Note: Descriptions are shown in the official language in which they were submitted.
1$1;~ '7
1 The hydropyrolysis of spent pulping liquors result-
ing from the pulping of various vegetative fibrous structures
is known in the art. Hydropyrolysis involves the heating under
high temperatures and pressures for various time intervals, the
spent or waste pulping liquor which produces a reaction product
which includes a char-slurry mixture and various uncondensable
organic gases The objective in hydropyrolysis is to separate
the organic values from the inorganic values in the waste li-
quor. The inorganic values are normally used in the regenera-
tion of a pulping liquor and the organic values are used for
the generation of energy or the production of other useful pro-
ducts. One of the other useful products which may be produced
from the organic values is activated carbon
In the so-called kraft pulping process, wood of var-
ious species is reacted with an aqueous solution of sodium hy-
droxide and sodium sulfide, which are the active ingredients,
and lesser amounts of sodium carbonate, sodium sulfate and other
salts which fail to convert to active ingredients in the chemi-
cal preparation steps of the pulping chemicals. In the hydro~
pyrolysis of kra~t waste black liquor the object is to separate
the sodium and sulfur rom the organic materia]s derived from
the wood and to recover the sodium and sulfur in forms suitable
for the regeneration of the pulping chemicals.
The present invention teaches a process of pretreat-
ing the incoming waste black liquor ~rom the kraft pulping pro-
cess with chemicals derived from the hydropyroloysis reaction,
so that the filtrate which is separated from the char produced
as a result of the hydrop~rolysis reaction, is more like the
so-called green liquor from the conventional process. The pre-
treatment of the waste black liquor adjusts the nature of the
-2-
'7
1 green liquor so that it can be converted to the so-called white
liquor The conversion of the green liquor to the white li-
~uor is made primarily by reacting lime (Ca(OH)2) with the green
liquor, which includes sodium carbonate and sodium bicarbonate,
to produce sodium hydroxide and c~lcium carbonate.
The pretreatment of the waste black liquor prior to its
entering the hydropyrolysis reaction, also, affects the physi-
: cal characteristic of the final char produced. For the sake of
example only, without the pretreatment of the present invention
~he resulting char may be very unmanageable in the sense that
it may be quite viscous or sticky, With the pretreatment, theproblems are obviated,
In some instances the char, which is produced by the
hydropyrolysis reaction, contains too high a percentage of
sodium which leaves too much ash in the furnace in the event
it is burned as fuel The present invention discloses a method
of generating a chemical to wash the sodium from the char which
chemical is a product of the hydropyrolysis reaction
The present disclosure, by way of pointing out one of
the best modes of carrying out the teachings of the present
invention, portrays the invention as it ~pecifically relates to
the treatment of spent liquors from a kraft pulping process;
however, it will be apparent to those skilled in the art that
the invention as equally relates to improvements in those hy-
dropyrolysis reactions where waste liquor is treated which uti-
lizes sodium and sulfur compounds as pulping agents Such other
p~ocesses include, but are not limited to:
1. The polysulfide pulping process, in which
a portion of the sulfur is present as
polysulfide~ ~a2Sx~
~$~83.'7
1 2. The hydrogen sulfide pretreatment pulping
process, in which at least a portion
of the sulfur in the form of hydrogen
sulfide gas is impregnated into the
wood prior to heating with an alkali,
3. The sulfite pulping process, in which
the pulping mixture contains sodium
sul~ite and free sulfur dioxide. Base
elements other than sodium may be '
employed such as calcium, magnesium or
ammonia.
4. The bisulfite pulping process, which
employs sodium (or calcium, magnesium,
ammonium) bisulfite and no excess sulfur
dioxide.
5. The neutral sulfi~ semichemical
process, which uses sodium (magnesium,
ammonium) sulfite buffered with sodium
carbonate.
60 The green liquor semichemical process,
which uses sodium carbonate and sodium
sulfide.
All of the above pulping processes and the many other
variants and hybrid processes can be used to pulp materials
other than wood. The starting material may be sugar cane ba-
gasse, straw, reeds, grasses, flax or any ~egetative fibrous
structure. In every case, pulping results in a spent liquor
containing dissolved organic matter and the constituents of
the pulping solution. Hydropyrolysis is applicable to process-
ing all of these spent liquors, and the improvements disclosed
.
--4--
'7
1 in the present disclosure are applicable in all instances to
the waste liquors discussed.
Other objects and a fuller understanding of this inven-
tion may be had by referring to the following description and
claims, taken in conjunction with the accompanying drawings, in
which: -
Figure 1 is a flow sheet illustrating the teachings
of pretreating the waste black liquor from a kraft pulping pro-
cess with a material produced from the hydropyrolysis reaction
and includes the acid washing of the char to reduce the sodium
content;
Figure 2 is a flow sheet similar to Figure 1, but
sh~wing an em~ddiment of the invention which omits the acid
washing of the char;
Figure 3 is a flow sheet illustrating a modification
of the invention illustrated in Figures l and 2 and shows an
improvement in the pressure reduction system reducing the pres-
sure of the material leaving the hydropyrolysi~reactor with
Figure 3 omitting the acid washing of the char; and
Figure 4 is a flow sheet combining the pressure re-
duction system shown in Figure 3 and the acid washing of thechar illustrated in Figure 1.
The improvements to which the present invention re-
late~ will be specifically described in conjunction with the
pretreatment of waste black liquor from the kraft pulping pro-
cess prior to the liquor being subjected to a hydropyrolysis
reaction. The treatment is made by materials produced from the
products of the hydropyrolysis reaction. In the process dis-
closed in the f~ow sheet of Figure 1, waste black liquor from
a kraft pulping mill at about 25 percent s~lids is treated by
'7
1 mixing it with the NaHSO3-Na2SO3 solution leaving the acid wash
stage of the filtration and acid washing system. The black
liquor is then introduced into a heat exchange system under a
pressure of about 3000 psig, heated to a temperature in the
range of from 630 F to 640 F, reacted, cooled to about 250 F
and separated into a vapor and a char-slurry product. The char-
slurry product is then separated into a char and a liquid (fil-
trate) and the char is then washed in two stages with water and
in one stage with the acid stream leaving the sulfur recovery
system.
The vapor product is separated into a condensate and
a gas. The gas is then burned or combusted so that energy
and sulfur can be recovered. The liquid (separated from the
char product) can then be sent to a clarifier and from it a fil-
trate results, essentially free of undissolved carbon, which is
then reconstituted to an effective pulping liquor.
Referring specifically to the flow sheet shown in
Figure 1, it will be seen that waste black liquor from a kraft
pulping process is introduced through 21 into a treatment tank
20, which is mechanically agitated and from there the black
liquor (through 25) is introduced into hydropyrolysis reactor
30 by means of a transfer pump 22, a high pressure pump 24 and
through a cross heat exchanger 26 and a preheater 28. The
conditions and parameters of the hydropyrolysis reaction per se
are known to thoseskilled in the art and aredisclosed inUnited
StatesPatent No. 3,762,989 issued October 2,1973, in the name of
Winfried G. Timpe. The incoming waste black liquor is pretreated
~ 6-
rr'7
1 in the treatment tank 20 with chemicals produced from the hydro-
pyrolysis reaction. The pretreatment chemicals will be discussed
hereinafter.
Under normal circumstances the waste black liquor en-
- tering the system from the kraft mill will be that coming from
the soap skimmer and normally will have a temperature in the
range of from 1800 F to 2000 F. The material is added to pre-
treat the waste black liquor in treatment tank 20, It is pre-
ferable that the treatment tank be designed to provide a hold-
ing time of on the order of thirty minutes for the pretreat-
ment of the waste black liquor. The high pressure pump 24 is
designed to increase the pressure of the pretreated waste black
; liquor to a pressure in the range of from 1000 to 3500 lbs. per
square inch, The operating pressure is controlled higher than
the pressure of saturated steam at the chosen operating temper-
ature in order to prevent flashing of the liquid. The cross
heat exchanger 26 utilizes the product stream from the hydro-
pyrolysis reactor 30 to bring the incoming waste black liquor
up to an intermediate temperature and a final preheater 28
serves to bring the waste black liquor up to the reaction tem
perature, which is in the range of from 490O F to 700O F and
to a preferable range of from 6300 to 6400 F.
The reactor 30 has been indicated only generally on
the flow sheet and the preferred type of reactor is a tubular-
flow reactor; however, by the same token a stirred-tank reactor
might be utilized or a plurality of such reactors in series,
The feed of the strea~ entering the reactor may be on the order
of 610 F and at this temperature, the reaction rate is nor-
mally rapid enough to insure a reasonable residence time require-
ment, The preferred residence time of the black liquor within
`7
1 the reaction zone is on the order of from 3 minutes to 3 hours
to obtain the required results.
The reaction product leaving the reactor 30 is passed
through the cross heat exahanger 26 by way of 31 to heat the
incoming waste black liquor traveling to the hydropyrolysis re-
actor 30 and from there it is passed through a slurry cooler 32
and cooled down by co~ling water which is passed through the
cooler in heat exchange relationship as indicated. The temp-
erature is reduced to in the range of from 150 F to 3000 F at
a constant pressure which is about that of the reactor 30. The
reaction product includes a char-slurry mixture as well as
noncondensable organic gases formed during the hydropyrolsi~s re-
action~ The reaction products stream is next passed through a
pressure letdown valve 34 and intr~duced into a flash gas se-
parator 36. For the sake of example, the reaction;; products
stream, in passing through the pressure letdown valve 34, is
reduced from about 2800 psig to about 4 psig. The action of
the pressure letdown valve 34 is normally controlled by sensing
the pressure of the waste black liquor stream between the high
pressure pump 24 and the reactor 30. The flash gas separator
36 should operate at atmospheric pressure and as a result when
the reaction product enters the separator 36, it is flashed at
atmospheric pressure and the char-slurry mixture leaving the
bottom of the flash gas separator 36 through line 45 is cooled
to on the order of about 212 F. The noncondensable gases
formed in the hydropyrolysis reaction and some water vapor
leave overhead from the flash gas separator 36 through line 47.
It is desirable, therefore, that there should be interposed
some type of flash separator to receive these vapors and con-
dense whatever condensable materials (such as water) are car-
'7
1 ried overhead by the flashing. Such a flash separator has notbeen shown in Figure 1, but will be appreciated by those
skilled in the art.
A portion of the noncondensable organic gases produced
in the hydropyrolysis reaction which pass overhead are com-
busted at 39 in the preheater 28 and serve the purpose of add-
ing extra energy to the waste black liquor to bring it up to
the desired reacticn temperature when it enters the reactor 30.
The rest of the gases are combusted in a boiler to produce
steam. The burning or combustion of these noncondensable gases
produces a mixture of gases which include sulfur dioxide, car-
bon dioxide and nitrogen~ The noncondensable organic gases
leaving overhead from the flash gas separator 36 and traveling
to the preheater 28 include methane, ethylene, ethane, hydro-
gen sulfide, propane, CH3SH, C4H12, C2H50H and (CH3)2S.
The gases of combustion which result at 39 in the pre-
heater 28 are transported by way of line 42 and are introduced
into the bottom of an SO2 absorber 41. These gases include SO2,
CO2, and N2. The S02 absorber should be some type of contact
tower which is satisfactory to absorb the S02 gas into a con-
centrated wash solution emanating from the char-slurry end of
the system. In point of theory either a packed or tray tower
is satisfactory to accomplish this end result.
The char-slurry mixture which is removed from the bot-
tom of the flash gas separator 36 is transported through line 45
to a first vacuum filter 44 where the filtrate is removed from
the char through 52. The filtrate from filter 44 includes a
concentrated solution comprising Na2C03 and NaHCO3. The char
is washed by wash water from line 46 which wash water originates
from a fresh water source which washes the char at a second
l filter 48. The washing of the char at the first filter 44
produces a weak filtrate having the same chemical components
as the filtrate leaving line 52, which is transported through
line 49 to the top of the SO2 absorber 41 where it is inter-
mixed with the S02 gases from the preheater 28. The intermix-
ing of the SO2 gases with the weak filtrate from filter 44 pro-
ceeds generally along the line of the following reaction:
Na2C03 ~ S~2 > Na2S03 ~ C2
Ma2SO3 + S2 + H2O >2NaHSO3
This SO2 treated weak filtrate is then taken from the
bottom of the SO2 absorber through line 50 and introduced into
what is referred to as an acid repulp tank 51 which tank also
receives the char from filter 44. The acid repulp tank 51 is -~
mechanically agitated and the purpose of mixing the char with
the above described material from the S02 absorber is to remove
sodium retained in the char. The reason fo~r removal of the so-
dium from the char is that in the eJent of the sodium content of
the char is quite high and the char is to be used as a fuel,
the removal of the sodium at this point reduces the amount of
ash resulting ~rom the burning of the char.
The char-slurry mixture from the acid repulp tank is
next ~ansferred through line 53 to the second filter 48 where
the filtrate is separated from the char and the char is washed
again from the fresh water source and is then passed on for fur-
ther processing. The filtrate passing through the second fil-
ter 48 is t~ans~erred through line 55 to the treatment tank
20 where it is utilized to pretreat the incoming waste black
liquor which is subsequently transported to the reactor 30
where it undergoes the hydropyrolysis reaction. The !mechanism
of removal of the sodium from the char in the acid repulp tank
--10--
1 51 is thought to be essentially a reaction of the sodium bi-
sulfite produced in the S02 absorber with a "sodium" group on
the char to convert the "sodium" group on the char to an "OH"
group with a subsequent conversion of the sodium bisulfite to
sodium sulfite
The reason for the pretreatment of the waste black
liquor prior to its entering the hydropyrolysis reaction is to
bring the ~a2S concentration of the filtrate leaving the ~ilter
44 through 52 up to an acceptable level so that with the addi-
tion of the proper chemicals the filtrate from 44 can be con-
verted from a green liquor to a white liquor suitable for thepulping of wood products in accordance with kraft pulping pro-
cess technology. The conversion of the filtrate from filter 44
is essential~y effected by reacting lime with the filtrate.
Under normal circumstances only about 85% of the required sulfur
for pretreatment is produced by burning the uncondensable or-
ganic gases at 39. The additional 15% required ~an be added to
~: the gas stream in line 47 as elemental sulfur which can, also,
be burned at 39.
:~ The concept of direct absorption of S02 gas resulting
from the combustion of various sulfur bearing gases into the
waste black liquor can be utilized in pretreating the liquor
but this concept is undesirable because it is also necessary to
absorb carbon dioxide in the combustion gases and this is unde-
sirable because if carbon dioxide is absorbed it reduces t~ pH
which tends to precipitate lignin from solution When this hap-
pens the lignin tends to clog the piping which is undesirbble,
In the present disclosure carbon dioxide produced in
the combustion in the preheater 28 is avoided in the black li-
quor because when the combusted gases are transmitted to the
--11--
L ;~
1 SO2 absorber, the carbon dioxide simply passes out the stack 43
of the absorber because it is not absorbed in the weak filtrate
coming from filter 44 through l.ine 49.
In order to assist those skilled in the art in prac- :
ticing the invention a material balance at various places in
the flow sheet of Figure 1 is given below in Table I. This ma-
terial balance is given when a pH of 3.1 is present in the li-
quid flowing in lines 50 and 55 This materialbalance will
change with a change in pH of stream 50. Amounts below are
given in weight percent
TAB~E I
TOTAL FLOW
LOCATION SOLIDS Na S C(lbs/hr)
21 25.00 5.55 0.79 10.70400.00
11.10 2.96 4.14 0.8365.97
23.03 5.20 1.27 9.31465.97
49 10.03 3,01 1,37 1.2267.78
42 93.41 -- 2.51 2.25101~41
43 91.09 -- 0.0052.45101.41
; 50 13.49 3.01 5.12 0 9267.78
31 23 03 5.19 1 27 9.31465.97
47 -- -- -- -- 11.97
22.07 5 25 0 78 8 96454 00
It will, thus, be seen from the above discussion of
the invention in conjunction with Figure 1 that within the
closed system disclosed, a new and novel means has been provid-
ed for pretreating the waste black liquor which is to undergo
the hydropyrolysis reaction, which pretreatment produces a fil-
trate that can be conveniently treated to put it in condition
to be used as a pulping chemical and, also, serves to adjust
'7
1 the characteristics of the char produced in the hydropyrolysis
reaction so that it is in its most convenient form. This con-
cept is accomplished by taking the noncondensable organic gases
resulting from the hydropyrolysis reaction, burning the same to
produce heat which can be used in r,aisin,g the black liquor to
the hydropyrolysis reaction temperature, and then conducting the
gases of combustion to an absorber where S02 is absorbed into
a filtrate resulting from the washing of the char produced in
the hydropyrolysis reaction, This absorption produces a mixture
of sodium bisulfite and sodium sulfite which is intr~duced di-
rectly into the waste black liquor to pretreat the same or be-
fore int~roduction into the waste black liquor is utilized by
mixing it with the char to reduce the sodium content of the
char and is subsequently introduced into the waste black liquor
to pretreat.
Figure 2 of the drawings is an illustration of the
present invention where the sodium content of the produced char
is such that it is at an acceptable level. In this instance it
is not necessary to wash the char with the product produced
in the S02 absorber and under these circumstances the sodium
20 bisulfite produced in the S02 absorber 41 is directly introduced
into the waste black liquor in the treatment tank 20 by way of
line 58. Under these circumstances the flow sheet of Figure 2
differs from Figure 1 in that the acid repulp tank 51 and the
second filter 48 have been deleted. In other respects, the sys-
tem is the same in that the char-slurry product from the bottom
of the flash gas separator 36 is passed ~rectly to filter 44
producing a filtrate as indicated. The char as separated from
the filtrate is washed with fresh water producing a weak fil-
trate which is transported to the S02 absorber 41 by way of line
-13-
- . . ~
l 49 where it absorbs and reacts with the S02 from preheater 28
to produce the sodium bisulfite.
The flow sheet of Figure 3 illustrates an improve-
ment in the processes illustrated in Figures 1 and 2 and is
best compared with flow sheet of Figure 2 to aid in an under-
standing of the impr~vements made in Figure 3. The improvement
of Figure 3 is in the pressure reduction system for reducing to
atmospheric pressure the char-slurry reaction product leaving
the reactor 30 The system of Figure 3 improves the char-
slurry composition which in Figure 2 leaves the flash gas se-
parator 36 through line 45. The improvement resulting in theprocess illustrated in Figures 3 and 4 produces advantageous
results in that it reduces the organic carbon content in the
filtrate which is regenerated to produce a kraft pulping li-
quor and, also, reduces the sodium bicarbonate level in the
filtrate. Additionally,the sulfur level is reduced in the char
which is quite advantageous in that when the char is utilized as
a fuel, the amount of S02 resulting from burning is reduced.
Referring specifically to Figure 3, the same identi-
fying numbers have been applied to the components in Figure 3
that have been shown in Figures 1 and 2 and which are the same;
however, when variations in the structure have been utilized,
new identifying numerals have been utilized.
It will be seen that the reaction product exiting the
hydropyrolysis reactor 30 is transferrred by way of line 61 in-
to the cross heat exahanger 26 where a portion of its heat is
given up to the waste black liquor prior to the liquor being
subjected to the hydropyrolysis reaction. The cross heat ex-
changer shown herein is slightly different than illustrated in
Figures l and 2, but for all intents and purposes it is the
sa~e and so, therefore, the same reference numeral has been
~ -14-
1 used in identifying this element.
After the reaction product exits the cross heat ex-
changer 26 it travels to what has been identified as a first or
number one pressure letdown valve 62 from whence it is intro-
duced into a flash gas separator 64 sometimes referred to as a
flash tank. A reduction in temperature and pressure results
when the reaction products pass through the pressure letdown
valve number one and are introduced into the flash gas separa-
tor 64. The temperature reduction is on the order of from 0 F
to 4000 F and the pressure reduction is on the order of from
500 to 3000 psig depending upon the pressure of the hydropyrol-
ysis reaction taking place in the reactor 30. As the reaction
products are introduced into the flash gas separator 64 at the
high temperatures indicated and under only partial pressure re-
duction extensive flashing of the reaction products and a wet
calcination reaction of sodium bicarbonate results, The flash-
ing and wet calcination reactlon reduce the temperature on the
order of those referred to above,
The flashing vapor overhead is vented through line 63
to what has been referred to as pressure letdown valve number
two 70. It is sometimes desirable to reduce the temperature of
the flashing vapor and to the extent that the vapor need be
cooled, a cooler 67 has been interposed in line 63. The second
pressure letdown valve 70 reduces the vapor pressure to about
atmospheric pressure in a condenser 72 and the condenser 72
serves the purpose of separating the noncondensable gases from
the condensable liquids which comprise primarily water, The
noncondensable gases are then transmitted by way of line 47 to
the preheater 28 where they are burned at 39 in the manner in-
dicated in the discussion of Figures 1 and 2.
1 The bottoms from the flash gas separator 64 which com-
prise primarily the char-slurry mixture are traveled through
line 68 and preferably through the cross heat exchanger 26 to
assist in heating the incoming waste black liquor prior to its
entering the hydropyrolysis reactor 30. After the heat exchang-
er the char-slurry mixture is cooled to a desired temperature
in the slurry cooler 32 to a temperature of on the order of from
1200 to 2120 F, The char-slurry is then passed through pres-
sure letdown valve number three 79 and iintroduced to vacuum
filter 44 where the filtrate is separated from the char and the
char is washed by a source of fresh water.
The pressure reduction system illustrated in Figure 3
acts at high temperatures (400O to 700O F) and high pressures
(1000-6000 psig), These high temperatures and partial pressure
reduction cause flashing and a partial wet calcination reaction.
The flashing appears to strip volatile organics and volatile
sulfur compounds from the slurry, The wet calcination decom-
poses the sodium bicarbonate in the slurry to sodium carbonate.
The hi~h temperature of the initial pressure reduction causes
the wet calcination reaction to proceed very quickly. The main
decomposition reaction is believed to be:
2NaHC03 ~ Na2CO3 + H2O + C2
As mentioned above, this flashing procedure and wet
calcination improves the filtrate and char produced in that the
organic c~r~bon and sodium bicarbonate in the filtrate are great-
ly reduced and the sulfur level in the char is reduced,
These improvements are important to a hydropyrolysis
recovery system, because it is desirable to remove as much or-
ganic carbon and sodium bicarbonate from the filtered mother li-
. .
-16-
1 quid (filtrate) as possible. The filtrate is returned to the
mill operation for treatment to regenerate it into a pulping
liquor and lowering the bicarbonate level reduces the amount
of treatment that has to be used to regenerate the filtrate
into a pulping liquor. The organics in the filtrate represent
an unnecessary recirculation of materials and this the reason
~or reducing the organics. Reduction in the sulfur level of
the char is desirable particularly where the char is used by
burning as a fuel.
A typical operation utilizing the modified pressure
reduction system shown in Figure 3 is illustrated in Table II
below, In this case there was no slurry temperature reduction
prior to the initial pressure reduction and the temperature was
on the order of 6000 F. The pressure was reduced 1000 psig into
the flashing tank from a pressure of about 2500 psig, The
flashing and wet calcination reduced the slurry temperature
50 F. The falshing operation concentrated the slurry but the
filtrate still enjoyed a 1,7% reduction in organic carbon and a
better than 20% decomposition of the sodium bicarbonate to so-
dium carbonate. The char produced had its sul~ur level de-
creased by 17.9%.
T~BLE II
Without Pressure Reduction of Fiqure 3
Fil~rate - 2,37% organic carbon
Filtrate _ 50.88 g/l ~a2Co3
Filtrate - 71.57 g/l ~aHCO3
Unwashed char - 3.13% Sulfur (DW~)
With Pressure Reduction of Fiqure 3
Filtrate - 2.33% organic carbon
Filtrate _ 76.32 g/l Na2C3
Filtrate - 56.50 g/l NaHC03
Unwashed Char - 2.57% Sulfur (DWB)
-17-
hP~`7
1 Figure 4 illustrates the pressure reduction system
shown in Figure 3; however, it illustrates the incorporation of
the pressure reduction system into the system found in Figure
1, namely, the acid washing of the char that is produced in the
hydropyrolysis reaction with the acid washing being accomplished
by the sodium bisulfite produced in the SO2 absorber 41. In
all other respects, the pressure letdown system operates in the
same manner as the system shown in Figure 3 and the acid wash-
ing of the char is accomplished as illustrated in Figure 1.
It will, therefore, be readily appreciated by those
skilled in the art that the present invention discloses a unique
process and system i-or pretreating the waste black liquor ema-
nating from a process wherein any of various vegetative fibers
may be digested by a pulping liquor wherein the pulping liquor's
main active ingredients are sodium and sulfur. The waste black
liquor is conveniently treated by chemicals produced as products
of the hydropyrolysis reaction and by means of the pretreatment
pulping chemicals recovered can be more easily and economically
regan~rat~ to a pulping liquor and the physical and chemical
nature of the char and subsequent products produced from the char
can be controlled within meaning$ul bounds
While we have illustrated and described a preferred
embodiment of our invention, it will be understood that this
; is by way of example only and not to be construed as limiting.
-18-
