Keys To The Success Of Pope Scientific’s Wiped-Film Stills

Distillation is one of the most important means of separation and purification of liquids in chemical processing. In a mixture of different chemicals, the various components will have differing characteristics, including physical properties such as levels of volatility.  A component with greater volatility than another (or “lighter” than the other), will boil at a lower temperature than the other. If the mixture is maintained at a temperature in between the boiling points (BP) of two components, the lighter component will vaporize while the heavier component(s) will not. If a surface (condenser), is placed in the vapor space and is kept cooler than any of the boiling points, the vapor will condense and can be collected as a liquid distillate with a composition having a greater percentage of the lighter component. The BP’s can be reduced by lowering the prevailing pressure, thus vacuum conditions are often used.

For simple molecules such as low molecular weight solvents, the temperature and the duration of the time of exposure to elevated temperature is typically not critical, in other words, prolonged boiling times, will not degrade these chemicals due to their thermal stability.  However, there exist chemicals of very high BP’s and molecular weights (MW) that are prone to chemical degradation by cracking, reacting or polymerizing.  These cannot survive the required elevated boiling temperatures for extended periods. The degradation effect is enhanced with increased temperatures, but often overlooked is the effect of the duration of time of the exposure to heat, which when extended, can cause far more degradation than the temperature level itself.

For these delicate materials, a special distillation technique is required.  The technique must include; provision for a very short period of heat exposure, a highly efficient mechanism to allow sufficient evaporation to occur within this short period, and high vacuum capability to reduce the extreme boiling points of the components.  Pope wiped-film stills are specially designed to include these and other important features.

Wiper Diagram

Wiped-Film Distillation Process

Feed liquid is continuously fed onto the inner wall of a heated vertical cylinder where it is immediately spread evenly as a thin film by rotating wipers around and down the cylinder.  The wipers are specially designed with diagonal slots, each of which causes a highly turbulent micro-mixing of the film. This dynamic causes a high degree of mass and heat transfer to take place, quickly heating the liquid and greatly increasing the effective surface area of the liquid-vapor interface, necessary for allowing the more volatile molecules to escape and vaporize rapidly from the liquid, (as opposed to being held back from reaching the interface by significant depths of liquid as is the case in, for example, a batch mode boiling vessel).  It is this efficiency that allows even high percentages of volatiles to evaporate within a matter of only seconds of travel in the cylinder.  The less volatile, heavier components don’t evaporate and remain as a liquid, leaving quickly out of bottom of the heated cylinder as residue.

Heated walls (orange) and high vacuum (yellow) drive the more volatile components (distillate) to the closely positioned internal condenser as the less volatile components (residue) continue down the cylinder. The resulting fractions from high vacuum distillation separate, and then exit through individual discharge outlets. Depending on the application, the desired product is either the distillate or the residue fraction. Small amounts of condensable low MW compounds collect in cold trap upstream of the vacuum system. For high solvent loads, an optional external condenser may be installed immediately.

The success of the wiped film technique is further enhanced by other special design features including capabilities of high temperatures to beyond 300°C and high vacuum down to 0.001 torr, a requirement for high boiling point distillates.  The high vacuum capability also allows substantial reduction in evaporation temperatures.  Yet another important design feature positions condensers a close distance from the evaporation surfaces, reducing pressure drop at high vacuum, allowing increased vapor transport efficiency and thus increased throughput through the still system. All is accomplished within an average pass through the system measured in seconds, decreasing thermal product degradation by levels of magnitudes compared to other distillation techniques.   Plus, the design allows for a straightforward scaleup from lab to pilot to production scale.  In addition to equipment provision, Pope has hands-on experience via our toll distillation, process development and testing services.  You can rely on Pope to work with your team from concept to commercialization!

Determining Which Fractional Distillation Process to Use: Batch or Continuous Mode

Introduction

Distillation is an important method for the separation and purification of liquids in a wide range of industries and laboratories.  For optimal purification in distillation, a vertical column is incorporated into the equipment with either a series of internal stacked plates (trays) or else filled with one of various types of structured or dumped-in packing.  The purpose of the trays or packing is to provide a high degree of vapor-liquid contact which results in multiple equilibrium stages or “theoretical plates”, each one leading to increasing purification all the way up the column, with the purest and lowest boiling point (BP) vaporized component(s) escaping the top of the column to contact a condenser where it leaves the column as a liquid distillate.  The overall result is that specific components of the starting composition have been separated, or fractionated, giving this process the name, fractional distillation.  Fractional stills often operate under pressure or vacuum and have a reflux device section for controlling and optimizing purity vs. throughput rate, however, these and several other topics of distillation equipment are not covered here. In this article, we will take a look at the two different types of fractional distillation; batch mode and continuous mode.

Batch Distillation Mode

Continuous Fractional Distillation Systems

Batch mode is the normal starting point of any distillation process and is the “simpler” of the modes.  Here, a fixed amount of feed material, (a batch) is loaded into a boiling vessel (pot), onto which is mounted a column with a condenser at the top.  The pot is heated and after a certain amount of time, the liquid begins to boil, and portions of it vaporize and travel up the column.  (One well-known example of batch mode is the traditional making of “moonshine” where a fermented mixture is loaded into the pot, heated, and an ethanol-enriched distillate is collected). The first feed components to vaporize are ones with lower boiling points (BP) than the others.  These components move up the column, with the lowest BP component in the feed becoming increasingly purified by means of fractionation finally being condensed and leaving the rest of the mixture as described above, to be collected in a distillate receiver.  This will continue until the first component of lower BP is depleted in the boiling pot.

At this point, if the process is allowed to continue, the next component of BP greater than the first begins to make the travel all the way up to the condenser to be condensed, leaving as another distillate fraction that can be collected in a different receiver than the first.  Some feed materials may contain many components and this means of separating several of them, one after the other, in order of increasing BP, can be continued until the desired product components have been collected at which point, the distillation process is ended by shutting off the heating of the boiling pot.

Examples of batch stills are many and can include anything from isolation of a flavor component in an extracted natural botanical source to recovering certain solvents from a waste mixture for reuse.  A key characteristic of batch mode is that the composition of the feed material in the pot is constantly, incrementally changing throughout the duration of the run.  The composition in the pot at the moment will be different an hour from now or even just a few minutes from now.  The distillate collected will also change in composition over time as each subsequent component is distilled away.  Thus, this is not a steady-state process.

In situations where the quantity of starting feed material becomes quite large, for example well beyond 1000 liters, and feed lots must be processed frequently, for example, at least every day, batch mode will become quite limited in addressing production requirements.  Another method for fractional distillation is then needed and this is when continuous mode must be considered.

Continuous Distillation Mode  

Continuous mode fractional distillation can handle very large quantities of feed without the need for very large boiling vessels.  Instead, feed is pumped at a set flow rate into the distillation system which has a column and condenser which can be similar to a batch-type setup.  However,  in this case, a reboiler replaces the boiling vessel and this is outfitted with apparatus for continuous discharge of residue, often referred to as a “bottoms stream”.  The feed is preheated and enters the column at a height selected to optimize the overall process efficiency.  Distillate leaves the system from the condenser, similar to the case of batch mode.  So, in continuous mode, there is one stream entering the system and two streams exiting the system, the distillate, and the residue.

A key characteristic of continuous mode is that the compositions of these streams and at any point within the distillation equipment do not change over time as in batch mode, instead they remain constant, in a steady state throughout the entire run, the duration of which may go on for a considerable time.  Oil refineries are a well-known example; these typically operate 24/7, being stopped only for maintenance or other technical reasons.  There are exceptions to the 3-stream scheme described above; for example, in addition to the distillate collected at the top of the column, there may be multiple take-off points (side-streams) at various heights of the column.  In the case of oil refinery columns, these will be multiple component cuts including gasoline, diesel, and mixed solvents in the upper column region, and oil cuts of increasing weight and viscosity as the locations get closer to the bottom of the column.  It is important to note that these are never pure single-component cuts, but collections of many components of somewhat similar BP, for example, one stream may become further processed to become 10W-40 motor oil, and another stream may be used to create a variety of lighter lubricants, etc.  The heaviest components of all become the materials used in tars, asphalts, and the like.

Other continuous fractional applications may involve feed streams with several, but far fewer individual components than are found in crude petroleum oil.  Examples include many specialty chemicals such as pharmaceutical intermediates and electronics materials manufactured in reactors.  The target product(s) may be somewhere in the middle of the range of the BP’s of several byproducts that must be removed.  For high-purity products, side draws cannot be utilized, these will not be pure enough.  To isolate pure components, more than a single column is needed, the number being dependent on the number of components in the feed.  For example, the first column may be used to distill away several components which are lower in BP than the desired product.  The residue can then serve as feed to a second column which will distill away the product plus a byproduct of BP near that of the product.  This distillate stream may be fed to a third column which separates the product away from the byproduct that is co-distilled from the second column.  This type of separation of pure components is more easily done with a single batch still if heat sensitivity is not a problem, (waiting for the product to start coming off the column and collecting it separately), however, as mentioned earlier, this is not practical with very large feed quantities; instead, a battery of staged continuous mode columns is called for.

There are many considerations that go into the design of a fractional column distillation system. These factors can vary depending on the scale at which you are manufacturing. When working at a commodity scale it often makes sense to manufacture in a continuous fashion, however, on the pilot and small production scale an important consideration is whether to use a batch approach. In addition to scale, there are a few other factors that play a considerable role such as the quantity of material, purity, energy use, and how many components need to be separated.  These are highlighted in the figure below:

Energy Usage

Energy consumption in continuous distillation is lower than that of batch distillation.  Heat recovery can be utilized, and the process loops optimized for both product quality and energy consumption. Energy usage can also be optimized by both preheating the feed material and selecting the best column entry point for the feed stream, reducing the condenser and reboiler duties.  In batch mode, more energy is required in startup and because much of the heat is applied to evaporating the same material portions multiple times; this effect is reduced in continuous mode operation.

Process Development and Scaleup; Lab, Pilot Plant, and Production Scale

As mentioned, in nearly all cases, batch mode is the starting point for fractional distillation.  Even if the long-range goals call for very large production quantities and continuous mode installation will eventually be needed, new product development typically starts with lab scale and this will be batch mode.  Lab scale equipment with boiling vessels of less than 1 liter up to 12 liters is appropriate for process feasibility studies. The lab-scale distillation experimentation may be part of an extensive general product development project.  For example, it may be found that upstream chemistry and reactions need to be changed to allow proper distillation and other downstream operations work well, followed by more experimentation, etc.  If lab studies are successful, it is normally advisable to move on to stainless steel (or other higher alloy if necessary) pilot scale equipment.  This may include batch stills from a few liters in size up to 100 liters or more.  It may also include pilot scale continuous stills with column diameters from 1” to 12” and feed rates from 1 to a few hundred liters/hr. Equipment of the larger end of these ranges can also serve as small or mid-size production systems as needed.  Examples of such equipment are shown.

Pope Scientific offers batch fractional stills in glass from 1 to 22 liters and in stainless steel from 2 to 2000 liters or larger.  Continuous fractional still systems are available in stainless steel with from 1” to 24” column diameters.  All of the above equipment is normally designed and manufactured in skid-mounted modular turnkey form. Instrumentation can range from control elements only to fully integrated PLC control systems.  Semi-turnkey systems and core distillation components alone are also available.  During pilot plant studies, it may be found that the distillation system may need to be reconfigured somewhat in order to optimize the process.  Pope takes this into account in the design of pilot plants, for example, allowing the ability to add or decrease column height or addition of collection receivers, etc.  Control systems for continuous systems are more complex as there are several control loops required to achieve a steady state.  However, the programs required for batch systems can be more extensive than for continuous, depending on what the user wants to accomplish. This is because there are usually several time and event-based changes in operating parameters during the run which may be automated, with routines set up by the operator before the run begins.

Pope maintains a staff highly knowledgeable in applications assistance, chemical engineering, chemistry, and chemical equipment operation, providing the customer with a partner for matching processing needs to equipment optimized for the lab, pilot, or production project. In addition to the provision of equipment, Pope can offer lab and pilot scale studies, depending on the nature of the specific applications.  Experienced personnel in mechanical and electrical engineering, assembly, quality assurance, and documentation assure the equipment will work well from delivery and startup and will last for decades.

The Benefits of Utilizing Data Acquisition Systems in Distillation Processing with Yokogawa

Data acquisition systems (DAQ) have been around for a long time; however many people aren’t familiar with this technology when inquiring about distillation equipment. Programmable Logic Controllers (PLCs) are used often in chemical processing for controlling key variables such as pressure, flow rates and temperature. While most PLCs will have some sort of data logging capabilities, it is typical that the software associated with the PLC is very cumbersome. Often a programmer will have to spend a good deal of time to incorporate the basic trend information that would come standard on a DAQ system. In addition, data logging on a PLC is often just a CSV file that is dumped to a location after one run. The CSV file must then be further manipulated to show the data on a graph or chart. An actual data recorder can do a much better job with the visualization of data without hours of additional programming. A DAQ system allows the user to look back at any timeframe of data from 60 seconds to possibly several days prior. This data can also be set to be shown as trend information, bar graphs or just plain figures straight out of the recorder. A DAQ system can exist as hardware within a control panel in which it is viewed from a web browser or as external hardware with its own housing touchscreen control. Having such ease of access to these heavily customizable reports can prove to be very useful to an operator, which is why Pope likes to include both in our control systems.

Having hardware and software record data for you, as opposed to someone in the lab handwriting these bits of information, provides many benefits. Ease of access to data can be a strong point for the recorders. If a DAQ system is uploading this information to a hard drive or server, there are many ways this information can be shared and analyzed throughout a group or company from remote locations. This data can be used to ensure reliability or to improve process efficiency. Having access to this data also allows the user to make predictions in maintenance. For example, if a certain heating or cooling element starts to take more time to reach its target temperature, it could imply that that element is starting to fail. Knowing this ahead of time allows a company time to order replacements before a failure actually occurs. This avoids any sort of downtime.

Most DAQ systems will have similar capabilities as those mentioned above – so why do we specifically like Yokogawa’s DAQ? The two main reasons are its easy-to-use interface as well as its ability to comply with GMP environments. Yokogawa has done an excellent job incorporating universal gestures into its touchscreen technology. The same swipe and pinch motions people use every day on their cell phones are incorporated into their recorders. The display is very intuitive in addition to being very bright and colorful. There is no shortage of customization options when it comes to viewing your data. Bar, trend, and digital graphs can be set up to view whatever variables are desired. As noted, the DAQ can be connected via ethernet to view data in real-time from anywhere, or more traditionally exported on a scheduled basis. This data can also be exported in different files types to be observed in other means. In regards to its GMP capabilities, Yokogawa DAQ systems can be compliant with CFR 21 part 11. This specific compliance is the FDA’s regulations for electronic documentation and electronic signatures. Yokogawa’s advanced security add-on allows the users DAQ system to contain an encrypted data file to prevent any manipulation to the data recovered. It’s a one-way road that only allows data to be exported out in an XLS file type. This add-on also contains an onboard audit trail that tracks what changes are made, who made them and when they were made allows for full traceability.

For all of these reasons, Yokogawa DAQ systems are a great asset to distillation equipment. Next time you’re inquiring about a Pope distillation system, be sure to ask how Data Acquisition Systems can be used with your process. Our experts are available to answer any questions and provide recommendations.