Thermophysical Modeling: Why was UREA modified for the MSE model

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The Background

Circa 2009 OLI Systems was asked by one of its customers to model the decomposition of Urea in aqueous solutions. OLI undertook this modeling and created a model. The basic chemistry of urea decomposition is as follows:

Urea: (NH2)2CO

Water: H2O

Ammonium Carbamate: NH4NH2COONH2

Ammonia: NH3

Carbon Dioxide: CO2

(NH2)2CO + H2O = 2NH3 + CO2

There are several side reactions which OLI attempted to model as well. Two molecules of urea can combine to form a species called biuret with a loss of an ammonia molecule.

Biuret: (CONH2)2

The reaction is:

2(NH2)2CO = (CONH2)2 + NH3

In addtion, another side reaction is the formation of ammonium cyanate from urea.

Ammonium Cyanate: NH4OCN


Finally Ammonia Cyanate decomposes to isocyanic acid and ammonia

Isocyanic Acid: HOCN


The Problem

Almost immediately after the urea decomposition work had been completed there were complaints from the user community. In normal Ammonia and/or CO2 scrubbing or stripping unit operations, OLI was now predicting urea formation. This is not normally reported in the field. In fact if a model had NH3 or CO2 then urea would be formed. In addition, all the side products would also be formed. This concerned the OLI user community so much that a temporary fix was created.

OLI created the species UREAINERT for both the vapor and aqueous phase. It was thought that if the user wanted to use Urea as a species then the inert species would solve the problem. Of course it did not in that if carbon dioxide and ammonia were present then "Active" urea would appear along with the side products.

There was no immediate fix to this problem.

The solution

With the release of the OLI software for 9.2.1 a massive amount of work was undertaken to fix this problem. The first issue was that since the formation of urea and its decomposition products only occur at elevated temperatures (relative to most stripping operations) these species were moved to a new databank called UREA. This databank is now available to all users of OLI software as an OLI provided databank in a similar fashion that GEOCHEM is used. The following species were moved to this new databank:

OLI Tag Name Formula Display Name
NH4OCN NH4OCN Ammonium cyanate
BIURET.1H2O (CONH3)NH.1H2O Biuret monohydrate
OCNION OCN-1 cyanate ion (-1)
HNCO3 (HNCO)3 Cyanuric acid
HNCO HNCO isocyanic acid
UREA CO(NH2)2 Urea
UREANH3 CO(NH2)2.NH3 Urea Amine

Since these species were removed, the entire databank had to be re-regressed to literature data to reflect that these species had been removed. Then the data in the urea databank had to be re-regressed to the new data to be consistent.