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Steady-State Molecular Diffusion

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Steady-State Molecular Diffusion Steady-State Molecular Diffusion This part is an application to the general differential equation of mass transfer. The objective is to solve the differential equation of mass transfer under steady state conditions at different conditions (chemical reaction, one dimensional or more etc.). Two approaches will be used to simplify the differential equations of mass transfer: 1. Fick’s equation and the general differential equation for mass transfer can be simplified by eliminating the terms that do not apply to the physical situation. 2. A material balance can be performed on a differential volume element of the control volume for mass transfer. One dimensional mass transfer independent of chemical reaction The diffusion coefficient or mass diffusivity for a gas may be experimentally measured in an Arnold diffusion cell. This cell is illustrated schematically in Figure 1. The narrow tube, which is partially filled with pure liquid A, is maintained at a constant temperature and pressure. Gas B, which flows across the open end of the tube, has a negligible solubility in liquid A and is also chemically inert to A. Component A vaporizes and diffuses into the gas phase; the rate of vaporization may be physically measured and may also be mathematically expressed in terms of the molar mass flux. Fig. 1, Arnold diffusion cell 1 Required: to get the flux and concentration profile for this system. Assumptions: 1. Steady state conditions 2. Unidirectional mass transfer in z direction 3. No chemical reaction occur The general differential equation for mass transfer is given by: 휕푐 ∇. 푁⃗⃗ + 퐴 − 푅 = 0 퐴 휕푡 퐴 In rectangular coordinates this equation is: 휕 휕 휕 휕푐 푁 + 푁 + 푁 + 퐴 − 푅 = 0 휕푥 퐴,푥 휕푦 퐴,푦 휕푧 퐴,푧 휕푡 퐴 Apply the above assumptions to this equation: 푑 ∴ 푁 = 0 푑푧 퐴,푧 It means that the molar flux of A is constant over the entire diffusion path from 푧1 to 푧2. The molar flux is defined by the equation: 푑푦 푁 = −푐퐷 퐴 + 푦 (푁 + 푁 ) 퐴 퐴퐵 푑푧 퐴 퐴 퐵 According to the conditions of the system (B is insoluble and chemically inert to A) ∴ 푁퐵 = 0 푑푦 푁 = −푐퐷 퐴 + 푦 푁 퐴 퐴퐵 푑푧 퐴 퐴 푐퐷퐴퐵 푑푦퐴 푁퐴 = − 1 − 푦퐴 푑푧 To get the flux 푁퐴 the above equation has to be integrated between 푧1 and 푧2 by using the boundary conditions: 2 at 푧 = 푧1 푦퐴 = 푦퐴1 at 푧 = 푧2 푦퐴 = 푦퐴2 푧2 푦퐴2 푑푦 푁 ∫ 푑푧 = 푐퐷 ∫ − 퐴 퐴 퐴퐵 1 − 푦 푧1 푦퐴1 퐴 푐퐷퐴퐵 (1 − 푦퐴2) 푁퐴 = 푙푛 (1) 푧2 − 푧1 (1 − 푦퐴1) The above equation (equation 1) is commonly referred to as equations for steady-state diffusion of one gas through a second non diffusing gas or stagnant gas. Absorption and humidification are typical operations defined by this two equation. Some important notes: Concentration for gas phase: 푝 푝 Total concentration: 푐 = Concentration of A: 푐 = 퐴 푅푇 퐴 푅푇 푝 푦 = 퐴 퐴 푝 Example 1: A tank with its top open to the atmosphere contains liquid methanol (CH3OH, molecular weight 32g/mol) at the bottom of the tank. The tank is maintained at 30 °C. The diameter of the cylindrical tank is 1.0 m, the total height of the tank is 3.0 m, and the liquid level at the bottom of the tank is maintained at 0.5 m. The gas space inside the tank is stagnant and the CH3OH vapors are immediately dispersed once they exit the tank. At 30 °C, the vapor pressure exerted by liquid CH3OH is 163 mmHg and at 40 °C the CH3OH vapor pressure is 265 mmHg. We are concerned that this open tank may be emitting a considerable amount of CH3OH vapor. a. What is the emission rate of CH3OH vapor from the tank in units of kg CH3OH /day when the tank is at a temperature of 30 °C? State all assumptions and boundary conditions. b. If the temperature of the tank is raised to 40 °C, what is the new methanol emission rate? (or calculate the % increase in the rate of emission for 10 °C increasing in temperature 3 Solution: Assume methanol is A and air is B Basic assumptions: 1. Steady state conditions 2. One dimensional mass transfer in z direction 3. B insoluble in A (푁퐵 = 0) 4. No chemical reaction occurs The mass flow rate = mass flux × area 휋 푎푟푒푎 = 푑2 = 0.785 푚2 4 The mass flux = 푚표푙푒 푓푙푢푥 × 푀. 푤푡 = 푁퐴 × 푀. 푤푡 = 푁퐴 × 32 푑푦 푁 = −푐퐷 퐴 + 푦 (푁 + 푁 ) 퐴 퐴퐵 푑푧 퐴 퐴 퐵 According to the conditions of the system (B is insoluble and chemically inert to A) ∴ 푁퐵 = 0 푑푦 푁 = −푐퐷 퐴 + 푦 푁 퐴 퐴퐵 푑푧 퐴 퐴 푐퐷퐴퐵 푑푦퐴 푁퐴 = − 1 − 푦퐴 푑푧 4 푐퐷퐴퐵 (1 − 푦퐴2) 푁퐴 = 푙푛 푧2 − 푧1 (1 − 푦퐴1) 푝 1 푐 = = = 4.023 × 10−2 푚표푙/퐿 = 40.23 푚표푙/푚3 푅푇 (0.082)(303) Length of the diffusing path: 푧2 − 푧1 = 3 − 0.5 = 2.5 푚 푝 163 푦 = 퐴 = = 0.2145 퐴1 푝 760 Since the CH3OH vapors are immediately dispersed once they exit the tank ∴ 푦퐴2 = 0 The diffusion coefficient 퐷퐴퐵 is obtained from the Hirschfelder equation: 1 1 1/2 0.001858 푇3⁄2 [ + ] 푀퐴 푀퐵 퐷퐴퐵 = 2 푃 휎퐴퐵 훺퐷 From table (6) Component 휎 휀/푘 Air 3.617 97 methanol 3.538 507 흈 + 흈 흈 = 푨 푩 = ퟑ. ퟔퟎퟏ 퐀̇ 푨푩 ퟐ 휀 Ω is a function of 퐴퐵 퐷 푘 휺 휺 휺 푨푩 = √ 푨 푩 풌 풌 풌 휺 푨 = √ (ퟗퟕ)(ퟓퟎퟕ) = ퟐퟐퟏ. ퟕퟔퟑퟒ 푲 풌 풌푻 ퟑퟎퟑ = = ퟏ. ퟑퟔퟔ 휺푨푩 ퟐퟐퟏ. ퟕퟔퟑퟒ 5 훀푫 = ퟏ. ퟐퟓퟑ 풆풓품풔 (풇풓풐풎 풕풂풃풍풆 ퟕ) Substitute in Hirschfelder equation: 푐푚2 푚2 ∴ 퐷 = 1.66 = 1.66 × 10−4 퐴퐵 푠 푠 −4 푐퐷퐴퐵 (1 − 푦퐴2) 40.23 × 1.66 × 10 1 푁퐴 = 푙푛 = 푙푛 [ ] 푧2 − 푧1 (1 − 푦퐴1) 2.5 (1 − 0.2145) −4 2 푁퐴 = 6.45 × 10 푚표푙/푚 ∙ 푠 The mass flux = 6.45 × 10−4 × 32 = 2.064 × 10−2 푔/푚2 ∙ 푠 푔 푔 The mass flow rate = 2.064 × 10−2 × 0.785 푚2 = 1.62 × 10−2 = 1.39 푘푔/푑푎푦 푚2∙푠 푠 For part b: The same procedure but the diffusion coefficient will be calculated at 313 K We can use the following equation or Hirschfelder equation 3/2 푃1 푇2 Ω퐷,푇1 퐷퐴퐵,푇2,푃2 = 퐷퐴퐵,푇1,푃1 ( ) ( ) 푃2 푇1 Ω퐷,푇2 In part a also we can use this equation and get the value of 퐷퐴퐵 at 298 K from table 3 and correct it at 313 K. Answer for part b is 2.6 푘푔/푑푎푦 Equimolar Counter diffusion This type of mass transfer operations is encountered in the distillation of two constituents whose molar latent heats of vaporization are essentially equal. The flux of one gaseous component is equal to but acting in the opposite direction from the other gaseous component; that is, 푁퐴 = −푁퐵 6 For the case of one dimensional, steady state mass transfer without homogeneous chemical reaction, the general equation: 휕푐 ∇. 푁⃗⃗ + 퐴 − 푅 = 0 퐴 휕푡 퐴 is reduced to: 푑 푁 = 0 푑푧 퐴,푧 The above equation specifies that 푁퐴 is constant over the diffusion path in the z direction. Since: 푑푦 푁 = −푐퐷 퐴 + 푦 (푁 + 푁 ) 퐴 퐴퐵 푑푧 퐴 퐴 퐵 푑푦 ∴ 푁 = −푐퐷 퐴 퐴 퐴퐵 푑푧 For constant temperature and pressure system 푑푐 ∴ 푁 = −퐷 퐴 퐴 퐴퐵 푑푧 The above equation can be integrated using the boundary conditions at 푧 = 푧1 푐퐴 = 푐퐴1 at 푧 = 푧2 푐퐴 = 푐퐴2 푧2 푐퐴2 푁퐴 ∫ 푑푧 = −퐷퐴퐵 ∫ 푑푐퐴 푧1 푐퐴1 퐷퐴퐵 푁퐴 = (푐퐴1 − 푐퐴2) (2) 푧2 − 푧1 For ideal gas system, the above equation is written in the following form: 퐷퐴퐵 푁퐴 = (푝퐴1 − 푝퐴2) (3) 푅푇(푧2 − 푧1) 7 Equations 2 and 3 are referred as the equations for steady state equimolar counter diffusion without homogenous chemical reaction. Equation 2 is used for diffusion in liquid and equation 3 is used for diffusion in gas. The two above equations may be used to describe any process where the bulk contribution term is zero (as equimolar counter diffusion, diffusion of solute through a solid, diffusion through stationary liquid). Concentration profile The concentration profile for equimolar counterdiffusion processes may be obtained by substituting equation 푑푐 푁 = −퐷 퐴 퐴 퐴퐵 푑푧 into the differential equation which describes transfer in the z direction 푑 푁 = 0 푑푧 퐴,푧 푑 푑푐 ∴ (−퐷 퐴) = 0 푑푧 퐴퐵 푑푧 푑2푐 퐴 = 0 푑푧2 The above equation may be integrated to yield: 푐퐴 = 퐶1푧 + 퐶2 The two constants of integration are evaluated, using the boundary conditions: at 푧 = 푧1 푐퐴 = 푐퐴1 at 푧 = 푧2 푐퐴 = 푐퐴2 8 Example 2: Consider the process shown in the figure below. A slab contains parallel linear channels running through a nonporous slab of thickness 2.0 cm. The gas space over the slab contains a mixture of A and B maintained at a constant composition. Gas phase species A diffuses down a straight, 1.0-mm-diameter channel. At the base of the slab is a catalytic surface that promotes the isomerization reaction A(g) → B(g). This reaction occurs very rapidly so that the production of B is diffusion limited. The quiescent gas space in the channel consists of only species A and B. The process is isothermal at 100 °C and isobaric at 2.0 atm total system pressure. The bulk composition over the slab is maintained at 40 mol% A and 60 mol% B. The molecular weight of species A and its isomer B is 58 g/mol.
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