Note: Descriptions are shown in the official language in which they were submitted.
A PROCESS FOR PRODUCING ~ULP FROM A LIGNocELLuLost~-~o ~ [~ ~ G
-
MATERIAL
This invention relates to a process for producing pulp from a
lignocellulosic-containing material. More particularly, this
invention relates to a process for producing dissolving pulp from a
lignocellulosic-containing material.
Wood pulp containing high levels of alpha cellulose, which is used in
the manufacture of various cellulose derivatives for various end uses
or products, is known in the art as dissolving pulp. Other terms
synonymous with dissolving pulp are chemical cellulose and special
high alpha pulp. Two processes are in general use for the manufacture
of dissolving pulp viz:
1) The acid sulphite process, the development of which started at
about the beginning of this century (Rydholm, S.A., Pulping
Processes, p. 280, Interscience Publishers, New York-
London-Sydney 1~65); and
2) The prehydrolysis - Kraft process, the development of which
started in 1929 (Rydholm referred to above - p 281)
The latter process makes use of an acidic pretreatment
("prehydrolysis") step in order to remove hemicellulose prior to the
alkaline pulping (delignification) step. South African Patent 88/4037
discloses the prehydrolysis - neutral sulphite - anthraquinone process
(PH-NS-AO) for the manufacture of "hemicellulose hydrolysate and
special pulp" (high alpha grade). This prehydrolysis step performs
essentially the same function as the prehydrolysis step prior to the
.~raft (Sulphate) pulping method for the manufacture of dissolving
grade pulp; while the neutral sulphite-anthraquinone de1ignification
steo is essentially the same as the process also known as the
,~ ~5
- 2 --
semi-alkaline sulphite-anthraquinone (SAS-AQ) process, first reported
at a technical conference in 1979 by Raubenheimer, S and Eggers, S.H.,
(both being research workers employed by SAPPI LIMITED, the present
applicant herein) viz the 11th European ESPRA Meeting, Maastricht, The
S Netherlands, during May 1979.
In the aforementioned patent specification, the washing of the cooked
chip material is reported to have taken place over a period of 10
hours, which is clearly impractical; and certain of the tests show a
Kappa number which is not as low as could reasonably be required or
expected.
It has been found by the inventor of the present invention that the
aforementioned process could be improved when applied to timber
presently used for dissolving pulp manufacture in South Africa viz Gum
(various Eucalyptus species - mostly E. grandis) and Wattle (Acacia
mearnsii) and it is therefore an object of the present invention to
provide a process which overcomes, at least partly, the
disadvantage(s) of the aforementioned process and hence of the prior
art.
According to one aspect of the present invention, there is provided a
20 process for producing dissolving grade pulp, the process including -
applying a prehydrolysing step to a lignocellulosic-containing
material for example hardwood.
partly dissolving the lignin contained in the prehydrolysed
material (- a pulping step); and
carrying out an alkali extraction step.
About 2% to 5%, preferably about 3%, by mass of alkali may be used to
carry out the alkali extraction step.
The alkali used may preferably be sodium hydroxide.
The alkali extraction step may be carried out at a temperature of
about 50C to 100C, preferably about 70C to 80C, for a period
sufficient to give optimal lignin removal.
According to another aspect of the present invention, there is
provided a process for reducing the lignin content (as expressed by
the Kappa number) of unbleached pulp intended for dissolving grade
pulp manufacture, the process including applying an alkali extraction
step to the pulp.
About 2% to 5%, preferably 3%, by mass of alkali may be used to carry
out the alkali extraction step.
The alkali used may preferably be sodium hydroxide.
The alkali extraction step may be carried out at a temperature of
about 50C to 100C, preferably 70C to 80C, for a period sufficient
to give optimal lignin removal.
Spent liquor from the pulping step may be removed from a digester,
treated with alkali, and re-introduced to the digester to lower the
Kappa number of the pulp and/or to improve evaporation of the spent
liquor.
After re-introduction of spent liquor to the digester, the digester
may be operated at a temperature ~ 100C and at e1evated pressure.
About 10% to 15% a1kali by mass of pulp may be used.
-- 4 --
The alkali used may preferably be sodium hydroxide.
The invention extends to a dissolving grade pulp whenever produced by
a process as herein described and/or exemplified.
The invention will now be described and illustrated in greater detail
with reference to the fol10wing non-limiting examples:
EXAMPLE 1
Prehydrolysis and pulping were carried out under laboratory conditions
on run-of-mill (ENSTRA) Eucalyp-tus wood chips. The prehydrolysis
conditions and yields are shown in Table 1 hereunder.
Table 1
~ __ _.__ _
R~ l~io. ~ydrolysis agent Tlme e~ Time at Yield
% on wood temp. (mins) temp. (hrs) (%)
__ ~__ __ __,
P1 2% - H2S04 ~0 to 15$ C 2 at lS5 C 73,1
P2 1~ - S2 60 to 16~ C 2 at 165 C 70,S
P3 0,2S~ - S0~ 60 to 165 C 2 a~ 165 C 80,4
P4 0.'53 - St 60 ~o 165 C 3 at 165 C 77,6
PS ___ 60 ~o 150 C 2 at 150 C 85,:5
(Pl - P4 used 4,0/1 L/W ratio; P5 used 5,0/1 L/W ratio)
-- 5 --
Thereafter pulping was carried out on material from three of the
prehydrolysis runs, yielding the pulping results shown in Table 2
hereunder.
Table 2
.__ . _ ~
Cook Prehydrolysis C~ar~e as Na20( ~ ) Y~eld ( ~ ~ 1;appa
No. Conditions ~ Screened Rejects No.
_ _ ~ __ ~ _
C1 AsP3- Ta~le 1 17 4 _ 51,97 0,1 ~2,5
C2 As P4 - ~able 1 20 2, 52, 5 ~9, 44 0, 02 87, 9
. C3 /~ ~7 4 _ 50,96 0,52 ~Z,9
The following process conditions applied:
0, 2% Anthraquinone in all cases
3,5/1 L/W ratio in all cases
85 mins to temperature (175C) in all cases
3 hours at 175C for Cl and C3
5 hours at 175C for C2
The Kappa number from cook no. Cl was relatively high.
Virkola has reported Kappa numbers in the low twenties after the
pulping step but how such figures were obtained is uncertain.
IVirkola, N-E and Tikka, P.O. - Tappi Pulping Conference, 1988; New
Orleans U.S.A. October 30 - November 3 p 255 . In South Africa patent
No. 88/4037 (with Virkola and Tikka as inventors) continuous washing
for 10 hours after pulping is reported.]
tJ fi,~'`'` ~3~ ~
- 6 --
For cook number C2 the chemical charge was increased~ caustic soda was
added and the pulping time was increased significantly in order to
reduce the Kappa number which, contrary to expectations, increased.
It is believed that this increase was due to the harsher prehydrolysis
conditions resulting in higher deactivation of the lignin.
Cook number C3 gave a lower Kappa number than Cl possibly because of
the milder prehydrolysis conditions.
In order to reduce the Kappa number a caustic extraction step was
applied.
This yielded the results shown in Table 3 below.
Table 3
. _ .. __~
Cook E:str~ction I Extraction lCappa llo. Yield on unbl.
. . . . ___ __
No. i~o~ m~lu3 ~etore- After pulp (~)
_ ~ ~
C2 E1 3~ ~laOH*87, g Z6, 6 N. D.
C2 EZ Wate- 87, 9 66, 6 ~.D.
C1 E3 3~ NaO~ 42tS 16,9 N.D.
C3 E4 3~ YaOH 3Z,9 17,6 95,71
The following process conditions applied:
* - Percentage on pulp
Time - l hour
Temperature - 70C
Consistency - lO%
-- 7 --
It was apparent that some of the residual lignin in the unbleached
pulp was rather loosely held and could be removed to a large extent
with a caustic soda extraction step (followed by washing).
Two bleaching runs were done, the results of which are shown in the
following table:
Table 4
~ __
Unbl. pu1p IB1. No. 5equenco Bright- ~lpha Y1c1d % Visc.
~ ness ~) ellu- llm -r cm~/g
~___ ._ ___
C1 ¦ B1 C8,; - E4,~ - D~,5 87,6 ~S,S~ 89,37 37,~ 528
E4 1 82 Ci,; - ~-2, 7 - DO, 8 89,i 96,5~ 97,36 40,i 848
_~ ___ _. ~
Bleach number Bl was done before the positive effect of an extraction
step after pulping was established. Because of the high unbleached
Kappa number, bleach chemical charges were extremely high. Both the
alpha cellulose content and the pulp yield looked promising. Bleach
number B2 was done using an extracted unbleached pulp with a Kappa
number of 17,6. Both the alpha cellulose content and pulp yield were
high.
-- 8 --
EXAMPLE 2
RESULTS OBTAINED WITH EUCALYPTUS AND WATTLE W00D
-
(RUN-OF MILL - SAICCOR)
PREHYDROLYSIS STEP
Table 5 hereunder sets out prehydrolysis conditions and results.
Table 5
__ __~ ~
Timber Rurl No. 52 charge L/~ Time to Tiale at Spent l.$q Yl~ld
( ~ ) ratio te~p Tcmp (Dlin~ ) pH (% )
. ~ins )
__ _ _ __ _ __ ~__
Gurn . P60,2 593/l 6~ 120 ~ lS0C 2,49 83,36
Gum P72,Z 5,3~1 6~ 120 ~ 150C Z~66 83,60
(~m P80,2 5,3/1 6~ 120 ~ 150C 2,64 83,gl
Gum ?90,2 5,3/l 60 120 @ 150C 2~67 83tS2
Qn~ P1~~ Q,2 5~3/l 60 120 ~ 150~C 2,52 82,70
W.~ t t leP1 t 0, 2 S, 3/1 60 1 2D e 1 so~c 2, 78 7g, :i2
The spent hydrolysis liquor from run no. P8 ~as titrated against
standard NaOH solution in order to get an indication of the caustic
soda consumption of such 1iq~or. This would be relevant in the case
where the prehydrolysis step is done in the same vessel or digester as
the pulping step and the acidity of the liquor remaining after
draining of -the bulk of the prehydrolysis liquor has to be neutralized
when adding pulping chemicals. Neutralization required 2,49 NaOH per
litre of prehydrolysis liquor.
~ ~ J ~
_ 9 _
PULPING STEP
Pulping conditions and results are given in Table 6 hereunder.
The initial concentration of NazSO3 (as NA20) at a charge of 17% at a
L/W ratio of 3,5/l was 48,6 g/l. In order to reduce the Kappa number,
cook number C5 was done at the same initial concentration as the other
cooks but at a much higher charge (-L/W ratio increased to 5,3/1).
The anthraquinone charge was also increased to 0,2% (on timber) and
the time at top temperature was increased to 4 hours. The Kappa
number could be lowered to 28,7 but this is still relatively high for
20 a bleachable hardwood pulp. See hereunder under the heading
"Extraction Step".
~ ~ ~ 9~
~ _ O~
- ~ - - - - ~
~ ~; ~ ~ In
~ ~ - - o~ o
c~ ~ t~
:~ ~ " ~ n
C O O O C~ ~ Q .
~ el~ o ~ o o o o ô
_ ~ ..
~ ~ ~ O
~ - _ ,r, a n~ a ~
~: _C ~
.. , = . ..... -.
i~`,
_ ~ O O O O O O O
.n ,-, ~ ~ r'~
. . .
~ ô c~ ~^ ô o ~ -
c~ ~ ~
~5 ~ z o
_ ~ ~ _ ~ ~D ~ ~ ~ ~ er
u ~ o
æ z
I ~ ~ cocn CQ~
~ - -
a
~ W 'J' ~
1 1
THE EXTRACTION STEP
Table 7
-
~ _ I
&t.ac~ion ~b. E~ E~ E7 E8 E9 ¦ E10
. Cook .~'o. C4 C5 C6 -C, ~8iC~ C10
Initial Kappa`~. 40,S 28,7 37,8 34,1 ~,7 31,7
.~aOH ~S) 3 ¦ 3 3 3 3
i l~m?era~uTe (C) 70 i 70 7a 7Q ~0 70
¦! Consistency (S~ 10 1 10 10 10 10 10 .
l Re~er,~ion ~hr3 1 1 1 1 1 1 .
I ~inal pH 17,~0 12,44 12,~7 1Z,SO 1~,34 129iO
K2p?a .~o. 14,; lZ,4 15,2 14,0 ~0,3 16,0
Yic'd (~) ~3,.3 N.D. g4,~3 ~S,~1 95,26 96,7~
~'iscosity (c~3/g) 998,~ 1 - lQ42,~ 1095,g 102S,7 1184,&
_ ~ 1573 l663 154S 1812
. . .
The Kappa number drop during the extraction step was such that it was
obviously more advantageous to use such a step than to try to reduce
the Kappa number by using a more severe pulping step. This is clearly
shown by comparing E5 and E6 in Table 7.
SINGLE VESSEL PROCEDURE
Cook number Cll ( see below) was done in order to simulate the
procedure when doing prehydrolysis, pulping and the extraction step(s~
in a single vessel (- batch digester, no washing between steps), as
follows:
- 1 2 - ~ ~ s i~
PREHYDROLYSIS STEP:
Wood charge - 1400 9 O.D.
L/W ratio - 5,3/1
S02 (%) - 0,2
Time to 150C (mins) - 60
Time at 150C (hrs) - 2
Liquor drained (ml) - 5500
Liquor remaining (ml/calculated) - 1920
Spent liquor pH - 2,66
PULPING STEP:
Assumed prehydrolysis yield (%) - 84
O.D. timber left (9) - 1176
NazSOl charge (% Na20) - 17%
Na2CO3 charge (% NazO) - 4%
NaOH charge for neutralization
1,920 x 2,4 9 - 4,608 9
A.Q. (%) - 0,15
L/W ratio - 3,5/1
Time to 175C (mins) - 80
Time at 175C (hours) - 3
Spent liquor residual sulphite (g/l NazO) - 9,05
Spent liquor pH - 8,89
Spent liquor volume (ml) - 1800
Spent liquor left (ml/calculated) - 2316
The amount of NaOH necessary to compensate for the acidity left after
draining of the excess prehydrolysis liquor, came to 0,3% (as Na20) on
O.D. - prehydrolised wood material.
f ~ 3 ~ ~
- 13 -
EXTRACTION STEP:
Spent pulping liquor from cook number C6 was titrated against NaOH in
order to determine the amount of caustic soda necessary to raise the
pH to 11 (the approximate pH at which all NaHCO3 will be converted to
Na2 CO3). This figure came to 129 NaOH per litre of spent pulping
liquor.
Assumed pulping yield (%) - 53
O.D. material left (9) - 623,28 (assumed)
NaOH (/0) ~ 3
NaOH for raising pH of remaining pulping liquor to 11
= 2,316 x 12 = 27,792 9 = 4,46%
Consistency (%) - 10
Time at 80C (mins) - 20
final pH - 11,66
Kappa no. - 13,4
Yield % - Screened - 42,59
- Rejects - 0,02
The final Kappa number of 13,4 was in line with figures obtained doing
the three steps separately with washing between each step. The Na'`H
necesscry tc raise the pH of the remaining pulping liqllor ~o 11 cam"e
to about 4,5,'0 on O.D. pulp.
OXYGEN DELIGNIFICATION
- In order to further lower the Kappa number, four of the unbleached
pulps were subjected to oxygen delignification. See Table 8 below.
- 14
Table 8
_ __ _
Oxygen bleach no. , 01 . .02 03 04 .
Startlng pulp no. EB C11-- ~,. ~70
Startin~ K~ppa No. 14,0 13,4 Zo,3 lb
NaOH ~) l 1 2 1,5
Mg as MgS04 (~) O,05 Nil Nil Nil
Consistency (~) lO 10 lO 10
Time to temp. (mins) 22 16 26 26
Time a~ tamp. (mins) ~0 3C 30 3n
Top te~perature (~C) llO llS 118 llS
2 pr~ssure (kPa) 6gO 690 6gO 69~
Spent liquor pH 9,66 9,S8 9,57 9,88
Yield (%) 97,86 g8 9 25 9~,45 96,37
Viscosity (cm3/g) 973,6 g46,8 784,8 1037,8
D.P. 1459 1415 1lSU lS66
~appa ~umber ¦ 6,6 S,l _ 3,6 4,2
These pulps delignified easily to low Kappa numbers during an oxygen
delignification step.
BLEACHING
Some caustic soda extracted pulps and the four oxygen delignified
pulps were subjected to full bleaching. See Table 9 below.
15 -
j _ _ _ _ _ _ _ . _
V~ ~ ~ ~-- O
O rl_ 0~ ~O Ln ~ ~ r_ I
.
_ _~
~: ~ oo r~ cn
.~ C~J CO I ~ ~ C`
o ôôo_o~
_.
s o
.~ I~
._. ~. _ ~ .
. .
o I_ r~ ~ . a-- co
c~ ~ r~
1." i ~ ~
CJ'~ Cll ~) a7 o) ,
____,_____ ~
~ôo^
~ ^ ^ ~ T
2 '-- ~ ~
~J ~O ~ O O I
O ~ O O
J N C~J --
I al o ~ o ~ ~
O O
. -~
~. ¦
_ O ~> d' Ln ~ r~
c~ z ~n c~ c~ ~ ~ ~
,~"1 _.~
- 16
EXAMPLE 3
:
In order to further illustrate the effect of an alkali extraction step
after the pulping step and its interaction with the prehydrolysis
step, a separate set of experiments was done using a new sample of
run-of-mill (SAICCOR) Eucalyptus chips.
Prehydrolysis conditions were varied keeping L/W ratio constant at
5,0/1 and temperature rise time at 1 hour. (Table 10)
Table 10
Prehydrolysis I ~ydrolysis I Te~p~ ¦T1me at ¦Yield¦Effluent
10 No I agent (% on wood) tC1 ¦teEp (mins)l (O ¦ p~ !
,2 SO~ :~0 1120184,63 2,71
~ ,3 SO~ 60 1',0 78,11 2,S0
2'' 3,~ SO~ ',0 ' ao 73,34 .,37
~1_ û,' SO~ 0 120 77,14 2,30
215 ~ure water j150 ~20 88,53 3,24
?1- 3,2 SO~ 0 60 ~0,08 ',74
Hydrolysed material was soaked f-or 30 minutes in water after treatment
berore washing and spin drying.
Material originating from some of the prehydrolysis runs was subjected
to pulping. (Table 11)
f'~ s` ~ ~
17 -
The following process conditions applied except for C17
3,5/1 L/W ratio
0,1% Anthraquinone
16% Na2S03 as Na20
5 4% Na2C03 as Na20
80 minutes to temperature (175C)
3 hours at 175C
C17 was done using original wood material which had not been subjected
to a prehydrolysis step. In this case the chemical charge was
10 increased to 18% Na2S03 (as Na20) and 4,5% Na2C03 (as Na20) keeping
the anthraquinone charge and other conditions as above.
Table 11
_ .
Cook No.¦ Pre- ~ ~ield ~%) ' Spent Liquor ¦
hydrolysis Nb. i Screened ¦ Rejects 1 Re5~ Sul~hite P~ No
as Na2O (g/l)
_
'2 1 ?12 1-_,,2 1',04 3,07 ~3,83 35,.
¦ ~'3 1 ?'3 ,~2,:~ ll 0,80 16"~ 8,88 7;,3
~7 1 ?1~ ,cl !" 81 '5,'3 3,~39 '70,:
C'~ I ?'5 i3"5 '',60 ~,~6 8,86 1 33,/
~'6 ?16 1-',01 ~0 6,~0 9,02 ! ~o,i
20C~, I9,08 ~_,02 1'3,~8 3,16 ~ 8~8
It was obvious that the prehydrolysis conditions had a large influence
on delignification response in the pulping step. Harsher
prehydrolysis conditions lead to higher pulp Kappa numbers and higher
levels of residual sulphite in the spent liquor indicating greater
~.d '~ 3 ~.
- 18 -
deactivation of the lignin. This lignin had lost some of its fibre
bonding properties however as the rejects obtained from pulping the
prehydrolysis material were much lower than the rejects obtained from
pulping the unhydrolised material in spite of the latter's lower Kappa
5 number compared to figures obtained from pulping some of the
prehydrolised material.
The pulps obtained as above were subjected to an alkaline extraction
step using sodium hydroxide as the alkali. Extraction was done in
plastic bottles in a constant ternperature water bath. (See Table 12)
The following constant process conditions applied:
60 minutes retention time
10~o consistency
temperature 70C
Table 12
.
.. j .. j . , ---- 1
I E~traction, Ccok INaO~¦Effluent ¦ ~appa Rappa ND ~ield %
, ~0 i~ ~ ,D~ ' Drop Sta~e¦Screened on O.Dq ¦
! w~
. , i . .... , .. I
_ ~ 96,00¦ 42,02
Ii " ,_8 5~ a,o 97,,6¦ 42,70
~ -,? :2,5 36,51~ '2,24
_~ '2 ~ "9 35,311 41,'2
Jq ~ ' 39r061 ~6,2S
, ~ 5 ~ ; ' ~ 3~,_ 35,o 32,91l 30,77
_ ,~ , '3,0 -~,/ 38,96 36,68
~ ~ ^ ,O _ _ 97,77 14,15
1 _~9 , _ ~ '112,i/ _1,- ," 9,01
!
~ ? ~? ~ 3
- 19 -
Comparing Kappa numbers of Ell, El2, E13 and El4 shows that the Kappa
number drop during the extraction step is dependent on the amount of
alkali charged. In a separate experiment lOOg O.D. of prehydrolysis -
S.A.S.-A.Q. gum pulp of 38,3 Kappa number was washed with 1 litre of a
3g/l NaOH solution at 70C on a Buchner funnel. After washing with
water the Kappa number of the resulting pulp was 25 indicating that
merely washing with alkaline wash water can also effect a substantial
Kappa number drop.
Comparing other extraction step Kappa numbers in Table 12 also
indicates that the extraction step brings widely varying pulping step
Kappa numbers, originating from varying prehydrolysis conditions, much
closer together. This is a positive result but yield figures indicate
that large yield losses can result from too harsh a prehydrolysis
step. This step, however, must be sufficiently harsh (pH, temperature
and time) to effect adequate hemicellulose removal
The Kappa number drop obtained from the unhydrolised pu1p (Cook C17)
on extraction also indicates the different nature of the lignin in
this pulp compared to the lignin in pulp which had been hydrolised.
If this material had been obtained by applying a hydrolysis step prior
to the pulping step, the Kappa number drop on extraction would have
been more pronounced considering the unextracted Kappa number of this
pulp,
The above results were obtained from pulp which had been well washed
after the pulping step in order to demonstrate the positive effect of
an alkaline extraction step.
When running batch digesters using this proces~ the preferred
procedure would be to drain the digester after the prehydrolysis step
of the prehydrolysis spent liquor. (This liquor would be used for
- 20 -
recovery and beneficiation of products dissolved and extracted from
the wood).
The pulping chemicals (Naz503, ~a2 C03, and Anthraquinone) would then
be charged (possibly together with a small amount of NaOH to
compensate for the residual acidity of the prehydrolysis step). After
a penetration period and ?ulping ~ime, the spent liquor would be
pumped from the digester. Alkali (,~aOH) would then be added to this
liquor and reintroduced to the digester followed by circulation Cor a
period. This would complete the cook.
Ihe addition of alkali to the spent liquor and reintroduction of this
mixture to the digester serves two purposes.
a) An effective extraction is accomplished which lowers the Kappa
number prior to the oxygen delignification step (or other step
depending on the bleach sequence). The pH adjusted spent liquor
can be introduced at nigh temperature because the free alkali
concentration is low which otherwise would have resulted in
cellulose yield loss at high temperature. The results appear to
indicate that the lowering of the Kappa number is mostly the
result of fibre swe11ing at the higher pH. Pathways are opened
as a result of the swelling which allow removal of some of the
lignin molecules which are loosely held.
bj Spent S.A.S.-A,Q. pulping liquor can be difficult to evaporate
to high solids levels without forming troublesome precipitates.
his can be overcome by adjusting the pH of the spent liquor to
about 11 prior to evaporation.
~ 3
- 21
In a continuous digester introduction of pH adjusted spent liquor to
the wash zone can accomplish both the extraction step Kappa reduction
benefit and the enhanced liquor evaporation properties resulting from
the higher pH.
It appears from the above that the process of the present invention
can produce high alpha-cellulose pulp at a high pulp yield. An
alkaline extraction step following pulping proved to be satisfactorily
effective in removing residual lignin.
An oxygen bleaching step also proved effective in further lowering the
Kappa number to acceptably low levels prior to full bleaching. Full
bleaching of such pulps required fairly low charges of
chlorine-containing chemicals, thereby providing a process which has a
low pollution potential. "Non polluting" bleach chemicals like ozone
and peroxide could also be considered.
Although cer~ain embodiments only of the invention have been
illustrated above, it will be appreciated by any person skilled in the
art that other possibilities and variations of the invention are
possible. Such possibilities and variations are therefore to be
considered as falling within the spirit and scope of the present
invention as defined in the appended claims.
