1. Introduction - Firenze University Press

Fig. 3. Triple Pressure Bottoming Steam cycle: Pressure Levels are 100, 25, 5 bar [10]
3.4. Optimization of AZEP 100
A local optimum is determined with sequential quadratic programming (SQP) using the inbuilt
ASPEN Plus ® Optimizer. Overall, the objective is maximal efficiency keeping the ITM size fixed.
The power cycle has 14 optimization variables – 5 variables for the topping cycle, and 9 for the
bottoming cycle. To overcome numerical difficulties and limitations of ASPEN Plus ® , the power
cycle is optimized in two steps. First, only the topping cycle is optimized. Then, only the bottoming
cycle is optimized using the inlet streams to the bottoming cycle – "GTEXH" and "PRODBOTM" –
as input specifications, fixed to the optimum operating condition of the topping cycle. In principle,
this two-stage method does not guarantee local optimization of the entire cycle [25]. However, as
the efficiency of the gas turbine "GT" in the top cycle is higher than that of the bottoming cycle, it is
more efficient to extract work through the gas turbine in the topping cycle than the bottoming cycle.
In other words, to attain maximum efficiency of the total power plant, maximum possible power
extraction should take place in the top cycle, transporting minimum thermal energy to the
bottoming cycle. Thus, sequential optimization of the topping cycle followed by the bottoming
cycle is believed to give the optimum of the entire power cycle.
3.4.1. Optimization of Top Cycle
The one-dimensional intermediate fidelity ITM model makes it impossible to optimize the ITM
geometry. Attempting to minimize the pressure drop would result in an infinite number of permeate
and feed channels, which are extremely small in length. Moreover, the ITM is an expensive
component, so optimization of the ITM size would require accurate estimates for its cost which are
not available since ITM is a very new technology. Hence, the topping cycle is optimized by varying
operational parameters such as mass flow rate, temperature and split fractions, while the ITM size is
kept fixed. More specifically, the degrees of freedom are the mass flow rates of "AIRMCM" and
"AIRREST", the split fraction of "B10", and the cold side outlet temperature of "BHEX". Varying
"AIRMCM" varies fuel flow rate since flue flow rate is stoichiometrically related to the amount of
O2 separated in ITM to ensure complete combustion.
As the mass flow rate of "AIRMCM" increases, the amount of oxygen separated by a fixed size
ITM also increases. This corresponds to an increase in fuel flow rate and greater compressor power
(decrease in efficiency). At the same time, the power output from the turbine increases (increase in
efficiency). The combination of these opposing effects provides scope for optimization.
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