1. PRODUCTION OF FORMALDEHYDE AND UREAFORMALDEHYDE PRE-CONDENSATE (UFC)
1.1 PLANT DATA
The production plant consists of no. 4 units, FOR1 – FOR2 – FOR3 – FOR4 with a production capacity expressed as aqueous solution of formaldehyde 36wt%:
The 4 units are completely independent from each other and all equipped with a DCS process control system by Yokogawa that manages the process control as well as the automatic safety blocks of the plant in case of out of range conditions.
1.2 METHANOL VAPORIZATION AND MIXTURE PREPARATION
Liquid methanol is fed to the plant by means of centrifugal pumps, and injected in the evaporator/over- heater, which consists of a jacketed pipe heat exchanger.
The methanol vaporization in first stage, takes places by the condensation of steam (ca. 10bar) in the jacket side of the heat exchanger. In the second stage of the same heat exchanger the methanol is overheated until ca. 160°C, before to be mixed up with the process air.
A group of centrifugal fans feeds the gas, necessary for the oxidation of methanol. The gas is a partial recirculated gas with poor oxygen content, mixed with the necessary fresh air to ensure the minimum oxygen content for reaction.
Before entering in the reactor head, the overheated methanol gas is injected and mixed in the total gas flow.
The oxidation reaction of methanol to formaldehyde is executed in the tubular reactors of the plant with inside the tubes a dosed amount of catalyst (Fe-Mo oxides). The gas mixture of methanol and air passes the reactors in the direction from reactor head to the bottom, performing the chemical reaction inside the tubes at the active position of the catalytic bed.
The reaction is exothermal and the developed heat of the reaction is removed from the reactor in continuous to keep the reaction temperature constant.
A fluid heat-exchanging media, constituted by a molten salt mixture of potassium nitrate and sodium nitrite, is kept in circulation by means of a propeller pump in between the reactor tubes. The molten salt mixture on his turn is cooled by transforming a controlled amount of water in steam inside coils in the center of the reactor, emerged in the molten salt bath.
The gases are further cooled down in a group of tube and shell heat exchangers in the outlet of the reactor.
In the first step, water will be transformed in steam in the shell side. In a second heat exchanger the incoming gases to the reactor are pre-heated by cooling down the outlet gases.The steam, produced in the reactors and in the first heat exchanger in the reactor outlet, is collected in a steam/condensate separator to redistribute the steam in the plant. 1.4 ABSORPTION
The formaldehyde leaves the reactors in gas phase to be absorbed in water in the absorption column. The column exists of 2 sections:
Section 1Is divided in 2 recycles which are composed by a circulation pump, a plate heat exchanger, a liquid distributor and a column packing.
Section 2In this section is installed a series of perforated trays, each one provided with cooling coils and downcomers. This section acts to the definitive and final absorption of the residual formaldehyde, still present in the gaseous phase after the 1st section of the column.
The absorbing liquid (water) enters in the head of the 2nd section of the column to ensure a minimum liquid level on all trays. The quantity of liquid is in function of the minimum requirements of the column and of the desired concentration of the final product.
The gases enter from the bottom of the column and pass through the 2 sections, loosing the formaldehyde and heat into the liquid.
The most concentrated liquid is in the bottom of the column, which is the final product.
1.5 PRODUCTION OF UFC
UFC is a pre-condensation product of formaldehyde and urea, containing a high amount of formaldehyde and urea, always remaining an aqueous solution. UFC is the main raw material for the production of UF Resins. UFC can be produced in the same plants as the formaldehyde, only by changing the absorption parameters. Instead of absorbing the formaldehyde in pure water, the water is reduced to keep only the head of the column with a minimum layer of liquid to absorb the last formaldehyde before exiting from the column head, and in a lower tray an aqueous solution of urea is injected in the column as absorbing liquid.Absorbing the formaldehyde in a urea solution instead of water, changing the temperature sets of the trays cooling and the recycles and controlling the pH of the liquid in the column it is possible to produce UFC in the same plant.
1.6 EMISSION AND OFF PRODUCT TREATMENT
To ensure the necessary oxygen in the reaction gas mixture of air and methanol, it is necessary to purge on the recycle gases and let enter fresh air, as described in §1.2.The purge gas of the production unit, together with other gasses from aspirations, are united in one collector that leads to an incinerator.
In the incinerator all residual amounts of formaldehyde, unreacted methanol and other by-products are completely oxidized to CO2 and H2O before its definitive discharge in the atmosphere.
Following process, first step is the methanol vaporization and mixture preparation. Plant is feed with liquid methanol, by means of centrifugal pumps. Methanol is injected in the evaporator/over-heater, which consists in a jacketed pipe heat exchanger. Inside this exchanger methanol is vaporized by heating at 160 °C degrees, while steam is condensed, at 10 bar, in the heat exchanger jacket. Then methanol is mixed with process air. A group of centrifugal fans feeds the air, necessary for the oxidation of methanol. Gas is partial re-circulated, with a poor oxygen content, mixed with the necessary fresh air to ensure the minimum oxygen content for reaction. Before entering in the reactor head, the overheated methanol gas is injected and mixed in the total gas flow.
Next process step is reaction. An oxidation reaction of methanol to formaldehyde take place inside reactors, feed with a dosed amount of catalyst (Fe-Mo oxides). The gas mixture of methanol and air passes through reactors from head to the bottom, performing the chemical reaction. Reaction is exothermal and developed heat need to be removed continuously from reactor, to keep reaction at constant temperature. A cooling fluid, done with a mixture of potassium nitrate and sodium nitrite, is kept in circulation, by means of pumps, between reactor tubes. Molten salt mixture is cooled by water evaporation, which circulate inside coils, that are positioned in reactor center. Gases are cooled again in a group of heat exchangers. The steam, produced in reactors and heat exchanger, is collected in a steam/condensate separator and then redistribute to plant.
Third step is absorption. Formaldehyde that leaves reactors as gas phase needs to be absorbed in water by an absorption column. Column has two sections. First section is divided in two recycles, which are connected with a circulation pump, a plate heat exchanger, a liquid distributor and a column packing. Second section has a series of perforated trays, each one provided with cooling coils. This section perform final absorption of residual formaldehyde, still present in gaseous phase. Absorbing liquid enter in column second section to ensure a minimum liquid level on all trays. Liquid quantity depends on minimum column requirements and final product desired concentration. Gases enter from column bottom and pass through two sections, leaving formaldehyde in the liquid phase.
Third step could be used for urea-formaldehyde pre-condensate (UFC) production, by using appropriate process modifications and parameters settings. UFC is a pre-condensation product of formaldehyde and urea, containing a high amount of formaldehyde and urea, that remain an aqueous solution. UFC is the main raw material used for UF Resins production. UFC can be produced by same plants used to product formaldehyde, just with a change of absorption parameters. Instead of a complete formaldehyde absorbing, water is reduced to keep only head of the column with a minimum layer of liquid while, in a lower tray, an aqueous solution of urea is injected inside column as absorbing liquid. Absorbing formaldehyde in an urea solution, allow the production of UFC with same plant.
To ensure necessary oxygen in reaction, it is necessary to purge recycle gases. Production unit purge gas and other gasses from aspirations, are sent by a collector to incinerator. There all residual formaldehyde, un-reacted methanol and other by-products are oxidized to CO2 and H2O, before their discharge to atmosphere.
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Balance Raw Materials & Utilities
Ref. Production of Formaldehyde 45% - 28,000 TPY
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