Trading Off Fixed versus Operating Costs

As discussed in Section 2.5, the minimum allowable composition differences can be used to trade off fixed versus operating costs. TVpically, an increase in j leads to an increase in the MOC of the network (see Figs. 3.12 and 3.14) and a decrease in the fixed cost of the system. Hence, the minimum allowable composition differences can be iteratively varied until the total annualized cost of the system is attained (see Fig. 2.13). 

The other common approach for trading off fixed costs against operating costs is mixing of waste streams. In some cases, the plant operation and the 

Data of Waste Streams for DepheaollzaUon Example with Mixing of Waste Streams 

Flowrate Supply composition Target composition Stream Description G,, kg/s (mass fraction) y (mass fraction) yl 

CHAPTER THREE Synthesis of mass-exchange networks 

DiBia of Waste Streams for D heHoUzatimBi Example nUxlag of W te Streams 

Sutam Description Flowrate Gi, kg/s Supply cmnpositirin (mass fraction) y Target composition (mass fraction) yf 

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Hint vary the minimum allowable composition differences to iteratively trade off fixed versus operating costs). 

Ehie to the dilute nature of the stream, we may assume that the latent heat of condensation for MEK is much smaller than the sensible heat removed from the gas. Therefore, we can apply the procedure presented in Section 10.5. Once a value is selected for ATj ", Eq. (10.19) can be used to determine T and Eqs. (10.3b) and (10.7) can be employed to calculate the value of Since the bounds on p (and consequently on AT ) are tight, we will iterate over two values of 0.96 and 1.00 (Ar2 " = 90.0 and 94.5 K). The other iterative variable is AT, . Both variables are used to trade off fixed versus operating costs. As an illustration, consider the following iteration AT," = 5 K and... 

Trading off fixed versus operating costs using mass-load paths... 

Fl urf 2.13 Using mass transfer driving force to trade off fixed cost versus operating cost. 

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