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A well designed batch plant can improve profitability

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A well designed batch plant can improve profitability Powered By Docstoc
					  Batch PLANT
         A well designed batch plant
          can improve profitability
  High quality mixed batch is essential for producing quality glass and
maximising production yields. Here D D Burgoon* of Toledo Engineering
Company points out that the glass making process starts at the batch plant
     and describes the critical features of good batch plant design.
        n Mr Burgoon’s article in Glass



I
                                                                                                      Fig 1. Basic process steps of a glass
        International, September 19991, the                                                         manufacturing operation.
        point was made that the glass man-
        ufacturing process is only as good                                                              0.0002 gm/cc range. Increasingly, direct
        as the weakest element in the                                                                   glass analysis, such as x-ray defraction,
process, that is, batching, melting/condi-                                                              allows direct tracking of the key oxide
tioning, forming, annealing, cold end                                                                   percentages. With the desired glass char-
treatment, or packaging (fig 1). Similarly,                                                              acteristics established, the design of the
batch plant performance is only as good                                                                                         batch weighing sys-
as its weakest element, whether it be,         silo inlet to minimise                                                           tem can proceed.
materials handling, weighing, mixing or        segregation during                                                                 Feeding devices
the electronic controls (fig 2). This article   filling. In addition,                                                             (not conveying
is written to highlight the essential ele-     particle size for all                                                            devices) are used to
                                                                               Fig 2. Basic process steps of a glass batching
ments of good batch plant design.              raw materials needs          operation.
                                                                                                                                feed raw materials
                                               to be in the same                                                                to the scale(s). The
Materials handling                             range and this is controlled during the                 feeder type, for example, vibratory,
During unloading of the various raw mate-      procurement phase.                                      screw or gate is dependent on the raw
rials, care must be taken to avoid both           The final control stage in segregation                material characteristics. Free flowing/low
contamination and segregation.                 takes place at the silo bottom, where mass              permeability materials are usually han-
Contamination shows up in the glass as         flow devices such as bin activators, special             dled by vibratory feeders (fig 3), whereas
off-chemical composition, or as stone type     flow control inserts and special bin bottom              erratic flowing/high permeability (easily
defects. Segregation will show up as stria-    shapes are used to minimise or eliminate                aerated) materials are handled by special
tion (cord or ream).                           funnel flow discharge. Ideal mass flow in a               screw feeders, with a flush control device
   Contamination can usually be avoided        silo would be for all material in the cylin-            (fig 4).
with good operational practices, such as       drical section to draw down at the same                     All scale feeding devices must be sized
making sure the unloading hopper is            rate, thereby keeping the surface profile                for both feed rate and necessary weigh-
clean before use, using a non-residual         unchanged. Achieving mass flow at dis-                   ment resolution (fig 3). The design para-
or non-contaminating bucket elevator           charge helps to compensate for segrega-                 meter for necessary resolution (variation
boot, and using a distribution system          tion created during the filling operation.               in angle of repose at cut-off, plus feeder
that cannot leak between adjacent silo         Furthermore, mass flow assures a uniform                 coast) are usually more than sufficient to
positions. For critical applications, dedi-    source of material at the weigh feeder                  satisfy the feeder capacity requirements.
cated unloading facilities are used for        inlet, which is essential for accurate,                     Many years ago, TECO developed the
each material.                                 repeatable ingredient weighments.                       Superfine feeder for handling fine
   Each raw material is subject to segrega-                                                            mesh/high permeability materials (fig 4).
tion during the silo filling and withdrawal     Batch weighing                                          A rotary vane feeder serves as an air-lock
operations. Silos should be kept as full as    Before designing the batch weighing sys-                to guard against material flushing. The
possible to minimise segregation created       tem, the glass technologist establishes the             rotary vane feeds a small surge hopper
by the impact of the material striking the     batch composition required to obtain the                and when full, the material overflows and
pile during the fill operation. In some         desired glass characteristics. The technol-             provides a fast-feed rate for filling the
applications, diffusers are installed at the   ogist also establishes the statistical toler-           scale. Once near the desired weight, the
                                                       ance limits to which each raw                   rotary vane feeder stops and a special
                                                       material must be weighed in order               screw feeder continues to feed material
                                                       to stay within the suitable range of            from the surge hopper, until the batch
                                                       properties. One common property                 weight is satisfied. A safety butterfly valve
                                                       used for statistical process control is         closes to assure that no additional material
                                                       glass density. Typical glass density            will flow to the scale. The Superfine feeder
                                                       limits for container glass are in the           can deliver almost any feed rate capacity
                                                       0.0020 gm/cc range, whereas the                 desired. This feeder type has been used in
                                                       limits for float glass are in the                conjunction with minor ingredient scales,
                                                         Fig 3. Vibratory feeder with cut-off
                                                                                                       as small in capacity as 20lb. with scale res-
                                                      resolution shown.                                olution of 0.02lb.

GLASS INTERNATIONAL NOVEMBER/DECEMBER 2000                                                                                                       21
                                                      Modern batch plant for the production of ‘E’ glass

  Batch PLANT                                       (continuous) glassfibre.




  Fig 4. Superfine feeder with                       mixer. In recent years, however, batch
rotary vane feeder, surge w/                         plant structural height (and cost) are
overflow, special screw                                  being reduced, by eliminating the
feeder and butterfly shut-
                                                           batch surge hopper and mounting
off valve.
                                                              load cells under the mixer or
   The glass compo-                                              pneumatic blender/trans-
sition and statistical                                              porter. This requires spe-             attention at the time of design is the elec-
limits set by the                                                    cial know-how to negate               tronic control system. Strain gauge load
glass technologist                                                   erroneous forces created              cells and digital scales are universally used
provide the basis                                                    by mixer motion, con-                 for weight detection. PLCs, with update
for determining scale resolution and                                 duits and piping, which               time in the 25 to 50 millisec range, are
accuracy, to which each ingredient is                                can affect the scale accu-            commonly used to obtain adequate con-
weighed. Scale resolutions of 1:5000 are                             racy and repeatability.               trol resolution for the weighing process.
common and in some cases, 1:10,000 or                                                                      Each ingredient is weighed under the
more are used. The weigh hopper must                                   Mixing                              supervision of real-time statistical control,
also be designed to assure cleanout at dis-                               The design of the mixing         which monitors each weighment and auto-
charge. The weighing system must be                   system is critical since it is the final stage        matically adjusts the ingredient cut-off, in
enclosed to contain dust, thus the scale              in developing necessary mixed batch                  line with the trend of previous batches,
system must be designed to be immune                  homogeneity. Today, pneumatic blending               thereby minimising process upsets.
from internal pressure effects, which can             is typically used for fine mesh ingredient               The furnace operator is commonly
be created by the operation of a dust col-            applications, like E-glassfibre batch, and            responsible for the batch plant opera-
lector, bucket elevator, and/or mixer.                the turbine-type mixer for the coarse                tion. Information and data from the
Pressure effects on an improperly                     mixes, such as soda-lime batch. Both                 batch plant is transmitted electronically
designed weighing system can create                   methods fluidise the batch and introduce              to the furnace control room, as well as to
errors that are not immediately apparent,             a turbulent force to create the mixing               the plant and company-wide informa-
since the scale would be a combination                action. The mixing time to achieve the               tion systems.
gravimetric plus pressure force detector.             desired degree of mixed batch homogene-
One method of obtaining pressure immu-                ity is established at start-up.                      Results
nity is to have the weigh feeders enter the              Cullet is generally introduced into the           To test how well a new batch plant (or
hopper side (not through the top) (fig 5),             process after the mixing operation, to               modernisation) is performing, one can
or by using a feeder ring, which is a pres-           reduce wear and maintenance of the                   compare glass density variation with a
sure counter-balancing scheme.                        mixer, and to avoid cullet/mixed batch               counterpart, that is, the obsolete batch
   A check scale is a vital element of any            segregation. TECO prefers to batch-                  plant or a similar facility within the
batch processing plant and it is a simple             weigh cullet and blend it with the mixed             organisation. TECO recently provided,
task if a separate batch surge hopper is              batch as it is conveyed to the storage bin           on a design/build basis, a new batch
used between the weigh gallery and the                at the furnace.                                      plant for an existing three-furnace glass
                                                                   Mixed batch, regardless of how          container plant. 30 days before
                                                                it is mixed, is subject to segrega-        switchover to the new batch plant, the
                                                                tion during conveying to storage           glass density data was noted for all three
                                                                at the furnace, and subsequent             furnaces. After switchover, the density
                                                                delivery to the furnace charger.           was noted for an additional 30 days. The
                                                                Proper design of the conveying             results of the ‘before and after’ glass den-
                                                                system and transfer points will            sity conditions are shown in fig 6.
                                                                minimise segregation. Most soda-           Obviously, the -30 day is glass density
                                                                lime batch is wetted with water in         before switchover and +30 day are the
                                                                the mixer, as a final step to pro-          results with the new batch plant. An
   Fig 5. Pressure balanced vs pressure sensitive weigh        vide cohesion, minimise particle            approximate ten-fold improvement in
hopper/feeder arrangement.                                     segregation, and dusting.                   glass density variation was obtained with
                                                                               Typically, mass flow         the new batch plant.
                                                                               bins are used for              Who said well performing batch plants
                                                                               batch/cullet storage        aren’t beneficial? In the above example,
                                                                               at the furnace. This        the ten-fold improvement in glass density
                                                                               is very important for       stability increased the pack rate a few per-
                                                                               batches that cannot         centage points, which had a positive and
                                                                               be wetted and must          direct impact on profitability.
                                                                               be conveyed dry.
                                                                                                           References
                                                                             Electronic                    1 Better plant performance begins at the
                                                                             controls                      batch house, D D Burgoon, Glass
                                                                             The last process ele-         International September 1999, pg 9.
                                                                             ment requiring
                                                                                                           * D D Burgoon
                                                                               Fig 6. Glass density his-
                                                                                                           Toledo Engineering Co Inc, Toledo, Ohio,
                                                                             tory 30 days before/after
                                                                             new batch plant.              USA. Fax+1 419 537 1369.

22                                                                                                     GLASS INTERNATIONAL NOVEMBER/DECEMBER 2000

				
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