CA2090832A1 - Method of monitoring ph in caustic liquor wet oxidation - Google Patents
Method of monitoring ph in caustic liquor wet oxidationInfo
- Publication number
- CA2090832A1 CA2090832A1 CA002090832A CA2090832A CA2090832A1 CA 2090832 A1 CA2090832 A1 CA 2090832A1 CA 002090832 A CA002090832 A CA 002090832A CA 2090832 A CA2090832 A CA 2090832A CA 2090832 A1 CA2090832 A1 CA 2090832A1
- Authority
- CA
- Canada
- Prior art keywords
- wet oxidation
- carbon dioxide
- caustic
- effluent
- process according
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F11/00—Treatment of sludge; Devices therefor
- C02F11/06—Treatment of sludge; Devices therefor by oxidation
- C02F11/08—Wet air oxidation
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
Abstract
D.N. 2473 METHOD OF MONITORING PH
IN CAUSTIC LIQUOR WET OXIDATION
ABSTRACT
A process is described for monitoring pH in caustic liquor wet oxidation systems. An offgas carbon dioxide baseline value is established for the operation system where oxidized liquid effluent is at pH 7 or above. Offgas carbon dioxide content deviating by a selected proportion above the baseline vale indicates a drop in liquid effluent pH to 7 or less. Sufficient alkalinity is added to the raw wastewater to bring the effluent pH to 7 or above and prevent excessive corrosion to the materials of construction of the nickel-based wet oxidation system.
IN CAUSTIC LIQUOR WET OXIDATION
ABSTRACT
A process is described for monitoring pH in caustic liquor wet oxidation systems. An offgas carbon dioxide baseline value is established for the operation system where oxidized liquid effluent is at pH 7 or above. Offgas carbon dioxide content deviating by a selected proportion above the baseline vale indicates a drop in liquid effluent pH to 7 or less. Sufficient alkalinity is added to the raw wastewater to bring the effluent pH to 7 or above and prevent excessive corrosion to the materials of construction of the nickel-based wet oxidation system.
Description
_ 2090832 U.~ 4/~
-1- .
METHOD OF MONITORING P~ IN C~USTIC LIQUOR WET OXID~TION
BACRGROUND OF T~IE ~ ENTION
1. Field of the Invention.
This invention relates to a method for protecting the materials of construction of a wet oxidation system treating 5 caustic wastewaters, particularly caustic sulfide wastewaters.
-1- .
METHOD OF MONITORING P~ IN C~USTIC LIQUOR WET OXID~TION
BACRGROUND OF T~IE ~ ENTION
1. Field of the Invention.
This invention relates to a method for protecting the materials of construction of a wet oxidation system treating 5 caustic wastewaters, particularly caustic sulfide wastewaters.
2. Information Disclosure Statement.
A variety of caustic wastewaters are generated which require treatment before the wastewater is released to the environment. These caustic wastewaters are generated in the 10 petrochemical industry, petroleum refining, pulp and paper manufacture and various chemical manufacturing processes. The caustic solutions are commonly used to remove acidic components such as hydrogen sulfide, H25, mercaptans, RSH, phenols, ArOH, and organic acids, RCO2H, from gas and li~uid 15 streams.
The contaminated caustic wastewaters represent a formidable disposal problem due to their caustic content as well as the acidic components therein. Neutralization of the caustic wastewaters b~ acid addition can result in release of 20 the acidic components. Therefore it is essential to convert the acidic components to a form suitable for release to the environment. Further, there may be additional components present in the caustic wastewater which adds to the Chemical Oxygen Demand (COD) of the wastewater. These components 25 include various carbonaceous materials including oils and pol ~ere.
D.N. 2473 Wet oxidation is the preferred method of treatment for caustic wastewaters since the products of oxidation are inorganic sulfate, carbon dioxide and water. Also, the oxidation is carried out within a closed system which prevents 5 transfer of pollutants to the a-tmosphere. The highly alkaline nature of these caustic wastewaters requires special materials of construction for wet oxidation systems employed in their treatment. The nickel-based alloys, such as Inconel 600, are well suited to withstand the elevated temperatures and 10 pressures employed in the wet oxidation process for caustic wastewater treatment.
In U.S. Patent No. 3,761,409 McCoy et al. disclose a continuous process for the air oxidation of sulEidic, ammoniacal sour water where feed water is adjusted to a pH
between about 6 to 13 and the oxidation occurs at 250F to 520F at 75 to 800 psig with up to 500~ excess oxygen based on the stoichiometric conversion of sulfide to sulfate.
Chowdhury in U.S. Patent No. 4,350,599 discloses wet oxidation of caustic liquor where carbon dioxide generated by the oxidation is used to reduce the pH of the caustic feed liquor to below ll. Maintaining the feed below pH ll.0 but above 7.0 prevents corrosion of the less expensive stainless steel wet oxidation system.
As mentioned above, tha nickel-based alloys are resistant 25 to corrosion by caustic sulfide wastewaters under wet oxidation conditions, provided the pH of the wastewater is maintained on the alkaline side, that is above pH 7. The wet oxidation of sulfide wastewaters generates acidic species which consume alkalinity. Depending on the components 3 present their conCentration~ 3nd th ph of the causti~
I),N. ~4/~
~090~32 sulfide wastewater, wet oxidation may produce an oxidized wastewater in which the pH is acidic, i.e. all alkalinity is consumed, and which is highly corrosive to the nickel-based wet oxidation system.
Beula et al. in U.S. Patent No. 5,082,571 have devised a process which relates the species present in the caustic liquor to the amount of caustic required to maintain an excess of alkalinity in the liquor during wet oxidation treatment.
This process allows a nickel-based allo~ wet oxidation system 10 to safely treat caustic sulfide liquor without excessive corrosion to the materials of construction of the system. The process requires extensive analysis of the raw feed liquor and gives best results with a constant composition feed. Problems can result where feed composition changes and alkalinit~
15 consuming species increases, causing a drop in the pH of the oxidized wastewater.
To overcome this prbblem, I have devised a method of determining a drop in pH for a cau~tic wastewater undergoing wet oxidation treatment which allows for the adjustment of the 20 raw feed composition, i.e. pH, before corrosion to the materials of construction of the wet oxidation system occurs.
It must be recognized that the corrosion problems need careful consideration in that the integrity of the pressurized wet oxidation system is important for both safety and economic 25 reasons. -~UMMARY OF THE INV~NTION
The invention comprises a process for preventingcorrosion to the materials of construction of a nickel-base alloy wet oxidation system treating raw causti~ wastewaters at . ,:: . . .............. .. .
D.N. 2473 2~9V832 elevated temperature and pressure comprising the steps;
(a~ establishing a flow of caustic wastewater and oxy~en containing gas through said wet oxidation system to produce an oxidized gas/liquicl mixture;
(b) separating said oxidized gas/liquid mixture into an oxidized liquid phase effluent and a gaseous phase effluent;
(c) measuring the carbon dioxide content of said gaseous phase effluent to establish a baseline carbon dioxide content value while the pH of said system liquid effluent remains at 7 or above; and (d) adding sufficient alkalinity to said raw caustic wastewater to maintain said system liquid effluent pH at 7 or above, upon the carbon dioxide content of said gaseous phase effluent exceeding said baseline value by a selected proportion, thereby preventing excessive corrosion to the material of construction of said wet oxidation system.
In an alternative embodiment of the invention, a flow of clean water and oxygen containing gas is established through the system and caustic wastewater is injected into the system at the reactor or any point upstream thereof.
~RIEF DE8CRIPTIO~ OF THB DRAWING
The FIGURE shows a general schema~ic diagram for a wet 25 oxidation system used to treat caustic wastewaters.
D.N 2473 2~90832 DE~C~IPTION OF THE PREF~R~E~ E~BODIMENTS
The invention is applicable to all caustic wastewaters treated by wet oxidation. It is particularly applicable to caustic sulfide scrubbing liquor and the invention will be 5 descri~ed as applied to such a wastewater.
The FIGURE shows a schematic flow diagram for a wet oxidation system used for treatment of caustic sulfide scrubbing liquors. Referring to the FIGURE, raw caustic sulfide liquor from a storage tank 10 flows through a cond~it 0 12 to a high pressure pump 14 which pressurizes the liquor.
The raw liquor is mixed with a pressurized oxygen-containing ga~, such as air, supplied by a compressor 16, within a conduit 18. The mixture flows through a heat exchanger 20 where it is heated to a temperature which initiates oxidation.
5 The heated mixture then flows through a second heat exchanger 22 which provides auxiliary heat for startup of the system.
For waste with low COD content, auxiliary heating may need to be continuously applied through the second heat exchanger 2~
in order to maintain the desired operating temperature for the 0 wet oxidation system. The heated feed mixture then enters a reactor vessel 24 which provides a residence time wherein the bulk of the oxidation reaction occurs. The oxidized liquor and oxygen depleted gas mixture then exits the reactor through a conduit 26 controlled by a pressure control valve 28. The hot oxidized effluent traverses the heat exchanger 20 where it is cooled against incoming raw liquor and gas mixture. The cooled effluent mixture flows through a conduit 30 to a separator vessel 32 where liquid and gases are disengaged.
The liquid efrluent exits the separator vessel 32 through a er conduit 3~ while the gases are Ve ted through an Upper D.N. 2473 20~832 conduit 36. The carbon dioxide content of the gases are continuously measured by a carbon dioxide monitor 38, located within the upper conduit 36. The carbon dioxide monitor 38 may be any of the commercially available instruments well known in the industry.
It is imperative that an excess of alkalinity be maintained throuyhout the nickel-based alloy wet oxidation system when treating caustic sulfide liquor. The excess alkalinity maintains the liquid phase at pH 7 or above and prevents corrosion of the nickel-based alloy system.
The raw caustic wastewater may contain carbonate or bicarbonate salts depending upon the pH of the liquid.
Additionally, carbon dioxide is generated by oxidation of carbonaceous compounds in the waste liquor. The carbon dioxide generated may be absorbed by the caustic solution within the system as carbonate/bicarbonate also.
It is well known that as a carbonate solution is made acidic, carbon dioxide gas is generated. Acid addition to a carbonate solution protonates the carbonate ion, giving carboniG acid which decomposes to water and carbon dioxide which is liberated from solution. The proportion of carbonate, bicarbonate and carbonic acid present in a liquid as a function of pH can be readily calculated. Here the total carbonate concentration is denoted as C~. The dissociation constants for carbonic acid ~H2CO3~ are:
Kl = 4.3 X lO-7 and K2 = 5.6 X lO-Il and {~ opH
According to acid-base equilibrium principles, it follows that:
D.N. 2473 2~90832 {H2CO3~ = Cl~ [ {H+}2 / {H+~2 + Kl{H+} ~ K~K~ ]
{HC03} = C~ [ ~l {H+}/ {H+}2 + Kl{H`~} + KIK2 ]
{CO3~} = Cc~b [ KIK2 / {H+}2-~ Kl{Ht} + KIK2 ]
Using the first two eguations, the ratio of concentration of 5 carbonic acid to bicarbonate can be calculated over the pH
range 5 to 10. This ratio, from the first 'wo above equations, simplifies to {H2CO3,'~HCO3} = {H+}2/{H+}KI= {H+}/K
which gives the following:
Table 1 ~ oa Lo pH {H } {H2CO3}l{HCOJ}
l X 10~ 23.25 6 1 X 106 2.325 7 1 X 107 0.2325 8 1 X 10-8 0.02325 9 1 X 109 0.002325 1 X 10-l 0.0002325 _~_~ x~ .
Thus at pH 7, the ratio of {H2C03}/{HC03-} is about 0.25 (1:4) or 20% H2C03present. At pH 8 the ratio is only 0.023 (1:50) or 2% H2CO3 present. Should the pH of the liquid within the 20 wet oxidation system drop to 7 or lower, the carbonic acid formed decomposes to carbon dioxida and water, with the carbon dioxide cntering the gas phase. Further, the residence time of the gas phase within ths wet oxidation sy~te= is ~uch les~
. :
D.N. 2~73 20~832 than that of the liquid phase. Wet oxidation systems for caustic wastewaters are desiyned for a reactor vessel residence time of about 30 minutes to 120 minutes. Depending upon the strength of the waste, the gas phase residencs time in the reactor vessel is about 5 minutes or less. Thus, any carbon dioxide driven into the gas phase is quickly carried through the system to the separator vessel 32 where it reports in the gas phase and is detected by the carbon dioxide monitor 38 in the upper gas conduit 3~. `
In imple~enting the invention, a flow of caustic wastewater and oxygen containing gas is established through the wet oxidation system at selected elevated temperature and pressure. The operating temperature may be as low as 10SC
(221F) or as high as 300C (572F~. Operating pressure may 15 vary from about 45 psig (310 kPa) up to 4,000 psig (27,57~
kPa) depending on the oxygen containing gas used in the system. The oxidized gas liquid mixture is separated into an oxidized liguid phase effluent and a gaseous phase effluent.
The carbon dioxide content of the gaseous phase effluent is 20 monitored to establish a baseline carbon dioxide content value while the pH of said system liquid effluent remains at 7 or above. There is generally little carbon dioxide in the gaseous phase when the oxidized liquid phase is caustic.
There may be certain wastes which do produce a nonzero carbon 25 dioxide of~gas contest, so a baseline carbon dioxide level in the offgases is established.
Should the carbon dioxide content of the ofPgases exceed the baseline value by a selected proportion, an alarm means is activated to warn that additional alkalinity must be added to ~30 e r~w caustlc w~s~ewater to mai~ta the system liquid ~.N. ~4/~
2~832 effluent pH at 7 or above, thereby preventing excessive corrosion to the material of construction of the nickel-based alloy wet oxidation system.
In an alternative embodiment, a flow of clean water is 5 first established through the wet oxidation system by filling the storage tank 10 with clean water and using the feed pump 14 to pump the clean water through the system. An oxygen-containing gas, supplied by the compressor 16, is mixed with the clean water within the conduit 18. The temperature and l0 pressure within the system are elevated by the auxiliary heat exchanger 22. Concentrated wastewater then is added to the system by means of a second feed pump (not shown) at any polnt as far downstream as the reactor vessel 24. Alternative points of addition for the wastewater are shown as 40 and 42.
In this embodiment the flow of clean water from the water pump 14 is required to dilute the concentrated wastewater within the system and provide sufficient liquid water for evaporative cooling and heat removal from the reactor vessel 24. The flow of clean water is also required to traverse the process heat exchanger 20 and recover the heat from the hot oxidized effluent leaving the system where the point of waste injection is beyond the process heat exchanger 2~.
Oxygen-containing gas addition points may be varied as well, depending on the characteristics of the particular 25 wastewater treated by the system and the point of addition of the wastewater. These alternative points are denoted as 4~
and 46. A wastewater which fouls heat exchangers when heated with limited oxygen would dictate that the oxygen containing gas be added upstream of the wastewater point of addition.
;30 0th wastewaters beco=e extremely corr-sive when heated in D.N. ~473 2~9~32 the absence of dissolved oxygen, thus dictating the addition of oxygen containing gas to the wastewater upstream of any heating device. Certain wastes which are difficult to dissolve, slurry or suspend in water can be injected directly into the reactor vessel. In this situation the oxygen containing gas may be added directly to the reactor vessel 2 or at any point upstream of the reactor vessel.
EXA~IPLE
A sample of spent caustic scrubbing liquor was obtained 0 from a petrochemical plant. The liquor contained about 22 g/l of COD, mainly a mixture of sodium sulfide, Na2S, and sodium hydrogen sulfide, NaHS. The liquor had a pH of 13.63 and contained about 15 g/l of sodium hydroxide, NaOH, and about 3.2 g/l of sodium carbonate, Na2COl. The sulfides present will consume alkalinity on wet oxidation and lower the pH of the oxidized effluent. If sufficient alkalinity is not available, the pH will become acidic and damage the materials of construction of the wet oxidation system.
Samples of the caustic wastewater were partially neutralized with sulfuric acid to reduce the alkalinity available in each. The samples were each placed in an autoclave, pressurized with sufficient air to oxidize all COD
contained, and heated at 16~C 1194F) for five minutes. After cooling, the carbon dioxide and oxygen content of the offga~es were measured by gas chromatography. The pH of the oxidized liquid phase was also determined. The results of these analyses are shown in Table 2.
LJ.N. ~4/~
2~9~32 Table 2 ~_~ ~
Run No. 1 23 4 pH of Feed13.63 13.35 13.30 13.21 pH of Oxidized12.36 8.62 7.95 2.63 Offgas Co2, %o.o0 o.00 0.26 0.44 Offgas 2~ %11.40 lZ.62 11.30 11.05 ~.
In runs No. 3 and 4, the pH of the oxidized liquor phase drops to below about pH 8.0, and the carbon dioxide content of the gaseous phase increases above the 0.00 ~ baseline value found for runs No. 1 and 2. The carbon dioxide evolved in a 10 continuous flow system is detected in the offgases and addition of alkalinity to the raw feed will maintain the oxidized effluent in the desired p~ operating range.
The alkalinity added may be in the form of alkali metal hydroxides, such as sodium hydroxide or potassium hydroxide, 15 or alkali metal carbonate or bicarbonate, such as sodium or potassium carbonate or bicarbonate~ Alkaline earth metal hydroxides, such as magnesium or calcium hydroxide, may also be used but these forms of alkalinity are less preferred.
From the foregoing description, one skilled in the art 20 can easily ascertain the essential characteristics of the invention and, without departing frcm the spirit and scope thereof, make various changes and modifications to adapt it to various usages.
A variety of caustic wastewaters are generated which require treatment before the wastewater is released to the environment. These caustic wastewaters are generated in the 10 petrochemical industry, petroleum refining, pulp and paper manufacture and various chemical manufacturing processes. The caustic solutions are commonly used to remove acidic components such as hydrogen sulfide, H25, mercaptans, RSH, phenols, ArOH, and organic acids, RCO2H, from gas and li~uid 15 streams.
The contaminated caustic wastewaters represent a formidable disposal problem due to their caustic content as well as the acidic components therein. Neutralization of the caustic wastewaters b~ acid addition can result in release of 20 the acidic components. Therefore it is essential to convert the acidic components to a form suitable for release to the environment. Further, there may be additional components present in the caustic wastewater which adds to the Chemical Oxygen Demand (COD) of the wastewater. These components 25 include various carbonaceous materials including oils and pol ~ere.
D.N. 2473 Wet oxidation is the preferred method of treatment for caustic wastewaters since the products of oxidation are inorganic sulfate, carbon dioxide and water. Also, the oxidation is carried out within a closed system which prevents 5 transfer of pollutants to the a-tmosphere. The highly alkaline nature of these caustic wastewaters requires special materials of construction for wet oxidation systems employed in their treatment. The nickel-based alloys, such as Inconel 600, are well suited to withstand the elevated temperatures and 10 pressures employed in the wet oxidation process for caustic wastewater treatment.
In U.S. Patent No. 3,761,409 McCoy et al. disclose a continuous process for the air oxidation of sulEidic, ammoniacal sour water where feed water is adjusted to a pH
between about 6 to 13 and the oxidation occurs at 250F to 520F at 75 to 800 psig with up to 500~ excess oxygen based on the stoichiometric conversion of sulfide to sulfate.
Chowdhury in U.S. Patent No. 4,350,599 discloses wet oxidation of caustic liquor where carbon dioxide generated by the oxidation is used to reduce the pH of the caustic feed liquor to below ll. Maintaining the feed below pH ll.0 but above 7.0 prevents corrosion of the less expensive stainless steel wet oxidation system.
As mentioned above, tha nickel-based alloys are resistant 25 to corrosion by caustic sulfide wastewaters under wet oxidation conditions, provided the pH of the wastewater is maintained on the alkaline side, that is above pH 7. The wet oxidation of sulfide wastewaters generates acidic species which consume alkalinity. Depending on the components 3 present their conCentration~ 3nd th ph of the causti~
I),N. ~4/~
~090~32 sulfide wastewater, wet oxidation may produce an oxidized wastewater in which the pH is acidic, i.e. all alkalinity is consumed, and which is highly corrosive to the nickel-based wet oxidation system.
Beula et al. in U.S. Patent No. 5,082,571 have devised a process which relates the species present in the caustic liquor to the amount of caustic required to maintain an excess of alkalinity in the liquor during wet oxidation treatment.
This process allows a nickel-based allo~ wet oxidation system 10 to safely treat caustic sulfide liquor without excessive corrosion to the materials of construction of the system. The process requires extensive analysis of the raw feed liquor and gives best results with a constant composition feed. Problems can result where feed composition changes and alkalinit~
15 consuming species increases, causing a drop in the pH of the oxidized wastewater.
To overcome this prbblem, I have devised a method of determining a drop in pH for a cau~tic wastewater undergoing wet oxidation treatment which allows for the adjustment of the 20 raw feed composition, i.e. pH, before corrosion to the materials of construction of the wet oxidation system occurs.
It must be recognized that the corrosion problems need careful consideration in that the integrity of the pressurized wet oxidation system is important for both safety and economic 25 reasons. -~UMMARY OF THE INV~NTION
The invention comprises a process for preventingcorrosion to the materials of construction of a nickel-base alloy wet oxidation system treating raw causti~ wastewaters at . ,:: . . .............. .. .
D.N. 2473 2~9V832 elevated temperature and pressure comprising the steps;
(a~ establishing a flow of caustic wastewater and oxy~en containing gas through said wet oxidation system to produce an oxidized gas/liquicl mixture;
(b) separating said oxidized gas/liquid mixture into an oxidized liquid phase effluent and a gaseous phase effluent;
(c) measuring the carbon dioxide content of said gaseous phase effluent to establish a baseline carbon dioxide content value while the pH of said system liquid effluent remains at 7 or above; and (d) adding sufficient alkalinity to said raw caustic wastewater to maintain said system liquid effluent pH at 7 or above, upon the carbon dioxide content of said gaseous phase effluent exceeding said baseline value by a selected proportion, thereby preventing excessive corrosion to the material of construction of said wet oxidation system.
In an alternative embodiment of the invention, a flow of clean water and oxygen containing gas is established through the system and caustic wastewater is injected into the system at the reactor or any point upstream thereof.
~RIEF DE8CRIPTIO~ OF THB DRAWING
The FIGURE shows a general schema~ic diagram for a wet 25 oxidation system used to treat caustic wastewaters.
D.N 2473 2~90832 DE~C~IPTION OF THE PREF~R~E~ E~BODIMENTS
The invention is applicable to all caustic wastewaters treated by wet oxidation. It is particularly applicable to caustic sulfide scrubbing liquor and the invention will be 5 descri~ed as applied to such a wastewater.
The FIGURE shows a schematic flow diagram for a wet oxidation system used for treatment of caustic sulfide scrubbing liquors. Referring to the FIGURE, raw caustic sulfide liquor from a storage tank 10 flows through a cond~it 0 12 to a high pressure pump 14 which pressurizes the liquor.
The raw liquor is mixed with a pressurized oxygen-containing ga~, such as air, supplied by a compressor 16, within a conduit 18. The mixture flows through a heat exchanger 20 where it is heated to a temperature which initiates oxidation.
5 The heated mixture then flows through a second heat exchanger 22 which provides auxiliary heat for startup of the system.
For waste with low COD content, auxiliary heating may need to be continuously applied through the second heat exchanger 2~
in order to maintain the desired operating temperature for the 0 wet oxidation system. The heated feed mixture then enters a reactor vessel 24 which provides a residence time wherein the bulk of the oxidation reaction occurs. The oxidized liquor and oxygen depleted gas mixture then exits the reactor through a conduit 26 controlled by a pressure control valve 28. The hot oxidized effluent traverses the heat exchanger 20 where it is cooled against incoming raw liquor and gas mixture. The cooled effluent mixture flows through a conduit 30 to a separator vessel 32 where liquid and gases are disengaged.
The liquid efrluent exits the separator vessel 32 through a er conduit 3~ while the gases are Ve ted through an Upper D.N. 2473 20~832 conduit 36. The carbon dioxide content of the gases are continuously measured by a carbon dioxide monitor 38, located within the upper conduit 36. The carbon dioxide monitor 38 may be any of the commercially available instruments well known in the industry.
It is imperative that an excess of alkalinity be maintained throuyhout the nickel-based alloy wet oxidation system when treating caustic sulfide liquor. The excess alkalinity maintains the liquid phase at pH 7 or above and prevents corrosion of the nickel-based alloy system.
The raw caustic wastewater may contain carbonate or bicarbonate salts depending upon the pH of the liquid.
Additionally, carbon dioxide is generated by oxidation of carbonaceous compounds in the waste liquor. The carbon dioxide generated may be absorbed by the caustic solution within the system as carbonate/bicarbonate also.
It is well known that as a carbonate solution is made acidic, carbon dioxide gas is generated. Acid addition to a carbonate solution protonates the carbonate ion, giving carboniG acid which decomposes to water and carbon dioxide which is liberated from solution. The proportion of carbonate, bicarbonate and carbonic acid present in a liquid as a function of pH can be readily calculated. Here the total carbonate concentration is denoted as C~. The dissociation constants for carbonic acid ~H2CO3~ are:
Kl = 4.3 X lO-7 and K2 = 5.6 X lO-Il and {~ opH
According to acid-base equilibrium principles, it follows that:
D.N. 2473 2~90832 {H2CO3~ = Cl~ [ {H+}2 / {H+~2 + Kl{H+} ~ K~K~ ]
{HC03} = C~ [ ~l {H+}/ {H+}2 + Kl{H`~} + KIK2 ]
{CO3~} = Cc~b [ KIK2 / {H+}2-~ Kl{Ht} + KIK2 ]
Using the first two eguations, the ratio of concentration of 5 carbonic acid to bicarbonate can be calculated over the pH
range 5 to 10. This ratio, from the first 'wo above equations, simplifies to {H2CO3,'~HCO3} = {H+}2/{H+}KI= {H+}/K
which gives the following:
Table 1 ~ oa Lo pH {H } {H2CO3}l{HCOJ}
l X 10~ 23.25 6 1 X 106 2.325 7 1 X 107 0.2325 8 1 X 10-8 0.02325 9 1 X 109 0.002325 1 X 10-l 0.0002325 _~_~ x~ .
Thus at pH 7, the ratio of {H2C03}/{HC03-} is about 0.25 (1:4) or 20% H2C03present. At pH 8 the ratio is only 0.023 (1:50) or 2% H2CO3 present. Should the pH of the liquid within the 20 wet oxidation system drop to 7 or lower, the carbonic acid formed decomposes to carbon dioxida and water, with the carbon dioxide cntering the gas phase. Further, the residence time of the gas phase within ths wet oxidation sy~te= is ~uch les~
. :
D.N. 2~73 20~832 than that of the liquid phase. Wet oxidation systems for caustic wastewaters are desiyned for a reactor vessel residence time of about 30 minutes to 120 minutes. Depending upon the strength of the waste, the gas phase residencs time in the reactor vessel is about 5 minutes or less. Thus, any carbon dioxide driven into the gas phase is quickly carried through the system to the separator vessel 32 where it reports in the gas phase and is detected by the carbon dioxide monitor 38 in the upper gas conduit 3~. `
In imple~enting the invention, a flow of caustic wastewater and oxygen containing gas is established through the wet oxidation system at selected elevated temperature and pressure. The operating temperature may be as low as 10SC
(221F) or as high as 300C (572F~. Operating pressure may 15 vary from about 45 psig (310 kPa) up to 4,000 psig (27,57~
kPa) depending on the oxygen containing gas used in the system. The oxidized gas liquid mixture is separated into an oxidized liguid phase effluent and a gaseous phase effluent.
The carbon dioxide content of the gaseous phase effluent is 20 monitored to establish a baseline carbon dioxide content value while the pH of said system liquid effluent remains at 7 or above. There is generally little carbon dioxide in the gaseous phase when the oxidized liquid phase is caustic.
There may be certain wastes which do produce a nonzero carbon 25 dioxide of~gas contest, so a baseline carbon dioxide level in the offgases is established.
Should the carbon dioxide content of the ofPgases exceed the baseline value by a selected proportion, an alarm means is activated to warn that additional alkalinity must be added to ~30 e r~w caustlc w~s~ewater to mai~ta the system liquid ~.N. ~4/~
2~832 effluent pH at 7 or above, thereby preventing excessive corrosion to the material of construction of the nickel-based alloy wet oxidation system.
In an alternative embodiment, a flow of clean water is 5 first established through the wet oxidation system by filling the storage tank 10 with clean water and using the feed pump 14 to pump the clean water through the system. An oxygen-containing gas, supplied by the compressor 16, is mixed with the clean water within the conduit 18. The temperature and l0 pressure within the system are elevated by the auxiliary heat exchanger 22. Concentrated wastewater then is added to the system by means of a second feed pump (not shown) at any polnt as far downstream as the reactor vessel 24. Alternative points of addition for the wastewater are shown as 40 and 42.
In this embodiment the flow of clean water from the water pump 14 is required to dilute the concentrated wastewater within the system and provide sufficient liquid water for evaporative cooling and heat removal from the reactor vessel 24. The flow of clean water is also required to traverse the process heat exchanger 20 and recover the heat from the hot oxidized effluent leaving the system where the point of waste injection is beyond the process heat exchanger 2~.
Oxygen-containing gas addition points may be varied as well, depending on the characteristics of the particular 25 wastewater treated by the system and the point of addition of the wastewater. These alternative points are denoted as 4~
and 46. A wastewater which fouls heat exchangers when heated with limited oxygen would dictate that the oxygen containing gas be added upstream of the wastewater point of addition.
;30 0th wastewaters beco=e extremely corr-sive when heated in D.N. ~473 2~9~32 the absence of dissolved oxygen, thus dictating the addition of oxygen containing gas to the wastewater upstream of any heating device. Certain wastes which are difficult to dissolve, slurry or suspend in water can be injected directly into the reactor vessel. In this situation the oxygen containing gas may be added directly to the reactor vessel 2 or at any point upstream of the reactor vessel.
EXA~IPLE
A sample of spent caustic scrubbing liquor was obtained 0 from a petrochemical plant. The liquor contained about 22 g/l of COD, mainly a mixture of sodium sulfide, Na2S, and sodium hydrogen sulfide, NaHS. The liquor had a pH of 13.63 and contained about 15 g/l of sodium hydroxide, NaOH, and about 3.2 g/l of sodium carbonate, Na2COl. The sulfides present will consume alkalinity on wet oxidation and lower the pH of the oxidized effluent. If sufficient alkalinity is not available, the pH will become acidic and damage the materials of construction of the wet oxidation system.
Samples of the caustic wastewater were partially neutralized with sulfuric acid to reduce the alkalinity available in each. The samples were each placed in an autoclave, pressurized with sufficient air to oxidize all COD
contained, and heated at 16~C 1194F) for five minutes. After cooling, the carbon dioxide and oxygen content of the offga~es were measured by gas chromatography. The pH of the oxidized liquid phase was also determined. The results of these analyses are shown in Table 2.
LJ.N. ~4/~
2~9~32 Table 2 ~_~ ~
Run No. 1 23 4 pH of Feed13.63 13.35 13.30 13.21 pH of Oxidized12.36 8.62 7.95 2.63 Offgas Co2, %o.o0 o.00 0.26 0.44 Offgas 2~ %11.40 lZ.62 11.30 11.05 ~.
In runs No. 3 and 4, the pH of the oxidized liquor phase drops to below about pH 8.0, and the carbon dioxide content of the gaseous phase increases above the 0.00 ~ baseline value found for runs No. 1 and 2. The carbon dioxide evolved in a 10 continuous flow system is detected in the offgases and addition of alkalinity to the raw feed will maintain the oxidized effluent in the desired p~ operating range.
The alkalinity added may be in the form of alkali metal hydroxides, such as sodium hydroxide or potassium hydroxide, 15 or alkali metal carbonate or bicarbonate, such as sodium or potassium carbonate or bicarbonate~ Alkaline earth metal hydroxides, such as magnesium or calcium hydroxide, may also be used but these forms of alkalinity are less preferred.
From the foregoing description, one skilled in the art 20 can easily ascertain the essential characteristics of the invention and, without departing frcm the spirit and scope thereof, make various changes and modifications to adapt it to various usages.
Claims (10)
- I claim:
l. A process for preventing corrosion to the materials of construction of a nickel-base alloy wet oxidation system treating raw caustic wastewaters at elevated temperature and pressure comprising the steps;
(a) establishing a flow of caustic wastewater and oxygen containing gas through said wet oxidation system to produce an oxidized gas/liquid mixture, (b) separating said oxidized gas/liquid mixture into an oxidized liquid phase effluent and a gaseous phase effluent;
(c) measuring the carbon dioxide content of said gaseous phase effluent to establish a baseline carbon dioxide content value while the pH of said system liquid effluent remains at 7 or above; and (d) adding sufficient alkalinity to said raw caustic wastewater to maintain said system liquid effluent pH at 7 or above, upon the carbon dioxide content of said gaseous phase effluent exceeding said baseline value by a selected proportion, thereby preventing excessive corrosion to the material of construction of said wet oxidation system. - 2. A process according to claim l wherein said caustic wastewater is a sulfidic scrubbing liquor.
- 3. A process according to claim l wherein said elevated temperature is between about 105°C and 300°C.
D.N. 2473 - 4. A process according to claim 1 wherein said elevated pressure is between about 45 psig and 4,000 psig.
- 5. A process according to claim 1 wherein said oxygen containing gas is air.
- 6. A process for preventing corrosion to the materials of construction of a nickel-based alloy wet oxidation system treating raw caustic wastewaters at elevated temperature and pressure comprising the steps;
(a) establishing a flow of clean water and oxygen containing gas through said wet oxidation system;
(b) adding a caustic wastewater to said system at a point within a reactor vessel or at a point as far upstream of said reactor vessel as a process heat exchanger, to produce an oxidized gas/liquid mixture;
(c) separating said oxidized gas/liquid mixture into an oxidized liquid phase effluent and a gaseous phase effluent;
(d) measuring the carbon dioxide content of said gaseous phase effluent to establish a baseline carbon dioxide content value while the pH of said system liquid effluent remains at 7 or above; and (e) adding sufficient alkalinity to said raw caustic wastewater to maintain said system liquid effluent pH at 7 or above, upon the carbon dioxide content of said gaseous phase effluent exceeding said baseline value by a selected proportion, thereby preventing excessive corrosion to the material of construction of said wet oxidation system.
D.N. 2473 - 7. A process according to claim 6 wherein said caustic wastewater is a sulfidic scrubbing liquor.
- 8. A process according to claim 6 wherein said elevated temperature is between about 105°C and 300°C.
- 9. A process according to claim 6 wherein said elevated pressure is between about 45 psig and 4,000 psig.
- 10. A process according to claim 6 wherein said oxygen containing gas is air.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| TW082102666A01A TW234149B (en) | 1992-04-01 | 1994-05-12 | Method of monitoring pH in caustic liquor wet oxidation |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US86173992A | 1992-04-01 | 1992-04-01 | |
| US861,739 | 1992-04-01 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2090832A1 true CA2090832A1 (en) | 1993-10-02 |
Family
ID=25336621
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002090832A Abandoned CA2090832A1 (en) | 1992-04-01 | 1993-03-02 | Method of monitoring ph in caustic liquor wet oxidation |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0564115A1 (en) |
| KR (1) | KR930022079A (en) |
| CA (1) | CA2090832A1 (en) |
| TW (1) | TW221304B (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP3553720B2 (en) * | 1996-01-31 | 2004-08-11 | 新日本石油化学株式会社 | Wet oxidation method |
| JP3565972B2 (en) * | 1996-01-31 | 2004-09-15 | 新日本石油化学株式会社 | Wet oxidation method of waste soda |
| CN102690016B (en) * | 2011-03-24 | 2013-12-25 | 中国石油化工股份有限公司 | Treatment recycle method of sewage from oil refinery |
| CN104045210B (en) * | 2011-10-13 | 2015-07-29 | 中国石油化工股份有限公司 | The treatment for reuse method of oil refining-Ethylene Complex unit sewage |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DD236513A1 (en) * | 1985-04-26 | 1986-06-11 | Bauwesen Ingbuero Veb | CIRCUIT ARRANGEMENT OF DOSING DEVICES IN CLOSED HEATING WATER CIRCUITS |
| US5082571A (en) * | 1991-05-13 | 1992-01-21 | Zimpro Passavant Environmental Systems Inc. | Caustic sulfide wet oxidation process |
-
1993
- 1993-03-02 CA CA002090832A patent/CA2090832A1/en not_active Abandoned
- 1993-03-12 EP EP93301906A patent/EP0564115A1/en not_active Withdrawn
- 1993-04-01 KR KR1019930005492A patent/KR930022079A/en not_active Application Discontinuation
- 1993-04-09 TW TW082102666A patent/TW221304B/zh active
Also Published As
| Publication number | Publication date |
|---|---|
| KR930022079A (en) | 1993-11-23 |
| TW221304B (en) | 1994-02-21 |
| EP0564115A1 (en) | 1993-10-06 |
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