High-pressure letdown rate

The pressure Pq before the pressure letdown valve is high enough to prevent any vaporization of feed at its temperature Tg and composition Xqj (mole fraction jth component). The forcing functions in this system are the feed temperature Tq, feed rate F, and feed composition x j. Adiabatic conditions (no heat losses) are assumed. The density of the liquid in the tank, is assumed to be a known function of temperature and composition. 

In a plant, both the steam pressure and temperature may be reduced for process steam use. This is because the process equipment is not rated for the high pressure and temperature. Also, steam latent heat at lower pressure is higher so less steam circulation is required with lower pressure steam. The device for reducing pressure is the steam letdown valve. 

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