25-08-2017, 09:32 PM
n many gas-liquid and gas-liquid-solid reactions where pure gas is used in reaction and gas phase conversion is very low, it is desirable to operate the reactor in dead end mode or to recycle the unreacted gas. This condition arises in number of industrially important reactions such as hydrogenation, alkylation, ethoxylation, ammonolysis, oxidation with pure oxygen, hydrochlorination, etc. For successful design of self inducing reactors, it is necessary to predict characteristics like gas induction rate, mass transfer coefficient, overall gas hold-up, and power consumption. In the present study, mass transfer characteristic of hollow pitched blade down flow impeller has been investigated. Experiments were carried out with 100 L tank having 0.5 m diameter and 800 L tank having 1 m diameter. Impeller diameter in both the cases was 0.25 m.
In many gas-liquid and gas-liquid-solid reactions where pure gas is used in reaction and gas phase conversion is very low, it is desirable to operate the reactor in dead end mode or to recycle the unreacted gas. This condition arises in number of industrially important reactions such as hydrogenation, alkylation, ethoxylation, ammonolysis, oxidation with pure oxygen, hydrochlorination, etc. For successful design of self inducing reactors, it is necessary to predict characteristics like gas induction rate, mass transfer coefficient, overall gas hold-up, and power consumption. In the present study, mass transfer characteristic of hollow pitched blade down flow impeller has been investigated. Experiments were carried out with 100 L tank having 0.5 m diameter and 800 L tank having 1 m diameter. Impeller diameter in both the cases was 0.25 m.
Impeller was located at 0.17 m from tank bottom and liquid height was 0.5 m during experiments in 100 L tank. Two different blade angles were investigated and impeller speed was varied between 2.5-7 r/s and power consumption in the range of 0.75 to 10 kW/m3. Mass transfer coefficient was measured using steady state hydrazine oxidation technique with air-water system. The value of kLa for PBTD 45° and PBTD 60" were found to vary between 0.02-0.14 s-1 and 0.02- 0.22 s-1, respectively.
Rushton type disk impeller is widely used for gas dispersion operations. In the present investigation mass transfer and hydrodynamic characteristics of gas inducing type of disk impeller (double disk impeller) has been investigated. Experiments were carried out with 0.5 m i.d. vessel having double disk impeller with diameter of 0.17 m. The double disk impeller studied was further modified. It was found that modified impeller gives 12% more gas induction vis-à-vis the double disk impeller at the same power input. Mass transfer coefficient for the double disk impeller and the modified double disk impeller was measured with steady state hydrazine oxidation technique. Maximum oxygen transfer efficiency for double disk impeller and modified double disk impeller was found to be 1.22 kg O2/kWh and 1.32 kg O2/kWh, respectively. In order to generate scale-up information for modified double disk impeller, experiments were performed in 800 L tank having i.d. of 1m and impeller diameter 0.33 m. Mass transfer correlation has been proposed which predicts kLa within 15% of measured value and can be used for design andscale-up of self inducing reactors.
Gas inducing impeller in combination with other axial flow impeller is widely used in industry where there is a need for good gas dispersion, higher mass transfer rate and solid suspension. In the present study, hydrodynamic and mass transfer characteristics of self inducing impeller in combination with axial flow impeller have been investigated. Experiments were carried out in 1m i.d. tank with gas inducing impeller diameter of 0.25, 0.33 and 0.5 m and axial flow impeller (lower impeller) diameter of 0.25 and 0.33 m. Liquid height in tank was varied at 1, 0.9 and 0.8 m with gas inducing impeller clearance of 0.7 m from bottom and axial flow impeller clearance of 0.33 m from bottom. Power consumption was varied up to 3.2 kW/m3, corresponding kLa was found to 0.075 s-1. Out of various combinations investigated, hollow PBTD45° with hydrofoil impeller was found to give best mass transfer coefficient at given power input per unit volume of liquid. Correlations for Mass transfer coefficient and power consumption have been proposed, which can be used for design of self inducing reactors.