Process for oxidation of black liquor

High recovery of useful energy from the heat of reaction in the oxidation of black liquor is obtained by integrating the oxidation into the multiple effect evaporation system of the pulp mill recovery sequence. The heat of reaction is thereby recovered as flash steam, which when combined with the vapors from an appropriate evaporator body enables recovery of its energy through further evaporation. The oxidation reaction may be carried out on the liquor leaving the second effect of the evaporation sequence for maximum energy recovery, or in accordance with an alternative embodiment, the reaction is carried out between the first effect and the flash tank with preferable addition of a second flash tank in series with the first.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the oxidation of sulfur and compounds 
thereof contained in black liquor as practiced in pulp mills for recovery 
of sulfur values and the elimination of malodorous emissions and is 
particularly concerned with a unique system for recovery of high value 
energy from the heat evolved in such oxidation reaction, which heat would 
otherwise be dissipated. 
2. Prior Art 
The oxidation of black liquor is currently being employed in the pulping 
industry and has been described in numerous publications. 
In a typical sequence of operation for cellulosic fiber liberation, as in 
the manufacture of paper products by the kraft pulping process, the raw 
wood chips or pieces are cooked or digested in a solution of one or more 
sulfur compounds. Thereafter the mixture of delignified fibers and 
treating liquid is sent to a blow tank for pressure reduction, and next 
transferred to a multiple stage washing facility in which the fibers are 
separated from the spent chemical-laden wash water filtrate, which 
filtrate is designated as "weak black liquor". In certain typical plants 
this black liquor is next subject to concentration in a multiple effect 
evaporation facility. 
Since the spent treating liquid from the digester contains sodium sulfide 
and other sulfur compounds which are of themselves malodorous or which 
form hydrogen sulfide and/or other malodorous sulfur compounds released to 
the atmosphere during the pulp mill operation, it has become the pevailing 
practice to subject the sulfur-laden spent digestion liquid to oxidation 
at some selected stage in the sequence, to convert the sulfide and/or 
other sulfur compounds therein to more stable compounds such as 
thiosulfates and/or sulfates. 
The point in the mill operation sequence at which the oxidation of this 
so-called "black liquor" is to be best carried out as well as the manner 
of carrying out the oxidation step, has been the subject of extensive 
investigation. Thus, among the various proposals advanced, it has been 
suggested by some proponents that the oxidation step be applied to the 
weak black liquor from the washing step and prior to concentration. 
Drawbacks encountered in this procedure led some mills to resort to 
previous concentration of the liquor and to subject the obtained "strong 
black liquor" to the oxidation step. Subsequently, it has been proposed, 
for example in accordance with U.S. Pat. No. 4,058,433, that oxidation be 
carried out in the weak black liquor preferably at a point between the 
outlet of the blow tank and the inlet of the pulp washers, preferably 
employing oxygen in high concentration as opposed to the more usual use of 
air for the purpose. 
Various types of oxidizing reactions have heretofore been used or proposed 
for use in the oxidation of black liquor. In U.S. Pat. No. 4,058,433, 
above-referred to, a long narrow counter-flow reactor column is advocated. 
Other types of oxidizing vessels and arrangements are depicted in U.S. 
Pat. Nos. 3,362,868; 3,549,314; 3,709,975; and 3,928,531. In accordance 
with the disclosure in U.S. Pat. No. 3,709,975 certain of the objections 
and drawbacks of prior black liquor oxidation processes are stated to be 
avoided by resort to a multi-stage oxidation technique employing 
oxygen-rich gas, stated to be applicable to both weak and strong black 
liquor. 
Systems and conditions for black liquor oxidation with molecular oxygen are 
described by Cooper et al in TAPPI 56, No. 6, June 1973 at pages 100 to 
103 and in AIChE Symposium Series, Vol. 69, No. 133 at pp 106-115. 
In none of the described techniques of the above-cited patents or other 
known prior art disclosures and practices of the pulp industry, is there 
found any description of a black liquor oxidation process particularly 
aimed at maximizing recovery of energy released in the oxidation reaction. 
Typically in conventional kraft mill recovery systems which employ air or 
oxygen black liquor oxidation (BLOX) only a fraction (about 20%) of the 
heat evolved in the oxidation reaction is recovered as high value energy. 
The remainder is either lost as vented water vapor or recovered as low 
value heat in the condenser cooling water. 
SUMMARY OF THE INVENTION 
In accordance with the process of the present invention most of the heat 
evolved in the oxidation of black liquor (as up to or approaching 100%) is 
recovered as high value energy. This desired objective is accomplished by 
the unique process sequences of the present invention wherein the BLOX 
reaction is integrated into the multiple effect evaporator system wherein 
the heat of reaction is recovered as flash steam. The obtained flash steam 
is combined with the vapors from an appropriate evaporator body to recover 
its energy through further evaporation. The BLOX reactor can be located 
such that flashed vapors are combined with vapors leaving an evaporation 
effect located upstream of the oxidation stage in the direction of black 
liquor flow, said evaporation effect being operated at higher pressure 
than that of the effect into which the black liquor is first introduced. 
Maximum energy recovery is attained when the oxidation reaction is applied 
to the liquor leaving the second effect of evaporation. When this oxidized 
liquor is then added to the first effect in the evaporation sequence, the 
heat of reaction is recovered by a reduction in evaporator steam demand. 
In accordance with an alternative embodiment, the oxidation of the black 
liquor may be carried out between the first effect and the flash tank. In 
carrying out this alternative, maximum energy recovery is achieved by 
addition of a second flash tank in series with the first. In this manner, 
the flash steam from the first tank receiving the oxidized liquid is added 
to the vapors leaving the first evaporator effect and the flash steam from 
the second tank is used to supplement the vapors from the second 
evaporator effect. While operation with a single flash tank is also 
possible, such operation obtains reduced energy recovery. 
In the foregoing summary reference is made to "first effect", "second 
effect", etc. of a multiple effect evaporator system. It will be 
understood the numbering sequence employed is in the direction of steam 
flow as is conventional in this art. Thus, in a reverse flow multiple 
effect evaporator system the initial dilute liquid to be subjected to 
concentration enters the evaporation system at the highest numbered effect 
while the steam is supplied at the opposed end of the sequence to the 
"first effect", so that in a six effect system the initial dilute liquid 
flow is increasingly concentrated as it flows from effect #6 to effect #1.

DETAILED DESCRIPTION 
For a fuller appreciation of the advantages afforded by the present 
invention, it is instructive to examine the overall energy and material 
balances applicable. 
In kraft pulp mill recovery processes, black liquor is concentrated to 
about 65% solids before it is fed to the recovery boiler for recovery of 
inorganic chemicals. Two methods of concentration are in general use. 
1. Concentration to 50% solids in multiple effect evaporators followed by 
further concentration to 65% using indirect contact (forced circulation) 
evaporators. 
2. Concentration to 50% solids in multiple effect evaporators followed by 
direct contact evaporation in which the liquor is contacted with recovery 
boiler flue gas to achieve a 65% solids concentration. 
When direct contact evaporation is used, black liquor oxidation (BLOX) is 
required to prevent H.sub.2 S formation which is caused by reaction 
between sodium sulfide and carbon dioxide in the flue gas. 
EQU Na.sub.2 S+CO.sub.2 +H.sub.2 O.fwdarw.Na.sub.2 CO.sub.3 +H.sub.2 S 
BLOX prevents this reaction by converting the sodium sulfide to sodium 
thiosulfate. 
BLOX is exothermic having a heat of reaction of about -2780 BTU/lb Na.sub.2 
S when adjusted for some organic reaction.sup.(1). As a result, the 
caloric value of the black liquor is reduced significantly resulting in a 
loss in steam production in the recovery boiler. In the existing art, only 
a fraction of this heat of reaction is recovered. 
FNT .sup.(1) Grace, T. M. in Forum on Kraft Recovery Alternatives, The 
Institute of Paper Chemistry, Appelton Wisc. 1976, p. 253. 
In FIG. 1 of the accompanying drawings, the following legends are employed: 
F is the weight amount of weak black liquor; 
C is the weight of the combined condensates; 
P is the weight amount of oxidized liquid product leaving the multiple 
effect evaporation system; 
W is weight amount of water vapor contained in the vented gas; 
V.sub.6 is the weight amount of vapor from evaporation effect (#6) sent to 
condensation. 
S is the weight amount of steam supplied to the first effect of the 
multiple effect evaporator system. 
K is the heat removed by condensation. When the weight unit is pounds, K is 
expressed in BTU/lb. 
In the energy and material balances shown below, the energy content of the 
air and or oxygen streams have been neglected as well as the effect of 
reacted oxygen on the mass of product. Since the balances are to be used 
to compare systems giving identical product, these corrections are small. 
By material balance, neglecting reaction oxygen 
EQU F=W+P+C (1) 
and energy balance gives 
EQU FH.sub.f +SL.sub.s =.DELTA.H+WH.sub.W +PH.sub.p +K+CH.sub.c (2) 
and, 
K=V.sub.6 L.sub.6 (3) 
where 
L=Latent heat of vaporization at condenser pressure, 
H=enthalpy in BTU/lb and 
.DELTA.H=heat of reaction. The subscripts refer to the streams as indicated 
in FIG. 1. 
Combining and solving for SL.sub.s gives 
EQU SL.sub.s =.DELTA.H+P(H.sub.p -H.sub.c)+F(H.sub.c -H.sub.f)+W(H.sub.W 
-H.sub.c)+V.sub.6 L.sub.6. (4) 
For practical reasons, there is an upper limit to the solids concentration 
of the feed to the direct contact evaporator.sup.(2). Thus BLOX systems 
should be compared at identical feed and product compositions. The 
important differences, therefore, are the quantity of vapor evaporated in 
the 6th effect and the water vapor in the vent gas. 
FNT .sup.(2) Bart, R. et al, in Forum on Kraft Recovery Alternatives. The 
Institute of Paper Chemistry, 1976, p. 69. 
For Air BLOX system, the term W(H.sub.W -H.sub.c) represents a significant 
energy loss which is not recoverable as high value energy. 
For oxygen BLOX systems, there is little or no venting of water vapor and 
thus the influence of BLOX on evaporator operating characteristics 
determines how efficiently the heat of reaction can be utilized. 
Systems Without BLOX 
Systems having direct contact evaporation but not using BLOX form a useful 
basis for comparison of the performance of various BLOX systems. 
To simplify the analysis, evaporator performance for mills without BLOX 
will be approximated by 
EQU F-P=E.sub.n S 
where E.sub.n is called the steam economy and is typically about 5 for 
kraft pulp mill evaporators under normal conditions. Thus, steam demand is 
##EQU1## 
for mills without BLOX 
FIG. 2 depicts a typical strong black liquor oxidation system using air. 
The multiple effect evaporators are numbered 1 to 6. The weak black 
liquor, as shown, is simultaneously charged to the evaporated effects #6 
and #5 and therefrom serially in descending number sequence into and 
through the other evaporator effects of the series discharging from 
evaporator effect #1 into a flash tank. Evaporator effect #1 is heated by 
a steam coil and the vapor products discharged overhead from evaporator 
effect #1 flow in indirect heat exchange with the liquid in evaporator 
effect #2, and in turn the vapor overhead from each of effects numbered #2 
to #5 is employed in heating the next higher numbered evaporator effect in 
ascending numerical sequence. The vapor overhead from evaporator effect #6 
is sent to a condenser. The vapor product flashed off in the flash tank is 
sent to join the vapor overhead discharged from evaporator effect #2 and 
passes into evaporator #3 in indirect heat exchange with the liquid 
therein. 
The liquid residue from the flash tank (P.sub.o) enters the oxidation 
reactor, where it is oxidized by contact with a flowing stream of air. The 
water vapors are vented from the oxidation reactor and, in some cases, the 
liquid is discharged into a BLOX polishing reactor for further contact 
with oxygen, the resulting "polished" liquid (P) being then sent to the 
direct contact evaporator. 
Material and energy balances over the BLOX reactor in an arrangement such 
as that depicted in FIG. 2, are shown below: 
##EQU2## 
and, if the oxidation reactor is operated at equal inlet and outlet 
temperatures as recommended by Christie.sup.(3), 
FNT .sup.(3) Christie, R. D., Sulfide Increase Following Weak Black Liquor 
Oxidation, P&P Magazine, Can. 73, No. 10, October 1972, pp 74-78. 
##EQU3## 
for the evaporators it can be shown that 
##EQU4## 
Noting that (H.sub.w -H.sub.c).congruent.L.sub.w and substitution into 
equation 4 gives 
##EQU5## 
therefore 
##EQU6## 
for air BLOX systems. 
In the typical system depicted in FIG. 3, oxidation of the weak black 
liquor is carried out before the liquor enters the multiple effect 
evaporator, using high concentration oxygen gas instead of normal air. The 
arrangement of the evaporator effects and flow patterns of liquid and 
vapor are otherwise the same as in the FIG. 2 scheme. Since the enthalpy 
of the evaporator feed liquor is higher than normal due to the heat of the 
reaction, the steam demand of the evaporators is reduced by an amount 
which compensates for the water removed as a result of flashing of the 
reaction heat in effects 5 and 6. Detailed multiple effect calculations 
are required for precise calculation of the steam demand; however, the 
following analysis is a reasonable approximation. 
The evaporator feed is distributed between effects 5 and 6 as follows: 
______________________________________ 
Feed to effect #6 = aF 
Feed to effect #5 = (1 - a) F 
Total water removed = F - P 
Water removed due to steam demand .congruent. E.sub.n s (see note)* 
##STR1## 
##STR2## 
##STR3## 
Thus: 
##STR4## 
##STR5## 
Or: 
##STR6## 
______________________________________ 
Note that the assumption that the water removed due to evaporator steam 
demand = E.sub.N S is not strictly correct because a disproportionate 
amount of steam is required to heat the liquor to the boiling point in 
passing from effect to effect. 
Table I summarizes the foregoing analysis of the known art. Note that the 
evaporator steam requirement is lowered when BLOX systems are installed in 
a mill with direct contact evaporation. However, this reduction in steam 
demand is insufficient to make up for the loss in heating value of the 
black liquor. Therefore, the net result is that existing BLOX systems 
cause a net loss in available steam. 
TABLE I 
__________________________________________________________________________ 
Effect of BLOX on Net Steam Production 
For Known Art Mills 
Mill Before Oxygen BLOX 
BLOX Air BLOX 
a = 1 a = 0.5 a = 0 
__________________________________________________________________________ 
Weak Black Liquor Flow 
LB/ADT 20,000 20,000 20,000 20,000 20,000 
K/MT 10,000 10,000 10,000 10,000 10,000 
Weak Black Liquor 
Concentration % 15 15 15 15 15 
Concentration Entering Direct 
Contact Evaporator % 
50 50* 50* 50* 50* 
Black Liquor Na.sub.2 S Loading 
LB/ADT 180 180 180 180 180 
(K/MT) (90) (90) (90) (90) (90) 
Total Water Removed 
LB/ADT 14,000 14,000 14,000 14,000 14,000 
(K/MT) (7,000) 
(7,000) (7,000) (7,000) (7,000) 
Latent Heat of Vaporization 
BTU/LB H.sub.2 O 1,000 1,000 1,000 1,000 1,000 
(Kc/K) (554.3) 
(554.3) (554.3) (554.3) (554.3) 
BLOX Efficiency % 99 99 99 99 
BLOX Heat of Reaction 
MM BTU/ADT -0.495** 
-0.495** 
-0.495** 
-0.495** 
(Kc/MT) (-1.37 .times. 10.sup.5) 
(-1.37 .times. 10.sup.5) 
(-1.37 .times. 10.sup.5) 
(-1.37 .times. 10.sup.5) 
Evaporator Steam Required. 
MM BTU/ADT 2.800 2.701 2.701 2.651 2.602 
(Kc/MT) (7.76 .times. 10.sup.5) 
(7.49 .times. 10.sup.5) 
(7.49 .times. 10.sup.5) 
(7.35 .times. 10.sup.5) 
(7.21 .times. 10.sup.5) 
Recovery Boiler Steam Production 
MM BTU/ADT 13.100 12.600*** 
12.600*** 
12.600*** 
12.600*** 
(Kc/MT) (3.63 .times. 10.sup.6) 
(3.49 .times. 10.sup.6) 
(3.49 .times. 10.sup.6) 
(3.49 .times. 10.sup.6) 
(3.49 .times. 10.sup.6) 
Net Steam Available for Process 
MM BTU/ADT 10.300 9.899 9.899 9.949 9.998 
(Kc/MT) (2.86 .times. 10.sup.6) 
(2.74 .times. 10.sup.6) 
(2.74 .times. 10.sup.6) 
(2.76 .times. 10.sup.6) 
(2.77 .times. 10.sup.6) 
__________________________________________________________________________ 
ADT = Air dried ton (of pulp) 
K/MT = Kilograms per metric ton of pulp (airdry basis) 
Kc/K = Kg. cal./Kg. 
Kc/MT = Kilogram calories per metric ton of pulp (airdry basis) 
*Not corrected for Na.sub.2 S.sub.2 O.sub.3 formation Reaction: 2780 
**Heat of Reaction 2780 .times. 180 .times. .99 .times. 10.sup.-6 MM 
BTU/ADT Includes correction for some organic oxidation (see Grace 
abovecited at page 253) 
***Corrected for loss in heating value due to BLOX 13,100 (2780 .times. 
180 .times. 10.sup.-6) Correction .noteq. heat of reaction because BLOX 
polishing operation eliminates remaining 1% of .DELTA.H. 
Translating these data to a 1000 t/d kraft mill, having a recovery system 
generating 10,300 MM BTU/day of process steam, air BLOX reduces the 
available steam by 401 MM BTU/day (101.times.10.sup.6 kilogram 
calories/day). Assuming that evaporator feed is split equally between 
effects #5 and #6, prior art oxygen BLOX reduces available steam by 351 MM 
BTU/day (88.5.times.10.sup.6 kilogram calories/day). 
In the embodiment of the present invention as illustrated by the flow 
diagram of FIG. 4, the oxidation reactor is placed in line between effect 
#1 and effect #2 of the multiple effect evaporator sequence. As shown, the 
weak black liquor admitted may be split between evaporator #6 and 
evaporator #5. The initial steam coil heating is applied as heretofore 
explained in connection with the known operations depicted in FIGS. 2 and 
3, the evolved vapor overhead, as before, passing from one lower to the 
next higher of the evaporator effects in ascending numerical order. From 
effect #2 the residual liquor enters the oxidation reactor where it is 
oxidized with high purity oxygen gas. The oxidized liquor is then sent to 
the first effect evaporator (#1) for heating by contact with the steam 
coil therein and the liquid therefrom discharged into a flash tank, 
followed by BLOX polishing before being sent to the direct contact 
evaporator. 
In the process as shown by the flow diagram of FIG. 4, only one flash tank 
is employed, placed intermediate to the #1 evaporator effect and the BLOX 
polishing unit. The embodiment illustrated in FIG. 5 utilizes two flash 
tanks in series. In the FIG. 5 embodiment, the oxygen reactor is placed in 
line of liquid flow between the #1 evaporator effect of the multiple 
effect sequence and the next adjacent flash tank (#2). The residual liquid 
thus flows sequentially from the first effect evaporator to the oxidation 
reactor and the oxidized liquor then flows into the #2 flash tank. The 
vapor overhead from the #2 flash tank is sent by conduit to indirect heat 
exchange in the #2 evaporator effect, these vapors being previously joined 
by the vapor discharged from the #1 effect evaporator. From the #2 flash 
tank the unvaporized liquid portion passes into flash tank #1, the vapor 
overhead from that tank joining the vapors discharged from #2 evaporator 
enroute to the heat exchange coil in #3 evaporator. As before, the liquid 
discharged from #1 flash tank is sent to BLOX polishing and thence to the 
direct contact evaporator. 
The embodiment of FIG. 6 uses the same location of the oxygen BLOX reactor 
as that in FIG. 5, but employs a single flash tank following the oxidation 
reaction. In this embodiment, the vapor overhead, as shown, from the flash 
tank is joined to the vapor overhead from #2 evaporator effect and the 
both vapors are used in heating the liquid in effect #3. 
In each of these embodiments according to the invention and as is 
illustrated in FIGS. 4 through 6, the BLOX reaction is incorporated into 
the multiple effect evaporator system in such manner as to allow recovery 
of the heat of reaction to replace a portion of the evaporator steam 
requirement. In operation of either the alternative embodiments 
illustrated in FIGS. 4 and 5, virtually all of the heat of reaction can be 
recovered. In both these cases, the net effect is to convert the heat of 
reaction to steam which supplements the vapor leaving the first effect of 
evaporation. Thus: 
______________________________________ 
F - P = E.sub.n (L.sub.s S + (- .DELTA.H)) 
or 
##STR7## 
______________________________________ 
which is clearly more efficient recovery of high value energy than is 
possible by the hitherto known systems of the prior art. 
In the method illustrated by the alternative modification of FIG. 6, using 
a single flash tank, the efficiency does not equal that obtained by the 
methods of FIGS. 4 and 5, but even operation according to FIG. 6 provides 
a higher efficiency than that obtained by the known methods of the prior 
art. The following analysis gives a reasonable approximation of steam 
demand when operation is conducted by the FIG. 6 embodiment. 
______________________________________ 
Total water removed = F - P 
Water removed due to steam demand 
= E.sub.n S 
Evaporation in Flash Tank due to heat of reaction 
##STR8## 
Evaporation in effects 3, 4, 5, 6 due to condensation of reaction heat 
flashed in F.T. (number of effects influenced .times. evaporation per 
effect) 
##STR9## 
##STR10## 
##STR11## 
______________________________________ 
which is still more efficient than the known art but not as efficient as 
the system of FIGS. 4 and 5. 
The liquid from condensation in each of the multiple effect evaporators may 
be individually discharged or combined for discharge through a common 
manifold. 
In Table II below, the analysis of the process following the embodiments 
illustrated in FIGS. 4, 5 and 6 respectively is summarized. It will be 
noted that in each instance, according to the invention, a reduction in 
evaporator steam demand is achieved nearly equal to the loss in black 
liquor heating value. Thus, the net available steam is nearly identical to 
that generated by a mill without BLOX. 
TABLE II 
__________________________________________________________________________ 
EFFECT OF BLOX ON NET STEAM PRODUCTION 
FOR MILLS USING THE PROCESS OF THIS INVENTION 
OXYGEN BLOX 
Between Effect #1 and Flash Tank 
Mill Before 
Between Effects 
2 Flash Tanks 
1 Flash Tank 
BLOX 1 & 2 (Fig. 4) 
(Fig. 5) (Fig. 6) 
__________________________________________________________________________ 
Weak Black Liquor Flow 
LB/ADT 20,000 20,000 20,000 20,000 
K/MT 10,000 10,000 10,000 10,000 
Weak Black Liquor 
Concentration % 15 15 15 15 
Concentration Entering Direct 
Contact Evaporator % 
50 50 50 50 
Black Liquor Na.sub.2 S Loading 
LB/ADT 180 180 180 180 
(K/MT) (90) (90) (90) (90) 
Total Water Removed 
LD/ADT 14,000 14,000 14,000 14,000 
(K/MT) (7,000) 
(7,000) (7,000) (7,000) 
Latent Heat of Vaporization 
BTU/LB H.sub.2 O 1,000 1,000 1,000 1,000 
Kc/K (554.3) 
(554.3) (554.3) (554.3) 
BLOX Efficiency % 98 98 98 
BLOX Heat of Reaction 
MM BTU/ADT -0.495 -0.495 -0.495 
(Kc/MT) (136 .times. 10.sup.3) 
(136 .times. 10.sup.3) 
(136 .times. 10.sup.3) 
Evaporator Steam Required 
MM BTU/ADT 2.800 2.310 2.310 2.375 
(Kc/MT) (7.76 .times. 10.sup.5) 
(6.40 .times. 10.sup.5) 
(6.40 .times. 10.sup.5) 
(6.58 .times. 10.sup.5) 
Recovery Boiler Steam Production 
MM BTU/ADT 13.100 12.600 12.600 12.600 
(Kc/MT) (3.63 .times. 10.sup.6) 
(3.49 .times. 10.sup.6) 
(3.49 .times. 10.sup.6) 
(3.49 .times. 10.sup.6) 
Net Steam Available for Process 
MM BTU/ADT 10.300 10.290 10290 10.225 
(Kc/MT) (2.86 .times. 10.sup.6) 
(2.85 .times. 10.sup.6) 
(2.85 .times. 10.sup.6) 
(2.83 .times. 10.sup.6) 
__________________________________________________________________________ 
In the practice of the invention, any type of multiple effect evaporator 
line may be employed as commonly used by the pulp industry. In designing a 
new mill, however, the heat transfer surface of the first effect can be 
reduced, as will be seen from Example 2 below. The evaporators should be 
capable of bringing a black liquor feed of normally about 15% solids 
concentration to a 50% product concentration. 
In the embodiment of FIG. 5, the vapor line should be connected to the 
vapor space of evaporator effect #1 for maximum efficiency. Pressure must 
be maintained equal to the pressure in effect #1 by providing a throttling 
(pressure control) valve between flash tank #1 and flash tank #2. The 
flash tanks employed in the embodiments of FIGS. 4 and 5 are identical in 
design to those commonly employed to step down evaporator product. In 
operation of the FIG. 6 embodiment, however, the flash tank must be 
designed for a higher capacity to handle the additional flash vapors 
resulting from the heat of the oxidation reaction. 
Any of the various known types of closed oxidation reactors may be employed 
in practice of the invention. Also, the operating conditions for the 
oxidation reaction may be such as are known for BLOX processes employing 
oxygen gas of high concentration (see e.g. Cooper et al, cited above). The 
operating pressure, of course, must be high enough to prevent flashing 
within the reactor. Specific conditions that may be employed in operation 
in accordance with the FIG. 4 embodiment, are as follows: 
______________________________________ 
Reactor inlet temperature 
225.degree. F. 
(107.22.degree. C.) 
Reactor outlet temperature 
300.degree. F. 
(148.89.degree. C.) 
Boiling point rise 7.degree. F. 
(3.89.degree. C.) 
Equilibrium steam temperature 
293.degree. F. 
(145.degree. C.) 
Pressure to prevent flashing 
46 psig (3.22 kg/cm.sup.2) 
Reactor pressure must exceed 
46 psig 
Residence time in reactor 
1 second to 5 minutes 
______________________________________ 
An example of conditions applicable in operation of the FIG. 5 embodiment 
is as follows: 
______________________________________ 
Reactor inlet temperature 
252.degree. F. 
(122.22.degree. C.) 
Reactor outlet temperature 
360.degree. F. 
(182.22.degree. C.) 
Boiling point rise 11.degree. F. 
(6.11 .degree. C.) 
Equilibrium steam temperature 
349.degree. F. 
(176.11.degree. C.) 
Pressure to prevent flashing 
119 psig 
Reactor pressure must exceed 
119 psig 
Residence time in reactor 
1 second to 5 minutes 
______________________________________ 
In operating the FIG. 5 embodiment, throttling valves will be required to 
step down products of the BLOX reactor and a feed pump must be used to 
pressurize the reactor. 
Stated more generally, BLOX utilizing the system of FIG. 4 may be operated 
at temperatures in the range of 200.degree. to 340.degree. F. (93.degree. 
to 170.degree. C.) and pressures of 45 to 100 psig. Somewhat higher 
pressures and temperatures are generally indicated for BLOX in the systems 
of FIGS. 5 and 6, as 225.degree. to 400.degree. F. (105.degree. to 
205.degree. C.) and pressures in the range of 50 to 200 psig (4.4 to 14.6 
atmospheres). 
Pressure in the BLOX reactor should be maintained above the equilibrium 
partial pressure of the black liquor in the reaction zone. Operation below 
such pressure will cause flashing or vaporization within the reactor and 
consequent loss in reactor efficiency. The temperature should be 
maintained as high as possible for maximum reaction rates. Temperatures in 
excess of about 400.degree. F. (205.degree. C.) may cause excessive 
organic oxidation and thus be wasteful of oxygen. Sufficient retention 
time must be allowed for the sulfide oxidation reaction to take place to 
substantial completion. It has been indicated that a retention time of 1 
second to 5 minutes will obtain 98% conversion of a strong black liquor. 
Of course, no harmful effects are had by extending the residence time. 
To obtain complete oxidation according to stoichiometry, a mole of oxygen 
is required for each mole of sulfur, so that the oxygen requirement per 
weight amount of Na.sub.2 S is 32/78 or 0.41 times the weight of the 
sulfide. In normal operation it is preferred to employ oxygen in 20-40% 
excess, i.e. in the range of 0.49 to 0.57 times the weight of sodium 
sulfide. Too much oxygen of course if wasteful. 
While a polishing reactor is shown in the several figures of the drawings, 
it will be understood that the same is optional and becomes necessary only 
when the remaining sulfide concentrations in the black liquor requires 
such additional oxidation treatment because of possible reversion or 
incomplete oxidation in earlier stages of oxidation of the liquor. If 
needed or desired, any of the known types of polishing reactors may be 
employed. 
For best results in practice of the invention oxygen gas of high purity 
should be employed in the BLOX reactor, preferably as of no less than 95% 
O.sub.2, the rest of the gas being composed of nitrogen, argon or other 
inert gases. 
In the following examples certain of the advantages obtained in operating 
in accordance with the invention are set out. Example I analyzes the net 
reduction in available steam after BLOX installation in a typical kraft 
pulp mill, utilizing the system depicted in FIG. 4. Example II analyzes 
the reduction in heat transfer surface required in the first effect, when 
using the FIG. 4 process as compared with the FIG. 5 process. Example III 
demonstrates that practice of the present invention allows increased 
evaporator capacity and consequently increased pulp production. 
EXAMPLE I 
A 1000 ton/day (907 metric tons/day) kraft pulp mill having direct contact 
evaporation, installs oxygen black liquor oxidation using the process 
shown in FIG. 4. 
Before Installation of the BLOX 
Evaporator steam economy=5 
Weak black liquor solids=15% 
Solids in feed to direct contact evaporator 50% 
Weak black liquor flow 20 MM lbs/day (9.027.times.10.sup.6 Kg/day) 
Na.sub.2 S in black liquor 180 M lbs/day (81,648 Kg/day) 
Evaporator steam 2800 MM BTU/day (7.056.times.10.sup.8 kg. cal/day) 
Steam produced in recovery boiler 13,100 MM BTU/day (3.3.times.10.sup.9 kg. 
cal/day) 
Net Process steam production 10,300 MM BTU/day (2.60.times.10.sup.9 kg. 
cal/day) 
After Installation of BLOX 
Weak black liquor solids 15% 
Solids in direct contact evaporator feed 50% (uncorrected for Na.sub.2 
S.sub.2 O.sub.3) 
Efficiency of 1st BLOX reactor 98% 
Na.sub.2 S after evaporation 3.6 M lbs/day (1633 kg/day) 
Na.sub.2 S after2nd BLOX reactor Nil 
O.sub.2 required (40% excess O.sub.2) 103 M lbs/day (46,721 kg/day) 
Evaporator steam 2310 MM BTU/day (5.82.times.10.sup.8 kg. cal/day) 
Steam produced in recovery boiler 12,600 MM BTU/day (3.175.times.10.sup.9 
Kg. cal/day) 
Net process steam production 10,290 MM BTU/day (2.59.times.10.sup.9 Kg. 
cal/day) 
Net reduction in available steam 10 MM BTU/day (2.52.times.10.sup.6 Kg. 
cal/day) 
Energy to operate O.sub.2 generator 120 MM BTU/day (3.02.times.10.sup.7 kg. 
cal/day) 
EXAMPLE II 
The use of this invention reduces the heat transfer surface which is 
required in the first effect of evaporation. A 1000 T/d kraft pulp mill 
having the same operating characteristics as in Example I has the first 
effect heat transfer surface requirements as follows: 
1000 t/d mill without BLOX 
______________________________________ 
Solids in feed to direct 
contact evaporator 50% 
Boiling point rise in flash tank 
13.degree. F. (7.22.degree. C.) 
Liquor temperature in flash tank 
231.degree. F. (110.56.degree. C.) 
Solids in feed to flash tank 
49.3% 
Boiling point rise in Effect # 1 
13.degree. F. (7.22.degree. C.) 
Liquor temperature in Effect # 1 
252.degree. F. (122.22.degree. C.) 
Steam temperature (35 psig) 
281.degree. F. (138.33.degree. C.) 
Heat transfer surface in Effect # 1 
20,000 SQFT 
(1858 sq. meters) 
Steam demand 2800 MMBTU/day 
(7.06 .times. 10.sup.8 Kg. cal/day) 
1000 t/d mill using process of Fig. 4. 
Steam demand 2310 MMBTU/day 
(5.82 .times. 10.sup.8 kg. cal/day) 
##STR12## 
(1533 sq. meters) 
______________________________________ 
Heat transfer surface of other evaporation effects not changed. 
1000 t/d mill using process of FIG. 5 
______________________________________ 
Solids in feed to direct contact 
evaporator 50% (uncorrected) 
Boiling point rise in flash tank # 2 
13.degree. C. (7.22.degree. C.) 
Liquor temperature in flash tank # 2 
231.degree. F. (110.56.degree. C.) 
Solids in feed to flash tank # 2 
49.3% 
Boiling point rise in flash tank # 1 
13.degree. F. (7.22.degree. C.) 
Vapor temperature in flash tank # 1 
239.degree. F. (115.degree. C.) 
Liquor temperature in flash tank # 1 
252.degree. F. (122.22.degree. C.) 
##STR13## 
##STR14## 
= 0.516 MM lbs/day (2.34 .times. 10.sup.5 kg/day) 
Solids in feed to flash tank # 1 
45.4% 
Boiling point rise in Effect # 1 
11.degree. F. (6.11.degree. C.) 
Vapor temperature in Effect # 1 
239.degree. F. (115.degree. C.) 
Temperature in Effect # 1 
250.degree. F. (121.11.degree. C.) 
Steam temperature (35 psig) 
281.degree. F. (138.33.degree. C.) 
Steam demand 2310 MM BTU/day 
(5.82 .times. 10.sup.8 kg. cal./day) 
##STR15## 
15,500 sq ft (1440 sq meters) 
______________________________________ 
Heat transfer surface of other evaporation effects not changed. 
EXAMPLE III 
The use of this invention allows increased evaporator capacity up to a 
practical limit which will differ for individual mills. If pulp mill 
production is limited by the evaporator capacity, as is frequently true, 
this invention permits increased pulp production to be achieved. The 
theoretical upper limit to pulp production resulting from this invention 
is shown below for the mill of Example I. 
1000 t/d mill without BLOX 
All operating conditions identical to those in Examples I and II. 
Theoretical Maximum Pulp Mill Capacity using Process of FIG. 4 
______________________________________ 
Heat transfer capability of Effect # 1 
2800 MM BTU/day 
(7.06 .times. 10.sup.8 kg. cal/day) 
Heat transfer actually used at 100 t/d 
2310 MM BTU/day 
(5.82 .times. 10.sup.8 kg. cal/day) 
##STR16## t pulp/day (1100 metric tons) 
O.sub.2 required at 1000 t/d 
103 M lbs/day 
(46,721 Kg/day) 
O.sub.2 required at 1212 t/d 
125 M lbs/day 
(56,700 kg/day) 
______________________________________ 
Practical limitations such as entrainment, heat transfer capacity of 
effects #2, 3, 4 etc, and vapor handling capacity of the evaporators will 
likely limit the pulp mill capacity to somewhat less than 1212 t/d (1100 
metric tons/day). 
Theoretical Maximum Pulp Mill Capacity Using Process of FIG. 5 
______________________________________ 
Heat transfer capability of effect #1 resulting from 
##STR17## 
(7.53 .times. 10.sup.8 Kg.cal./day) 
Heat transfer actually used at 1000 t/d = 2310 MM BTU/day 
(5.82 .times. 10.sup.8 kg. cal./day) 
##STR18## = 1295 t pulp/day (1175 metric tons/day) 
O.sub.2 required at 1000 t/d 
= 103 M lbs/day 
(46,721 kg/day) 
O.sub.2 required at 1295 t/d 
= 133 M lbs/day 
(60,329 kg/day) 
______________________________________ 
As before, practical limitations will limit the pulp mill capacity to less 
than 1295 t/day (1175 metric tons/day). 
While the preferred locations of the BLOX reactor integrated into the 
evaporator system are as hereinbefore specifically described and 
illustrated in FIGS. 4 to 6, it will be apparent that certain other 
possible locations are within the scope of the invention and would provide 
a higher energy efficiency than that of previous known systems. The 
following additional locations are suggested: 
1. Location of the BLOX reactor between evaporator effects #1 and #2 and 
employing a single flash tank, so that the vapors discharged from such 
tank are added to the vapors leaving effect #1. 
2. Location of the BLOX reactor between effects #2 and #3, or between #3 
and #4, etc., providing that the heat of reaction is used to supplement 
the vapors leaving an evaporation effect operated at higher pressure than 
the feed effect. 
3. Conducting BLOX in two or more reactors which are located in any 
combination of the above locations and in some instances even applied to 
the yet weak black liquor. This arrangement could have the advantage of 
removing a bottleneck in an existing mill if a particular evaporator 
effect exhibits poor heat transfer. Moreover, an arrangement using two 
BLOX stages, with the first stage between effects #2 and #1 and the second 
stage between effect #1 and the flash tanks, would attain energy recovery 
equal to that obtained in the designated preferred embodiments of the 
invention. 
By the practice of the present invention the following advantages are 
achieved: 
1. High recovery--as up to about 100%--of the energy which is otherwise 
wasted in prior known BLOX systems. 
2. Increase in potential pulp production as a result of increased 
evaporator capacity under appropriate circumstances. 
3. Reduction in reversion incident to the high temperatures employed in the 
BLOX reaction; see papers by Bart et al and Christie, cited above. 
4. Reduction in heatng surface requirement in the first evaporation effect. 
While the invention has been particularly described with respect to its 
application to pulp mill recovery systems, the principles thereof could be 
applicable to any chemical process having an exothermic reaction and in 
which evaporation is employed as part of the processing sequence.