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Hi Dane
While vacuum recovery seems simple in concept, it is not that easy to make work efficiently for methanol. The reason is, you should have the pump between the pot and the condenser, so it will be running at 65C and upwards, with concentrated methanol fumes being pumped. Suitable pumps for this type of service are hard to find and very expensive. OK, how about if we move the pump to the cool side of the condenser? Hmmm, if you have the pump on the outlet of the condenser, you will be running the condenser at reduced pressure, which means the vapour in the condenser will condense at a lower temperature than if the condenser were at atmospheric pressure. This means you need to cool the jacket to a lower temperature. This all adds up to vacuum distillation being less efficient than traditional distillation, for methanol, anyway. The energy you will need to put into the vacuum pump and/or extra chilling may as well just be put in as heat, using the heater! Simpler system and better efficiency. A scheme which uses a simple venturi vacuum pump which operates at atmospheric pressure is shown here. With this you will be able to start recovering methanol from 40C upwards. Hope that helps. Regards GL |
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Thanks. That's why I ask here before trying to experiment on my own. : )
The more I know about biodiesel, the less I know about biodiesel |
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member 2008 sponsor |
Graham, You say not to have the venturi hooked to the cool end of the condenser. In the state diagram it is drawn hooked to the cool side, albeit an indirect route. Is the indirect route the key? Ive had my venture connected directly to the cool end of the condenser since i started. I'm wondering if that is why I'm not having much success in distilling methanol? |
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Hi Jon
It's OK with the venturi, because everything is working at atmospheric pressure. The placement is only an issue when using a true vacuum pump, to draw the tank and vapour pressure down. That doesn't happen in our case. we are only re-circulating, not drawing a vacuum. Hope that helps, graham |
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member 2008 sponsor |
Oh...Then what do you think my problem is aside from the huge head room in my tank? Every time I attempt distillation I only get a few liters, not the gallons I'm expecting.
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Hi Jon
My guess is you may have methanol condensing on the walls / pipework of the tank , returning to the biodiesel. The upper pipework and tank surface need to be very well insulated with a thermal blanket, like this ...  Are you 100% sure all is well insulated? What temperature are you heating to? All the best, Graham |
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member 2008 sponsor |
OK, I need to do a bit more insulating on my pipework and processor. That may be where its lacking.
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Graham,
you need to look seriously at what you are saying about vacuum distilation being ineffficent especially on Methanol. I know of no plant that puts the pump between the pot and the condenser, I work in chemical plants installing and maintaining vacuum distillation systems spacifficly grade 3 solvents all over Australia, which by the way are replacing less efficient atmospheric and pressure systems due mainly to comply with EPA emmissions and occupational health and safety. The type of pump used is generally the key, Piston or Diaphram pumps are not suitable because of the pulsing nature of operation, fumes, should they get in to the pump will destroy them and are a fire hazard, water ring or oil filled type are best suited and you need to do a bit of calculation so the pump can maintain vacuunm as the batch heats up to negate any pressure whilst maintaining a full vacuunm, you don't need a big pump if you use a thermostat to controll heating. You can use Vacuum to load your vessel with oil, you can do away with mechanical mixing by bubbling through the bottom of your vessel often used as a back up in industry when stirers and pumps fail, bubble mixing can get quite agressive, you can load your methodoxide in to a sealed vessel by sucking it in, every last drop no hands, You can even Bubble wash your bio under vacuunm. Infact you can do away with your mixing pump all together and use a 1/4 of the power with a 1/8 hp vacuum pump up to 400L capacity because you are only pulling the vacant space between the roduct and the top of the vessel, condencer and recovery vessel. For efficiency IT'S HARD TO BEAT, for safety if set up correctly using a vapor trap on the end of the condenser before the recovery vessel and a water scrubber infront of the vacuunm pump your nose will tell you how efficent it is, no fumes= no losses = SAFETY + efficency even if you don't factor in the energy or time saving. Distilation starts under vacuum at a lower temprature hence the need to fill the reactor and know how much heat expancion will increase the bio level in your reactor, you need to insulate the top of the vessel to minamise reflux a sight glass will amaze you and help in getting the right level to minamise reflux, this happens regardless of of what method you use to distill, raising the level is easy under vacunm just suck in some finished bio if you have some. I keep 20l for this because the glycerol quantaties when drained off vary depending on the feed stock. Try and reverse the process with atmospheric Distilation I recovered 90% methanol Measured accuratly in an hour just to see if it could be done before a slight leak started the batch foaming probably caused by a tiny bit of water forming soap,it would take for ever under atmosphere. As a foot note I can recover by atmosphere,pressure or vacuum using the same equipment just by disconnecting the Vacuum pump I use a seven 12mm straight tube water cooled condencer 1350 mm long and there is no need to chill the water even when the vessel is taken to over 110c to dry my oil under vacuum. it is necessary to moniter condencer cooling water I stared with 200L no good then I recirculated the water in our garden pond and used the pond pump and filter roughly 800L never got above 30c in summer. Ferraloil. |
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Hi Ferraloil
I agree that distilling from the same pot with and without vacuum pump will give faster distillation - but this is not because vacuum distillation is more efficient. It is because you are adding more energy. Vacuum distillation won't give higher efficiency than atmospheric distillation, all other things being equal. It can make it faster, whilst limiting the temperature of the mix, but it won't be more efficient, because it is more complex and has more opportunity for losing energy. If you want a faster process, you need to add more energy to it to get you there faster, assuming all losses are equal in both processes. There may not be many plants which put the pump between pot and condenser, but it would be a better place, given a suitable pump. Why? Because putting the pump on the condenser outlet side puts the recovered liquid under reduced pressure, which encourages it to vapourise, not condense. This is opposite to want we are trying to achieve. But it is done this way because it is kinder and less demanding on the pump. --------------------- Let's look at both processes side by side ... Both begin the same - a tank of methanol laden biodiesel at 55C Both end the same - a tank of de-meth'd biodiesel at room temperature and a tank of methanol at room temperature. We have the same 'journey length'. How do we get from A to B faster, in any journey, and use less energy? Doesn't matter if it is a journey of methanol from one tank to another, or a car journey from one town to another. We take a more direct route, and we conserve energy wherever possible. We freewheel down hills, and we don't use a tow-truck to help us along. The more transfers we make on the way, the more energy we use. The more wheels or engines we involve, the more energy we lose. Simplicity is the key to efficiency. One inlet, one energy source, one outlet and as little wasted energy as possible in between. --------------------- Let's compare losses in our two systems I'll assume a closed pot, with no liquid circulation, in both cases. We know we need a certain amount of energy to get from A to B. If one process uses more energy than the other, there must be waste, or loss involved. Where is this likely to come from? 1. Atmospheric distillation energy loss... Because we run the pot at a higher temperature, heat loss is likely to be greater in an atmospheric system, given the same insulation in both systems. Where will heat loss come from? The tank has the biggest surface area, so it the main candidate for focusing on reducing heat loss. Thicker insulation. The heater will be inside the liquid, so all its heat will transfer to the liquid. Energy is being put into the pot. If our insulation is perfect, energy will only leave the pot in the form of methanol vapour, en route to the condenser. Now we come to the condenser, and we heat up the water in the jacket as the methanol condenses inside. That water now has greater energy potential, so can be used for something, perhaps to pre-heat the next batch. Coming out the end of the condenser is methanol at the same temperature as the water going into the jacket, assuming the condense is sufficiently long. So our losses are from the tank (Easy to cure, with more insulation), and we have moved energy to the cooling water - which is easy to capture and re-use. 2. Vacuum distillation energy loss... Our tank is the largest likely source of lost heat, so we need to insulate it, though not as much as for atmospheric distillation, as the pot temperature is lower. If we are drawing vapour at reduced pressure through the condenser, we will need to cool the condenser to a lower temperature than we would be able to with atmospheric distillation. This is simply related to the boiling/condensation point being lower at lower pressure. If we draw a lower vacuum, we need a lower max. temperature for condensation to occur. Drawing a vacuum needs a vaccum pump, to which we need to supply energy. A pump is not a perfectly efficient machine so energy will be lost as heat, unless we recover heat from the pump's motor. The efficiency of the system will be affected by the efficiency of the vacuum pump, and/or by the efficiency of our heat collection process for the pump's wasted heat. It would be fairly complex to build a heat recovery system to capture waste heat from the vacuum pump's motor, not practical for home-brew processes, certainly. Therefore, we have energy loss which does not exist in atmospheric distillation processes. ------------------------- Conclusion I believe that a simple, well insulated atmospheric distillation process is potentially more efficient and cost effective than a vacuumm distillation process, especially if you put that easily-captured warm water to use. Bear in mind that for the homebrewer, the vacuum pump of choice is a piston pump from a refrigerator. Liquid ring pumps are not available to the average homebrewer, and this forum attracts many more low volume home producers than it does high volume commercial producers. Atmospheric distillation has less capital cost, has less likelihood of failure, and requires less maintenance, hence is likely to be more suited to home recovery of methanol. Granted, a large plant with more complex infrastructure, tighter timescales and tighter emission controls may find a vacuum process more appropriate - but I would still argue that it is inherently less efficient and more prone to failure due to its increased complexity. That is why my personal approach has been to suggest to fellow home-brew members that in general, vacuum distillation may not be as appropriate as it first seems. Interesting debate, though! |
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Bravo fellas, great discussion. I just tuned in to research this very question!
Cheers, Farmer |
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Just a couple of my thought on Vacuum Distillation
If you boil liquid in a pot, and have a condenser on the vapour outlet what happens? Well the liquid boils, produces vapour, which then condenses back to liquid. Why does the vapour flow from the pot to the condenser? Liquid boils and produces vapour – as vapour occupies a greater volume than liquid, this produces an increased pressure at the pot (we would see this more if the pot were sealed). This slight increased pressure pushes the vapour to the condenser, where the vapour condenses – as liquid occupies much less volume than vapour this produces a vacuum (if you put some water in an old oil can, heat it up until it boils and seal on the lid, the can collapses as it cools). Hey now we’re pushing (boiling) and pulling (condensing) and the vapour is flowing! To get the liquid to boil we need to add energy (heat) to · Raise the liquid to boiling temp – mass x specific heat x change of temp required. · Evaporate the liquid – mass x latent heat of vaporisation. The first of these terms reduces if we operate under vacuum as the boiling point is lower at lower pressure. To get the vapour to condense we need to reverse the second term. In reality, to stop it evaporating again we take it a bit lower (like the first step) – especially if the vac is applied after the condenser collection pot (running the vac pump there means that the pressure is lower in the collecting pot than the condenser, so the boiling point is lower, so the liquid re-evaporates). If we could run the vac pump between the boiler and the condenser, we lower the pressure in the boiler (good) but raise it in the condenser (making condensation easier). However, due to contamination issues within pumps, potential leakage routes and flammability issues, this is not common practise in industry. Now which is more efficient? Vac or no vac? Well how much does your electricity cost? I worked on a chemical plant which was a net exporter of 5MW of electricity…go figure. Which is safer? Well the vac system operates at lower temperatures, possibly below methanol flash points…… BUT this is not the complete story! The methanol has to diffuse through a mass of liquor with which it is miscible. This liquid diffusion step is often rate limiting. I won’t go into the theory, but it depends on temperature cubed, and pressure is completely irrelevant. Ooops, this means running a hotter still is faster – but a colder one will get there. In the final stages of evaporation – the last bit of methanol is difficult to pull out of its miscible state, an equilibrium exists between that in the liquid and that in the vapour. We can upset this equilibrium by introducing a carrier gas (air), the equilibrium tries to correct by pushing more methanol into the vapour. I am not surprised that this is what GL has found empirically. |
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Hi' Foggy,
Efficency these days is measured in cost of operation instalation of equipment maintanance, now more than ever Environmentally (Polution)risk to plant ( Insurances) leagal liability to operators. If we are talking back yard operations (home brewers) there are only two of the above that count, cost of construction and power consumption, and all to often without serious concideration to the risks involved as demonstrated within the pages and some posts on this site. As for cost comparisons of your system of atmospheric recovery, how can it be cheaper running a circulation pump of atleast 500w being conservative, Heater elements of 2500w + for several hours at elevated temps, when my system uses a 250W vacuum pump a cooling water pump of 95w ( we have serious water restrictions over hear so recycling water is paramount) 2x2500 w in series elements for at max 1 hour at lower Tempratures on a 300l batch, and on top there is no need to run at full vacuum further reducing the load or power consumption on the pump by regulating vac using a simple valve bubbling up through the bottom eliminating the need for a 500+w circulating pump = lower running cost? As for aplication of vacuum after the condenser RE-EVAPORATING condensate (methanol in this case)that is simply not possible with a properly constructed system and it's simple no great expence, all that is required is a "u" tube or a Vapor trap on the end of the Condenser that fills and remains full as soon as distilation starts, yes it dose try to re- evapourate for about 30 seconds until the trap fills, and that is evident through a viewing port in my sealed recovery vessel, of course the way to totally prevent this is to fill the vapour trap about 300ml with methanol before distilation starts, initially the walls of the recovery vessel and the lower temp inside the vessel soon condense any vapours before they enter the water scrubber on their way through to the vacuum pump where all that goes out in to the atmosphere is warm air where most of the heat is generated in the vac pump itself. By the way I have used my reactor set up in Atmospheric Mode using a circulating pump, raised Pressure Mode 25kpa with circulating pump and vacuum only, there are two major differences time and energy input, guess which is the quickest most energy efficient with highest and SAFEST recovery rates. Theories, Every one has one or two, putting them to practical use or aplication is entirely another matter, this makes me wonder if you ever had any practical experience with such a system and operated it on any scale other than glass ware. Like most home brewers my vessel was built from disgarded bits and pieces,As for Graham suggesting the choice of vacuum pump being a piston type from a refrigerator for the home brewer indicates a complete lack of understanding of why this type of pump or any piston or diaphrame pump should NEVER BE USED on any solvent is to me bewildering, given he professes knowledge of METHANOL distilation under vacuum.(THIS IS SOWING THE SEEDS OF DESASTER should some one take it on board). There are heaps of cheap oil rotary vane types avaliable check out E-BAY, or if you really want a cheap one try a vac pump off a jap diesel engine they are some times built in to the altinator, all that is needed is to put a small 1/2 L oil tank on them to keep the rotor submerged. Most chemical plants burn off vapours and fumes in boilers to generate electricity or steam from antiquated systems that shows quite the opposite if they produce enough to export, unless it's part of process that has no other use, out hear we call those plants oxidisers and rarely if ever do they break even due to high maintenance.so what did the 5MW cost to produce? Sure there are problems with recovery no matter what way you go and from what I have experienced is that it must be done as Quickly and at as lower stable temprature as possible you don't want to pull of water with the methanol, that can be done later if necessary. ferraloil. |
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Hi' Foggy,
Efficency these days is measured in cost of operation instalation of equipment maintanance, now more than ever Environmentally (Polution)risk to plant ( Insurances) leagal liability to operators. If we are talking back yard operations (home brewers) there are only two of the above that count, cost of construction and power consumption, and all to often without serious concideration to the risks involved as demonstrated within the pages and some posts on this site. As for cost comparisons of your system of atmospheric recovery, how can it be cheaper running a circulation pump of atleast 500w being conservative, Heater elements of 2500w + for several hours at elevated temps, when my system uses a 250W vacuum pump a cooling water pump of 95w ( we have serious water restrictions over hear so recycling water is paramount) 2x2500 w in series elements for at max 1 hour at lower Tempratures on a 300l batch, and on top there is no need to run at full vacuum further reducing the load or power consumption on the pump by regulating vac using a simple valve bubbling up through the bottom eliminating the need for a 500+w circulating pump = lower running cost? As for aplication of vacuum after the condenser RE-EVAPORATING condensate (methanol in this case)that is simply not possible with a properly constructed system and it's simple no great expence, all that is required is a "u" tube or a Vapor trap on the end of the Condenser that fills and remains full as soon as distilation starts, yes it dose try to re- evapourate for about 30 seconds until the trap fills, and that is evident through a viewing port in my sealed recovery vessel, of course the way to totally prevent this is to fill the vapour trap about 300ml with methanol before distilation starts, initially the walls of the recovery vessel and the lower temp inside the vessel soon condense any vapours before they enter the water scrubber on their way through to the vacuum pump where all that goes out in to the atmosphere is warm air where most of the heat is generated in the vac pump itself. By the way I have used my reactor set up in Atmospheric Mode using a circulating pump, raised Pressure Mode 25kpa with circulating pump and vacuum only, there are two major differences time and energy input, guess which is the quickest most energy efficient with highest and SAFEST recovery rates. Theories, Every one has one or two, putting them to practical use or aplication is entirely another matter, this makes me wonder if you ever had any practical experience with such a system and operated it on any scale other than glass ware. Like most home brewers my vessel was built from disgarded bits and pieces,As for Graham suggesting the choice of vacuum pump being a piston type from a refrigerator for the home brewer indicates a complete lack of understanding of why this type of pump or any piston or diaphrame pump should NEVER BE USED on any solvent is to me bewildering, given he professes knowledge of METHANOL distilation under vacuum.(THIS IS SOWING THE SEEDS OF DESASTER should some one take it on board). There are heaps of cheap oil rotary vane types avaliable check out E-BAY, or if you really want a cheap one try a vac pump off a jap diesel engine they are some times built in to the altinator, all that is needed is to put a small 1/2 L oil tank on them to keep the rotor submerged. Most chemical plants burn off vapours and fumes in boilers to generate electricity or steam from antiquated systems that shows quite the opposite if they produce enough to export, unless it's part of process that has no other use, out hear we call those plants oxidisers and rarely if ever do they break even due to high maintenance.so what did the 5MW cost to produce? Sure there are problems with recovery no matter what way you go and from what I have experienced is that it must be done as Quickly and at as lower stable temprature as possible you don't want to pull of water with the methanol, that can be done later if necessary. The attatchment is of a mates recovery system mine is almost identical in operation, the vac pump is not shown but has only a 5/16 plastic tube from the top of the scrubber to the pump, not big at all for the work it dose. ferraloil.  Bio_reactor-6.jpg (106 Kb, 64 downloads) |
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member 2008 Sponsor |
For what is worth, I have used both virgin soybean oil and finished biodiesel as the lubricating oil for my vacuum pump. Both the SBO and BD lubricate reasonably well. The recovered methanol is added to the next batch. So what if the methanol contains a little SBO or BD. |
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Hi Ferraloil
You misunderstood me. Let me clarify ... I didn't suggest anyone should use a piston pump for vacuum distillation. On the contrary. I said a piston pump is what most people can lay their hands on. For this reason I DISCOURAGE people to attempt vacuum distillation, because a piston pump is not suited to the task. Hope that clarifies it. |
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Hi' Graham'
It May have been a misunderstanding on my part but if you have ever seen the nightmare of some ones face falling apart through a flash not quite an explosion of vapors off less than 1L of ethanol from a spill tray that blew back in to the 180L drum being decanted that was luckly still near full, all because some turky stole a copper terminal block the grounding cables were attatched to, tied them off and left them dangaling in mid air,the victom this guy had a LEL meter strapped to his belt as we all do,may be you can understand my paranoia. Sure the conditions have to be right for this type of thing to happen I have seen the right pump for the right aplication installed in the wrong place and the reverse as well, mostly dammaging plant and equipment,bear in mind this happens in industry,what chance has a back yard brewer got unless there is some first hand experienced input. Based on technical Advice there are cheaper ways to distill Methanol that can be set up safely, but for safety you can't beat vacuum if it's set up properly. The echonomical costs of operation are if not better, then worth the extra few cents per kwh in safety, each to their own. All that aside I have taken note of the cheapness of some of the processors,they are cheap, cheap in construction cheap in operation and cheap on SAFETY,how many if any have the most basic safety feature of including static straps and how to use them, Water flowing through plastic lines generates static, dry powders flowing generate Static,and methanol is recovered in to covered but not sealed containers, bio is pumped in to containers from reactors, Static takes the path of least resistance and can jump incredible gaps. It may never happen, but for the cost of a couple of sets of Jumper leads that are cheaper than gas masks or fans, its deadly amusing, where is the input? Just to clarafy, Cheap dose not have to mean inefficient or dangerous, I have built a couple of reactors for the cost of plumbing fittings, Cheap in this case is Cheap on basic safety. Perhaps there should be a topic on the hidden or overlooked potential hazards,eliminating hazards could lead to a higher quality of reactors and processes with a higher Quality of Bio Diesel, most all of us know the hazards of the chemicals we use, you yourself have recognised one of the worst dangers of turning on heaters on low levels in vessels and come up with with a level indicator perhaps some one has the know how of designing a simple low cost low voltage relay circuit using a level indicator that could drop the power of a remote contactor to the elements a safety interlock? by remote I mean set up at a safe distance contactors throw sparks,intrinsically safe units cost a bomb.(forgive the bad spellin) I have a full length glass level indicator, I have accidently almost emptied my reactor with the heaters on I saw the heater indicator light on and nearly had a heart attack. Ferraloil. |
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Hi Ferraloil,
Efficiency can be measured in many ways - depending on where you place your priorities. Industry follows legislative limits as well as costs - they are rarely altruistic due to their need to turn a profit. The cost of power (or even just electricity costs) are not the only costs involved. You may well be right in regard to homebrewers, however I think there are at least some who are concerned with their legal responsibilities and some who care about the environment, but you clearly know more than me on this. The reason for the vapour trap, is to retain a heel of liquid which allows it to cool slightly below the boiling point - known in chem. eng design as sub-cooling.....Chem eng 101. I have designed built and run many such systems. You are right and I tried to explain why you are right - the sub cooling is essential, and could be a safety factor IF the pump cannot handle methanol vapour. Certainly no argument as to which is quickest and SAFEST - as to energy efficiency that would depend on the level of vac you require; the lower you require the pressure the greater the inefficiencies in producing it, but NOW that you are quoting some numbers your argument gains credibility - what pressures do you operate vac at? What I posted was a simplified view of basic engineering thermodynamics. I think your view of glassware as being small scale is naive, but no most of my experience is large scale involving thousands of tons per year. Exporting 5MW gives a little bitty clue as to scale.The last piece of glassware I used was 5 stories high and @ 10ft wide. Pump compatibility relies on many things, and you are right to urge caution when selecting one for methanol duty. All oil filled rotary vane types suffer with oil contamination (unless people sub-cool with a vapour trap as you do). Then there is the issue of oil disposal, etc. Personally I preferred this type over water filled liquid ring pumps, but specification depends on duty. That’s not strictly true. If the stream has enough value the preferred choice is to recover it. HMIP in the UK have moved away from recommending thermal oxidisers as best practise. The process to which I referred involved the burning of liquid ammonia to produce NOx then scrubbing these out to produce nitric acid - some of which got concentrated and some went on to react with ammonia to produce ammonium nitrate, which we then dried. Just in terms of the fertiliser think around 2million tonnes per year. I was shift manager for 5 years. Other parts of the plant did other things, but most of my solvent experience comes from my 8 years at Ciba - and that solvent was too damn expensive to burn. Oh and my veg oil experience comes from 4 years as engineering manager of one of Croda's plants. I also have had roles within other parts of the chemical industry. In all that time I have only come across three plants (that is plants within sites containing a number of plants), which had thermal oxidisers to destroy end fumes... as you say usually not cost effective options. Actually, in my experience, better separation, and therefore quality, usually results from slower distillation, but if it works for you then great - please post more details. As I have said my experience comes from industry NOT homebrew, and I appreciate the differences. I also believe that there is no one solution to any engineering problem. Cheers, Paul |
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