Reducing Cycle Time
Often the easiest way to improve throughput is to reduce cycle time and this can be done just as often working on the work-up, isolation, or purification sequences as changing the actual chemical reactions.
By cycle time, I mean the time it takes to run a batch when the reactor, reactor accessories, and peripheral equipment, such as centrifuges, filters, driers etc. are deployed as continuously as possible making the end product of that step. This includes cleaning, verifying, noting the results of the previous run …everything that must be done. The goal of reducing cycle time is to compress more process step runs into the same amount of time.
Some chemical process steps contain operations in which solution or slurried reactor contents are concentrated by removing some volatiles. If this concentrating consumes a significant amount of time, making it more efficient might go a long way or even all the way towards increasing throughput by reducing the cycle time.
In the fine chemical industry most evaporations are either semi- or fully batch. While the product from the evaporation step can either be the overhead or the pot contents, the liquid to be separated is usually either wide-boiling or has a non-ideal vapour/liquid equilibrium (such as an azeotrope), which results in a more discreet separation between mixture components.
Processes where liquid concentration is a predominant aspect actually use special equipment to remove unwanted fluid. A typical industrial evaporator has tubular heating surfaces, a vessel to hold the charge and sweep away vapour from liquid, and a condenser (heat exchanger) to condense the lighter overhead fraction. These units can operate at atmospheric or elevated pressures, but are often run under vacuum to reduce the system temperature. This unit operation can be run continuously, semi-batch-wise, or fully batch-wise.
Most multi-purpose plants do not have access to such equipment; however, even in a process that is predominantly chemical reactions, getting access to such specialized but common equipment may be advantageous if part of a workup or isolation is a laborious concentration.
In batch chemical reactors commonly encountered in the typical multipurpose fine chemical plant heating is commonly simply coming from the reactor walls.
Still even here the operation of concentrating solutions or slurries by batch distillation, can often be shortened. In most cases, these practical approaches are applicable to both vacuum and atmospheric evaporators. With respect to the evaporation the question needs to be asked, “Where are we with respect to this system’s capacity?”
If an important element of the problem is the rate of heat transfer from a jacketed reactor’s walls, injecting live steam into the reactor is another way to supply heat. By using what is rudimentary steam distillation the vapour fraction of the organic volatile which you are trying to remove is reduced-yes- but the actual rate of heat transfer and hence mass vaporized per unit time may more than compensate for that. So long as your condenser can handle it, steam distillation can be combined with reduced pressure to give vacuum steam distillation. Contrary to what you might think, distilling with steam may actually cause less degradation than ordinary distillation because the temperature of the steam may be much less than the temperature of the jacket’s walls.
Even if water has deleterious effects on the charge you are trying to concentrate, inert gas sparging can still speed up an operation. We know this because there are anecdotal reports that reactions that use volatile catalysts can stall on scale-up because the higher rate of inert gas sparging provided in the pilot plant unexpectedly reduced the charge of catalyst.
If the process is currently set up as atmospheric evaporation, the obvious change would modify the process train such that it can become a vacuum operation. The first and hopefully most obvious issue here is to ensure that all equipment in the evaporator train is rated for vacuum service. If not, this option may require a substantial capital investment and other means of achieving desired capacity may need to be investigated first. However, if the process is already configured for vacuum evaporation, it can simply be run at lower pressures, thus allowing the system to operate at lower temperatures, keeping the product cooler.
Take into consideration that running at lower pressure may mean a decrease in vapour density and, thus, an increase in vapour loading to the condenser to achieve the increase! Ensure that the condenser and utility streams are capable of handling such an increase.
Temperature Sensitivity Issues
When the product is a solute, one needs to know the yield/product losses due to the heat-treatment of temperature-sensitive materials. Perhaps the recovery-yield can be improved by more precise temperature control and throughput increased just by raising the overall yield. Even if the system’s overall temperature is not deleterious to the product, ensure that local hot spots in the base heater are not degrading some material. One way to resolve this is to ask: "Can we run the process at a lower temperature (which usually means running at lower pressure), such that we can keep the product cooler?" Again, this may extend the cycle time, but if yield improvements are large enough to counteract those losses, this could be an elegantly simple change which alters throughput in the right direction.
More Precise Heating with a Tempered Loop
An option that provides benefits both in temperature control and heat-transfer efficiency is to install what is called a tempered loop feeding your base heater on the liquid-service side of the heat exchanger. In a tempered loop, a pump re-circulates the bulk of the heat-transfer fluid through the reactor jacket at an increased rate, with a small bleed-in of cold or hot utilities to achieve accurate temperature control. The increased mass of re-circulating fluid buffers the temperature, improving temperature control, which, in turn, permits running a little closer to any limiting temperature values (examples might be the freezing point of the condensate mixture or degradation of a heat-sensitive material due to a minor control upset on the base-heater service fluid). Higher fluid velocities and a higher corresponding Reynolds number positively affect the fouling resistance, as well as the overall heat-transfer coefficients and, thus, the heat exchanger efficiency.
However, with the tempered loop, as with all changes, there are checks that need to be made to ensure that the system works properly. Be careful that the heat exchanger is sized to handle increased flow rates, velocities, pressures and pressure drops. This is something where you need the input of a chemical engineer. It can be problematic, and maybe dangerous, that after changing to a tempered loop, the heat exchanger's pressure-relief setting and system pressure are too close, causing the system to relieve the pressure with any pressure spike.
Accurate temperature control is an area to look at early on in evaluating capacity increases or when troubleshooting temperature problems (such as freezing condensate) in heat-transfer systems. For a relatively small investment, the reward can be large in terms of condenser freeze protection, additional heat duties and overheat protection needed for temperature-sensitive materials.
Cleaning
In cleaning, some of the desired material is removed from the equipment and ends up in the waste. In flushing (treated further below) residual desired material is removed from the equipment and combined with the major recovered amount.
Another area often overlooked when searching for more capacity, perhaps more related to specific process materials than the unit operation itself, is the cleaning cycle time. Assess the impact the cleaning cycle has on the overall cycle time and then investigate what changes can be made to simply reduce this time. Assess the appropriateness of the cleaning agents, analyze the cleaning sequence, the quantities used, etc. Optimize the water flush/rinse times or volumes to achieve more production time. Reducing the amount of idle time increases the number of lots processed.
When repeating certain steps with identical runs one after the other only a partial cleaning may be needed and only a visual inspection rather than the more thorough swabbing and chemical analysis. This saves a lot of time that was being taken into account measuring throughput.
Product Flushing
Particularly for batch or semi-batch processes, eliminating or minimizing physical product losses in between batches from inadequate flushing of precipitated materials can increase the recovery yield. Changing flushing techniques or solvents can do this.
Whereas extending flush time may cut some capacity by extending cycle time, it may still be justified if there is sufficient increased recovery.
Look closely at the product/solute properties. If the product is sticky, look for a flushing agent that is compatible with downstream processes but will aid in removing such particles prior to their transfer to the next process step.
Another example where extra flushing may pay dividends is recovering residual products from charcoaling and other solid adsorbent treatments.