Greetings to Everyone! Today we will be learning about one of the important separation technique in the field of natural gas industries i.e., distillation. Though, I used particularly as natural gas industries, distillation can be used to separate homogeneous fluid mixtures of our interest. The separation technique could be employed for a variety of purposes, including refining of oil, purification of water or to create alcoholic beverages. Distillation is a physical process and it uses heat as a source to separate or extract desirable pure chemical from a mixture of liquids.
We shall be learning about the applications of distillation in natural gas system, brief operation of distillation separation process, components in a distillation column and the types of distillation column. APPLICATIONS OF DISTILLATION IN NATURAL GAS SYSTEM: Now, coming to the applications of this distillation in natural gas systems, we have basically acid gas removal like CO2, H2S and nitrogen removal. OPERATION OF DISTILLATION SEPARATION The basis of the separation is the difference in the volatilities of the components of a mixture. Those mixtures which have relative volatilities quite away from unity or one can be separated by distillation. The process requirement for distillation are some creation of vapor and liquid phases. Distillation can only be carried out when we have vapor and liquid phases. We can not carry out distillation only with single phase. So, to create these two phases, we can either partially vaporise a liquid feed or partially condense a vapor feed or in the complete or partial liquid vaporisation in the reboiler. We shall see what a reboiler is and similarly, we can have complete or partial vapor condensation in condenser.WHAT WE SHALL LEARN?
Then we have the mixing of two phases; that means after creating the two phases, we have to ensure that the two phases come to very intimate contact for long enough time. So, that all the components whether higher volatile components can have a chance to go to the vapor phase, but preferentially the higher boiling point components can have time to come to the liquid phase. Lastly, we should have enough surface area so that the two phases can come to intimate contact with each other.
WHAT IS A DISTILLATION COLUMN?
The distillation process is carried out in a distillation column. It is a cascaded device to carry out several condensations and evaporations subsequently on different plates or trays. Now, doing so we find the vapor becomes richer in more volatile components and liquid becomes richer in less volatile components. The temperature and pressure keeps changing in the column from top to bottom, we find both temperature and pressure increases from the top to bottom of the distillation column. The lowest temperature and pressure are in the condenser and the highest temperature and pressure are in the reboiler.
STREAMS IN DISTILLATION COLUMN
The basic streams which are needed in a typical distillation column are a feed stream which will be having two or more than two components which will be again separated into two or more product streams. There could be more than one feed stream in a column. Then, we will be having some bottom product which will always be a liquid and this will be containing the heavier components in a mixture. We will also be having a overhead distillate which will be having the lighter components obtained as liquid, vapor or both.
TYPICAL CONFIGURATION OF A DISTILLATION COLUMN
We find that the column has one feed coming inside and then according to the feed location we are dividing the column into two sections -
A) Rectifying section: It is also known as enrichment section and this is the portion above the feed. It serves to remove the lighter components from the up-flowing vapor. This basically means that the lighter components from the liquid phase will go more to the vapor phase
B) Stripping section: The stripping section is below the feed stage and it serves to remove the lighter components from the down-flowing liquid. This means that when the liquid is coming down the lighter components or low boiling point components will be going more from the liquid phase to the vapor phase. Thereby making the liquid richer in the high boiling point components.
Next, we have this top vapor taken to a condenser.
Condenser: Each condenser condenses the overhead vapor partially or completely and send part of liquid back to the column as reflux. This condenser may be a partial condenser or a total condenser. For now, we will not be discussing about these two in depths. But for basic knowledge, just remember that total condenser condenses the vapor completely whereas partial condenser condenses vapor partially.
Now, we see that there is a reflux drum which is carrying the condensed liquid from the condenser.
Reflux drum: Its primary job is to hold the condensed vapor to provide the reflux and it serves as a distribution point for reflux and distillate.
At the bottom, the liquid comes down and goes to a reboiler.
Reboiler: The reboiler is operated by adding some external heat as the condenser is also operated by taking out the heat by some kind of cooling medium. The reboiler revaporizes the bottom liquid partially or completely and send a part or whole of the vapor back to the column.
Then from the reboiler, we are getting the bottom product. This is a typical configuration of a distillation column and variations may be found as per the different types of separations by distillations in different applications.
There are various types of distillation column. Broadly, they are divided into 2 types -
- Tray or plate column : It is a vertical assembly of plates or trays. Here, the vapor and liquid are contacted on each plate and the liquid flows down the tower under the force of gravity. The vapor flows upward under the force of a slight pressure drop from plate to plate. The vapor passes through openings in each plate and contacts the liquid flowing across the plate.
- Packed column : This kind of columns are filled with some sort of divided solids, shaped to provide a large particle-surface area. The liquid flows down over the surface of the solids and is exposed to the vapor, which flows upward through the open channels not filled by packing or liquid. The large surface area per unit volume of the column is necessary for maximum mass transfer between the vapor and the liquid phase.
Distillation - An overview by ScienceDirect
Reflux and distillation by RSC education
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