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Category Archives: Safety
On the release of hazardous energy
What should you do if a raw material for a process is explosive? Good question. Just because a material has explosive properties does not automatically disqualify it for use. To use it safely you must accumulate some information on the type and magnitude of stimulus that is required to give a hazardous release of energy.
But first, some comments on the release of hazardous energy. Hazardous energy is that energy which, if released in an uncontrolled way, can result in harm to people or equipment. This energy may be stored in the form of mechanical strain of the sort found in a compressed spring, a tank of compressed gas, the unstable chemical bonds of an explosive material, or as an explosive mixture of air and fuel. A good old fashioned pool fire is a release of hazardous energy as well. The radiant energy from a pool fire can easily and rapidly accelerate past the ignition point of nearby materials.
Accumulating and applying energy in large quantities is common and actually necessary in many essential activities. In chemical processing, heat energy may be applied to chemical reactions. Commonly, heat is also released from chemical reactions at some level ranging from minimal to large. The rate of heat evolution in common chemical condensation or metathesis reactions can be simply and reliably managed by controlling the rate of addition of reactants where two reactants are necessary.
There are explosive materials and there are explosive conditions. If one places the components of the fire triangle into a confined space, what may have been conditions for simple flammability in open air are now the components for an explosion. Heat and increasing pressure will apply PV work to the containment. In confinement, the initiation of combustion may accelerate to deflagration or detonation. The outcome will minimally be an overpressure with containment failure. If the contents are capable of accelerating from deflagration to detonation, then loss of containment may involve catastrophic failure of mechanical components.
Rate control of substances that autodecompose or otherwise break into multiple fragments is a bit more tricky. This is the reaction realm of explosives. The energy output is governed by the mathematics of first order kinetics, at least to some level of approximation. In first order kinetics, the rate of reaction depends on both the rate constant and the intitial concentration of one reactant. Regarding the control of reactions that are approximately first order in nature, some thought should be given to limiting the reaction mass size to that which is controllable with available reactor utilities. A determination of the adiabatic ΔT will give information that will tell you if the reaction will self-heat past the bp of your solvent system.
There is a particular type of explosive behavior called detonation. Detonation is a variety of explosive behavior that is characterized by the generation and propagation of a high velocity shock through a material. A shock is a high velocity compression wave which begins at the point of initiation and propagates throughout the bulk mass. Because it is a wave, it can be manipulated somewhat. This is the basis for explosive lensing and shaped charges.
Detonable materials may be subject to geometry constraints that limit the propagation of the shock. A cylinder of explosive material may or may not propagate a detonation wave depending on the diameter. Some materials are relatively insensitive to the shape and thickness. A film of nitroglycerin will easily propagate as will a slender filling of PETN in detcord. But these compounds are for munitions makers, not custom or fine chemical manufacturers. The point is that explosability and detonability is rather more complex than you might realize. Therefore, it is important to do a variety of tests on a material suspected of explosability.
A characteristic of high order explosives is the ability to propagate a shock across the bulk of the explosive material. However, this ability may depend upon the geometry of the material, the shock velocity, and the purity of the explosive itself. There are other parameters as well. Marginally detonable materials may lose critical energy if the shape of the charge provides enough surface area for loss of energy. The point is that “explosion” and “detonation” are not quite synonymous, and care must be exercised in their use. The word “detonation” confers attributes that are unique to that phenomenon.
Explosive substances have functional groups that are the locus of their explosibility. A functional group related to the onset of explosive behavior, called an explosiphore (or explosaphore), is needed to give a molecule explosability beyond the fuel-air variety. Obvious explosiphores include azide, nitro, nitroesters, nitrate salts, perchlorates, fulminates, diazo compounds, peroxides, picrates and styphnates, and hydrazine moieties. Other explosiphores include hydroxylamino. HOBt, a triazole analog of hydroxyamine, hydroxybenzotriazole, has injured people, destroyed reactors and caused serious damage to facilities. Hydroxylamine has been the source of a few plant explosions as well. It is possible to run a process for years and never cross the line to runaway.
Let’s go back to the original question of this essay. What do you do if you find that a raw material or a product is explosive? The first thing to do is collect all available information on the properties of the substance. In a business organization, upper management must be engaged immediately since the handling of such materials involves the assumption of risk profiles beyond that expected.
At this point, an evaluation must be made in relation to the value of the product in your business model vs the magnitude of the risk. Dow’s Fire and Explosion Index is one place to start. This methodology attempts to quantify and weight the risks of a particular scenario. A range of numbers are possible and a ranking of risk magnitude can be obtained therein. It is then possible to compare the risk ranking to a risk policy schedule generated beforehand by management. The intent is to quantify the risk against a scale already settled upon for easier decision making.
But even before such a risk ranking can be made, it is necessary to understand the type and magnitude of stimulus needed to elicit a release of hazardous energy. A good place to start is with a DSC thermogram and a TGA profile. These are easy and relatively inexpensive. A DSC thermogram will indicate onset temperature and energy release data as a first pass. Low onset temperature and high energy release is least desirable. High onset temperature and low exothermocity is most desirable.
What is more difficult to come to a decision point on is the scenario where there is relatively high temperature onset and high exothermicity. Inevitably, the argument will be made that operating temperatures will be far below the onset temp and that a hazardous condition may be avoided by simply putting controls on processing temperatures. While there is some truth to this, here is where we find that simple DSC data is inadequate for validating safe operating conditions.
Onset temperatures are not inherent physical properties. Onset temperatures are kinetic epiphenomena that are dependent on sample quality, the Cp of both the the sample and the crucible, and the rate of temperature rise. What is needed once an indication of high energy release is indicated by the DSC is a determination of time to maximum rate (TMS) determination. While this can be done with special techniques in the DSC (i.e., AKTS). TMR data may be calculated from 4 DSC scans at different rates, or it may be determined from Accelerated Rate Calorimetry, or ARC testing. Arc testing gives time, temp, and pressure profiles that DSC cannot give and in my mind, is the more information-rich choice of the two approaches. ARC also gives an indication of non-classical liquid/vapour behavior that is useful. ARC testing can indicate the generation of non-condensable gases in the decomposition profile which is good to know.
Other tests that indicate sensitivity to stimulus is the standard test protocol for DOT classification. Several companies do this testing and rating. There are levels of testing applied based on the result of what the lower series tests show. Series 1 and 2 are minimally what can be done to flesh out the effects of basic stimuli. What you get from the results of Series 1, 2, and 3 are a general indication of explosabilty and detonability, as well as sensitivity to impact and friction. In addition, tests for sensitivity to electric discharge and dust explosability should be performed as well.
The Gap test, Konen test, and time-pressure test will give a good picture of the ability to detonate, and whether or not any explosability requires confinement. The Konen test indicates whether or not extreme heating can cause decomposition to accelerate into an explosion sufficient to fragment a container with a hole in it.
BOM or BAM impact testing will indicate sensitivity to impact stimulus. Friction testing gives threshold data for friction sensitivity.
ESD sensitivity testing gives threshold data for visible effects of static discharge on the test material. Positive results include discoloration, smoking, flame, explosive report, etc.
Once the data is in hand, it is necessary to sift through it and make some determinations. There is rarely a clear line on the ground to indicate what to do. The real question for the company is whether or not the risk processing with the material is worth the reward. Everyone will have an opinion.
The key activity is to consider where in the process an unsafe stimulus may be applied to the material. If it is thermally sensitive in the range of heating utilities, then layers of protection guarding against overheating must be put in place. Layers of protection should include multiple engineering and administrative layers. Every layer is like a piece of Swiss cheese. The idea is to prevent the holes in the cheese from aligning.
If the material is impact or friction sensitive, then measures to guard against these stimuli must be put in place. For solids handling, this can be problematic. It might be that preparing the material as a solution is needed for minimum solids handling.
If the material is detonable, then all forms of stimulus must be guarded against unless you have specific knowledge that indicates otherwise. Furthermore, a safety study on storage should be performed. Segregation of explosable or detonable materials in storage will work towards decoupling of energy transfer during an incident. By segregating such materials, it is possible to minimize the adverse effects of fire and explosion to the rest of the facility.
With explosive materials, electrostatic safety is very important. All solids handling of explosable solids should involve provisions for suppression of static energy. A discharge of static energy in bulk solid material is a good way to initiate runaway decomposition in an energetic material. This is how a material with a high decomposition temperature by DSC can find sufficient stimulus for an explosion.
Safe practices involving energetic materials require an understanding the cause and effect of stimulus on the materials themselves. This is of necessity a data and knowledge driven activity. Along with ESD energy, handwaving arguments should also be suppressed.
Unfortunate trigonometry and dynamic blind spots.
Near miss on the highway last night. I averted a high speed T-bone impact by a meter or two as Music from the Hearts of Space played on the radio. Good gravy- I was crashing to space music. My Cherokee is quite stable in an emergency braking maneuver moving straight forward. But if swerving is required, then vehicle sway begins to couple into the steering. Never thought of this control problem before.
Imagine you are sitting at a stop sign waiting to turn left onto a 4 lane highway. To your left is a deceleration lane for traffic moving left to right. Now, imagine your line of sight as a line extending from your eyes across your left shoulder and into the distance. Got that?
Now, imagine a car in the deceleration lane on your left slowing to make a right turn at your intersection. A line can be drawn from your eyes to that car. As the car moves toward you, the line extending from your eyes to that car sweeps clockwise across the landscape. Still with me?
Now consider this. There exists a third vehicle (me) moving from left to right exactly along your line of sight, but behind and eclipsed by the vehicle in the turn lane. As the turn lane vehicle slows down in preparation for a turn, the third vehicle is at a distance and speed such that it continues to remain along the sight line yet remain eclipsed by the turning vehicle. As the turning vehicle approaches, it’s angular size increases, obstructing even more space behind it.
That is the condition I was in last night. The perversity of trigonometry allows for a set of velocities and alignments that presents a dynamic obstruction of view to evolve. If the driver at the stop sign grows impatient and attempts to cross the two lanes of traffic before the vehicle in the turn lane completes the turn, he is doing so on the assumption that the blind space is clear. Last night I was in that blind space while an impatient driver pulled in front of me.
As the driver pulled out, he saw me and hesitated in my (right) lane, causing me to apply heavy braking and to swerve around his rear and away from oncoming traffic. Fortunately for all of us, he stepped on the gas and got out of the way. With the sway of my vehicle with the maneuvering loads, I was on the verge of a loss of control and may have hit him anyway.
It is interesting how a steady state cruise along the highway can evolve into a complex set of events in just seconds. In fact, I had anticipated this problem of visibility as I approached the intersection. I disengaged the cruise control and began to decelerate when I saw that the waiting vehicle could not see me. I now think that my deceleration kept me in the blind spot even longer, aggravating and tightening the coupling of the event. I was barely able to avoid this failure mode even though I was aware of it. That is scary.
Taking the dragon out for a walk
Plan on working with HF? A friend who was president of an HF manufacturing company once gave me some valuable advice. He said there are several things to do before the plastic bottle of HF arrives. First, have a well ventilated fume hood available. Next, read up on HF first aid. Try to identify a hospital ER that could cope with an HF incident. How do you do that? You call and ask questions. Get some calcium gluconate salve. Learn what to do with it. If you have an incident, you will need to get decontaminated before you arrive at the hospital, otherwise there may be delays in getting teatment by the medical staff.
Here is my personal policy. You follow your own policies. If you handle HF and do not have a specific response plan, get one in place. If you handle this acid, you need to have a plan.
Do not rely on the local fire department to know what to do. They’ll want to take charge as soon as they arrive. Time will be lost as they ignore the staff of chemical experts standing right there while they confer on a plan. I’ve seen variants of this many times. It might transpire that the firemen will be ordered to stand clear of you until their commander has a plan for dealing with the contamination. So there you’ll sit.
Your main concern in a major splash incident is to get decontaminated. Your lab buddies who are there with you need to know how help you with this so there is no delay in getting you decontaminated. Do not wait for the fire department to come decontaminate you. Strip off contaminated clothing and get under the shower pronto, even if you have to use your one good arm to drag yourself there.
HF is a weak acid with a pKa of 3.17. It is somewhat skin permeable and will cause deep tissue injury. In addition to the general hydrolytic havoc associated with an acid exposure, HF delivers fluoride which scavenges calcium and will precipitate calcium fluoride in your tissues. That is what sets an HF exposure apart. This link to Honeywell Specialty Materials is especially well written and informative.
Avoid inhalation exposure and provide for splash protection. If you are heating it, consider using a full face respirator with appropriate cartridges when opening the sash of the hood when handling the reaction mixture. Wear a long rubber or plastic gloves and apron and make sure that your lab coat is non-absorbant. Be fastidious.
Don’t be afraid of HF. It is a lot like a table saw. You just have to know how to behave around it. And like a table saw, it’ll take body parts or worse from the careless or the complacent. You have to handle it carefully every single time. Be in the moment. Don’t get distracted by talkative bystanders. Pay attention to what you’re doing.
Materials of Construction
One of the things you have to consider when scaling up a chemical process is the composition of the wetted or exposed surfaces in the reaction vessel, associated feed piping, gaskets, and overhead vapor spaces. Common materials of construction subject to wetting are steel (various types), glass, Hastelloy(s), tantalum, titanium, PTFE, Viton, and various polymers found in hoses.
Metal batch reactors are subject to erosion over time. Vessel walls can be tested for thickness periodically. Glass coated reactors are very useful for their broad applicability to many kinds of reactions, but have drawbacks of their own. Glassed vessels are sensitive to very high and very low temperatures as well as thermal gradients across the vessel wall. It is possible to crack the glass coating and have it flake away, exposing the underlying metal to corrosion.
We are all trained to do chemistry in glass reactors, but it should be pointed out that much chemistry can be performed in steel vessels. While you want to give some thought to the use of hydrogen, for the most part metal pots are well suited for reaction under neutral or reducing conditions. That is, metal hydrides, Na, carbanionic species BuLi and RMgX, alkoxides, etc., are well tolerated in wetted-metal pots.
Oxidizing or acid halide producing reaction systems are problematic for metal pots, however. Acidic corrosive reaction mixtures can attack the wetted metal parts of the reactor system. Acidic chlorides in particular are quite corrosive to various grades of steel. It is especially problematic when you’re talking about shell and tube condensers. The tubes are often very thin for good heat transfer, leading to the possibility of the introduction of chiller fluids into the reactor if corrosion chews through the tubes. If the chiller fluids are protic and the pot is full of MeLi, then the batch may be lost or an unplanned reactive hazard event may take place.
Condenser surfaces can be subject to more corrosion that you realize. This is the location where hot concentrated corrosive gases will condense, after all. To extend the life of the condenser, special materials of construction may be used. Tantalum and PTFE can be used when the cost is justified. With exotic materials of construction come exotic prices.
There is more to consider than corrosion. Polymer transfer lines will generate static electric hazards via the isolation of charge on nonconductive surfaces. Tranferring hydrocarbon solvents from a drum or cylinder to a reactor through nonconductive plumbing can generate significant hazardous energy and certainly enough to be incendive. Grounded metal piping can prevent part of this problem. However, discharging a flammable liquid into an air filled space may lead to an incendive discharge as well. It is important that all atmospheres over flammable liquids be inerted. While you may not be able to stop static discharges, you can certainly keep the fire triangle for being formed.
Operators are often alarmed by the sight of a glassed reactor with stirring toluene in it generating sparks by discharge through the glass coating. While this may be hard on the glassing by forming pinholes, unless there is an explosive material in solution, the lack of a complete fire triangle means that the sparks cannot lead to ignition of the toluene.
Remember not to take your material to high viscosity or dryness in a large reactor. You might end up rolling your solid material into a giant bowling ball and bending your agitator shaft. Maybe even slamming it into the reactor wall. A very expensive mistake.
On the manufacture of hazardous materials
How does one decide if a given compound is too hazardous to manufacture at a particular site? The answer to this question is much harder to arrive at than you might imagine. It is very easy to spout glib, hand waving statements about risk analysis and risk based process safety. It is quite another matter to actually conceive of experiments to tease out the safety data and compile it into knowledge based practice. For the manufacturer there are two kinds of operating hazards to contend with- 1) physical hazards, and 2) regulatory hazards. Getting into trouble with either can bring your operation to a halt.
In general, there are two kinds of GO / NO GO approaches to the question of going forward with any given material. One method applies some kind of quantitative risk analysis based on accumulated knowledge combined with hazard thresholds defining acceptable risk. Regulatory compliance and insurance issues may apply, or not.
The other general approach is simply a management decision. The board of directors or CEO decrees that we’ll go forward and do what it takes to operate safely. Or management decrees that we will not go forward with the manufacture. We’ll let someone else have that plum.
I recall being at a propellants conference a few years ago where a representative from a solid propellants manufacturer asked me if we would consider making lead styphnate. I paused for a moment, as if to be carefully weighing my answer, and replied with a flat ‘no’. The fellow wasn’t surprised and went on his merry way. This was the exercise of an informal method of process safety. Decline to make the obviously hazardous materials.
The threshold for the definition of hazardous materials varies considerably within industries and between them. The spread of hazard types across the manufacturing world is large and perhaps confusing. Two of the broad types of manufacturing hazards are hazardous energy and toxicity. Hazardous energy is found in operations with high pressure, flammable materials, mechanical energy, chemical reactivity, electrical energy, and explosive materials which is a combination of chemical reactivity and mechanical energy.
Toxicity
Toxic hazards are a group that cover a wide range of physiological effects and modes of dosing. Toxicity issues relating to manufacture can be a very complex matter and it is best to involve experienced hands to sort out the good sense fom the nonsense. To a large extent, the maufacture of toxic materials is covered by the proper application of personal protection equipment (PPE), good plant hygiene, and a process that keeps toxic materials contained to the greatest extent possible. The pharma people know all about this activity. But in the specialty chemicals business, a good deal of the chemical intermediates that go out the door have poorly understood toxicology.
Products that are made for dispersal into the environment are subject to much greater oversight by EPA. But the same is not true for a great many chemical intermediates. Chemical intermediates flow through different regulatory pipelines with some under thorough regulatory scrutiny and others considerably less so. Pharmaceutical intermediates may or may not be covered under FDA GMP rules. Very early intermediates may be items of commerce and not subject to the Byzantine ways of GMP. Later intermediates may have FDA requirements that handcuff you to the bedpost. It is possible to have a very prosperous career outside of the GMP world.
Chemicals that are not for pharmaceutical or pesticide use may be listed under TSCA. Chemical Abstracts Service maintains the list and access to entries is had through the CASRN, or the CAS registry number. TSCA is a type of oversight promulgated by the EPA and is intended to provide scrutiny in regard to worker exposure and environmental release during the execution of a chemical process. EPA does consider the toxicology and environmental literature and is able to model the fate of a release into the air or water by calculation.
While specialty chemicals are subject to TSCA regulations and an approval by EPA, only cursory toxicological examination is customarily performed. TSCA approval is either in the form of a listing or through a low volume exemption. PSM regulations promulgated by OSHA provide regulatory crossfire on the manufacturer in that OSHA regulations require enough safety testing as to provide a safe working environment. So together, OSHA and EPA cover a great deal of area in manufacturing safety. The rules are meant to be proactive, but they also provide substantial penalties for infractions. There is much more depth to TSCA and PSM than I have mentioned here, obviously. It is important to have people on staff who specialize in regulatory affairs.
Testing for toxicological effects is time and resource consuming. Much planning must go into such testing and it must be started well in advance of plant operations. Substances that pose a potential risk to workers via chronic occupational exposure during manufacture and handling are good candidates for such testing. However, if the substance is not a commodity chemical and if the substance is made only during infrequent campaigns for a limited group of users, it is less than likely that it will have been tested. The best approach to manufacturing a substance with little data available on toxicity is through the use of precautionary guidelines with layers of protection for the operators. That is, design a process that prevents exposure of the workers to the product and offers redundancy in engineering and administrative controls. The coverage must include production operators, maintenance crews, warehouse workers, chemists, and engineers.
Hazardous Energy
Reactive hazards and hazardous energy issues can and should be investigated by the manufacturer to the greatest extent possible. While such activity can be farmed out to commercial labs, it is very important for management to grasp the benefis of in-house expertise. Depth of knowledge is important in understanding and preventing upset conditions. But the accumulation of such depth of knowledge is expensive and subject to throttling by management. It always involves accruing more information than is apparently needed, at least initially.
Science is to a large extent about understanding boundary conditions. In the same way, chemical safety requires understanding the conditions for the release of hazardous energy, decomposition, or other undesirable attributes. What you’ll find quite often is that a single measurable attribute is not enough to assemble a complete picture of reactive hazards. Most reactive hazards are understood by assemblig a composite of several kinds of experimental results for a more complete appreciation of the dimensions of the reactivity.
To find such boundary conditions one needs to conceive of experiments to tease out the effects. Some kinds of information relating to safety issues can be obtained by instrumentation. Differential Scanning Calorimetry (DSC) is one such technique that gives a quantitative picture of the heat evolution of a substance while it is being heated over a planned temperature range. Thermogravimetric Analysis (TGA) of a test substance gives an indication of mass loss as a function of temperature. Accelerating Rate Calorimetry (ARC) shows heat flows into or out of a sample while recording sample cell pressure. ARC goes a bit further than DSC in that the evolution of non-condensable gases can be inferred by the shape of a derived Anton curve. ARC also gives an indication of time to maximum rate (TMR), which is a useful parameter in determining the maximum temperature or residence time for a reactive material or mixture. Reaction Calorimetry (RC1) shows the heat flux profile of an actual reaction mixture over the course of reagnt dosing. RC1 may be used to look for the accumulation of energy in a reactor. There are other tests available, but I cannot attest to them on the basis of personal experience.
Explosivity
Noninstrumental methods of safety appraisal include the tests for explosive properties. There are well defined protocols for explosive testing and they are applied in layers. It is very important for people handling new materials that may have explosive properties to understand the various assays for explosivity.
Explosivity (or explosability) may be manifested in many ways and there are tests to tease out sensitivity to a measured stimulus. The key point I’m trying to make is that explosivity is a composite property sensitive to multiple kinds of stimulus and physical circumstances. Many materials are explosive only according to a few kinds of tests.
Safety testing for materials that may be energetic include BOM (Bureau of Mines) or BAM fall hammer tests and the BAM friction test. These tests do as the names suggest- look for thresholds for sensitivity to impact or friction.
The Koenen test looks for explosivity when a material is heated under partial confinement, i.e., material is packed in a metal tube with a small hole in the end. Materials that are merely flammable will decompose and vent through the orifice. Compounds that are explosive may cause the Koenen tube to burst.
The time/pressure test is used in DOT classification and consists of a pressure vessel fitted with nichrome wire and a pressure sensor. The sample is heated with the nichrome wire or flame and the pressure is monitored. A pressure rise from 100 to 300 psi in 30 msec or less is regarded as having rapid deflagration properties and an qualifies as a positive indication for explosivity for transportation purposes. For the process chemical industry, this test gives an indication of the potential for rapid gas formation and the unwanted PV work it may do on your equipment.
There is a series of tests used for DOT classification of explosive properties that will give useful insight for those who propose to manufacture intended or unintended energetic materials. It is useful to have material tested to assemble the composite picture of the materials sensitivity. Questions to ask are: 1) does the material show any positive indications at all? 2) If explosive indications are found, is confinement required? 3) Does the material show any detonability at all? 4) Can you fnd any sensitizers or catalysts to explosivity? 5) Does the material transition from deflagration to detonation? 6) Is the material sensitive to stimulus by electrostatic discharge (ESD)? 7) What temperature gives a time to maximum rate (TMR) of 24 hours? 8) Do the decomposition products contain non-condensable gases? There are more questions to ask. Remember not to confuse detonation with explosion.
For the chemist interested in manufacturing a product that has known reactive hazards associated with it, it is useful to have collated the data. The application of knowledge of reactive hazards depends greatly on the kind of equipment to be used and the kind of chemistry to be performed. It is possible nonetheless to make a few useful generalizations.
Accumulation of Hazardous Energy
The execution of a chemical process usually requires that two or more substances be put into physical contact in a solvent. This is a point at which hazardous energy may be evolved. Obviously, for promptly reacting systems the rate of heat generation must be less than the rate of heat removal to avoid a runaway situation. But special care must be taken for reactions that are not prompt and that might allow for the accumulation of unreacted material in the vessel. This unreacted material in a reation vessel represents an accumulation of potentially hazardous energy. Good process R&D will identify reactions with latent periods or reactions that are particularly slow to start. Problematic reactions require good in-process checks to ascertain the state of the rection. Very often, a heat kick is all you need to see to know the reatcion has begun.
Grignard reagent reactions are notorious for being slow to start, tempting operators to “goose” the reaction by adding more RX to the pot. Above about 10 % of RX over Mg, the potential for a runaway initiation is very high. It is best to limit RX addition to a maximum of 5 %. If no initiation is observed after a reasonable attempt, the chemist must be awakened and hauled to the plant to provide on the spot guidance. Generally, initiation is a matter of time. But but sometimes parlor tricks must be used to activate the Mg. These are well known. It is always best to use these activation tricks prior to addition RX to the pot because otherwise a rapid consumption of RX may occur.
Solids Handling
It is always more desirable to handle sensitive or reactive materials in solution. They can be piped around under inert atmosphere and generally protected from environmental problems. However, sometimes there is no way to avoid the handling of reactive solids. That is, solids that are sensitive to O2 and/or water. The sensitivity may only go as far as quality control and specification problems. Or reactive hazards may be in play.
Solids handling is problematic in certain operations. Charging a reactor with reactive solids requires specialized solids handling equipment. Even non-reactive solids present a problem in handling. Dumping solids into an open manway can result inan incendive electrostatic disharge. It’s made more serious if there is a flammable solvent in the vessel. Here is th rule- you don’t add solids into a reactor manway of there is the possibility of explosive dusts or flammable solvent in the pot.
Filtration is another problematic operation. Well, let’s say that opening the filter with reactive materials in it is a problem. If you use BuLi orRMgX, you probably have to do a filtration at some point. Unless you quench the BuLi or RMgX in the pot, you are likely to have a hot filter cake. While I cannot divulge any particular methods here, I can say that managers have to address this issue one way or another. It is especially exciting if the hot cake is wet with a flammable solvent. So, ESD and other ignition sources must be delt with when the filter is opened. Operators must be grounded and all locations for possible charge isolation must be accounted for. It is best to open a filter in a location where having a hot cake fire is acceptable.
Filter cakes may be waste or product, depending on the circumstance. Drying operations in the filter must account for the accumulation of electrostatic energy as the material dries. It is important to have decay times for the solids if they are potentially energetic. Energetic materials that accumulate static must be allowed to decay their charge prior to handling. Of course, the prevention of charge accumulation is best. Propellant folks will coat granulated or pelletized product with charcoal or grapite to render the solids conductive.
Packaging
Chemists really hate to have to worry about packaging, but I can attest this is an activity that attracts quality control problems. Obviously reactive materials must be compatible with the package materials of construction. Containers must seal properly. Steel drums are useful for many kinds of materials, but the bungs can and do leak with temperature changes, so they can pull in moist air. In terms of reactive hazards associated with containment, usually some choices have to be made. What kind of leak scenario is plausible and does the proposed container pose any special weaknesses? Drums are notoriously susceptable to being speared by forklifts. Cylinders too.
What hazards are present for a workman who opens the drum with the hazardous material? Does the operator have to open the drum and put in a dip tube for pumping out the material? Perhaps a cylinder with a built in dip tube is safer.
Another matter to consider, especially with solids, is the issue of static charge generation during filling operations. Is the container or liner conductive or dissapative? Are ESD procedures in place for safe handling? Liquids can generate considerable static energy, especially when low dielectric constant liquids travel through a plastic pipe. Transfer of flammable organic fluids should take place in grounded or bonded conductive pipe to the greatest extent possible to avoid charge isolation.
All equipment should be grounded or bonded via a ground that is periodically tested for integrity. Everything should be at the same potential as the ground. Cement floors are dissipative, but painted cement floors are not. Wooden pallets and fibreboard packaging are dissipative when sitting on bare concrete.
On the pitfalls of process intensification
As any process development chemist knows, there is motivation to optimize a chemical process to produce maximum output in the minimum of reaction space. In the context of this essay, I’m referring to batch or semi-batch processes. Most multipurpose fine chemical production batch reactors have a capacity somewhere between 25 and 5000 gallons. These reactors are connected to utilities that supply heat transfer fluids for heating and cooling. These vessels are connected to inerting gases- nitrogen is typical- and to vacuum systems as well.
Maximum reactor pressure can be set as a matter of policy or by the vessel rating. Organizations can, as a matter of policy, set the maximum vessel pressure by the selection of the appropriate rupture disk rating. Vessel pressure rating and emergency venting considerations are a specialist art best left to chemical engineers.
Reactor temperatures are determined by the limits of the vessel materials and by the heat/chiller source. Batch reactors are typically heated or chilled with a heat transfer fluid. On heating, pressurized steam may be applied to the vessel jacket to provide even and controlled heating. Or a heat transfer fluid like Dowtherm may be used in a heating or chilling circuit.
Process intensification is about getting the maximum space yield (kg product per liter of reaction volume) and involves several parameters in process design. Concentration, temperature, and pressure are three of the handles the process chemist can pull to increase the reaction velocity generally, but concentration is the important variable in high space yield processes. Increasing reaction temperatures or pressures might increase the number of batches per week, but if more product per batch is desired and reactor choices are limited, then eventually the matter of higher concentration must be addressed.
The principle of the economy of scale says that on scale-up of a process, not all costs scale continuously or at the same rate. That is, if you double the scale, you double the raw material costs but not necessarily the labor costs. While there may be some beneficial economy of scale in the raw materials, most of the economy will be had in the labor component of the process cost. The labor and overhead costs in operating a full reactor are only slightly greater than a quarter full reactor. So, the labor component is diluted over a greater number of kg of product in a full reactor.
The same effect operates in higher space yield processes. The labor cost dilution effect can be considerable. This is especially important for the profitable production of commoditized products where there are many competitors and the customer makes the decision solely on price and delivery. Low margin products where raw material costs are large and relatively fixed and labor is the only cost that can be shaved are good candiates for larger scale and higher space yield.
But the chemist must be wary of certain effects when attempting process intensification. In general, process intensification involves increasing some kind of energy in the vessel. Process intensification through increased concentration will have the effect of increasing the amount of energy evolution per kilogram of reaction mixture.
Energy accumulation in a reactor is one of the most important things to consider when attempting to increase space yield. It is crucial to assure that process changes do not result in the accumulation of hazardous energy.
Energy accumulation in a reactor occurs in several ways. The accumulation of unreacted reagents is a form of stored energy. The danger here is in the potential for a runaway reaction. Accumulated reagents can react to evolve heat leading to an accelerated rates and eventually may open further exothermic pathways of decomposition. As the event ensues, the temperature rises, overwhelming the cooling capacity of the reactor. The reactor pressure rises, accelerating the event further. At some point the rupture disk bursts venting some of the reactor contents. Hopefully the pressure venting will result in cooling of the vessel contents and depressurizing the vessel. But it may not. If the pressure acceleration is greater than the deceleration afforded by the vent system, then the reactor pressure will continue to a pressure spike. This is where the weak components may fail. Hopefully, nobody is standing nearby. Survivors will report a bang followed by a rushing sound followed by a bigger bang and BLEVE-type flare if the system suffers a structural failure.
Energy accumulation can manifest in less obvious ways. Here is an example. Assume a spherical reaction volume. As the radius of the sphere increases, the surface area of the sphere increases as the square of the radius. The volume increases as the cube of the radius. So, on scale-up the volume of reaction mixture (and heat generation potential) will increase faster than the heat transfer surface area. The ratios are different for cylindrical volumes, but the principle is the same. Generally the adjustment of feed rates will take care of this matter in semi-batch reactions. Batch reactions where all of the reagents are added at once are where the unwary and unlucky can get into big trouble.
Process intensification via increased concentration may have deleterious effects on viscosity and mixing. This is especially true if slurries are produced and is even worse if a low boiling solvent is used. Slurries result in poor mixing and poor heat transfer. Low boiling solvents may be prone to cavitation with strong agitation, exacerbating the heat transfer problem. Slurry solids provide nucleation sites for the initiation of cavitation. Cavitation is difficult to detect as well. The instinct to increase agitator speed to “help” the mixing may only make matters worse by increasing the shear and thus the onset of cavitation.
Denser slurries resulting from process intensification are more problematic to transfer and filter as well. Ground gained from higher concentrations may be lost in subsequent materials handling problems. Filtration is where the whole thing can hang up. It is important for the process development chemist to pay attention to materials handling issues before commiting to increased slurry densities. Crow is best eaten while it is still warm.
Flight Profile of Cactus Flight 1549
The YouTube video below is a reconstruction of the flight of US Airways Flight 1549, referred to as Cactus. It is interesting to note how the pilot acted to conserve his altitude by careful energy management. After the dual engine flameouts the pilots established an optimum glide to maximize flight time. They did not bank the aircraft anymore than they had to- banking without power consumes altitude. While one pilot was flying the airplane the other was consulting the manual for emergency restart of the engines. The captain evidently knew right away that the only option for maximum survivability was to set the plane in the river.
Near the end the pilots found themselves coming in a bit fast so they brought the nose of the aircraft up and porpoised ~250 ft or so before locking on 130 kts indicated for their glide to the surface. They were mindful of bringing the aircraft to the site of the accident as slowly as possible. KE = (1/2) mv^2.
Note how he dips the tail in the water first while keeping the wings absolutely level. This brought the aircraft into the water along the longitudinal axis and thus averted a cartwheeling accident. The engines become powerful drag devices once they are in water.
Lowest Common Denominator
What is happening in the chemical world is that the safety people are taking control. Everything is dumbing down to the point where the safety of a facility is being judged on the basis of what the least qualified deem as safe.
I just received an MSDS for the Buchner funnel (!$#%!!) I recently purchased from Aldrich. The MSDS lists zero’s for Health, Flammability, and Reactivity both for HMIS and NFPA ratings. Thank heavens for that. It does recommend “suitable storage” and that it be kept “tightly closed”. It is silent, though, on the matter of repeatedly jabbing the pointy end into your eye.
I gotta get out of the chemical business if this is where it is going. Administrative controls on common laboratory activity requires management by a dedicated staff member in order to maintain a favorable paper trail and stay in compliance with the ever growing web of regulation. OSHA, EPA, Homeland Security, as well as state and local agencies who want to inspect this or that or place tax stamps on your balances.
How did civilization get this far along without the legions of officious ninnies who want to exert control over everything you do? Chemical labs have inherent hazards, depending on the work that is being done in them. The cost of achieving de minimus risk for the lowest common denominator is quite high. Risk ends up being transferred to countries who reside on the other side of the curve- those who have little care for people.
CSB Reports
Being a reactive hazards person, I try to keep up on the reports posted by the US Chemical Safety Board (CSB). In my view, the CSB does exemplary work in root cause analysis of what are often very complex events leading to disaster. I wholeheartedly recommend that people in the process side of chemistry peruse the many reports and videos posted on the CSB website.
The development of any technology in the real world involves what I refer to as
“the discovery of new failure modes”.
While it is possible to anticipate many kinds of failure modes, it often happens that plant operations will present the opportunity to line up the planets in a particular way that was left out of the failure analysis.
A recent account from the CSB is the report on the T2 Laboratories accident in late 2007 in Jacksonville, FL. This accident killed 4 employees and injured 32 in many of the adjacent businesses. The explosive yield was estimated by the CSB investigators to be equivalent to 1450 lbs of TNT.
What is most instructive about this incident is the extent to which the thermokinetic behaviour was unknown to the owner/operators. This accident illustrates that thermal decomposition modes leading to runaway can happen despite a large number of successful runs.
I won’t go into too much detail since the report itself should be read by those interested in such things. But the upshot is that the reactor contents (MeCp dimer, Na, and diglyme) accelerated to a temperature that lead to the exothermic reaction of sodium metal and solvent diglyme. The reaction contents accelerated, raising the temperature and pressure to the rupture disk yield pressure of 400 psi. However, the acceleration was far too energetic for this safety device. The vessel exploded, hurling fire and fragments off the site. Just prior to the explosion, the owner/engineer directed the operators to leave the control room, saying prophetically, “there is going to be a fire”.
While the owners did perform some process development and did have the used vessel professionally inspected, what was left out was a study of the aptitude of the reaction to self-heat into a runaway condition. The company rightly anticipated the exothermicity of the sodium reaction with MeCp monomer and in fact, relied on the exotherm to raise the rxn temperature to a level where the economics would be more favorable. But what nobody at T2 anticipated was the runaway potential of the reaction of the sodium with the diglyme. No doubt they thought that the cooling jacket would prevent temperature excursions leading to a runaway.
The various glymes are often chosen as reaction solvents owing to their diether character as well as their high boiling points. Troublesome compounds or reactions requiring a polar solvent can be dissolved at high temperature and reacted in this high boiler. In certain cases, reactions can be run in a glyme and the product conveniently distilled out of the reaction mixture. Perhaps this is what they were doing in the MCMT process, I don’t know. This level of detail was not provided in the report.
Lamentations on Chemistry 