ABSIM - modular simulation of advanced absorption systems

Abstract The computer code ABSIM has been developed for simulation of absorption systems in a flexible and modular form, making it possible to investigate various cycle configurations with different working fluids. Based on a user-supplied cycle diagram, working fluid specification and given operating conditions, the program calculates the temperature, flowrate, concentration, pressure and vapor fraction at each state point in the system and the heat duty at each component. The modular structure of the code is based on unit subroutines containing the governing equations for the system's components. A main program calling these subroutines links the components together according to the cycle diagram. The system of equations for the entire cycle is thus established, and a mathematical solver routine is employed to solve them simultaneously. Property subroutines contained in a separate database serve to provide thermodynamic properties of the working fluids. ABSIM has been employed over the past decade by many users worldwide to simulate a variety of absorption systems in different multi-effect configurations and working fluids. The paper will describe the current capabilities of the program and recent improvements made in it. Improvements to the method of cycle specification and solution have enhanced considerably the convergence capability with large and complex cycles. Additional units and working fluids have been added, resulting in much-enhanced simulation capability and applicability. A Windows version has recently been developed with an improved user-interface, which enhances user-friendliness considerably. It makes it possible to create the cycle diagram on the computer screen, supply the data interactively, observe the results superimposed on the cycle diagram and plot them. The paper describes examples of simulation results for several rather complex cycles, including lithium bromide–water double-, triple- and quadruple-effect cycles and ammonia–water GAX, branched GAX and vapor exchange (VX) cycles.