- Pumping Speed (also referred to as displacement or swept volume)
- Depth of Vacuum
There are also other factors associated with vacuum pumps, with the most significant being what capabilities the pump has to best handle the process from a perspective of sustainable performance and resistance to failure in the application it is being applied in. This is the case in every industry that utilizes vacuum pumps and is more of a factor in the Botanical Processing Industry given that the pumps are being utilized for the extraction of Volatile Organic Compounds to end up with a purified end product.
The main challenge to using vacuum pumps in botanical processing is that ethanol, cannabinoids, terpenes, and other Volatile Organic Compounds can condense inside the vacuum pumps as a sticky byproduct and cause poor performance, damage, and eventually vacuum pump failure.
The simplest understanding of how a low pressure enhances the process is from a grade school chemistry experiment that introduced a low pressure in a beaker of water and the water would boil at a lower temperature. This is a simple demonstration of the vapor phase curve, wherein every substance will change from a solid to a liquid to a gas at a specific temperature and pressure, The temperature at which the phase change occurs is (generally) reduced when the pressure is reduced. At sea level, water is solid below 0℃, liquid between 1-99℃ and gas at 100℃ (212℉), however, reducing the pressure moves the boiling point to a lower point on the curve as shown below.
The reason a lower pressure is desired in botanical processing is that the key component of the process involves removing Volatile Organic Compounds with heat, however, too much heat can damage cannabinoids and terpenes. This results in dark-colored concentrates vs. the desirable pure honey-colored product. This is where the use of low pressure allows the solvents to be effectively removed while keeping the required heat to a minimum.
Many different steps in the process utilize vacuum pumps; as with temperature, the same is the case with pressure – too low a pressure results in the removal of some substances that are desirable to be retained in the end product. Therefore, the different steps of the production process require different levels of low pressure (vacuum) to produce the desired end product. The key here is that the end product will differ depending on what you are aiming for. In the end, a combination of heat and low pressure remove (extract) specific Volatile Organic Compounds along the steps of the process to produce a specific desired end product – however, the right combination of both results in the most efficient process that produces the best end product.
Here’s a list of some of the processes & equipment where vacuum pumps are utilized to either enhance the process, or when low pressure is required to produce a pure clean product:
- Solvent Recovery
- Rotary evaporators
- Falling film evaporators
- Decarboxylation
- Wiped Wall evaporation
- Vacuum ovens
- Biomass Drying systems
- Freeze Dryers
Also, for the sake of understanding pressure, and in the case of negative pressure that is the subject matter herein, there are many different pressure measurement scales – in this case, all measurements are given in Torr, which is a measurement of pressure developed by Evangelista Torricelli, an Italian physicist, mathematician, and student of Galileo; he is best known as the inventor of the Barometer. The Torr scale is quite simple, shown below is the portion of the scale in which is the range that vacuum pumps work in.
- +2 (10+2)
- +1 (10+1)
- 0 (10+0)
- -1 (10-1)
- -2 (10-2)
- -3 (10-3)
- -4 (10-4)
- Etc…
At sea level, Atmospheric Pressure is 760 torr. As you increase in altitude, the pressure is reduced or lowered. When you drive into the mountains, say 11,000 ft., the atmospheric pressure is 502.9 torr. When you’re in an airplane flying at 33,000 ft., the atmospheric pressure outside the plane is 197.1 torr.
At the point that you reduce below 1 torr, the measurement is changed to Millitorr. The point negative of 1 torr is 999.99 millitorr or 9.99X10-1. The -1 scale goes down to 100 millitorr, (1×10-2) The point negative of 100 millitorr the pressure is 99.9 torr, or 9.99×10-2. The -2 scale goes down to 10 millitorr (1×10-2). The point negative of 10 millitorr is 9.99 torr or 9.99X10-3. Below 1 millitorr you get to the -4 scale. This scale continues to decrease as the pressure decreases. Think ( -17 scale) The pressure on the moon is 2×10-12. Even though it is a low pressure, it is still not a perfect vacuum. The pressure in deep space is considered to be in the -17 scale. We have been asked through the years about a pump that can produce a perfect vacuum – however, it is simply not possible.
Botanical Processing Vacuum Pump Types & Their Attainable Pressure Ranges:
- 7.6×10+2 to 4.0×10+2 (760 torr to 400 torr)
- Diaphragm
- Liquid Ring pump
- 2.0×10+1 to 3.5×10-2 (20 torr to 35 millitorr)
- Single-Stage Rotary Vane
- 1.5×10-2 to 2.0×10-3 (15 millitorr to 2 millitorr)
- Dry Scroll Pump
- Air-Cooled Multi-Stage Dry Pump
- Dry Screw Pump
- 8×10-4
- Two-Stage Rotary Vane
- 5×10-6
- Molecular Drag
- 1×10-9 to 1×10-11
- Diffusion
- Turbo-molecular
Various vacuum pumps can be utilized in botanical processing processes to produce different levels of pressure and required pumping speed. The list below is generally arranged by the attainable pressures from highest to lowest.
Diaphragm pumps for Botanical Processing
These pumps have become heavily used as they can operate successfully for extended periods of time when chemical version pumps are used. The chemical version pumps utilize Teflon Diaphragms as the Teflon is impervious to solvents. The disadvantage of Diaphragm pumps is that they are limited in the negative pressure (depth of vacuum) and the pumping speed (displacement). To attain the lowest possible pressures, these pumps are staged with up to eight stages, and to increase the displacement, they are combined (series), and Roots-type boosters are added before the diaphragm stages. Even with this, due to the compression ratio and operating pressure required with Roots boosters, the displacement is limited to about 6 cfm (Cubic Feet per minute)
As with all of the vacuum pumps discussed in this paper, Diaphragm pumps will eventually fail when materials are condensed in the “head space” which is the space that sucks in and then discharges the gases being pumped. Since diaphragm pumps operate with rods (like a piston), when the pumping chamber becomes filled with a solid, there are typically two potential resultant failures:
- The Teflon diaphragm wears, sometimes to a point of destruction where a hole is developed
- The rod and/or rod bearings become worn when the diagram comes up against a solid object and the stroke of the rod is reduced, causing stress on the rod and/or rod bearings.
The operating principle of a diaphragm pump is much like any reciprocating piston apparatus, wherein the rod produces a “stroke”, and the stroke moves the diaphragm in and out. If the space that is reserved for the transfer of gases is occupied by a solid mass, the rod and diaphragm push against the solid mass, thus causing a bending motion on the rod. This ultimately results in wear on the rod and bearings.
Liquid Ring pumps for Botanical Processing
These pumps are attractive to use as they are generally less affected by the ingestion and resulting condensing of volatile gases, however, they are dependent on the medium (liquid), which is typically water, being maintained at a specific temperature. If it becomes too elevated, the “ring” will cavitate (or collapse). The typical method of temperature control is via make-up water, which becomes costly and adds to issues with disposal when the liquid becomes contaminated with solvents and other products produced by the process. The other method of temperature control is via a chiller or cooling tower, both of which have adverse desires based on energy costs and water usage due to evaporation. In addition, if the compounds that are being pumped become too concentrated inside the pump (mixing with the water or other medium that is used to make the “ring”) the internal components of the vacuum pump can become affected – most typically from corrosion. These pumps are available in Cast Iron, Brass, Stainless Steel, or a combination of these materials.
In addition to cavitation from high temperature, the ring can also cavitate if the pressure is too low, so as a result, the pump is limited in the ultimate pressure it can attain as it always requires a leak of gas into the inlet.
Liquid ring pumps are available in single- and two-stage versions. Single-Stage versions are capable of 100 torr. Two-Stage Versions are capable of 25 torr.
These pumps can also be combined with a roots booster to increase pumping speed and reduce pressures at the inlet. They can also be combined effectively with multiple roots booster pumps of reduced displacement in series to produce pressures in the 10-3 torr range. However, these combined systems can get very large and expensive.
Single-Stage Rotary Vane pumps for Botanical Processing
Single Stage Rotary Vane pumps are usually applied to the first stage of the evaporation process as the process demands a pressure that is not too low. Referencing the outline of common pump types + their pressure ranges above, you’ll notice these pumps have a wide range of pressure – this is due to the fact that there are really two types of these pumps, with the first type typically being utilized in botanical processing:
- Course Vacuum (20 torr to 250 millitorr)
- High vacuum (35 millitorr)
One type is more of a high vacuum pump capable of pressures of 35 millitorr, and they resemble the two-stage rotary vanes discussed below, however, they only have one rotor/stage. The other type of single-stage rotary vane, such as that utilized in this process, is capable of pressures of only 250 milltorr.
Rotary Vane pumps incorporate a large oil reservoir which delivers a significant amount of lubrication to the rotor and vanes, coupled with an internal oil separation system to remove the oil vapors from the exhaust stream. The high content and flow of lubricating fluid equate to longer pump life as the oil takes longer to contaminate and the higher flow through the pump keeps the oil from solidifying inside the rotor and cylinder. Again, as with all of the pumps utilized in botanical processing, eventually, the process compounds condense inside the pump (Compression cycle) which in this case causes contamination of the oil, resulting in the requirement of frequent oil changes. Without frequent oil changes and other proper maintenance, eventually, the pump will fail due to the buildup of contaminated oil. If that happens, it can be a very “dirty” and difficult job to clean the pump components as needed in order to perform an effective and quality rebuild.
Dry Scroll pumps for Botanical Processing
Dry Scroll pumps have become very popular in the botanical processing industry since they require no lubricating oil. Although they have become popular, they are actually not a good solution for the applications in the industry. Dry Scroll pumps are best suited to very clean applications that do not have particulates, condensable vapors, or anything else in the gas stream that would be ingested by the pump. This is due to the design function of the Dry Scroll pumps in that they have intersecting “scroll” chambers that are sealed at the intersecting point with a Teflon seal (tip seal). This Teflon seal must be in constant contact with the opposing scroll chamber. It is this point (where the contact is made) that is a failure point for the scroll pumps. A typical frustration for users is that they purchase the pump and it works very well for a specific length of time (depending on the application) before it ultimately fails. However, after the dry scroll pump fails, the pump is rebuilt and then either does not pull effective vacuum, or the effective life of the pump is shorter than when the pump was new. Simply said, when used for botanical processing, the performance of dry scroll pumps degrades more quickly which causes a more frequent need for rebuilds.
Air-Cooled Multi-Stage Dry pumps for Botanical Processing
Another type of dry vacuum pump that has begun to be utilized is the Air-Cooled Multi-Stage Dry pump. Like the dry scroll pumps discussed above, these pumps are also considered as “POU” due to the fact that they require no water or other facilities besides AC power to operate. The purpose of identifying these pumps as Air-Cooled is that there are also water-cooled versions of these pumps available.
Air-Cooled Multi-Stage Dry pumps are not completely oil-free like the scroll pumps discussed above, as they have lubricating fluid in the gear chambers and/or in the bearing chambers. In most cases, the gear chambers have oil and the opposing bearing chamber has greased bearings. In either case, the lubricating oil is not in the swept gas stream, so the volatiles being pumped through the vacuum pump are not coming in direct contact with the lubrication and therefore not affecting/contaminating the lubricating fluid. Consequently, there is a significantly reduced opportunity to have vacuum pump oil contamination of the product.
These pumps have produced some effective results, in some applications, that in some cases may be more attractive than rotary vane pumps. However, they will also eventually fail, and when they do, the rebuild process is more specialized than vane pumps and scroll pumps and therefore can be significantly more expensive.
Our initial application of one of these pumps was as a replacement of a rotary vane pump on vacuum ovens that were used to process botanicals into concentrates. In this case, the end-user customer was not using cold traps on the rotary vane pumps, and they were likely not changing the oil as frequently as they should have been. The results were failed pumps in 2-3 months. They installed a Multi-Stage Air-Cooled Dry pump that we provided, and the initial pump ran for over 8 months with zero maintenance.
However, like all pumps that are ingesting volatile compounds, the compounds condensed in the final stages of compression causing the pump to lock up. The internal clearances of multi-stage dry pumps are extremely tight, and as contamination builds up inside these pumps, the pumps lock up, unable to turn. This results in the need for complete disassembly and cleanout of the pump.
The good news is there is generally no damage/wear to the internal components, and the pumps are returned to original operating specifications with no degradation of performance or operating life. Subsequent installs of these pumps in this application were equipped with gas ballast function, and the life of the pumps typically exceeded 14 months of operation with zero maintenance.
We have seen multi-stage dry pumps utilized in other applications (such as wiped wall evaporation) with less favorable results, with some failures happening in as little as 2 months and with similar causes such as the buildup of deposits in the final stages of the pump, causing the pumps to lock up.
Two-Stage Rotary Vane pumps for Botanical Processing
Two-Stage Rotary Vane pumps are among the most common pumps utilized in the botanical processing industry. They are relatively inexpensive, reliable, able to reach low pressures, and they can be coupled with roots boosters to increase the pumping speed. These pumps are available in a very wide range of quality and price points. A very low price is often indicative of the quality, but on the flip side, a very expensive price may not necessarily correlate to a better/higher quality pump. Ultimately, you should be sure that you are purchasing your rotary vane pumps from a trusted and knowledgeable source (such as Highvac.)
The biggest detriment to utilizing Rotary Vane Pumps (abbreviated as RVP) in this industry is that the performance of the pump is drastically affected when pumping volatiles. The reason for this is that vacuum pumps are basically a compressor running backward. If you’ve ever operated a basic air compressor, you will know that the compressor tank (ballast reservoir of compressed air) gets water in it that must be drained – when the compressor is running, it condenses the water vapor into liquid and deposits it into the compressor tank. The vacuum pump is doing the same thing, however in this case the liquid is not water – in most cases, it is ethanol or terpenes plus some traces of oil from the product. In the case of the vacuum pump, these liquids are being routed in the oil reservoir of the pump, mixing with the oil. This turns the excellent high quality / low vapor pressure vacuum oil that you are buying into a nasty mess of high vapor pressure (poorly lubricating) fluid. The end result is a pump that will not reach the base pressure that you’re trying to get to, and a soon-to-be broken vacuum pump.
However, if you start with a good quality rotary vane pump, add a high-quality cold trap in front of the pump coupled with the use of an effective gas ballast valve and frequent oil changes, these pumps are known to run for 1 year or more on vacuum ovens and evaporation systems. On the other hand, as outlined above, improper care and protection of these pumps equate to quick failure. As this is an all-too-common experience with the end-users of these pumps, you will find that most resellers/distributors of these pumps typically offer rebuild kits on their web stores.
End-users may attempt to rebuild their pumps in-house, and they are relatively easy to rebuild given a prerequisite of at least some basic mechanical aptitude. That being said, RVPs can be a bit complex owing to the small bits and pieces inside the pumps, and improper assembly (missing or misplaced parts) can result in frustration when the pump is restarted and the performance is not as expected/required. In addition, improper installation of the shaft seal can result in annoying oil leaks after rebuilding.
We have seen social media posts of rebuild training classes offered by some OEM’s, and we have seen the “flushing” of the pumps with ethanol where the pumps are drained of the contaminated lubrication fluid, filled with ethanol, operated for a short period of time, then the ethanol is drained and the process repeated a few times. New oil is put back into the pump, the pump is operated for a short period of time, and the oil is drained again. This process is repeated a few times until the pump is able to attain the required pressures. Although this process can work, it is not recommended and is very time-consuming.
Dry Screw Vacuum pumps for Botanical Processing
If you are really paying attention to the sequence of the different vacuum pumps discussed above, you may notice that this type of pump is out of sequence. Our sequence started with the pumps that create the least vacuum (higher pressures) and progressed to the pumps that create a deeper vacuum (lower pressures). There are a few reasons for ordering Dry Screw Vacuum Pumps after two-stage rotary vane pumps – if you refer to the data above, you will see that rotary vane pumps, in perfect conditions, will create lower pressures than Dry Screw pumps. However, as we have mentioned above, in the botanical processing industry, the conditions for the rotary vane pumps are less than perfect (think: volatiles.)
Dry Screw Pumps are very new to this industry, only recently being introduced as viable alternatives to many of the pumps outlined above. The reason for this is that until recently, dry screw pumps were only available in water-cooled versions, and in pumping speeds exceeding those typically utilized in this industry. This resulted in pumps that were very expensive in comparison to the pumps outlined above, and therefore not very available on a “retail” basis. As evaporation/extraction equipment has become larger, some very large displacement (250 m3/h) Dry Screw pumps have begun to be utilized. However, these pumps are very expensive and far too large for the typical wiped wall system or vacuum oven.
Based on our vast experience with vacuum pumps and what makes them reliable (or not-so-reliable), we have proudly begun supplying our Air-Cooled Dry Screw pumps specifically designed for this industry. Our smallest air-cooled dry screw pump is 25 m3/h and our largest version is 100 m3/h. We also have begun supplying our water-cooled versions that start at 80 m3/h and go all the way up to 800 m3/h.
You may be wondering “what is so great about dry screw pumps as compared to dry scroll pumps or the other pumps identified and outlined above?” Fair question, however, please do not confuse Dry Screw vacuum pump technology with Dry Scroll vacuum pumps- they are light years away from each other when it comes to technology. Dry Scroll pumps have, by design, internal components that are in constant contact with other components, and therefore constantly wearing components. As opposed to this, Dry Screw pumps have no internal components that make contact other than bearings and seals. Of all the pumps outlined above, Dry Screw pumps are the utmost of simplicity, following the K.I.S.S. Principle.
Dry screw pumps consist of twin intertwining screws, surrounded by a single housing, with the screws being suspended by opposing bearing “head” plates. Gears retain the timing between the screws as they rotate at high speeds.
Note: there are also some screw pumps that differ from the pumps mentioned above – wherein they are “cantilevered” pumps that have the screws overhanging one of the two head plates. The manufacturers of these pumps will expunge specific benefits of not having bearings on both ends of the screw rotors, however, the K.I.S.S. Principle mentioned above is no longer a factor with these pump designs.
That said, Dry screw pumps of either type lend themselves to a very unique ability to apply the methodology of “in-situ” cleaning of the internal components. Cleaning any of the pumps mentioned above on a regular basis will make them last longer, however, with any other pump mentioned above, this would either be not possible or difficult at best, as mentioned with the dual-stage rotary vane pumps. We like to say, “a clean pump is a (happy) running pump.”
As mentioned above – all of these pumps are eventually going to get contaminated internally and thus fail. This is owing to the fact that they are by design ingesting gases, and in the case of the botanical processing industry, volatile and condensable gases.
Dry screw pumps are no different, however, the in-situ cleaning process is simple, and takes little or no time and effort. The cleaning process is as simple as sucking some solvent (such as ethanol) through the pump inlet at the end of each day, effectively cleaning the deposits that have built up on the screws and housing throughout the day’s processing. The solvent exits the pump exhaust, and due to the elevated operating temperature of the pump, any residual solvent evaporates quickly, with the pump returning to normal operation in less than 30 minutes.
We have demonstrated this process effectively, with the initial pump trial being installed on a large (150 liter) mixing decarb vessel. The pump was operated without cleaning and failed in about 1 week. Pictures of the screw rotors below show the buildup of the process resins on the screw rotors. When the pump was shut off after a week on the decarb vessel, it failed to start the following morning.
The pump was taken apart, cleaned, and returned to service on the decarb vessel. It has been in operation for well over 10 weeks with no indication of imminent failure, no oil changes, and consistent and reliable vacuum pressures. It has been cleaned on a daily basis at shut down every evening and restarted without issue every morning during this time. Based on the principle that “a clean pump is a (happy) running pump” with daily cleaning, there is no reason that this pump is susceptible to failure in the short term.
All this said, this pump design, like the multistage dry pumps mentioned above, has bearings in the head plates at either end of the pump rotors, and the oil should be inspected and serviced on a regular basis. Typical would be quarterly oil changes.
Our Air-Cooled Dry Screw (abbreviated as ACS) vacuum pump technology is a viable replacement to operate in a variety of applications that the alternative pump technologies outlined in this paper are currently being utilized in. Another unique benefit of our ACS vacuum pump technology is that the level of vacuum/pressure can be effectively controlled via rotational speed with a frequency control driver.