Reactors packed bubble bed

Packed Bubble Bed Reactor (BBR) This is a tubular flow reactor with concurrent up-flow of gas and liquid (Figure 3.11). The catalyst bed is completely immersed in a continuous liquid flow while gas rises as bubbles. Some applications of BBR are the catalytic denitrification of aqueous nitrate solutions and the hydrogenation processes. 

THREE-PHASE FIXED BEDS TRICKLE-BED AND PACKED BUBBLE-BED REACTORS... 

Three-Phase Fixed Beds Trickle-Bed and Packed Bubble-Bed Reactors... 

In general, the material balances and the corresponding solutions for trickle and bubble bed reactors are the same, under the assumption that the plug-flow condition holds for both phases. Of course, the appropriate correlations should be used for the estimation of mass transfer coefficients. However, in packed bubble bed reactors, the liquid-phase is frequently found in a complete mixed state, and thus some adjustments have to be made to the aforementioned models. Two special cases will be presented here. 

Concerning packed bubble bed reactors, the evaluation of the Peclet number of the liquid-phase is important in order to decide if we have to use a plug- or backmixed-flow model. The liquid-phase can be considered well mixed if (Ramachandran and Chaudhari, 1980)... 

Gas-liquid mass transfer in packed bubble bed reactors... 

Liquid-solid interfacial area in trickle and packed bubble bed reactors By definition, the total external particle area per reactor volume is (Smith, 1981)... 

Note that the parameter VL/VR in a slurry reactor is the fractional liquid holdup. In the packed bubble bed reactor and the trickle-bed reactor, under complete recycling of the liquid stream, VL/VR is the ratio of total volume of the liquid that is processed (recycled) to the volume of reactor, and is always greater than 1. By recycling, it is possible to process a larger volume of liquid than the reactor volume by having a surge tank in the recycle line. 

Packed bubble bed reactor under complete mixing of both phases. 

In the following sections, the solutions of the models as well as examples will be presented for the case of trickle-bed reactors and packed bubble bed reactors. Plug flow and fust-order reaction will be assumed in order to present analytical solutions. Furthermore, the expansion factor is considered to be zero unless otherwise stated. Some solutions for other kinetics will be also given. The reactant A is gas and the B is liquid unless otherwise stated. 

Evaluate the time needed to achieve the same conversion in a packed bubble bed reactor of the same dimensions under complete recycle of the liquid phase with VL/VR = 2. 

Packed bubble bed reactor with complete recycle The model solution for the specific reactor and constant gas-phase concentration is (eq. (5.151))... 

Packed bubble bed reactor The pressure drop in the packed bubble bed reactor is evaluated using the Huntington correlation (eq. (3.395)). The parameters needed are the following (eqs. 3.396, 3.397, and 3.398) ... 

Concerning packed bubble bed reactors, the evaluation of the Peclet number of the liquid phase is important in order to decide if we have to use a plug- or backmixed-flow model. For the specified Reynolds number, the Peclet number for the liquid phase using the Stiegel-Shah correlation (eq. (3.422)) is 0.15, much lower than in the trickle bed, which was expected as the backmixing in the liquid phase in packed bubble bed reactors is relatively high. The liquid phase can be considered to be well mixed if (Ramachandran, and Chaudhari, 1980) (eq. (3.423))... 

Note that the mass transfer coefficients are higher in the packed bubble bed reactor compared to those in the trickle-bed reactor under the same conditions, as expected. 

Note that in the packed bubble bed reactor, the conversion achieved is higher by 14.02% due to the higher mass transfer coefficients compared to the trickle bed, especially the gas-liquid one, which is 3.61 times higher in the packed bubble bed reactor. 

In the case of saturated liquid feed, the conversion achieved is almost identical for both reactors. This is why in the respective reactor model, the gas-phase mass transfer is theoretically infinite and the difference in the liquid-solid mass transfer between the reactors is small, only 1.21 times higher than that in the packed bubble bed reactor. 

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