Active vs. Passive Butane Recovery

Rather than address every thread one by one, I wish to address this discussion with its own thread and provide some academically grounded insight.


Active vs. passive recovery is analogous to the concepts of CNG vs. LNG when discussing natural gas. CNG, which uses pump compression, has a smaller capital cost which makes it suitable for small/mid scale applications. Whereas LNG (liquefied natural gas) dominates at very large to medium scale, and can be stored in very large low pressure cryo tanks, rather than CNG tanks which become impractically thick at large scale.

A similar issue can be seen with butane/propane extraction, at small scales the speed provided by a vapor recovery pump vs. capital investment is favorable, esp. for old school setups involving TR21’s and steel (rusting) butane tanks. However as the scale increases it becomes problematic for a number of reasons, the first of which is grounded in some basic and universally accepted physics.

Enter Choked Flow Theory: (

Summarizing, in a gas flow situation the fluid is compressible and in situations where flow is forced through an orifice once the downstream pressure (p2) (after the orifice, such as at the inlet of the recovery pump) drops below roughly 50% of the upstream pressure (p1) the flow will “choke”. What this means is that a shockwave forms in the orifice, which limits the ability of gas molecules on the downstream side of the orifice to communicate with the upstream side, thusly decoupling the downstream pressure (Now at P2 < ~50% P1) from the upstream pressure. Once the mass flow rate reaches the point at which choking occurs, no further amount of pumping speed will affect the flow rate!!! The pump is maxed out and it can’t pump any harder.

For the given upstream pressure and temp (P1 & T1) no force in the universe can increase the mass flow rate once the choke point is reached.

Counter points:

  • Ahahh you say!I will pre-cool the butane prior to the choke point, thereby increasing the density and subsequent mass flow rate! Yes this is possible and valid (Density increase due to temp decrease), but really just illustrates my entire point, pre-cooling the gas requires a cooling system or at least water tower/evaporator to provide heat rejection. You could just provide more cooling, and liquefy the butane and have a much simpler extractor flow diagram. Adding yet more lines, and yet more intercoolers creates a rats nest of extra bullshit.
  • Or you could say, I will use propane to boost the pressure P2! This is also possible but really doesn’t address the core issue and 100% butane has been shown to be a better solvent for quality and yield.

Further discussion points/assertions:

  • Passive systems which provide a large enough un-restricted orifice between the recovery pot and condenser and enough refrigeration power are much simpler and less complex on the extractor side.
  • Choked flow only refers to the idealized case of a flat plate with an orifice in the center, most extractors include fluid lines to connect the tanks and pumps, the more pumps and intercoolers are required the more lines must be used which creates additional flow loss via frictional ( pressure loss which I have not even included for the purpose of this simplified analysis.
  • Using liquid CO2 or LN2 is incredibly inefficient and costly, even at the bulk rate of $0.5 per gallon and commercial power cost of 7.68 Cents / KwH on the basis of operating cost alone.
    • Other analysis I have seen points to roughly 1 gallon of LN2 being required to chill 2 gal of ethanol to -40 Deg from 30 Deg C
    • On the basis of operating cost alone electric refrigeration is ~8 times more efficient based on preliminary analysis of our systems, and reaches lower temps in some models (-48, -60 Deg C).
    • Including CAPEX and waste heat to determine the payoff point for the refrigeration system vs. LN2 indicates the quickest payoff point would be reached with our 15 ton Piston system in as little as 4 months. (granted this is complex analysis and has many underlying assumptions, systems that are configured differently have higher CAPEX and payoff occurs within 6-12 months)
  • Refrigeration systems (such as those offered by GPS LLC) can be configured to recover their waste heat and route it back into the process, producing significant additional energy savings.
  • A passive extractor with a large near direct connection between the recovery pot/FFE and butane condenser is the ideal solution, simpler and safer to operate.
    • No electrical pump is required in the C1D1 Space, reducing noise and hazards, only a means of transferring the glycol/refrigerant from the external refrigeration unit into the C1D1 space is required.
    • Removing the recovery pump removes the need for air compressors and associated noise inside and outside the extraction space
    • Some(all?) pumps are dirty and shed Teflon and metal shavings into the oil (TR21)

Final assertion:

Once the total capital, ownership and operating costs are taken into account, passive electrical refrigeration is by far the safest, cleanest and most efficient method, far beyond LCO2, LN2, pneumatic or electrical pumping in anything beyond small scale applications. With financing available to get over the initial investment there is no rational reason(beyond refrigeration system production/installation lead times) to use the other methods with higher operating costs, when that time and money could be spent paying of the electrical system and reaching the breakeven point within 4 months to a year(depending on configuration).

Only by an overly narrow analysis of extractor/pump cost and operating costs can butane pumps be viewed as an appealing option by parties likely lacking the right information without considering the overall processing system as a whole.

(End rant)

(I understand my analysis is shooting from the hip here, but if anyone wants to split hairs and do more calculations you will not find huge differences)

(I am working on a longer white paper on this, if anyone wants to see my underling analysis and spreadsheets DM me)


So a passive, short transfer, hard piped system with the largest diameter possible would make the fastest CLS.


Jesus you guys dont give up with the bro science.


Wide till the condensor where the gas needs to get in contact as fast as possible with the cold surface
So a lot off thiner pipes or plates


Yes, the larger the orifice (A) the larger the flow rate. So even if you run into the issue of a choke point (based on pressure difference) increasing the diameter of your flow orifice Willa improve flow rate


If the recovery vessel is kept @ -80c there might not need to be a coil. :thinking: Thought this design out a while ago then saw Bhogart moved to all hard lines on their system.


It will work but I think it be better if a forced touching off cryo surface is implemented either by coil/ angel/ tube in shell like the glass coldtrap s
For a coil I now use a sshotwater boiler set in a maner that by means of gravity the coil empty s itself of liquids
We are used to putting the coil in a bucket having to push all condensate out
By cutting the botom of a hot water boiler tank and flipping it I have the coil set in. The tank(bucket) and place dry ice in there


That was my theory when designing our system. However, maintaining -80c tank temperature, while recovering up to 10 lbs per minute is literally impossible with dry ice and open jackets. The coils are necessary to increase surface area, but don’t be fooled, they do not aid in reducing dry ice consumption, only rather allowing you to recover faster.


There seems to be several branches to these issues worthy of detailed cost, energy, product quality and engineering analysis.

  1.   Solvent pre-chilling (Ethanol is the most demanding for this)

a. LN2 or Electric Refrigeration with chilled Ethanol or butane at (-40,-48 or -60 Deg C)
b. In the case of hydrocarbon you could run warm(room temp) Butane and CRC later…(ick)

  1.   Solvent Recovery (This one is tough because it is heavily influenced by extractor design, bizzybee systems would be good comparison benchmark as they can run LN2 or electric and are designed with large connections between vessels)

a. Passive via Electric Refrigeration or LN2 (Ethanol or butane)
b. Active via Pump (Hydrocarbons only)
i. Type of Pump (TRXX, MXX, HaXXXX, CoXXX) (Others?)
ii. Electric or Air Compressor driven (Includes power/capital costs)

  1.   Solvent Heating Evaporation

a. COTS stand alone heat source (Electric or natural gas, costs)
b. Waste heat recovery from electric refrigeration system
c. Combination of a and b

  1.   Overall complexity of extraction system and process

a. Review which parts are required for each possible configuration and estimated costs from various vendors, extractors heat exchangers etc. Difficult to do a true apples to apples comparison.


Why do you prefer ln2 over lc02? Or is that just what you used in your trials as a control?

LN2 was supposed to be the control but I guess we could include both if there is interest.

Was just curious.

What does CLS stand for?

Closed Loop System


Here to follow.
Running a passive cls currently eith just bho

I appreciate your explanations
What I am curious about is in the heat exchanger you use in your unit
What material is it made of ?

What is the heat transfer on butane and propane and the 30/70 mix ?
What diameter is the tubing ?
In a perfect situation what would the max condensation rate be ?

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This is helpful, thank you.

Their calculations support the same trends I was getting which is total ownership and operating cost of a mechanical refrigeration system which is ~8 times lower than LN2.

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For the food grade direct (R-507) refrigeration heat Ex see pg 2 of the spec sheet for a picture:

The heat Ex is double pass and uses seamless 316 tubing, and has ASME and CRN numbers. It can be optionally electropolished for an additional 26,430.-USD

See example PID below:


I’m confused by your CNG vs LNG analogy.

When you talk about CNG being better for small scale and LNG “dominating” at large scale, what exactly are you talking about? Fleets of vehicles? Or the general transmission and storage of natural gas on a global scale?

Either way your association of pumps (CNG) with smaller capital costs and smaller scale doesn’t seem to make sense here.

Recovery rate (speed) is generally not determined or provided by any pump/compressor or lack thereof. It’s determined by vapor generation in the evaporator or collection pot.

A pump that’s undersized will act as a restriction but a pump that’s oversized will generally not increase recovery rate although there may be some minor “scavenging” effect like a car’s exhaust manifold.

The recovery setup of an extractor is all about overall system design and workflow.

Generally at smaller scales in cannabis extraction the recovery pumps used (TR21, CMEPOL) often act as a major restriction due to their small pumping capacities and people see an increase in recovery rate switching to passive but sacrifice certain operating abilities.

Fluid characteristics and flow characteristics are two different things.

Just because the fluid itself is compressible, doesn’t mean the flow is.

Whether or not a flow is considered compressible is directly related to the value of the Mach number (speed) of the flow not the properties of the fluid itself.

Mach number is the ratio of flow velocity past a boundary compared to the speed of sound in that fluid.

Choked flow only occurs when the exit velocity from the orifice is at or very close to Mach 1, or 100% the speed of sound in that fluid. This is related to the critical pressure ratio across the orifice like you mentioned, but where are you thinking this is occurring in an extraction system? Why do you think the inlet of the pump would cause this?

I don’t see how the inlet to the compressors used in extraction would cause choked flow as you describe.

I think it’s possible you might be confusing choked flow through an orifice with compressor choking or stonewalling. Compressor choke is a property of dynamic (centrifugal) compressors, not positive displacement compressors like the ones we typically use in extraction.

Although some people erroneously think so, a gas compressor (pump) is not used in an extraction system to speed up recovery rate.

I agree that it’s very important to not have any restrictive orifices in the vapor recovery stream. The typical Swagelok GS series process valves that are used do indeed have a restrictive orifice size at the larger connection sizes and I make sure to use 63 series or full port valves on my vapor side to avoid flow restriction.

I’m not saying it’s impossible to have choked flow conditions in extraction systems, especially huge falling films and very large collection pots but I think it’s very unlikely.

Compressible flow and choked flow seem to be more an issue in high speed (mach 1) aircraft, jet engines, rocket nozzles and high pressure gas pipelines (I am familiar with orifice plates).

I would say this really depends on the specific situation.

For a very large batch or continuous system with intermediate tanks and falling film evaporators it could be a good choice to just go passive. Just like for ethanol and other hydrocarbons.

I personally think that for batch systems up to very large batch sizes that want to rely on a one tank closed loop system (recovering into the same tank you flood from), it will be simplest and most efficient to have a vapor recovery pump in the loop, probably as part of a hybrid recovery setup.

I think the real answer for ultimate efficiency with a balance of overall system ease of workflow and recovery rate a hybrid recovery circuit would be the best choice, see my post in the other active/passive thread.

There are definitely periods during my (active) recovery toward the top of the bell curve of the difference between vessel pressures where a passive circuit would make sense to add and I have pondered putting a simple bypass line with a one-way check valve around the pump to take full advantage.

As systems and vessels get larger, I think it becomes more difficult to keep the entire volume of solvent at low enough vapor pressure on the recovery side to keep up peak passive recovery efficiency. Not to mention vapor assisting. This is all compounded if the operator wishes to use a high propane mix.

I think your assertions about noise and hazards from recovery pumps and air compressors is somewhat moot. Obviously some people are more sensitive than others but I have worked in shops my whole life and don’t have any problems with these issues. I know the old Haskel’s can be a little loud in the operating booth with all the knocking but pumps like Corken, Blackmer and LeRoi are very quiet pumps and not a problem at all.

With respect to teflon and metal shavings, I don’t think the effect is really that bad. Especially with a proper pump like a Corken that is actually designed to pump hazardous gases like ammonia for years and years. A diaphragm pump like an MVP is probably even less prone to shedding contaminates.

Also, most extracts are filtered down to 1 micron these days with the advent of CRC which would handle any detritus that would find its way in.

Honestly, I think most contaminates come into the system on the input material itself.

I remember I was out in the hills one time and I saw some guys working on a greenhouse frame around plants and had to take a grinder to something they were working on. They had setup a blanket to act as a shield but I’m sure some metal shavings made their way onto the plants.

I understand that you’re trying to do some marketing here for your company and that’s all good.

It’s not a surprise to me that a mechanical chiller will be cheaper over time than a constant consumable like liquid air gases. That seems very obvious and I don’t think anyone is debating that fact.

However there are still several arguments for why someone would want to stick with liquid air gases over a mechanical chiller like financial ability, equipment reliability, electrical hookup availability, ability to modify the existing building, etc.

Marketing this from the angle that chiller systems like the ones you offer save money in the long run vs a constant consumable makes sense.

I think using this logic to say that passive is a better or simpler option for batch extraction systems is a stretch and may be the result of some confirmation bias you have toward the products you’re trying to sell.

The active/passive debate is much more about workflow design, throughput and system design simplicity, IMO, than it is about overall recovery rate.