The heat capacity flow rate is given by CP (kW/K) = FCp(T-To). The heat exchanger network featured four cold streams with combined enthalpy of 141,195.65 kW, and ten hot streams having total enthalpy of 138,583.12 kW. It is clear from the capacity of hot and cold streams presented in Table 4 that the load available in all hot streams is more than those available in the cold streams.
In order to generate targets for minimum energy targets, the ΔTmin value was set for the problem, with an initial value of 10 oC. The ΔTmin or minimum temperature approach is the smallest temperature difference that was allowed between hot and cold streams in the heat exchanger where counter-current flow was assumed. This parameter reflects the trade-off between capital investment (which increases as the ΔTminvalue gets smaller) and energy cost (which goes down as the ΔTmin value gets smaller). For the purpose of this study, typical ranges of ΔTmin values that have been found to represent the trade-off for each class of process have been used.
The retrofitted HEN grid diagram for the base case design is shown in Figure 5. It can be observed from the CDU network that there is little restriction on heat exchange between the cold process and the large hot process streams. This results in wastage of useful heat energy which is also unsafe for the environment and the plant operators. Hence, in the retrofitted network, cold utility (cooling water - CW) was introduced in order to make up for the deficient heat sink (cold process streams) to exchange heat with the heat source (hot process streams). The temperature versus enthalpy plot or composite curve is shown in Figure 6.