Page 237 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

(at the lowest temperature) and rejects heat to the secondary working fluid that rejects its heat to cooling
water at 45°C. A simplified diagram of a cascaded refrigeration system is shown in Figure 8.5.

Figure 8.5 Schematic Diagram of a Simple Cascaded Refrigeration System

Steam Production. Steam is produced by the evaporation and superheating of specially treated water.
The fuel that is used to supply the energy to produce steam is by far the major operating expense.
However, water treatment costs can be substantial depending on the supply water composition and the
degree of recovery of condensed steam in process heat exchangers. As shown in Table 8.3, for large
chemical plants, steam is often required at several different pressure levels. However, it is often
generated at the highest level and then let down to the lower pressure levels through turbines. These
turbines produce electricity used in the plant. A typical steam generating facility is shown in Figure 8.6.
Because there are losses of steam in the system due to leaks and, more important, due to process users not
returning condensate, there is a need to add makeup water. This water is filtered to remove particulates
and then treated to reduce the hardness. The latter can be achieved by the addition of chemicals to

precipitate magnesium and calcium salts followed by filtration. These salts have reverse solubility
characteristics and therefore precipitate at high temperatures. Alternatively, an ion-exchange system can
be employed. The solids-free, “soft” water is now fed to the steam generating system. The thorough
treatment of the water is necessary, because any contaminants entering with the water will ultimately
deposit on heat-exchanger surfaces and boiler tubes and cause fouling and other damage. Another
important issue is the dissolved oxygen and carbon dioxide that enter with the makeup water. These
dissolved gases must be removed in order to eliminate (reduce) corrosion of metal surfaces in the plant.
The removal occurs in the deaerator, in which the makeup water is scrubbed with steam to de-gas the
water. Oxygen scavengers are also added to the circulating condensate to remove any trace amounts of
oxygen in the system. Amines may also be added to the water in order to neutralize any residual carbonic
acid formed from dissolved carbon dioxide. Finally, blowdown of water from the water storage tank
(situated near the boiler) is necessary to remove any heavy sludge and light solids that are picked up as
steam and condensate circulate through the system [12]. The problems associated with the buildup of
chemicals become even more troublesome in high-pressure (>66 bar) boilers, and several solutions are
discussed by Wolfe [13].

Figure 8.6 Typical Steam Producing System for a Large Chemical Facility