U.S. patent application number 10/686527 was filed with the patent office on 2004-07-15 for systems and methods for generating polysulfides.
Invention is credited to Duarte, Daniel, Epiney, Michel, Sundaram, V.S. Meenakshi.
Application Number | 20040134794 10/686527 |
Document ID | / |
Family ID | 32179783 |
Filed Date | 2004-07-15 |
United States Patent
Application |
20040134794 |
Kind Code |
A1 |
Sundaram, V.S. Meenakshi ;
et al. |
July 15, 2004 |
Systems and methods for generating polysulfides
Abstract
A system and method for generating polysulfide includes at least
one vessel for containing the pulping liquor, and at least one
oxidation promoter element including an oxidation promoter adhered
by a coating material to a substrate. The oxidation promoter
element may be affixed within the vessel or, alternatively, movable
between two or more vessels to facilitate polysulfide generation
and/or recovery of the oxidation promoter in at least two
vessels.
Inventors: |
Sundaram, V.S. Meenakshi;
(Burr Ridge, IL) ; Duarte, Daniel; (Clarendon
Hills, IL) ; Epiney, Michel; (Duvernay (Laval),
CA) |
Correspondence
Address: |
Linda K. Russell, Patent Counsel
Air Liquide
Suite 1800
2700 Post Oak Blvd.
Houston
TX
77056
US
|
Family ID: |
32179783 |
Appl. No.: |
10/686527 |
Filed: |
October 16, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60420144 |
Oct 22, 2002 |
|
|
|
Current U.S.
Class: |
205/688 ;
204/242 |
Current CPC
Class: |
D21C 11/0057 20130101;
C01B 17/34 20130101; B01J 37/0219 20130101; B01J 37/0225 20130101;
B01J 23/34 20130101 |
Class at
Publication: |
205/688 ;
204/242 |
International
Class: |
C25D 017/00; C25C
007/00 |
Claims
What is claimed:
1. A process for generating polysulfide in a pulping liquor,
comprising the steps of: a. providing a pulping liquor containing
sodium sulfide; b. providing at least one oxidation promoter
element comprising an oxidation promoter adhered by a coating
material to a substrate; c. contacting the at least one oxidation
promoter element with the pulping liquor so as to generate
polysulfide in the pulping liquor.
2. The process of claim 1, wherein the pulping liquor comprises
white liquor, green liquor, black liquor, or orange liquor.
3. The process of claim 1, wherein at least some of the sodium
sulfide is oxidized to generate polysulfide in the pulping liquor
by the oxidizing promoter.
4. The process of claim 3, wherein a majority of the sodium sulfide
is oxidized by the oxidizing promoter.
5. The process of claim 1, wherein the substrate is selected from
the group consisting of metals, organic materials, polymers,
ceramic, brick, thermosetting plastic, and combinations
thereof.
6. The process of claim 1, wherein the coating material is selected
from the group consisting of organic polymers, inorganic polymers,
ceramic, brick, thermosetting plastic, resin types, and
combinations thereof.
7. The process of claim 1, wherein the oxidizing promoter is
selected from the group consisting of transition metals, transition
metal oxides, and adsorbents and combinations thereof.
8. The process of claim 1, wherein at least a portion of the
oxidation promoter is at least partially embedded in the coating
material.
9. The process of claim 1, wherein at least some of the oxidation
promoter is fully encapsulated by the coating material.
10. The process of claim 9, wherein preparation of the at least one
oxidation promoter element comprises the steps of: a. providing a
matrix of the oxidizing promoter in the coating material; b.
applying the matrix to the substrate; and c. hardening the coating
material.
11. The process of claim 1, wherein the at least one oxidation
promoter element is a plurality of oxidation promoter elements
contacting the pulping liquor.
12. The process of claim 11, wherein any one of the plurality of
oxidation promoter elements includes a major dimension that is less
than about 40 cm.
13. The process of claim 1, wherein the pulping liquor is contained
within at least one vessel and the at least one oxidation promoter
element is positioned within the at least one vessel.
14. The process of claim 13, further comprising the step of fixing
the at least one oxidizing promoter element to the vessel.
15. The process of claim 13, further comprising the step of
positioning the at least one oxidizing promoter element at a bottom
of the at least one vessel.
16. The process of claim 13, further comprising the step of
positioning the at least one oxidizing promoter element in the at
least one vessel such that the at least one oxidizing promoter
element is mobile in the at least one vessel.
17. The process of claim 13, wherein the substrate is configured as
a plurality of strips circumferentially disposed about and attached
to a ring, and each of the plurality of strips is at least
partially coated with the coating material and the oxidizing
promoter at least partially embedded in the coating.
18. The process of claim 17, further comprising the step of
rotating the substrate within the vessel.
19. The process of claim 1, wherein an oxidative state of the
oxidation promoter is increased by contacting at least one
oxidizing agent with the oxidation promoter.
20. The process of claim 19, wherein the at least one oxidizing
agent includes at least one of sodium hydroxide and an
oxygen-containing gas.
21. The process of claim 19, wherein the at least one oxidizing
agent includes oxygen-enriched air.
22. The process of claim 19, wherein the at least one oxidizing
agent and the pulping liquor simultaneously contact the at least
one oxidation promoter element.
23. The process of claim 22, wherein at least some of the sodium
sulfide is oxidized to create polysulfide by the at least one
oxidizing agent.
24. The process of claim 19, wherein the at least one oxidizing
agent is contacted with the at least one oxidation promoter element
when the at least one oxidation promoter element is in minimal or
no contact with the pulping liquor, and the pulping liquor is
contacted with the at least one oxidation promoter element when the
at least one oxidation promoter element is in minimal or no contact
with the at least one oxidizing agent.
25. The process of claim 19, wherein the at least one vessel
comprises a first vessel including a first oxidation promoter
element and a second vessel including a second oxidation promoter
element, and the method further comprises: d. providing pulping
liquor to the first vessel to facilitate polysulfide production
within the first vessel when the second vessel is provided with the
at least one oxidizing agent to increase the oxidized state of the
second oxidation promoter element; and e. providing pulping liquor
to the second vessel to facilitate polysulfide production within
the second vessel when the first vessel is provided with the at
least one oxidizing agent to increase the oxidized state of the
first oxidation promoter element.
26. The process of claim 1, wherein a temperature at which the
polysulfide is generated ranges from about 50.degree. C. to about
120.degree. C.
27. The process of claim 1, wherein a temperature at which the
polysulfide is generated ranges from about 75.degree. C. to about
85.degree. C.
28. A system for generating polysulfide from pulping liquor
containing sodium sulfide, comprising: at least one vessel for
containing the pulping liquor, comprising an inlet to facilitate
the flow of pulping liquor into the at least one vessel, and an
outlet to facilitate the flow of pulping liquor from the at least
one vessel; and at least one oxidation promoter element comprising
an oxidation promoter at least partially embedded in a coating
material that at least partially coats a substrate, the oxidation
promoter promoting the oxidation of the sodium sulfide to generate
polysulfide; wherein the at least one oxidation promoter element is
positioned within the at least one vessel to contact the pulping
liquor.
29. The system of claim 28, further comprising at least one
oxidizing agent and a conduit for receiving the at least one
oxidizing agent, one end of the conduit fluidly communicating with
the vessel.
30. The system of claim 28, further comprising a mixing element
positioned within the at least one vessel for mixing the pulping
liquor.
31. The system of claim 30, further comprising a hollow shaft
disposed at least partially within the at least one vessel, the
mixing element being disposed at an end of the hollow shaft.
32. The system of claim 31, further comprising at least one
oxidizing agent and a conduit for receiving at least one oxidizing
agent, one end of the conduit fluidly communicating with the
vessel, and the hollow shaft having a first aperture adjacent an
upper end of the shaft and a second aperture adjacent a lower end
of the shaft so as to allow the at least one oxidizing agent to
flow through the first aperture and exit through the second
aperture.
33. The system of claim 28, wherein the at least one vessel
comprises first and second vessels, the at least one oxidation
promoter element comprises a first oxidation promoter element
positioned within the first vessel and a second oxidation promoter
element positioned within the second vessel, and the system is
configured to provide pulping liquor to the first vessel when the
second vessel contains the at least one oxidizing agent and provide
pulping liquor to the second vessel when the first vessel contains
or receives the pulping liquor.
34. The system of claim 28, wherein the at least one oxidation
promoter element comprises a plurality of oxidation promoter
elements having a major dimension of less than about 40 cm.
35. The system of claim 28, wherein the at least one oxidation
promoter element includes a plurality of substrates disposed about
and attached to a ring, and each of the plurality of strips is at
least partially coated with the coating material and the oxidizing
promoter at least partially embedded in the coating.
36. The system of claim 35, wherein the ring is further attached to
a shaft, and the shaft is in communication with a drive element
that rotates the shaft and the plurality of strips in the at least
one vessel.
37. A system for generating polysulfide from pulping liquor
containing sodium sulfide, comprising: a polysulfide generation
zone that receives pulping liquor including sodium sulfide to
facilitate the generation of polysulfide; a recovery zone that
receives at least one oxidizing agent; and at least one oxidation
promoter element that is movable between the polysulfide generation
zone and the recovery zone.
38. The system of claim 37, wherein the at least one oxidizing
agent includes at least one of sodium hydroxide and an
oxygen-containing gas to increase the oxidative state of the at
least one oxidation promoter upon contacting the at least one
oxidizing agent with the at least one oxidation promoter.
39. The system of claim 37, wherein the at least one oxidation
promoter element comprises a substrate with a coating material
disposed thereon, wherein the coating material includes an
oxidation promoter that promotes oxidation of the sodium sulfide in
the pulping liquor to generate polysulfide.
40. The system of claim 37, wherein the substrate is rotatably
secured to a support member to facilitate movement of the substrate
between the polysulfide generation zone and the recovery zone.
41. The system of claim 39, wherein the polysulfide generation zone
includes at least one vessel containing an inlet to facilitate the
flow of pulping liquor into the polysulfide generation zone and an
outlet to facilitate the flow of pulping liquor out of the
polysulfide generation zone, and the recovery zone includes at
least one vessel that receives the at least one oxidizing
agent.
42. The system of claim 39, wherein the polysulfide generation zone
comprises two vessels that each receive a pulping liquor containing
sodium sulfide, the recovery zone comprises a vessel that is
disposed between the two vessels of the polysulfide generation zone
and is configured to receive the at least one oxidizing agent.
43. The system of claim 42, wherein the at least one oxidation
promoter element includes a first oxidation promoter element that
is independently movable between one of the vessels of the
polysulfide generation zone and the vessel of the recovery zone and
a second oxidation promoter element that is independently movable
between the other of the vessels of the polysulfide generation zone
and the vessel of the recovery zone.
44. The system of claim 39, wherein the polysulfide generation zone
is adjacent the recovery zone and the substrate of the at least one
oxidation promoter element comprises at least one disc that is
disposed between and extends into both the polysulfide generation
and recovery zones, the at least one disc being rotatable with
respect to the polysulfide generation and recovery zones to
selectively situate the at least one oxidation promoter element
within each of polysulfide generation zone and the recovery
zone.
45. The system of claim 44, wherein the at least one disc comprises
a plurality of spatially separated rotatable discs.
46. The system of claim 37, further comprising: at least one baffle
disposed within at least one of the polysulfide generation zone and
the recovery zone to facilitate mixing of fluid flowing around the
at least one baffle.
47. A method of generating polysulfide from a pulping liquor
containing sodium sulfide, the method comprising: a. providing
pulping liquor to a polysulfide generation zone; b. providing at
least one oxidizing agent to a recovery zone; and c. selectively
moving at least one oxidation promoter element between the
polysulfide generation zone and the recovery zone; wherein
polysulfide is generated from sodium sulfide within the pulping
liquor when the at least one oxidation promoter element is situated
within the polysulfide generation zone, and the oxidative state of
the at least one oxidation promoter is increased when the at least
one oxidation promoter is situated within the recovery zone and in
contact with the at least one oxidizing agent.
48. The method of claim 47, wherein the at least one oxidizing
agent includes at least one of sodium hydroxide and an
oxygen-containing gas.
49. The method of claim 47, wherein the at least one oxidation
promoter element comprises a substrate with a coating material
disposed thereon, wherein the coating material includes an
oxidation promoter that promotes oxidation of the sodium sulfide in
the pulping liquor to generate polysulfide.
50. The method of claim 49, wherein the substrate is rotatably
secured to a support member to facilitate movement of the substrate
between the polysulfide generation zone and the recovery zone.
51. The method of claim 47, wherein the polysulfide generation zone
includes at least one vessel containing an inlet to receive pulping
liquor into the polysulfide generation zone and an outlet to remove
pulping liquor from the polysulfide generation zone, and the
recovery zone includes at least one vessel to receive the at least
one oxidizing agent.
52. The method of claim 47, wherein the polysulfide generation zone
comprises two vessels that each receive pulping liquor, the
recovery zone comprises a vessel that is disposed between the two
vessels of the polysulfide generation zone and is configured to
receive the at least one oxidizing agent.
53. The method of claim 52, wherein the at least one oxidation
promoter element includes a first oxidation promoter element that
is selectively moved between one of the vessels of the polysulfide
generation zone and the vessel of the recovery zone and a second
oxidation promoter element that is selectively moved between the
other of the vessels of the polysulfide generation zone and the
vessel of the recovery zone.
54. The method of claim 47, wherein the polysulfide generation zone
is adjacent the recovery zone and the substrate of the at least one
oxidation promoter element comprises at least one disc that is
disposed between and extends into both the polysulfide generation
and recovery zones, and the at least one disc is rotated with
respect to the polysulfide generation zone and the recovery zone to
selectively situate the at least one oxidation promoter element
within each of the polysulfide generation and recovery zones.
55. The method of claim 54, wherein the at least one disc comprises
a plurality of spatially separated discs that are rotated with
respect to the polysulfide generation zone and the recovery
zone.
56. The method of claim 47, further comprising: d. providing at
least one baffle within at least one of the polysulfide generation
zone and the recovery zone to facilitate mixing of fluid flowing
around the at least one baffle.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Patent Application No. 60/420,144, entitled "Surface Coated
Products Used in Polysulfide Generation and Methods for Their
Production" and filed Oct. 22, 2002. The disclosure of the
above-mentioned provisional application is incorporated by
reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] In the conventional kraft cooking process, two chemicals,
namely sodium hydroxide (NaOH) and sodium sulfide (Na.sub.2S), are
used to delignify wood chips. During the course of the reaction,
part of the undesired fraction of wood, lignin, is solubilized and
removed. However, cellulose and hemicelluloses, which are desirable
polysaccharide components of the wood, are also attacked. Hence,
one of the goals sought during cooking is to protect this fraction
in order to achieve a better process yield.
[0003] The weight contribution of these components varies with each
wood species, but it is usually around 70%. In an industrial
process, however, the amount retained is often around 45-50%.
Typically, 80% of the lignin, 50% of the hemicelluloses and 10% of
the cellulose are removed. The hemicelluloses are easily attacked
because they are low molecular weight sugars that are more
accessible than the crystalline cellulose. They are removed by what
is called "alkaline peeling," which occurs at the reducing end
group of the polymeric chain.
[0004] It is well known in the art that sodium polysulfide
(Na.sub.2S.sub.n wherein n.gtoreq.2), herein referred to as
"polysulfide," can be introduced in a digester to increase the
carbohydrate yield in the kraft cooking process. This prevents the
degradation of the polysaccharides and increases the lignin yield.
Haegglund first discussed this concept in 1946 (Svensk Papperstidn.
49(9): 191, 1946). Polysulfide can be generated by different means.
In one approach, polysulfide is formed by adding elemental sulfur
to white liquor. White liquor, as it is commonly known in the art,
is the liquid that comes in contact with the pulping materials,
generally wood chips, during the cooking process. Adding elemental
sulfur to the white liquor, however, may lead to sulfur imbalances
in the chemical recovery cycle, leading to sulfur build-up that
will eventually be released to the atmosphere as sulfur gas
emission. For this reason, this approach has very limited
industrial interest.
[0005] A second approach involves chemically oxidizing the sodium
sulfide present in the white liquor to polysulfide. The resulting
liquor containing polysulfide is known in the art as orange
liquor.
[0006] Several variations of this oxidation method producing
polysulfide have been published:
[0007] 1) In U.S. Pat. No. 3,470,061 Barker discloses a method
using inorganic manganese oxides as the oxidant. The oxidation
occurs in an external recycle loop after the catalyst has been
separated and dried. One of several drawbacks to this process,
however, is that the presence of sodium sulfide in the white liquor
prior to the clarifier can result in problems with the lime
mud.
[0008] 2) In U.S. Pat. No. 3,860,479, Barker et al. discloses a
method in which manganese dioxide is an oxidation catalyst
comprising a slurry or packing in a tower. The catalyst contacts an
oxygen source that helps it maintain its oxidative state. This
process shares many of the drawbacks of U.S. Pat. No.
3,470,061.
[0009] 3) In U.S. Pat. No. 4,024,229 Smith discloses a method to
produce polysulfide through chemical oxidation, using particulate
carbon coated with PTFE as the catalyst. The method is said to
reduce the production of sodium thiosulfate
(Na.sub.2S.sub.nO.sub.3), an undesirable compound. One drawback to
this process is that the catalyst bed has to be regenerated due to
particles of calcium carbonate deactivating the catalyst.
[0010] 4) In U.S. Pat. No. 4,855,123 Suzuki et al. discloses a
method similar to that of U.S. Pat. No. 4,024,229 but uses
activated carbon for the catalyst. This invention has the same
drawbacks as U.S. Pat. No. 4,024,229.
[0011] 5) In U.S. Pat. No. 5,624,545 Landfors et al. discloses a
method to produce polysulfide through electrolysis of the white
liquor. This method involves high capital and energy costs.
[0012] 6) In U.S. Pat. No. 5,082,526 Dorris discloses a method to
produce polysulfide in the presence of lime mud. One disadvantage
of this method is that it requires a long oxidation time, leading
to a lower selectivity because of over-oxidation and thermal
degradation of polysulfide. Another disadvantage is that the white
liquor must be sent with its lime mud contents to the oxidation
process, which increases oxygen usage and equipment cost.
[0013] As described, there are several processes for producing
polysulfide from white liquor to increase the cellulose and
hemicelluloses yield in kraft cooking. However, these processes are
often either complicated or not cost-effective.
OBJECTS AND SUMMARY OF THE INVENTION
[0014] Therefore, in light of the above, and for other reasons that
become apparent when the invention is described, an object of the
present invention is to provide a simple, efficient, long-lasting,
and cost-effective system and process to oxidize sodium sulfide to
become sodium polysulfide in kraft cooking liquors without the
drawbacks of prior art methods.
[0015] Another object of the present invention is to provide a
system and process that increases the production of polysulfide and
minimizes the amount of sodium thiosulphate produced.
[0016] A further object of the present invention is to provide a
system and process that is flexible in its application, because it
will not be limited by the presence of lime mud in the white liquor
and can be used prior to and/or after clarification.
[0017] The aforesaid objects are achieved individually and in
combination, and it is not intended that the present invention be
construed as requiring two or greater of the objects to be combined
unless expressly required by the claims attached hereto.
[0018] In accordance with one embodiment of the present invention,
a process for generating polysulfide in a pulping liquor includes
the steps of: providing a pulping liquor containing sodium sulfide,
providing at least one oxidation promoter element including an
oxidation promoter adhered by a coating material to a substrate,
and contacting the oxidation promoter element with the pulping
liquor so as to generate polysulfide in the pulping liquor.
[0019] In accordance with another embodiment of the present
invention, a system includes at least one vessel for containing the
pulping liquor, including an inlet to facilitate the flow of
pulping liquor into the vessel, and an outlet to facilitate the
flow of pulping liquor from the vessel. The system further includes
at least one oxidation promoter element including an oxidation
promoter at least partially embedded in a coating material that at
least partially coats a substrate, where the oxidation promoter
promotes the oxidation of the sodium sulfide to generate
polysulfide. The oxidation promoter element is positioned within
the vessel to contact the pulping liquor.
[0020] One or more oxidation promoter elements may be disposed in
one or more vessels, with the oxidation promoter elements further
being affixed or movable (e.g., rotatable) within the vessels. The
vessels may further be configured to receive at least one oxidizing
agent to increase the oxidative state of the oxidation promoter
disposed in each vessel.
[0021] In accordance with a further embodiment of the present
invention, a system and corresponding methods for generating
polysulfide in accordance with the present invention include a
polysulfide generation zone that receives pulping liquor including
sodium sulfide to facilitate the generation of polysulfide, a
recovery zone that receives at least one oxidizing agent, and at
least one oxidation promoter element that is movable between the
polysulfide generation zone and the recovery zone. The oxidation
promoter element preferably includes a substrate with a coating
material disposed thereon, where the coating material includes an
oxidation promoter that promotes oxidation of the sodium sulfide in
the pulping liquor to generate polysulfide. Polysulfide is
generated from sodium sulfide within the pulping liquor when the
oxidation promoter element is situated within the polysulfide
generation zone, and the oxidative state of the oxidation promoter
is increased when the oxidation promoter is situated within the
recovery zone and in contact with the at least one oxidizing
agent
[0022] The polysulfide generation and recovery zones can each
include one or more vessels, and the oxidation promoter element can
include a plurality of elements, where each oxidation promoter
element is movable between a vessel for the polysulfide generation
zone and a corresponding vessel for the recovery zone.
[0023] The above and still further objects, features and advantages
of the present invention will become apparent upon consideration of
the following detailed description of specific embodiments thereof,
particularly when taken in conjunction with the accompanying
drawings wherein like reference numerals in the various figures are
utilized to designate like components.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a schematic of the polysulfide formation process
using the oxidation promotion element, according to the
invention.
[0025] FIG. 2 is a side view in elevation of the oxidation promoter
element in a vessel according to the invention.
[0026] FIG. 3 is a cross-sectional view of a first embodiment of
the oxidation promoter element.
[0027] FIG. 4 is a cross-sectional view of a second embodiment of
the oxidation promoter element according to the invention.
[0028] FIG. 5 is a perspective view of a third embodiment of the
oxidation promoter element according to the invention.
[0029] FIG. 6 is a cross-sectional view of FIG. 5.
[0030] FIG. 7 is a view in perspective of an embodiment FIG. 5 in a
vessel.
[0031] FIG. 8 is a view in perspective of another embodiment of
FIG. 5 in a vessel.
[0032] FIG. 9 is a side view in elevation of an embodiment of the
substrate.
[0033] FIG. 10 is a side view in elevation of an embodiment of the
oxidation promoter element.
[0034] FIG. 11 is a perspective view of a sleeve configuration
incorporating a number of elements of FIG. 10.
[0035] FIG. 12 is a side view in elevation of the oxidation
promoter element of FIG. 9 fixed to a vessel according to the
invention.
[0036] FIG. 13 is a side view in elevation of the oxidation
promoter element fixed to a vessel.
[0037] FIG. 14 is a side view in elevation of the oxidation
promoter element in a washer configuration.
[0038] FIG. 15 is a side view in elevation of a system including a
vessel with inlet and outlet ports and the oxidation promoter
element.
[0039] FIG. 16 is a side view in elevation and partial section of a
system including a vessel containing an oxidation promoter element
in the form of a sleeve, a hollow shaft, and an impeller.
[0040] FIG. 17 is a side view in elevation and partial section of
the system of FIG. 16, further including an oxygen-containing gas
supply conduit.
[0041] FIG. 18 is a side view in elevation and partial section of
the system of claim 17, further including a caustic agent (sodium
hydroxide) supply.
[0042] FIG. 19 is a side view in elevation and partial section of a
dual vessel configuration of vessels with inlet and outlet ports, a
Hollow Shaft, an impeller, and the oxidation promoter element.
[0043] FIG. 20A is a cross-sectional view of a dual-vessel system
including a recovery zone and a polysulfide generation zone.
[0044] FIG. 20B is a top plan view of the system of FIG. 20A.
[0045] FIG. 21 is a cross-sectional view of a system including a
plurality of polysulfide generation zones separated by a recovery
zone.
[0046] FIG. 22 is a top plan view of the system of FIG. 21.
[0047] FIG. 23 is a partially exploded view in perspective of a
system including a polysulfide generation zone and a recovery zone
and rotatable discs.
[0048] FIG. 24 is a cross-sectional view of the system of FIG.
23.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0049] As seen in FIG. 1, the method of the invention includes an
oxidation promoter element 1 contacting sodium sulfide 2 contained
within a pulping liquor, which is oxidized to create polysulfide 3.
The oxidation promoter element 1 includes a substrate, a coating
material, and an oxidation promoter. The oxidation promoter element
1 may oxidize the sodium sulfide 2 itself, or it may catalyze
oxidation of the sodium sulfide 2 by an oxidizing agent 4. If an
oxidizing agent serves as the oxidation promoter element 1, its
oxidation state may decrease during the sodium sulfide reaction,
but the oxidation state is recovered by oxidizing the oxidation
promoter element 1 itself back to the beginning or initial
oxidation state.
[0050] As shown in FIG. 2, the system of the invention includes the
oxidation promoter element of FIG. 1 contained within a vessel 5
containing pulping liquor 6 where it is oxidized to generate
polysulfide. Vessel 5 includes an inlet 7 and an outlet 8, through
which the pulping liquor flows into and out of the vessel 5, at
which time it includes polysulfide generated by oxidation of sodium
sulfide. The vessel is a non-limiting configuration that may
further include a process line, a tank, or a column.
[0051] While practice of the invention may be for oxidizing sodium
sulfide in various solutions, the solution discussed herein is
pulping liquor. Preferably, the pulping liquor to be oxidized is
what is commonly known in the art as "white liquor," although other
pulping liquors may be used, such as "green liquor" or "black
liquor". After oxidation of the sodium sulfide is achieved, the
liquor contains polysulfide, and is commonly known in the art as
"orange liquor." The phrases "white liquor", "green liquor", "black
liquor", and "orange liquor" are well known in the kraft pulping
art as defined, for example, by Grace, T. M. and Malcolm, E. W.,
"Pulp and Paper Manufacture", Volume 5, 3rd Edition, 1989, and
Dorris, G. M, "Oxidation of White Liquor in the Presence of Lime
Mud", Pulp and Paper Canada, 95:10 (1994), the disclosures of which
are incorporated herein by reference in their entireties. The white
liquor is generally clarified in a clarifying tank prior to
oxidation; however, unclarified liquor may also be used. One
advantage to the invention is that it may perform to generate
polysulfide from sodium sulfide in unclarified liquor. Normally the
presence of lime mud in the unclarified liquor would interfere with
sodium sulfide contacting an oxidizing agent, but use of the
invention allows enough contact points between the oxidation
promoter element and the sodium sulfide for sodium sulfide
oxidation to occur and generate polysulfide.
[0052] Other advantages to the invention include how it limits
possible negative outcomes to the sodium sulfide oxidation process.
If sodium sulfide and a newly formed polysulfide remain in contact
with an oxidizing agent for an extended period of time, the
selectivity of polysulfide formed may decrease. Selectivity is
defined by ((polysulfide formed)/(converted sulfide))/100. This
differs from yield, which is defined by ((polysulfide
formed)/(initial sulfide))/100. High selectivity indicates that a
large content of the sodium sulfide was converted to polysulfide,
while high yield only indicates that a large content of sodium
sulfide underwent conversion but did not necessarily form
polysulfide. Low selectivity may result if sodium sulfide is
exposed to an oxidizing agent for too long, causing the sodium
sulfide to be converted into undesirable products that contribute
to the process deadload. One of these undesirable products is
sodium thiosulfate (Na.sub.2S.sub.nO.sub.3 where n.gtoreq.2),
herein referred to as "thiosulfate," which not only leads to
process deadload but also is corrosive.
[0053] The present invention helps to minimize the possible
occurrence of these outcomes. The white liquor contacts the
oxidation promoter element for a length of time sufficient to allow
the sodium sulfide to form polysulfide. The oxidation promoter
element causes polysulfide to form relatively quickly. As a result,
it limits the number of side reactions that could occur with the
sodium sulfide, such as thiosulfate formation. The contact time
between the oxidation promoter element and the sodium sulfide is
short enough to prevent a substantial amount of thiosulfate from
forming, and selectivity is increased. Furthermore, the invention's
efficiency in oxidizing sodium sulfide leads to forming a high
ratio of active to inactive polysulfide.
[0054] As best shown in FIG. 3, the oxidation promoter element 1 is
made of a substrate 9, a coating material 10, and an oxidation
promoter 11. The substrate 9 serves as a solid support for the
oxidation promoter element 1 and may be comprised of any one or
more of a variety of materials, including metals, thermosetting
plastics, nylons, clays, bricks, ceramics, and the like.
Preferably, the substrate is stainless steel.
[0055] The coating material 10 adheres the oxidation promoter 11 to
the substrate 9 and at least partially covers the substrate 9. The
coating material 10 may be comprised of any one or more of a
variety of materials, including organic polymers, inorganic
polymers, metals, clays, bricks, ceramics, thermosetting plastics,
resin types, and the like. Preferably, coating material 10 is an
organic polymer. A specific example of an epoxy resin for use as a
coating material is a formulation sold under the trade name
Impreglon.TM. (e.g., Impreglon.TM. 817), available from Impreglon,
Inc., Houston Tex.
[0056] The oxidation promoter 11 promotes the oxidation of sodium
sulfide in the pulping liquor. The oxidation promoter 11 is at
least partially embedded in the coating material 10 such that a
portion of the oxidation promoter 11 is available to contact the
sodium sulfide, while another portion interfaces with the coating
material 10. The oxidation promoter 11 may be comprised of
transition metal oxides, transition metals, or adsorbents. As a
transition metal oxide, the oxidation promoter 11 serves as an
oxidant to oxidize sodium sulfide. A preferred transition metal
oxide is manganese dioxide. In addition, transition metals or
adsorbents catalyze the oxidation of sodium sulfide by an oxidant.
Suitable transition metals include platinum, nickel, cobalt, and
the like. Suitable adsorbents include carbon, silica, aluminum,
zeolytes, and the like. Whether the sodium sulfide is oxidized
directly by oxidation promoter element 1, or the oxidation promoter
element catalyzes this oxidation, both serve to promote oxidation
of the sodium sulfide.
[0057] To at least partially embed the oxidation promoter 11 within
the coating material 10, the coating material 10 and the oxidation
promoter 11 may be subjected to different treatments. For example,
the coating material 10 can be formed via a spray, followed by
pressing the oxidation promoter 11 on the coating material 10. The
coating material 10 and oxidation promoter 11 are then baked
together. Alternatively, the coating material 10 may not be baked
and the oxidation promoter 11 may be pressed into the coating
material 11. The coating material 10 may also be made of ceramic
pre-forms with the oxidation promoter 11 blasted on top of the
coating material 10, followed by backing the pre-forms.
[0058] As seen in FIG. 4, the oxidation promoter 11 and coating
material 10 may be configured as a matrix composition. To
illustrate, the oxidation promoter 11 may be embedded in a matrix
comprised of the coating material 10 that coats the substrate 9. A
portion of at least some of the oxidation promoter 11 is left
uncovered by the coating material 10 so that it may contact the
sodium sulfide. In order to prepare the matrix such that at least
some of the surface of the oxidation promoter 11 is exposed, the
outer surface of the matrix may be mechanically abraded,
sandblasted, or selectively dissolved to uncover the oxidation
promoter 11.
[0059] Aside from the previously discussed relationship between the
substrate 9, the coating material 10, and the oxidation promoter
11, the oxidation promoter element 1 may be structured in various
configurations that apply to open or closed systems. For example,
the oxidation promoter element 1 may comprise a screen placed
perpendicularly to the flow of sodium sulfide in a process line
such that the sodium sulfide flows through the screen.
Additionally, as shown in FIG. 5, the oxidation promoter element
may possess what is herein called a "bead" configuration 12.
Illustrated in the embodiment of FIG. 6, the substrate 9 is formed
into a spherical shape, forming a bead. Bead 12 is then covered
with the coating material 10 having the oxidation promoter 11
partially embedded therein. The bead 12 may be any shape or size
that allows the sodium sulfide to form polysulfide. For example,
the bead may be shaped like a sphere, an ellipsoid, a flattened
disc, a fiber, or may possess an irregular shape. Preferably, the
bead has a major dimension of less than about 40 cm. More
preferably, the major dimension of the bead is less than about 20
cm. Most preferably, the major dimension of the bead is less than
about 2 cm.
[0060] As shown in FIG. 7, a number of beads 12 could be adhered to
a solid support 14 (here, a series of rods) that is fixed with
vessel supports (a rod 22 and a base 23) to the bottom of a vessel
21. Also, as in FIG. 8, the beads 12 may float freely in vessel 21
so that the liquid containing sodium sulfide 18 may flow around the
suspended beads. The beads may also occupy a packed column, similar
to a configuration described in U.S. Pat. No. 3,860,479 to Barker
et al., which is incorporated herein by reference in its
entirety.
[0061] Another configuration of the oxidation promoter element is
herein called a "sleeve" configuration. It is prepared by first
shaping the substrate 9 into a rectangular coupon 19, as shown in
FIG. 9. As illustrated in FIG. 10, each coupon 19 is then covered
with the coating material 10 having the oxidation promoter 11
partially embedded therein. As shown in FIG. 11, a series of these
coupons 19 are placed in parallel and circumferentially disposed
about an axis of a ring 20, securing them to one another, and
forming a sleeve 24. The coupons 19 are spaced apart on the ring 20
so that the liquid containing sodium sulfide may flow between the
coupons 19. For example, the rings may be spaced about 0.25 cm
apart. The coupons, while rectangular, may possess other shapes,
and may be of any size. The ring center (and thus sleeve 24) is
open such that the sodium sulfide is allowed to flow therein and
thereout, maximizing the surface contact of the sodium sulfide with
oxidation promoter element (i.e. the sleeve).
[0062] The oxidation promoter element may be stationary or moving
within a vessel. To illustrate a stationary structure, as shown in
FIG. 12, the oxidation promoter element 1 is placed in a vessel 21
such that it rests in a stationary position on the bottom of the
vessel 21 in any suitable manner. As shown in FIG. 13, the element
(here, a sleeve) may be fixed to a vessel by attaching one end of a
rod 22 to the ring 20 of sleeve 24, while fixing the other end of
the rod 22 to the vessel 21 using a support base 23. One of
ordinary skill in the art will understand that many other suitable
structures for the supports are suitable for practice of the
invention.
[0063] An embodiment in which the oxidation promoter element moves
in relation to a vessel 21 is shown in FIG. 14. Sleeve 24 is placed
horizontally within a vessel 21. The sleeve 24 has a shaft 25 in
its center connected by several rods 26 to the shaft ring 20. The
shaft 25 extends beyond the vessel 21 through a vessel aperture 27
and is connected to a rotor 28. The rotor 28, driven by a motor 29,
turns the shaft 25, causing the sleeve 24 to rotate within the
vessel 21. This is herein referred to as the "washer"
configuration.
[0064] The oxidation promoter element may be used to produce
polysulfide in a batch process, a continuous process and/or a
combination thereof. A batch process may include a single vessel or
multiple vessels in which polysulfide is created in the white
liquor. FIG. 15 illustrates a batch process with a single vessel
21, in which white liquor enters the vessel through an inlet 30 and
contacts the oxidation promoter element. One skilled in the art
will recognize that the manufacturing of polysulfide and selection
of an amount of oxidation promoter to use per amount of sodium
sulfide to be converted to polysulfide is a function of well known
operating parameters, such as the reaction kinetics of converting
sodium sulfide to polysulfide and the amount of oxidation promoter
required per amount of sodium sulfide provided in the liquor.
[0065] In polysulfide mode, the white liquor enters the vessel
until a specific parameter to stop the white liquor flow is
fulfilled. These parameters may include vessel level (determined by
a level detector on the inner wall of the vessel) and/or time of
flow (determined by a timer in communication with a white liquor
flow control valve). After the required retention time for the
white liquor to remain in the vessel expires, the white liquor
containing polysulfide, now called orange liquor, exits the vessel
through an outlet 31. To illustrate a continuous process with a
single reactor, a vessel may be constructed with a similar
configuration as the batch process in FIG. 15, except that the
white liquor entry into and exit out of the vessel 21 is generally
ongoing. For both processes, the white liquor flow into and out of
the vessel 21 is controlled to allow sufficient time for the sodium
sulfide to form polysulfide without proceeding to thiosulfate
formation.
[0066] As sodium sulfide is oxidized to form polysulfide, it is
advantageous to create as many contact points as possible between
the sodium sulfide and the oxidation promoter element to make
polysulfide formation more efficient. One way to achieve this is to
agitate the sodium sulfide with a rotating device. As seen in FIG.
16, the sleeve 24 can be placed in a stationary position in a
vessel 21 by fixing the sleeve ring 20 to a rod 22 that, in turn,
is attached to the bottom of the vessel 21 using a support base 23.
A hollow shaft ("shaft") 32 is provided with a three-blade impeller
33 positioned at its end. Shaft 32 is a hollow, rod-shaped device
that allows material to enter through a shaft inlet 34, flow
through the shaft 32, and exit through a shaft outlet 35 at the
shaft's end. The hollow shaft 32 is more fully described in U.S.
Pat. No. 6,517,729, which is incorporated herein by reference in
its entirety. In the embodiment of FIG. 16, the shaft 32 and
impeller 33 are inserted through the central portion of the sleeve
24; however, they may be located at any point in the vessel 21.
When the white liquor enters the vessel 21 through an inlet 30, the
impeller 33 agitates the liquor such that it travels around the
vessel 21, increasing its contact with sleeve 24. Any material
introduced through shaft inlet 34 is also mixed into the white
liquor as the material exits through shaft outlet 35. Once reacted,
the solution exits the vessel via outlet 31. Mixing may also be
achieved with the washer configuration shown in FIG. 14 as the
sleeve rotates within the vessel.
[0067] In addition to mixing, the efficiency of forming polysulfide
in the white liquor is increased when the oxidation promoter is a
transition metal oxide. The transition metal oxide acts as an
oxidizing agent that oxidizes sodium sulfide to create polysulfide,
and it may do this in less than about 20 minutes (e.g., about 10-20
minutes). More specifically, with manganese dioxide as the
oxidation promoter, it only takes about 1 minute for 1 gram of
manganese dioxide per liter of white liquor to produce 0.77 grams
of polysulfide per liter of white liquor. Because the transition
metal oxide is reduced when it oxidizes the sodium sulfide, its
oxidative state needs to be restored so that more sodium sulfide
may be oxidized. This is achieved by exposing the transition metal
oxide to an oxidizing agent. Suitable oxidizing agents include
O.sub.2, O.sub.3, oxygen-enriched air, peroxides, and the like. One
preferred oxidizing agent is oxygen-enriched air. For example,
oxygen-enriched air may be obtained by premixing air with oxygen at
selected proportions to achieve a desired result, namely,
restoration of the transition metal oxide to a selected oxidative
state. The source of oxygen supply can be obtained, e.g., from
various locations within the pulp and/or paper mills. This obtained
oxygen supply can also be enriched with a suitable supply of fresh
oxygen, as desired.
[0068] Controlling the temperature of the reaction liquids can also
increase efficiency. Since the production of polysulfides is
dependent on temperature and the oxidation reaction of white liquor
is exothermic, it is important to control the temperature to
minimize oxidation of Na.sub.2S to sodium thiosulphate
(Na.sub.2S.sub.2O.sub.3). For this reason, the system vessel may be
equipped with heating jackets and cooling coils to maintain the
desired temperature. The temperature of the reaction liquids is
preferably kept in the range of from about 50.degree. C. to about
120.degree. C., and more preferably from about 75.degree. C. to
about 85.degree. C. The manufacturing of polysulfide, selection of
an amount of oxidation promoter and/or oxidizing agent to use per
amount of sodium sulfide to be converted to polysulfide is
described in detail in copending U.S. patent application Ser. No.
09/784,076, filed on Feb. 16, 2001, which is incorporated herein by
reference in its entirety.
[0069] There are several ways to expose the transition metal oxide
to an oxidizing agent. For example, as in FIG. 17, a system
includes a vessel 21 with a stationary sleeve 24 positioned
therein. A hollow shaft 32 with impeller 33 is further positioned
within the sleeve. When the system performs as a continuous process
to generate polysulfide, an oxygen-containing gas such as
oxygen-enriched air may be added to the vessel. While 70% oxygen
content is preferred in the oxygen-containing gas, oxygen contents
greater or less than 70% may be used. Pure oxygen may also be used,
but oxygen-enriched air is preferred in this configuration in order
to avoid over-oxidation of sodium sulfide, which may lead to
substantial thiosulfate production. A sodium sulfide containing
liquid (here, white liquor) 37 enters the vessel through inlet 30.
Once the vessel is filled, oxygen-containing gas from a conduit or
carrier line 80 enters the vessel 21 through an inlet 36 such that
it is mixed into the white liquor 37. The oxygen-containing gas
flows up through the liquor and occupies headspace 38 in the vessel
above the white liquor's surface. Alternatively, the
oxygen-containing gas may be introduced directly into the headspace
38. The oxygen-containing gas then enters the shaft inlet 34 and
flows through the shaft 32, exiting through the shaft outlet 35
near the impeller 33. When the oxygen-containing gas exits the
shaft 32, the rotating impeller 33 mixes the gas into liquor 37 and
distributes it around the sleeve. In this manner, a convection
current is created in which oxygen-containing gas is dispersed to
the middle and periphery of the vessel 21 and then the unreacted or
remaining gas rises up to the surface of liquor 37 into headspace
38. It then re-enters the hollow shaft 32 through the shaft inlet
34. The reacted liquor then exits through an outlet 31. This
distribution improves the availability of the oxygen-containing gas
to oxidize the sodium sulfide and also to help the transition metal
oxide recover its oxidative state. Thus, in the continuous process,
polysulfide and recovery modes occur simultaneously.
[0070] Recovery of the oxidation promoter's oxidative state may
also be performed in a batch process. The system embodiment of FIG.
18 includes a vessel 21 with a stationary sleeve 24 positioned
therein. In this batch process, the white liquor enters the vessel
through an inlet 30, and remains in the vessel for a time period
sufficient to create a desired amount of polysulfide. The reacted,
orange liquor is then discharged from the vessel through an outlet
31. This is referred to as the "polysulfide mode". After the orange
liquor exits the vessel 21, a caustic agent such as sodium
hydroxide is added to the vessel through an inlet 39. An
oxygen-containing gas enters vessel 21 from a conduit 80 through an
inlet 36, such that it is mixed into and flows up through the
sodium hydroxide to occupy the headspace 38 in the vessel above the
sodium hydroxide's surface. The oxygen gas then enters the shaft
inlet 34, flows through the shaft 32, and exits through shaft
outlet 35 near the impeller 33. Preferably, pure oxygen or very
highly oxygen-enriched air is used because the process is not in
polysulfide mode when the oxygen-containing gas is added, so there
is no concern about forming thiosulfate. The sodium hydroxide and
the oxygen-containing gas react with the transition metal oxide so
that it recovers its oxidative state. This is referred to as the
"recovery mode". Those skilled in the art will understand that when
adsorbents as the oxidation promoter, they need not be oxidized.
The retention time of the sodium hydroxide and oxygen gas in the
vessel should be sufficient for the oxidative state of the
transition metal oxide to be recovered, which may be as little as
about 3-5 minutes.
[0071] Following this recovery, the sodium hydroxide, plus other
remaining vessel contents, are discharged from the vessel 21
through an outlet 40. Although FIG. 18 shows different inlets 30,
39 for the white liquor and the sodium hydroxide flows,
respectively, both flows may enter the vessel through the same
inlet, so long as the two flows are not mixed. This can be
achieved, e.g., by using flow-control and check valves. Similarly,
both the polysulfide and sodium hydroxide outlet flows may exit the
vessel 21 through a same outlet. The discharge from the recovery
process may be sent to a sodium hydroxide holding tank 41. To
reduce costs, the contents of the sodium hydroxide holding tank 41
may be re-used several times for recovery mode. After the sodium
hydroxide and other contents have exited the vessel 21, white
liquor is once again added to the vessel through inlet 30 to repeat
the batch process in polysulfide mode.
[0072] Polysulfide generation and recovery of the oxidation
promoter may be carried out separately, yet simultaneously, by
utilizing at least two vessels. In this embodiment, while one
vessel is in polysulfide mode, the other vessel or vessels is/are
in recovery mode. Each of the vessels need not have identical
configurations or other parameters. When a particular vessel has
completed its respective mode, the remaining contents of that mode
are discharged, as described in the batch process above. This
discharge may or may not occur at the same time and is generally
dependent on the process design. After the contents in each vessel
have been at least partly discharged, each vessel is filled with
the necessary contents for the other mode, which then proceeds in
each vessel. Thus, if a vessel completes polysulfide mode, at least
part of, if not all of, the liquor is discharged and the vessel is
filled with sodium hydroxide and an oxygen-containing gas to carry
out recovery mode.
[0073] The embodiment of FIG. 19 illustrates a multi-vessel system.
In this system, vessel 1 21a and vessel 2 21b are part of a
dual-vessel configuration, in which each vessel contains a hollow
shaft 32a, 32b equipped with an impeller 33a, 33b. The shaft is
positioned with a sleeve 24a, 24b. Vessel 1 21a is filled with
white liquor through an inlet 30a and then the polysulfide mode is
performed. During this time, vessel 2 21b is filled with sodium
hydroxide through a second inlet 39b. Oxygen-containing gas enters
vessel 2 21b from a carrier line 80b through a gas inlet 36b, such
that it is mixed into and flows up through the sodium hydroxide to
occupy the headspace 38 in the vessel above the sodium hydroxide's
surface. The oxygen-containing gas then enters the shaft inlet 34b,
flows through the shaft 32b, and exits through shaft outlet 35b
near the impeller 33b. Thus, vessel 2 21b performs and completes
the recovery mode.
[0074] After each mode is completed, the polysulfide products are
discharged through the outlet 31a of vessel 1 and the recovery
products are discharged through outlet 40b of vessel 2. Then,
vessel 1 21a can be filled with sodium hydroxide through an inlet
39a, and oxygen-containing gas can enter from a carrier line 80a
through a gas inlet 36a so that recovery mode will proceed in
vessel 1 21a, similar to how recovery mode proceeded in vessel 2
21b. During this time, vessel 2 21b is filled with white liquor
through an inlet 30b so that polysulfide mode will proceed in
vessel 2 21b, similar to polysulfide mode in vessel 1 21a. To
reduce cost, the carrier lines 80a, 80b may be connected to a
single oxygen-containing gas source 44. In this system, like the
single-vessel continuous process, polysulfide mode is not
interrupted for the purpose of recovering the oxidative state of
the oxidation promoter. Like the single-vessel batch process,
however, the oxidation promoter is always in its most reactive
state when white liquor enters the vessel 21a or 21b to generate
polysulfide. As described above, instead of using separate inlet
and outlets for the white liquor and sodium hydroxide, a single
inlet and/or outlet may be used.
[0075] Another system configuration establishes a "recovery zone"
and a "polysulfide generation" zone, wherein the oxidation promoter
element is capable of moving between the two zones. The term
"polysulfide generation zone" refers to any vessel or portion of
the system that generates polysulfide from sodium sulfide in the
pulping liquor, whereas the term "recovery zone" refers to any
vessel or portion of the system that facilitates the recovery of
the oxidation promoter element to its initial oxidative state
(i.e., its oxidative state prior to promoting generation of
polysulfide).
[0076] One embodiment of the configuration is shown in FIGS. 20A
and 20B. In this embodiment, a tank 82 is partitioned into two
separate vessels. Vessel 1 21a (the polysulfide generation zone)
includes an inlet 30 and an outlet 31. Vessel 2 21b (the recovery
zone) includes a carrier line 80, an inlet 39 and an outlet 40. A
supporting member 48 is positioned above tank 82. Flag members 47a,
47b are attached to support member 48. Each flag includes an arm
46a, 46b connected to the support member at one end and to a coupon
19a, 19b at the other. The coupons are similar to those described
above. The size of the coupons is not particularly limited, and may
be of any shape. The arms, moreover, are capable of substantial
rotation (e.g., about 270.degree.) about support member 48 such
that the coupons can be rotated from vessel 1 21a to vessel 2 21b.
In use, the white liquor enters vessel 1 21a through inlet 30 and
sodium hydroxide enters vessel 2 21b through inlet 39.
Oxygen-containing gas from conduit 80 enters vessel 2 21b through a
gas inlet 36, such that it is mixed into and flows up through the
sodium hydroxide.
[0077] The flags are suitably positioned such that coupon 19a of
flag 47a is contained in vessel 1 21a while coupon 19b of flag 47b
is contained in vessel 2 21b. This enables the flag contained in
vessel 1 to be immersed in the white liquor, and the flag contained
in vessel 2 to be immersed in the sodium hydroxide solution. The
flags remain immersed for the desired time period (i.e., the time
necessary to produce polysulfide). Preferably, vessel 1 21a is
sparged with an inert gas, such as nitrogen, to decrease or limit
entry of oxygen into vessel 1 21a.
[0078] When the white liquor has reacted to the desired degree, it
(now in the form of orange liquor) may be removed from vessel 1 21a
via outlet 31. The sodium hydroxide, moreover, is discharged using
outlet 40. Alternatively, as described above, instead of using
separate inlet and outlets for the white liquor and sodium
hydroxide, a single inlet and/or outlet may be used. Moreover, to
reduce operating costs, the sodium hydroxide may be used for
several recovery processes before being discharged. Thus, recovery
vessel 2 21b can be a batch tank, while the liquor flows
continuously through vessel 1 21a. However, it is noted that each
vessel may be batch, semi-batch or continuous.
[0079] While draining, the position of the flags can be reversed.
That is, the flags can be repositioned such that coupon 19a of flag
47a is now contained in vessel 2 21b (immersed in the sodium
hydroxide) and coupon 19b of flag 47b is contained in vessel 1 21a
(immersed in the white liquor). White liquor again fills vessel 1
21a, and the process can be repeated. Thus, vessel 1 21a generates
polysulfide and vessel 2 21b performs and completes recovery of the
oxidation promoter. Though the current embodiment discloses two
flags, any number of flags may be utilized (e.g., three or more).
The flags may be of any shape or size, and may be positioned in any
desired configuration to enable their selective immersion in either
the recovery zone or the polysulfide generation zone. Additionally,
it is noted that, depending upon a particular processing scenario,
the flags may be rotated one or more times during a reaction
cycle.
[0080] FIGS. 21 & 22 disclose a configuration including a
plurality of polysulfide generation zones separated by a recovery
zone. Referring to FIG. 22, a tank 82 is partitioned into three
vessels. Specifically, a recovery zone vessel 92 is positioned
between two polysulfide zone vessels 90, 94. A first support member
54 is positioned directly over a partition wall separating the
first polysulfide zone vessel 90 and the recovery zone vessel 92.
The first support member is suitably positioned to permit the
rotation of flags into both the first polysulfide zone vessel 90
and the recovery zone vessel 92. Similarly, a second support member
56 is suitably positioned directly over the partition wall
separating the recovery zone vessel 92 and the second polysulfide
zone vessel 94 to permit the rotation of flags into both the
recovery zone vessel 92 and the second polysulfide zone vessel 94.
The polysulfide zone vessels 90, 94 include inlets 30a, 30b and
outlets 31a, 31b (though a single port may serve as both an inlet
and and outlet). The recovery zone vessel 92 includes a fluid inlet
39, a fluid outlet 40, and a gas inlet 36 through which
oxygen-containing gas delivered via conduit 80 enters.
[0081] The number and position of flags are not limited. In the
current embodiment, at least one flag is positioned within the
recovery zone vessel and at least one flag is positioned within one
or both the first and second polysulfide zone vessels. Referring
particularly to FIGS. 21 and 22, flags 47a-d are rotatably attached
to support member 54 in alternating positions. Specifically, flags
47a, 47c are positioned such that coupons 19a, 19c are contained in
the first polysulfide zone vessel 90 (to enable immersion in the
white liquor), and flags 47b, 47d are positioned such that coupons
19b, 19d are contained in recovery vessel (to enable immersion into
the sodium hydroxide). Similarly, flags 47e-h are rotatably
attached to support member 56 in alternating positions, with flags
47e, 47g positioned such that coupons 19e, 19g are contained in the
second polysulfide zone vessel 94 (to enable immersion into the
white liquor), and flags 47f, 47h are positioned such that coupons
19f, 19h are contained in the recovery zone vessel 92 (to enable
immersion in the sodium hydroxide).
[0082] In use, the polysulfide generation zone vessels 90, 94 are
filled with white liquor via inlets 30a, 30b. The recovery zone
vessel 92, moreover, is filled with a caustic agent such as sodium
hydroxide, and an oxygen-containing gas flows through the sodium
hydroxide, entering via gas inlet 36. As explained above, coupons
19a, 19c, 19e, 19g are positioned within the polysulfide generation
zone vessels 90, 94 and coupons 19b, 19d, 19f, 19h are positioned
within the recovery zone vessel. The coupons in the polysulfide
zone vessel contact the white liquor for a time suitable to
generate polysulfide products. The polysulfide products are then
discharged through an outlet 31a, 31b and, similarly, the recovery
products can be discharged through outlet 40, if desired. While
discharge takes place all of the flags are rotated such that
coupons 19a, 19c, 19e, 19g are positioned within the recovery zone
vessel 92, while coupons 19b, 19d, 19f, 19h are positioned within
the first and second polysulfide zone vessels 90, 94. The recovery
zone and polysulfide zone vessels are then refilled with their
respective fluids, oxygen-containing gas flows through gas inlet
36, and the recovery zone and polysulfide zone processes continue.
While in this embodiment, the vessels can be filled and discharged
simultaneously. Alternatively, the vessels may be filled or
discharged in any other manner. In addition, the flags may be
rotated at any one or more time periods between filling and
discharging of the recovery and polysulfide generation zone
vessels.
[0083] As discussed above, the efficiency of performing polysulfide
generation and recovery can be increased by creating as many
contact points as possible between the oxidation promoter element
and the reaction fluids, e.g., by agitating the fluids. Preferably,
this is accomplished in a continuous process whereby the white
liquor and the sodium hydroxide solution flow through vessel 1 21a
and vessel 2 21b. Consequently, one of more baffles may further be
used in any of the vessels to increase the efficiency of the
system. In the embodiment shown in FIGS. 21 and 22, the vessels 90,
92, 94 include baffles 50a-i at spaced locations disposed along the
length of the vessels. The baffles are suitably spaced to permit
flags 47a-h to rotate between two neighboring baffles. The baffles
typically do not extend the full height or width of the vessel. The
baffles, moreover, extend from the top, bottom and/or sides of the
vessel, preferably the entire width of the vessel such that fluids
flow over and/or under each baffle. However, it is noted that the
baffles may be designed in any suitable manner to permit the
reaction fluids to flow over, around, above, below and/or through
the baffles so as to become agitated and increase the interaction
between the oxidation promoter element and the reaction fluids.
[0084] The movement of fluid in one of the tank vessels is shown by
dashed line 100 in FIG. 21. In particular, three baffles 50a-c
extend from a bottom wall of vessel 90 at spaced locations such
that when flags 47a, 47c or flags 47b, 47d are rotated to permit
their respective coupons to be situated in vessel 90, the flags
extend between two neighboring baffles or a baffle and a side wall
of vessel 90. Thus, the combination of baffles and flags in vessel
90 creates a tortuous flow path for fluid flowing between inlet 30
and outlet 31. As previously noted, baffles may be placed at any
one or more suitable locations within each vessel to establish any
desired flow path for fluid flowing within and the vessels and
contacting the coupons disposed therein.
[0085] FIGS. 23 and 24 disclose a system configuration having a
recovery zone and a polysulfide-generating zone that includes a
rotary disc. Here, a tank 82 is provided with inlet 30 and outlet
31 ports. The oxidation promoter element, moreover, includes a disc
substrate. The size and/or geometry of the disc substrate is not
limited to any particular configuration. Preferably, a plurality of
discs is provided having a diameter of about four meters and a
thickness of about 0.6 centimeters. In addition, each disc
substrate in the plurality may comprise identical or differing
coating formulations placed thereon. In the embodiment illustrated
in FIG. 24, the oxidation promoter element includes a plurality of
disc substrates coated with a coating material having the oxidation
promoter partially embedded therein (as described above). The
oxidation promoter may be disposed at any selected locations within
the coating and along the disc substrates. For example, the
oxidation promoter may be disposed over substantially the entire
surface area of the disc substrates or, alternatively, over
selected portions of the disc substrates. The one or more oxidation
promoter discs 60a-f are connected to a rotary shaft 62. Shaft 62
is driven by a conventional motor (not shown), and is suitably
positioned to permit a portion of the disc to be contained within
the tank and a portion of the disc to be exposed outside the tank.
If more than one disc is present, the discs are spaced apart (e.g.,
0.5 inches apart) to enable air and liquid flow between them.
Situated above the discs and connected to the tank is a pressure
controlled ventilation (PCV) chamber 86. An oxygen-containing gas
is supplied to the chamber via conduit 80. Chamber 86 controls the
flow of oxygen-containing gas over the exposed disc portions.
[0086] In use, the white liquor enters tank through inlet 30,
flowing through the tank and between discs 60a-f. The discs are
positioned such that a portion of each disk is immersed in the
white liquor. The immersed portions contact the flowing white
liquor, generating polysulfide products. The reacted, orange liquor
continues to flow through tank 82, and can be discharged through
outlet 31. After the desired reaction time expires, the discs can
then be suitably rotated to expose the reduced portion of the disc
(i.e., the portion that was immersed in the white liquor) to the
oxygen-containing gas flow of the PCV chamber, as well as to
immerse the recovered portion (i.e., the portion originally exposed
to the oxygen containing gas) in the white liquor. Alternatively,
discs 60a-f can be continuously rotated at the same or differing
rates to facilitate the continuous performance of the polysulfide
generation and recovery zones. The system of FIGS. 23 and 24 is a
continuous process, with liquour continuously flowing through the
system and the oxidation promoter element being selectively
positioned in either or both of the recovery and polysulfide
generation zones during the process.
[0087] An exemplary embodiment utilizing the system of FIGS. 23 and
24 is described below, in which about 33 liters per second of white
liquor is continuously processed, utilizing MnO.sub.2 as the
oxidation promoter element, to form orange liquor containing about
5 grams per liter of polysulfide. Each disc in the system has a
diameter of 4 meters and is situated within the system such that
about half the surface area of the disc extends within the tank and
the other half extends within the ventilation chamber (i.e., about
half the surface area of each disc is exposed at any given time to
white liquor flowing through the system). Each disc has a thickness
of 0.635 centimeters, with a gap between neighboring discs also
being 0.635 centimeters. The thickness of the coating on each disc
substrate, including the oxidation promoter element, is 0.079
centimeters. The coating can be any of the previously described
coatings (e.g., a formulation sold under the tradename
Impreglon.TM. 817, available from Impreglon, Inc., Houston
Tex.).
[0088] To produce polysulfide at the rate of about 5 grams/liter
(measured as sulfur) in the orange liquor when processing white
liquor at a flow rate of about 33 liters per second, where a
retention time in which white liquor contacts the rotating discs is
about 1 second, requires a total surface area of the discs, and
thus the area of exposure to the oxidation promoter element, of
from about 1,250 square meters to about 2,500 square meters. The
total number of 4 meter diameter discs required to ensure a
sufficient disc surface area for exposure to the white liquor is
about 100 discs to about 199 discs. The discs are secured to a
shaft to facilitate continuous rotation of the discs at a rate of
about 10 revolutions per minute (RPM) to about 1000 RPM, preferably
about 10 RPM to about 500 RPM, and most preferably about 150 RPM to
about 500 RPM. A selected rotational rate of the discs ensures
effective conversion of sodium sulfide to polysulfide by the
oxidation promoter element portions of each disc that are submerged
in the tank while simultaneously regenerating oxidation promoter
element portions of each disc that are situated in the ventilation
chamber.
EXAMPLES
[0089] A single-vessel configuration was used for the following
examples in which polysulfide is generated in accordance with the
present invention. The vessel for each experiment was either a
2-liter glass vessel or a 2.5-liter stainless steel vessel. Process
conditions were controlled via a control panel.
[0090] The vessel was equipped with a heating jacket, a cooling
coil, and a PID temperature controller to maintain the desired
temperature. Temperature regulation is important because
polysulfide production is dependent on temperature, and white
liquor oxidation is an exothermic reaction. Lack of temperature
regulation may result in producing thiosulphate.
[0091] For the oxidizing promoter element, the substrate was
stainless steel and formed into coupons, approximately
1-inch.times.9-inch with 1/8-inch thickness. The coating material
was composed of polymeric formulations, including carbon, silicon,
and iron, available from Impreglon, Inc., Houston, Tex. This
coating material was sprayed on the substrate. The oxidation
promoter, MnO.sub.2 in powder form, was applied to each of the
coated substrate coupons, where it became fixed to the coating
material upon contact. The amount of MnO.sub.2 needed for the
experiment was determined based on the 1:1 molar ratio of MnO.sub.2
to sodium sulfide proposed by Barker in U.S. Pat. No. 3,470,061,
which is incorporated herein by reference in its entirety. The
coupons were then baked in an oven, after which the finished
MnO.sub.2-coated coupons were fixed to two stainless steel rings to
form what is referred to as the "sleeve" configuration of the
oxidizing promoter element (i.e., similar to the configuration
described above and illustrated in FIG. 11).
[0092] For each experiment, the sleeve was inserted into the vessel
such that it rested at the bottom of the vessel. The vessel was
filled with 1 liter of clarified white liquor from Potlach or E. B.
Eddy and then sealed and purged with nitrogen for 15 minutes to
remove excess oxygen in the vessel above the white liquor surface.
An outlet vent at the top of the vessel was open to allow the
nitrogen and excess oxygen to escape. After the nitrogen purge, the
outlet vent was closed and the nitrogen pressure was increased to
15 psig. The liquor was then heated to the desired set point,
approximately 80.degree. C. During this heating stage, the mixing
speed was 500 rpm.
[0093] When the desired temperature set point was reached, a liquor
sample was withdrawn from the reactor. This sample was coded as
time (t)=0 minutes since no oxidation had yet taken place. The
mixing speed was then increased to 1500 rpm, and oxygen was
introduced into the reactor through an open tube immersed in the
liquor. The oxygen flow rate was based on system pressure and
monitored with a mass flow controller. The gas-liquid mixing was
done using a Hollow Shaft of 3/4-inch diameter and 7-inch length
having attached at the end a 6-blade Ruston impeller of 13/8-inch
diameter and 3/4-inch height, which was also hollow so that gas
could flow through it. When oxygen surfaced above the white liquor
in the headspace of the vessel, it entered the hollow shaft through
an opening near the top of the vessel and exited at the end of the
hollow shaft submerged in the white liquor.
[0094] For each experiment, a sufficient number of samples were
collected to produce a concentration vs. time curve. To prevent
sample degradation, the samples were "capped" with nitrogen and
immediately quenched in an ice bath prior to analysis, as described
below.
[0095] The presence of polysulfide species was determined by
gravimetric analysis. This consists of acidifying the filtered
liquor sample in order to precipitate the elemental sulfur, which
is the excess sulphur of the polysulfide. Each sample tested for
polysulfide was first diluted with water and acidified in the
presence of EDTA. The resulting milky solution was then filtered
under vacuum. Finally, the polysulfide precipitate was dried and
weighed.
[0096] The results are as follows:
1 Experiment #1 Polysulfide Time Concentration (min) (g/L) 0 0.2 7
2.8 9 3.5 11 4.2 14 5.0 17 6.2 20 7.2 25 7.8 30 4.8
[0097]
2 Experiment #2 Polysulfide Time Concentration (min) (g/L) 0 0.8
6.5 1.9 12 2.7 17 3.4 20 4.4 23 5.1 26 6.0 29 6.8 32 7.8 36 6.7
[0098]
3 Experiment #3 Polysulfide Time Concentration (min) (g/L) 12 2.6
16 4.6 20 5.6 24 6.6 27 7.3 30.5 8.3 33 7.7 36 6.4
[0099]
4 Experiment #4 Polysulfide Time Concentration (min) (g/L) 0 0.410
6 1.670 12 3.010 16 3.950 23 5.830 26 7.030 29 7.630 32 7.670 36
5.610
[0100]
5 Experiment #5 Polysulfide Time Concentration (min) (g/L) 0 0.4 6
2.1 12 3.6 18 5.6 24 7.4 27 9.0 30 8.1 32 6.2 34 5.5 37 4.0
[0101]
6 Experiment #6 Polysulfide Time Concentration (min) (g/L) 0 0.7 6
1.7 12 3.9 18 5.5 22 7.7 25 7.9 28 8.5 32 6.9 36 4.5
[0102] Having described preferred embodiments of systems and
methods for generating polysulfide, variations and changes will be
suggested to those skilled in the art in view of the teachings set
forth herein. It is therefore to be understood that all such
variations, modifications and changes are believed to fall within
the scope of the present invention as defined by the appended
claims.
* * * * *