Chapter 4, Problem 4.49P

Boilers are used in most chemical plants to generate steam for various purposes, such as to preheat process streams fed to reactors and separation units. In one such process, steam and a cold process fluid are fed to a heat exchanger where enough energy is transferred from the steam to cause a large fraction of it to condense. The uncondensed steam is vented to the atmosphere, and the liquid condensate is recycled to a deaerator into which another liquid stream (makeup water) is fed. The makeup water contains some dissolved impurities and other chemicals that help prevent deposition of solids on boiler walls and heating elements, which would lead to a reduction in operating efficiency and eventually to safety hazards, possibly including explosions. The liquid leaving the deaerator is the feed to the boiler. In the boiler, most of the water in the feed evaporates to form steam, and some of the impurities in the feedwater precipitate to form solid particles suspended in the liquid (kept in suspension by the chemical additives in the makeup w ater). The liquid and suspended solids are drawn oft' as boiler blowdown, either in manual bursts or w ith a continuous blowdown system.

A diagram of the system is shown below. The symbol I is used for combined impurities and chemical additives. The makeup water contains 1.0 kg 1/2.0 X 10³ kg HjO, and the ratio in the blowdown is 1.0 kg 1/3.5 X 10² kg HiO. Of the steam fed to the heat exchanger, 76% is condensed.

1. In your own words, describe why makeup water, chemical additives to the makeup water, and blowdown are necessary in this process. Speculate on the probable disadvantage of making the 1/HsO ratio in the blowdown (I) too small, and (ii) too large.
2. Assume a basis of calculation and draw and fully label a flow chart of the process. (When you draw the heat exchanger you can omit the process fluid, which plays no role in the problem.)
3. Carry out a degree-of-freedom analysis and outline a solution procedure (which equations would you write in which order to calculate all of the unknow ns on the chart?).

(d, Calculate the ratio (mass of makeup water/lOOkg steam produced in boiler) and the percentage of the boiler feedwater taken off as blowdown.

(c) A proposal has been made to use highly purified water as makeup. List the benefits that would result from doing so and the most likely reason for not doing it.