Thermo-Cognitive Advanced Process Control (TC-APC) models, simulates and optimizes your process in real time to improve yield, conversion and throughput, and reduce energy use, with a quick payback.
Reaction selectivity, nitration and Grignard
Temperature buys conversion and hazard in the same movement. Nothing tells you which one you have just bought.
Here the margin is not only money. It is the reason nobody has been hurt.
So it stays wide, and it should, until something can say where you actually are.
One knob, two outcomes, and only one of them is visible.
In a great many industrial reactions the thing you want and the thing you are afraid of respond to the same variable in the same direction. Push the temperature, the feed rate or the dosing and you get more conversion. You also get more of the side chemistry, and the side chemistry usually accelerates faster than the product does.
The two are not symmetrical. Yield that you did not get is a smaller drum. The side chemistry, in the reactions on this page, can be an energetic byproduct or a runaway. One of those is an accounting entry and the other is an incident.
So the plant runs well back from the boundary, and it is right to. What it cannot do is know how far back it is, because the quantity that would tell it is not measured while the reaction is running.
Published analysis, in Chemical Weekly, October 2025.
The two curves do not have the same shape. The product saturates. The side chemistry keeps climbing, which is why the boundary exists and why it is treated as absolute.
The boundary never moves. The only thing that can move is how well you know where you are standing relative to it.
Nitration
Where the chemistry favors the isomer you did not want.
The thermodynamics are against you
In aromatic nitration the distribution of isomers is set by the chemistry, and in several commercially important cases it favors the one you are not selling. The valuable isomer is the minority product before you do anything at all.
Temperature narrows the gap
Which is why operators reach for it. More conversion, and a shift in the balance. It is the obvious lever and it is the only one the control system really offers.
And opens a second door
The same temperature accelerates the formation of polynitrated byproducts, which are energetic materials. So plants deliberately under-convert, accepting product left in the residue as the price of staying clear of them.
Product left in the residue is a smaller drum, and it appears in no report.
The other failure is not the kind you write up afterward.
So the deliberate under-conversion is not conservatism to be engineered away. It is a correct decision made without the information that would let it be a smaller one. A kinetic model of the system, carrying the competing pathways and the temperature dependence of each, can say where the batch actually is at any moment rather than where the recipe assumes it is.
Grignard
The problem is not the reaction. It is the wait.
Initiation is unpredictable. Sometimes it starts almost at once, sometimes after a long induction period, and the same vessel with the same charge will not always behave the same way. Once it does start it can proceed quickly and release a great deal of heat.
That puts the operator in an awkward position during the wait. Dose cautiously and you may sit there a long time, with batches that run long and occasionally do not initiate at all. Dose more freely and unreacted material accumulates in a vessel that has not yet begun to react, which is the condition every process safety assessment is written about.
What the control system is doing
Following a timer, or a fixed recipe written for the slowest initiation anybody has seen. It has no way to tell an induction period that is about to end from one that is going to continue, because nothing in the loop is watching for the difference.
What initiation actually looks like first
It announces itself before it arrives, in small movements in temperature and in the electrical character of the mixture, which are far below the threshold of anything the recipe reacts to. A model that knows what it is looking for can see them.
If initiation can be detected as it begins rather than inferred after it is obvious, then dosing and agitation can be modulated to match it, and the accumulation that makes the hazard never builds in the first place. That is a prevention argument rather than a yield one, and it is the right way round for this reaction.
Condensation
And a third, where the window is in time rather than temperature.
Dose too slowly
The reaction becomes sluggish and the side reactions, which are not waiting for you, take a larger share of the material.
Dose too quickly
Unreacted material accumulates and turns up later as impurity, in a batch that looked perfectly normal while it was being made.
And the quench has to be timed
Too early and yield suffers. Too late and purity does. The moment of maximum yield and acceptable purity is real, it is short, and it is not the same moment in every batch.
This is the same problem as every other application on this site. The variable that determines the money is not measured, so the plant runs a fixed recipe with a margin built into it. What differs here is that part of the margin is protecting something other than the yield.
What TC-APC does about it
It tracks the reaction, not the clock.
A kinetic model, not a schedule
The competing pathways and their temperature dependence are carried in the model, so the state of the batch is estimated continuously instead of assumed from elapsed time.
Small moves, continuously
Feed and temperature profile are adjusted in small increments against that estimate, rather than in the step changes a recipe produces when it reaches its next instruction.
The constraint is hard, never traded
The hazardous regime enters the objective function as a boundary that cannot be crossed, not as a term weighed against yield. There is no setting in which the engine trades safety margin for conversion.
The gain, where there is one, comes from spending less of the batch further from the boundary than you needed to be, because you now know where you are. It does not come from moving the boundary.
What this cannot do, and must not be asked to do.
It is not a safety system
Your safety instrumented system, your relief design, your interlocks and your hazard assessment are what protect the plant. This is a supervisory layer above the control system and it writes nothing into any of them. If anybody offers you a controller as a substitute for that, decline it.
It cannot change the chemistry
Where the isomer distribution or the thermodynamics cap what is achievable, no control policy moves that. The answer there is different chemistry or a different route, not better control of this one.
It cannot make a badly instrumented vessel safe
An estimator on a reactor with poor temperature coverage or sluggish measurement will report accurately that it cannot see enough. That is useful information and it is a reason to stop, not to continue carefully.
It does not replace your laboratory
In-process analysis stays the reference. The estimate interpolates between results and the disagreement between the two is displayed rather than hidden.
The mark
Find the optimum. Hold it while it moves.
The curve is a yield or selectivity response. The amber point is the optimum, and the dashed drop is where it sits on the operating axis.
The second half of the line is the part that matters. Finding an optimum is a study, and studies end up in drawers. Holding one while the feed, the fouling, the weather and the wear keep moving it is a control problem, and that is the work.
Start with the published analysis
Optimisation of chemical processes using hybrid technology
The nitration, condensation and Grignard cases as they appeared in Chemical Weekly, October 2025, alongside carbonation and sugar crystallization.
Or start with your own batch records
Send batch data from one reaction with the in-process results beside it
We will tell you how much of the batch is being spent further from the boundary than it needed to be, with the uncertainty stated openly. No charge, and no commitment either way. Including the answer that there is nothing here worth pursuing.
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