Operating line ( Absorption)

Like distillation, operating lines can also be found in absorption. The basic principle to obtain the operating lines is the same. If we do the mass balance of the solute absorbed, we will end up with one equation, a linear equation.

Before we have a look at the mass balance, it is important to introduced a new term; loading. Loading is defined in the same way as molar fraction is. Normally a big capital X is used to represent loading, while a small one, x represents molar fraction.

Molar fraction, x = mole of solute / total mole available

Loading, X = mole of solute / (total mole available - mole of solute)

If molar fraction is given, it can simply be converted into loading via a very simple equation:

X = x/(1-x) or

x = X/(1+X)

Again X is for liquid phase and Y is for gas phase. There is however one important assumption to do the mass balance around the absorption column: Absorbent does not evaporate and carrier gas is not absorbed at the same time. This allows us to say that gas, G and liquid flow rate, L remain unchanged through out the column.

We know already G(in) = G(out) and it also valids L(in) = L(out)

Mass balance using loading:

G*Y(n+1) + L*Xo = G*Y1 + L*Xn

With some rearrangement we will get this final equation:

Y(n+1) = L/G *(Xn-Xo) + Y1

This is a straight line on a Y against X diagram.

Absorption

Absorption is one of many unit operations used in process industries. It is a separation process that involves a mass transfer from a gas to a liquid. The solute which is absorbed is called absorptive and the solvent which absorbs solute is referred as absorbent. The reversed process (mass transfer from a liquid to a gas) is desorption.

These are the applications of (physical) absorption:

1) Air purification
2) Odor control
3) Purification in refinery and petro-chemical industry

Meanwhile desorption is sometimes used in:

1) Regenerating of loaded absorbent
2) Removing of solved components from liquids

There are some similarities between distillation and absorption:

1) Both involve two phases, liquid and gas
2) Both phases are normally saturated

With that being said absorption differs from distillation in many ways:

1) Feed in distillation is mostly liquid, vapour is sometimes generated as a part of the liquid feed
2) The operating temperature for absorption is normally lower than that for distillation
3) Liquid to gas ratio in absorption is mostly lower than that in distillation

Physical absorption and chemical absorption:

a) Physical absorption: where absorbed solute does not react with the aqueous solution
b) Chemical absorption: solute is absorbed and then reacts with the aqueous solution

Physical absorption is normally preferred at high pressure while at low pressure chemical absorption works best. However the regeneration in case of chemical absorption is really challenging.

Dividing-wall column

A dividing-wall column or divided-wall column is a type of new column to be used in separating a multicomponent mixture. It is however not widely used in industry as people are still doing some researches on that topic.

It looks exactly similar to the normal distillation column. The only noticeable and probably main difference is that a vertical wall is installed or welded inside the column that separates the internal column into two regions.

Since distillation is responsible for the largest fraction of huge amount of energy consumed in process industries, it then becomes a major concern and primary target of energy saving effort in industrially developed countries. This separation process also is the most widely used in industry compared to other unit operations and needs most of the time a large expenditure of capital.

It is important to do many improvements in this area as the distillation applications will definitely increase from year to year. Therefore distillation systems that are sustainable and economically feasible must designed and if they are proven to be successful, they must be introduced. This is the reason why a dividing-wall column is introduced as it is said that this type of column is capable of reducing both capital and operating costs.

The welded wall separates the column into two sections namely a feed section and a side draw section. The former acts as a prefractionator and the latter allows the separation of high purity intermediate-boiling component in a ternary mixture, for example.

Example of a dividing-wall column:

So only one column is required to separate a mixture that contains A,B and C. Since we know that we need at least two columns to separate a ternary mixture via ordinary distillation, a dividing-wall column is obviously advantageous in terms of capital cost. The capital cost can be reduced by installing this type of column. In addition to that it has been proven that this type of column also consumes relatively less energy than the two-column system. The operating cost is said to be reduced up to 10%.
 le and economically feasible must be designed and introduced

Therefore distillation systems that are

sustaina

ble and economically feasible must be designed and introduced.

Number of theoretical stages

As mentioned several times this McCabe-Thiele diagram can be used to determine the number of theoretical stages too. Knowing the number of theoretical stages is crucial in order to get a rough estimation of the cost and other aspects of distillation.

First of all, mark all those given or known compositions in the diagram. Then draw a feed line from the feed composition that depends on the feed phase. In this example we again use a saturated-liquid feed. So the feed line will be a vertical line.

Secondly draw an operating line for the rectifying section, provided we know the reflux ratio that we want to operate at. Since it is known, we can simply calculate the slope and/or the y-intercept of the operating line. Draw the line until it intercepts with the feed line.

Next step is to connect the intersection point with the bottom composition, forming another straight line. This is the operating line for a stripping section of the column.

Last step is to draw steps in between those operating lines and the equilibrium curve. This is drawn in red. In total we need 10 stages, 9 stages and 1 reboiler as reboiler is counted as a stage as well.

Minimum reflux ratio ii

Sometimes the equilibrium curve does not look really curvy. The more curvy the equilibrium curve is, the better is the separation of the key components. Previously we already learned how to determine the minimum reflux ratio in case of an easily-separated mixture. This time we will discuss how to determine the same parameter when the equilibrium curve differs or gets less curvy.

Example:

Notice that the equilibrium curve gets closer to the bisecting line as X-value approaches one. This is a completely different mixture in comparison to the one that we used as an example in the previous entry.

First step is to mark all those given compositions such bottom-, feed- and distillate composition. In reality only the distillate composition is required. However there is no harm in marking all those points on the diagram.

Secondly from that distillate composition draw a tangent to the curve and extrapolate it until we have the value of y-intercept, a.

Since we know a is equal to xd/(v+1) , we can easily determine its minimum reflux ratio, provided xd is known before.

Minimum reflux ratio

With the help of McCabe-Thiele diagram we can also determine the minimum reflux ratio. It is again pretty simple and direct. One only has to know how to use the diagram.

First of all, we need to mark all those points that we know such as distillate-,bottom- and feed composition. This time feed composition and feed phase are important and cannot be neglected.

Secondly draw a feed line from the feed composition point marked in the diagram. In case of having a saturated-liquid feed, a vertical line is drawn until it touches the equilibrium curve.

Then draw an operating line from the distillate composition that passes through the intersection point between feed line and equilibrium curve.

After that extrapolate that operating line until it intersects with y-axis at point a.

Knowing the value of a, the minimum reflux ratio can be calculated. We have seen that the y-intercept of an operating line for a rectifying section is equal to xd/(v+1). Since xd is normally known or given, v or v(min) can easily be determined.

Minimum number of stages

In this entry we will learn how to determine the number of stages from a very popular diagram used in distillation field, McCabe-Thiele diagram. It is actually pretty easy and direct if one knows how to use it.

First of all we have to know what it means by minimum number of stages. It is actually another way of saying that we have an infinite amount of reflux ratio. We know that the slope of the operating line for a rectifying section is equal to v/v+1, this means the slope will be one in case of having a very high reflux ratio. The operating line will simply follow the y=x line in the diagram.

Secondly we need to mark all those important points such distillate composition, feed composition and bottom composition. Feed composition is however not needed in this case. Feed has no influence on the minimum number of stages if we have an infinite reflux ratio.

The next step is to draw stages in the diagram. It is worth mentioning that the order to start drawing the stage does not matter. We can either start stepping off from distillate or from bottom. The result remains the same and will not change at all.

Hence for this example we need minimum 3 stages without considering the tray efficiency. Normally we need more than 3 if we take the tray efficiency into account.

Feed composition

A feed is necessary in order to perform the distillation. The phase state of the feed must not necessarily be liquid. It can be vapour as well.

Depending on the feed temperature or pressure, a liquid-feed can either be a sub-cooled liquid or saturated liquid. A saturated liquid simply means that it is available at its boiling point and about to vaporise.

In addition to that, a feed can also be a vapour. Again, depending on temperature or pressure, that feed can also contain two phases in a certain ratio, it can be a saturated vapour and a superheated vapour. These different types of feed are important if we want to use McCabe-Thiele diagram in order to determine parameters such reflux ratio and number of stages, just to name a few.

With the help of total mass balance and the mass balance for each component, a feed line can be obtained. It is a again a straight line. To be able to extract necessary information from the McCabe-Thiele diagram, both operating line and feed line are crucial.

Depending on the feed phase, the feed line can either have a positive slope or a negative slope(zero and infinite also possible). In order to see the influence of the feed on the feed line in the McCabe-Thiele diagram, a diagram would be helpful:

There are 5 different feed lines, a,b,c,d,e in the diagram above.

a) sub-cooled liquid feed
b) saturated liquid feed
c) two phases feed
d) saturated vapour feed.
e) superheated vapour feed

A/C-Path Modification

Since this type of column sequence needs 6 heat exchangers, other alternative sequences must be found so that the energy requirement can be lowered. Plus this A/C-Path also uses 3 columns which means not only operating cost is higher than other sequences, its capital cost is relatively higher as well. There are two alternatives available:

A/C-path: 6 heat exchangers and 3 columns

First modification: 4 heat exchangers and 2 columns (simply coupling two columns). This arrangement is superior to A/C-path.

Second modification: 3 heat exchangers and 2 columns (with side rectifying and side stripping). This sequence is superior to the first modification.

Column's arrangement

If we have a three-component mixture and we want to separate them into their pure products respectively, we need at least two columns. It is however possible to use only one column for this kind of mixture. The only disadvantage is that the purity of the intermediate-boiling component will always be low!

One simple equation is available to be used in order to know how many columns are needed:

Number of columns needed = Number of components - 1

So in case of having three different components, as mentioned we need 2 columns (3-1 = 2).

There are however many ways to rearrange these two columns. The number of rearrangement also can be easily calculated using one simple equation:

Number of sequence = (2(p-1)!)/p!(p-1)!

with p = number of component

Hence for a ternary mixture, we have 5 different ways to arrange the two columns needed. In this entry 3 of them will be shown. There are also known as:

1) A-path
2) C-path
3) A/C-path

A-path: where the separation is done in a descending order of relative volatility

 C-path: where the separation is done in an ascending order of relative volatility

A/C-path: Separation of the B  from A and C is done first.

This type of sequence consumes a lot of energy as it has 6 heat exchangers compared to only 4 in A-path and C-path respectively.

Operating region

In the last three posts we talked about reflux ratio or reboiler ratio (depending on column's section). We know that this parameter has a big influence on cost and quality of the product. There are however several factors or parameters that are just as important as reflux ratio for distillation fields. They tell us whether the column will run smoothly or not. In other words they affect the efficiency of the column.

Those factors are :

1) Liquid load, L

2) Gas load, G

If we plot G/A against L/A in a diagram, we will obtain this kind of diagram:

If gas load is too low, it will lead to weeping. Weeping means liquid starts to fall through the perforations as gas flow is not sufficient to hold it up.

If gas load is too high, then we will have a situation called flooding. It is the opposite of weeping. Instead of leaking through, liquid gets carried upwards together with the vapour/gas.

If liquid load is too low, there will be no homogeneous distribution of liquid on each tray or plate. Hence the mass transfer quality becomes poor.

If liquid load is too high, we will have a similar situation with weeping.

The shaded area is the region where distillation should be operated or run. This guarantees a good separation efficiency.

Operating line ii

In the previous entry we have discussed how to obtain the equation for the operating line of the rectifying section. Now we will derive the second operating line but for the bottom part of the column. This column's part is also known as stripping section.

Here is again a picture to ease the derivation:

For the stripping section, we have a new parameter called reboiler ratio. It is actually quite comparable to reflux ratio. In fact the definition is also similar.

Reboiler ratio w = gas goes back to the column / liquid which is taken out at the bottom column

or

w = G/B

Total mass (flow) balance:

What comes in = what goes out

hence:

G + B = L

Mass balance for one component (the more volatile one):

G*y + B*xb = L*x

Combining these three equations and with some re-arrangements we will obtain our goal which looks like below:

y = (w+1)/w *x - xb/w

This is again an equation for a straigh line with the slope w+1/w and has a y-intercept -1/w.

Operating line

Basically an operating line is a tool which can be used for example in a Mc-Cabe Thiele diagram to determine parameters such as reflux ratio, reboiler ratio and so forth.

Therefore knowing how to derive and then draw it on that diagram is necessary. Otherwise you cannot gain much from that Mc-Cabe Thiele diagram. It is a very useful diagram although it is only valid in a certain case. One important assumption that enables us to work with it is that the evaporation enthalpy of each species is all equal where this is of course not the case in reality.

Let us look at this diagram once again and derive the equation we need:

Total mass (flow) balance:

What goes in = what comes out

hence:

G = L + D

Mass balance for one component ( the more volatile one):

G*y = L*x + D*xd

and then we know that reflux ratio is defined:

v = L/D

Combining all these three equations and do some re-arrangement, you will get this equation looking like this:

y = (v/v+1)x + xd/(v+1)

This is a linear equation and a straight line with the slope v/v+1 and the y-intercept xd/v+1 is to be expected. This is however valid for a rectifying section ( column's part above the feed).

Reflux ratio

In distillation there are lots of important which might affect process's efficiency, operating cost and capital cost. One of them is reflux ratio. This ratio is already defined in a certain way.

Below is the picture of the upper part of the column ( column section above the feed ):

Reflux ratio is defined like this:

v = liquid which is going back to the column, R / liquid which is taken out, D

Normally the reflux ratio is 1.5 to 2 times bigger than the minimum reflux ratio.
It is worth mentioning  that if reflux ratio is too low, it is bad and if it is too high, it is also problematic. Below is the graph that can explain why the right range of reflux ratio's value is crucial:

Theoretically high reflux ratio is good to obtain a pure top product. However high reflux ratio also means high operating cost. More liquid remains in the column therefore more energy is needed to reboil it.

If it is too low, capital cost increases significantly. This is because more stages are needed to obtain the same product's quality. Minimum reflux ratio means the number of stages that we need is infinite. In practice we want to avoid a situation with high number of stages and high energy consumption.

Heuristic Rules

Heuristic rule is a set of rules that can be used in order to solve problems which might occur in distillation fields. It also helps in terms of running distillation efficiently and optimising the process.
This is taken from those people who worked in the industry or are now working there. In other words these rules are based on experience. By applying these rules we can rule out those process which are not promising.

These are examples of those rules. There are  a lot of them. However in the post i would like to highlight only five of them.

1) Remove thermally unstable, corrosive, or chemically reactive components early in the sequence.

This is just to avoid or lower the risk. The risk of something unwanted to happen is higher if it is not removed at the beginning of the sequence.

2) Remove final products one by one as distillates.

The final product must be drawn from the top of the column and not at the bottom. This is due to the fact that the product might get contaminated if it is taken out from the bottom of the column because of the wall corrosion for example.

3) Sequence separation points to remove, early in the sequence, those components of greatest molar percentage in the feed.

By removing those components with the higher amount from the column, there is no need to use a bigger or thicker column for the next separation. The bigger the column is, the higher is the capital cost.

4) Do the separation points in the order of decreasing relative volatility.

The split of components with higher relative volatility is done first. The easiest split is to dealt first.

5) Sequence separation points that favour near equimolar amounts of distillate and bottoms in each column.

This means:

if we have  0,3 mol A, 0,3 mol B and 0,2 mol C, 0,15 mol D and 0,05 mol E, we split A and B first as the amount of A and B is the same. Hence it favors the equimolar split.

It is however worth mentioning that not all of them can be fulfilled at the same time. Sometimes only some of them are important and the rest is not. 

Overcoming Azeotrope iii

The third and the last solution to be discussed here in order to overcome the azeotrope problem is:

3) Addition of entrainer: Almost similar to the second method where we add a third component. In this case we also add another component with certain properties. The difference is that the third method adds a so-called entrainer which forms a low-boiling heterogeneous azeotrope with one of our key component. This will then lead to the formation of two phases. These two phases can be separated easily with the help of decanter for example.

Another method which might be possible to break an azeotrope is using membrane. The problem with this method is that membrane is really expensive. So to use a really big membrane (which also means big area) is really not practicable and not a smart move. Depending on the situation these are normally the choices that we have. After considering all of those concerned aspects the best one will be picked or chosen.

Overcoming Azeotrope ii

Since the previous method does not work with all azeotropes, we need to have a look at other methods.

The second method to be dealt with in this entry is:

2) Extractive distillation : Unlike pressure swing, there is no need to alter the operating pressure. All we have to do is to introduce a third component ( in case of having a mixture of two liquids ). This third component is also called separating agent. It is non-volatile and has normally a high boiling point than the key components. Besides that it is also important to mention that this separating agent must not form any azeotrope with the key components.

Let us assume we have a mixture of two liquids A and B.

A: low-boiling point
B: high-boiling point

A and B form an azeotrope at a certain composition.
By introducing a third component, C, the order of boiling point will look like this:

A: lowest-boiling point
B: intermediate-boiling point
C: highest-boiling point

The function of the separating agent is to alter the relative volatility of the mixture. Normally this method is used when we have a mixture whose relative volatility is very low or close to one. There are other factors that we have to consider in choosing the right separating agent.

1) It must not be toxic and harmful
2) It must be cheap
3) It must be easily and readily available
4) It must not react chemically with the key components

Column 1, C1 is able to separate A after adding C as the relative volatility changes or increases.
The bottom product from C1 is fed into the second column where B and C is separated as distillate and bottom product respectively. C is the recycled into the first column. The second column, C2 is also known as the solvent recovery column.

Overcoming Azeotrope

There are several solutions that can be applied to overcome this azeotropic problem.

One of them is:

1) Pressure-swing: Some of the azeotropes change their composition when operating pressure is changed. For example the azeotropic composition of an ethanol-water mixture at 1 bar is different compared to that of the same mixture at 8 bar.

The first column is operated at 1 bar. The column is able to separate pure A1 and azeotropic composition B1 as a top product. The distillate is then fed into the second column which is operated at a higher pressure, 8 bar. It is able to separate pure A2 and the second azeotropic composition B2. From this diagram we can clearly see that by changing the pressure the azeotropic composition shifts to the right.

This method of overcoming azeotropic composition is known as 'pressure-swing distillation'. It is however only applicable to those azeotropes that change their composition with pressure. 

Azeotrope iii

The previous entry dealt with the first type of azeotrope. This time we will go through the second one.

2) This type of azeotrope is caused by a strong negative deviation from Raoult's law.

P-x diagram in this case will look like this:

Since we know the overall pressure is lower than that of an ideal mixture, we can expect to see a minimum point. This again marks the azeotropic composition of the liquid mixture.

T-x diagram:

Since the vapour pressure at the minimum point is the lowest, a maximum boiling temperature at exactly the same composition is expected. The azeotrope has the highest boiling point. That is why this type of azeotrope is also called 'maximum-boiling azeotrope'.

If we perform the separation process via distillation with feed composition lower than azeotropic composition, we will get azeotrope at the bottom and pure A at the top of the column. In this case Azeotrope will always come out at the bottom.

Similarly it is also impossible to obtain both pure A and B regardless of the feed composition.

Next we will have a look at several ways or solutions that have been applied to overcome this problem.

Azeotrope ii

There are two types of Azeotrope.

1) The first one is caused by the strong positive deviation from the Raoult's Law.

P-x diagram will look similar to this:

There is a maximum point in this diagram. This maximum point represents the azeotropic composition. The total vapour pressure curve or line can easily be done with the help of Dalton's law which states that the sum of every partial pressure must be equal to the total pressure.

T-x diagram:

If a mixture has a high vapour pressure, it also means that this mixture has a low boiling temperature. So it is not surprising to see a minimum point in a Temperature-composition diagram. In this case the azeotrope boils at a lower temperature than those two pure components. This type of azeotrope is also called as 'minimum boiling azeotrope'.

If we have a feed composition lower than azeotropic composition, by performing a distillation you end up having the azeotrope at the top and pure A as a bottom product.

If we have a feed composition higher than azeotropic composition, you also end up having the azeotrope as a top product and pure B at the bottom of the column.

Therefore it is not possible to have both pure A and B at the same time if a mixture of two or more liquids shows an azeotropic composition.

Azeotrope

As a result of the very strong deviation from Raoult's law, we will have a situation called Azeotrope.

The word actually comes from greek and has a literal meaning of boiling remains unchanged.

In other words azeotrope is actually a mixture of two liquids or more that has a constant boiling temperature either lower or higher than the boiling point of pure components.

The composition at which this situation is observed is called azeotropic composition.

It is a bit problematic to deal with as the mixture can no longer be separated to their pure components with the help of distillation. No matter how you do or perform the distillation, you will end up having that azeotropic composition. It is also worth mentioning this is the composition at which the liquid phase and vapour phase both have the same composition.

Example:  

A mixture of Ethanol and Water is the most classical example of a mixture that has an azeotropic composition at around 95 weight percent of Ethanol and 5% water.

Boiling point of pure water 100° C

Boiling point of pure ethanol 78.5°C

At this composition the mixture has a boiling temperature of 78,2°C.

This is lower than the boiling points mentioned above.

Next entry will deal with two different types of Azeotropes in detail.

Pressure deviation ii

Last time we talked about the first deviation from Raoult's Law.
This time we will discuss the second one. The deviation from Raoult's Law is important to know because some mixtures form an azeotrope. We will talk about that one in the next entry.

Another type of pressure deviation is:

1) Negative deviation: This deviation occurs when the intermolecular forces between component X and Y are stronger than those intramolecular forces in pure liquid X and pure liquid Y. In other words the resulting vapour pressure is lower than that of ideal mixtures.

Below are some of the mixtures that show this type of pressure deviation:

1) Nitric acid-Water mixture
2) Hydrochloric acid-Water mixture
3) Acetone-Chloroform mixture

In order to identify the difference between these two mixtures (an ideal mixture and a mixture with negative deviation), let us have a look at the P-x diagram.

P-x diagram for an ideal mixture:

P-x diagram for a mixture that shows a negative deviation:

If the deviation from Raoult's law is too strong, we will then have a situation which is not desired (sometimes it cannot be avoided as well) in the field of distillation. Stay tune!

Pressure deviation

As mentioned in the previous post in this blog, in reality there is no liquid mixture that behaves ideally. This time we will have a closer look at real liquid mixtures.

In the case of an ideal solution: The intermolecular forces ( the forces between component X and component Y) in the mixture are more or less equivalently strong as those forces in pure liquid X and in pure liquid X.

There are two different types of deviation. Let us look at the first one first.

1) Positive deviation: This deviation from Raoult's law occurs when the intermolecular forces in the mixture are weaker than those in pure liquids. This also means that the resulting vapour pressure of these mixtures is generally higher than that of ideal mixtures.

Below are the examples of the mixture that exhibit this kind of deviation:

a) Benzene-Water mixture
b) Ethanol-Water mixture
c) Pyridine-Water mixture

P-x diagram for an ideal mixture of X and Y:

P-x diagram for a mixture with positive deviation:

As you can clearly see, the total vapour is higher in the second case than that in an ideal liquid mixture. The second deviation will be dealt in the next post.

Pressure

Pressure and partial pressure are also important parameters in the distillation field.

Partial pressure : The pressure that would be exerted by one of the gases in the mixture if it alone occupied the volume of the mixture at the same temperature.

There are several useful laws that are related to the partial pressure,
For example:

1) Raoult's Law : It states that the partial pressure of the component i in an ideal mixture is equal to the product of mole fraction of that component i in the mixture and its vapour pressure of the pure component i.



It is also important to know what does an ideal mixture exactly mean. There are two different types of mixture. One is ideal and another one is real mixture. There is no such thing as ideal mixture in reality. However some mixtures get quite close to being ideal. These are the mixtures of two or several molecules that are fairly similar to one another.

Examples of ideal mixture:

1) Hexane and cyclohexane
2) Hexane and Heptane
3) Benzene and Toluene

2) Henry's Law : It states that the amount of gas dissolved in a liquid at one particular temperature is directly proportional to the partial pressure of the gas which is in equilibrium with that liquid.



It is worth mentioning that Henry's law is valid for an ideal dilute solution. This means it is applicable to a mixture where the solvent is in excess and the mole fraction of the component is approaching zero. Henry's constant in this gas law depends on type of solute, temperature and pressure.

Distillation

Distillation - A process of separating components having different boiling points.
                      It is a very important process and widely used in chemical industry.

There are two different operating modes of distillation:

1) Continuous mode : The distillation plants are operated 24 hours per day, 7 days per week and sometimes there is a period where the operation has to be stopped for the maintenance purposes such as cleaning the internal column's structure.
                                 
This mode is normally preferred if we want to have a high-capacity.

2) Batch mode :  In contrast to the previous mode, the mixture is fed again to the column only when the separation of the previous mixture in the distillation still already finished.

It is an important mode for the seasonal production of low capacity.

There are lots of factors that one should consider and should be aware of, so that the separation process takes place efficiently. The existence of different column types is attributed to these factors such as pressure drop and column's cost.

Types of column :

1) Spray column : The liquid is sprayed inside the column with the help of liquid distributor and the gas enters the bottom of the column and rises upwards.
 
This particular type of column is normally used at a very low operating pressure.

2) Bubble column : Opposite to the previous column, this column is filled with the liquid mixture and the inlet gas stream is again introduced at the bottom of the column. This leads to bubbling.

If the distillation is to be performed at a high pressure, then this column works well.

3) Tray column : The internal of the previous two columns is empty. They are not filled with anything. That is the major difference between them and tray column. This type of column is filled with trays in a certain spacing range.

4) Packed column : The column is filled with certain packings. The mass transfer between liquid and gas normally takes place at these packings' surface. Packed column is normally cheaper than tray column. The major downside of this column is the pressure drop is sometimes high.

Types of trays and packings :

Again there are different types of trays and packings that have been manufactured up to now.
For example:

Trays

1) Sieve tray - cheap, high pressure drop, poor efficiency
2) Valve tray - cheaper than bubble cap tray, decent pressure drop, good efficiency
3) Bubble cap tray - expensive, low pressure drop, good efficiency

Packings

1) Rings made of metal, ceramics and plastics - for random packed column
2) Ceramics saddle - for random packed column
3) High capacity packing - for structured packed column

Sulzer and Montz are among the most know engineering firms that produce packings and trays for distillation column.