Steady State Modeling of Urea Synthesis Loop

Urea, which is known to be an important petrochemical product, is mainly
used as fertilizer. Urea (NH2CONH2) is produced commercially by reaction of
ammonia (NH3) and carbon dioxide (C02), under conditions depending on each
particular plant technology. There are a lot of urea synthesis technologies available
such as Snamprogetti process, Stamicarbon process and etc. In most operating process
the synthesis reaction is carried out in the liquid phase, at pressure from 13 to 25 MPa
and at temperature between 170°C and 200°C. In this study, a simulation is developed
specifically for the high pressure urea synthesis section of the ABFPlant; which adopt
Stamicarbon process. In this study, the formation of ammonium carbamate is
considered to occur through the heterogeneous reaction of carbon dioxide and
ammonia. In present study, the urea reactor is divided into three reactors namely the
equilibrium reactor where the ammonium carbamate formation take place, and two
continuous stirred tank reactors in which the urea formation and biuret formation are
taking place respectively. The validity of the proposed simulation was demonstrated
using the actual plant data from ABF Plant. From the Aspen HYSYS simulation
result, it shows that the simulation could predict the behavior of the urea reactor as
well as the urea synthesis section as per literature but could not give the accurate
value. The CO2 conversion in the first equilibrium reactor is calculated by Aspen
HYSYS to be approximately similar as found in the ABF Plant; that is 60% (with
5.2% error) while Polymath 5.1 calculates the C02 conversion in the equilibrium
reactor to be approximately 60% which also agrees with the Aspen HYSYS and the
ABF Plant data. For the second CSTR reactor, HYSYS and Polymath 5.1 calculate
the conversion of ammonium carbamate to urea to be 92.75%) and 94% respectively
which agreed with each other. The overall conversion of CO2 to urea in the high
pressure urea synthesis loop with the recycle stream is calculated by HYSYS to be
approximately 89.73%) which is 10% higher than is found in the ABF Plant. The urea
yield formed in the high pressure urea synthesis loop is calculated by HYSYS to be
90.86%) which is 5.5%) higher than is found in the ABF Plant. From the study
conducted, it is found that this simulation could be used to predict the CO2conversion
as well as urea yield obtained while the throughput is varied within 10%o error.