FSC 432
Petroleum Processing

Answers to Lesson 3 Exercise 2

PrintPrint

Answers to Lesson 3 Exercise 2

Question 1

A refinery has access to two different crude oil stocks A and B with the following compositions:

Naphthenes % Aromatics %
Crude A 10 60
Crude B 60 10
  1. What type of crude oils are A and B according to the ternary classification based on composition?
  2. What type of crude would be obtained if A and B are blended in a proportion of A/B =2/3?

Answer:

  1. What type of crude oils are A and B according to the ternary classification based on composition?
    A= Aromatic: 60%, Naphthenes: 10%
    Aromatic-Intermediate crude (Aromatics >50%, Paraffins >10%)
    B= Aromatic: 10%, Naphthenes: 60%
    Naphthenic crude (P+N >50%N>PN>40%)
  2. What type of crude would be obtained if A and B are blended in a proportion of A/B =2/3?
    In the blend with A/B = 2/3: A: 40%, B=60%
    Binary Blend C:  Naphthenes = (0.4)(10) + (0.6)(60) = 40%
    Aromatics = (0.4)(60) + (0.6)(10) = 30%,  P=30%
    Binary Blend C: Border-line between Paraffinic-Naphthenic (A<50%, P<40, N<40%) and Naphthenic ((P+N>50%, N>P, N>40%) crude oils.

You may solve the problem graphically using a ternary diagram, as described below.

Video: FSC 432 Blend Triangle (4:43)

Blend Triangle
Click here for transcript of Blend Triangle.

Here is an example of how we can use a ternary diagram to determine the blend compositions from the individual crude oils. On the ternary diagram you see two regions shaded. The gray shaded region is the aromatic-intermediate region marked by A dash I. And the green shaded region, the right hand side corner, is the naphthenic region. So let's start with placing our individual crude oils A and B on this ternary diagram.

For A, we have 10% naphthenes. So we should first find the 10% naphthene line on this diagram. It is against the corner of naphthenic crudes, 100% naphthenic corner, and parallel to the line that joins aromatic and paraffinic corners. That is the first major tick on this group of lines. So that is the 10% naphthene line.

Now, the crude A also has 60% aromatics. So we can now find that line on the diagram, starting from the top corner, aromatic corner, which is 100%, coming down to find the 60% line. An intersection of that line, 60% aromatic with 10% naphthenics, will identify or will determine, locate crude A on the ternary diagram. You can see that the crude A falls in the aromatic-intermediate crude region, so it's an aromatic-intermediate crude.

Now let's place crude B on this diagram. Crude B has 10% aromatics and 60% naphthenes. Let's first find the 10% aromatics line right across from the aromatics corner, the line that is parallel to the line combining paraffinic and naphthenic corners. That's the major tick line, 10% from the bottom of the diagram. So on this line all the points will have 10% aromatics in essence.

Now let's find the other line, so they can place crude B on this 10% aromatic line. Naphthenes are 60%. So, if you locate the 60% naphthenes line, the intersection of these two lines, 10% aromatic and 60% naphthenes will place B in the naphthenic region. So crude B is a naphthenic crude.

All the mixtures or blends we can make from crude A and crude B will fall on a straight line that combines or connects point A with point B. So you can see, there is an intermediate region here between aromatic-intermediate and naphthenic crudes. And that is paraffinic-naphthenic, P dash N. So you can see, there are three possibilities for the final blend C. It could be aromatic-intermediate, paraffinic-naphthenic, or naphthenic crude, depending on the ratio of A to B.

So, the given ratio is A to B is 2 over 3. When we place that ratio all on the blue line in this plot, you can see that point C the blend falls on the boundary between paraffinic-naphthenic and naphthenic region, right on that green line. So the blend C could be between paraffinic-naphthenic and naphthenic classifications. This is a borderline case.

So if you would like to make a paraffinic-naphthenic crude from these two crudes A and B, then you need to increase the concentration of A in the blend. But if you would like to end up with a naphthenic crude in the blend, then you need to increase the concentration of B. So you can essentially traverse this line between A and B by changing the ratios of these two crudes in the final mix.

Question 2

The compositions of three crude oils available to a refinery are as follows:

Crude A: 60%wt paraffins, 20%wt naphthenes
Crude B: 50%wt aromatics, 30%wt paraffins
Crude C: 10%wt paraffins, 20%wt naphthenes

The refiners would like to maintain a weight ratio of 1/1, Crude B/Crude C in a ternary blend of the oils A, B, and C. What would be the minimum concentration of Crude A (% wt) in a ternary blend that could be classified as paraffinic oil?

Answer:

aromatics naphthenes paraffins
Crude A 20%wt 20%wt 60%wt
Crude B 50%wt 20%wt 30%wt
Crude C 70%wt 20%wt 10%wt

Set A+B+C = 100g and B=C

Paraffin balance:

0.60A + 0.3(100-A)/2 + 0.1(100-A)/2 > 40
1.2A + 30 - 0 .3A + 10 - 0.1A >80
.8A > 40

A>50, Therefore, A must be greater than 50% to maintain a paraffinic crude blend.