Note: Descriptions are shown in the official language in which they were submitted.
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PRESSUE~E DIFFUSER CONTROl[.LER
BACKGROUND AND SIJMMARY OF THE INVENTION
This invention rela ces to a method for
maintaining all flows in and out of pressurized
5 diffuser machines constant and continuous and of
stabilized consistency thereby facilitating
incorporation of such a machine or machines in a pulp
mill process.
Conventional dewatering machines based on the
rotating drum principle, such as vacuum washers, ai~
pressure washers, drum presses, wash presses and the
like, share the co~mon drawbacks that the outlet
consistency is undstermined and that the filtrates
must exit into atmospheric filtrate tanks. The outlet
consistency varies as a result of temperture
fluctuations, changes in the production flow xate,
variances in pulp drainage characteristics and
mechanical factors associated with the process
machinery.
To compensate for these problems, the
conventional methods employed filtrate tanks as surge
absorbers beca~se the exiting filtrate flow varied
with its discharging consistency. These large
filtrate tanks must be equipped with level
recorders. The tanks must be of considerable
dimensions to absorb the mentioned fluctuations and to
allow suff icient deaeration of the liquor. As a
further drawbac~, the large size resuits in
significant heat losses.
The conventional filtrate tanks also require
adequate pump caparity and pressure head controls to
both dilute the incoming stock and circulate the
e~cess (surplus) liquor countercurrently back into the
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process. Considerable pressure head on the pump or
pumps is required sinre the pressure process i5
conventionally kept we~l above the static head of the
filtrate tank which was exposed to the atmosphere.
Introduction of a continuous diffuser which
did not require any dilution of the incoming stock of
medium consistency coupled with some control over
outlet consistency by adjusting the extraction flow
relative to wash liquor flow marked an improvement
over the conventional processes. However, problems
with pressure loss in the stock flow and air
extrainment in the stock flow remained~ Since pulp
su~pensions of~en contain liquids with fairly high
surface tensions, the entrained air causes the
suspension to become foamy, elastic, to emit smelly
gases and hard to dewater causing treatment problems
in succeeding process machinery. Stacking a series of
diffusers in ~tages within a common tower casing was
only a partial soluton because of practical
limitations on the number of stages in one tower.
Additionally, the filtrate flow or extraction
flow from the conventional diffusers remained
intermittent because of the operating principle of the
machine. The intermittent flow required periodic
injection of li~uor to the interior oE the diffuser
screens. Conventional systems consequently needed
adequate liquor tanks, level recorders, pumps and
other equipment.
Furthermore, the conventional diffusers
needed extraction liquor flow control and separate
wash liquor flow control to the preceding stage in
multi-stage arrangements.
According to the method of the present
invention, all flows in and out of the diffus~r are
kept constant and continuous to eliminate the problems
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commonly associated with diffusers. Consequently,
pressurized diffusers may now be easily incorporated
in a pressuri~ed system thereby eliminating the need
for intermediate ~urge tank~, reducing pump power
demand to a minimum, and totally eliminating the air
entrainment and gas emission problems since the
present invention no longer requires atmospheric
disc~arge~ Level controllers will no longer be
required and the number of flow controllers is almost
halved since the extraction controller for one stage
now concurrently serves as th2 wash liquor controller
for the preceding stage~ Variat;ons in pulp charac-
teristics, production rate and temperature will not
af~ect pulp consistency since they will be governed
purely through flow control using straightforward
instrumentation regulatorsD
According to the pres:ent invention, there is
provided a method of treating cellulosic pulp suspensions
flowing in a relatively continuous stream through.
pressurized diffusers equipped with pulp inlets and outlets
along with treatment liquid inlets and outlets, which
method comprises feeding the pulp stream through the pulp
inlet into the diffuser, withdrawing the trea-ted pulp
through the pulp outlet, governing the pulp withdrawal flow
rate through the pulp outlet by controlling the pulp feed
flow rate into the pulp inlet, es-tablishing consistency
control over pulp withdrawn through the pulp outlet by
controlllng the liquid flow rate from the liquid outlet.
Additional cellulosic flow treatment comprises feeding
treatment liquid through the liquid inlet and may also
include chemical
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admission and/or temperature control over the
treatment (wash-displacement) liquid relative to the
characteristics of ~he pulp withdrawn through the pulp
outlet. Moreover, adapting the method to a multi-
s~age pressurized diffuser system comprises connectingthe desired number o such diffusers in serie5 using
the pulp withdrawn from one diffuser as the pulp eed
f or the 5 uc~eeding diffuser with optional interme~iate
temperature and chemical control of the treatment
liquid relative to the pulp flow ~hrough the pulp
outlet. Further, passing the pulp through an enlarged
portion of the feed pipe or a vessel between the
diffusers provides a desired retention time between
liquid extractions. The agitator in the outlet end of
the first diffuser in series measures pulp
concentration.
BRIEF DESCRIPTION OF' THE DRAWINGS
For the purposes of description, the method
for maintaining constant conti~uous cellulosic flows
is shown schematically in the accompanying drawings:
FIGURE 1 is a side view, partly in cross-
section and partly in elevation, of details of an
exemplary diffuser apparatus utilizable in practicing
the method of the invention;
FIGURE 2 is illustrative of a single
pressurized diffuser stage showing the flow control
method;
FIGURE 3 is a drawing vf the flow control
process with independent control of screen speed;
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FIGUR~ 4 is a drawing of the control process
with independent screen speed, temperature and
chemical addltion control and i5 shown on the sheet illus-
trating figure 6;
FIGURE ~ is a drawing of a multiple stage
arrangement using the flow control method; and
FIGURE 6 is a drawing of a multi-stage
arrangement with flow control showing intermediate
chemical admission, heating and cooling control, and
retention~
DETAI~ED D~SCRIPTION OF THE DRAWINGS
The present inventive method maintaining
constant and c~ntinuous all pulp or cellulosic flows
in and out of a pressurized diffuser. An exemplary
diffuser utilizable ;n practicing the present
invention is illustrated in FIGURE 1.
The exemplary diffuser 10 illustrated in
FIGURE 1 comprises a conical housing 11 with top part
11' and bottom part 11'' having a pulp inlet 12 and
outlet 13, with a powered rotatable scraper 14 mounted
in the bottom of housing 11 facilitating discharge o~
pulp from the ho~sing 11 after treat~lent. A withdrawn
liquid outlet 17 extends from the top part of the
ho~sing 11. A screen body 18 is mounted inside and
substantially concentric with the housiny 11, the body
18 having per~rations or apertures formed therein to
allow passage of liquid therethrough but prevent the
passage of suspended solids therethrough. The
bearings 19 and 19' are provided between the body 18
and housing 11 to allow relative linear movement of
screen 18 with respect to housing 11 in dimension ~A",
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The screen 18 is rigidly attached to one end
thereo~ to a reciprocal sbaf~ 23, which shaf~c 23
passes through a seal 24 at the top o the housing
11. A conventional drive device 25 (such as shown in
U.S. Patent 4,041,560) reciprocates the shaft 23 in
dimension A. Normally, drive 25 will operate to move
the screen 18 downwardly at about the same speed as
the pulp flow from inlet 12 to outlet 13, and ~hen
move it upwardly much more quickly, and then restart
the downward movement. The speed and nature of
operation of the drive 25 may be varied as desired to
accomplish the objectives of the present invention, as
may be the details, dimensions, and relative spacings,
of the housing 11 and screen 18.
For treatment of suspension passing through
the housing 11, a treatment liquid inelt 26 is
provided~ Treatment liquid introduced through inlet
26 is spread out by baffles 27 arranged circumferen-
tially around a concentric li~uid distribution body 15
fastened to the housing part 11'' by ~.hree arms 15' of
which one is hollow and in connection with inlet ~6.
As shown in FIGURE 2, e~emplary apparatus
utilizable in prac~icing the method of the invention
includes pressurized diuser 10/ and associated inlet
and outlet flow controls and the lîke. The pulp flow
~slurry) from a continuous digester, a storage tower
or the like enters pipeline 28, flows through stock
pump 29 (which alternatively may comprise a valve),
and enters the top of the vertically aligned
pressurized diffuser 10 and flows pa~t movable screen
18 therein. The pulp flow ent~ring pressurized
diffuser 10, possibly of varying consistency/ is
stabiliz~d in consistency under its passage through
the diffuser 10 by means of fine adjustment o ~he
extraction flow 30 through outlet 17 relative to wash
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li~uor flow 31 into inlets 26 of said pressurized
diffuser 10. The bottom discharging agitator 14 of
pressurized diffuser 10 gives a very reliable output
signal for consistency control to a consistency
controller 22 and control ratio relay 33. ~his signal
adjusts the set point of control ratio relay 33
between the wash liquor flow control FIC 34 and the
extraction liquor flow control FIC 35. Control ratio
relay 33 may have a limited range, for example,
0.9-1.3, to keep extraction flow deviations within the
desired tolerances.
The discharge flow 3~ from outlet 13 is
measured by pulp discharge flow monitor 37 and a
signal is transmitted to pre-set pulp flow controller
FIC 38 which controls the stock pump 29 feeding the
diffuser 10 [or controls a valve, depending on the
pressure in the preceding vessel and the pressure
demand for succeeding processes]. From pulp f low
controller FIC 38, a proportional signal is
transmitted to pulp flow ratio relay 39 which in turn
controls the set point of the wash liquor controller
FIC 34 which governs the wash liquid flow to the
pressure difuser lOo
Wash (or displacement) liquor flow controller
FIC 34 receives signals from wash l.iquor sensing means
40 and pulp flow ratio relay 39 to send a composite
signal to control ratio relay 33. The composite
signal through wash li~uor controller FIC 34 governs
the flow of wash liquor to the pressurized diffuser 10
through wash liquor control means 41~
The composite signal is sent from wash liquor
controller ~IC 34 to control ratio relay 33. A signal
is then transmitted from control ratio relay 33 to
extraction flow controller FIC 35. The combination of
the signals from extraction flow sensing means 42 and
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rom the control ratio relay 33 through extraction
flow con~roller FIC 35 governs the extraction flow
control means 43.
Thus simultaneous pulp flow and consistency
con~rol is achieved by regulating the extraction
liquor flow relative to the wash liquor flow wi~h
reference to the pulp discharge flow, which in turn
governs inlet pulp flow and wash liquor flow. That
is, i~ the consistency of the incoming pulp stream
tends to drop, the extraction flow will increase to
maintain constant discharge consistency, while the
pulp flow entering the diffuser will increase
correspondingly so as to maintain a constant ~iber
flow discharging from the diffuser.
In FIGURE 3, the flow control system operates
generally according to the descript;on of FIGURE 2,
moreover to op~imize performance of pressuriz~d
diffuser 10, the speed of screen 18 may be
automatically adjusted to match that of the average
pulp stream speed through pressurized diffuser 10 by
means of a pressure differential control PDC 44 which
regulates the drive 25 (e.g. t regulates the flow of
oil when drive 25 is a hydraulic cylinder) relative to
the pressure differential between the top and bottom
of diffuser 10, taken at point~ 45, 46. The
optimization process is independent of the flow and
consistency control system described in FIGURE 2.
FIGURE ~ shows the 10w and consistency
control system as shown in FIGURE 2 and the pressure
differential control PDC 44 optimization method ~hown
in FIGURE 3 with the addition of chemical admission
controller FIC 47 and stock temperature controler TIC
48 which are connected to the wash liquor line 31 just
prior to the wash liquor control 41. Displacement or
wash liquor enters pipeline 49 and chemical additions
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made at point 50 are controlled by chemical admission
controller FIC 47 which receives the necessary data,
or example, on p~ or conductivity, from transmitter
51 which may be located directly in the discharging
pulp stream 36. The chemical admission controller
combines the signal f om transmitter 51 with one from
chemical flow sensor means 52 to determine the ~mount
of chemical admissions through chemical flow control
53 (which may be a valve or pump)~ Passing wash
(displacement) liquor through heat exchanger 54
permits the indirect heating or cooling of the pulp
stream itself passing ~hrough the pressurized diffuser
10~ The temperature of the heat exchange medium is
regulated through temperature controller TIC 4B and
attendant flow control 55 wherein the temperature data
transmitter 56 may be located in the discharging pulp
stream 36. The location of the temperature data
transmitter 56 allows automatic temperature control of
the pulp stream entering succeediny treatment steps by
controlling the temperature of the pulp stream
discharging from the preceding diffuser.
A multi-stage pressure diffuser arrangement
employing the flow control method can be understood
with reference to FIGURB 5. While FIGURE 2
illustrate6 in detail the flow control system of a
single diffuser, FIGURE 5 illustrates a multi-stage
arrangement. The wash liquid flow controller 34 of
Section A becomes the extraction flow controller for
diffuser 10' of Section B, and similarly the wash
liquid flow controller 34' of Section B becomes the
extraction flow controller for diffuser lQI' of
Section C. Via control line 57, the ratio relay 39
sends identical signals to the subsequent flow
controllers 34', 34'~, so that they are automatically
set at the same flow rate.
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Note the absence in FIGURE 5 of any
intermediate liquor or surge tanks. There~ore, the
smaller boos~er pumps 58, 58', 58 " only have pres~ure
heads sufficient to compensate for decreases in
pressure through, or example, control valves 41, 41'
and 41'' and pipings. For example, booster pumps 58,
58', 58l' can raise the wash (displacement) liquor
pressure from diffuser 10' to that in the preceding
diffuser 10. This additional pressure head need
ba~sically correspond to pulp line friction losses
between the diffusers.
Since all wash (displacement) and extraction
liquor flows should be of constant magnitude in a
multi-stage arrangement, the extraction flow 30 from
the first pressurized diffuser 10 which governs the
pulp flow consistency of the system should be free of
surge effects. To this end, extraction flow 30 from
diffuser 10 should enter a surge or liquor tank 59.
Tank 59, the only one required in the flow control
system, may be pressurized if the diffusers 10, 10',
10 " are connected in the blow line from a pressurized
vessel (e.g., continuous digester or oxygen reactor).
A vapor or gas cushion 60 acting on the gas relief
flow 61 through valve 62 enables tank 59 to absorb
surges. Gas analy2ers 63 may be incorporated in the
gas relief line 61 if desired. This prevents surge
flow variances from affecting succeeding process
steps. The other constant flow and consistency
effects are obtained according to the steps described
with FIGURE 2.
In section C of FIGURE 5, the dischar~ing
pulp stream 36'' ~rom the final diffuser 10'' may flow
directly to any final treatment or processing stage or
into a storage tower. Should pressure diffusers be
utilized for high temperature washing (above 100C)~ a
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discharge pressure control PIC 64 governs the pressure
of the discharging stream 36'' relative to that oE
final pressure diffuser 10l' by controlling valve
65. It is understood that the pres~nt invention is
not limited to a three-stage system.
A multiple ~tage arrangement employing the
flow control method with intermediate chemical
admission to the wash ~displacement) liquor is
illustrated in FIGURE 6. In principle, the chemical
admission controller 47' operates in a similar manner
to that described with FIGURE 4. The indireck heating
and cooling of the wash (displacement) liquor in the
heat exchange 54, and associated components also
operate in the same manner as in FIGURE 4.
Utilization of the optional hydraulically filled
retention vessel 66, however, necessitates that the
chemical admission controller FIC 47' combines the
signals received from transmitter 52 and ratio relay
67 which in turn receives a signal proportional to the
pulp flow in line 36. A change in the pulp flow will
thus result in an immediate and proportional change in
chemical admission flow. In due time a possible ine
correction in the setting of the ratio relay 67 will
take place throu~h the incoming signal from the
chemical sensor 51 located in the pulp discharge line
from retention vessel 6S. The effect of additional
retention time on the chemical properties of the pulp
will thus be detected and compensated Eor.
In tne multiple stage arrangement as
illustrated in FIGURE 6, it is p.referable to maintain
all flows in and out of the diffusers constant in
order not to induce any chan~es in consistency
throughout the sy~tem once stabilized in the ~irst
stageO Since the system according to FIGURE 6
incorporates the addition of a smaller flow of
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chemical solution to the displacement liquid, an equal
small flow o extraction liquid from a succeeding
s~age is bypassed and introduced with the chemical
additions to the displacement liquid to any preceding
stage of a corresponding pH-rating. This consistency
preserving extraction liquor bleed is controlled by
flow controller FIC 69 with flow sensor 70 and control
valve 710 The flow through line 72 is kept identical
to that through line 49 by means of ratio relay 68
transmitting a 1:1 relay signal from flow controller
47' to flow controller 69.
All intermediate stages will have
arrangements similar to that described above whenever
additives come into the picture. This also applies to
the system illustrated in FIGURE 4.
The systems as described above are capable o~
practicing the method of the invention so that the
flow rate of the pulp being withdrawn through the pulp
outlet 36 by controlling the flow rate (by conrol oE
pump ?9) of pulp fed into the first stage difuser 10;
and are capable of setting and controlling the
consistency of the pulp being withdrawn through
outlets 36, 36', 36'' by controlling the flow rate
(via flow controller 35) of withdrawn liquid rom
outlet 17 of first stage diffuser 10 through conduit
30. The nature of the pulp treatment and the
qualities o~ the pulp withdrawn from each stage are
controlled by adding desired chemicals to the
treatment liquids introduced into inlets 26, 26'; 26' 3
through lines 31, 31', 32'', and controlling the
temperature o the treatment liquid via tempe~ature
controllers 48, 48', 48l' and associated heat
exchangers 54, 54', 54 " , The qualities o~ ~he
withdrawn pulp in lines 36, 367, 36'' are sensed (by
components 51, 51', 51~' and 56, 56', 56''), ~nd
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control of the treatmen~ liquid chemical addition and
temperature is effected in response to such ~ensing.
While the invention has been herein shown and
described in what is presently conceived to be the
most preferred and practical embodi~ent thereof, it
will be apparent to those of ordinary skill in the art
that many modifications may be made thereof within the
scope of the invention, which scope is to be accorded
the broadest interpretation of the appended claims as
to encompass all equivalent methods and procedures.
