorinated biphenyls (PCBs). Flow in these kilns is cocurrent. Major advantages include high temperature, long residence time, and flexibility to process gas, liquid, solid, or drummed wastes. 

When petroleum coke is utilized for anode and electrode production and some specialty applications, it is necessary to calcine it to remove moisture and hydrocarbon VCM. Product qualities, along with production rate, are based on feedstock composition, kiln temperature profile, kiln residence time and cooling procedures. The two methods available for calcining coke commercially are the rotary kiln (5 ) shown in Figure 8 and the rotary hearth (6J shown in Figure 9. 

Rotary kiln incineration at a temperature range of 820-1,000 °C and residence times of seconds for liquid and gaseous wastes and hours for solids can totally destroy dinitrocresols. Fluidized bed incineration at a temperature range of 450-980 °C and residence times of seconds for liquid and gaseous wastes and longer for solid wastes can also destroy dinitrocresols. Mixing dinitrocresols with a more flammable solvent may facilitate incineration. Containers used for dinitrocresols that are not to be reused can be disposed by burial in a designated landfill (HSDB 1994). 

As described previously pyrolysis is a process that thermally degrades organic waste at high temperatures in absence of air and oxygen. This process can be carried out in a rotary kiln reactor or in a fluidized bed. In a rotary kiln process the feed material is conveyed through a rotating drum (i.e. reactor) and is then pyrolysed in the hot atmosphere into gas and solid residues. The residence time of the reaction is dependent on the rotating... 

For shredder light fractions, a mixture of polyolefins and polyurethanes containing as well as polyamides, PVC, polystyrene and blends, Wanzl et al. [23, 24] and Basel [25] have shown for the treatment of the pyrolysis gases from rotary kiln pyrolysis that by using dolomite beds at 500°C and a residence time of 20 s, chlorine content can be reduced from 1000 ppm to below the detection limit of 1 ppm. 

Modern rotary kiln plants have capacities of up to ca. 500 t/d, continuous grate furnaces up to 1000 t/d. The residence times are less than 1 hour. In kettles throughputs up to 150 t/d with residence times of several hours are aimed for. Box shaft kilns are no longer viable due to the lengthy (several day) calcination times. 

It is expected that the heating value of the waste fed to the incinerator may vary substantially. Therefore, a rotary kiln incinerator would be more appropriate since it can handle a large range of waste heating values. The residence time of a rotary kiln incinerator may be changed by adjusting the rotational speed of the kiln. 

The flow of the combustion gases in direct-fired systems may be concurrent (i.e., the gases flow in the same direction as the feedstock) or countercurrent (i.e., the gases flow opposite to the movement of the feedstock). The benefits of a concurrent system are higher treatment temperatures, longer residence times, and treatment of fines. Countercurrent systems offer more efficient heat transfer, and lower volumes and temperatures of off-gas. ° In large part due to the beneficial aspects of the off-gas and consequent simplified APC system, countercurrent systems are used more commonly in rotary kilns. 

There is a wide variation in the system throughput for rotary kiln TDUs with values ranging from 5 to 150tph. However, most systems operate in the 15-30tph range.f The actual throughput achieved depends not only on the design capacity of the rotary kiln, but also on the treatment criteria. Treatment criteria determine the maximum soil depth in the kiln and the required residence time. 

All rotary kilns operate to achieve a desired soil output temperature to ensure complete treatment of the contaminated soil. The specific soil exit temperature is determined during startup testing to ensure the soil is properly treated in the TDU. This temperature generally falls into two groups based on residence time (1) less than or equal to 260° C and a residence time of approximately 6-7 min and (2) greater than 260°C and a residence time of approximately 15-20min. ... 

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