(315d) On the Stability of Pulse-Jet Regenerated Bag Filter In Case of One Regeneration Pulse At Upper Pressure Drop Limit

Pulse-Jet regenerated bag filters are increasingly becoming an integral part
of process plants. The problems with their operation directly influence the
process plant operation and may involve loss of production time. Therefore, a
long term stable operation of the filter plant is desirable for the
manufacturers as well as process operators. Intrinsically the bag filter
operation is semi-continuous which consists of alternate periods of filtration
(cake formation) and regeneration (cake detachment). The dust deposited during
filtration period (filter cake) is a source of higher pressure drop which must
be removed at certain operating points determined either by a preset upper
pressure drop level or constant filtration time. The filter cake is removed by
short duration high pressure reverse pulse-Jet during regeneration period. The
regeneration period is normally very short as compared to filtration period for
a stable filter operation. The plant may run unstable due to variation of the
operating parameters. The instability may be temporary where the filter reaches
a new stable steady state, reversible where system can be restored to its
initial conditions by manipulating the operating parameters, or it may be
irreversible so that the parameters like pressure drop keeps on increasing
beyond the allowable limits of the plant. Then shutdown of the plant remains
the only choice.

The pulse-jet bag filters can be operated (1) between upper and lower
pressure drop limits with no control on number of pulses, or (2) at upper
pressure drop limit with a fixed number of pulses without regulating the lower
limit, or (3) for fixed filtration time and fixed number of pulses. The
stability is measured by  (a) stable and steady filtration cycle time and
number of cleaning pulses per filtration cycle, (b) by the steady filtration
cycle time and a steady lower pressure drop limit, or (c) steady upper and
lower pressure drop limits respectively depending on the bag cleaning sequence
(1), (2) or (3).

While the stable operation is desirable, it is observed that the filter
de-rails under many operating situations. Knowledge of the operating conditions
which have potential for instability is important. Few attempts are reported in
literature for the prediction of stable versus unstable filter operation [1-
4]. Experimental investigation of the filter stability while operated in mode
(1) is presented elsewhere [5]. Study of pulse-jet bag filter over a range of
operating parameters which may or may not lead to unstable filter operation for
the cleaning mode (2) is presented in this work.

The test facility consists of three rows of bags. The set up is presented
elsewhere [6] while the operating conditions are listed in Table 1.

Table  SEQ Table \* ARABIC 1: The range of operating parameters

The difference between the static pressure developed by the pulse-jet and DP_max is the detachment over pressure (force
intensity) responsible for the cake detachment. Based on the experimental
investigations, the filtration time per pulse, detachment over pressure and
velocity are plotted in Fig. 1. The operation is observed unstable at
detachment over pressure below a threshold. At higher detachment over pressure,
the operation is stable, however, two interesting situations are observed:
 first at 41 mm/s, a stable operation (a) went unstable (a to a1) because
of reducing pulse pressure from 4 bar to 3 bar at 2200 Pa DP_max
. Reducing DP_max could not
stabilize the operation (a1 to a2). Only increasing the pulse pressure from 3
to 4 bar stabilizes the operation (a) at the same
velocity. The second at 68 mm/s and 2300 Pa DP_max
, stable operation (b) turns unstable (b to b1) and

(b1 to b2) when DP_max
is reduced. The cycle time per pulse is so less that there is little filter
cake formed during a filtration cycle. Decreasing the upper pressure drop limit
increases the detachment over pressure but decreases the cake area load
further. The over pressure can not remove the cake when there is no significant
cake on the filter bag, therefore, the operation leads to continuous pulses.

Figure  SEQ Figure \* ARABIC 1: Effect of operating parameters on bag
filter stability

This mode of regeneration control has less tolerance because regenerated
area per cycle is fixed. If cake detachment becomes ineffective due to some
reason, the lower pressure drop keeps on increasing and leads to either short
filtration cycles or continuous pulses.

Many combinations are identified when the operation de-rails from stable to
unstable operation on changing one of the operating parameters. Filtration
velocity, jet pulse pressure and upper pressure drop limit have potential of
de-railing stable operation to unstable. There exists a threshold detachment
over pressure below which the operation leads to unstable condition at certain
filtration velocity. This threshold increased with increasing velocity. 
Increasing cake detachment pressure by merely decreasing the upper pressure
drop level will not work once the operation goes unstable. There is a certain
lower limit on the upper pressure drop below which the operation runs unstable
at specific velocity. The upper limit on the upper pressure drop level is
expected above 2300 Pa however. Reducing upper pressure drop limit leads the
system to unstable operation at higher velocity when filtration cycles are
already short.

Thus conclusion can be made that the detachment of a thicker cake is more
efficient as compared to a thinner cake at low cake area loads. There are set
of conditions when operation becomes unstable irrespective of the path to reach
those conditions. One can say that stable/unstable operation depends on prior
set of conditions, therefore.

Acknowledgements

Authors acknowledge the project funding by Austrian Science Foundation (FWF)
under project P 16313-No.7, financial support of Higher Education Commission, Islamabad, Pakistan, supply of needle felts by M/S Inspec
Fibers (Lenzing, Austria), and M/S Alicona Imaging (Graz, Austria) for microscopic analysis of
needle felts.

References

[1] Leith D. and Ellenbecker
M. J. Theory for pressure drop in a pulse-jet cleaned fabric filter.
Atmospheric Environment, 14(7):845?852, 1980.

[2] Morris K. and Millington C.A. Modeling fabric filters. Filtration and Separation, pages 478?483, November/December, 1983.

[3] Krammer G., Kavouras
A., and Saleem M. Low frequency pressure drop patterns during continuous filter
operation. In Proceedings of the International Congress of
Particle Technology, PARTEC, 2004.

[4] Kavouras A. and Krammer
G. A model analysis on the reasons for unstable operation of
jet-pulsed filters. Powder Technology, 154:24?32, 2005.

[5] Saleem M.,  Krammer
G.,  Ijaz A., Effect of operating parameters on
stability of pulse jet bag filter: an experimental study, World Filtration
Congress, 14-18 April, 2008, Leipzig, Germany.

[6] Saleem M., Krammer G., R¨uther
M., and Bischof H. Optical measurement of cake
thickness distribution and cake detachment on patchily cleaned commercial bag
filters. In Proceedings of the International Conference and Exhibition for
Filtration and separation Technology, volume II, pages 51?58, Wiesbaden, Germany, October, 2005.