Читать книгу Process Intensification and Integration for Sustainable Design - Группа авторов - Страница 45

PRI is a relative comparison. The exact values from the calculations are not meaningful, only the relative comparison between different process routes, or in this case different incoming feeds. Only streams where a significant change in composition, pressure, or fluid density occurs were included in the calculation.

One significant weakness of PRI is that is uses the overall average value of each property for all the streams, effectively treating every stream considered as if it has the same flow rate (and thus same potential risk). This leads to misleading results; to give a more truthful picture, the values of each of the four properties used to calculate PRI (mass heating value, fluid density, pressure, and explosiveness) were multiplied by their flow rates as presented in Table 2.9.

Table 2.9 Results from modified process route index calculations.

	Feed #1	Feed #2	Feed #3	Feed #4	Feed #5	HA feed
Overall	19.48	27.85	31.83	34.03	34.43	26.83

The lower methane cases are less safe to process. This is because the flows are generally larger in these designs because dehydration is more difficult, and because the fractionation process has higher flow rates due to higher NGL content. Nonetheless there is really not a significant difference in these process designs from a safety perspective. Therefore economic criteria should be the main concern for deciding whether or not to treat streams.