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I’ve been using a Mettler-Toledo (MT) RC1e reaction calorimeter for about 6 years. Our system came with MT’s iControl software, RTCal, and 2 feed pumps with balances. Overall it has proven its worth for chemical process safety and has helped us understand and adjust the thermal profile of diverse reactions. Like everything else, MT’s RC1e has many strengths and a few weaknesses.

The RC1e’s mechanical side seems reasonably robust. Our instrument sits in a walk-in fume hood resting on a low lab benchtop supported by an excess of cinder blocks- it is a heavy beast. During installation we discovered that the unit would not achieve stable calibration with the hood sash down. The control box mounted on the instrument didn’t work properly on installation. After a trip to the repair shop, the box was returned as functional but without finding the fault.

Recently we had a mixing valve fail in the heat transfer plumbing, resulting in down time. Diagnosis of this was unsuccessful over the email and phone, necessitating a service call. Parts may not be inventoried in the US and consequently must come from Switzerland. Expect Swiss prices and less than snappy delivery. Hey, it’s been my experience.

A chiller unit is required for RC1 operation and can add 15-30k$ to the setup cost. Users will have to contend with the loss of floor space in the lab for the chiller and RC1. Chillers can take many hours to get down to the set temperature. Given that RC1 experiments can also be lengthy, plan accordingly. Our (brand new Neslab 80) chiller requires nearly 2 and 1/2 hours to get from +20 C to -20 C, which is the upper chiller temperature we use, depending on the reaction chemistry. For reactions that are on the sporty side, we’ll drop the chiller to – 50 C.  This is near the  minimum temperature for the water-based chilling fluid we use. Early on I opted for an aqueous lithium formate solution with a very low freezing point. It’s a little spendy, but a pool of it on the floor cannot warm up to become combustible and an ESD ignition hazard. Also, it is odorless.

The chiller required the wiring-in of a dedicated single-phase 240 VAC circuit. With the chiller using single-phase and the RC1e using 3-phase 240 VAC, it is important to assure that one cannot inadvertently connect into the wrong power circuit (idiot proofing). The chiller plug design should already prevent this. It is critical that the electrician is alert to this and does NOT jury-rig the plugs to use the same style of connectors because he has only one style in the parts bin.

Some comments on the collection and interpretation of RC1 thermograms.

  • It is critical that those who request RC1 experiments understand the limitations of the instrument. For instance, we use a 2 Liter reaction vessel with a 400 mL minimum fill volume. Refluxing is not allowed owing to the huge thermal noise input from the reflux return stream. Special equipment is said to be available for reflux.
  • Experiments must be carefully designed to elicit results that can answer questions about feed rates and energy accumulation.
  • Like many instruments, the RC1 needs a dedicated keeper and contact person for inside and outside communication. A maintenance logbook should be kept next to the instrument if for no other reason than to pass along learnings from previous issues.
  • If thermokinetic measurement is part of your organization’s development SOP, someone on staff should be reasonably familiar with chemical thermodynamics. That can be a chemical engineer, as may often be the case.
  • The users of thermal data are likely to need help with interpretation of the results. Be prepared to offer advice on interpreting the data, taking care not to over-interpret. If you don’t know, say so. It is easier to claw back “I don’t know” than “yeah, go ahead and do that …”.
  • Do not be anxious to singlehandedly bear the weight of responsibility for safety. Safety is a group responsibility.
  • Be curious. How do the insights and learnings from the data translate into best practices? What changes, if any, can the process chemists make to nudge the process for better safety and yields? A credible specialist in RC can make comments or ask questions that lead to better discussions on thermal hazards. Be a fly in the ointment.
  • Never forget that a reaction calorimeter is a blunt instrument for the understanding of a reaction. An RC1 thermogram is a composite of overlapping solution-phase phenomena. Interpretation of results can be greatly refined by pulling timely aliquots for NMR, GC/MS, or HPLC analysis.
  • A database should be constructed to collect and immortalize learnings from all safety work and RC1 learnings fall into that group.

There is the question of who collects and presents the data. An engineer or a chemist? Engineering thermodynamics is a big part of a chemical engineer’s education and skill set. As a plus, an engineer can take thermal data and apply it to scale-up design for safety and sizing of equipment and utilities. You know, the engineering part.

Do not be anxious to singlehandedly bear the weight of responsibility for safety. Alpha males- are you listening??  Safety is a group responsibility that should originate from a healthy group dynamic.

There’s a good argument for a chemist to conduct RC experiments as well. A trained synthesis chemist is qualified to conduct chemical reactions within their organization. That includes sourcing raw materials, handling them, running the reaction, and safely cleaning up the equipment afterwards. But interpreting RC1 data has a large physical chemistry component. In my experience, run of the mill inorganic/organic synthesis people may have seen PChem as an obstacle rather than a focus in their college education. Their skill set is in instrumental analysis like NMR and chromatography, mechanisms, and reaction chemistry. I would recommend having a PhD chemist with a focus on thermo in a leadership role when calorimetry is a key part of a busy process safety environment.

Safety data can be collected and archived all day long. The crucial and often tricky part is how to develop best practices from the data. I would offer that this is inherently a cross-disciplinary problem. Calorimetric data from reaction chemistry can be collected readily, especially with the diverse and excellent instrumentation available today. Adiabatic temperature rise, ΔTad, can be determined by a chemist, but it’s the engineers who understand how the equipment may respond to a given heat release. A smooth and efficient technology transfer from lab to plant happens when good communication skills are used. Yes, SOP’s must be in place for consistency and safety. But the positive effect of individuals who have good social skills and are prone to volunteering information cannot be underestimated.

 

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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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