Flash Vaporizer Dynamics
experiment water, or an ethanol/water mixture, is pumped into a
glass vessel containing a 1000 W electrical heater driven by a
variable transformer or by a commercial (Omega) PID controller. The
heater effluent flows to a 150 ml glass flash vaporizer. Vapor from
the vaporizer is condensed in a glass condenser, and the liquid
bottoms flow to a glass cooler. Flow rates are measured by
collecting and timing. With an ethanol feed the steady state
behavior of the flash vaporizer can be studied. If the feed is
water the apparatus becomes essentially a linear flow heater
controlled by a PID controller. Step response runs can be made
using the variable transformer to generate steps in heater power, or
by stepwise changes in feed rate. The students can set the PID
gains and the controller set point, and observe sluggish control,
good control, oscillatory decay of the error toward zero, or limit
cycle oscillations depending on the PID gains. A parameter
estimation program can be used to fit step response data.
Overhead and bottoms samples are most conveniently analyzed for ethanol using a gas chromatograph (optional).
Similar control studies can be done using an ethanol/water feed, but the system is no longer linear due to heat of vaporization and vapor/liquid equilibrium effects.
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