Top circulation line cooling for a modified cook digester

In the production of cellulose pulp (e.g. kraft pulp) utilizing a continuous digester having a number of different feed points for cooking (e.g. white) liquor and utilizing a high pressure feeder, the volume of cool white liquor that is applied to the feed system is reduced compared to conventional processing. This can cause excessive hammering, and damage the high pressure feeder and adjacent piping and equipment. In order to avoid this, liquid being recirculated from the top of the digester back to the high pressure feeder is cooled by passing it into a heat exchanger into heat exchange relationship with a cooler liquid, with the flow of coolant automatically controlled by sensing the temperature of the recirculated liquid. The temperature in the feed system can further be lowered by cooling the cooking liquor before it is added to the pulp slurry, as by passing it to a flash tank so that its temperature is reduced at least 10.degree. C., and the flashed steam can be used in an evaporator.

BACKGROUND AND SUMMARY OF THE INVENTION 
During conventional continuous chemical pulp production, particularly in 
kraft cooking, the entire cooking liquor (e.g. white liquor) charge is 
added to the feed system, which includes the high pressure feeder and the 
circulation line to the top of the digester either with or without an 
impregnation vessel. However over the last decade two significant advances 
have taken place in continuous chemical pulp production technology which 
have changed this. First the MCC.TM. digesters, and method, developed by 
Kamyr, Inc. of Glens Falls, N.Y., added white liquor into a central 
recirculation loop within the digester. Subsequently, EMCC.RTM. digesters 
and processes, also developed by Kamyr, Inc., provided for introduction of 
white liquor into the bottom (wash) circulation loop. While these 
digesters and systems have been commercially successful because they 
enhance the quality of the pulp produced, one unexpected problem resulted 
from the introduction of the cooking liquor at multiple points, instead of 
the entire white liquor charge being added to the feed system. 
According to the present invention, it has been determined that, in 
continuous digesting systems where a plurality of feed points for the 
cooking liquor are provided, since the volume of relatively cool cooking 
liquor supplied to the feed system is reduced, higher temperatures occur 
in the top circulation line, i.e. the line returning liquid separated from 
the chips in the top of the digester to the high pressure feeder (either 
with or without an impregnation vessel). This increases the potential for 
hydraulic hammering due to liquor flashing in the line, introducing 
loading on the adjacent equipment and piping, and providing potential 
hammering which can damage the high pressure feeder and adjacent piping 
and equipment. According to the present invention, the temperature of the 
liquid in the recirculation line, and circulation line, is kept low enough 
so as to avoid hydraulic hammering due to liquor flashing. 
According to the present invention there is provided a method of feeding 
comminuted cellulosic fibrous material to a continuous digester having a 
plurality of feed points for cooking liquor, and utilizing a high pressure 
feeder. The method comprises the following steps: (a) Entraining 
comminuted cellulosic fibrous material in liquid to produce a slurry, and 
feeding the slurry to the top of the digester using the high pressure 
feeder. (b) Adding some cooking liquor to the slurry as part of the liquid 
entraining the material. (c) Separating some of the liquid from the slurry 
at the top of the digester. And, (d) recirculating the separated out 
liquid from the top of the digester to the high pressure feeder. According 
to the invention, the hydraulic hammering can be prevented when using one 
or both of the following techniques: the recirculating liquid can be 
cooled (by passing it into heat exchange relationship with a cooler 
liquid), and/or the cooking liquor may be cooled before it is added to the 
slurry (e.g. by flashing the cooking liquor to reduce its temperature, and 
produce flashed steam which may subsequently be used in an evaporator). If 
flashing of the cooking liquor is utilized, typically it is flashed when 
it has a temperature of about 90.degree. C., and the temperature thereof 
is reduced by at least about 10.degree. C., which can be enough--either 
singly or in combination with cooling of the recirculating liquid from the 
top of the digester--to avoid hydraulic hammering. The vacuum required for 
such flashing is provided through the connections made to the evaporator 
system. 
According to another aspect of the present invention, a cellulosic pulp 
producing system is provided which comprises the following elements: A 
substantially upright continuous digester. A high pressure feeder. A 
circulating line operatively extending from the high pressure feeder to 
the top of the digester. A recirculating line operatively extending from 
the top of the digester to the high pressure feeder. A separator for 
separating liquid from a slurry containing cellulosic fibrous material and 
liquid, the separator disposed at the top of the digester and connected to 
the recirculating line. Means for adding cooking liquor to slurry being 
transported by the high pressure feeder to the top of the digester. And, 
heat exchanger means operatively disposed in the recirculating line for 
reducing the temperature of liquid being recirculated from the digester to 
the high pressure feeder. 
The system may also comprise means for sensing the temperature of liquid in 
the recirculating line, means for regulating the flow rate of coolant to 
the heat exchanger means (e.g. a valve), and means for controlling the 
coolant flow rate regulating means in response to the temperature sensing. 
An impregnation vessel may be disposed in the recirculating and 
circulating lines between the high pressure feeder and the continuous 
digester, in which case the heat exchanger means is typically in the 
recirculating line between the impregnation vessel and the high pressure 
feeder. 
According to yet another aspect of the present invention a cellulose pulp 
producing system is provided comprising: A substantially upright 
continuous digester. A high pressure feeder. A circulating line 
operatively extending from the high pressure feeder to the top of the 
digester. A recirculating line operatively extending from the top of the 
digester to the high pressure feeder. A separator for separating liquid 
from a slurry containing cellulosic fibrous material and liquid, the 
separator disposed at the top of the digester and connected to the 
recirculating line. Means for adding cooking liquor to slurry being 
transported by the high pressure feeder to the top of the digester. And, 
means for cooling the cooking liquor before supplying it to the means for 
adding cooking liquor. The cooking liquor cooling means preferably 
comprises a flash tank, including a steam discharge, and the steam 
discharge is operatively connected to evaporators. 
It is the primary object of the present invention to provide a method and 
apparatus for avoiding hydraulic hammering or the like in modern 
continuous digesting systems in which a plurality of feed points for 
cooking liquor are provided. This and other objects of the invention will 
become clear from an inspection of the detailed description of the 
invention and from the appended.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 schematically illustrates a first embodiment of apparatus according 
to the present invention, for feeding comminuted cellulosic fibrous 
material (e.g. wood chips) to a continuous digester, and treating the wood 
chips to produce pulp, such as sulfate pulp, sulfite pulp, or the like. 
The conventional components of the apparatus of FIG. 1 include the air lock 
10 and chips bin 11 which receive chips from a source and then pass them 
through a chip meter 12 and low pressure feeder 13 into a horizontal 
steaming vessel 14, the chips being discharged into a chute 15 connected 
to a conventional high pressure feeder 18. A high pressure pump 19 is 
connected to one port of the high pressure feeder 18, while a low pressure 
pump 20 is connected to another port thereof. The pump 20 also is 
operatively connected to a sand separator 21, which in turn is connected 
to an in-line drainer 22. A level tank 23 and a pump 24 are also provided, 
and white liquor from a source 25 (or a like cooking liquor depending upon 
which pulping process is utilized) is ultimately entrained with the chips 
discharged by the high pressure feeder 18. Line 26 connected to the pump 
24 leads to the top of an upright continuous digester, and white liquor is 
also added at one or more additional points to the digester 29, such as to 
line 27 connected through a pump to the line 28. 
Additional conventional components of the system of FIG. 1 include the line 
30 for circulating cellulosic fibrous material (chips) entrained in liquid 
to the top of the digester 29, at which point some of the liquid is 
separated from the chips/liquid slurry by the conventional top separator 
31, and then is returned by the top circulation line 32 to the high 
pressure inlet pump 19 of the high pressure feeder 18. Heaters 33, 34, and 
35 are provided for heating liquid withdrawn from various screens 
associated with the digester 29, which liquid is then circulated back to 
the digester 29 to effect cooking or the like. Also black liquor is 
withdrawn from the digester 29 and flashed in the first and second flash 
tanks 36, 37. An outlet device 38 discharges pulp from the bottom of the 
digester 29 into a discharge line 39, after which the pulp is passed on to 
subsequent treatment stages, such as washing, storage, and bleaching 
stages. 
The conventional components of the system illustrated in FIG. 1 comprise a 
single vessel hydraulic "MCC".TM. Kamyr, Inc. digester system. Since white 
liquor is added at different points in the system, the temperature of the 
slurry or liquid in the lines 30, 32 may increase undesirably so that it 
flashes into steam and causes hammering, and perhaps damage, in and to the 
high pressure feeder 18 and to line 30. In order to avoid this adverse 
consequence, according to the present invention the recirculating liquid 
in line 32 is cooled so that the temperature of the slurry in the high 
pressure feeder 18 and being fed in line 30 to the top of the digester 29 
is low enough to avoid hydraulic hammering due to liquor flashing. 
The apparatus (cooling means) for accomplishing the desired result 
according to the invention is shown generally within the dotted line box 
42 in FIG. 1. It includes a conventional heat exchanger 43 through which 
the recirculating line 32 passes between the top separator 31 and the low 
pressure inlet pump 19 for the high pressure feeder 18. Coolant, which can 
be any available mill water, or cooler wash water from upstream brown 
stock washers or the like (e.g. liquid in route to recovery) or any 
process stream that could benefit from heating, passes through line 45 
into heat exchange relationship with the hot liquor in the line 32, 
thereby significantly cooling the liquor in line 32. The coolant passes 
through an automatically controlled valve 46, controlled by flow 
controller 47, which receives input from the temperature indicator 48 
operatively connected to the line 32. The temperature indicator 48 senses 
the temperature of the liquid being recirculated in line 32, and adjusts 
the flow of cooling liquid from coolant source 44 through valve 46, 
depending upon the sensed temperature. In this way, the temperature in the 
line 32 is lowered to the extent necessary to prevent hydraulic hammering, 
but yet is maintained high enough so that substantial amounts of energy 
are not wasted, or a great deal of additional energy need not be supplied 
to the chips to heat them to cooking temperature. 
The apparatus illustrated in FIG. 2 is the same as that illustrated in FIG. 
1 except that it is for a two vessel hydraulic Kamyr MCC.TM. digester 
system. In the FIG. 2 embodiment components comparable to those in the 
FIG. 1 embodiment are illustrated by the same reference numeral only 
preceded by a "1". 
The only significant difference in the embodiment of FIG. 2 is that a 
conventional impregnation vessel 50 is provided in the circulating and 
recirculating lines 130, 132. That is the circulating line 130 extends 
from the high pressure feeder 118 to the top separator 51 of the 
impregnation vessel 50, and recirculating liquid is withdrawn through line 
52 at the top of the impregnation vessel 50. The chips slurry discharged 
by the outlet device 53 at the bottom of the impregnation vessel 50 passes 
the chips in circulating line 54 to the top of the digester 129, while 
liquid recirculated from the top of the digester 129 in line 132 passes 
back to the vessel 50. In this case, the high pressure feeder 118 is 
protected by the cooling mechanism 142 being provided in line 52, as 
illustrated in FIG. 2. That is the cooling means 142 (substantially 
identical to cooling means 42) is between the impregnation vessel 50 and 
the high pressure feeder 118 in the circulating and recirculating loops 
for supplying slurry to and withdrawing liquid from the continuous 
digester 129. 
In addition to the apparatus according to the invention illustrated in 
FIGS. 1 and 2 (namely the cooling means 42, 142), according to the present 
invention another mechanism (FIG. 3) may be utilized for lowering the 
temperature of the liquids associated with the high pressure feeder 18, 
118, and thereby avoiding hydraulic hammering. Alternatively, in some 
situations the cooling means 42, 142 may not be necessary and the 
mechanism illustrated in FIG. 3 may be used in its place. 
FIG. 3 illustrates a mechanism for cooling the white liquor that is 
supplied to the chips that are passing in the circulating line to the top 
of the digester 29, 129, taking into account that the volume of this white 
liquor is much less than in conventional digesters (that is those without 
the MCC.TM. or EMCC.RTM. process improvements of Kamyr, Inc.). 
As illustrated in FIG. 3, hot white liquor from the pulp mill 
recausticization and white liquor storage facilities, typically at a 
temperature of about 90.degree. C., is flashed in a conventional flash 
tank 61. The vacuum required for this flashing is provided by a vacuum 
pump in the attached evaporator. The flashed steam, at approximately 
80.degree. C., goes into line 63, while the cooled white liquor passes 
into line 25 (that is the cooled white liquor becomes the source of white 
liquor illustrated at the left hand bottom of FIG. 1). The temperature of 
the white liquor is reduced by at least 10.degree. C. by flashing. 
The steam in line 63 from flash tank 61 is combined with steam from green 
liquor flashing, or other steam sources, illustrated schematically at 64 
in FIG. 3, and then passes via line 65 to a plurality of evaporators 66 
through 69. Thus the steam from white liquor flashing is utilized to 
supplement the heat requirements of evaporators, such as the evaporators 
66 through 69. 
It will thus be seen according to the present invention that the 
temperature in the top circulation line of an MCC.TM. or EMCC.RTM. 
digester is controlled so as to avoid hydraulic hammering. While the 
invention has been herein shown and described in what is presently 
conceived to be the most practical and preferred embodiment 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 
so as to encompass all equivalent structures and methods.