Formaldehyde is a main constituent for manufacturing valuable chemical like melamine, urea and
phenolic resins. It is available in 37% aqueous solution. It is identified by some of it properties like
molecular weight about 30.3, boiling point-118 o C, melting point at -19 o C and density of 0.815 at -
20 o C.
Formaldehyde Manufacturing Technologies:
Out of two important methods of manufacture formaldehyde in large scale, methanol process
route is optimized and established for industrial production.
1. Non catalytic oxidation of methane/ butane or propane.
2. Dehydrogenation of methanol by catalytic oxidation.
1. Silver catalyst process
2. Metal oxide catalyst process
A process for the catalytic conversion of methanol to formaldehyde wherein a gas stream containing
methanol and oxygen is contacted with a silver catalyst in a first conversion zone containing a silver
catalyst to produce an effluent containing formaldehyde, unreacted methanol, and oxidation byproducts;
and contacting the first zone effluent with a copper catalyst in a second conversion zone for further
conversion of the unreacted methanol. No molecular oxygen is added to the first stage effluent or to the
second conversion zone.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention broadly relates to a process for the catalytic conversion of methanol to formaldehyde. In
particular, the present invention relates to a process for preparing formaldehyde from methanol by
contacting methanol and oxygen in contact with a first catalyst bed containing a silver catalyst, and
passing the effluent from the first conversion into contact with a second catalyst bed containing an
elemental copper catalyst
2. Description of Related Art
There are primarily two commercially accepted processes for converting methanol to formaldehyde. The
first utilizes a silver catalyst and operates in an oxygen lean atmosphere. The second utilizes a metal
oxide catalyst and operates in a methanol lean atmosphere. The first process involves passing a mixture
of methanol vapor and air over a fixed bed silver catalyst at approximately atmospheric pressure and
absorbing the product gases in water. The mechanism by which methanol is converted to formaldehyde is
believed to be a combination of two reactions separately involving the dehydrogenation and partial
oxidation of methanol
CH3 OH→HCHO+H2 (dehydrogenation)
CH3 OH+1/2O2 →HCHO+H2 O (partial oxidation)
Both single stage and multiple stage silver-catalyzed processes are known. Single stage operation is
widely used but generally suffers from the disadvantage that rather high amounts of unconverted
methanol are contained in the desired formaldehyde product emerging from the catalyst bed. For many
applications, methanol is an undesirable contaminant in the formaldehyde product and must be separated
from the recovered formaldehyde solution. Such recovery often entails a substantial investment in
distillation facilities and energy to carry out such separations. Formaldehyde product specifications
requiring a methanol content of no greater than 2% by weight are not uncommon.
Catalytic oxidation process is an optimized production method. Let us discuss about the technology in this
process. It is a simple process as per the stoichiometric reaction steps shown below.
1. Oxidation of methanol with oxygen present in air
CH3OH + ½ O2 → HCHO + H20 ∆H = -37 Kcal
2. Pyrolysis endothermic reaction
CH3OH → HCHO + H2 ∆H = +20 Kcal
3. Side reaction – complete combustion producing heat energy
CH3OH + 3/2 O2 → 2H2O +CO2 ∆H = -162 Kcal
Silver Catalyst Method: Initially Air compressed to pressure 0.2 atm by an air compressor and fed to
the bottom of methanol vaporizer. The ratio of methanol and air maintained about 35-45%. This mixture
heated to the reaction temperature 550-600 o C by series of preheater before entering into the
silver catalyst reactor. The catalytic reactor is fixed bed type filled with silver catalyst used for converting
methanol to formaldehyde. The fraction of conversion depends on the catalyst type and the temperature
of the reactor at fixed operating pressure. Advance reactor controls both reactions,
exothermic combustion reaction and endothermic dehydration reaction simultaneously in single step.
The product stream sent to purification and recovery section. Unreacted methanol fed back to the process
at methanol vaporizer. Recycle stream contain 15% unreacted methanol of feed to vaporizer.
Formaldehyde obtained as heavy end of alcohol stripper column. It is in the form of aqueous containing
63% water. Overall process has yield ranging from 85-90% on weight basis.
Technology challenges on formaldehyde catalytic oxidation are selection of catalyst and air to methanol
ratio. When metal oxides are mixed, and then there are chances of obtaining greater than 90% overall
yield of desired product. Reducing the concentrations of CO + CO2 in the streams is one of the
demanding tasks for a process engineer.
Methanol stripper: In above flow diagram the unreacted methanol is stripped out from formaldehyde
solution and recycled to the process. The downstream from methanol stripper requires further purification
column to remove water and increase the purity of formaldehyde or adjust it concentration as per
customer order. However by using a single vacuum distillation column the separation can be achieved
eliminating the two stripping columns.
Metal oxide catalyst process: Iron–molybdenum oxide catalyst replaced the costly silver improving the
methanol conversion to 99% it surface is very much suitable for formation of H2CO. The reactions in the
shell and tube reactor are completely exothermic. 250-345oC temperature is maintained in the reactor by
removing the excess energy from reactor tube using dowtherm oil.
الغرفة و قابل لألشتعال ذو رائحة نفاذه ،يتكون من األكسجين و الكربون و الهيدروجين ،
و هو يعتر من أبسط األلدهيدات ،وزنه الجزيئي ، 30.03درجة HCHOصيغته الكيميائية
انصهاره 92-درجة سيليزية و درجة غليانه 19.5-درجة سيليزية .يحضر الفورمالدهيد على
نطاق تجاري من خالل تسخين الهواء الجاف و بخار الكحول الميثيلي في وجود عامل حفاز
مثل النحاس او الفضة .وحاليا امكن صناعة الفورمالدهيد من خالل التفاعل المباشر ألول
أكسيد الكربون مع غاز الهيدروجين بوجود عامل حفاز عند درجة حرارة تتراوح ما بين 200
و 300درجة سيليزية و تحت ضغط 7757تور .الفلورمالديهيد النقي نشط جدا كيميائيا و
يمكن بلمرته بسهوله و هو أيضا قابل للذوبان بسهوله في الماء مكونا محلوال يعرف باسم
الفورمالين و أيضا قابل للذوبان في الكحول و اإليثير .يستخدم الفورمالدهيد على نطاق واسع
في الصناعات الكيميائية حيث صناعة المركبات الكيميائية فهو يستخدم كثيرا في صناعة
الراتنجات الصناعية .الدراسات الحديثة أثبتت بأنه مادة مسرطنة لذلك يجب التعامل معه بكل
حذر
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