GAS MIXING DESIGN PROGRAM INSTRUCTIONS The Gas Mixing program is on disk with the name GASMIXER.EXE. The other files on the disk called GAS3.PIC and GAS3.INP are also necessary for the program to operate. The file with the extension .PIC are a file of screens used by the program that are read into the high memory space of your computer during the initiation stage of the program. They must be present on the disk or in the subdirectory of the computer that you use to initiate the operation of this program. The Gas Mixer program is started by inserting the disk into you A: drive after you have booted up your computer. Give the command at the DOS prompt A:> GASMIXER and the program will begin. The program should preferably be operated from a subdirectory on your computers hard disk to maximize its preformance. M A I N M E N U The Main menu for the Gas Mixing program will come on and present you with alternate selections, to operate the program. A new user may wish to start by loading a file from the disk by using the option 6 below. The Disk related functions (Options 5,6,and 7) are identical to the descriptions given in the Blending Program, except that, the extension .GAS is added to all saved data files instead of .MIX. Option 8 (New Case) will blank out all data in memory and permit a new case to be entered from scratch. Option E will exit the program. You will be asked to confirm your decision to quit to guard against accidental loss of data due to key stroke error. The Vessel Program (Option 1) is also identical to the program provided with the Blending program and will not be described in the following text. The Liquid data is also essentially identical to the program provided with the Blending program and will not be described; except that provision was made to also input Surface Tension data. Surface tension data is needed for gas bubble and hold up correlations. New data must be entered in the following order. The Vessel data must be entered first, followed by the Liquid properties then the Gas properties, and finally the Agitator data. G A S P R O G R A M The gas input program is selected by selecting Option 3 from the Main Menu. A data value must be entered for each data point requested by the program. Enter all data before F10 Key to terminate input and calculate the gas property and bubbler results. The following data are required as input. INPUT DATA Temperature Deg F Henry's Law constant ( Atm/Lb-Mole Ft3) An optional window is provided in the program to calculate the Henry law constants from gas solubility data by pressing the F2 key. Compressibility Factor usually 1.0 Diffusivity of gas in solvent ( cm2/sec) Press the F1 key to calculate this if unknown. It is calculated by the method of Wilke and Chang as describe in section 14 of Perry's 4th Edition and in considerable detail in page 95 of Multiphase Chemical Reactor's -- Giannetto & Silveston. Psia - Outlet- This is the operating pressure in the upper head, vapor space. The pressure at the bottom sparger outlet will be calculated from the liquid head in the vessel. The bottom sparger is assumed to be located at the bottom vessel tangent line. Mole % Reactant in the gas at the inlet (ie Sparger Outlet) and at the Outlet (upper head - vapor space) Lb-Moles/Hr (in and out) This is the total gas rate to the reactor. The sum of the gas reactants and inerts. Molecular Weight of the gas in and out of the reactor. CALCULATED GAS PROPERTIES After the F10 key is pressed. The program will proceed to calculate the Inlet, Outlet and Log Mean Average values for the gas properties as follows: Lbs of Gas, Standard Cubic Feet/ Minute of Gas, Actual Cubic Feet/Minute of Gas, Partial Pressure of the Reactant (psia), Superficial Velocity of the gas in feet per second. ( This is the Actual volume of the gas divided by the vessel cross sectional area) , and finally the Gas energy transferred to the liquid by the expanding gas in terms of BHP/ 1000 gallons. BUBBLER RESULTS The program calculates the key parameters of a bubbler reactor from the liquid, gas and vessel properties. A bubbler reactor is a system without an agitator. All mixing is performed by the rising gas bubbles. The gas is assumed to be evenly dispersed across the vessel cross section,and the Vessel is assumed to be vertical. Hold Up Fraction -- The gas hold up fraction is calculated for the Air Water system and is based upon a curve fit of the data presented in Akita and Yoshita -- Jan 65 AIChE Journal. This method can be changed to the Method proposed by Hughmark that incorporates the surface tension and specific gravity of the liquid into the Holdup correlation. In either case the Hold Up fraction is principally a function of the gas superficial velocity. Bubble Diameter -- The bubble diameter is based upon an assumed size of 1/8 of and inch for the air/water system as recommended by Fair. This bubble size is then corrected for the specific gravity and surface tension of the liquid according to Calderbank's equation. Db = (0.125/12)*(Surf.Ten/72)^0.6*(SpGravity)^-0.2 The program will however not calculate or use a value less than 1/8 inch unless you override it. The size of the bubbles has a major effect on the calculated Klas and other data. The user can override the size of the Sauter mean Bubble diameter calculated by the program from the command line. I do not suggest doing this unless you have experimental data. Bubble sizes of less than 1/8 inch are not likely with most systems. You may however wish to make changes to this value to determine the sensitivity of your design to the bubble size, or to increase it to a higher value. Liquid Height -- The Liquid height is calculated in inches above the bottom vessel seam for both the clear liquid height and the gassed volume. You should check that your have provided enough free volume in your vessel design to separate the gas. Flow Regime -- This will be Quiescent if the average superficial velocity is less than 0.2 ft/ sec. Otherwise it will be turbulent. If the flow reqime is turbulent then the correlations used in the bubbler reactor models have been extended beyond the experimental data in the literature and may not be accurate. There is a higher level of risk in designing in this region. Interfacial Area (Ft2/Ft3) This is the total interfacial available for mass transfer per cu. ft of volume. The calculation of the interfacial area is given by the following formula: Area = 12.0*6*HoldUp/Bubble Diamter MASS TRANSFER COEFFICIENTS The program calculates the Kga, Kla, rate of gas reaction per unit volume and total LbMoles of gas reacted per hour for both the Air Sulfite System and for the gas and liquid properties that describe your system. Essentially all the data presented for bubbler reactors in the literature is based upon the Air Sulfite system. This is also true for the agitated reactor correlations. Kla Air-Water This is calculated by the procedures recommended in Fair's Article which in turn is based upon the Froessling Two-Film model. the equation is: Kla = (12*Diff*Holdup)/(Bubble Diam)^2*[1+0.276*Nre^0.5*Scl^0.33] Where Nre = Bubble Reynolds Number Scl = visc liquid / (diffusivity * liq density) This correlation was checked against the data presented in Akita and Yoshita and it agrees over a wide data range. The Kla for the specific system are calculated from the Kla of air and water by multiplying the value by: [ Diff System / Diff Air water ] ^ 0.5 The values for Kga are calculated from the respective Kla's by with values for the reactant average partial pressure and Henry law constant. Kga Air/Water = Kla / 1.115*10^4 Na Air/Water = Kga * 0.12(ave pp atm)*1*10^-4 Moles Air Reacted = Na * Liquid Volume in Ft3 For the Specified System Na = Kla(system) * (PPave in Atm) / Henry Constant Kga(system) = Na(system)*10^4 / (PPave in atm) The following literature articles were used in the development of this portion of the program. J.R. Fair Chemical Engineering July 3 and July 17 1967 G.A. Hughmark I&EC Process Design and Development 1966 Akita and Yoshida AIChE Journal Jan 1965 Shah and Deckwer AIChE Journal May 1982 Tilton and Russell Chemical Engineering 1982 The interested user should obtain copies of these article to fully understand the theory of bubble reactor design. Shah and Deckwers survey article lists all the major theory in this area. A G I T A T O R P R O G R A M The Agitator design program is addressed from the Main Menu after the Gas data has been entered. This program allows you to calculate the improvement of the Mass transfer coefficients by the use of an agitator. The program is based upon the assumption that only Rushton type ie Flat Blade disk turbines are used for the bottom turbine. They are generally considered the most effective turbine for gas dispersion. The upper turbines in the reactor can be of different types. You will have the option of using flat blade, 45 degree axial, hydrofoil or propeller types for the upper agitators. AGITATOR BHP The program data entry begins by requesting an input value for the gassed BHP/1000 gallons to the lower agitator. A window pops up that will recommend a value based upon the gas rate specified in the previous bubbler design program. The Bhp/1000 gallons must generally exceed the gas Bhp energy provided by the expanding gas in order to be effective in dispersing the gas. The lowest effective value is the Flooding number. The program will generally select a value in the Moderate range. The BHP/1000 gallons for Uniform Distribution is the highest practical value. Power Numbers above this will not provide an increase in the Process results, since the gas will be completely and uniformly dispersed at this value. The BHP/1000 gallons values required for gas dispersion are also a function of the Diameter of the Impeller to the Tank diameter. The Designer should normally select a value somewhat above the Minimum Flooding value up to the Moderate range as given by this entry screen. This screen is addressable for changes from the command line after data entry by pressing the 'B' {Bhp} key. TURBINE BLADE DATA The Program is based upon the assumption that the agitator is a Rushton type Flat blade disk turbine. The disk turbine can be specified with an alternate number of blades and blade widths. A Window pops up to enable you the define the turbine. The Default values of 6 blades at a Blade width to diameter ratio of 0.2 is given, but these values can be changed. Blade Width ratio down to 0.125 can be used for high shear application. The Sacrifice for this is less circulation. Blade widths above 0.2 do not yield a process improvement. The program will use the turbine blade description to calculate the shear rates and the power requirements of the turbine. The turbine power number of the standard turbine with 6 blades and a W/D blade ratio of 0.2 is 5.0. The power number is corrected for the number of blades by taking the ratio to 6 blades to the 0.8 power. The Power Number is corrected for the blade width by a linear relationship. AGITATOR SUMMARY SCREEN The program calculates the agitator results after the foregoing data has been entered. The program will size the diameter of the required gas dispersion agitator based upon the desired BHP/1000 gallons. The initial diameter will be approximately at an impeller to tank diameter of 0.33 and the agitator RPM will be selected from the standard AGMA speeds to get the closest match to the BHP/1000 gallon value selected. The diameter and RPM can be altered from the command line if values other than those selected by the design section of this program are desired. Mass Transfer Coefficients The Kga,Kla, and Moles of gas reacted are calculated for the air sulfite system as well as the specified system. The values for the Air Sulfite system are based upon the correlations presented by OldShue in his book 'Fluid Mixing Technology' and from other sources. The Kga is a function of both the gas superficial velocity and the gas energy in BHP/ 1000 gallons. The Kga and Kla of the specified system are calculated from the Air Sulfite data by the diffusivities to the 0.5 power as discussed in the previous section on the bubbler reactor models. BOTTOM AGITATOR The program calculates the following information for the bottom agitator. The BHP/1000 gallons for the flooded and for uniform distribution. The diameter of the turbine required for the specified BHP/1000 gallons and the ratio of this diameter to the diameter of the vessel. The RPM selected is displayed and the BHP/1000 gallons for both the gassed condition and the results in liquid with no gas present. The latter value is the BHP/1000 gallons if the selected agitator is operated without gas flow at the selected RPM. It is often more than twice the gas bhp /1000 gallon requirement since the presense of gas in the agitator will severely reduce the liquid circulation and power dispersion. The Gas Holdup ratio is based upon the correlation of Foust HC Processing Nov 7 as follows: Holdup = 4.25*(BHp/1000gal)^0.47 *(Superficial Velocity)^0.53 If this formula gives a lower value then developed by the bubbler model correlation in the previous section then the hold up for the bubbler model is used. The P/Po ratio is the power ratio of the gassed condition to the pure liquid condtion. This value was developed from correlations provided in the Chemineer series in Chemical Engineering in 1976 and by Michel and Miller in AIChE Journal in May 1962. The Torque in ft-lb is generated for the Bhp in the gassed condition. The Torque in liquid only is calculated by prorating the torque gassed by the ratio of the liquid to the gassed BHP requirements. The gas Number is a number used in the calculations that is defined as the ratio of the gas flow rate to the product of the rpm and turbine diameter. The gas mixing intensity number is a ratio used by Chemineer in their Chemical Engineering Articles. The Tip speed of the Agitator is calculated. The pumping flow rates, Flow Number of the impeller, Agitator Reynolds Number, Prandtl Number and the heat transfer coeffients are calculated for both the lower and upper turbines. The Shear both Max and Average are also calculated. The average shear is equal to the Maximum shear / 2.3. Shear Max is calculated by the formula: Shear = [9.7*RPS*(D/T)^0.3 ] / (W/D) ratio of blades The user is refered to R.L. Bowens article in Chemical Engineering June 9 1986. For an excellent discussion of Shear sensitive systems and how it is to be calculated. UPPER AGITATOR The upper agitator sizing calculations are started by selection of the letter U from the command line. The upper agitator selection screens will come on and will request information on the type of agitator and the number of agitators to be used in the upper sections of the reactor. You may select from the alternative of Flat or Disk Blade Turbines, Axial 45 degree turbines, propeller or hydrofoil designs. The number of blades and the blade widths can also be specified. The computer will make recommendations on the number of agitators to be used, but this can be overridden by the user. The agitator sizing will be based upon the agitator RPMs selected for the lower agitator. It is assumed that the upper agitators and the lower gas dispersion agititor are on the same shaft. The Agitator RPM can only be changed from the lower agitator command line. The turbine diameters are limited to a maximum size of 0.4 times the tank diameter for the computer sizing calculations. The diameter can however be specified at any value from the commmand line. This upper agitators are assumed to be located in the liquid bubble swarm above the dispersing agitator. Consequently, the amount of gas entering the impeller is much less than the bottom agitator. The power ratio is based upon the holdup value generate by the bottom agitator. The upper agitator calculation results include a summary of the agitator Bhp's for both the gassed and liquid condition for the number and type of agitators selected. The same type of data are calculated for the upper agitator as was presented for the lower agitator including, flow pumping rates, and heat transfer calculated values.