(18g) Mass Transfer in Bubble Column With and Without Internals

Mass transfer in bubble column with and without internals

Multiphase reactions are backbone of chemical industry. They are frequently encountered in many petrochemical processes, biochemical processes, waste water treatment, and in production of various organic compounds. Fischer Tropsch (FT) synthesis is one of such complex widely used multiphase reaction. This reaction has gained importance due to its ability to convert synthesis gas derived from natural gas or from biomass sources to liquid transportation fuels. This reaction is highly exothermic; about 170 kJ of heat is released in conversion of one mole of carbon monoxide. Bubble column reactors are preferred choice of reactor for Fischer Tropsch synthesis due to their superior heat and mass transfer properties. In order for successful scale up and optimum performance of the reactor it is important to have fundamental understanding of the reactor. Due to their application in wide systems bubble column reactors have been extensively studied in past by many researchers. Various correlations for mass transfer coefficients have been developed for bubble column reactors. Effect of various operating parameters like pressure, superficial gas velocity, presence of solids catalyst, gas phase density, and effect of column diameter on volumetric mass transfer coefficient has been studied. However, most of the work done on mass transfer in bubble column reactor has not accounted for presence of cooling internals. As mentioned earlier FT synthesis is highly exothermic; heat removal is one of the key issue in designing of bubble column reactor for FT synthesis. In order to remove this heat vertical heat exchanging internals are installed in the reactor. Previous work revealed that presence of internals affects both gas and liquid velocity profiles as well as bubble dynamics. Effect of internals on liquid circulation velocity has been studied using CFD simulations. These changes are expected to reflect on volumetric mass transfer coefficient (). To best of authors knowledge there has not been any study on effect of internals on overall volumetric mass transfer coefficient k_La in pilot scale bubble column. In present work effect of superficial gas velocity on volumetric mass transfer coefficient is studied in pilot scale bubble column. Present work aims to provide better understanding of effect of vertical cooling internals at different superficial gas velocities ranging from 20 cm/s to 45 cm/s. All the experiments are performed in churn turbulent regime. Knowledge of mass transfer coefficient in this regime is important for both FT synthesis and Methanol synthesis. This study aims to improve understanding of mass transfer coefficients in churn turbulent regime and in presence of vertical heat exchanging internals.

Experiment were carried out in transparent plexi glass column with and without vertical heat-exchanging internals and diameter of 45 cm. Oxygen-enriched-air method was implemented to evaluate volumetric mass transfer coefficients. The change in dissolved oxygen concentration is monitored with neo fox optical probe developed by ocean optics. For evaluating mass transfer coefficients CSTR model is used.

Change in oxygen concentration with time at different axial and radial positions with and without internals was observed. It was found that concentration at different locations in the reactor was identical in this regime. The overall volumetric mass transfer coefficients at different radial positions are not significantly different from each other. This was expected as the concentration profiles were similar at different axial positions. Similar trend follows at axial values as well. This justifies selection of CSTR model to evaluate mass transfer coefficient. Also it can be seen that the value of mass transfer coefficient remains constant at different superficial gas velocities. Unlike to trend in bubbly regime where value of mass transfer coefficient increases with increase in superficial gas velocity. The change in interfacial area is mainly responsible for changes in k_La. The interfacial area is directly proportional to gas holdup and bubble size. In bubbly regime slight changes in superficial gas velocity results large change in gas holdup; when superficial gas velocity is changed from 2 cm/s to 8 cm/s gas hold up at the center of the column changes by 0.25. In the churn turbulent when superficial gas velocity changes from 30 cm/s to 45 cm/s the corresponding change in gas holdup is only 0.08. Also the change in mean bubble chord length at the center of the column is very small 0.04 cm when superficial gas velocity changes from 20 cm/s to 45 cm/s. The mean bubble size almost remains constant due to rapid bubble breakup and coalescence which is observed in churn turbulent regime. Hence it can be said that due to relatively less changes in gas holdup and also not much change in mean bubble size the gas liquid interfacial area remains same and we are not able to see much change in volumetric mass transfer coefficient in absence of internals at high superficial gas velocities.  

Volumetric mass transfer coefficients in presence of internals slightly less compared to volumetric coefficients without internals. The volumetric mass transfer coefficient k_La is combination of mass transfer coefficient ‘k_L’and interfacial area ‘a’. Higbie’s theory for mass transfer is well accepted to calculate mass transfer coefficient in bubble column, according to this theory mass transfer coefficient is inversely proportional to the contact time of liquid surrounding the bubble, which is inversely proportional to bubble rise velocity. It is well known that larger bubbles have higher rise velocity than smaller bubbles hence larger bubbles are expected to have higher value of k_L. According to previous research work it is shown that average chord length is reduced in presence of internals, decrease in the average bubble chord length, implying that mass transfer coefficient k_L should also decrease. In accordance with other mass transfer theories volumetric mass transfer coefficient k_L is also function of turbulent intensity, which suggests that the presence of internals should decrease k_L due to decrease in turbulent intensity. On other hand interfacial area which is directly proportional to gas hold up is increased in presence of internals due to increase in gas hold up.

Hence it can be concluded that decrease in k_L is balanced by increase in interfacial area and the volumetric mas transfer coefficient is almost unchanged in presence of internals.