November 20, 2019

Falling Film Evaporators: The Ultimate Guide

The use of solvents is a critical step in the overall industrial process of cannabis extraction. There are several types of solvents that can be used, although supercooled/cryogenic ethanol is one of the most effective and efficient for large-volume THC/CBD extraction. Not only is it fairly quick, but it can also be easily scaled up to boost production volume. 

However, once the solvent has done its part and the raw materials have been extracted from the cannabis plant material (also known as biomass), what should manufacturers do with the used solvent? It’s wasteful to just dump it out and not reuse it, so the process of solvent recovery becomes crucial.

Falling film evaporators are industrial machines that use evaporation and condensation under vacuum conditions in order to reclaim and save the used solvent. Cannabis manufacturers can then use this reclaimed solvent for further extraction runs, thereby increasing their return on investment (ROI) and maintaining profitability.

WHAT KINDS OF SOLUTIONS СAN BE RECOVERED WITH SOLVENT RECOVERY?

The following types of solvent solutions can be successfully recovered:

  1. Non-azeotropic
  2. Aqueous non-azeotropic
  3. Heterogeneous non-azeotropic
  4. Aqueous homogeneous azeotropic 

Depending on the type of cannabis extraction system you are using, you should be able to recover all of the above types of solvents. This also includes ethanol extraction systems, even if the ethanol is supercooled or cryogenic. 

Furthermore, the boiling point of a solvent can be easily, safely, and effectively lowered through the use of a vacuum. In other words, any solvent recovery system that operates in a vacuum can vaporize the solvent solution and then reclaim it for future use. Additionally, if a recovery system is properly operated, then recovery efficiencies of at least 80% have been reported (with some even operating at higher rates). 

In most cases, however, recovering 75% of the volume of solvent at full chemical purity is the result of this type of system. In other words, you would be losing about 25% of the solvent during the recovery process, although this is vastly better than losing 100% by not performing solvent recovery at all. 

The benefits of using ethanol as a solvent in cannabis extraction are numerous, and it’s quickly gaining popularity as one of the most effective, efficient, and safest solvents a producer can use. Furthermore, ethanol solvent recovery works exceptionally well with falling film evaporators. Falling film evaporators get their name from the film that the solvent creates inside the vertical tubes where the liquid solvent is vaporized into a gas (before it is condensed at the end of the recovery process). Ethanol forms a thin “film” easily (due to properties in its viscosity), meaning that the heated tubes in the falling film evaporator can vaporize it more quickly than if it were collected in a large volume at one time.

WHAT ARE THE BENEFITS OF SOLVENT RECOVERY

Before we go into the details of how falling film evaporators work, let’s take a look at the overall process of solvent recovery. There are a number of benefits, including: 

  1. It reduces the cost of solvent and allows producers to reuse material. 
  2. It also reduces the cost of transportation and disposal of materials. 
  3. The reporting requirements for the Environmental Protection Agency (EPA) are also lowered as there are less solvents and/or chemicals that need to be disposed.
  4. It allows producers to maintain a high return on investment (ROI) by reducing waste and optimizing the overall extraction process. 
  5. There is hIgher efficiency in the manufacturing process due to the use of chemicals that are still effective and usable after recovery.

With that being said, if you wanted to calculate the ROI for your solvent recovery system, you would have to take the following criteria into consideration: 

  1. Amount of solvent used
  2. How much waste products (solvent solution) is created during the process
  3. The cost of disposing these waste products
  4. Any and all capital or annual costs

If properly implemented and operated, a solvent recovery system can greatly boost ROI to keep your operations streamlined and your costs low.

WHAT EXACTLY IS A FALLING FILM EVAPORATOR?

A falling film evaporator is a type of solvent recovery system. Essentially, it is a system that uses a vacuum to lower the boiling point of the solvent solution, vaporizes it with the use of a heating source, and then turns it back into a liquid form at cooling condensers. This allows for a recovery and reclamation process in which manufacturers can save and reuse high volumes of the original solvent solution. 

Remember, the working principle of falling film evaporators is that the internal environment is operating under a vacuum, thereby lowering the boiling points of solvents like ethanol. As you may imagine, using a vacuum in this manner means that the heat energy expenditure is demonstrably lower (since you don’t have to heat the liquid solvent nearly as much).

Additionally, having the solvent form a thin, evenly-distributed film within the interior of the vertical tubes is absolutely critical in order for the process to occur efficiently and effectively. This film occurs inside of multiple vertical tubes that have a heated surface. Furthermore, the liquid solvent is poured into the tubes and flows downward due to gravitational pull. 

Because a heating source is used to heat up the sides of the vertical tubes, the solvent solution starts as a liquid and is vaporized into a gas within the tubes. This liquid/vapor mixture makes its way down the vertical tubes due to a mixture of gravitational pull and negative vacuum.

WHAT IS THE SIGNIFICANCE OF THE “FILM” INSIDE THE VERTICAL TUBES?

The formation of this “film” is absolutely critical for the proper function of a falling film evaporator. Furthermore, it bears repeating that this film must be thin and evenly-distributed; this maximizes the efficiency of the heat expenditure by “spreading out” the liquid solvent over a wider surface area (the interior of the multiple vertical tubes).

Additionally, as we mentioned above, the act of heating the liquid solvent via the surface of the vertical tubes causes vaporization, and the separation between the two phases of matter (liquid and gas) occurs within the vertical tubes. However, the downward vapor velocity increases because of the principle of the conservation of mass. This creates a positive feedback loop where the shear force that is acting on this film increases as well as the downward velocity. In other words:

Increase in vapor velocity → Increase in shear force

So what does this mean? Well, as the downward velocity increases, the film becomes increasingly thinner, thereby increasing the turbulent flow. This is how the entire falling film evaporation system is able to maintain the very high heat transfer coefficients that are necessary in order for it to work effectively!

So, it follows that the heat transfer coefficient on the heated side of the vertical tube (the interior wall or the evaporating side) is determined by hydrodynamic conditions of this film. Remember, this heated side is the side that comes in contact with the liquid solvent and the film that forms as it’s poured down the length of the vertical tubes. So what do we mean by “hydrodynamic conditions”? Well, the flow of the film can be laminar in the case of high viscosity or low mass flow. This means that heat transfer is determined via conduction, and that as mass flow increases, the heat transfer coefficient decreases. In other words:

High mass flow → low heat transfer coefficient

As the mass flow becomes higher and higher, then the film becomes increasingly turbulent. In other words:

Higher mass flow → higher film flow turbulence

As these turbulent conditions increase, the heat transfer coefficient also increases accordingly with increased flow. 

So what does all this mean? It’s just basically saying that the use of the film alongside the heated walls of the vertical tubes creates a system that causes a positive feedback loop, making it inexpensive in terms of the expenditure of heat and energy. To put it simply, this is a highly efficient way of recovering solvent that uses the natural hydrodynamic properties of the film to complete the process.

WHAT ARE THE COMPONENTS IN A FALLING FILM EVAPORATOR?

Let’s break the components down as follows: 

  1. The machine will always have a feed at the top (this can also be referred to as the head of the evaporator). This is where the used solvent is poured in before it is vaporized. If supercooled/cryogenic ethanol is used, then it can be poured in while still cold; the vacuum conditions and the fact that it forms a thin, evenly-distributed film within the vertical tubes means that it will quickly be vaporized by the heating components.
  2. After the feed, the solvent pours down a series of vertical tubes. How many of these vertical tubes are there in a falling film evaporator? Well, it depends on the model and machine. The most common configuration is four rows of tubes arranged in a staggered pattern, with each row having 12 tubes in four pass arrangements. Having multiple tubes increases the efficiency – the higher volume of original used solvent, the more surface area you need in order to create a thin, evenly-distributed film. Having more tubes equals more surface area, thereby requiring less heat energy to vaporize the liquid solvent. Furthermore, there is usually a spray ball or internal distributor near the feed (or head) that ensure that the liquid solvent is evenly distributed between all the tubes.  
  3. These rows of tubes are arranged within a large, tubular structure known as the evaporator (because this is where the liquid solvent is vaporized). In some cases, it’s also known as the fluid distributor or, if they have a concentric shape, ferrules. This portion has to be designed carefully in order to maintain an even distribution of the liquid solvent along the walls of the vertical tubes. Remember, the key words for the “film” formed in falling film evaporation are thin and evenly-distributed; maintaining these criteria for the liquid solvent during the process of vaporization creates an effective and efficient system. 
  4. The liquid solvent is vaporized via the heating medium. Remember, the vacuum conditions keep the natural boiling points lower, although heat energy is obviously still needed. It is usually placed outside of the tubes, thereby heating the surface of the vertical tubes. Due to the fact that high heat transfer coefficients are needed to achieve well-balanced heat transfer resistance, the heating medium that is most frequently used is condensing steam. There is frequently a valve to pump this steam into the evaporator and to surround the fluid distributor or ferrules. 
  5. Within the tubes, this vaporization is partial, so the liquid product and solvent vapors both flow down the evaporator in a parallel flow. At the bottom of the evaporator there is an outward valve that can extract the concentrated product and then the rest of the vapor/liquid solvent solution is pumped to the separator. Remember, everything is happening in a negative vacuum, so it’s easy to pump this mixture into a separate component. Here, the vapors and liquids are separated, giving you a recovered solvent that has only lost a small proportion of its total volume. Depending on the parameters and type of the falling film evaporator, the skill of the operator, the type of solvent, and various other factors, the efficiency of this reclamation can be in the range of 75 to 90% (meaning that only about 10 to 25% of the solvent is lost during the reclamation process). 

So that is, in a nutshell, how a falling film evaporator works. As you can see, it’s a fairly simple machine. Again, it bears repeating that it works on the fundamental concept that the liquid solvent forms a thin and evenly-distributed film alongside the interior of the vertical tubes, all of which are heated by a source (usually steam) which then vaporizes the solvent. The thinner and more evenly-distributed the solvent film, the more efficient the process becomes.

WHAT ARE THE ADVANTAGES OF A FALLING FILM EVAPORATOR?

  1. They have high heat transfer coefficients.
  2. The residence time on the surface of the heated tubes is short. In other words, the short contact time for the solvent will minimize the effects of thermal degradation. 
  3. Due to the system being a vacuum, there is a low pressure drop. 
  4. The evaporation ratios remain consistently high. 
  5. They are highly cost-effective and efficient for large-scale, high-volume extraction operations. 
  6. Due to their high-speed and high-volume production, they are best for bulk cannabis distillate or isolate. 
  7. There is a low probability of fouling occurring due to the low vertical tube wall temperature.
  8. Foaming products are reduced due to high vapor shear. 
  9. The turndown on recirculation is almost infinite.

WHAT ABOUT OTHER SOLVENT RECOVERY SYSTEMS?

Falling film evaporators are an effective and efficient means for solvent recovery. However, they are not the only machines available! There are also wiped film evaporators which use wiper blades to “smear” the liquid solvent into a thin film onto the walls of a heated tube. This can increase or decrease the residence time in order to speed up or slow down the rate of evaporation. 

Additionally, there are also climbing film evaporators or climbing film plate evaporators. These, in essence, are the same as the falling film evaporators we described above, except that the liquid solvent is introduced not at the head (or top) of the evaporator, but at the bottom. 

Finally, producers can also use rotary evaporators (or rotovaps) in the process of solvent recovery. These machines use gentle, mechanical agitation to evaporate the liquid solvent, avoiding the use of heat energy entirely. These are also especially popular with cannabis extraction or THC/CBD production, especially if the manufacturer is looking to create a final product that has its terpene and flavonoid profile entirely intact. The use of gentle agitation is better for small-scale shatter production, but the needs of the producer ultimately decide which solvent recovery system works best. 

In conclusion, falling film evaporators are excellent for large-scale, high-volume extraction processes. They work exceptionally well with ethanol solvent recovery and can help a producer boost their ROI and maximize profits. The most important takeaway from this article is understanding that the mechanism of action for falling film evaporation is the formation of the “film” on the interior of the vertical tubes: it has to be thin and evenly-distributed in order for the heat energy expenditure to stay low. Additionally, everything will occur in a negative vacuum in order to keep boiling points low, making it an excellent solvent recovery system for temperature-sensitive solutions.

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