Pressure Drop and Liquid Hold-Up

Specchia, V., and Baldi, G., Pressure drop and liquid hold-up for two-phase concurrent flow in packed beds. Chem. Eng. Sci. 32, 515-523 (1977). 

The second section presents a review of studies concerning counter-currently and co-currently down-flow conditions in fixed bed gas-liquid-solid reactors operating at elevated pressures. The various consequences induced by the presence of elevated pressures are detailed for Trickle Bed Reactors (TBR). Hydrodynamic parameters including flow regimes, two-phase pressure drop and liquid hold-up are examined. The scarce mass transfer data such gas-liquid interfacial area, liquid-side and gas-side mass transfer coefficients are reported. 

M.H. Al-Dahhan and M.P. Dudukovic, Pressure drop and liquid hold-up in high pressure trickle-bed reactors, Chem. Engng. Science, 49 (1994) 5681-5698. 

I. Iliuta, F. Larachi and B.P.A. Grandjean, Pressure drop and liquid hold-up in trickle flow reactors improved Ergun constants and slip correlations for the slit model, Ind. Engng. Chem. Res., 37 (1998) 4542-4550. 

The dynamic formulation of the model equations requires a careful analysis of the whole system in order to prevent high index problems during the numerical solution [96]. As a consequence, a consistent set of initial conditions for the dynamic simulations and a suitable description of the hydrodynamics must be introduced. For example, pressure drop and liquid hold-up must be correlated with the gas and liquid flows. 

Continuous stirred-tank reactors (CSTRs) have been routinely employed for producer gas fermentations. A two-stage reactor system has also been used to maximize ethanol production and minimize the formation of byproducts. Carbon monoxide and hydrogen conversions of 90% and 70%, respectively, were observed in the first reactor, while they were about 70% and 10% in the second reactor. High ethanol-to-acetate ratios were achieved by the use of such a dual reactor system. Bubble colunms are also commonly used for industrial fermentations. A comparative study was performed between a CSTR and a bubble column reactor for CO fermentation using Peptostreptococcus productus. Higher conversion rates of CO were observed with the bubble column without the use of any additional agitation. Producer gas fermentation with packed bubble colunms and trickle bed reactors has also been studied. The trickle bed reactor has a low pressure drop and liquid hold-up, and the conversion rates were the highest compared to CSTRs and bubble columns. 

This FDPAK programme provides chemical engineers with a fast calculation the hydraulic parameters of any types of column internals (more than 200) and allows the user to assess the individual types of packings in terms of their maximum capacity, pressure drop and liquid hold-up. The experience of the past 20 years has shown that the programme is also suitable for teaching purposes at colleges and universities. 

The correlations derived in Chaps. 2, 3 and 4 can be used to calculate the apparatus diameter and determine the pressure drop and liquid hold-up in a given operating range and at flooding point. 

W.J.A. Wammes, J. Middekamp, W.J. Huisman, C.M. Debaas and K.R. Westerterp, Hydrodynamics in a cocurrent gas-liquid trickle bed at elevated pressures, Part 2 liquid hold-up, pressure drop, flow regimes, AIChE Journal, 37, 12 (1991) 1855-1862. 

Dhanuka, V.R. and J.B. Stepanek, "Gas and Liquid Hold-up and Pressure Drop Measurements in a three-Phase Fluidized Bed" in Fluidization, eds. Davidson and Keairns, 179-183, (Cambridge University Press, 1978). 

Various aspects of three-phase fluidization have been the subject of numerous investigations due to its high potential for industrial applications in areas such as catalytic processes, hydrocracking and desulphurization of petroleum products, coal liquefaction, hydrogenation of unsaturated fats, and production of calcium bisulfite liquor. Extensive studies have been published concerning the hydrodynamics of three-phase fluidized beds, such as expansion [l-S], pressure drop [ 9 J, gas and liquid hold up [10-14], minimum fluidization velocity [izj and axial mixing [8, 10, 14-17]. 

The principal considerations that determine (or at least influence) the choice between packed and tray columns are the ability of either t) e to handle a given gas or liquid load, or range of loads, and the pressure drop per theoretical stage. Somewhat lower on the scale of priorities is the ability to handle corrosive fluids, liquids containing solid impurities or components with a potential to crystallize, and liquid hold-up. 

The pressure drop due to acceleration is important in two-phase flow because the gas is normally flowing much faster than the liquid, and therefore as it expands the liquid phase will accelerate with consequent transfer of energy. For flow in a vertical direction, an additional term — AZ y must be added to the right hand side of equation 5.5 to account for the hydrostatic pressure attributable to the liquid in the pipe, and this may be calculated approximately provided that the liquid hold-up is known. 

A well-substantiated correlation for air-water systems taken from the trickle bed literature (Morsi and Charpentier, 1981) was used for the volumetric mass transfer coefficients in the / , and (Rewap)i terms in the model. The hi term was taken from a correlation of Kirillov et al. (1983), while the liquid hold-up term a, in Eqs. (70), (71), (74), (77), and (79) were estimated from a hold-up model of Specchia and Baldi (1977). All of these correlations require the pressure drop per unit bed length. The correlation of Rao and Drinkenburg (1985) was employed for this purpose. Liquid static hold-up was assumed invariate and a literature value was used. Gas hold-up was obtained by difference using the bed porosity. 

Only a limited number of attempts have been made to correlate the liquid hold-up or the equivalent in the pipe. Again it seems clear that the liquid hold-up will be significally influenced by the flow pattern, and that either separate correlations will be required for each flow pattern, or a master correlation incorporating those variables which influence flow pattern will need to be developed. With one or two exceptions the few existing correlations suffer from the same kind of disadvantage as those for the prediction of pressure drop. ... 

The terminology used by various authors for visual description of upward vertical two-phase gas-liquid flow has already been given in Fig. 1. Typical pressure drop and hold-up ratio curves are shown in Fig. 2. Inasmuch as the liquid feed rate is constant in Fig. 2, the pressure-drop... 

Recently Iliuta and Larachi [44] developed a generalized slit model for the prediction of frictional two-phase pressure-drop, liquid hold-up, and wetting efficiency in TBR operated under partially-and fully wetted conditions. This proposed model mimicked the actual bed void by two geometrically identical inclined slits, a wet slit and a dry slit (see Figure 5.2-14). 

In the first slit, the liquid wets the wall with a film of uniform thickness the gas being in the central core (wet slit). The second slit is visited exclusively by the gas (dry slit). The high-pressure-and high-temperature-wetting efficiency, liquid hold-up and pressure-drop data reported in the literature for TBR in the trickle-flow regime were successfully forecasted by the model. 

For the TBR design the dynamic liquid hold-up is a basic parameter because it is related to other important hydrodynamic parameters (including the pressure drop, wetting, and mean-residence-time of liquid). 

R.A. Holub, M.P. Dudukovic and P.A. Ramachandran, A phenomenological model for pressure drop, liquid hold-up and flow regimes transition in gas-liquid trickle flow, Chem. Engng. Science, 47 (1992) 2343-2348. 

Referring to Fig. 4.3 these values lie in region II indicating that the reaction is only moderately fast and that a relatively high liquid hold-up is required. A packed column would in any case therefore be unsuitable. We therefore conclude from the above considerations that an agitated tank, a simple bubble column or a packed bubble column should be chosen. The final choice between these will depend on such factors as operating temperature and pressure, corrosiveness of the system, allowable pressure drop in the gas, and the possibility of fouling. 

In principle, any catalyst bed used for reactive distillation or trickle bed operation can also be applied in reactive stripping. The performance will depend mainly on the optimal ratio between catalyst hold-up, the gas-liquid and the liquid-solid interface. However, recycling of the strip gas flow makes a low pressure drop (and therefore a high voidage) especially beneficial. In countercurrent operation, flooding - a well-known problem - must be avoided. The present studies have focused on structured catalyst supports, developed for either reactive distillation or reactive stripping, with a particular emphasis being placed on the use of so-called film-flow monoliths. 

Reactive absorption processes present essentially a combination of transport phenomena and reactions taking place in a two-phase system with an interface. Because of their multicomponent nature, reactive absorption processes are affected by a complex thermodynamic and diffusional coupling which, in turn, is accompanied by simultaneous chemical reactions [14—16], Generally, the reaction has to be considered both in the bulk and in the film region. Modeling of hydrodynamics in gas-liquid contactors includes an appropriate description of axial dispersion, liquid hold-up and pressure drop. 

The rate-based stage model parameters describing the mass transfer and hydrodynamic behavior comprise mass transfer coefficients, specific contact area, liquid hold-up, residence time distribution characteristics and pressure drop. Usually they have to be determined by extensive and expensive experimental estimation procedures and correlated with process variables and specific internals properties. 

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