Page 123 - Sustainability in the Process Industry Integration and Optimization

100 Cha p te r F o u r

steam pressure levels, which are represented by their corresponding
saturation temperatures. The main distribution levels are high-
pressure (HP) steam at a saturation temperature of 200°C , medium-
pressure (MP) steam at 170°C, and LP steam at 115°C. In addition,
very-high-pressure (VHP) steam at 250°C is generated by the steam
boilers, and CW is also available.
The Site Source Profile in Figure 4.67 can generate at most 2 MW
of HP steam, another 2 MW of MP steam, and 15 MW of LP steam.
The rest of the heat sources will need to be served by the CW, which
completes the Site Source CC. The Site Sink CC, indicates needs for 1
MW of LP steam, 6.5 MW of MP steam, 2.5 MW HP steam, and the
remaining 5 MW demand to be satisfied by VHP steam. The two site
CCs can be overlapped in the same way as were the process CCs,
thereby illustrating the Total Site heat recovery possible through the
steam system. Figure 4.68 shows the corresponding site CCs
overlapped. The amount of heat recovery for the Total Site is indicated
by the amount of overlap between the CCs. Heat recovery is
maximized when the two Site Composite Curves touch and cannot
be shifted further. The area where the curves touch, which is usually
confined between two steam levels (here, the MP and LP levels), is
the Total Site Pinch. The steam mains located at the Site Pinch feature
opposite net steam loads; in other words, the steam main above the
Site Pinch is a net steam user while the one below the Site Pinch is a
net steam generator. Just as with the Process Pinch, the site Pinch
divides an overall heat recovery problem into a net heat source and a
net heat sink.

250 VHP

200 HP
Temperature [°C] 150 MP

LP
100

Heat recovery via steam
CW
50
−25 −20 −15 −10 −5 0 5 10 15 20 25
Enthalpy [MW]
FIGURE 4.68 Targeting heat recovery.