Chemical Engineering May 2013 - 37

η
η
Upper limit
Manual set point
Lower limit
Reject point
Time
Figure 2. With manual control, upper
and lower control limits can often be exceeded,
allowing for out-of-specification
product
Process measurements
Automatic control of process fluid viscosity
ensures consistent product all
the time, reduces product hold times,
and can eliminate human errors and
expensive sample testing (Figures
2 and 3). Also, it provides for a complete
record of how the process varied
over a span of time, instead of at just
one point in time. In a plant environment,
there are many ways that viscosity
can be measured, such as by
a rotational viscometer, a vibrating
element and by a falling object. Understanding
whether a true, defined
shear-rate measurement is needed,
or if you are really just looking for
set-point control, is beneficial when
choosing the right type of instrument
for your application.
Process measurements are made
inline or in a flow loop. A bench-top rotational
viscometer can be used for offline
or near-line measurements, where
a sample of the process fluid is drawn
and tested under controlled conditions
(using the same bath temperature,
shear history, shear rate and so on).
Inline viscometers are immersed in
the process stream and measure continuously
under process conditions.
Installation can be in a side-stream,
in the main flow stream or in a tank.
It is important to consider how cleaning
and maintenance of these devices
might occur, if necessary, when deciding
on the installation.
It is also important to make sure
that a representative sample of the
fluid will be measured. Possible concerns
about stratification, mixing and
turbulence should be considered. The
instrument will measure the product
with which it makes contact, so making
sure the fluid that the instrument
" sees " is the material that you want
it to measure, is a primary consideration.
The demands of laboratory
versus process environments are difUpper
limit
Automatic set point
Lower limit
t
t
Time
ferent, so it is unlikely that the same
equipment can be used for both styles
of measurement or that the exact
same measured value will be generated.
However, if done properly, the
results of both laboratory and inline
measurements will follow the same
trend, making inline measurement
useful for ensuring consistent production
quality.
Choosing an instrument for inline
measurement. When evaluating
an instrument for inline viscosity
control, there are several parameters
that must be considered to provide
the proper installation. The answers
to these questions will eliminate some
types of viscometers, and aid in defining
the specifications of the final instrument.
These questions include
the following:
* What are the minimum, maximum
and average pressure and
temperature requirements for the
application?
* What is the expected viscosity range,
and control set-points desired?
* What are the minimum, maximum
and typical flowrates in the
process?
* What is the area electrical classification
(NEMA 4; NEMA 7; ATEX,
for example)?
* What are the necessary materials of
construction, and recommended seal
and elastomer materials? (This can
often be determined based on what
the plant is using for other process
equipment in the area, such as
pumps.)
* Where will the instrument be
mounted? This will determine the
style of instrument to be used. Examples
include a tank/flange mount
(Figure 4); a flow-through housing
(Figure 1); a probe style for barrels
(Figure 5); and mounting from the
top of an open tank
For process control measurements,
the critical factors are stability, repeatability
and sensitivity to changes
in viscosity. A stable, repeatable reading
from an instrument that is senFigure
3.
Better product
and process
control can be
obtained with
inline measurements
sitive
to change in the process will
allow the engineer to properly control
the process.
Applications
Most products are formulated to flow,
spray or coat in a controlled manner.
Monitoring viscosity at critical shear
points ensures that the product will
act the same way every time for the
user. This is the most tangible indicator
of quality. With the increase
in standardization initiatives, such
as ISO 9000 and process analytical
technology (PAT), there is an increasing
use of viscometers to establish
and document the desired properties
of products. To a much larger extent,
the use of viscometers for quality control,
and in particular, the use of inline
viscometers, wherever possible,
to automate the process of controlling
desired fluid properties is on the rise.
Quality, consistency and customer acceptance
require testing and control of
key parameters, of which viscosity is
certainly an important one.
Some typical operations where process
viscosity control can be important
include the following:
Determining the endpoint. For
applications involving chemical reactions,
viscosity of a product is continuously
monitored in-tank and the
process is either stopped, or the next
steps are taken once a specific viscosity
limit is reached.
In addition to determining the endpoint
of chemical reactions, this approach
is also used to determine the
endpoint of blending operations, such
as the blending of multiple ingredients
in a batch process. One example is
synthetic-fiber manufacturing. Latex,
spandex and other synthetic materials
are used to manufacture fibers, which
are stretchable, rugged and used in
many applications such as clothing.
The manufacturing process is carried
out in a reactor, where both temperature
control and tight viscosity control
are required over the steps and additions
made during the process.
ChemiCal engineering www.Che.Com may 2013 35
Viscosity
Viscosity
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Chemical Engineering May 2013

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