Have the fractions, and the tail tested, if you get further bogus results, confront the lab. It's basically a closed system, nothing is lost, only possibly converted.
-
ICMag and The Vault are running a NEW contest! You can check it here. Prizes are seeds & forum premium access. Come join in!
The Clear .............Thread!
- Thread starter LostTribe
- Start date
HashoftheTitans
Member
I have had this happen multiple times. Every time I ask more questions to the trim provider and it turns out that the material is old. I have been told by a few that it could be the trans delta 8 and 9 isomers that don't show up on test results.
Digital Man
Member
How do you like the spinning band system you are using?
Recommend it over a Pope for example ?
BlackestofOps
Member
I am not a fan of the Pope systems. I like the BR quite a bit. The data gathered is great. The throughput is less than 500ml/hr, however. To get really tight fractions on a first pass, it's very slow, but doable.
If anyone is going to get one, PM me. I get a small kickback. I'll share my data files thus far with you.
thump easy
New member
Im looking already the 2000 and 5000 is to slow looking already for upgrade im gona look into it.. Thanks gor the info..
Digital Man
Member
You need to get up to (50) posts before you can receive PM's. Check your profile and I'll drop you a note. Thanks .....
thump easy
New member
I bought two head a 5 liter and 2 liter best price so far on the heads from gold leaf after my experience with across international riping me on 731$ next day delivery and getting to me 2 weeks later.. and the 5 litter being to fucking big for the mantle not sitting correctly on the base. but there are alot of science guys blowing glass out hear just found out... Not gona cussifie anyone but man..... Its ok i guess it cost me all together alot of money but there are options for shure you gota dig deeper im waiting for someone to come up with a terp classs not slabing the material in a vac oven and cold traping before you hit the pump. And then desolving it back in ethenol to dewax and then catching fractions is there another way easer saving time buy a day or 2? Im looking for terp whole salers like direct from china india or brazzil were its distilled first had mabe a final refinement before mixing?? Were would one find this type of training?
mobin
Member
there are enough shitty food grade terps available domestically to warrant not getting even shittier ones abroad.
you want terps? run steam through plant material below 50*c and continue until you can no longer see anything coming along (you'll see oil bubbles in the water stream collecting in the condenser)
don't pay someone to teach you that. you're gonna need to know things like this on your own...there's always more to learn.
don't learn off the lungs of your customers.
thump easy
New member
Nice Thank You!!!
Digital Man
Member
What does doing that to your plant material prior to extracting do to your final yields .... if anything ? Just wondering if you're losing a bunch of trichomes in the attempt to capture terps via steaming.
thump easy
New member
Im gona try it i build a big still ill let you know my return problem is the eth is expensive its almost best to brew your own. But the white material cloudyness in the eth is i wounder if flavor has anything to do with it as spirts have there own flavor from corn sugar or cain sugar its really starting to turn into chemistry. Local brewry shops sell hops terps for 7 dollars but there stores are as packef as hydro stores they also want to infuse weed terps into beer as the brewing community are just as intrested in the terp profiles pluss the got free beer on tap in the back best place ever i love it...
mobin
Member
that steam will melt the triches into a pool of oil in the flask below....ready for processing....will be full of water soluble garbage so act accordingly.
this shit is time consuming as fuck and ends up becoming a labor of love so it would behoove you to start with something nice.
Digital Man
Member
Thanks much !!!!
purplepassion
Member
I'm looking for a good 2-5l short path setup, any suggestions? Lots of outdoor bho to work with.
thump easy
New member
Does the ethenol vary in process if it is tequila or sugar was or grapeseed all 200 proof?
200 proof for our purposes is the same. There are minor fractional differences at a PPB level.
To achieve maximum separation by fractional distillation:
1. The column must be flooded initially to wet the packing. For this reason it is customary to operate a still at reflux for some time before beginning the distillation.
2. The reflux ratio should be high (i.e. the ratio of drops of liquid which return to the distilling flask and the drops which distil over), so that the distillation proceeds slowly and with minimum disturbance of the equilibria in the column.
3. The hold-up of the column should not exceed one-tenth of the volume of any one component to be separated.
4. Heat loss from the column should be prevented but, if the column is heated to offset this, its temperature must not exceed that of the distillate in the column.
5. Heat input to the still-pot should remain constant.
6. For distillation under reduced pressure there must be careful control of the pressure to avoid flooding or cessation of reflux.
From page 10 of PURIFICATION OF LABORATORY CHEMICALS Fifth Edition
Wilfred L. E Armarego/Christina Li Lin Chai
"Heat input to the still-pot should remain constant."
Seems to back up my contention that power (watts) to the heater should be dialed down to only what is necessary to achieve the desired steady boiling level. A PID controller alone drives the heater/mantle with pulses of too much power, the energy level of the boiling liquid is driven up, allowed to fall, driven up, allowed to fall, over and over.
I've posted up extensively on this subject here,
https://www.icmag.com/ic/showthread.php?t=338633&page=25
1. The column must be flooded initially to wet the packing. For this reason it is customary to operate a still at reflux for some time before beginning the distillation.
2. The reflux ratio should be high (i.e. the ratio of drops of liquid which return to the distilling flask and the drops which distil over), so that the distillation proceeds slowly and with minimum disturbance of the equilibria in the column.
3. The hold-up of the column should not exceed one-tenth of the volume of any one component to be separated.
4. Heat loss from the column should be prevented but, if the column is heated to offset this, its temperature must not exceed that of the distillate in the column.
5. Heat input to the still-pot should remain constant.
6. For distillation under reduced pressure there must be careful control of the pressure to avoid flooding or cessation of reflux.
From page 10 of PURIFICATION OF LABORATORY CHEMICALS Fifth Edition
Wilfred L. E Armarego/Christina Li Lin Chai
"Heat input to the still-pot should remain constant."
Seems to back up my contention that power (watts) to the heater should be dialed down to only what is necessary to achieve the desired steady boiling level. A PID controller alone drives the heater/mantle with pulses of too much power, the energy level of the boiling liquid is driven up, allowed to fall, driven up, allowed to fall, over and over.
I've posted up extensively on this subject here,
https://www.icmag.com/ic/showthread.php?t=338633&page=25
Last edited:
what is hold up?
3. The hold-up of the column should not exceed one-tenth of the volume of any one component to be separated.
3. The hold-up of the column should not exceed one-tenth of the volume of any one component to be separated.
hold-up is mentioned four times - here is the full context from pages 10-12, I boldfaced the mentions:
One of the most widely applicable and most commonly used methods of purification of liquids or low melting solids (especially of organic chemicals) is fractional distillation at atmospheric, or some lower, pressure. Almost without exception, this method can be assumed to be suitable for all organic liquids and most of the low-melting organic solids. For this reason it has been possible in Chapter 4 to omit many procedures for purification of organic chemicals when only a simple fractional distillation is involved - the suitability of such a procedure is implied from the boiling point.
The boiling point of a liquid varies with the 'atmospheric' pressure to which it is exposed. A liquid boils when its vapour pressure is the same as the external pressure on its surface, its normal boiling point being the temperature at which its vapour pressure is equal to that of a standard atmosphere (760mm Hg). Lowering the external pressure lowers the boiling point. For most substances, boiling point and vapour pressure are related by an equation of the form,
logp = A+BT(f+273), where p is the pressure, t is in C, and A and B are constants. Hence, if the boiling points at two different pressures are known the boiling point at another pressure can be calculated from a simple plot of log p versus \l(t+273). For organic molecules that are not strongly associated, this equation can be written in the form,
log p = 8.586 - 5.703 (T + 213)1(t + 273)
where T is the boiling point in C at 760mm Hg. Tables 3A and 3B give computed boiling points over a range of pressures. Some examples illustrate its application. Ethyl acetoacetate, b 180° (with decomposition) at 760mm Hg has a predicted b of 79° at 16mm; the experimental value is 78°. Similarly 2,4-diaminotoluene, b 292° at 760mm, has a predicted b of 147° at 8mm; the experimental value is 148-150°. For self-associated molecules the predicted b are lower than the experimental values. Thus, glycerol, b 290° at 760mm, has a predicted b of 146° at 8mm: the experimental value is 182°.
Similarly an estimate of the boiling points of liquids at reduced pressure can be obtained using a nomogram (see Figure 1).
For pressures near 760mm, the change in boiling point is given approximately by, lf = a(760-p)(f + 273)
where a - 0.00012 for most substances, but a = 0.00010 for water, alcohols, carboxylic acids and other associated liquids, and a = 0.00014 for very low-boiling substances such as nitrogen or ammonia [Crafts Chem Ber 20 709 1887 ]. When all the impurities are non-volatile, simple distillation is adequate purification. The observed boiling point remains almost constant and approximately equal to that of the pure material. Usually, however, some of the impurities are appreciably volatile, so that the boiling point progressively rises during the distillation because of the progressive enrichment of the higher boiling components in the distillation flask. In such cases, separation is effected by fractional distillation using an efficient column.
Techniques.
The distillation apparatus consists basically of a distillation flask, usually fitted with a vertical fractionating column (which may be empty or packed with suitable materials such as glass helices or stainless-steel wool) to which is attached a condenser leading to a receiving flask. The bulb of a thermometer projects into the vapour phase just below the region where the condenser joins the column. The distilling flask is heated so that its contents are steadily vaporised by boiling. The vapour passes up into the column where, initially, it condenses and runs back into the flask. The resulting heat transfer gradually warms the column so that there is a progressive movement of the vapour phase-liquid boundary up the column, with increasing enrichment of the more volatile component. Because of this fractionation, the vapour finally passing into the condenser (where it condenses and flows into the receiver) is commonly that of the lowest-boiling components in the system. The conditions apply until all of the low-boiling material has been distilled, whereupon distillation ceases until the column temperature is high enough to permit the next component to distil. This usually results in a temporary fall in the temperature indicated by the thermometer.
Distillation of liquid mixtures.
The principles involved in fractional distillation of liquid mixtures are complex but can be seen by considering a system which approximately obeys Raoult's law. (This law states that the vapour pressure of a solution at any given temperature is the sum of the vapour pressures of each component multiplied by its mole fraction in the solution.) If two substances, A and B, having vapour pressures of 600mm Hg and 360mm Hg, respectively, were mixed in a molar ratio of 2:1 (i.e. 0.666:0.333 mole ratio), the mixture would have (ideally) a vapour pressure of 520mm Hg (i.e. 600 x 0.666 + 360 x 0.333, or 399.6 + 119.88 mm Hg) and the vapour phase would contain 77% (399.6 x 100/520) of A and 23% (119.88 x 100/520) of B. If this phase was now condensed, the new liquid phase would, therefore, be richer in the volatile component A. Similarly, the vapour in equilibrium with this phase is still further enriched in A. Each such liquid-vapour equilibrium constitutes a "theoretical plate". The efficiency of a fractionating column is commonly expressed as the number of such plates to which it corresponds in operation. Alternatively, this information may be given in the form of the height equivalent to a theoretical plate, or HETP. The number of theoretical plates and equilibria between liquids and vapours are affected by the factors listed to achieve maximum separation by fractional distillation in the section below on techniques.
In most cases, systems deviate to a greater or lesser extent from Raoult's law, and vapour pressures may be greater or less than the values calculated. In extreme cases (e.g. azeotropes), vapour pressure-composition curves pass through maxima or minima, so that attempts at fractional distillation lead finally to the separation of a constant- boiling (azeotropic) mixture and one (but not both) of the pure species if either of the latter is present in excess.
Elevatiion of the boiling point by dissolved solids. Organic substances dissolved in organic solvents cause a rise in boiling point which is proportional to the concentration of the substance, and the extent of rise in temperature is characteristic of the solvent. The following equation applies for dilute solutions and non-associating substances:
M*Dt/c = K
Where M is the molecular weight of the solute, Dt is the elevation of boiling point in C, c is the concentration
of solute in grams for lOOOgm of solvent, and K is the Ebullioscopic Constant (molecular elevation of the boiling point) for the solvent. K is a fixed property (constant) for the particular solvent. This has been very useful for the determination of the molecular weights of organic substances in solution.
The efficiency of a distillation apparatus used for purification of liquids depends on the difference in boiling points of the pure material and its impurities. For example, if two components of an ideal mixture have vapour pressures in the ratio 2:1, it would be necessary to have a still with an efficiency of at least seven plates (giving an enrichment of 2 to the seventh power = 128) if the concentration of the higher boiling component in the distillate was to be reduced to less than 1% of its initial value. For a vapour pressure ratio of 5:1, three plates would achieve as much separation.
In a fractional distillation, it is usual to reject the initial and final fractions, which are likely to be richer in the lower-boiling and higher-boiling impurities respectively. The centre fraction can be further purified by repeated fractional distillation.
To achieve maximum separation by fractional distillation:
1. The column must be flooded initially to wet the packing. For this reason it is customary to operate a still at reflux for some time before beginning the distillation.
2. The reflux ratio should be high (i.e. the ratio of drops of liquid which return to the distilling flask and the drops which distill over), so that the distillation proceeds slowly and with minimum disturbance of the equilibria in the column.
3. The hold-up of the column should not exceed one-tenth of the volume of any one component to be separated.
4. Heat loss from the column should be prevented but, if the column is heated to offset this, its temperature must not exceed that of the distillate in the column.
5. Heat input to the still-pot should remain constant.
6. For distillation under reduced pressure there must be careful control of the pressure to avoid flooding or cessation of reflux.
Types of distillation
The distilling flask. To minimise superheating of the liquid (due to the absence of minute air bubbles or other suitable nuclei for forming bubbles of vapour), and to prevent bumping, one or more of the following precautions should be taken:
(a) The flask is heated uniformly over a large part of its surface, either by using an electrical heating mantle or, by partial immersion in a bath above the boiling point of the liquid to be distilled.
(b) Before heating begins, small pieces of unglazed fireclay or porcelain (porous pot, boiling chips), pumice, diatomaceous earth, or platinum wire are added to the flask. These act as sources of air bubbles.
(c) The flask may contain glass siphons or boiling tubes. The former are inverted J-shaped tubes, the end of the shorter arm being just above the surface of the liquid. The latter comprise long capillary tubes sealed above the lower end.
(d) A steady slow stream of inert gas (e.g. N2, Ar or He) is passed through the liquid.
(e) The liquid in the flask is stirred mechanically. This is especially necessary when suspended insoluble material is present.
For simple distillations a Claisen flask is often used. This flask is, essentially, a round-bottomed flask to the neck of which is joined another neck carrying a side arm. This second neck is sometimes extended so as to form a Vigreux column [a glass tube in which have been made a number of pairs of indentations which almost touch each other and which slope slightly downwards. The pairs of indentations are arranged to form a spiral of glass inside the tube].
For heating baths, see Table 4. For distillation apparatus on a micro or semi-micro scale see Aldrich and other glassware catalogues. Alternatively, some useful websites for suppliers of laboratory glassware are www.wheatonsci.com, www.sigmaaldrich.com and www.kimble-kontes.com.
Types of columns and packings. A slow distillation rate is necessary to ensure that equilibrium conditions operate and also that the vapour does not become superheated so that the temperature rises above the boiling point. Efficiency is improved if the column is heat insulated (either by vacuum jacketing or by lagging) and, if necessary, heated to just below the boiling point of the most volatile component. Efficiency of separation also improves with increase in the heat of vaporisation of the liquids concerned (because fractionation depends on heat equilibration at multiple liquid-gas boundaries). Water and alcohols are more easily purified by distillation for this reason.
Columns used in distillation vary in their shapes and types of packing. Packed columns are intended to give efficient separation by maintaining a large surface of contact between liquid and vapour. Efficiency of separation is further increased by operation under conditions approaching total reflux, i.e. under a high reflux ratio. However, great care must be taken to avoid flooding of the column during distillation. The minimum number of theoretical plates for satisfactory separation of two liquids differing in boiling point by It is approximately (273 + t)/3It, where t is the average boiling point in C.
The packing of a column greatly increases the surface of liquid films in contact with the vapour phase, thereby increasing the efficiency of the column, but reducing its capacity (the quantities of vapour and liquid able to flow in opposite directions in a column without causing flooding). Material for packing should be of uniform size, symmetrical shape, and have a unit diameter less than one eighth that of the column. (Rectification efficiency increases sharply as the size of the packing is reduced but so, also, does the hold-up in the column.) It should also be capable of uniform, reproducible packing.
The usual packings are:
(a) Rings. These may be hollow glass or porcelain (Raschig rings), of stainless steel gauze
(Dixon rings), or hollow rings with a central partition (Lessing rings) which may be of porcelain, aluminium, copper or nickel.
(b) Helices. These may be of metal or glass (Fenske rings), the latter being used where resistance to chemical attack is important (e.g. in distilling acids, organic halides, some sulfur compounds, and phenols). Metal single-turn helices are available in aluminium, nickel or stainless steel. Glass helices are less efficient, because they cannot be tamped to ensure uniform packing.
(c) Balls or beads. These are usually made of glass.
Condensers. Some of the more commonly used condensers are:
Air condenser. A glass tube such as the inner part of a Liebig condenser (see below). Used for
liquids with boiling points above 90°. Can be of any length.
Coil condenser. An open tube, into which is sealed a glass coil or spiral through which water
circulates. The tube is sometimes also surrounded by an outer cooling jacket. A double coil condenser has two inner coils with circulating water.
Double surface condenser. A tube in which the vapour is condensed between an outer and inner water-cooled jacket after impinging on the latter. Very useful for liquids boiling below 40°.
Friedrichs condenser. A "cold-finger" type of condenser sealed into a glass jacket open at the bottom and near the top. The cold finger is formed into glass screw threads.
Liebig condenser. An inner glass tube surrounded by a glass jacket through which water is
circulated.
Vacuum distillation. This expression is commonly used to denote a distillation under reduced pressure lower than that of the normal atmosphere. Because the boiling point of a substance depends on the pressure, it is often possible by sufficiently lowering the pressure to distill materials at a temperature low enough to avoid partial or complete decomposition, even if they are unstable when boiled at atmospheric pressure.
Sensitive or high boiling liquids should invariably be distilled or fractionally distilled under reduced pressure. The apparatus is essentially as described for distillation except that ground joints connecting the different parts of the apparatus should be air tight by using grease, or better Teflon sleaves. For low, moderately high, and very high temperatures Apiezon L, M and T greases respectively, are very satisfactory. Alternatively, it is often preferable to avoid grease and to use thin Teflon sleeves in the joints. The distilling flask, must be supplied with a capillary bleed (which allows a fine stream of air, nitrogen or argon into the flask), and the receiver should be of the fraction collector type. When distilling under vacuum it is very important to place a loose packing of glass wool above the liquid to buffer sudden boiling of the liquid. The flask should be not more than two-thirds full of liquid. The vacuum must have attained a steady state, i.e. the liquid has been completely degassed, before the heat source is applied, and the temperature of the heat source must be raised very slowly until boiling is achieved.
If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg 2 respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range 10~ mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10~ mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used.
Special oil or mercury traps are available commercially and a liquid-nitrogen (b -209.9 C) trap is the most satisfactory one to use between these and the apparatus. It has an advantage over liquid air or oxygen in that it is non-explosive if it becomes contaminated with organic matter. Air should not be sucked through the apparatus before starting a distillation because this will cause liquid oxygen (b -183 C) to condense in the liquid nitrogen
trap and this is potentially explosive (especially in mixtures with organic materials). Due to the potential lethal consequences of liquid oxygen/organic material mixtures, care must be exercised when handling liquid nitrogen. Hence, it is advisable to degas the system for a short period before the trap is immersed into the liquid nitrogen (which is kept in a Dewar flask).
Spinning-band distillation. Factors which limit the performance of distillation columns include the tendency to flood (which occurs when the returning liquid blocks the pathway taken by the vapour through the column) and the increased hold-up (which decreases the attainable efficiency) in the column that should, theoretically, be highly efficient. To overcome these difficulties, especially for distillation under high vacuum of heat sensitive or high-boiling highly viscous fluids, spinning band columns are commercially available. In such units, the distillation columns contain a rapidly rotating, motor-driven, spiral band, which may be of polymer-coated metal, stainless steel or platinum. The rapid rotation of the band in contact with the walls of the still gives intimate mixing of descending liquid and ascending vapour while the screw-like motion of the band drives the liquid towards the still-pot, helping to reduce hold-up. There is very little pressure drop in such a system, and very high throughputs are possible, with high efficiency. For example, a 765-mm long 10-mm diameter commercial spinning-band column is reported to have an efficiency of 28 plates and a pressure drop of 0.2mm Hg for a throughput of 330mL/h. The columns may be either vacuum jacketed or heated externally. The stills can be operated down to 10~ mm Hg. The principle, which was first used commercially in the Podbielniak Centrifugal Superfractionator, has also been embodied in descending-film molecular distillation apparatus.
One of the most widely applicable and most commonly used methods of purification of liquids or low melting solids (especially of organic chemicals) is fractional distillation at atmospheric, or some lower, pressure. Almost without exception, this method can be assumed to be suitable for all organic liquids and most of the low-melting organic solids. For this reason it has been possible in Chapter 4 to omit many procedures for purification of organic chemicals when only a simple fractional distillation is involved - the suitability of such a procedure is implied from the boiling point.
The boiling point of a liquid varies with the 'atmospheric' pressure to which it is exposed. A liquid boils when its vapour pressure is the same as the external pressure on its surface, its normal boiling point being the temperature at which its vapour pressure is equal to that of a standard atmosphere (760mm Hg). Lowering the external pressure lowers the boiling point. For most substances, boiling point and vapour pressure are related by an equation of the form,
logp = A+BT(f+273), where p is the pressure, t is in C, and A and B are constants. Hence, if the boiling points at two different pressures are known the boiling point at another pressure can be calculated from a simple plot of log p versus \l(t+273). For organic molecules that are not strongly associated, this equation can be written in the form,
log p = 8.586 - 5.703 (T + 213)1(t + 273)
where T is the boiling point in C at 760mm Hg. Tables 3A and 3B give computed boiling points over a range of pressures. Some examples illustrate its application. Ethyl acetoacetate, b 180° (with decomposition) at 760mm Hg has a predicted b of 79° at 16mm; the experimental value is 78°. Similarly 2,4-diaminotoluene, b 292° at 760mm, has a predicted b of 147° at 8mm; the experimental value is 148-150°. For self-associated molecules the predicted b are lower than the experimental values. Thus, glycerol, b 290° at 760mm, has a predicted b of 146° at 8mm: the experimental value is 182°.
Similarly an estimate of the boiling points of liquids at reduced pressure can be obtained using a nomogram (see Figure 1).
For pressures near 760mm, the change in boiling point is given approximately by, lf = a(760-p)(f + 273)
where a - 0.00012 for most substances, but a = 0.00010 for water, alcohols, carboxylic acids and other associated liquids, and a = 0.00014 for very low-boiling substances such as nitrogen or ammonia [Crafts Chem Ber 20 709 1887 ]. When all the impurities are non-volatile, simple distillation is adequate purification. The observed boiling point remains almost constant and approximately equal to that of the pure material. Usually, however, some of the impurities are appreciably volatile, so that the boiling point progressively rises during the distillation because of the progressive enrichment of the higher boiling components in the distillation flask. In such cases, separation is effected by fractional distillation using an efficient column.
Techniques.
The distillation apparatus consists basically of a distillation flask, usually fitted with a vertical fractionating column (which may be empty or packed with suitable materials such as glass helices or stainless-steel wool) to which is attached a condenser leading to a receiving flask. The bulb of a thermometer projects into the vapour phase just below the region where the condenser joins the column. The distilling flask is heated so that its contents are steadily vaporised by boiling. The vapour passes up into the column where, initially, it condenses and runs back into the flask. The resulting heat transfer gradually warms the column so that there is a progressive movement of the vapour phase-liquid boundary up the column, with increasing enrichment of the more volatile component. Because of this fractionation, the vapour finally passing into the condenser (where it condenses and flows into the receiver) is commonly that of the lowest-boiling components in the system. The conditions apply until all of the low-boiling material has been distilled, whereupon distillation ceases until the column temperature is high enough to permit the next component to distil. This usually results in a temporary fall in the temperature indicated by the thermometer.
Distillation of liquid mixtures.
The principles involved in fractional distillation of liquid mixtures are complex but can be seen by considering a system which approximately obeys Raoult's law. (This law states that the vapour pressure of a solution at any given temperature is the sum of the vapour pressures of each component multiplied by its mole fraction in the solution.) If two substances, A and B, having vapour pressures of 600mm Hg and 360mm Hg, respectively, were mixed in a molar ratio of 2:1 (i.e. 0.666:0.333 mole ratio), the mixture would have (ideally) a vapour pressure of 520mm Hg (i.e. 600 x 0.666 + 360 x 0.333, or 399.6 + 119.88 mm Hg) and the vapour phase would contain 77% (399.6 x 100/520) of A and 23% (119.88 x 100/520) of B. If this phase was now condensed, the new liquid phase would, therefore, be richer in the volatile component A. Similarly, the vapour in equilibrium with this phase is still further enriched in A. Each such liquid-vapour equilibrium constitutes a "theoretical plate". The efficiency of a fractionating column is commonly expressed as the number of such plates to which it corresponds in operation. Alternatively, this information may be given in the form of the height equivalent to a theoretical plate, or HETP. The number of theoretical plates and equilibria between liquids and vapours are affected by the factors listed to achieve maximum separation by fractional distillation in the section below on techniques.
In most cases, systems deviate to a greater or lesser extent from Raoult's law, and vapour pressures may be greater or less than the values calculated. In extreme cases (e.g. azeotropes), vapour pressure-composition curves pass through maxima or minima, so that attempts at fractional distillation lead finally to the separation of a constant- boiling (azeotropic) mixture and one (but not both) of the pure species if either of the latter is present in excess.
Elevatiion of the boiling point by dissolved solids. Organic substances dissolved in organic solvents cause a rise in boiling point which is proportional to the concentration of the substance, and the extent of rise in temperature is characteristic of the solvent. The following equation applies for dilute solutions and non-associating substances:
M*Dt/c = K
Where M is the molecular weight of the solute, Dt is the elevation of boiling point in C, c is the concentration
of solute in grams for lOOOgm of solvent, and K is the Ebullioscopic Constant (molecular elevation of the boiling point) for the solvent. K is a fixed property (constant) for the particular solvent. This has been very useful for the determination of the molecular weights of organic substances in solution.
The efficiency of a distillation apparatus used for purification of liquids depends on the difference in boiling points of the pure material and its impurities. For example, if two components of an ideal mixture have vapour pressures in the ratio 2:1, it would be necessary to have a still with an efficiency of at least seven plates (giving an enrichment of 2 to the seventh power = 128) if the concentration of the higher boiling component in the distillate was to be reduced to less than 1% of its initial value. For a vapour pressure ratio of 5:1, three plates would achieve as much separation.
In a fractional distillation, it is usual to reject the initial and final fractions, which are likely to be richer in the lower-boiling and higher-boiling impurities respectively. The centre fraction can be further purified by repeated fractional distillation.
To achieve maximum separation by fractional distillation:
1. The column must be flooded initially to wet the packing. For this reason it is customary to operate a still at reflux for some time before beginning the distillation.
2. The reflux ratio should be high (i.e. the ratio of drops of liquid which return to the distilling flask and the drops which distill over), so that the distillation proceeds slowly and with minimum disturbance of the equilibria in the column.
3. The hold-up of the column should not exceed one-tenth of the volume of any one component to be separated.
4. Heat loss from the column should be prevented but, if the column is heated to offset this, its temperature must not exceed that of the distillate in the column.
5. Heat input to the still-pot should remain constant.
6. For distillation under reduced pressure there must be careful control of the pressure to avoid flooding or cessation of reflux.
Types of distillation
The distilling flask. To minimise superheating of the liquid (due to the absence of minute air bubbles or other suitable nuclei for forming bubbles of vapour), and to prevent bumping, one or more of the following precautions should be taken:
(a) The flask is heated uniformly over a large part of its surface, either by using an electrical heating mantle or, by partial immersion in a bath above the boiling point of the liquid to be distilled.
(b) Before heating begins, small pieces of unglazed fireclay or porcelain (porous pot, boiling chips), pumice, diatomaceous earth, or platinum wire are added to the flask. These act as sources of air bubbles.
(c) The flask may contain glass siphons or boiling tubes. The former are inverted J-shaped tubes, the end of the shorter arm being just above the surface of the liquid. The latter comprise long capillary tubes sealed above the lower end.
(d) A steady slow stream of inert gas (e.g. N2, Ar or He) is passed through the liquid.
(e) The liquid in the flask is stirred mechanically. This is especially necessary when suspended insoluble material is present.
For simple distillations a Claisen flask is often used. This flask is, essentially, a round-bottomed flask to the neck of which is joined another neck carrying a side arm. This second neck is sometimes extended so as to form a Vigreux column [a glass tube in which have been made a number of pairs of indentations which almost touch each other and which slope slightly downwards. The pairs of indentations are arranged to form a spiral of glass inside the tube].
For heating baths, see Table 4. For distillation apparatus on a micro or semi-micro scale see Aldrich and other glassware catalogues. Alternatively, some useful websites for suppliers of laboratory glassware are www.wheatonsci.com, www.sigmaaldrich.com and www.kimble-kontes.com.
Types of columns and packings. A slow distillation rate is necessary to ensure that equilibrium conditions operate and also that the vapour does not become superheated so that the temperature rises above the boiling point. Efficiency is improved if the column is heat insulated (either by vacuum jacketing or by lagging) and, if necessary, heated to just below the boiling point of the most volatile component. Efficiency of separation also improves with increase in the heat of vaporisation of the liquids concerned (because fractionation depends on heat equilibration at multiple liquid-gas boundaries). Water and alcohols are more easily purified by distillation for this reason.
Columns used in distillation vary in their shapes and types of packing. Packed columns are intended to give efficient separation by maintaining a large surface of contact between liquid and vapour. Efficiency of separation is further increased by operation under conditions approaching total reflux, i.e. under a high reflux ratio. However, great care must be taken to avoid flooding of the column during distillation. The minimum number of theoretical plates for satisfactory separation of two liquids differing in boiling point by It is approximately (273 + t)/3It, where t is the average boiling point in C.
The packing of a column greatly increases the surface of liquid films in contact with the vapour phase, thereby increasing the efficiency of the column, but reducing its capacity (the quantities of vapour and liquid able to flow in opposite directions in a column without causing flooding). Material for packing should be of uniform size, symmetrical shape, and have a unit diameter less than one eighth that of the column. (Rectification efficiency increases sharply as the size of the packing is reduced but so, also, does the hold-up in the column.) It should also be capable of uniform, reproducible packing.
The usual packings are:
(a) Rings. These may be hollow glass or porcelain (Raschig rings), of stainless steel gauze
(Dixon rings), or hollow rings with a central partition (Lessing rings) which may be of porcelain, aluminium, copper or nickel.
(b) Helices. These may be of metal or glass (Fenske rings), the latter being used where resistance to chemical attack is important (e.g. in distilling acids, organic halides, some sulfur compounds, and phenols). Metal single-turn helices are available in aluminium, nickel or stainless steel. Glass helices are less efficient, because they cannot be tamped to ensure uniform packing.
(c) Balls or beads. These are usually made of glass.
Condensers. Some of the more commonly used condensers are:
Air condenser. A glass tube such as the inner part of a Liebig condenser (see below). Used for
liquids with boiling points above 90°. Can be of any length.
Coil condenser. An open tube, into which is sealed a glass coil or spiral through which water
circulates. The tube is sometimes also surrounded by an outer cooling jacket. A double coil condenser has two inner coils with circulating water.
Double surface condenser. A tube in which the vapour is condensed between an outer and inner water-cooled jacket after impinging on the latter. Very useful for liquids boiling below 40°.
Friedrichs condenser. A "cold-finger" type of condenser sealed into a glass jacket open at the bottom and near the top. The cold finger is formed into glass screw threads.
Liebig condenser. An inner glass tube surrounded by a glass jacket through which water is
circulated.
Vacuum distillation. This expression is commonly used to denote a distillation under reduced pressure lower than that of the normal atmosphere. Because the boiling point of a substance depends on the pressure, it is often possible by sufficiently lowering the pressure to distill materials at a temperature low enough to avoid partial or complete decomposition, even if they are unstable when boiled at atmospheric pressure.
Sensitive or high boiling liquids should invariably be distilled or fractionally distilled under reduced pressure. The apparatus is essentially as described for distillation except that ground joints connecting the different parts of the apparatus should be air tight by using grease, or better Teflon sleaves. For low, moderately high, and very high temperatures Apiezon L, M and T greases respectively, are very satisfactory. Alternatively, it is often preferable to avoid grease and to use thin Teflon sleeves in the joints. The distilling flask, must be supplied with a capillary bleed (which allows a fine stream of air, nitrogen or argon into the flask), and the receiver should be of the fraction collector type. When distilling under vacuum it is very important to place a loose packing of glass wool above the liquid to buffer sudden boiling of the liquid. The flask should be not more than two-thirds full of liquid. The vacuum must have attained a steady state, i.e. the liquid has been completely degassed, before the heat source is applied, and the temperature of the heat source must be raised very slowly until boiling is achieved.
If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg 2 respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range 10~ mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10~ mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used.
Special oil or mercury traps are available commercially and a liquid-nitrogen (b -209.9 C) trap is the most satisfactory one to use between these and the apparatus. It has an advantage over liquid air or oxygen in that it is non-explosive if it becomes contaminated with organic matter. Air should not be sucked through the apparatus before starting a distillation because this will cause liquid oxygen (b -183 C) to condense in the liquid nitrogen
trap and this is potentially explosive (especially in mixtures with organic materials). Due to the potential lethal consequences of liquid oxygen/organic material mixtures, care must be exercised when handling liquid nitrogen. Hence, it is advisable to degas the system for a short period before the trap is immersed into the liquid nitrogen (which is kept in a Dewar flask).
Spinning-band distillation. Factors which limit the performance of distillation columns include the tendency to flood (which occurs when the returning liquid blocks the pathway taken by the vapour through the column) and the increased hold-up (which decreases the attainable efficiency) in the column that should, theoretically, be highly efficient. To overcome these difficulties, especially for distillation under high vacuum of heat sensitive or high-boiling highly viscous fluids, spinning band columns are commercially available. In such units, the distillation columns contain a rapidly rotating, motor-driven, spiral band, which may be of polymer-coated metal, stainless steel or platinum. The rapid rotation of the band in contact with the walls of the still gives intimate mixing of descending liquid and ascending vapour while the screw-like motion of the band drives the liquid towards the still-pot, helping to reduce hold-up. There is very little pressure drop in such a system, and very high throughputs are possible, with high efficiency. For example, a 765-mm long 10-mm diameter commercial spinning-band column is reported to have an efficiency of 28 plates and a pressure drop of 0.2mm Hg for a throughput of 330mL/h. The columns may be either vacuum jacketed or heated externally. The stills can be operated down to 10~ mm Hg. The principle, which was first used commercially in the Podbielniak Centrifugal Superfractionator, has also been embodied in descending-film molecular distillation apparatus.
Last edited:
TestyCalls
New member
Still unclear about what "hold up" is.
