CONVENTIONAL INSTRUMENTATION M E R L E F. B R O W N
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Enthoipy computer
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I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
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temperature variation. Also B.t.u. content is changed by phase change (liquid to vapor) of the flowing stream. Enthalpy takes into account both of these types of heat changes and, therefore, is a true measure of flow-heat content. For distillation columns and heat exchangers, enthalpy changes are now being used to control heat added. The electronic multiplier-divider with other electronic computing elements is ideally suited for continuous enthalpy control, particularly where. distillation feed streams must be partially vaporized at the bubble point prior to entering the column. The liquid feed stream must contain enough heat to tlash into a vapor state as it enters the column. In Figure 1, the flow of saturated steam is used in a preheater to keep enthalpy of the feed stream at a predetermined constant value. Assuming that heat losses of the process piping are s m a l l enough to be neglected, enthalpy of the feed stream entering the column is simply equal to the heat content of the feed upstream of the preheater plus the heat added by the preheater. The following equgtion can be used to compute enthalpy of the feed stream entering the column. The computed enthalpy is then used as the process variable in a simple control loop (Figure 1). The set point of this control loop would determine the heat content of the feed stream entering the column. Basic heat balance equation:
M F 4 T iMQJL mass flow of feed stream mass flow of saturated steam specificheat of feed stream ( T I - TO). T I = temperature of feed stream, before the preheater; and TO = base temperature HI = enthalpy of feed stream, entering the column (above base temperature) P. = steam pressure K , = saturated steam constant
equilibrium ( H , is now a preset constant). The steam flaw requirement is then used to set the set point of the controller (cascade control). The computer equation would now be:
M, =
MIHr
- M,CAT P A
In many applications, in addition to the feed preheat, there is also a bottom’s product heat exchanger (2) which must be considered. In this case, the heat added by the flow through the exchanger must also be taken into account. The enthalpy computer is just one of many applications to which electronic static computing instruments
Miff, M, M,
= = = C, = AT =
Converting the basic equation for enthalpy control:
H,
M&&T
= -
+M Z A total B.t.u. per pound of feed
Simplifying the equation:
H,
=
CAT
+ M.P.K. -= total B.t.u. per pound of feed MY
may be applied. Some outstanding features of this type of computer applications are: -Maximum reliability -System accuracy as high as 1/2% -No moving pacts, such as bellows, diaphragms, servomotors, slidewires, or nozzle baffle assemblies --Essentially zero deadband and response times of approximately 3 db. to 4 c.p.8. -Maximum instrument density. The enthalpy computer in the example used can be packaged in a case, 9 X 6 X 15inches
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A schematic diagram showing the electronic computing instruments necessary to solve the equation for enthalpy is shown in Figure 2. If the heat content requirements of feed stream are known or can readily be predetermined, an alternate control system can be used. The computer is set up to determine the amount of steam required to maintain
The enthalpy computer application is designed to acquaint process instrument engineers with new potential tools which the electric instrument systems offers. The process economics of an application will determine the justification for an analog computer. Most major manufacturers of electronic instrumentation have systems application engineering able to assist the customer in “building up” an electronic analog computing system.
Mnle F. Brown is an application engineer, Instrument Dcpmhncnt, Gmnal Hectric Co., West Lynn, Mass.
(1) Ogleaby, M.W.,Lupfer, D. E.,C a f d E n g . (February 1962). (2) Perry, J. H. (ed.), Chemical Engineem Handbook, 3rd ed.
REFERENCES AUTHOR
VOL 55
NO. 9
SEPTEMBER 1963
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