Note: Descriptions are shown in the official language in which they were submitted.
_ 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.
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
2-
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
-3-
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.
