Hold-up, in packing

Total liquid hold-up in packed bed, h[ = static hold-up, hg, plus operating hold-up, ho [64, 66]. 

Jesser, B. W. and J. C. Elgin, Studies of Liquid Hold-up in Packed Towers, Trans. Amer. Inst. Chem. Engr., 39, No. 3 277 (1943). 

Otake, T. and K. Okada, Liquid Hold-up in Packed Towers, Operating and Holdup Without Gas Flow, Soc. Chem. Engrs. Qapan) 17, No. 7, 176 (1953). 

Gayler, R., Roberts, N. W. and Pratt, H. R. C. Trans. Inst. Chem. Eng. 31 (1953) 57. Liquid-liquid extraction. Part IV. A further study of hold-up in packed columns. 

R. Gayler, N. W. Roberts, H. R. C. Pratt, Liquid-Liquid Extraction. A further Study of Hold-up in Packed Columns. Trans. Am. Inst. 

Ortiz-Arroyo, A. Larachi, F. Iliuta, I. Method for inferring contact angle and for correlating static liquid hold-up in packed beds. Chem. Eng. Sci. 2003, 58 (13), 2835-2855. 

Literature research has shown that there are well-founded theoretical and empirical methods, which can be used to describe the Uquid hold-up in packed columns. In the... 

Dispersed Phase Hold-Up in Packed Columns Containing Random and Structured Packings... 

Pendulum Pulsator. Technical Information - ENVIMAC Engineering GmbH, 46149 Oberhausen Gayler R, Roberts NW, Pratt HRC. Liquid-Liquid Extraction Part IV. A Father Study of Hold-Up in Packed Columns. Trans. Inst. Chem. Engrs. 31(1953), p. 57-68... 

Liquid hold up in a tower represents the liquid held in the void spaces of the packing during operating conditions. At flooding, essentially all of the voids are filled with liquid. 

Fig. 8.15. Experimentally determined liquid hold-up in katapak-S packing. (From Ref. [21].)...

In the absence of liquid flow the gas hold-up is 0.58. This value is the porosity of the packing determined with a 10% error by a volumetric method. For constant gas velocity at 0.037 m/s measurement of the total hold-up in the gas phase have shown that the pressure had no effect on this hydrodynamic parameter in the range 0.1 to 1.3 MPa. This result can be explained by the fact that the hold-up only depends on the residence time, which is constant for a given gas velocity in the pressure range studied, as shown in table 1. 

The study under pressure with countercurrent flow has shown that the operating pressure has no effect on hold-up. In fact for a liquid flow rate of 4.63 kg/rrPs and constant gas velocity of 0.037 m/s, liquid hold-up is 0.21 for pressure ranging between 0.1 and 1.3 MPa. This result is in agreement with the conclusions of Van Gelder el al. [1] concerning the influence of pressure on liquid hold-up in a packed column operating with cocurrent flow. Larachi el al. [4] report the same observations in a fixed bed column with cocurrent flow. 

K.B. Van Gelder and K.R. Westerterp, Residence time distribution and hold-up in a cocurrent upflow packed bed reactor at elevated pressure, Chem. Eng. Technol. N°13, 27(1990). 

In addition to these pressure drop models, models to represent spreading of liquid in packed beds because of spatial variation in flow resistance are needed. In a randomly packed bed, the void fraction is not uniform. This implies that some flow channels formed within a packed bed offer less resistance to flow than other channels of equal cross-sectional area. Liquid will tend to move toward channels of lower resistance, leading to higher liquid hold-up in such channels. Thus, even if the initial liquid distribution is uniform, inherent random spatial variation of the bed leads to non-uniform liquid flow. Yin et al. (2000) assumed that the dispersion coefficient for liquid phase volume fraction is linearly proportional to the adverse gradient of... 

Another important consideration in deciding on packing material is how long the packing will hold up in a hot alcohol solution. Durable packing like stainless steel may last indefinitely but is expensive initially. Thus, cost-benefit ratio must be considered. 

For purposes of comparison it is convenient to calculate the static hold-up, ox)erating hold-up and total hold-up per theoretical or practical plate. The total hold-up varies with the load, but the literature gives little definite information on this relationship. For an Oldershaw sieve-plate column with a diameter of 28 mm, containing 30 actual plates, Collins and Lantz [200] published the values shown in B. 92. The total hold-up lies between 43 and 60 ml, depending on the load, so that it can be taken to be 1.4—2.00 ml for each actual plate and 2.5—3.5 ml for each theoretical plate. According to measurements of the author s, the total hold-up of packed columns is about the same, as wHl also be. seen from Fig. 96, which indicates... 

Shiotani, T. and Yamane, T. A horizontal packed-bed bioreactor to reduce carbon dioxide gas hold-up in the continuous production of ethanol in immobilized yeast cells. Eur. J. Appl. Microbiol. Biotechnol. 13 (1981) 96-101. 

Tray colunms operate with about 40 mm of liquid on the tray and for this reason have a higher hold-up than packed columns. This is not an important handicap in continuous distillation, where ideally the composition is constant in any part of the system once equilibrium is established. In batch distillation a high inventory in the column makes a sharp separation more difficult to achieve. 

---