I have an ethoxylation process with temperature split range control question.

First I would like to introduce this process a little bit. The ethoxylation process is to produce non-ionic surfactants and PEG(polyethylene glycol). Fatty alcohols, or alkyl phenol are typically used as reactant with ethylene oxide. The reactor type is semi-batch liquid stirred reactor, equipped with a circulation loop to increase the reactor heat exchange capacity, and liquid EO is continuous dispersed into the gaseous one, followed by post-treatment operation such as catalyst removal to achieve the required quality of the product. The usual operation pressure in this process is 4~5 bara. Nitorgen will be purged to the reactor to assure the EO gases composition within the safety limits. Besides that, the reactor temperature is between 120 degree C to 180 degree C and is kept constant by external regulation.

This PID(TIC-120) controller to keep the reactor temperature constant. The heat removal is made by water recirculation cooler and heat addition is made by water recirculation heater. TIC-120 measures the temperature at inlet of reactor heat exchanger by the temperature transmitter TE/TIT 120 and acts on following valve,TV-120A, TV-120B and TV-120C. ( TIC -120 output and TV-120A, B and C position is on the attached pdf) TIC-001 measures the reactor bottom line temperature by TE/TIT-001 and act on set-point of 120 in cascade mode.

My question is the temperature control loop T-120.
Why do we have to set TIC-120 middle output as 50% instead of 70% to make valve(ball valve) action?
My reason is that the slower valve should require longer time to react.
It seems to for system take longer time to shut the TV-120A(steam valve close/open) so set TIC-120 output middle value as 70% seems more approaiate.

You will probably have a reason that you entered this question. It appears to me that it isn't just curiosity on the TIC-120 percentage setting for the valve TV-120A.

It appears to me that there are some mistyping errors in your data:
?    In the table TV-120A is closed at 50% of TIC-120, which conflicts with the graphic.
?    In the graphic it seems that the "TIC-120 output" and "Valve position" are mixed up.

If I understand your data correctly, following appears to happen starting with full heating:
?    At 0% TIC steam is full open on the steam HX. There is no cooling by the CW HX. The valve TV-120B is fully open.
?    If the temperature of the reactants at the discharge of the Reactor HX becomes too high, the TIC will give a higher value e.g. 20%. The steam supply will be reduced. Part of the water returning from the Reactor HX will be diverted to the CW HX.
?    At 50% TIC there will be no steam supply. Part of the water is diverted to the CW HX for cooling and the remainder bypasses the CW HX through the valve TV-120B.
?    In the range 50 ... 100 % TIC the quantity of cooling is increased to full CW HX cooling at 100% TIC.

Whether a change in the closed position of valve TV-120A from 50% to 70% TIC would be beneficial, cannot be answered without detailed mass and heat balances.

You have an existing situation with HXs with a specific heat exchanging surface. An increase in TIC percentage for the closed position of TV-120A could be beneficial, if you have too abrupt cooling in the present situation, but note that flow quantities are changing.

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