When blending ingredients up-pumping technology can offer potential improvements in productivity, yield and profitability. Steve Spreckley, sales & application engineering manager, SPX Flow Technology, explains how
Multiphase processes, such as hydrogenation, fermentation, waste water aeration and solids make-down represent some of the most complex mixing processes. In these applications, intimate contact between gas, liquid and solid phases is required to achieve the desired process results. Depending on the application, various combinations of impellers can be employed.
Regardless of the specific process or impeller configuration, multiphase mixing is traditionally achieved by combinations of radial and down-pumping impellers. The contents of the tank are pumped by the impellers with the objective being to hold the air or gas down in the tank for as long as possible, suspending solids and blending the liquid components. Using impellers that pump upwardly, rather than radially or downwardly can provide a superior solution.
Up-pumping mixing technology uses multiple hydrofoil impellers in multiphase mixing applications. This reduces reaction times, increases yield, improves purity, enhances uniformity and lowers catalyst costs. Moreover, considerable mechanical advantages exist. Because fluid forces and torque fluctuations are reduced, there is less vibration and that improves seal and bearing life.
Empirical experience, together with results of formal tests, demonstrates that up-pumping technology offers advantages. Even so, many process engineers find the technology counter-intuitive. Multiple up-pumping impellers circulate the contents of the tank up through the impellers. If the goal is to hold the gas or air in the tank for as long as possible, then why pump it up and out of the tank?
A better understanding of how up-pumping works requires a close examination of fundamental mixing concepts as they relate to multiphase mixing. Often in multiphase mixing applications the objectives include: blending, suspension of solids, gas dispersion, mass transfer, gas absorption and heat exchange. Hydrogenation, for instance, requires proper hydrogen dispersion, good catalyst distribution, correct heat removal and good reactant blending. Similarly, the uniform distribution of air and nutrients is essential to the fermentation of biological media.
Although many observers regard mass transfer as the most important step in multiphase applications, the optimisation of any multiphase process must satisfy all of the concurrent process goals. For instance, improvements in blending or heat transfer can increase the process reaction rate, while not directly influencing mass transfer.
Traditional multiphase systems with combinations of radial and down-pumping axial flow impellers often create flow patterns within the tank that lead to staging, or dead zones, between the impellers. That results in sub-optimal blending, dispersion and solids suspension as well as a decrease in the resulting mass transfer.
Multiple up-pumping impellers, on the other hand, create a single loop flow pattern similar to that of a draft tube. Computational Fluid Dynamic (CFD) modelling techniques and laser velocimetry analysis have both predicted and confirmed the single loop flow pattern. In up-pumping systems, the rising gas is driven up through the impellers to the top impeller, which produces a strong recirculating flow. The velocity of the recirculating flow pattern is so great that the system’s contents (including entrained gas bubbles) actually accelerate back down the sidewall to the bottom of the tank. The bottom impeller reverses the flow and pumps the tank contents upward, once again creating the single loop flow (see Fig.1).
Enhanced mass transfer and gas hold-up: The flow pattern affects all the mixing objectives and in traditional systems the lowest impeller disperses gas as it is sparged into the tank. The objective of dispersion is to increase the gas-liquid interfacial area for greater mass transfer.
It is particularly important in coalescing systems, such as fermentation, because air bubbles tend to increase in size as they rise to the surface. Up-pumping systems maintain constant gas dispersion throughout the system and enhance the gas-liquid mass transfer.
An additional benefit, even without sparged air or other gas, is that the recirculating velocities of the upper impeller induce surface gas into the fluid. That results in higher gas hold-up. Gas hold-up has always been closely related to mass transfer performance and nowhere is that improvement in gas induction more evident than in hydrogenation.
Actual process results have confirmed that significantly less hydrogen sparging is required. In fact, some end-users have eliminated hydrogen sparging, and the associated high-costs of capital equipment, in favour of 100% surface induction.
Improved blending, suspension and heat transfer: The strong velocities down the sides of the tank wall in the single-loop pattern dramatically improve blending, solids suspension and heat transfer. In traditional multiphase mixing systems, characterised by tall, narrow tanks and distant impeller spacing, dead zones often develop between impellers.
With up-pumping systems, solids are distributed uniformly throughout the tank and blending is significantly faster and more uniform. In hydrogenation, this means that the catalyst is distributed evenly throughout the tank and is more fully utilised. Similarly, pH-sensitive processes such as fermentation also benefit. Enhanced blending performance maintains process pH uniformity throughout the tank.
Because of the strong uniform flow pattern, heat transfer also improves. Temperature fluctuations within full-scale tank installations have been measured at only +/-0.2ºC variance. These improvements combine to increase the productivity of the system.
Another significant advantage of up-pumping is that it cannot flood, regardless of the power level or gas rate. Flooding occurs when the gas (not the mixer) controls the flow pattern. The movement of the tank contents is chaotic, and, if solids are present they remain at the bottom of the tank. In large tanks, the only way of determining whether a tank is flooded is to see if there is geysering on the surface. If a tank is flooded intermittently, however, the surface is smooth even though no distinct flow pattern exists below the impellers and solids still remain on the bottom.
Finally, increases in power are directly associated with improvements in process operation. In all systems the power imparted by the impeller decreases on aeration and must be accounted for in the design. The power drop is defined as the ratio of gassed horsepower to un-gassed horsepower and is known as the K-factor. The lower the K-factor, the more the power drops off on aeration.
The K-factor of the up-pumping system has a much flatter response to gas rate increases than the K-factors of traditional systems. This allows greater operational flexibility and less dependence on the expensive gas interlock and/or speed control devices needed to prevent the overloading of traditional systems upon gas failure.
Up-pumping applications: Most multiphase systems are idiosyncratic and must be engineered to optimise results. Usually up-pumping systems begin as retrofits in older systems, but there is an increase in demand for new systems. SPX Flow Technology is seeing a number of companies running both types of systems side-by-side in order to quantify the production improvements of the technology.
In installations where several 50-hp to 300-hp hydrogenation reactors are in use, those reactors retrofitted with up-pumping technology revealed a five-fold increase in catalyst life and elimination of heating coil fouling, thereby reducing downtime. A sparger was no longer required because sufficient gas was drawn in from the headspace and batch time was reduced by 50%.
In a polymer oxidation plant using a 50-hp mixer, batch time was reduced from 12 to 1.5 hours and product purity improved so much that earlier batches were recalled. Elsewhere in a 750-hp fermenter containing viscous coalescing media, large gas bubbles were reduced significantly in volume while blending and temperature uniformity increased dramatically. As a result, a 30% improvement in yield occurred with a 50% reduction in power.
So long as a multiphase system is mass-transfer limited, up-pumping technology offers potential improvements in blending, heat transfer, solids suspension, gas induction and dispersion. Flooding, coalescence and shear can also improve.
This explains why the mass transfer coefficients of up-pumping systems are generally two-fold greater than in traditional systems. That in turn translates into significant improvements in system productivity, yield and profitability.
