Utility requirements

In some threshold problems, the hot utility requirement disappears, as in Fig. 6.10. In others, the cold utility disappears, as shown in Fig. 6.11. 

Example 6.1 The flowsheet for a low-temperature distillation process is shown in Fig. 6.19. Calculate the minimum hot and cold utility requirements and the location of the pinch assuming AT, m = 5°C. 

By comparison, Fig. 13.36 shows an exothermic reactor integrated below the pinch. Although heat is being recovered, it is being recovered into part of the process which is a heat source. The hot utility requirement cannot be reduced because the process above the pinch needs at least Q//m-,n to satisfy its enthalpy imbalance. 

Example 16.1 The process stream data for a heat recovery network problem are given in Table 16.1. A problem table analysis on these data reveals that the minimum hot utility requirement for the process is 15 MW and the minimum cold utility requirement is 26 MW for a minimum allowable temperature diflFerence of 20°C. The analysis also reveals that the pinch is located at a temperature of 120°C for hot streams and 100°C for cold streams. Design a heat exchanger network for maximum energy recovery in the minimum number of units. 

The amount of CO2 that must be removed from the gas stream is higher than for a natural gas feedstock. Process and utility requirements per 30,000 m (1.06 X 10 ft3) of 97% H2 gas are Hsted in Table 7. 

Table 7. Process and Utility Requirements of Producing Hydrogen by PSA ...

Effective utilization requires experienced or trained staff who may not be available in some companies. 

The cost of utilities is one of the most significant, yet difficult chores encountered in estimating operating costs. As discussed earlier, the amount of utilities required for both the process and the offsite areas must be estimated as accurately as possible. If utilities are generated in the project, the utilities required to operate the utility area must be included. Any increase in the project requires re-estiraating the utilities consumed in the utility area. This can result in a trial and error calculation to get the total cost of utilities. 

Process Conceptual Design Equipment selection and sizing Inventory of process Single vs. Multiple trains Utility requirements Overdesign and flexibility Recycles and buffer capacities Instrumentation and control Location of plant Preliminary plant layout Materials of construction As above plus equipment suppliers data, raw materials data, company design procedures and requirements... 

As can be seen from Fig. 10.4, the minimum cooling utility requirement is... 

The process engineer must summarize in some form the raw material and utility requirements for use by others. For example, the civil engineer is interested in waste water and sanitary sewer flows for proper layout studies. He is also in need of special requirements for site devel-... 

For checking and estimating purposes, follow the guides as to the number of stages, utility requirements, etc. presented herein. 

The utilities required for steam jet refrigeration operation often determine the selection of these units, between manufacturers and between types of refrigeration. As the chilled water temperature off of the unit approaches 32°F, the cost of the basic unit and its steam and cooling water requirements rise rapidly. 

Figure 11-9. Steam jet utilities requirements when producing 40°F chilled water temperature. (Used by permission Croll Reynolds Company, Inc.)...

Table 11-1 presents a few of the many possible operating conditions for these systems. The same refrigerating plant may operate through a wide range of pressures and temperatures. Table 11-2 presents a summary of utilities required for absorption plants. The steam indicated can be substituted by direct gas, waste heat, or other stream at an equivalent temperature level. 

Data for Determining Utilities Required for Absorption Plants... 

Diethanolamine is a favored absorbent due to its lower corrosion rate, smaller amine loss potential, fewer utility requirements, and minimal reclaiming needs. Diethanolamine also reacts reversibly with 75% of carbonyl sulfides (COS), while the mono- reacts irreversibly with 95% of the COS and forms a degradation product that must be disposed of. 

The air bleed is used to maintain a constant condition. However, a control valve may be used instead. Control or hand valves in the lower pressure vapor lines to an ejector are not recommended, as they must be paid for in system pressure drop and ejector utility requirements. 

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