| |
Solid Suspension
Industrial Portable
Mixers:
We have mixer selection
tables for suspending
solids however it would not be wise to publish them because they have
limitations. In other words, and more importantly, you need to know when
to apply them. In most cases, adding solids can turn the mixture into a
Slurry, which may act independently from an insoluble solid for example.
Solids Suspension Tables, for insoluble solids, generally
encompass, the degree of suspension, the percent by weight solids, the settling
velocity of the solids, versus the tank volume, but they do not encompass many other
parameters. The following is a general
discussion of solid-liquid mixing and the problems related to sizing a mixer
based upon these tables.
INFORMATION
REQUIRED TO DESIGN A MIXER:
 | Percent by Weight
of Solids |
| Solubility of the
solid for which the Solids are to be Suspended |
| The Tank Dimensions |
| The Type of Tank
Bottom to be Used:
 |
Cone: (Angle or
Cone Vertical Height) |
|
Dish Type: ASME
(1/6 Tank Diameter) or 2:1 or Semi-Elliptical (1/4 Tank Diameter) or
Hemispherical (1/2 Tank Diameter) |
|
Flat Bottom |
|
Sloped Bottom
& vertical Height of Slope |
|
Special |
|
|
Maximum Solid Size or
Size Distribution based percentages) |
|
Degree of Suspension
Required |
Degree of Suspension:
There generally are four degrees of suspension of free setting solids, which
include on-bottom suspension, off-bottom suspension, mid-depth suspension, and
uniform suspension. All degrees refer to the largest solids that must be
suspended. Solids of smaller size are suspended throughout each
level.
|
Uniform Suspension.
Uniform suspension is required when an overflow is used, where the largest
solids must be suspended to the level of the overflow weir. Generally
speaking this is very inefficient because it takes approximately five times
the amount of horsepower to achieve uniform suspension as compared to
off-bottom suspension. The preferred method is to utilize a down-comer
or a draw-off pipe or tube located at or near the tank bottom for
draw-off purposes in combination with off-bottom suspension. |
|
Off-Bottom Suspension.
Off-Bottom Suspension is where the largest solids are suspended just off the
tank bottom. This is the most common level of suspension used.
Generally speaking, it is more efficient power-wise to drive the flow of fluid
downward to the solid catalyst at the bottom of the tank, than it would be to
try to suspend these solids. |
|
On-Bottom Suspension.
On-Bottom Suspension is also very popular in that the largest solids are not
suspended per se, but are allowed to move upon the tank bottom, with the smaller
sized solids suspended at various degrees throughout the tank. The reasons
for this design parameter is again for economic reasons, as approximately 1/2
the horsepower is required as compared to off-bottom
suspension. |
Particle size
directly influences the mixer design or the initial cost of the mixer.
Think of a grain of sand. Due to its density & size it is quite
difficult to suspend as compared to the overall requirement of primary goal of
the mixing application. Said another way, in waste & water treatment
applications it is acceptable to allow a very small percentage of sand
accumulate in a large basin to continuously process say anoxic sludge which has
a significantly lower suspension requirement. Over time, say several
years, the basin would need to be shut down where the sand would be manually
removed rather than to incur the continual & initial design to remove the
sand.
Said another way, it
is quite common to process solids that are processed to 200 to 400 microns
for slurry mixing applications where the aggregate size is reduced
significantly prior to processing. Think of gold or copper ore that is
ground down prior to processing to remove the valuable pay dirt from the
tailings. In other words, size reduction is performed not just for
diffusion but for mixing pruposes (solid suspension) as
well.
06.22.20
|
