Secondary Loop Gas Fermentation Reactor Foam Control
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Solution Overview
Problem
Existing forced-circulation external-loop reactors face challenges in regulating gas holdup and mass transfer efficiency due to competing effects of loop pump speed, leading to substrate inhibition and inefficient foam control, which limits the commercial viability of gas fermentation processes.
Innovation Solution
The introduction of a secondary loop in the reactor system, which circulates fermentation broth from the downcomer to the top of the riser via a nozzle, breaking up foam and enhancing gas holdup, thereby decoupling the effects of loop pump speed on gas entrainment and liquid velocity, and improving mass transfer efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If loop pump speed is increased to enhance gas entrainment and improve mass transfer, then gas-liquid mass transfer rate increases, but liquid velocity increases which washes out gas bubbles quickly and decreases gas holdup
Solution Approach 1:
The circulation system is divided into two independent loops: primary loop for bulk liquid circulation and secondary loop for gas holdup enhancement. The secondary loop withdraws liquid from the downcomer and returns it to the top of the riser, creating additional gas entrainment without increasing primary loop pump speed, thus resolving the contradiction between mass transfer rate and gas holdup.
Solution Approach 2:
The invention changes the operational parameters by introducing a secondary circulation loop with its own pump, allowing independent control of gas entrainment and liquid velocity. This enables optimization of gas holdup through secondary loop flow rate adjustment while maintaining appropriate liquid velocity in the primary loop for substrate delivery.
2Productivity
If measures are taken to increase mass transfer surface area (increase gas holdup or decrease bubble size), then volumetric mass transfer coefficient increases, but large quantity of foam is generated which may block pipelines
Solution Approach 1:
The secondary loop extracts liquid from the downcomer section and returns it to the top of the riser, creating a separate circulation path that enhances gas entrainment and mass transfer surface area. This controlled extraction allows increased gas holdup without generating excessive foam in the main reactor volume, as the secondary loop manages bubble distribution and breakup.
Solution Approach 2:
The secondary loop acts as an intermediary system between the primary circulation and the gas-liquid interface. It mediates the gas holdup enhancement by providing additional liquid flow that entrains gas without directly increasing the primary loop's gas-liquid mixing intensity, thus improving mass transfer while controlling foam generation.
3Productivity
If mass transfer rate is increased above maximum reaction rate, then gas-liquid mass transfer efficiency improves, but substrate inhibition occurs causing slow microbial growth and culture death
Solution Approach 1:
The secondary loop creates local variations in gas holdup and substrate concentration within the riser. By returning liquid to the top of the riser, it creates zones with different mass transfer intensities, preventing uniform oversupply of substrate throughout the reactor. This local quality variation ensures that no region experiences excessive substrate concentration that would cause inhibition, while maintaining high overall mass transfer efficiency.
4Productivity
If higher pressure is used to enhance driving force for mass transfer, then saturation concentration increases, but compression cost increases
Solution Approach 1:
Instead of using static high pressure to enhance mass transfer driving force, the invention uses dynamic means (secondary loop circulation) to increase gas holdup and interfacial area. The secondary pump provides dynamic liquid flow that enhances gas entrainment and mass transfer without requiring high system pressure, thus avoiding compression costs while maintaining high mass transfer rates.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration effectively increases gas holdup and mass transfer rates, reduces substrate inhibition, and enhances the production of products like ethanol and acetic acid, while minimizing energy consumption and preventing foam accumulation.
Implementation Method 1
circulates fermentation broth from the downcomer to the top of the riser via a nozzle, breaking up foam
Implementation Method 2
enhancing gas holdup, thereby decoupling the effects of loop pump speed on gas entrainment and liquid velocity
Implementation Method 3
The efficiency of gas fermentation is known to be limited primarily by a low gas-liquid mass transfer rate due to the poor solubility of gaseous substrates
Implementation Method 4
forced-circulation external-loop reactor
Data Source
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AI summary
A reactor system is provided for improved fermentation of a gaseous substrate through the introduction of a secondary loop to a forced-circulation loop reactor. The reactor comprises a primary loop through which fermentation broth comprising a gaseous substrate is circulated through a riser segment and a downcomer section by a loop pump. Downstream of the loop pump a portion of fermentation broth is withdrawn from the downcomer section and is directed to the top of the reactor via a secondary loop. Further provided is a method for improving the mass transfer of a gaseous substrate to a fermentation broth in a fermentation vessel comprising a secondary loop. Further provided is a method for reducing foam in the headspace of a fermentation vessel comprising a secondary loop.