Russian Journal of Applied Chemistry, 2013, Vol. 86, No. 6, pp. 863−866.
Pleiades Publishing, Ltd., 2013.
Original Russian Text © L.N. Altshuler, 2013, published in Zhurnal Prikladnoi Khimii, 2013, Vol. 86, No. 6, pp. 922−925.
Phase Equilibria in High Pressure Melamine Synthesis
L. N. Altshuler
Research and Design Institute of Urea and Organic Synthesis Products, Dzerzhinsk, Russia
Received April 8, 2013
Abstract—A set of equations describing equilibrium compositions of liquid and gas phases in a reaction mixture
formed in the synthesis of melamine from urea at high pressures was derived. A numerical solution of the set in a
pressure range of 5–15 MPa, temperatures range of 350–400°C, and at an ammonia to urea ratio from 0 to 1 mol
enabled determining the equilibrium output of melamine and byproducts. It was shown that in the range of
parameters under study an increase in temperature and amount of ammonia introduced and a decrease in pressure
in melamine synthesis can lead to a higher content of impurities in the product.
According to the generally accepted conceptions
 synthesis of melamine from urea at high pressure
(5–15 MPa) and temperature of 350–400°C proceeds as
follows. Urea delivered to a reactor as a melt immediately
dissociates to form a gaseous mixture of ammonia and
Formed cyanic acid in the gas phase is decarboxylated
2HNCO → H
NCN + CO
with the formation of cyanamide, which is almost com-
pletely trimerized to form melamine:
NCN → С
A part of melamine is condensed to form of the liquid
The phase equilibria in the ammonia–melamine
and melamine–ammonia–carbon dioxide systems were
studied in [2–5] using static methods. The equilibrium
compositions of the phases in these systems at various
temperatures and pressures were found and it was shown,
e.g., that in the industrial synthesis of melamine at high
pressure the process takes place in a two phase system
liquid–gas, and therewith ammonia is soluble and carbon
dioxide, insoluble in the melamine melt.
In [6–8] a conversion of urea into melamine in indus-
trial synthesis without a separate analysis of liquid and
gas was found as well as an equation for calculating the
partial pressure of melamine in the melamine–ammonia
gas–liquid system . Moreover, the volume ratios of
liquid to gas phases were determined in the reactor of
melamine synthesis from urea  and calculation of the
equilibrium of melamine synthesis at low pressures in
the gaseous phase was performed .
Results of these studies are used in the design and
operation of the melamine plants. These results comple-
mented by data on the equilibrium compositions of the
liquid and gas phases and their relative proportions in the
real reactor of melamine synthesis from urea would solve
some other problems associated with the implementation
of the melamine synthesis and processing its products in
the industrial environment. The possibility of practical
use of the data on equilibrium compositions results from
the fact that, as shown in , the conversion of urea into
melamine in a ﬂ ow reactor is close to equilibrium already
at very low values of a residence time.
In this work, calculations of equilibrium composi-
tions and quantitative ratios of the phases in the reaction
mixture formed in the melamine synthesis from urea at