This paper will focus on analysis of a gas fired rotary kiln in the processing of Bentonite. A rotary kiln is a drier that is used for increasing the temperatures of materials in a process called calcinations through a continuous progression. During the processing of minerals such as Bentonite and cement, this kiln is appropriate. Its thermal efficiency ranges from 50% to 65%. This kiln has a cylinder that is slightly incline horizontally and rotates gradually about its axis. Materials that are to be processed are fed into the kiln through the upper part of the cylinder. These materials slowly move to the lower end of the cylinder as they undergo stirring and mixing. The stirring ensures that temperature is evenly distributed through the material being processed (Bolt & Van 147).
Usually, hot air is passed through the cylinder in a concurrent direction to that of the materials being processed. But in some cases, the hot air may pass through the cylinder in same direction as material in processing. The rotary kiln may source its hot air from an external source or may have a flame inside it that generates the flame. This flame is directed into the cylinder by firing pipes. The burning fuel used in this kiln may be oil, gas or pulverized coal.
The kiln shell is usually constructed from mild steel plates of a thickness of about 25mm. it may have a length of 230 meters and a diameter of 6 meters which is usually in the east-west direction to avoid formation of eddy currents. The kiln has refractory linings which insulates the shell made from steel from the escalated temperatures in the kiln. It also protects the kiln from corrosion from the materials being processed. Tyres and rollers are used in the kiln to allow rotational movements which should have minimal friction. It also has a drive gear which is turned using a single girth gear usually surrounded by a part of the kiln that is cooler (Anderson 649).
This paper requires an analysis of a gas fired rotary kiln which has the dimensions of 8 ft internal diameter and is 120 ft long. It has a burner on-high fire that contributes a 12(106) BTU/Hr to the kiln. The production of the plant is 100 tons of bentonite per day.
Reducing moisture content of Bentonite from 11% to 4%
The measurement of moisture content facilitates the control of the process of drying the Bentonite. This allows the drying process to be done continuously until the desired moisture contents have been reached. This is a better way since just fixing a time period may not be as accurate as desired.
Since the through-put of the hot air kiln plant is 100 tons of wet product per day, then drying it from 11% moisture to 4% moisture levels requires heat of:
Changing 100 tons per day to kg per day;
(100 × 1000) kg=100,000kg/24hr=4166.67kg/hr
4166.67kg of wet Bentonite contains 4166.67×0.11kg water=458.3337kg moisture
And:
4166.67× (1-0.11) =3708.3363 kg dry bone basis
As the final product should contain moisture content of 4% therefore the moisture product will be;
3708.3363/9.6=386.285 kg
Therefore the moisture eliminated = (458.333-386.285) =72.048 kg
Latent heat of evaporation =2257 kJ kg-1(at 100 °C).
This means that the heat necessary to be supplied from the drier = 72.048 × 2257 = 162,612.336 KJ
Volume of the kiln;
V = π (D/2)2 × (L) since kiln has 8ft internal diameter and is 120 ft long, then;
Volume =3.142 × (8/2)2 ×120 = 6032.64 ft3
Changing cubic ft into meter cube we get 170.83 m3
Loading capacity of kiln;
L = (C x f x t) ÷ (D x V) where C=capacity of kiln in Tons/hr
F = ton (kg) dry feed/ton (kg)
T = time of residence
D = dry feed ton/hr (kg) bulk density
V = kin internal volume (m3)
L = (4166.67 x 3708.33) ÷ (t x 6032.64 x 2.438 x 8) = 8405.6/t
Calculating Q internal energy developed in kiln;
Q = 1.1 x 106 x D3 (Kcal/hr)
Q = 1.1 x 106 x (8 x 2.4384) m = 21.4579 x 106
Kiln thermal loading = QP = Q/FP
Where FP = 0.785 x D2 ( thermal loading at cross section of burning )
And QP = specific internal thermal loading
FP = 0.785 x (8 x 2.4384)2 = 298.716
QP = 1.4 x 106 x D Kcal/m2hr
QP = 1.4 x 106 x (8 x 2.4384) m = 27.3112Kcal/m2hr
In this case, the plant processing moisture of bentonite was specified to be at 6% bone dry mass as the accepted maximum. The plant can further alter the production moisture to as low as 5%. In many cases, the final moisture levels specifications may be raised without any disadvantageous effects on post processes. The overall production can be considerably improved by easing of this requirement. A second drier installation can be very handy in removing the small last amounts of moisture. This format is mostly known as the two stages drying (Appelo & Postma 647). The reduction of airflow and heat requirements in using this method usually leads low energy consumption and higher production as the moisture removal is very small.
Mass balances for water and bentonite
Research has proved that inside bentonite, there are three types of different waters. Firstly is the re-saturation internal water. These waters are found in the spaces between each single interlayer of water. This isolation therefore gives these waters different characteristics from free water. Secondly is the external water categorized into two; electrical double layers and the free water. Bentonite interacts with ground water and pore water. Groundwater has water that flows through fracture and that which is stagnant found in the rock matrix pores (Appelo & Postma 647).
Depending of the chemical content, bentonites can swell to several times its initial volume when it comes into contact with water. Calcium bentonite swells to a smaller extent as compared to sodium bentonite. It can absorb water up to five times its weight. At maximum saturation it occupies 12 to 15 times it ideal volume.
Energy reduction
It is very important to identify the points of energy wastage in the production of bentonite in order to be able to make changes that are aimed at efficient use of energy. Measurements aids in the quantification and comparison with set industrial standards. Time utilization, machine efficiency and production of the machine are often overlooked by the use although it is an important aspect in energy conservation (Anderson 649).
Increasing the differential of temperature is one of the ways of saving on energy. This may increase the inlet temperature conditions or the may further reduce the outlet or exhaust temperatures. During drying, bentonite takes up both sensible heat (in order to reach operational temperatures) and latent heat of vaporization. Water evaporating from the bentonite leaves behind the latent heat of thereby reducing on the energy of the mass.
Reduction of moisture content that reached the drier could be another way of saving energy. The reduction of this load allows for better utilization of energy available on the drying process. Mechanical dewatering uses only one percent of the energy required to evaporate the same amount of water thereby it a good energy saving practice. Another way could be the utilization of dry air which essentially reduces the amount of air that is in need of heating and vaporization.
Proper use of instrumentation and control is very important in trying to save energy. Resistance thermometers are available which are more accurate though expensive as compared to the thermocouple thermometers. Technical modifications can also be done on the driers to improve on their efficiency. For instance, direct heating leads to a reduction of up to 35% to 45% of steam that is used for the primary requirements of fuel.
Proper house keeping and maintenance could reduce a lot of heat losses. Energy that is lost to the atmosphere by radiation or leakages is avoided by this practice. Proper insulation and repairs on leakages with the recommended material will help in containing the heat which would have rather been lost. The insulation thickness that is recommended for kilns is important to maintain as this aids in restraining heat losses. Maintenance of utility lines ensures that each component is working optimally. Usually losses that occur in heat loss in kilns are unrecoverable and many usually lead to high maintenance costs (Anderson 648).
Bentonite can be mixed with other soils to get the desired soil properties. It can be shipped to other destinations when dry to save on costs anmd for easy hanling. A very small amount of bentonite is required in producing a high quality seal as it can swell many times to its original volumes thus creating a tight fix.
Work Cited
Anderson, Greg. Thermodynamics of Natural Systems, 2nd Edition. New York: Cambridge University Press, 2005. 647- 648. Print.
Appelo, CAJ & Postma, Dieke. Geochemistry, groundwater and pollution, 2nd edition. Leiden: Balkema Publishers, 2007. 640-649. Print.
Bolt, G. & Van, Riemsdijk. “Surface chemical processes in soil. In: Stumm, W. Aquatic Surface Chemistry.” Chemical Processes at the Particle-Water Interface. United States: John Wiley & Sons, Inc. 1987. 127–164. Print.
Van, Olphen. “An Introduction to clay colloid chemistry.” For Clay Technologists, Geologists, and Soil Scientists. New York: John Wiley & Sons, Inc. 1967. 300-301. Print.
Transport in Variably Saturated Geologic Media. Version 1.2.1. Berkeley: Earth Sciences Division, Lawrence Berkeley National Laboratory, University of California. 2008. 195.
