Gas-liquid Co-injection Sparger In Bubble Column Reactors

Bubble column or slurry bubble column reactor is a common gas-liquid contact and reaction equipment.It is widely used in residue oil hydrocracking,P-xylene oxidation,Fischer-Tropsch synthesis and other fields due to simple structure,absence of any moving parts and excellent heat and mass transfer characteristics.Gas sparger is the most important accessory for the bubble column,which can achieve gas uniform distribution and enhance gas-liquid mass transfer.Furthermore,gas sparger determines the initial size of bubble,gas holdup distribution and gas-liquid mass transfer characteristics.For large scale bubble column,when designing the gas sparger,it is necessary to consider the gas uniform distribution,gas-liquid mass transfer and blockage prevention.However,only the first two requirements are considered in the published literature.In industry,increasing the gas nozzle/hole diameter is a measure most in use to prevent blockage,but it leads an increase of the bubble diameter and weakens gas-liquid mass transfer.Based on this,a kind of gas-liquid co-injection sparger is proposed in this thesis.The main idea is to inject gas and liquid into the reactor at high speed through a large diameter coaxial nozzle which can take advantage of liquid kinetic energy to break bubbles and enhance gas-liquid transfer.Gas-liquid coaxial nozzle can meet the requirements of blockage prevention,gas-liquid mass transfer and uniform distribution of gas and liquid.This thesis covers the following aspects:1.Experimental investigation on a gas-liquid co-injection bubble column.The cold-test experiment was carried out on a bubble column whose inner diameter is 183 mm,the average gas holdup and phase holdup of large and small bubble were measured by the bed expansion method and dynamic gas disengagement.Through comparing the average gas holdup,the ratio of small bubble phase,the Sauter mean diameter of bubbles and the total gas-liquid interaction area between gas jet alone and gas-liquid co-injection,we can find that:compared to the gas jet alone,gas-liquid co-injection make use of liquid kinetic energy to break bubbles,so it can increase the average gas holdup and the ratio of small bubble phase and decrease the Sauter mean diameter of bubbles,then the gas-liquid interaction area was increased significantly,which indicate that gas-liquid co-injection can effectively enhance gas-liquid mass transfer.2.CFD simulation of gas-liquid coaxial nozzle.We carried out CFD simulation with the VOF model to compare the bubble size of different nozzles and with or without liquid feed.The results showed that:compared to circular nozzle,gas injection through annular space of the coaxial nozzle can significantly reduce the bubble size,increase gas-liquid interaction area and enhance gas-liquid transfer.What's more,adding the liquid feed can further reduce the bubble size.3.Design and CFD simulation of large gas-liquid coaxial jet sparger.A large-scale gas-liquid coaxial jet sparger was design for a p-xylene(PX)oxidation reactor whose capacity is 1.2 million tons per year.The sparger takes advantage of gas-liquid coaxial nozzle to meet the requirements of gas-liquid mass transfer and blockage prevention and its structure is multi-ring which is conducive to gas and liquid uniform distribution.We carried out 3D transient simulation of the flow inside the sparger to explore the distribution performance of the sparger.The results showed that the maldistribution can be significantly addressed by replacing usual constant diameter distributor tube with variable diameter distributor tube whose diameter increases with ring diameter.In addition,we investigated the influence of load change on the distribution performance of the sparger and determined the operating flexibility range.