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Oxalic Acid
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MANUFACTURE
1. Oxalic acid from sodium formate
RAW MATERIALS
Basis: 1000kg oxalic acid dihydrate from sodium formate
Sodium formate 1325kg
Calcium hydroxide 650kg
Sulfuric acid 950kg
REACTION:
MANUFACTURE PROCESS
Sodium formate was obtained by the reaction of solid sodium hydroxide (97 - 98%) and carbon
monoxide at 200C temperature and 150psi pressure in an autoclave.
Sodium oxalate was obtained, at the completion of above reaction by reducing the pressure in
autoclave and increasing the temperature to 400C. Reaction was completed after the evolution of
hydrogen was ceased.
The reaction mixture was transferred to precipitator and calcium hydroxide was added under stirring.
The calcium oxalate was filtered and sodium hydroxide was concentrated for reuse. Filter cake of
crude calcium oxalate run to an acidifier with small amount of calcium carbonate and treated with
dilute sulfuric acid. Calcium precipitated out as calcium sulphate dihydrate (CaSO4.2H2O). Mother
liquor was run to a crystallizer and concentrated to specific gravity of 300Be. Crystallize oxalic acid in
the form of dehydrate was washed and dried.
The yield of oxalic acid dehydrate was about 80% by weight based on sodium formate. It is generally
sold and used, as the dihydrate, but heating the dihydrate to 100C where it loses 2 molecules of water
may form anhydrous oxalic acid.
The sodium oxalate may be acidified directly to oxalic acid by mixing 1 part sodium oxalate with 3
parts of methanol and 0.8 parts of concentrated sulfuric acid. Separate insoluble sodium sulfate, from
the methyloxallate- methanol-sulfuric acid solution. Filter sodium sulfate and hydrolyze the mother
liquor by the addition of 3.5 parts water to form oxalic acid and methanol. Distilled the methanol and
recycled to the process. Concentrate the crude oxalic acid solution and crystallized the oxalic acid.
Oxalic acid is also manufactured by the oxidation of molasses with nitric acid. Similarly, oxalic acid can
be obtained from many other organic compounds like glycol, alcohol, fats, oat hulls saw dust and other
cellulosic materials by oxidation with nitric acid.
BLOCK DIAGRAM:
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2. Oxalic acid from propylene
RAW MATERIALS
Basis: 1000kg oxalic acid from propylene
Propylene 460kg
Nitric acid 1375kg
Oxygen 870kg
REACTION
MANUFACTURING PROCESS
In this process oxalic acid was obtained by oxidation of propylene with nitric acid. It is the two-step
process, in first step propylene was converted into -nitratolactic α acid and second step leads to
oxidation of -nitratolactic α acid to oxalic acid.
In the first reactor, propylene was introduced where it was reacted with nitric acid. Propylene to nitric
acid molar ratio is kept at 0.01 - 0.5. The mixture from first reactor was run into second reactor where
it was reacted with oxygen. In this reactor vapor of nitric acid was continuously evolved from the top
of the vessel. The stream from vessel contains intermediate and trace amount of nitric acid was run
into reservoir tank where acid was evolved. -nitratolactic α acid from reservoir was then run into
autoclave where it was oxidized at 45 - 1000C in the presence of mixed acid as catalyst. In this step -α
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nitratolactic acid is converted into oxalic acid dihydrate. The crude product was then transfer into a
crystallizer where oxalic acid crystals obtained. The slurry from crystallizer is filtered and sent for
drying operation where dried oxalic acid is obtained.
BLOCK DIAGRAM
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3. Oxalic acid dimethyl oxalate process
RAW MATERIALS
Basis: 1000kg oxalic acid from methyl nitrite and carbon monoxide
Carbon monoxide 957kg
Methyl nitrite 1330kg
Water 395kg
REACTION
MANUFACTRING PROCESS
In this process, circulating gas containing CO and regenerated methyl nitrite from regeneration column
was pressurized and fed to the reactor. In this vessel dimethyl oxalate was produced while methyl
nitrite is mostly consumed. The dimethyl oxalate and unconverted mixture was run into the condenser
where methanol was added. Uncondensed vapor contains methyl nitrite, water and methanol was
drawn to regeneration column where NOx and oxygen are added to regenerate the methyl nitrite
which is to be recycled.
Condensed dimethyl oxalate from condenser was run into the distillation column where water vapors
were removed while dimethyl oxalate was sent for hydrolysis where water is added. After the
hydrolysis of oxalate, slurry was sent to the crystallizer where a crystal of oxalic acid is obtained. The
slurry is then filtered and passed from dryer and dried oxalic acid stored.
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BLOCK DIAGRAM
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4. Oxalic Acid from Molasses using Nitric Acid:
RAW MATERIALS:
1. Nitric Acid
2. Sulphuric Acid
3. Water
4. Molasses
5. Vanadium Pentoxide (Catalyst)
REACTION
C6H12O6 + HNO3 3 [COOH]2. 2H2O + 6NO
MANUFACTURING PROCESS:
Molasses is a viscous by-product of the refining of sugarcane, grapes, or sugar beets into sugar. The
word comes from the Portuguese―melaço, ultimately derived from Mel, the Latin word for "honey”.
The quality of molasses depends on the maturity of the source plant, the amount of sugar extracted,
and the method employed. Molasses are of various type based on plant material from which it is
produced. So different type of molasses produced base upon on various raw material are cane
molasses from sugarcane, beet molasses from sugar beet, grape molasses from grapes .Its major
constituents are- 1) Glucose–35.9% 2) Fructose-5.6% 3) Sucrose–2.6% 4) Water-23.5%
Initially molasses was preheated & temperature of it was increased from 37oC to 65.5oC. After that it was
fed into a CSTR. Simultaneously Nitric acid was also fed into the CSTR along with Vanadium Pentoxide,
which act as a catalyst. This mixture was mixed thoroughly & was allowed to react for 2-3 hours. After 2-3
hours, Oxalic acid, un-reacted molasses, un- reacted Nitric acid, Nitrogen Oxide was formed. Oxalic acid
along with un-reacted molasses, unreacted Nitric acid and Vanadium Pentoxide comes out from the bottom
section of CSTR and undergoes further separation process. During this process Vanadium Pentoxide gets
first get separated out with help of a filter. Oxalic acid & mother liquor (unreacted Nitric acid & molasses) is
separated in a 2-stage process. In the first stage, solution that has been filtered is fed into a crystallizer in
which oxalic acid crystals along with mother liquor comes out and further these are separated with help of a
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centrifuge. After Oxalic acid gets separated, to remove inclusion (process by which a solvent particles get
trapped inside a crystal) ,it is re-crystallized by adding hot water inside a crystallizer containing these Oxalic
acid crystals.
After separating out the mother liquor again, Oxalic acid crystals are sent into drier to remove the moisture
present on the surface of the crystals. Nitrogen Oxide gas which comes out from the top surface of CSTR
cannot be discharged directly to the atmosphere, as it can cause air pollution, so this gas was sent into a
compressor to increase the pressure & to a steam heater to increase the temperature. After that this gas was
sent to fluidized-bed reactor & in presence of Al203catalyst, Nitrogen dioxide gas is formed. Al203after this
get separated by cyclone separator. After this process, in an absorber, Nitrogen dioxide gas was allowed to
react with water sprayed inside the absorber to produce Nitric acid & Nitrogen oxide gas. Nitric acid
(20%)obtained which is less concentrated than the Nitric acid (95%)which was used as a fed can further be
used in other industrial process and Nitrogen oxide gas can be recycled back to stream leading to fluidized
bed reactor.
BLOCK DIAGRAM
Figure: Manufacture of oxalic acid using Molasses & Nitric Acid
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Equipments:
S – STEAM HEATER
R1 – CSTR
T – COMPRESSOR
F – FLUIDIZED BED REACTOR
R2 – ABSORBER
CS- CYCLONE SEPARATOR
N1 & N2 – CRYSTALLIZERS
C1 & C2 – CENTRIFUGE
TD- TUNNEL DRYER
ENGINEERING ASPECTS & TECHNOLOGY SELECTION
In propylene process, sulfuric acid is used which results into corrosion problem at the oxidation
step.
Oxidation reaction can be made speedy by using large reactor, thus increase in capital
investment.
Possibility of unstable by-product formation along with -nitratolactic α acid leads to explosion
or decomposition.
In dimethyl oxalate process, the manufacturing process is complex, time consuming and
requires highest capital investment.
In the sodium formate process, the temperature pressure conditions are quite high which might
lead to safety issues and high equipment cost.
Thus, considering the above drawbacks of the first three processes used in manufacturing
oxalic acid, we choose the 4th process i.e., Manufacture of oxalic acid using molasses & nitric
acid.
Reasons for selecting the 4th process are as follows:
1. Abundance of raw materials
2. Good yield of oxalic acid
3. Low cost of operation
4. No pollution caused during the process
5. Simplicity of operation
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