Yugang CO2 Reactor - A Guide

Contents:

Benefits of Yugang reactor
Calculate the reactor dimensions
Overflow mode, regulator mode, pH/CO2 controller
Overflow mode, step by step instruction
Regulator mode, pH/CO2 controller mode, step by step instruction
Addendum A: Adjustable power setting
Addendum B: Water flow in the reactor
Addendum C: Do we need a transparent reactor for observation?
Addendum D: Do we need a purge valve?
Addendum E: Physics explanation of reactor size calculation
Addendum F: Gas exchange in reactors, basics without math.
Addendum G: Q&A



Benefits of Yugang reactor

• Simple design rules to find the correct reactor dimensions for any tank or CO2 target.
• Concept can be scaled for arbitrarily small tanks or arbitrarily large tanks. Tested and in operation on tanks as large as 1000 gallons, only 40 minutes needed for target CO2.
• The reactor can be configured so that in the event of a failure, CO2 injection will not exceed a safety limit and fish cannot be gassed. Inherently safe.
• The reactor can be configured so that we do not need a precision regulator. We can use an affordable hobby grade regulator, because the reactor controls the CO2 injection rate and CO2 stability.
• No CO2 mist in the tank.
• Virtually no reduction of flow from pump, or back pressure.
• No noise.
• The reactor will purge itself, no need for manual purging.
• Compact (compared to vertical bubble reactors).
• No maintenance, and stable performance over time.
• Easy and cheap to build DIY, furthermore commercial versions already available in the marketplace.

Calculate the reactor dimensions

Summary:

• Calculate the Tank_Surface_Area (tank length * width)
• The target Reactor_Surface_Area between water and the gas pocket in the reactor (reactor length * reactor width) is proportional to the Tank_Surface_Area (for any given pH drop target).
• For a pH drop target 1.5, the Reactor_Surface_Area is estimated at Tank_Surface_Area / 17.7
• For a pH drop target 1.2, the Reactor_Surface_Area is estimated at Tank_Surface_Area / 35.4
• For a pH drop target 0.9, the Reactor_Surface_Area is estimated at Tank_Surface_Area / 70.8

Overflow mode, regulator mode, pH/CO2 controller

Three alternative approaches:

1. ‘Overflow mode’.
   We inject slightly more (5%) CO2 in the reactor than the CO2 absorption into the water. The CO2 reservoir remains always full, and every few minutes we see an excess CO2 bubble escape from the CO2 overflow. As the meniscus of CO2 will align itself with the overflow, the CO2-water absorption area will be constant. The CO2 injection rate is now set by the reactor geometry, i.e. the reactor surface area, and almost independent of changes in the CO2 flow from the regulator.
   
   The quality and stability of the CO2 regulator is no longer important, as long as it injects enough. Monitoring flow rate and using a bubble counter is no longer needed, just watch the overflow in action and if it ‘runs dry’ increase the injection a bit.
   
   
2. ‘Regulator mode’.
   We use the reactor below its capacity, so that the CO2 meniscuses does not reach the overflow (at least initially) and no CO2 bubbles escape from the overflow. The system will then stabilise when CO2 absorption = CO2 injection from the regulator, and we have a very similar function as with a traditional vertical reactor. It is now the setting of the CO2 regulator that drives how much CO2 is absorbed in the tank.
   
   Now, of course, we need a good quality CO2 regulator that is both short term stable, and has no long term (day-to-day and week-to-week) drift. We need to use a bubble counter to regularly monitor & fine tune the injection rate. While the Yugang reactor functions mostly similar as a vertical bubble reactor, it eliminates most of the traditional reactor’s challenges. No noise. No flow reduction for filter. No safety concern when a cylinder blow out or regulator malfunction causes too much CO2 injection. No CO2 mist in tank. The main factor that will drive both short term and long term CO2 stability are surface agitation / gas exchange, as well as instabilities and drift from the CO2 regulator.
   
   
3. ‘CO2/pH controller mode’.
   As Regulator Mode, no bubbles escaping from overflow, but rather than the regulator it is the CO2/pH controller that drives the CO2 injection rate. As with Regulator Mode, but instead using a CO2/pH controller to stabilise pH within a given bandwidth. While the Yugang reactor functions similar to a vertical bubble reactor, as per above, it eliminates most of the traditional reactor’s challenges. No noise. No flow reduction for filter. No CO2 mist in tank. With a proper setting of the overflow, it can be used to fully maintain the function of a correctly working pH controller, while mitigating risks sometimes associated with the CO2/pH controller. When the controller injects more than a safe level of CO2 (malfunction, probe or KH change) the CO2 overflow will act as a safety valve and limit the maximum CO2 that the Yugang reactor can inject. The main factors that will drive CO2 stability are the KH stability, calibration of pH probe, and upper/lower limits pH setting.

Overflow mode, step by step instruction

• Always monitor livestock health and warning signals in the process.
• With the initial set up and testing, it is useful to make sure the reactor is purged. This can be done with the optional gas purge valve, the reactor full of water, or alternatively blowing a higher volume of CO2 gas in the reactor for some time and have the escaping gas bubbles clean out the gas pocket until only pure CO2 remains.
• Inject sufficient CO2, so that the reactor fills to its maximum capacity and CO2 bubbles emerge from the outlet into the tank. Then slowly dial back until nearly no, or very few CO2 bubbles leave the reactor. This is the reactor working at its maximum capacity, and it is useful to note down the flow rate (bubble counter, or counting injected bubbles inside a transparent reactor) for future reference.
• Now increase the rate of injection just a little bit, 5-10%, so that from time to time a small gas bubble escapes from the reactor into the tank.
• This is now the optimal setting for overflow mode, and the CO2 injection rate in the water will be stabilised by the reactor and cannot further increase from this level that is considered to be safe (observe livestock). The regulator setting, or CO2 flow count can be mostly ignored and would only be checked or adjusted from time to time.

During the day, when CO2 is on we want to see probably once per minute a small bubble escaping the reactor, blown into the tank and escaping to the air above the tank. This guarantees that the absorption surface is constant, and hence the injection rate of CO2 into the water is constant and not dependent on regulator stability or controller.

Another method to verify if everything is OK is to check the size of the small gas bubble that remains after solenoid off, and remaining CO2 has been absorbed in the water. This typically takes around 20 minutes after solenoid off. The size of this remaining gas bubble is an indication of how much overflow there was during the day (purging gases that diffuse from water into the CO2 gas pocket), and that again is a measure of the stability of CO2 injection.

So in summary we have three alternative indicators to check overflow mode

1. Count bps from injected CO2. For any given size of the reactor we can test the bps when it starts to overflow, and use this as the reference for the future. Target 5-10% excess injection for the optimal trade of between stability and CO2 consumption.
2. Measure the time between bubbles escaping from the overflow, typically 1 small bubble per minute.
3. Observe the size of the remaining gas pocket after all CO2 has been dissolved. Smaller is better, typically less than 5% of the reactor volume.

Any of these three would do, but the easiest may be to have a quick check of 3, once per week or so. For non transparent plastic, the easiest is to observe 2 for a minute or so, and see if a bubble of gas escapes during that period.


Regulator mode, pH/CO2 controller mode, step by step instruction

• Always monitor livestock health and warning signals in the process.
• With the initial set up and testing, it is useful to make sure the reactor is purged. This can be done with the optional gas purge valve, the reactor full of water, or alternatively blowing a higher volume of CO2 gas in the reactor for some time and have the escaping gas bubbles clean out the gas pocket until only pure CO2 remains.
  Note: In case the reactor capacity exceeds a safe limit for livestock, and no purge valve or other means to remove gas from the reactor available, just ignore purging the reactor and allow more time for the setup process.
• Set the CO2 regulator, or CO2 controller, and the reactor will be ready to work.
• After some time, this may be hours or days, some gases will build up in the reactor and from time to time a bubble will escape from the reactor. Do not worry, it is the reactor purging itself and not be detrimental to injection or stability.

Addendum A: Adjustable power setting

Most reactors are built with cylindrical pipes, and the following method can be applied to rotate the asymmetrical water outflow piece for adjusting the reactors surface area and hence the reactor power:



If we would aim for a 50% reduction of the reactor power from its maximum setting, we would need to design the top of the water exit at 86% of the reactor radius:





Addendum B: Water flow in the reactor



The reactor’s operation is virtually independent of water flow as long as it’s not stagnant. Turbulence at the water/gas interface ceases when the water becomes stagnant, and a boundary layer limits gas diffusion into water. The reactor can handle almost any flow, and most users may find a bypass useful but not essential:

• A slow water flow in the reactor will ensure that water never splashes with high water pump capacity and make noise.
• With a slow water flow the water surface will be smooth with tiny ripples, and smaller gas bubbles will escape from the reactor exit in a more regular pattern.

It is important to set up the reactor in the tank, so that bubbles from the reactor exit can escape upward, not down. This is to avoid that bubbles cannot escape, they resist going down, especially at low water flow, and risk to create unwanted gas pockets in tubes. We should allow small bubbles to escape from the reactor and without obstruction be released in the tank where they can escape to ambient air.

Optimal water flow, minimal back pressure, will be when the bypass valve is nearly fully open, and we observe just a small flow in the reactor. The movement of injected CO2 bubbles and water ripple are good indicators of the flow in the reactor.




Another option, useful but for most users not essential, are 45 degrees “elbows” in the water inlet as well as the water outlet.

The benefit for the water inlet is to guide the water flow down into the lower half of the reactor, minimising any splashing and noise at high flow rates (without bypass, high powered water pump).

The benefit for the water outlet is to have gas starting to purge when the water level is at exactly half of the reactor diameter, using the full maximum reactor surface area.


Addendum C: Do we need a transparent reactor for observation?

All reactor operations can be perfectly done without any visual observation what is happening inside.
Some users may find it useful to diagnose problems taking an inside look with a transparent reactor, but with more experience and a better user guide as intended in this document it should be perfectly possible to operate a steel or any other non-transparent reactor.
A transparent reactor is a nice to have option, but not essential.

Addendum D: Do we need a purge valve?

As described in this document, the reactor can purge itself, even in a worst case situation when it is full of air when started. A purge valve is a nice to have, but not essential.
The best argument for having a purge valve is for new and less experienced users to eliminate causes of confusion, especially with reactors that are not transparent. When the reactor has been manually purged, full of water, any injected CO2 will immediately lead to absorption in the water and be noticeable with pH measurements and drop checkers. If a reactor has not been purged and contains a pocket of air, it may be slower to react and may lead to confusion as to what is happening.


Addendum E: Physics explanation of reactor size calculation

For anyone interested in the physics rationale, how we calculate the reactor dimensions, please refer to the below.

Let’s first recap what happens with CO2 in our tank, from the perspective of a ‘CO2 accountant’. When we inject CO2 (reactor, 100% absorption. Diffusers with bubbles escaping to surface are less predictable) in our tank, three things can happen to any molecule that we account for:

• CO2 concentration in the water column will increase, and as a consequence we see the pH drop.
• CO2 from the water column (that is no longer in equilibrium with the atmosphere) will outgas at the surface. The rate of outgassing is a function of the tank surface area, the CO2 ppm in the water, and the surface agitation. The rate of outgassing (gram CO2 per hour) is NOT dependent on the tank depth/volume.
• CO2 may be consumed by plants. Here it is important that plant consumption is generally much less (typically 10% or so) than the surface outgassing.

When we start from a fully outgassed tank (no CO2 injected for a couple of days), most of the injected CO2 will be accumulating in the water column, CO2 ppm increases and pH goes down. The rate at which CO2 ppm increases is of course smaller when we have a big tank, and it goes quicker for a small tank.

Continuing to inject, the tank CO2 ppm increases, we will see more and more outgassing at the surface up to the point where this outgassing more or less equals the rate of injection. It is important to remember that plant uptake, or life stock / microorganism respiration will not be a major factor here, and we will ignore these to keep the estimations as simple as possible.

The tank CO2 ppm will stabilise (we may call this ‘steady state’) once outgassing at the surface equals the rate of injection. Usually hobbyists target a steady state pH about 1.0 – 1.4 below the fully outgassed level, and we need a reactor that can inject as much as is needed to offset the outgassing.

Now remember that outgassing is a function of tank surface, CO2 ppm, surface agitation, but NOT of tank depth. This means that for calculating the required reactor capacity for maintaining a 1.5 pH drop steady state we only take into account the tank surface, not the volume.

During the night, when CO2 injection is off, the tank will continue to outgas and CO2 ppm will go down. The rate of outgassing (gram CO2 per hour) is mainly dependent on tank surface and agitation, and again not on tank volume (if we look in detail this is only an approximation, in reality there is a small dependency here). This means that for the reactor in the morning to replenish the outgassed CO2 we have mainly to take into account the surface area rather than the tank volume.

So in summary, both for maintaining a steady state (say constant 1.5 pH drop) and ramp up in the morning we want to scale the reactor capacity with the tank surface area. If we know how much reactor capacity was sufficient for a 1.5 pH drop in one tank, we can compare the surface area of another tank and estimate how much we need to scale up/down the reactor capacity.

The reactors capacity is proportional on the surface area between the flowing water and the CO2 pocket. As long as the water in the reactor is not stagnant (in which case we would be limited by diffusion at a boundary layer) we will not see much dependency on the flow. That is why it is recommended to have a gentle flow, as it minimises noise and splashing of water, while still optimal CO2 absorption and reactor capacity. So in summary, we find that the reactor capacity scales with the inner dimensions of our tube, i.e. length*diameter.

Now the estimations for the reactor dimensions are straightforward, as from the above follows that for any target pH drop we need to scale the reactor surface area to the tank surface area. Once we have measured the correct scaling factor for one tank, we can apply it for other tanks and achieve the same targeted pH drop for these.



Addendum F: Gas exchange in reactors, basics without math.

This paragraph is for the interested readers, and may be skipped as it is not essential for the know-how of operating the reactor in the aquarium.
More detailed physics and math has not been included below, and can be found in the ScapeCrunch post #156.

After starting to inject CO2 into the reactor, F1CO2, a reservoir of almost pure CO2 gas in the reactor builds up. As the partial pressure in the gas pocket is not in equilibrium with the water (Henry’s law) CO2 will be injected into the aquarium water (F4CO2).



As is also the case in bubble reactors, gradually we will see some oxygen and nitrogen outgas from the tank water into the pure CO2 pocket in the reactor (Henry’s law), F5Other.

It is important to note that F5Other is much smaller than F4CO2, around 3% as per initial measurements. Follow up measurements may give a more accurate value for F5Other/F4CO2, but are not likely to change the basic understanding or operation as long as the ratio remains small.

Let’s investigate first what happens when we inject a lot of CO2, then what happens when we inject a only a little CO2, to build up a qualitative understanding what happens in-between these two extremes. The math has been worked out and posted elsewhere, but is beyond the scope of this document.

If we inject a lot of CO2 in the reactor, the flow F1CO2 will fill the gas pocket fast, even before flow F5Other had a chance to add a significant volume of other gases to the gas pocket in the reactor. Therefore the gas pocket will be virtually pure CO2, and will grow until bubbles of pure CO2, F2CO2, will escape from the reactor exit. As the gas pocket is pure CO2, and the reactor geometry sets the reactor surface area, we conclude that the CO2 injection into the water, F4CO2, is constant and NOT any longer dependent on time or on the CO2 flow F1CO2.

If we inject just a little CO2 in the reactor, as compared to the maximum reactor capacity, the gas pocket will hardly grow as all CO2 will be absorbed in the water, F4 CO2. The operation is now very similar to a conventional bubble reactor. However as time passes, the diffusion of other gases from the water, F5Other, will slowly grow the gas pocket which in a cylindrical tube means increase the reactor surface volume. If we would not purge the reactor manually, as would be necessary for a vertical bubble reactor, the gas pocket will slowly continue to grow over several hours or days until the overflow F2CO2 plus F3Other will become active and create a stable situation where almost all CO2 will still be injected into the water, while the reactor purges itself from other gases than CO2. It is important to know that in this case the gas pocket is not pure CO2, and the lesser the CO2 injection rate the less pure it will be. The CO2 injection in the water, F4CO2, equals the injection rate in the reactor F1CO2.

If we start with a reactor that is filled with air, then inject CO2, the gas pocket will slowly grow, the reactor surface area will grow, until the condition is met that the CO2 flow into the reactor, F1CO2, equals the injection into the water, F4CO2. From then on the gas pocket continues to slowly grow until it starts to purge the reactor as described above. We see that the reactor handles air pockets without any issue, but the purging will be faster with a higher CO2 injection rate F1CO2 and will be sooner in a steady state mode of operation than with a low injection rate.

For practical purpose the most interesting situation is when we start with an air filled reactor, that we first purge with a high CO2 flow F1CO2, and then slowly dial back CO2 until only a few gas bubbles escape from the reactor exit. This is the compromise where the injection in the water, F4CO2, is constant and NOT any longer dependent on time or on the CO2 flow F1CO2, while minimising CO2 losses F2CO2. This is what we call “overflow mode” in this document.


Addendum G: Q&A

Q: What is the meaning of “Yugang” ?
A: Yùgāng 浴缸 is Chinese for tub or aquarium


Q: Where do I find detailed information, measurements and calculations?
A: On ScapeCrunch in the threads Horizontal CO2 Reactor - Yugang 鱼缸 Reactor and CO2 Spray Bar - a summary


Q: What is “good” CO2 in the planted tank?
A: Despite the assertion by experts and professionals in the hobby that up to 90% of problems in high-tech planted tanks originate from inadequate CO2 levels, surprisingly, there exists no definitive definition or procedure for ascertaining what constitutes optimal CO2 levels. Arguably, the stability and distribution of CO2 within the tank are more critical factors in determining optimal CO2 levels than the precise concentration.


Q: What is meant with CO2 stability?
A: CO2 stability means the dissolved CO2 concentration stays constant within the daily photo-period, day to day, and week to week. Plants benefit from this as it avoids the plant adjusting its machinery to capture CO2 for photosynthesis (RubisCo enzyme) and saves energy for improved growth and health. As long as we know that CO2 is stable, as the Yugang reactor in overflow mode will do, we do not necessarily need to measure and know the precise CO2 concentration.


Q: What is meant by “pH drop”
A: Dissolving CO2 in water will lower the pH, and the relation between CO2 ppm concentration and pH is described by a logarithmic function. A decrease of pH by 1, means a 10 fold increase in CO2 ppm. A decrease of pH by 0.3 means a doubling of CO2 ppm. For determining the pH drop, aquarium water should be outgassed for a long period in outdoor air, so that the water CO2 partial pressure is in equilibrium (Henry’s law) with the 400 ppm atmospheric CO2 concentration. The measured pH in the outgassed sample, compared to the aquarium water with injected CO2 is the pH drop, and is an indication of the dissolved concentration CO2.


Q: How to accurately measure and monitor CO2 ppm in the tank?
A: To accurately measure CO2 ppm it is essential to have proper tools as well as follow the correct measurement procedures. This can be challenging, even for experienced hobbyists or professionals. When ppm values are mentioned, it is not unlikely that these are believed to reflect reality, but are in fact the result of an imperfect process and not even nearly correct.
The good old drop checker is slow and not very accurate, but it is reliable compared to other methods.
pH probes are useful, but have many potential causes for error, including water chemistry, outgassing procedures, calibration.
Even very expensive professional CO2 meters need calibrations, can only be trusted with proper measurement procedures, and can be very slow or wrong especially at low ppm’s as in our hobby or stagnant water.
Should we be concerned with accurately measuring CO2 ppm, if what counts mostly is stability and actually we have mostly no idea if X ppm would be better than X+5 ppm or X-5 ppm CO2? Probably not.


Q: How accurately will I know CO2 ppm when using overflow mode?
A: Testing was done with the 17.7 ratio, and the 1.5 pH drop is an approximate target. The 1.2 and 0.9 pH drops are estimated from the 1.5 drop, using logarithmic dependance, and halving the reactor size in order to halve the injection rate. More measurements and fellow hobbyists could refine these, but it’s unclear if this adds much value since we’re not interested in precise CO2 ppm but rather stability.


Q: What CO2 regulator do I need?
A: Hobby-grade regulator manufacturers rarely publish data or specifications on stability, and there’s a reason for that. For a guaranteed reliable and stable regulator with a long lifespan, we are looking at expensive (semi-) professional products used only by serious hobbyists with deep pockets.
The Yugang Reactor ’s overflow mode is designed to ensure CO2 stability in the tank, regardless of the regulator’s stability. An affordable regulator is sufficient, even single stage, as we’ve mitigated CO2 dump risks and guaranteed reactor stability by its design.


Q: Compare performance to vertical bubble reactors (Cerges, Griggs)?
A: After some three decades experience in the hobby community there are hardly any reliable design rules or operating procedures for vertical reactors. The stability performance is limited by the regulator, they have no role in stabilising CO2 or to protect livestock from gassing. As they work as inverted airlift pumps they will always create a back pressure, and some designs with impellers or similar tricks may reduce water flow even more than that. Most won’t purge themselves, some will create noise especially when an gas pocket builds up over time. Its is really hard to build vertical reactors for large tanks, and generally they will be larger in size, need dedicated water pumps, or need more reactors in parallel, than a single comparable power Yugang reactor .
In summary, vertical reactors work but have unnecessary complications and miss most of the benefits of a Yugang reactor .


Q: Compare performance to CO2 diffusers?
A: Diffusers are useful and affordable for smaller tanks with limited requirements on tank CO2 stability. For these smaller tanks it is worth considering CO2 Spray Bar / Open Flow Reactor, that applies the same principles as the Yugang Reactor , including its stability and safety, and can be placed in the tank. If preferred, Yugang reactor can also be built very small, for example using a transparent box intended to be used to store foods, with a really small pump.


Q: Can I combine multiple smaller Yugang CO2 reactors?
A: Yes, but note that the second reactor in a chain should be at the same height, or higher than the first so that CO2 bubbles can move freely from reactor to reactor.


Q: What is the role of the tank surface agitation and flow?
A: Surface agitation enabling gas exchange is essential for the stabilisation of CO2 in the tank. This follows from simple math. A stagnant water column, without any turbulent water movement at the top will create a diffusion limited boundary layer that will seriously decrease gas exchange and hence should be avoided.
Gas exchange can be complex, especially in large oceans for climate change research. However, in our hobby, in a controlled aquarium with a reasonable turbulent surface agitation, it can assumed that gas exchange is mostly proportional to the tank’s CO2 ppm and surface area. As long as the surface isn’t stagnant and has some turbulent movement, agitation isn’t a significant factor.


Q: What are requirements on the water flow in the reactor?
A: As long as there is some flow in the reactor, the water is not stagnant, the reactors operation is virtually independent of the rate of flow. Same physics as for gas exchange in the tank, as long as we don’t have a diffusion limited boundary layer the gas exchange will be proportional to the reactor surface area between water and the CO2 pocket. For the interested physicists, also note that the CO2 concentration in the reactor, even at low flow, will generally be far less than the solubility of CO2 in water and thus the partial CO2 pressure in water is insignificant as compared to the nearly pure CO2 gas pocket.


Q: Can the reactor be used on small tanks?
A: The reactor can be built very small, using plastic or glass food storage box and internal bypass. Ideally manufacturer could offer it an an alternative to diffusers, and it could be just a few square cm small.


Q: What is the typical build time and cost?
A: It depends on the country where parts are sourced, and the choice of materials. The most expensive part is probably an acrylic tube for a transparent reactor. I build a transparent reactor for a 50 gallon tank for less than 15 USD material cost and one hour DIY. I built many prototypes and gained experience, but most would probably build it in less than 2 hours. Using plastic food containers and some plumbing pipe, I could build a reactor for a 50 gallon tank for less then 5 USD.


Q: Can I convert a Cerges vertical reactor to a Yugang reactor ?
A: Hobbyists have started just doing that. Place the Cerges on its side, and obtain a much stronger Yugang Horizontal reactor.


Q: The article mentions the reactor is compact - compared to what?
A: Estimating the average bubble size and count in a vertical reactor allows for calculating the total surface area of bubbles being absorbed in the water. This surface area is usually much smaller than the active area of a Yugang reactor of similar size, making vertical reactors less powerful for any given size.


Q: Can the reactor be integrated with aquarium canister filters?
A: Fluval FX4/6, and perhaps others, can be easily modified to include a Yugang reactor function that will be powerful enough for a 750 litre tank. The modification could be DIY, but ideally Fluval would offer an additional part to enable this function.