Distillation can be performed using a variety of techniques and equipment types. One of the most popular is Wiped Film Distillation or Thin Film Distillation. These two terms, like Wiped Film Evaporators and Thin Film Distillation Systems are often interchangeable.
These specific types of distillation systems are extremely effective for concentrating heat-sensitive materials. They are the preferred method for the distillation of high-boiling-point compounds in industrial or large-scale production.
Thin/wiped film systems often have a variety of names depending on the manufacturer of the system and what compounds are being distilled within them. Some alternative names are thin film, wiped film, rolled film, or just short path distillation.
One thing all these systems have in common, however, is the operating principle of creating a “thin” film inside the evaporator. When a compound is spread thinly, it makes it much easier to evaporate. What this means is that the temperature required for distillation can be lowered, and with the help of reduced pressure (using a vacuum system), high-boiling-point compounds can be effectively distilled without damage.
The low residency time inside the system allows compounds to be distilled and separated quickly, without degrading thermally-sensitive compounds. Most systems also utilize some level of automation (such as with our EliteLab® software), giving them the ability to run continuously. This means that they are more effective than batch-style distillation systems.
Understanding how a Thin/Wiped Film system operates is easier than most believe, especially because these systems can look extremely complex when viewed from the outside with no prior knowledge.
Although these systems can be complex in their piping and instrumentation, they are all very similar and share a few common parts which effectively allow them to create this “thin” film and distill a given compound.
Below, we will go into detail on the operation and layout of these systems as well as the chemical process that pertains to the distillation of high-boiling-point compounds within the thin/wiped film system.
Thin/Wiped Film Evaporator Nomenclature Explained:
Thin/wiped film systems can have a variety of names. This is because certain systems use different methods for creating this “thin” film inside the evaporator.
The most common name is Wiped Film. This refers to the use of a wiper blade to spread the compound evenly along the interior wall of the evaporator, thus creating a “wiped” film across the surface of the whole evaporator.
The term Thin Film can either describe the same operating principle or be applied more generally to any system that creates a “thin” film across the interior surface of the evaporator.
Another name that is sometimes used to describe these evaporators is Rolled Film. This corresponds to utilizing rollers, or hollow cylinders (often made of PTFE), stacked upon one another loosely around a rod for retainment.
These “rollers” are then strategically placed around a wiper basket in place of wiper blades. Rolled films offer an extended life-span for the wiper module, thanks to minimized forced contact with the outer surface of the evaporator body. When the wiper basket rotates, centrifugal force causes the rollers to come into contact with the outer surface, thus creating the thin/wiped film.
Thin/wiped film evaporators can also be used with an external condenser instead of an inner condenser, like the “short path” design. The external condenser would be much farther away from the area of evaporation, meaning there is a larger pressure drop between these two areas inside the system.
External-condenser wiped/thin-film systems work well for tasks like solvent recovery or devolatilization prior to distilling higher-boiling-point compounds inside of a “short path” evaporator. (Refer to diagram B below.)
Short Path Distillation:
As stated above, another common term many wiped film evaporator system manufacturers use is “short path” distillation. This is not to be confused with the more common batch-style system known as a short path.
However, both a benchtop short path system and a wiped film system utilizing a “short path” are similar in that there isn’t much distance from where the distilled compound is evaporated to where it recondenses in the form of a distillate. This reduces the pressure drop between the area of evaporation and the area of condensation, thus allowing thermally-sensitive compounds to be distilled effectively. (Refer to diagram A below.)
In the case of a “short path” wiped film, a condenser inside the evaporating column is used to re-condense whatever compound is being distilled. The distance between the wall of the evaporator and the inner condenser is usually only a few inches or centimeters.
Thus, the compound that is being distilled travels from the wall of the evaporator directly into the condenser in the middle of the evaporator column which is only a short distance for the vapor to travel. This is the concept behind the “short path” within a wiped film evaporator. (Refer to Diagram A below.)
Diagram A: “Short path” configuration of a Thin/Wiped Film
Diagram B: Thin/Wiped Film configuration (notice the external condenser)
Thin/Wiped Film Evaporator Components:
Thin/Wiped Film Evaporators can be extremely complex in their build and design, like those from HVE or InCon Process Systems, but almost all thin/wiped film systems have the same major components that allow them to function properly and effectively distill high-boiling-point compounds. Below, you will find a list of the major components and their functions within the system:
- Still: The main body or column of a Thin/Wiped Film evaporator is usually referred to as the still. This is the area of the system in which distillation actually occurs. The “still” body is usually one of the most complex sections of the system and includes a motorized wiper mechanism of some sort, a feed connection, an inner condenser (or external condenser depending on the configuration), and two discharges for the distillate and residue. The body of the still also has an outer jacket which is heated to create conditions for distillation.
- Feed: The feed supply is the crude oil that will be distilled within the evaporator. This has often been prepared or purified – prior to distillation – to maximize yield and consistency. In most systems, there is a pumping mechanism between the feed supply and the still body that helps maintain a consistent flow rate.
- Distillate & Residue: These are the two end products that the thin/wiped film creates. Distillate is the purified compound that was selectively collected based on boiling point; typically this is the target compound. Residue is composed of any feed constituents that did not evaporate during distillation; typically containing minimal amounts of the target compound. The residue may also be considered a waste product or saved for another material application. In continuous/automated systems, there are usually pumping mechanisms at the distillate and residue discharge points in the system. This allows for continuous feeding and is what gives wiped films an advantage over traditional batch distillation systems.
- Vacuum System & Cold Trap: The vacuum system is an integral part of thin/wiped film operation. The vacuum system is what allows distillation to occur at much lower temperatures. This is because compounds will evaporate at lower temperatures inside of a vacuum than they will at atmospheric pressure. The vacuum system is usually made up of one or more vacuum pumps as well as a secondary condenser or cold trap after the still body. The cold trap will trap any light compounds or vapors that made it past the internal or external condenser(s) prior to reaching the vacuum pump. This helps maintain vacuum levels and increases vacuum pump longevity.
Depending on the system and configuration, there may be many more parts/devices installed on a thin/wiped film system. However, the core parts are all detailed above. Understanding the major components of such systems can often shed light on nearly any system you might encounter in the real world.
Thin/Wiped Film Technical Characteristics:
Thin/wiped film systems are often built using two types of materials: glass and/or some kind of metal, typically stainless steel. Glass is usually used on smaller systems and can be cheaper than stainless steel. As systems increase in size, stainless steel becomes the best option due to its durability and temperature stability when compared to glass.
Because these evaporators have a variety of temperature zones throughout the system, it is imperative to have quality temperature control systems in place. Heating can be done via electric wraps, bands, or even by fluid circulation through jacketing.
Chilling is done using fluid circulation or using probe/immersion-style coolers. Each wiped film system has a variety of temperature control systems that allow it to operate at the correct distillation temperatures.
In the case of the system above, the larger fluid circulator is for the jacket around the still body, and the smaller one is for the inner condenser within the still body. The still body temperature would be set to the temperature required for the distillation of a given compound.
The inner condenser would be set to a lower temperature, namely the condensation point of the compound being distilled. The smaller circulator also controls the temperature of all the jacketed lines on the system. There is also one other circulator (not pictured above) that would chill the cold trap to protect the vacuum system. Some systems use dry ice or liquid nitrogen to chill the cold trap.
Some systems also have multiple “stages”.
These systems have two or more full evaporators in sequence. The first stage will remove a portion of the feed material to devolatize the feed to a desired degree, and a second (or third) distillation will be performed in the following stage to obtain the target compound.
Multiple stages reduce the need to do multiple passes through a single system, which can be more time-consuming. They also keep compounds inside the system, reducing the amount of exposure to the atmosphere which can degrade some compounds due to light and oxygen exposure.
Thin/Wiped Film distillation systems can differ greatly in construction material, functionality, and ancillaries used with the system.
These details will differ depending on the throughput required, feed material constituents, and the price point of the system. When researching types or manufacturers of thin/wiped film systems, it is always important to understand what the major parts of the system are, so that you can correctly identify the potential performance needed from the system and how much it may cost.
Thin/wiped film units can also greatly expand your processing speed and capabilities with a much smaller footprint compared to batch systems. This is due to their ability to run continuously and distill high-boiling point/thermally-sensitive compounds more efficiently than other types of distillation systems.