Chemical Engineering February 2017 - 63

TABLE 1. OPERATING COST COMPARISON: PIN MILL VS. AIR-SWEPT CLASSIFIER MILL
(SODIUM BICARBONATE FINE-CUT CAPACITY)
Pin mill
Fine-cut capacity
Exhaust fan
d10, d50, d90, µm
Resulting kWh used
1,500 lb/h
N/A
Operating cost of 1,500 lb/h at $0.13/kWh $2.39/h
the desired products. Thanks to
ongoing design and material improvements,
pin mills are trending
toward producing finer and finer
cuts, now typically in the 30-300µm
range.
When manufacturers are able to
combine a few of the features already
discussed, an assemblage of
key features can now be realized.
For instance, a vertical-shaft pin mill
that provides very fine particle distribution
can now have low maintenance
requirements, be cost-effective
and simple to operate. A final
comparison must be made on the
tradeoff between the cost to operate
in kilowatt hours and relative
fineness of the same material, because
a pin mill still cannot achieve
the low, tight particle size distribution
exactly like an air-swept classifying
mill.
Comparing key attributes
To compare the energy usage, particle
sizes, and operating cost of the
two technologies, laboratory testing
was carried out using a pin mill and
an air-swept classifying mill. Both
mills processed identical material
with corresponding throughput settings.
The simulation was based on
an industrial fluegas-treatment application,
where sodium bicarbonate
would be used for dry sorbent
injection. The resulting particle-size
distributions and energy consumption
were also observed so operating
costs and performance could
be compared between the two systems.
The test was run under similar
conditions and optimal horsepower
settings. Each mill design is flexible
and horsepower can be adjusted
for every application. Table 1 summarizes
results from a side-by-side
comparison test, showing energy
consumption per hour on a kilowatt
(kW) basis.
Energy cost per hour to mill 1,500
lb/h was calculated using a utility
cost of $0.13/kWh, by the following
equations:
Air-swept classifier mill
1,500 lb/h
20 HP @ 460 V (15 kWh)
d10 = 2, d50 = 13, d90 = 48 d10 = 2, d50 = 9, d90 = 22
18.35 kWh
51.84 kWh
$6.74/h
Horsepower formula (3-phase):
(1)
tive, pin mill technology may not be
quite ready to handle the challenge.
However, if there are allowances for
particle size in the upper limits of
the particle distribution, the pin mill
performance is very close to that of
the air-swept classifying mill. When
looking at only the average of particles
- the d50 value - fineness is
almost identical.
In the past, pin mills (Figure 4)
Where:
EFF = Motor efficiency (typically expressed
as percentage)
Conversion from HP to kW:
(2)
Cost to operate:
(3)
As demonstrated in the table,
using $0.13 per kWh, a $5.78/ton
savings is shown in this example.
To understand the savings, consider
a single-shift plant, assuming
8-hour days and 250 d/yr, the
annual savings would be $8,700
using a pin mill versus an air-swept
classifying mill.
However, observing the graph
in Figure 3, it is also important
to note that based on these results,
the pin mill never achieved
the maximum fineness that was
possible with the air-swept classifying
mill: 50% of the particles
produced by the pin mill exceeded
13 µm, compared to 9 µm with
the air-swept classifying mill; and
90% of particles from the pin mill
passed 48 µm, versus only 22 µm
for the air-swept classifying mill.
Regarding target fineness, the
takeaway is that when fineness of
20 µm for 90% or more of the volume
being processed is a key objecCHEMICAL
ENGINEERING WWW.CHEMENGONLINE.COM
FEBRUARY 2017
were only capable of producing top
sizes down to 75 µm - the theoretical
limit of fineness on a pin mill.
Equally for an air-swept classifying
mill, many believed the finest materials
it could achieve were in the range
of 20 µm. Today, the theoretical fineness
in air-swept classifying mills is
in the 5-10 µm range, with the practical
limit being around 20 µm. Pin
mills have achieved a practical range
down to 40 µm, which is very similar
to that of air-swept classifying mills
so the gap is narrow.
For users who want narrow particle-size
distributions, air-swept classifying
mills do it well consistently,
but advancements in pin mill technologies
show that they are now
able to produce comparable particle-size
distributions, too. Although
pin mills cannot do everything that
air-swept classifying mills can, in
some cases, the cost to operate the
latter outweighs the benefits of milling
performance. In these situations,
the advancements in modern pin mill
technologies can far outweigh the investment
and cost to operate an airswept
classifying mill.
Understanding the trade-off between
energy, fineness, and the
convenience features of a modern
pin mill relative to an air-swept classifying
mill may be an important step
in any plant's next design review. n
Edited by Suzanne Shelley
Author
Sam Rajkovich is the vice president
of sales & marketing of Sturtevant
(348 Circuit Street, Hanover,
A
15year
chemical and bulk solids veteran,
Rajkovich directs all international
and domestic
sales and
marketing strategies, overseeing the brand's development
and execution of sales strategies, key account
management, and management of the global sales
team. Rajkovich holds an MBA from The Fisher College
of Business at The Ohio State University.
63
MA 02339; Email:
srajkovich@sturtevantinc.com;
Phone: 781-829-6501; Website:
www.sturtevantinc.com).
http://www.sturtevantinc.com http://WWW.CHEMENGONLINE.COM
Chemical Engineering February 2017

Table of Contents for the Digital Edition of Chemical Engineering February 2017
