Last time we talked about the separation of crude into its various components by essentially boiling the crude. Every undergraduate book I ever read on the subject of distillation began with distilling alcohol…probably with the hopes of engaging beer swigging early twenty-something college students. I spent many hours in bars on Manhattan’s Upper West Side during undergrad (Columbia-go Lions) and that never inspired me.
Crude Distillation is actually very simple. Crude is first heated in a crude furnace and partially vaporized. The gas and liquid phase crude is then directed to a trayed distillation column where the smallest, lightest molecules boil first, rise through the perforated trays (trays with holes) to the highest point in the tower, and then leave said tower through a side draw. Same goes for the second lightest components, except they exit through a slightly lower draw in the tower, and the third all the way down the tower until you get to the heaviest components of the crude which do not boil at atmospheric pressure and 750-800F, typical crude tower feed inlet temperatures. Certainly, we could continue heating this heavy material, referred to as “raising heater coil outlet temperature”, until the non-boiling components reach their vaporization temperature. However, these heavier crude components would undergo thermal cracking (breaking apart of large molecules into smaller under high temperatures) or coking (polymerization of hydrocarbon molecules to form a “burned” or charcoal like substance) prior to vaporization-and both are really bad scenarios for a crude column-and the unit’s engineer…firing…
Crude Distillation Unit
Instead, this heavy material, referred to as Atmospheric Gas Oil (AGO), is pumped from the crude or atmospheric (atmospheric pressure-get it?) tower to a vacuum distillation column. In the vacuum column the exact same phenomenon happens except under a negative or vacuum pressure to allow heavier material to boil before cracking or coking. Vacuum distillation gives a little assistance to molecules transitioning from the liquid to the vapor phase by lowering the pressure, liberating these molecules. I’ll cover Vacuum Distillation in more detail in another post.
Take a look at the diagram of a typical crude distillation tower above. Try to trace the path of the crude oil through its separation into products and the right hand side of the page. First, the crude is pumped from a pipeline or storage (ever driven by a refinery and seen those huge tanks with the name of the refinery on the side? The largest ones are typically the Crude Tanks if the refinery has crude storage capacity) and is preheated through a series of heat exchangers which allow heat to be transferred from a hotter material to a cooler material. The crude preheat train serves two purposes:
1) Preheats the crude for the desalting process and ultimately the Crude Furnace
2) Saves energy costs-instead of preheating the crude with a smaller furnace prior to desalting, the crude is preheated via heat exchange with products
Desalting
Once the crude has been preheated, it enters the Desalter. Chemical Engineers aren’t that clever when it comes to coming up with names for various unit operations-a Desalter does just that. It “De-Salts.” Crude comes from all over the world, through a variety of deposits-some crudes are pumped from the ground, others from thousands of feet under the ocean, others and mined from the Canadian Oil sands. Wherever the crude was derived from, it comes along with a variety of contaminants that would wreak corrosion havoc on a carbon steel refining facility. So the desalting unit serves to remove salts and other contaminants from the feed by a) washing it with fresh water and b) passing an electric current through the crude to coalesce dissolved water droplets for easy removal before the crude leaves the Desalter.
Crude Furnace and Column Flash Zone
After desalting and additional preheat, the crude enters the furnace where it is heated to the desired feed temperature, say 750F degrees. The crude is partially vaporized when it enters the crude column’s flash zone. Here the vaporized material flashes or rises up the column’s tray, the lightest material reaching the top of the tower the heavier reaching the lower intermediate trays. Material that does not flash instantly flows to the bottom of the tower where it is steam stripped, to remove any entrained lighter material, and then pumped from the bottom of the tower.
Distillation Baby!
You’ll notice the crude column has two sources of heat-heat from the furnace at the feed entrance to the tower and heat from stripping steam at various points along the column. The temperature profile of the column (fancy way of saying the temperature from the top of the tower to the bottom) is maintained by these heat sources as well various heat sinks (heat removal). Examples of column heat sinks are the condenser at the top of the tower that cools top product before refluxing (returning) material back to the tower as well as column pumparounds which, in this case, remove heat from the column by exchanging it with cooler incoming crude. The temperature profile of the column is coolest at the top and hottest at the bottom near the furnace outlet. So vaporized material moves up the column it reaches a certain height until it cools and returns to its liquid phase. This liquid material collects on the “collector” (clever Chem-E’s) portion of a perforated distillation tray and is drawn off at that same height in the tower. So for example, kerosene is drawn off the tower, flows to a stripping column (where lighter materials are steam stripped out of the kerosene product) and goes to product. That Kerosene is drawn off at the point where kerosene cools and collects. Same for the naphtha, light gas oil, and heavy gas oil.
The naphtha cut seems unique because it leaves the top of the tower, is cooled in a condenser, and collects in a reflux drum before being drawn off to product but the exact same phenomenon is happening here. So what is reflux? The top reflux, in this case, returns cooled gasoline range material to the top of the column and condenses any heavier material thus purifying the overhead product. Pumparounds are similar to top column reflux except they are lower in the tower-they are also used to control the temperature profile of the tower and controlling product quality.
In addition to external reflux, there’s internal refluxing happening on each tray of the column. Each tray has perforation or hole in them to allow vaporized hydrocarbon to flow up the column. As material reaches the point where it condenses and falls back down on the tray, a liquid level is formed on the tray. Typically these holes have bubble or valve caps to allow vapor to move through the liquid on the tray. As the liquid vapor moves through the liquid some of the heavier material in the vapor is condensed by the liquid and at the same time some of the lighter material in the liquid is vaporized and joins the vapor up the column.
