Unisim Turorial

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UniSim Design Tutorial for CHEE 399



Queen’s University Department of Chemical Engineering





2008



Introduction

UniSim (very similar to HYSYS) is a program used to design chemical plants. It is built around:

• a library of the physical properties of a large number of chemical species
• a set of subroutines to estimate the behavior of many types ofplant equipment (heat exchangers, reactors, etc.)
• a graphical user interface to accept specifications for the case, and display results


The user describes the process in terms of pieces of equipment interconnected by process streams, and the program solves all the mass/energy/equilibrium equations, taking into consideration the specified design parameters for the units.



In thistutorial, you will be introduced to some of the basic features of UniSim. Once you have completed this tutorial, it is highly recommended that you attempt to work through the advanced tutorial (available for download on the course webpage), as it will introduce you to additional unit operations and features. Upon completion of both tutorials, you will be well-prepared to dig into the plantoptimization assignments.



Separating CO2 from Synthesis Gas

In this year’s Integrated Design Project, we are interested in developing a process to obtain pure hydrogen (H2) from Synthesis Gas (syngas) which, for the purposes of this assignment, is a mixture of CO2, CO, H2 and H2O. The reaction governing the ratio of these gasses is:

CO + H2O ↔ H2 + CO2

If we drive the reactionequilibrium towards the production of H2, we will also increase ratio of CO2 in the gas mixture. This CO2 is then can be removed from the syngas using Selexol, a liquid absorbent. Since Selexol is a physical absorbent, decreasing the temperature and increasing the pressure will drive CO2 to dissolve into the liquid. Downstream, the CO2 can then be removed by decreasing the pressure which will causethe solvent to degas.

The amount of CO2 that the Selexol is capable of absorbing can be calculated based on the principles of vapour/liquid equilibrium (VLE). Since Selexol and CO2 are not chemically similar, gas solubility cannot be accurately predicted using an unmodified Raoult’s law (which would assume the fluids are ideal). Our choice of property package will determine how the modelbehaves when operating outside of conditions where ideality holds. If we use the UNIQUAC (UNiversal QUAsiChemical) model, gas solubility can be calculated using a set of binary coefficients. Experimental values for these binary coefficients are unavailable in the UniSim database. The UNIFAC (UNiversal Functional Activity Coefficient) method can be used to generate semi-theoretical estimates ofthese binary coefficients based on molecular size and shape differences and molecular interactions in the system. These binary coefficients can then be validated by comparing the solubilities predicted by the simulation to the manufacturer’s data (refer to Genosorb_data.pdf – use Genosorb 1753). If the solubilities are not being predicted correctly, the binary coefficients can be modified usingan educated trial-and-error basis.

We will develop a model capable of performing the solubility calculation. The binary coefficients of the UNIQUAC model will be adjusted to match the manufacturer’s solubility data. These binary coefficients will then be input into the Integrated Design Project process model.

The following diagram shows the process model we will use in our solubilitycalculation for CO2 in Selexol.

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To ensure the Selexol is saturated with CO2, a stream containing excess amounts of CO2 is mixed with a stream of pure Selexol. This mixed stream will contain two phases: a liquid stream of Selexol saturated with CO2, and a vapour stream consisting of excess CO2. The excess CO2 (beyond saturation) is removed using a (drum) separator, leaving us with the...