GB1037626A - Process for absorbing formaldehyde from gases containing formaldehyde, with formation of a concentrated solution containing formaldehyde and melamine - Google Patents

Abstract

Formaldehyde is completely or substantially completely absorbed from a formaldehyde-containing gas in two or more absorption stages by a continuous process in which in the first stage the said gas is washed with an aqueous solution of formaldehyde and melamine and in the last stage gas not absorbed in a preceding stage is washed counter-currently with water; solution resulting from the absorption in the or each stage after the first is used in conjunction with part of the solution produced in the preceding stage as washing liquid in said preceding stage and that part of the solution produced in the first stage which is not so recirculated is discharged as final solution; melamine is added to the system to replace that discharged and the rate of supply of said melamine, and of the water to the last stage, and of the discharge of said final solution in relation to the rate of supply of formaldehyde to the system being such as to maintain in the first stage washing liquid 15 to 25 moles of formaldehyde per mole of melamine and 25 to 55% by weight of water. The absorption stages, which may be packed on plate columns, are preferably maintained at 45 DEG to 60 DEG C. Replacement melamine can be added to the solution passing to the first absorption column. The circulating absorption solutions should be stabilized by keeping said solutions alkaline, e.g. at pH 8, except where methyl alcohol is present in the gas supplied to the system.ALSO:Formaldehyde is completely or substantially completely absorbed from a formaldehyde-containing gas (5) in two or more absorption stages (1, 2) by a continuous process in which in the first stage the said gas is washed with an aqueous solution of formaldehyde and melamine and in the last stage gas not absorbed in a preceding stage is washed counter-currently with water; solution (10) resulting from the absorption in the or each stage after the first is used in conjunction with part of the solution (13) produced in the preceding stage as washing liquid in said preceding stage and that part of the solution produced in the first stage which is not so recirculated (6) is discharged as final solution; melamine is added to the system to replace that discharged and the rate of supply of said melamine, and of the water to the last stage, and of the discharge of said final solution in relation to the rate of supply of formaldehyde to the system is such as to maintain in the first stage washing liquid 15 to 25 moles of formaldehyde per mole of melamine and 25 to 55% by weight of water. The absorption stages, which may be plate or packed columns, are preferably maintained at 45 DEG to 60 DEG C. Replacement melamine can be added to the solution passing to the first absorption column. The circulating absorption solutions should be stabilized by keeping said solutions alkaline, e.g. at pH8, except where methyl alcohol is present in the gas supplied to the system. The numerals refer to Figs. 1 and 2 (not shown) which respectively illustrate a two stage and a three stage absorption process.