HTU of Acetone-Toluene-Water Extraction in a Pulsed Column

Transcription

1 HU of Acetone-oluene-Water Etraction in a Pulsed Column erdthai Vatanatham*, Pisan erasukaporn, and Paisan Lorpongpaiboon Department of Chemical Engineering, Kasetsart University ABSRAC he mass transfer correlation in a pulsed column particularly the HU is needed for column design. evertheless, scarce information is available. his work aims to study the HU of pulsed perforatedplate tower for liquid-liquid etraction. In the eperimental ternary system, water etraction solvent was used to etract acetone solute from toluene feed solvent in a counter-current etraction scheme. Operating variables such as pulsing frequency and amplitude, solute concentration, drop diameter, and flow rate of heavy liquid and light solution were investigated. Each eperimental variable was varied while holding other variables constant. he outlet concentrations of each component of the pulsedcolumn were analyzed by using a refractometer. HU and erwood number correlation was obtained. In addition, the mass transfer coefficient and ynolds number correlation was illustrated. he results are: HU 0.5 = 4.4 sh K ma = Key Words: Liquid-Liquid Etraction, HU, Perforated-Plate Column, and Pulsed ower.

2 Introduction Liquid-liquid etraction is a separation process that involves the transfer of mass from one liquid phase into a second immiscible liquid phase. he simplest one is a transfer of one component from a binary miture into a second phase. It is used primarily when distillation is impractical due to close relative volatility of the two components, both are nonvolatile components, or the components are heat sensitive. Continuous or differential contact equipment is arranged for countercurrent contact of the insoluble liquids without complete separation of the liquids from each other. he two immiscible liquids are in continuous contact through out their passage through the equipment. One of the phases can remain dispersed as droplets in the contactor while passing countercurrently to another continuous phase. Rate of mass transfer and driving force are related by a general epression (McCabe et al., 1993) as eq.(1). A = k ma (C A C Ai ) = k ( A - A e ) (1) From the mechanism of mass transfer, the coefficient k s would depend on the diffusivity D V, fluid velocity u, the viscosity µ, the density ρ, and some linear dimension D. For any given surface Dimensional analysis gives km u = k = ψ (D V, D, u, µ, ρ) Multiplying eq.() by ( Du ρ / µ )( µ / ρdv ) yields Duρ µ, () µ ρdv kmd D V Duρ µ = ψ, (3) µ ρdv hat is = ψ (, ) (4) Sc For a low viscosity drop falling through a viscous liquid, the velocity boundary layer in the eternal fluid almost disappears. Eternal liquid elements are eposed to the drop for a short time and the penetration theory applies. he effective contact time is the time for the drop to fall a distance of its own diameter. Equation (4) becomes = Dpuρ µ π µ ρdv 1 or, Sc = (5) Prediction of km is difficult for a practical application. A volumetric mass transfer coefficient, k m a, estimated from laboratory or pilot plant test is generally used in mass transfer calculations. Height of ransfer Units For differential contact etractor, the concept of mass transfer unit was developed many years ago. For liquid-liquid etraction in perforated-plate towers, mass transfer rates may be epressed in terms of overall heights of transfer units and successfully correlated for any tower and system ( Perry

3 and Green, 1997). Even though there are many methods for estimating the etraction rates, a pilotplant test of any new process is recommended. Application of mass transfer unit concept to pulsed column with perforated-plate can be done using eq.(1) and mass balance as follow. Mass balance of solute transferred between continuous phase and immiscible dispersed phase, in a differential section dh of a pulsed column is - d(r ) = - R d dr = A a S dz (6) and for one solute, dr = - a S dz (7) Substituting for dr into eq.(6) and integration with A from eq.(1), in terms of overall driving force, will obtain = R d Z or, 1 e K as(1 )( ) Z R d = 1 e K as (1 )( ) (8) In the H or and or or HU R and U R format Z = H = HU U (9) or or R R Equations (8) and (9) are useful to obtain values of HU and K from eperimental data. Dimensions of a pulsed column and equilibrium data are needed for the calculations (erasukaporn, and Lorpongpaiboon, 1996). he value of HU reflects the mass transfer rate and mass transfer coefficient. As seen in eq. (8), the HU is lower for higher mass transfer coefficient. In view of eq.(5), empirical correlations correlating HU and K A to and are tested in this work. Eperimental Procedure Etraction of acetone in ternary system of acetone-toluene-water was done in a glass pulsed column. Acetone was the solute and fed into the column at the bottom with toluene as feed solvent. Water was the etraction solvent and flowed down the column as a continuous phase. Dispersed toluene droplets of average sizes varied from mm rose through the column and perforated plates. he column is a perforated plate type with 50-mm nominal diameter size. Perforated holes are mm in diameter with 140 holes per plate of 50 mm diameter. here are 38 plates inside with a plate spacing of 50 mm. Plate free area is 5%. Height of two-phase contact in column is.45 meters. Liquid inside the column was pulsed with a piston pump. Pulsing frequency varied from 0.8 to.3 Hz while pulsing amplitude varied from cm. Concentrations of acetone in toluene and acetone in water were measured using refractive inde and calibration curve. he U R was calculated. hen HU R was obtained from eq. (9). After that K could be etracted out. sults and Discussions Eperimental results show that HU R decreases as increases. It is obvious that lower HU R reflects more efficient mass transfer rate or higher mass transfer coefficient. Meanwhile higher mass transfer coefficient brings higher. Comparison of eperimental HU R to raffinate yields a correlation HU R 0.5 = 4.4 sh (10)

4 which is shown in Fig.1, where HU R is in meter. he is estimated from dispersed droplet diameter, diffusivity of acetone in toluene, and K A which is obtained form eq.(1) through application of eperimental data. he eperimental data was obtained with droplet ynolds number varied from 5 to 18. For mass transfer coefficient, eperimental data show that it varries almost linearly with under the eperimental feed concentration of 13 to 47 weight percent of acetone in toluene feed solvent. he empirical epression, with K ma in m/s, is K ma = (11) which is shown in Fig.1, where K ma is in micrometer. his is in line with the epression shown in eq. (5), i.e. mass transfer coefficient increases with ynolds number. he dependency is stronger than in eq.(5) as it is a pulsed column in which breaking and forming of droplet depends strongly on the pulse action of the equipment. HU, m y = erwood umber Figure 1. lationship between Height of ransfer Unit and erwood umber Km, micrometer/s y = ynolds umber. Figure. lationship of overall mass transfer coefficient to ynolds umber. Conclusions Height of transfer unit for pulsed perforated-plate column was obtained from an application of mass transfer unit concept. It s value decreases as erwood number increases as epected. he mass transfer coefficient relation to ynolds number is also obtained. he relation is almost linear. he data were obtain with acetone-toluene-water system under a limited range of operating condition as epected in it s nature of application.

5 omenclature a Area of interface between phases per unit volume of equipment, m -1 C A1 Concentration of component A at location 1, moles per unit volume D Linear dimension or diameter, m D V Volumetric diffusivity, m /h E Etract rate, kg mol per unit time H or Height of transfer unit based on overall mass transfer and raffinate concentration, m/s HU R Height of transfer unit based on overall mass transfer and raffinate concentration, m/s k ma Mass transfer coefficient for component A, rate of mass transfer per unit area per unit concentration difference, or unit length per unit time K ma Overall mass transfer coefficient for component A, rate of mass transfer per unit area per unit concentration difference, or m/s k Mass transfer coefficient based on mole-fraction differences, kg-mol / m -s-unit mol fraction K A Overall mass transfer coefficient based on mole-fraction differences, kg-mol / m -s-unit mol fraction A Mass transfer flu of component A, moles of component A (solute) transferred in a unit time across a unit area or umber of transfer unit based on overall mass transfer and raffinate concentration ynolds number, Duρ / µ Sc Schmidt number, µ / ρdv erwood number, k C D / DV U R umber of transfer unit based on overall mass transfer and raffinate concentration R Raffinate rate, kg mol per unit time S Cross sectional area of column, m u Velocity, m/s A Mole fraction of component A e A Mole fraction of component A in another phase which is in equilibrium with y A µ Viscosity, kg/m-s ρ Density, kg/m 3 ψ Functions ferences Perry, R.H. and Chilton, C.H Perry s Chemical Engineers Handbook, McGraw-Hill, ew York. McCabe, W.L., Smith, J.C., and Harriott, P Unit Operations of Chemical Engineering, McGraw-Hill, ew York. erasukaporn, Pisan and Lorpongpaiboon, Paisan HU of Pulsed Perforated Liquid-Liquid Etractor: Acetone-oluene-Water, ChE Senior Project, Kasetsart University, Bangkok