High pressure reactor, liquid holdup

Figure 6. Liquid holdup for various gas and liquid flow rates in a high pressure reactor.

The average total liquid holdup in a trickle bed reactor decreases with increasing bed depth in a low pressure laboratory or pilot scale column. However, for commercial use where there is a moderate to high pressure input, the holdup is essentially constant (Figure 5). 

Liquid holdup is defined as the volume of liquid contained in the bed per unit bed volume. It is a function of the physical properties of the fluid phases and the bed characteristics. It is a basic parameter for reactor design, because it is related to other important parameters, namely, pressure gradient, gas-liquid interfacial area, the mean residence time of the liquid phase, catalyst loading per unit volume, axial dispersion coefficient, mass transfer characteristics, and heat transfer coefficient at the wall, etc. The optimal value of liquid holdup is desirable for better performance of TBR as a high value of liquid holdup will increase mass transfer resistance while too low a value of liquid holdup will decrease the proper utilization of the catalyst bed. Sometimes, the term total liquid saturation (j t) is used to describe the amount of liquid in the bed. It is defined as the volume of liquid present in a unit void volume of the reactor. Thus, the liquid holdup and total liquid saturation are related as ... 

As the fluids travel down the bed, the energy losses decrease the pressure. As the pressure is dropped, the gas expands. The expansion results in a decrease in liquid holdup. However, the relative pressure drop for the commercial size bed at a depth of 3.8 m is 1.9% of the initial value while for the laboratory column at the same depth this value is 73%. The gas in the low pressure bed consequently expands five fold from 200cc/g to 1000 cc/g, at a pressure of 4 atm, while the high pressure bed gas specific volume remains nearly constant at 23.8cc/g for a pressure of 35 atm. It is this large expansion which is responsible for the appreciable drop in liquid holdup for the small reactor. 

Al-Dahhan, M.H. Highfill, W. Liquid holdup measurement techniques in laboratory high pressure trickle bed reactors. Can. J. Chem. Eng. 1999, 77, 759. 

With Stamicarbon s pool condenser technology, condensation can be done very efficiently by reversing the former high-pressure carbamate condenser s process and steam side. The entire heat exchanging part is submerged in condensed carbamate. This pool-type condensation enables higher heat transfer, while staging two-thirds of the entire synthesis section s urea conversion in its liquid holdup. Thus, the urea reactor... 

The gas properties have no effect on liquid holdup at low pressure and low gas rates, when the liquid flow is affected only by gravity forces. At high gas velocity the holdup decreases because of shear at the gas-liquid interface. Several correlations have been proposed to account for the effects of liquid and gas properties on holdup, but these correlations are complex and quite different in form [20], which makes comparisons difficult. Furthermore, most of the data are from studies at ambient conditions using water or low-molecular-weight solvents. More data are needed from reactors operating at industrial conditions. 

Most commercial trickle-bed reactors operate adiabatically at high temperatures and high pressures and generally involve hydrogenation, oxidation, desulfurization, hydrocracking, etc. The most important hydrodynamic properties for trickle-bed reactors are i) the liquid holdup that controls the liquid-to-gas reactant... 

Al-Dahhan MH, Dudukovic MP. Pressure drop and liquid holdup in high pressure trickle-bed reactors. Chem. Eng. Sd. 1994 49 5681. 

Munoz, J.A.D., Alvarez, A., Ancheyta, J., Rodriguez, M.A., Marroqum, G. 2005. Process heat integration of a heavy crude hydrotreatment plant. Catal. Today 109(1 ) 214-218. Ring, Z.E., Missen, R.W. 1991. Trickle-bed reactors tracer study of liquid holdup and wetting efficiency at high temperature and pressure. Can. J. Chem. Eng. 69(4) 1016-1020. 

For instance, for a riser gas holdup, of 0.2 and a reactor height, of 10 m, the average liquid circulation velocity is 6.26 m/s. That is, liquid circulation velocity is very high. If a bubble is introduced in the downcomer, it is entrained downward if the liquid velocity is higher than the bubble rise velocity. It may be pointed out that the presence of gas on the downcomer side reduces the pressure driving force. However, the circulation continues even if part of the downcomer section is occupied by the gas phase. This is because (Figure 11.23b) part of the downcomer is bubble free. is given by... 

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