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I have spent some time researching basic magnesium chemistry. Not anything synthetic but more safety and thermochemically related. I am not able to give a lot of particulars motivating the study, but I can say that one should consider that nitrogen over activated magnesium may not be as innocent as you think. While lithium is widely known to react with nitrogen gas to form a passivating nitride layer, the reaction of dinitrogen with magnesium is rarely encountered.

Activated magnesium residues from a Grignard or other magnesium metallation reaction may self-heat to incandescence under a nitrogen atmosphere in the right circumstances. Activated residues left isolated on the reactor wall or other features in a nitrogen blanketed reactor during an aqueous quenching procedure may self-heat to incandescence. In the presence of reactive gas-phase components like water vapor in nitrogen, activated metals can self-heat over an induction period of minutes to hours or longer.

Many metals, including magnesium and aluminum, can be rendered kinetically stable to air or humidity by the formation of a protective oxide layer. Once heated to some onset temperature by a low activation reaction, penetration of the protective layer by reactive gas composition can occur, leading to an exothermic reaction.

Performing a “kill reaction” or a quench of a reactive metal at the bench or at scale is always problematic and requires the skill and close attention of the process chemists and operators. I guess what I’d like to pass on is that nitrogen is not an innocent spectator in the presence of finely divided, activated magnesium. Humid nitrogen can support a combustion reaction to produce nitrided magnesium once preheated to an onset temperature.

If you mean to kill any reactive residues, it is important to apply the quenching agent in such a manner that the heat generated can be readily absorbed in the quenching medium itself. A good example of a quenching agent is water. Often a reactive must be killed slowly due to gas generation or some particular. Adding a quenching agent to a solution or slurry by slow feed or titration may be your best bet. If you have another vessel available, a feed to a chilled quenching agent will also work.  Dribs and drabs of water on a neat reactive material will lead to hotspots that may be incendive.

I enjoy working with our RC1 reaction calorimeter. As we get more experience with thermal profiles of reactions, the utility of this instrument is made more evident. The Mettler-Toledo RC1 can be used to follow the heat evolution of a reaction for safety purposes, and/or it can be used to narrow in on optimum feed rates of reactants.

What is next on the agenda is to determine the heat transfer coefficient(s) and wetted heat transfer surface areas in selected reactors in order to gauge the upper heat load boundary that can be managed safely. There are many variables to contend with.  Inevitably, one has to pick a finite range of operating parameters to evaluate. Agitation rate, fill level, and heat transfer medium are variables to take into account.

So, down the merry path we go, learning more and more about applied thermodynamics and chemical engineering. I can dig it.

In my experience with people in different organizations in the context of training and expertise, I have come to notice that employees can be partitioned into two camps. There are those who wait to be trained and there are those who will not wait to for it.  As a rule, scientists and engineers are driven by curiosity and not a small portion of competitive spirit. This group will engage in self-study to acquire the necessary skills to push back the limits of their abilities.

An instrument like the RC1 requires that the user be familiar with the intimate details of the chemical transformation.  It is possible to alter the experiment profile on the fly, and that is not the work of a pure analyst following SOP’s. A chemist experienced in experimental synthesis with a broad background in material phenomena and descriptive chemistry is one who can steer the instrument and tease out key subtleties.

I recently had a reaction mixture in the RC1 that formed a slush at low temperature. At this temperature the heat flow trace was extremely irregular.  The reaction mass showed little visible sign of mixing.  Addition of reactant was followed by large magnitude, short coupled, exothermic swings. Apparently the heat of reaction was being released on a relatively small top portion of the reaction mass and eventually swirling towards the heat sensor strip with little dilution, giving exaggerated heat flow indications. With a Tr increase of 20 ºC and a higher mixing frequency, the mass began to thin a bit giving a vortex. The wild heat excursions disappeared.

What I take from this experience is that control problems might arise as a result of poor mixing leading to temperature or feed control inputs that are exaggerated as a result of being out of phase with the state of the reaction mass. An economic consequence might arise in the form of overly conservative metering of reactant, adding extra plant hours to the cost.

The concentration effects due to poor mixing can lead to localized enthalpic overheating and potential disturbances in the composition profile.  A reaction mixture with high viscosity or density in a solvent with low tensile strength (i.e., diethyl ether) can lead to cavitation and further exacerbation of mixing problems.

A poorly mixing slurry of reactive components in a low boiling solvent is a bad combination. Especially when the reactor is filled to afford low headspace. A temperature excursion can exceed the boiling point and cause the thick mixture to develop into a foam which can expand into the headspace or beyond.  This is the realm of heterogeneous flow and your emergency venting system may not be designed for such flows.  This is one of the many reasons that some operations define an operating temperature policy relating to the reaction temperature and the boiling point of the reaction solvent.

It is worth pointing out that process intensification is likely to lead to higher power densities (W/kg) in the reaction mass as well as solubility problems that can cause poor mixing and heat transfer. The RC1 can help the process chemist flesh out the merits of process intensification.

I had an evil thought just now as I attempt to write 2 reports simultaneously. Why do we keep using that superscripted circle in front of C (i.e., ºC) that designates “degree”?

What the hell? We don’t use it for the Kelvin temperature scale. And, who knows if the engineers use it for Rankine? The thing is useless like an appendix or a titular chairman. Get rid of it!

What do you think?

So I’ve been working out a process for the last few days. Among other things the compound is a ketal and it’s synthesis is pretty simple. Ketone and diol brew in a pot of refluxing hydrocarbon and through the magic of equilibration, the water and hydrocarbon vapors condense and phase separate in a Dean-Stark apparatus. The water phase drops to the bottom of a graduated collector and the progress of the reaction is monitored by watching the water volume accumulate. 

This reaction is straight forward enough that I can easily make up the procedure myself. So I calculated a favorite weight percentage for concentration and pitched in the reagents. I chose a few of my favorite acid catalysts as well for a series of trial runs. Everybody knows that these reactions run faster with an acid catalyst. Such mechanisms are used to torment sophomore organic students everywhere.

After satisfactory completion with a few catalysts, I decided to round out my table of data with a run without catalyst. What better way to show the critical nature of the catalyst than to run a blank?

As luck would have it, the reaction ran splendidly without added catalyst. In fact, there was precious little increase in yield over the test interval with added catalyst.  Even better, without the catalyst the color of the reaction mixture was lighter (the substrate is a little sensitive).

So I took the carbonyl reagent and shook it up with some water and plunged a pH probe into it. What I had assumed to be a neutral organic material was quite acidic on contact with water. Hmmm.

A dive into the literature (patents, actually) revealed that the history of the compound most likely involves exposure to HCl from a continuous acid hydrolysis and steam distillation. And the Aldrich bottle did say that ca 1 % water is present. A fact that I neglected in my haste to set up the reaction.

The upshot is that I didn’t anticipate that there was residual acid catalyst in the reagent itself.

This is good to know from the scale-up perspective. An acid catalyst probably won’t be needed and loading procedures and sourcing do not have to be done to use a separate catalyst.  

Now the trick is to determine if it is safe to combine all of the reagents in the reactor or if one needs to be fed in as the reaction proceeds. A run where all of the reagents are in the pot from the start is called a batch run.  A run where one or more reagents are fed into the vessel over the course of the reaction is called a semi-batch run.  The reaction rate is greatest if all of the reagents are present from the start, but it does represent an accumulation of energy in a low phi-factor vessel that could be a runaway hazard. I’ll have to noodle through this issue if this reaction gets scaled up.

Taking into account the phi factor, or the thermal inertia of the system, is one of the crucial details in scale-up. Eventually, you have to make a decision on whether to configure the run as a batch or a semi-batch process. The precautionary principle usually leads to semi-batch unless you can prove that a batch configuration is safe.

Running a process at reflux with a heated jacket relies on the overhead condenser to be the primary thermal safety device. This usually is very effective in knocking down condensable components in the gas phase. A good condenser has a huge effect on the heat balance of a reactor system.

Knocking down condensable components helps to regulate the pressure and temperature of the pot. The transition from liquid to gas phase carries heat away from the reaction mass quite effectively under ordinary conditions.

However, it is possible for a reaction to accelerate to a point where the condenser capacity is inadequate. At such a point the jacket may be filled with heating fluid and a switchover to chiller fluid may take a relatively long time. 

A reactor can behave as an adiabatic system if you pick a time interval that is short enough. So, a reaction mass that exotherms rapidly enough may find itself in an approximately adiabatic containment. In this condition, the reaction mass can accelerate with gusto as pressure and temperature ramp skyward, multiplying the reaction rate. Decomposition reactions kick in and non-condensable gases are evolved that further pressure the system. Hopefully, the rupture disk and vent were properly sized because there is going to be an administrative mess to clean up afterwards.

This scenario is one to be avoided. Reaction calorimetry and ARC testing give results that help tremendously with engineering around a runaway scenario. A parameter of particular interest in the adiabatic Time to Maximum Rate (TMRad).  TMR is extracted from the slope of a linear portion of an Antoine curve determined by ARC. A formula for the line can be substituted with a desired time and a temperature can be calculated.

A particularly useful value to come from this is the temperature affording a 24 hour TMR. Many companies will determine the 24 hr TMR and set a policy to operate at a set temperature margin below the 24 hr TMR temp:  a 60 C margin is not uncommon.


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