Gas - Liquid
The role of the mixer
is to increase the gas dispersion throughout the liquid and produce sufficient shear to break
the gas bubbles into a size that will give the greatest interfacial area for best mass transfer rate.
Some examples of gas dispersion operations are:
- Hydrogenation — vegetable oils using hydrogen.
- Carbonations — CO2 gas conversions of Hydroxides to Bicarbonates.
- Air dispersion for absorption or fermentation processes.
A mixer will increase efficiencies by decreasing the gas bubble size, therefore increasing the interfacial area of the gas for better mass transfer. Increasing the gas-liquid interface turbulence, thus reducing the liquid film resistance. Providing a uniform gas concentration throughout the tank contents.
The agitator influence a gas-liquid system in thew following ways:
- Increasing interfacial area for mass transfer contact by reducing bubble size, thus increasing exposed bubble surface area to the liquid.
- Increasing gas hold-up or residence time, allowing the gas to remain in contact with the liquid longer.
- Providing uniform contact of reactants throughout the vessel.
In order to achieve these three goals, both flow control and shear control are required in gas-liquid systems.
The basic approach to selecting the right mixer for an application requiring gas-liquid contacting is to decide from the process details whether the process is controlled by:
Physical Dispersion (Visual observation of gas -liquid in the vessel)
Mass Transfer (This mass transfer rate is a function of mixer horsepower, gas rate and a system driving force).
Majority (80 - 90% ) of gas-liquid systems requiring agitation can achieve the process result (e.g., mass transfer) by simply sizing for physical gas dispersion.
Gas - Liquid; Physical Dspersion
For every shear rate there is a corresponding shear stress. It is this shear stress, the product of shear rate and viscosity, which tends to tear and break a gas bubble.
The average shear rate at the impeller is a function of impeller speed only. The maximum shear rate around the impeller is a function of both the impeller speed and diameter. An additional range of shear rates is generated throughout the entire contents of the tank as the primary flow interacts with the bulk fluid flow, tank walls and bottom.
The compressor-energy of the gas is released when the gas is discharged near the bottom of a vessel. The energy of the gas is referred to as the isothermal expansion horsepower (IEHP) and is found to be directly related to the superficial gas velocity. (Superficial gas velocity is the volumetric flowrate of the gas at mid-depth divided by the cross sectional area of the tank.) Isothermal expansion horsepower is defined by the following equation:
IEHP = (0.064) (SCFM) In (P2/P1),
where P1 = ambient pressure, absolute
P2 = pressure at sparge location, absolute vessel head pressure plus hydrostatic head
The velocity head of the gas is negligible when compared to the pressure head.
The isothermal expansion horsepower per 1000 gallons can be approximated using the following simple equation:
IEHP/1000 gallons = 0.25 x Vf, where Vf is the superficial gas velocity in feet per minute.
Most compressors operate adiabatically. Adiabatic horsepower is defined by:
Adiabatic HP = (0.2268)x (SCFM)x [(P2/P1)^0.283 — 1]
From adiabatic horsepower the brake horsepower required by the compressor can be determined.
Physical dispersion of a gas can be characterized by "degree of dispersion" which is not only a function of horsepower but also impeller type. When the isothermal expansion horsepower is greater than the mixer horsepower input to the system, the gas controls the flow pattern of the tank and the mixer is said to be "flooded".
Varying degrees of dispersion, between flooding and uniform dispersion may be developed. They are a function of the impeller type, its inherent flow direction, gas rate and mixer power input into the system. It should be noted that nothing can be surmised about the mass transfer rate based solely on analysis of the flow pattern and flow regime of the gas-liquid system.
Gassed vs Ungassed HP
The power consumed by an impeller operating in a gas-liquid system is typically different from the power consumed when the system is running ungassed. In most cases the power consumption of the mixer is lower in the gassed condition than in the ungassed condition. However, some impellers can actually draw the same or greater power in the gassed state, when compared to the ungassed state, depending upon operating conditions.
In the flooded state the gas passes rapidly through the liquid, tending to geyser and "burp" at the surface. This occurrence illustrates that the gas is not adequately or uniformly dispersed. At this point, the mixer probably contributes very little to the process result, if anything. Furthermore, the power consumption of the mixer is greatly reduced, and the mixer is subjected to increased mechanical loads (fluid forces) that can be detrimental. A flooded condition is undesirable in almost all situations since gas dispersion is greatly reduced, regardless of the impeller being used.
The ratio of gassed shaft horsepower to ungassed shaft horsepower is called the "K" factor of the impeller, i.e. K = Pg/Po. For each impeller type, this factor is unique and is a function of gas rate, impeller diameter, speed and horsepower input.
Since loading the mixer depends on the gas rate, it is important to take the K factor effect into consideration when designing the mixer. We must ensure that the necessary amount of power is generated by the mixer in the gassed condition to overcome the isothermal expansion horsepower and obtain the required process result.
If the K factor is close to 1, the mixer should be loaded in the ungassed condition. But we must be sure that in the gassed condition, the mixer is capable of delivering the necessary power required to obtain the process result. If the K factor is significantly less than 1, the situation becomes more complex. It may be suitable to design the mixer with a 2-speed motor, the high speed is used during the gassed phase and the lower speed is used in the ungassed phase.
Loading a mixer based on the gassed horsepower and using a 2-speed motor may offer advantages such as a smaller mixer, lower operating cost, lower spare parts cost, and a smaller, less expensive starter box.
Another option is to have the mixer loaded for the gassed condition and gas-interlocked with the mixer power supply. Again, the customer can design the system such that if the gas shuts off, the mixer shuts off as well, ensuring that the mixer won't overload.
The main objective to keep in mind when deciding how to load a unit is to make sure that the mixer provides the necessary power input in the gassed condition to meet the process requirements without overloading the unit in the ungassed condition.
As a general rule, a gas velocity of 6 feet/min requires 3-10 HP per 1000 gallons to avoid flooding of the impeller in the gassed condition. To invest this amount of HP requires speeds from 125-190 RPM.
The gas factor is used to modify the HP for gassed conditions, the mixer will overload if run at the same speed in the ungassed state.
Surface Tension and Viscosity Effects
The surface tension between air and water is typical of many gas-liquid systems.
The flow pattern of the gas as well as the liquid is primarily determined by the balance between isothermal expansion horsepower (or superficial gas velocity) and mixer horsepower, regardless of viscosity or surface tension.
If the surface tension is reduced, the visible effect is a marked reduction in bubble size, and a reduction in the tendency for geysering at the surface. Because the finer bubbles tend to rise more slowly, the gas is also driven to lower levels in the tank. Liquid film mass transfer coefficients also increase.
The rise velocities of an air bubble through liquids of various viscosities can be estimated. A rise in velocity of 10' to 60' per minute is needed for gas bubbles to escape from the sparge ring and pass upward through the system. Bubbles 2" to 4" in diameter are common in high viscosity gas liquid systems. At the surface these larger bubbles look undispersed, but this is necessary for bubbles to escape in high viscosity systems.
Gas Disperison with Solids suspension
Determine the horsepower required for solids suspension and using the impeller diameter to tank diamter ratio ( D/T ) as selected for gas dispersion. If the mixer horsepower applied during gas injection is more than four times this amount, the solids suspension will be adequately achieved.
If the gassed horsepower level under the same conditions as above is between
three and four times the solids suspension power requirement, and the dispersion level chosen is intimate or uniform, the solids suspension objective will still be accomplished.
If the combined gas dispersion-solids suspension jobs do not fall in either of the
above categories, contact ZAIN.
Gas Disperison Spargers - Turbines
Gas is added to the system through a sparge ring or tube. A sparge ring is mounted 6"-10" below the turbine and its diameter should be the same as the impeller. It should have the gas outlet holes on the top and the total area of the holes should be greater than the inside diameter of the ring. Several drain holes are drilled in the ring bottom to allow draining totake place when the tank is emptied.
In cases where fine solids in the tank might settle out and plug the holes in the top of the sparge ring, it can be reversed so the holes are on the bottom. The efficiency of this reversed sparge ring suffers only slightly from the upright ring.
A tube or pipe can also be used to introduce gas into the system. The pipe outlet should be 1/2 the turbine diameter below the turbine and as close to the center as possible. Where steady bearings prevent placing the pipe outlet at the center, the pipe should be slanted into the center and end about two inches form the shaft. Sparge rings are generally accepted as being 20% more efficient for gas dispersion than
center inlets and should be used unless mechanical aspects prevent their installation.
Flat Blade Disc Turbines are most commnly used for gas dispersion. Their D/T should range from 0.23 to 0.35. With a square batch a single Flat Blade Turbine (FBT) mounted one diameter off-bottom should be used. When liquid depth approaches twice the tank diameter two turbines should be used with the top FBT having a liquid cover of at least one diameter.