Introduction to Crude Oil and Refining

Table of Contents

I. Introduction

II. Crude oil

a. Basic chemistry

i. Fractions

ii. Gravity

iii. Contaminants

b. Transport

i. Land

ii. Sea

III. Refining

a. Production chain overview and possible chokepoints

i. Distillation columns

ii. Vacuum flashers

iii. Cokers

iv. Fluid Catalytic Cracking units

b. Refinery profiles

IV. Conclusion

Figure1. The first four hydrocarbons are shown as well as a three dimensional representation of methane.

I. IntroductionThis primer is divided into a Crude Oil section and a Refining section. The former covers

the basics of crude oil chemistry and transport and can be skipped by anyone who is

familiar with the subject matter or is not familiar and does not care to become so. The

latter section focuses more on the original motivation of this project: to try to identify and

quantify potential chokepoints in the oil refining process.

II. Crude Oil

a. Basic chemistryCrude oil consists of a mixture of hydrocarbons, chains of carbon atoms bonded to each

other and to hydrogen atoms, with the major stipulation being that each carbon can form

four bonds and each hydrogen can form one bond. (See examples in Figure 1.)The

properties of the resulting molecules are defined mainly by the arrangement of atoms,

known as the form, and the number of carbon atoms, referred to as the size or weight.

The complexity of a hydrocarbon’s form increases greatly as the size increases.

i. FractionsDifferent end products such as gasoline, diesel, and jet fuel, are composed of different

weight portions, or fractions, of a hydrocarbon mixture. Let’s look specifically at

gasoline. Gasoline is composed of a mixture of hydrocarbons that are mostly between

2

Figure 2. Each fraction has its own boiling point.

Table 1. Boiling point ranges are shown for typical crude oil products.

four and twelve carbons in length. These “useful” hydrocarbons are contained in a

mixture of hydrocarbons that contain between one and several hundred carbons. To

separate the gasoline fraction from the others, refiners take advantage of boiling point

differences between fractions. As a rule, the heavier (i.e. higher number of carbons) the

molecule, the higher its boiling point. In pure substances such as water, the entire

solution will boil at one defined boiling point. In a mixture, each component will boil and

evaporate at its own defined boiling point as seen in Figure 2 below.

Hence, there is a rough temperature, called the cut point, at which everything in the

mixture containing one, two, or three carbons will boil away. There is another cut point at

which everything containing up to twelve carbons will boil away. Whatever has boiled

away between these two cut points is the fraction most commonly used for gasoline.

Table 1 shows typical cut points for gasoline and other products. This description greatly

simplifies things. The actual process is more complicated and we will look at it more

closely in the section on refining.

3

Table 2. Prices are in USD per barrel for the week of November 5th, 2010.

ii. GravityGasoline is the most commercially valuable crude product to refiners, and it is composed

of relatively light hydrocarbons. The refining section will discuss how refiners convert

long hydrocarbon chains into shorter ones, but for now we only need to know that that

process is expensive, and it would be more convenient and profitable to simply find a

crude composed mostly of these shorter chains. These light crudes do exist, but they are

more expensive than their long chain containing counterparts due to the higher demand

associated with crude that costs less to refine. Chemists usually use specific gravity,

defined below, to quantify how light or heavy a liquid is.

The higher a liquid’s specific gravity, the heavier, or denser, it is. Petroleum engineers

favor a different convention: API gravity. API (American Petroleum Institute) gravity is

measured in degrees (though there is no relation to temperature) and defined as follows:

The important thing to note is that the specific gravity is in the denominator, giving an

inverse relationship between density and API gravity. Hence, the higher the API gravity,

the lighter, or less dense, it is and the more expensive it will probably be. EIA data in

Table 2 demonstrates this correspondence for three types of Saudi Arabian crude.

This relationship holds generally, but as the next section will show, when it comes to

determining price, weight isn’t everything.

4

Table 3. Prices are in USD per barrel for the week of November 5th, 2010.

iii. ContaminantsIt’s not uncommon to see two crudes with the same API gravity that have different prices.

One example is seen in Table 3.

This discrepancy can have many causes, some rooted in market dynamics, some rooted in

physical properties of the two crudes. One of the most important causes that falls into the

latter class is the presence of non-hydrocarbon molecules, or contaminants, in the crude

oil mixture. Separating different hydrocarbons from one another is one concern for

refiners. Another is separating contaminants from the hydrocarbon mixture. It is

important to note that the contaminants are often chemically attached to hydrocarbons

(see Figure 3) so there is no simple filter that will allow hydrocarbons to pass, but catch

contaminants. If contaminants are not removed, they can poison catalysts and corrode

machinery in a refinery, thus giving refiners a financial incentive to remove some of

them. Also, an end product with contaminants will release those contaminants when

burned, which leaves an environmental footprint, thus giving the public incentive to

mandate the removal of harmful compounds. The contaminant most people are familiar

with, and the one that will receive the most focus here, is sulfur. (Nitrogen, oxygen and

heavy metals are also common contaminants.) For historical reasons, a crude that

contains less than 0.5% sulfur content is called sweet, while a crude with a higher crude

content is called sour. (Sometimes sour crudes are defined as having over 1.0% sulfur

content.) Sweet crudes are more attractive to refiners for the same reason light crudes are

attractive: they cost less to refine. Once again, the high demand drives prices up and we

can establish the general rule that the sweeter the crude, the more expensive it is.

5

Figure 3. Thiophene is one of many sulfur containing molecules found in crude oil.

b. Transport

i. LandOne major method of transporting crude is via pipeline. Pipelines have internal pumps

placed every 20 to 100 miles that facilitate oil movement, can transfer one or multiple

products, and are unidirectional in principal though flows can be reversed at substantial

cost if necessary. For long distances over land, pipeline transport is the most economical

way to move crude oil. Pipelines take raw crude from oil fields or coastal loading

facilities to refineries. End products can then be moved via pipeline to depots, where

specialized vehicles further distribute them. If oil is the blood of an energy dependent

nation, then refineries are the hearts, pipelines are both the veins that take the “bad”

blood to the heart and the arteries that take the “good” blood en masse close to where it’s

needed, and fuel trucks are the capillaries that finish the job by taking the blood to

specific destinations via networks consisting of myriad, short individual routes.

ii. Sea A second major method of transporting crude is via oil tanker. Like pipelines, tankers can

carry raw crude as well as refined product. This method is relatively straightforward,

though it presents a potential chokepoint even before the refining process has begun. The

Energy Information Administration details the major ones in Table 4 on the following

page.

6

Table 4. The EIA gives details on some of the world’s strategic energy chokepoints.

7

Figure 5. A simplified diagram (left) shows how a crude reaches the distillation column (right).

Figure 6. Typical outputs of a distillation column are given.

III. Refining

a. Production Chain Overview and Potential Chokepoints

i. Distillation columnsSo now the crude oil has reached the refinery. A pump moves the crude from a

storage tank to the distillation column, one of the most visibly distinctive features of a

refinery. (See Figure X.) Distillation columns are not viewed as choke points in the

refining process since a column can achieve its

objective independent of the chemical properties of

the crude that were discussed earlier. Hence, a distillation column processing a light,

sweet crude could easily switch to a heavy, sour crude.

Heating the crude on the way causes the input of the distillation column to be a

vapor-liquid mixture (Figure 5). Once inside the column, gravity guarantees that the

lightest vapors will rise to the top, the heaviest liquids will fall to the bottom and

everything in between will take an intermediary position. A breakdown of the products of

a distillation column and where they appear in the column relative to one another is

shown in Figure 6. After this initial separation, which is fairly simple and cheap, the

process becomes more complicated. An outline of a refinery’s operations is shown in

Figure 7 on the following page. Clearly, even in simplified form, refining isn’t simple.

8

Figure 7. A simplified schematic depicts a vacuum flasher’s role.

However, this diagram is a good starting point and will make more sense as we go along

and references to it are made.

ii. Vacuum flashers

As seen in Figure 6 on the previous page, the bottom stream in a distillation column is

designated for flashing. By definition, this stream consists of our heaviest hydrocarbons,

those with boiling points in excess of 800 degrees Fahrenheit. This represents quite a

wide range of hydrocarbons, and it’d be nice to separate them further. So why not raise

the temperature? The answer lies in a phenomenon known as cracking, the breaking apart

of large molecules into smaller ones. At temperatures approaching 900 degrees, the

normal boiling behavior does not occur; rather, the excessive heat causes carbon-carbon

bonds to break i.e. the molecule cracks. This process is quite lucrative when controlled.

(Imagine a relatively useless 80-carbon long chain being broken into 10 8-carbon long

chains, all of which can now be used in gasoline.) However, simply raising the

temperature leads to uncontrolled and unpredictable cracking. Hence, another solution is

9

needed that would allow a refiner to further separate this lower stream into useful

fractions without cracking the molecules. A vacuum flasher (Figure 7) is designed to do

just that. Physics dictates that a solution at atmospheric pressure will have a specific

boiling point. However, if the pressure is dropped below atmospheric, the solution can

boil at a lower temperature. So in a vacuum flasher, a vacuum pump is used to drop the

pressure of the chamber. Then the crude in the chamber will boil and can be separated in

a process similar to that used in a distillation column. The portion that boils, the distillate,

can be separated into a light fraction, which is often sent through the distillation column

again for another pass, and a heavy fraction, which is treated before it is further processed

because it contains many contaminants that can poison catalysts. At the bottom of the

flasher is the heaviest portion of the vacuum flasher input, which was itself the heaviest

portion of the distillation column input. This portion, appropriately named the flasher

bottoms, is sent on to the coker.

The vacuum flasher is a potential chokepoint in that it is tasked with processing

all of the crude in a mixture with density above a certain value, which we’ll call the

critical density (not an actual term in the petroleum industry). This amount might be 10%

in hypothetical Crude A. So if 10,000 barrels a day were sent through the distillation

column, 1,000 would have to go to the vacuum flasher. If the refinery was built to handle

this amount (and most refineries are built with specific crude streams in mind) then

imagine the consequences of switching to Crude B, a heavier stream where 20% of the

stream is above the critical density. If we want to fully utilize the vacuum flasher’s 1,000

barrel daily capacity, we can only run 5,000 barrels per day through our distillation

column. That leads to a massive decrease in other products. To avoid this we can keep the

10

Figure 8. A simplified outline of a refinery’s operations are given above.

distillation column at 10,000 barrels per day, but now we have 2,000 barrels of crude

11

that need to be processed by a unit with a 1,000 barrel per day capacity. The end result of

all this is a decrease in gasoline production and an increase in yield of our heavier, less

commercially viable product, residual fuel. Note the refinery can keep running, but not in

an economical fashion.

ii. CokersThe purpose of the coker is to carry out a process we had been avoiding so far:

cracking hydrocarbons. As mentioned, uncontrolled cracking is not lucrative. A useful

hydrocarbon, say one around 6 carbons long, can be cracked into three less useful (at

least with respect to gasoline production) 2 carbon long chains. The reason we don’t

mind cracking hydrocarbons in the coker is that at this point, essentially none of the

hydrocarbons are useful. Instead we see more molecules like cetane, which, when

cracked, gives products that fit into our previously defined profile for gasoline as

demonstrated in Figure 9. The coker output is comprised partly of these new

hydrocarbons, which are processed further to be blended into gasoline and other products,

and partly of petroleum coke, essentially pure carbon and the product from which the

coker derives its name. Coke, a product consumed mainly by cement and power plants, is

rarely the preferred product of a refinery.

12

The coker can be a chokepoint in a manner exactly analogous to that of the vacuum

flasher before it. Once it reaches full capacity, a refiner can’t increase the flow of crude

into the distillation column, and using a heavier crude is one way to push a coker to full

capacity.

iii. Fluid Catalytic Cracking

13

Figure 9. Cetane is cracked into octane, hexane and ethylene.

We’ll finish our discussion on potential chokepoints by talking about the fluid

catalytic cracking (FCC) unit. FCC units, as the name implies, offer another method of

cracking molecules as opposed to through the coker. In the FCC unit, catalysts are used

to crack long hydrocarbon chains in a more specific way than that of the coker. This

method is used for heavy gas oils and flasher tops, the crude that comes off the trays in

the vacuum flasher. (See Figure 7 for a reminder.) The chemistry behind this process isn’t

relevant here. What is relevant is the fact that, like other units downstream of the

distillation column, the FCC unit can fill up with the introduction of a heavier crude

stream to the refinery. When this happens, gasoline yield decreases and residual fuel

yield increases. Hence, the FCC unit is another potential chokepoint. This relationship is

presented in Figure 10, which demonstrates the detrimental effect on gasoline yield that

refiners face when certain units reach capacity. Figure 10 illustrates this problem in the

context of increasing distillation column utilization. Our context, that of constant

distillation column utilization but an increase in heavy fractions, is similar in principal so

the graph still applies.

b. Refinery Profiles

14

Figure 10. As the coker and fluid catalytic cracker reach capacity, gasoline yield falls.

Obtaining specific input and output information from refineries is no easy task. Reports

like the Worldwide Refining survey offer a view of output, but input information, details

on the specific crude streams entering a refinery, is tightly guarded and company

spokesmen consistently say that they are not authorized to reveal such information.

IV. ConclusionThe initial goal of this project, to identify and quantify chokepoints in the crude oil

refining process, was not fulfilled. However, a look at the refining process does give an

idea of which stages could be held up by changes to supply. Unfortunately, changes to

supply aren’t easy to track, but in the event that that information is available, general

statements can be made concerning a refinery’s ability to cope with such changes.

15

Sources

Petroleum Refining in Non-Technical Language – Leffler

Energy Information Administration

http://www.eia.doe.gov/dnav/pet/pet_pri_wco_k_w.htm

Association of Oil Pipelines

http://www.aopl.org/

Pet Coke Consulting

http://www.petcokeconsulting.com/primer/index.html

16