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Modeling Urea Processes: A New Thermodynamic Model and Software Integration Paradigm
(Special Shared Content with Virtual Materials Group)

Low and Medium Pressure Equilibrium

At low and medium pressures the mixtures are mostly concentrated solutions of water and urea with dissolved carbon dioxide and ammonia. A considerable body of work exists for sour water systems without dissolved urea (17, 18, 19, and 20). In this work, the model proposed by Edwards and co-workers (18) is used with specially determined interaction parameters between ammonia / urea and carbon dioxide / urea to properly account the presence of urea in the solution (21).

Equilibrium Reactor Modeling

A useful tool for mass and energy balances in a urea plant is an equilibrium reactor, which can be used to estimate the performance of actual reactors at optimum conditions (from a thermodynamic point of view). Which can be used as a first approximation for the synthesis reactor. Usually reactors with more than nine baffles approach the results one would get by assuming complete chemical equilibrium as reported by Uchino (5). Also, equilibrium reactors provide a convenient tool for initial studies on how water will affect the reactor performance and can replace empirical graphical relationships used in hand calculations (22, 23). For the we use the ionic reaction system defined by reactions 3-7. Comparisons between predicted and calculated results can be found in figure 9.

Figure 9: Error in Predicting CO₂ Conversion for Urea Equilibrium Reactor (24)

Modeling of specific urea processing unit operations

Several of the unit operations found in the urea process are not found in process simulators, and some ingenuity is required for their proper modeling. This section describes some of these unit operations and the steps taken for their modeling. The discussion is based on the Stamicarbon process.

Urea Synthesis Kinetic Reactor Model

Before the urea synthesis reactor model can be used for predictions, it needs to be tuned. There are two major parameters that are determined during the tuning process. These are a) determining the amount of ammonium carbamate in the reactor feed and b) the equivalent kinetic reactor volume. In order to do this, reactor performance and feed composition needs to be known for at least one operating point.

Determine the amount of Carbamate in the Feed

The feed composition is known in terms of CO₂ and NH₃ and not in terms of the amount of carbamate present. The first step is to use the UREA++ equilibrium reactor in order to compute the equilibrium carbamate leaving the reactor at the process reactor outlet temperature. In the equilibrium reactor, the urea reaction equilibrium constant efficiency is adjusted such that the actual CO₂ conversion is matched. Then the inlet carbamate content is adjusted (keeping the total amount of CO₂ and ammonia constant) to obtain an adiabatic reactor.

Reactor Kinetic Model

The plate type synthesis reactor can be modeled as a set of equilibrium and reactor stages. Since the Carbamate formation reaction is fast it can be modeled as an equilibrium reaction. The carbamate decomposition into urea is slow and is modeled as a kinetic (CSTR) reaction. The equilibrium constants for the carbamate formation are well known, as are the kinetic parameters for the carbamate decomposition into urea. It is found that for plate type reactors, 3 stages are often enough to model the synthesis reactor. A typical example is shown in Figure 10.

Figure 10: Kinetic Reactor Model

Determine kinetic reactor volume

The kinetic reactor volume of each stage can be adjusted such that the desired urea formation is achieved at the known process conditions. Thereafter the reactor model can be used for predicting the performance due to changing flows and compositions.

High Pressure Stripper Model

The high-pressure stripper is a carbamate decomposer. The high concentration of CO₂ pushes the carbamate decomposition toward completion. This unit-operation is a non-equilibrium process and cannot be modeled using standard equilibrium thermodynamics. The presence of the CO₂ strips the reactor products of its ammonia and CO₂. In addition, any CO₂ and ammonia produced by carbamate decomposition is also stripped by the flowing CO₂. This process seems to be mass transfer controlled, and it is currently modeled by assuming that all the free CO₂, ammonia and all the products of the decomposed carbamate get carried up with the stripping CO₂. Heat balances reveal that about 75% of the energy in the High Pressure Stripper is consumed by the carbamate decomposition and the rest is taken up as sensible heat. A component-splitter unit-operation such as the one provided by the HYSYS process simulator (25) is used to model this non-equilibrium process. Knowing the distribution of the energy for carbamate decomposition and sensible heat it is possible to create a semi-predictive model of the Stripper as steam and process flow changes.

High Pressure Scrubber

The vent from the synthesis reactor is scrubbed in this vessel. Some carbamate is formed and heat has to be removed from the system. There are two components to the removed heat: the sensible heat and the heat of reaction due to carbamate formation. The amount of carbamate formed can be back calculated from the process temperatures and the amount of heat supplied.

High Pressure Condenser

This unit operation supplies the feed to the synthesis reactor. As such the amount of carbamate formed and leaving this condenser is known (see Reactor Tuning). Hence this unit-operation can be modeled as a simple conversion reactor where the CO₂ conversion to ammonium carbamate is known.

Low Pressure Desorbers and Hydrolyzer Model

This part of the flowsheet can be directly modeled using Urea++. No special considerations are required. Predicted are within 0.5 °F of plant performance and predicted compositions are within 1% of plant measurements.

Software Implementation