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Abstract
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Probes
for monitoring electrostatic phenomena in challenging environments, such as
fluidized bed reactors. These probes include a coated or uncoated static
probe for measuring electric field and or particle charge state, an
oscillatory electric field probe for measuring electric field, a chopped
electric field probe for measuring electric field, and a radio-frequency
antenna probe for detecting electrostatic discharges.
BACKGROUND
Fouling in commercial fluidized bed reactors, including gas phase polymerization
reactors, is a significant operational issue. Fouling negatively impact
operational efficiency and ultimately requires time-consuming shutdown and
maintenance. Understanding the causal factors of fouling within the reactor
systems would be beneficial in reducing fouling. Fouling in fluidized bed
reactors can be strongly affected by physical processes within the fluidized
bed reactor, such as electrostatic charge and solids carryover within a
recycle loop. Commercial probes, including those commonly referred to as
static probes and acoustic probes, exist for measuring certain physical
parameters within fluidized bed reactors, such as electrostatic charge and
solids flow. In common practice, however, these probes do not reliably
directly measure these phenomena, and are instead dominated by noise and/or
artifacts in the signals. Thus, probe signals have proven to be of limited or
no value in monitoring the operational status of or diagnosing inefficiencies
in fluidized bed reactors. Electrostatic charge can affect commercial process
units such as chemical reactors, granular particle handling equipment,
transfer lines, holding tanks, and shipping containers, for example. The
types of operations can include fluidized bed reactors for producing a variety
of chemical products such as gas, liquid or solid products such as
polyethylene. Cryogenic processes or handling equipment are another notable
case where the dry environment can lend itself to electrostatic charge
buildup in at least some portions of a process, especially if solids such as
ice form due to the cryogenic conditions. The buildup of electrostatic charge
on particles, and/or process components results in the formation of an
electric field, which then exerts forces on particles or components within a
given process or system. Additionally, in cases where the electrostatic
charge is sufficient, electrostatic discharge events can occur, which by
themselves can be deleterious to reliable or safe operations, or simply an
indicator that electrostatic effects are present at a given moment. For
example, commercial polyethylene (PE) reactors utilize a fluidized bed to
suspend catalyst particles that grow into PE resin particles by converting
ethylene gas into polyethylene resin. Collisions between catalyst particles,
resin particles and also the reactor wall can result in the particles
becoming charged. The wall can also become charged wherever it has an
insulating coating or surface deposit or layer. If the net charge per volume
(ρ) in a cylindrical reactor is uniform, the electric field is given by:
E(r)=(ρr)/2∈, where r is the cylinder radius, E(r)
is the electric field as a function of reactor vessel radius and ∈
is the relative permittivity of the volume. This electric field is greatest
at the reactor wall, and creates a force (F) on the charged particles given
by F(r)=qE(r), where q is the particle charge. Both F and E are still a
function of radius as mentioned above. Particles with charge of the same sign
as the bulk net charge density experience a force towards the wall. If this
force is large enough, it can pin the charged catalyst and resin particles to
the wall, and they tend to grow into PE sheets (sheeting) that eventually
fall off and clog up the resin discharge system, forcing a shutdown of the
reactor. In addition, if the electric field is larger than the Paschen
breakdown strength of the gas in the reactor, electrical discharges, or
sparks can occur through the gas. Any isolated conductors in the reactor can
become charged by particle impact, and they can also spark to nearby metallic
objects. In addition, the insulating coating on the reactor wall can charge
to a level that supports propagating brush discharges across and through the
wall surface. It is desirable to instrument the reactor with sensors that can
indicate a highly charged condition, because that can eventually lead to
sheeting and a forced reactor shutdown. Advanced knowledge of a sheeting
condition allows operating parameters to be adjusted to eliminate the
condition. A highly charged reactor condition can be accompanied by sparking
inside the reactor, while lower levels of charge would not result in sparks.
Therefore, sparking can be used as an indicator of a highly charged reactor,
and indirectly, as a warning that the reactor is in a condition conducive to
sheeting. It is well known that electrical sparks emit electromagnetic waves,
typically in the radio frequency (RF) part of the electromagnetic spectrum
between about 100 kHz and 10 GHz. Due to the challenging environments
encountered in chemical process equipment, especially within a high
temperature fluidized bed with reactive gas mixture, no prior art exists for
detection of RF signals arising from electrical discharges. In simpler
environments, such as assembly rooms for sensitive semiconductor components,
some technology does exist. For these simpler environments, the current art
includes technology such as the 3M company's EM Aware1, which contains radio
frequency receivers with appropriate antennas used to detect sparks by
receiving these radio waves. The amplitude, spectral distribution and
radiation pattern of the emitted waves depends on the source of the spark.
13M™ EM Aware TNG ESD Event Monitor. Models 3M034-3-TNG, 3M034-030-TNG and
3M034-031-TNG As indicated in the EM Aware user guide published by the
manufacturer, this technology is intended to be used only as follows.
“Intended Use”—The 3M EM Aware TNG ESD Event Monitor monitors up to four key
parameters that keep you aware of critical symptoms of ESD problems: 1) ESD
events; 2) static voltages; 3) ionization balance; and 4) charge decay. The
thresholds for these parameters are fully adjustable to suit your needs. The
improved design features a metal case module with built-in LCD display, a
control joystick, remote antenna, power supply and a data output. The monitor
system must be installed as specified in this user's guide. It is intended
for use in the following environmental conditions only: (1) indoor use; (2)
altitudes up to 2,000 meters above sea level; (3) temperature range of 10° C.
to 40° C.; (4) maximum relative humidity of 80% for temperatures up to 31°
C., decreasing linearly to 50% relative humidity at 40° C.; and (5) pollution
degree two (office, laboratory, test station). It would be desirable to have
instruments which can reliably measure and monitor electrostatic phenomena
within these systems. These instruments could also be combined with new
methods for processing and interpreting probe signals in fluidized bed
reactor systems. It would further be desirable to have new methods, which may
rely upon the use of these instruments and processing methods to provide for
more efficient system operation and reliability.
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