In a multi-purpose chemical plant, there are only a fixed number of chemical vessels, each one with a fixed volume capacity, and each with its own fixed auxiliary facilities ( ie. stirrer types, materials of construction, distillation facility, heating, cooling). More of a chemical product can be produced in a reactor, whatever its type, by increasing the concentrations of reactants and reagents in the vessel; but, there are limitations to the extent to which this can be done.
Improving the throughput is not equally important for all the process steps in a particular synthetic sequence. It is nearly without exception that it is the earliest steps in a synthesis that turn out to need to be repeated multiple times to provide sufficient early intermediates to make the desired quantity of the ultimate desired product. It is reducing the number of repetitions of these early steps that can deliver cost savings.
Improving the throughput of a process step is frequently the work of a chemical engineer rather than a process chemist. Nevertheless, a chemist may be assigned such work or the scientist who developed the laboratory procedure upon which the process step is based may be able to provide valuable insight or even experimental assistance to speed up or improve the change.
The wish to improve the throughput of an early step in a process sequence comes after the process itself has been successful at scale. Therefore, changes that might improve the throughput and decrease the number of times an early step needs to be repeated absolutely must not involve reengineering the whole process. The changes need to be ones that can be expected to have essentially no impact on the quality or quantity of the output of the step being modified. For example, completely switching to a different solvent for the reaction would almost certainly be too much change.
Very often a reaction has been first scouted and subsequently improved in the laboratory under conditions where a reaction occurs under homogeneous conditions. Enough solvent is specified to completely dissolve all of the reactants and that amount of solvent sets a minimum volume constraint for any given amount of reactant(s). This limitation may not be real. So long as other constraints do not intervene, experimentation may be able to demonstrate that a reactant does not need to be completely homogeneously dissolved in solvent throughout all or some interval of the reaction time— but this needs to be established by appropriate laboratory experiments using the actual qualities of reactants and reagents that will be used in the plant or pilot plant. This increases the number of experimental runs— never a desirable thing— and requires that materials representative of what will be used at scale be already available to the lab. This is important because the physical properties of undissolved solids can change reaction rates substantially.
The most frequent impediment to increasing the concentration of reactants, reagents, (and catalysts) to increase the throughput and reduce the overall process cost is the exothermicity of the reactions involved. As the reacting species are brought more intimately into contact they react faster and more heat can be produced. The more exothermic a transformation the more severe this restriction becomes. Almost always there is a temperature range that must not be exceeded.
The primary function of a reaction solvent is to shape the physical environment under which the reactants meet. A very prominent secondary function is to provide the capacity to absorb and buffer the energy absorbed or released by the reaction.
If we want to increase the concentrations of the reactants in our reactor so that we can increase the batch size in our reactor this will produce a larger and more rapid exotherm. To keep the reaction temperature within a specified range therefore we need to remove heat from the reactor’s contents more quickly.
The most common way to do this without changing the reactor or its cooling facility is to periodically stop further reaction, cool the reactor contents to the bottom of the reaction’s acceptable range, and then cause the reaction to resume until the internal reaction temperature reaches the top of the acceptable range; then stop the reacting and repeat. This starting and stopping of the reacting process is most commonly done by starting or stopping the gradual addition of one of the reactants. An even more common variant is to adjust the addition so that exotherm and cooling capacity remain balanced within the acceptable range.
To an extent to be determined experimentally for any particular transformation higher throughput may be achieved at the expense of a longer addition time for reactant mixing together. As we shall see other changes can be blended in with these two.
For most general-purpose chemical reactors cooling is provided by an external cooling jacket, a liquid coolant flowing in that jacket, and external refrigeration capacity to recool the refrigerant. The cooling system has, within its own limits, an adjustable capacity to keep the portion of the reactor wall that is in contact with the reaction mixture cold. The effectiveness of cooling is not however a function of the refrigeration power alone. Cooling of the bulk of the reacting mixture will depend upon the effectiveness with which the bulk reaction mixture is brought into close contact with the cold refrigerated reactor wall. This will depend upon the quality of the stirring in the reactor itself. The stirrer in a given reactor is a given for our purposes. Its velocity has an upper limit and its mixing effectiveness will depend upon other things, most prominently the viscosity of the reaction mixture, which itself changes through the reaction period. It also depends upon the ratio of reactor volume/wall surface area.
It is an impending difficulty that in the plant the reactor volume/wall surface areas are much higher than in typical laboratory reactors. Cooling the plant can be much less effective because of this. On the other hand, stirring and refrigerating power may be substantially better in the plant than in the laboratory. I think it is going to be impossible to model in the lab.
What can be done is a very low-risk experiment at plant scale in which an acceptable increase in reactant/solvent ratio is applied combined with maximum cooling power and maximum stirring speed while controlling the rate of addition of the limiting reactant that will hold the reaction temperature within the allowable range. This experiment will provide you with the minimum addition time possible for the throughput you are trying.
Every solvent has its own heat capacity. Heat capacity is the number of calories per mole required to raise its temperature by 1 centigrade degree. Different solvents will have varying abilities to absorb heat but we have already agreed that changing the solvent for a reaction would be too great a change to countenance at this stage when a process has already been proven acceptable. Could we add some new solvent into the reaction mixture to produce some solvent blend with a higher heat capacity? This is not likely to work well. The addition required to substantially change heat capacity would probably be sufficient to substantially change the reaction conditions.
The heat capacity of the solvent is not the only means by which the exothermic energy of chemical change can be controlled. Every solvent has a particular heat of vaporization whereby calories are removed to boil that solvent. If a solvent mixture is used as a reaction mixture at the temperature that is its boiling point heat from the reaction mixture can be removed by using it to boil solvent up into an attached condenser where it is cooled and that liquid will flow back into the reactor. This source of cooling can supplement cooling provided by a cooling jacket.
The downside of this technique is that it only operates when the reaction temperature is the same as the boiling point of the solvent. The optimal solvent range is usually chosen to maximize the selectivity of the reaction not to match the solvent’s boiling point so this method is unlikely to be applicable for increasing the throughput of a process already optimized.
However, a variant of the method might be useful. Suppose we were to modify the reaction mixture by adding a small amount of a cosolvent that had a boiling point within the reaction’s optimal reaction temperature range. So long as this cosolvent does not form an azeotrope with the other reactor components, it will boil at its own boiling point, take up heat from the reaction mixture, and vaporize up into any attached condenser. In the condenser, the heat will be extracted and the condensed cosolvent returned to the reactor where it can repeatedly be revaporized removing heat each time this occurs. This removal of heat will modulate the reaction exotherm and add to the cooling capacity. After the reaction period, the cosolvent is evaporated off and not returned. The reaction mixture is unchanged! Because a small amount of cosolvent is evaporated, and condensed over and over again a small amount of cosolvent could provide a lot of cooling.
The cosolvent for this use may be many things; all that is required is that it be unreactive under the reaction conditions and that it has a satisfactory boiling point and heat of vaporization. The condenser’s cooling capacity must also be sufficient. This cosolvent need not be a pure substance. It could be a mixture of liquids of the correct composition to have a lower boiling azeotrope. Low boiling examples could be ethyl ether/isoprene bp. 33.2 C or ethyl ether/ methyl formate bp.28.2 C. It could even be condensed, precooled gas that is sparged into the fluid reactor contents and then vents without condensation; liquid nitrogen for example.
Some solvents can be partially frozen onto the reactor walls before beginning the addition of a reactant that starts the reaction process. Using this technique the heat of melting of that solidified solvent can be used to cancel the calories produced by the reaction. This modulation would very efficiently cancel the initial burst of heat from the reaction before the cooling from the cooling jacket kicked in. Only a limited number of solvents have freezing points and heats of fusion in useful ranges. Water, DMSO, glacial acetic acid, dichloroacetic acid, dioxane, ethylene glycol, formic acid, formamide, nitrobenzene, and glycerol. The melting point must be low enough and the heat of fusion high enough to be useful.
The limitation on throughput using a particular reactor is often the point of maximum volume in the procedure. This point of maximum volume often comes at the reaction quenching or extraction stage. In the strict sense, these points of maximum volume are for the process step not strictly speaking for the reaction period itself.
Let us look at the situation that arises where the point of maximum volume comes after the reaction mixture has been quenched with an equal volume of water to provide a two-phase mixture. It may be possible to transfer half of a reaction mixture into a second vessel (it need not necessarily have the same facilities that were required during the reaction phase proper ie. heat/cooling facilities; it may be no more than a stirred tank ) and then quench each portion separately in its own vessel and work up each vessel’s contents separately, thereby doubling the throughput of the reaction. In the same manner, a process step that has its point of maximum volume during a liquid-liquid extraction can have the reaction mixture divided between vessels and the extractions done separately each in its own vessel. The extracts can then perhaps even be recombined and the remainder of the isolation/purification done together.
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