Perancangan Alat Pengaduk [PDF]

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Perancangan Alat Pengaduk



ALUR PROSES DI INDUSTRI KIMIA



Pengertian Pengadukan 



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Mixing merupakan suatu proses pergerakan fluida atau padatan untuk mendapatkan kondisi yang diharapkan baik komposisi maupun temperatur. Proses pencampuran banyak dipergunakan pada industri kimia seperti pencampuran antara gas dengan cairan, cairan – cairan, padatan dengan padatan, padatan dengan cairan Bidang – bidang yang mengaplikasikan pengadukan seperti pada industri cat, industri kosmetik, farmasi, industri kertas,dll



Tujuan Pengadukan   



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Blending of two miscible liquids, such as ethyl alcohol and water. Dissolving solids in liquids, such as salt in water. Dispersing a gas in a liquid as fine bubbles, such as oxygen from air in a suspension of microorganisms for fermentation or for the activated sludge process in waste treatment. Suspending of fine solid particles in a liquid, in the catalytic hydrogenation of a liquid, solid catalyst particles and hydrogen bubbles are dispersed in the liquid. Agitation of the fluid to increase heat transfer between the fluid and a coil or jacket in the vessel wall.



Equipment for Agitation 



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Three-blade propeller agitator Paddle agitators Turbine agitators Helical-ribbon agitators



1. Three-blade propeller agitator



FIGURE 3.4-1. Baffled tank and three-blade propeller agitator with axial-flow pattern: (a) side view, (b) bottom view



2. Paddle Agitators  



often used at low speeds, between about 20 and 200 rpm. Two-bladed and four-bladed flat paddles are often used, as shown in Fig. 3.4-2a.



FIGURE 3.4-2. Various types of agitators: (a) four-blade paddle, (b) gate or anchor paddle, (c) six-blade open turbine, (d) pitched-blade (45 ) turbine.



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At low speeds mild agitation is obtained in an unbaffled vessel. At higher speeds, baffles are used -since, without baffles, the liquid is simply swirled around with little actual mixing. The paddle agitator is ineffective for suspending solids -since good radial flow is present but little vertical or axial flow. An anchor or gate paddle, shown in Fig. 3.4-2b, is often used. It is used with viscous liquids where deposits on walls can occur and to improve heat transfer to the walls. However, it is a poor mixer. Paddle agitators are often used to process starch pastes, paints, adhesives, and cosmetics.



3. Turbine Agitator 



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Turbines resemble of multibladed paddle agitators with shorter blades used at high speeds for liquids with a very wide range of viscosities. The turbines usually have four or six blades. Figure 3.4-3 shows a flat six-blade turbine agitator with disk. They are also useful for good gas dispersion; - the gas is introduced just below the impeller at its axis and is drawn up to the blades and chopped into fine bubbles. Often a pitched-blade turbine with only four blades is used in suspension of solids.



Figure 3.4-3. Baffled tank with six-blade turbine agitator with disk showing flow patterns: (a) side view, (b) bottom view, (c) dimensions of turbine and tank.



4. Helical-Ribbon Agitator 



used in highly viscous solutions  operates at a low rpm in the laminar region.  The ribbon is formed in a helical path and is attached to a central shaft.  The liquid moves in a tortuous flow path down the center and up along the sides in a twisting motion.  Similar types are the double-helical-ribbon agitator shown in Fig. 3.4-4b and the helical-screw impeller shown in Fig. 3.4-4c.



FIGURE 3.4-4. Other types of agitators: (a) highefficiency, three-blade impeller (b) double-helical-ribbon, (c) helical-screw.



BENTUK ALIRAN



Agitated Vessel Power Used in Agitated Vessel • In the design of an agitated vessel, an important factor is the power required to drive the impeller. • Empirical correlations have been developed to predict the powerrequired. '  N Re



Where:



Da2 N







---- Eq. (3.4-1)



Da = impeller (agitator) diameter (m) N = rotational speed (rev/s) ρ = fluid density (kg/m3) µ = fluid viscosity (kg/m.s) Reynolds



number,



N'



is defined as:



consumption is related to fluid density ρ, fluid viscosity µ, rotational speed N and impeller diameter Da ' N by plots of power number N p versus Re The power number is defined as: Power



P Np  N 3 Da5 (SI)



----------------- Eq. (3.4-2)



Pgc (English) Np  N 3 Da5 Where P = power (J/s) or (W). In English units, P = ft.lbf/s.



Heat Transfer in Agitated Vessel  



Often it is necessary to cool or heat the contents of the vessel during agitation. This is usually done by heat-transfer surfaces, which may be in the form of: 1) cooling or heating jackets in the wall of the vessel 2) coils of pipe immersed in the liquid.



Vessel with heating jacket   



When heating, the fluid entering is often steam, which condenses inside the jacket and leaves at the bottom. The vessel is equipped with an agitator and in most cases also with baffles. Correlations for the heat-transfer coefficient from the agitated Newtonian liquid inside the vessel to the jacket walls of the vessel have the following form:



Latihan Kelompok 



Suatu cairan metanol akan dipanaskan dalam suatu tangki pengaduk dari temperatur 30oC menjadi temperatur 60oC. Pengaduk yang dipergunakan 6BD dengan diameter impeller 30” serta kecepatan pengadukan 200 rpm. Tangki yang dipergunakan mempunyai volume 7,5 m3 dengan diamater tangki 2 m. Adapun pemanas di dinding dengan ketebalan pelat dinding 5 mm dengan bahan pelat SS serta pemanas steam pada temperatur 110oC. Berapakah waktu yang diperlukan dalam pemanasan di dalam tangki tersebut?