AN EFFECT OF BLADE GEOMETRY ON HEAT TRANSFER PERFORMANCE IN STIRRED VESSEL COAL WATER SLURRY SYSTEM USING COAL GASIFICATION

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1 AN EFFECT OF BLADE GEOMETRY ON HEAT TRANSFER PERFORMANCE IN STIRRED VESSEL COAL WATER SLURRY SYSTEM USING COAL GASIFICATION C.M.Raguraman 1, A. Ragupathy 2, R. Ramkumar 3, L. Sivakumar 4 1,2 and 3 Department of Mechanical Engineering, Annamalai University, Annamalai Nagar , Tamil Nadu, India 4 Formerly General Manager, (Corporate R&D), BHEL, Hyderabad, presently vice principal, Sri Krishna College of Engineering and Technology, Coimbatore, India. Abstract The effect of the geometrical parameter of blades on heat transfer co-efficient were experimentally studied for agitated vessels using coal slurry in coal gasification. The intensity of hear transfer during mixing of fluids depends on the type of the stirrer, the design of the vessel and conditions of the process. The type and size of the stirrer, as well as its location in the vessel, also affect the rate of hear transfer. In this study, the effect of some important design parameter such as the type of stirrer, angle and shape of blades, number of blades and location of stirrer, etc., were investigated and optimized. Besides, the Taguchi method can successfully be applied to heat transfer investigation to save energy, time and material in experimentation. Keywords: Heat transfer rate, Coal slurry, Taguchi method, Heat transfer coefficient. 1. Introduction Agitated jacketed vessels are generally used for processing liquid system and are suitable for carrying out reaction at isothermal condition where heat of reaction is high. Generally, heating or cooling is effected by jacketing the vessel [1]. Mechanically agitated reactors are widely used in the mining, food, petroleum, chemical, pharmaceutical, pulp, paper industries, and also using in power generation industries. In many process industries, mixing and heat transfer in stirred vessels are an important operation in both batch and continuous process.[1]. Heat transfer in stirred vessels is important because fluid temperature in the reactor is one of the most significant factors for controlling the outcome of process. The proper design of stirrer is important in obtaining satisfactory mixing results. Adequate stirring assures good mixing by providing a flow pattern that carries through out the entire batch. The type and size of the stirrer, as well as its location into vessels also affect the rate of heat transfer. Generally mechanical stirrers are used for homogenous suspension. The degree of mixing depends to a large extent, upon the design parameters of the vessel and the stirrers apart from the particle characteristics (size distribution shape and density) liquid properties (density, viscosity, surface tension) and process parameters as thy affect rheology and flow behavior of the slurry [2]. Researchers have mainly attempted to establish some correlations between the power number, Reynolds number, the geometry of the vessel and stirrer and the physical properties of the liquid and the particle [3-4]. The intensity of heat transfer during mixing of fluids like slurry depends on the type of the stirrer, the design of the vessel and condition of the processes. In this study the effect of some important parameter such as type of stirrer, angle and shape of the blades, number of blades, and location of the stirrer etc., were investigated and optimized [5]. ISSN:

2 In this work, the effect of four variables namely stirrer speed, stirrer location, D/d ratio and concentration of slurry on heat transfer were analyzed. Each of the above variables will be kept at three different levels and the level at which the maximum heat transfer rate achieved were identified using Taguchi method. Also in this work the fluid as a slurry is water with coal particle. Heat transfer rates in agitated vessels have been investigated for coal slurry in a flat bottom vessel equipped with of flat type agitation. 2. Experimental details The experimental setup consists of a cylindrical vessel, impeller, Variable speed DC motor, auto transformer as shown schematically in Fig.1. Fig.1. Experimental Setup The experimental setup consists of a cylindrical jacketed vessel, flat blade impellor with shaft, variable speed motor, variac, temperature indicator and pressure gauge. The jacket is on the out side the mixing vessel. The inlet of the jacket is connected to an electrically operated boiler, which produces steam continuously at constant pressure. A pressure gauge is connected to the jacket for monitoring the jacket steam pressure. A vent is provided for releasing non-condensable gas as well as maintains the pressure in jacket, A steam strainer is placed at the bottom to the vessel to collect the condensed steam. An agitator that is driven by a vertically mounted motor is used for stirring. The entire vessel is completely insulated. When the temperature reaches the saturation temperature of the water (100 deg. C) the time period (τ sec) was noted and this period is called heating period (Unsteady state). The cylindrical outside vessel of 210mm diameter, 6mm thick, and 160mm height with a flat bottom was used which is named as jacketed vessel. Similarly the cylindrical inside vessel of 110mm diameter, 6mm thick and 110mm height with same flat bottom was used. So the height of vessel is equal to diameter of the vessel (H=D)[6]. The different types of impeller used in process industries as shown in fig. 2. ISSN:

3 2.1 Types of stirrers Pitched blade turbine impeller Rushton turbine impeller Pfaudler Impeller Flat blade impeller Plate paddles impeller Smith turbine impeller Anchor impeller Propeller impeller Fig. 2 Types of stirrer Vessel: Mild steel vessel, cylindrical shape, flat bottom Size: 0.21m, diameter x 0.160m height x m thick, Provision for sampling two side outlets at bottom side for taking out the solids and liquid after each set of experiments. Stirrer: Driven by motor. Speed control by variac (0-220), speed measured by tachometer. Type and shape of blade four flat blade schematically shown in fig 3. Fig.3. Flat blade details Angle of blade : parallel to axis of the shaft, D Diameter of Vessel, d Diameter of impeller, No. of blades = 4, blade thickness=.1.6mm, Width of the impeller = 17.5mm, Location of impeller: 25%, 50% and 75% of height of vessel. 3. Coal - water slurry Coal gasification is a key processes for integrated and coal gasification combined cycle(igcc power plant. Being one of the most competitive and promising coal gasification technologies, the slurry feed type entrained flow coal gasification process has been extensively studied in the foreign countries. Coal water slurry (CWS) is a mixture of water and coal. First pulverize coal mix with water according to proper proportion. (Coal : water= (40 to 70) : ( 60 to 30) and their add a little additive ( about 1% of the total soiled weight), the coal water two phase flow is formed after strong agitation. In an entrained flow gasifer using coal-water slurry, it should be necessary to uniformly mix the coal water slurry with oxygen to obtain the higher carbon conversion in short residence time (0.4 5 s). The proximate and ultimate analysis of the coal used in the preparation of the slurries are given in table1. ISSN:

4 Table 1. Coal analysis details Proximate analysis Weight basis in % Moisture 6.64% Ash 48.71% Volatile matter 19.12% Fixed Carbon 25.53% Gross calorific value in Kcal/Kg Ultimate analysis Weight basis in % Carbon as C 34.53% Hydrogen as H 1.81% Nitrogen as N 1.05% Sulphur as S 0.47% Oxygen by difference 1.92% Fig. 4 Coal gasification system [8] Additive is the main factor of making CWS, which is also one of the key factors affecting CWS quality. Usually additive dosage accounts for about 1% of the total. Based on function, additives can be used like dispersant and stabilizing agent.. In this project, sodium carbonate and sodium salt of carboxymethyl cellulose (Na-CMC) is used respectively. The best dosage of dispersant and stabilizers is 0.75% by wt. of solids and 0.1% by wt. from total solids [9]. The concentration of coal slurry could be maintained from 40% to 70% which is enabling to feed into the gasification without any feeding problem [7]. So that the concentration chosen are 40%, 50% and 60% and the coal particle size is BSS 36(0.422mm) which is using for coal water slurry. ISSN:

5 4. Design of Experiment using Taguchi method C.M.Raguraman.et. al. / International Journal of Engineering Science and Technology The experiments were designed based on the orthogonal array technique. An orthogonal array is fractional factorial design with pair wise balancing property. Using orthogonal array design the effects of multiple process variables on the performance characteristic can be estimated simultaneously while minimizing the number of test runs. Taguchi s parameter design method is a powerful tool for optimizing the performance characteristic of a process [11-12]. The aim of a parameter design experiment is to identify and design the settings of the process parameters that optimize the chosen quality characteristic and are least sensitive to noise (uncontrollable) factors. The selection of control factors and their levels are made on the basis of some preliminary trial experiments conducted in the laboratory and also from literature review o the subject. Four control factors such as speed, location, D/d ratio and concentration of the coal slurry are selected for this study. Each of the four control factors is treated at three levels, as shown in table 2. The choice of three levels has been made because the effect of these factors on the performance characteristic may vary nonlinearly. An L9 standard orthogonal array as shown in table was employed for the present investigation Selection of variables and levels: The variables and corresponding levels which mainly affect the heart transfer rate in jacketed vessel are shown in table 2. Table 2. Variables and levels Sl.No Variables Levels I II III 1 Stirrer speed in rpm Stirrer location from free surface in cm D/d ratio Slurry concentration in % Table 3. Experimental arrays S.No Stirrer speed in rpm Stirrer location from free surface in mm D/d ratio Slurry concentration by mass in % The Taguchi approach can be utilized to arrive at the best parameter for the near optimum design configuration with the least number of analytical investigation [10]. An L9 standard orthogonal array was employed for the present investigation. But as per requirements, L7 is sufficient, but since there is no interaction affect in that array, L9 is selected. The main variables A, B and C are assigned to columns 1, 2 and 3 respectively. All other columns are kept dummy. As per L9 orthogonal array, the experimental arrays are shown in table 3. ISSN:

6 4.2 Steps in Taguchi Analysis After conducting the experiments, the following are the steps in analysis of results: Evaluation of S/N ratio and Mean at different levels of variables Formulation of ANOVA table Interpretation by graph Identification of Significant & Control variables Optimization of parameters 5. Result and discussion Experiments were carried out and the characteristics curves were drawn. Four curves are shown in fig. 9. It is clear that all the variables namely stirrer speed, location, D/d ratio and concentration influence the overall heat transfer co-efficient almost equal, but the speed of the stirrers has an edge over other three variables. The overall heat transfer co-efficient has a decreasing trend with increasing the stirrer diameter. But next to stirrer speed, the most affecting variables is D/d ratio for over all heat transfer co-efficient ISSN:

7 Effect of S/N ratio on overall heat transfer co-efficient 1 100rpm, 2 200rpm, 3 300rpm Level of Design Variable (Speed) Fig 5 Effect of different level on overall heat transfer co-efficient for speed Effect of S/N ratio on overall heat transfer co-efficient mm, mm, mm. Level of Design Variable (Location) Fig.6.Effect of different level on overall heat transfer co-efficient for location Effect of S/N ratio on overall heat transfer co-efficient 1 4.0, 2 2.0, Effect of S/N ratio on overall heat transfer co-efficient 1 40%, 2 50%, 3 60% Level of Design Variable (D/d ratio) Fig.7. Effect of different level on overall heat transfer co-efficient for D/d ratio Level of Design Variable (Concentration) Fig 8. Effect of different level on overall heat transfer co-efficient for concentration. Effect of S/N ratio on overall heat transfer co-efficient 1 Speed. 2 Location. 3 D/d ratio, 4 Concentration Level of Design Variable Fig.9. Effect of different level on overall heat transfer co-efficient Fig.10. Percentage of contribution on each design variables Overall heat transfer co-efficient. ISSN:

8 Table 4. Optimum working conditions for flat blade Variables Optimum level Optimum conditions Optimum value From the table 4, the maximum value of overall heat transfer co-efficient is obtained when the stirrer speed is 300rpm, location is 55.0mm, D/d ratio is 4.0 and slurry concentration is 50%. Finally the experimented value for overall heat transfer co-efficient optimum level was found for this type of impeller as W/m 2 K. 6. Conclusion Stirrer speed in rpm Stirrer location from free surface in mm D/d ratio Slurry Concentration 2 50 The purpose of the investigation is to determine the overall heat transfer co-efficient for stream augmented flows in agitated vessels with flat blade impeller. The rate of heat transfer is influenced by a number of physical and geometrical factors such as vessel configuration, impeller type and process fluid like slurry. The reliable selection, design configuration and specifications of such equipment are based on the ability to predict the behavior of the stirred material. The optimum condition for heat transfer from the surface of coal slurry were determined. The major conclusions derived from the present work are a) It is clear that all the four variable have almost equal effect on overall heat transfer co-efficient, vessel side heat transfer co-efficient of coal slurry but the speed has an edge over the other variables in this types of impeller. b) The higher agitation rate increases the rate of heat transfer. c) The work can be extended with some more types of stirrers like angled blade impeller, helical impeller, trapezoidal type impeller etc., and also various slurries like char coal with water can be tested to get the effect of material properties on heat transfer co-efficient. d) The jacketed vessel was heated by waste steam from steam turbine. So the waste heat energy also utilized. e) The Taguchi method can successfully be applied to heat transfer investigations to save energy, time and material in experimentation. 7. Reference [1] C.L.Rai, I.Devotta, P.G.Rao, Heat transfer to viscous Newtonian and non-newtonian fluids using helical ribbon agitator, Chemical Engineering journal 79: 73-77, [2] P.K.Biswas, S.C.Dev, K.M.Godiwalla, C.S.Sivaramakrishnan,Effect of some design parameters on the suspensions characteristics of a mechanically agitated sand-water slurry system, Materials and design,20: , [3] Gray JB, Oldshue JY, Mixing theory and practice, Vol.3, Academic press, Inc., 1986:16 [4] Halland Fa, Chapman FS, Liquid mixing and processing in stirred tank, Reinhold Publ. Corp [5] Joanna karcz, Fryderyk strek, Heat transfer in jacketed agitated vessels equipped with non-standard Bafles The chemical Engineering journal,58: , [6] Joanna karcz, Marta major, An effect of a Baffle length on the power consumption in an Agitated vessel, Chemical Engineering and Processing, 37: ,1998. [7] T. J. Park, J. H. Kim, J. G. Lee, J. C. Hong, Y. K. Kim, Y. C. Choi, Experimental studies on the characteristics of Entrained flow coal gasifier,energy conversion research department, korea Institute of energy research, Taejon, Koriea. [8] Y.C.Choi, X.Y.Li, T.J.Park, J.H. Kim, J.G. Lee, Numerical study on the coal gasification characteristics in an entrained flow coal gasifier, Fuel, 80: , [9] Eisa S.Mosa, Abdel-hady M. Salch, Effect of chemical additives on flow characterstics of coal slurries, Physicochemical problem of mineral processing, 42: , [10] Prabir Kumar Chaulia, Reeta Das, Process parameter Optimization for Fly Ash Brick by Taguchi Method, Materials Research, vol.11, No.2, , [11] Ealey Lance A., Quality by design Taguchi method and US industry, 2 nd edition. Sidney: Irwin professional publish in and ASI Press: 1994, P [12] Montgomery DC. Design and analysis of experiments. 4 th edition. New York: John wiley publication, ISSN: