8 replies to this topic

[prengr]

Dear all,

It's a short and basic question.

Why does we need the steam injection into crude oil column and so on?
What is the purpose of steam injected into column?

Thanks.

[Flame]

Any fractionation column consists of reboilers, which provide the heat duty required for separation. In case there are chances of the process fluid degrade at certain temperatures, option of reboiling such fluid at high temperature need to be replaced with some other mechanisms in order to achieve vaporisation. (For egs. higher feed preheating or Steam distillation)

Injecting steam serves two purposes, first and foremost is that steam lowers the partial pressure of the process fluid (steam & fluid are totally immiscible) and hence the boiling point. Therefore, the process fluid can start vaporising at much lower temperature (well below its degradation point) Secondly, hot steam is source of heat energy and this heat is transferred to the fluid to create partial vaporisation.

[prengr]

Dear Flame,

Thank you for your ansewr.
I have one more question.
As you mentioned, steam can reduce partial pressure and hence boiling point.
If so, why is the steam injected into especially heavy oil fractionation column like a crude oil column? In other words, why is not the steam injected into general column even though it can save the enegy if the steam is injected into other columns,? (General columns do not include that fluid reacts with H2O)

[Flame]

In response to your query, i would like to add the following, picked up from literature:

" Steam or inert distillation is commonly used in the following situations:

1. To separate relatively small amounts of volatile impurity from a large amount of material.

2. To separate appreciable quantities of higher boiling materials.

3. To recover high-boiling materials from small amounts of impurity which have a higher boiling point.

4. Where the material to be subjected to distillation is thermally unstable or will react with other components associated with it at boiling temperatures.

5. Where the material cannot be distilled by indirect heating even under low pressure because of high boiling temperature.

6. Where direct fired heaters cannot be used because of danger. "



Regards,

Edited by Flame, 15 March 2012 - 08:37 AM.

[prengr]

Dear Flame,

Thank you for your answer.

[J_Leo]

It would be very interesting if someone can give e

In response to your query, i would like to add the following, picked up from literature:

" Steam or inert distillation is commonly used in the following situations:

1. To separate relatively small amounts of volatile impurity from a large amount of material.

2. To separate appreciable quantities of higher boiling materials.

3. To recover high-boiling materials from small amounts of impurity which have a higher boiling point.

4. Where the material to be subjected to distillation is thermally unstable or will react with other components associated with it at boiling temperatures.

5. Where the material cannot be distilled by indirect heating even under low pressure because of high boiling temperature.

6. Where direct fired heaters cannot be used because of danger. "



Regards,

It would be very helpful if examples are given for each category listed.
For example, sour water stripper (1?), Diesel/Kerosene Stripper (1?)

[rche]

Hi Flame! I've read your comment saying that injecting steam lowers the partial vapor pressure of the process fluid. What does this mean? Does it have something to do with Dalton's Law of Partial Pressures?

[gegio1960]

Why does we need the steam injection into crude oil column and so on?
What is the purpose of steam injected into column?

Steam is injected into crude oil column (topping) only for stripping reasons, ie removing of light tails from residue and heavier products.
The fractionation heat is provided by furnace.
Steam has to be taken at a minimum since it has to be condensed and then it generates sour water (ie additional operating costs).
The steam stripping improves some qualities of the residue/heavy streams:
- the bottom steam reduces the content of gasoil in residue, facilitating the downstream Vacuum Distillation
- the side strippers steam improves (increases) flash points of middle distillates (ie gasoils and kerosene)

[kkala]

Following notes could be useful, complying with above posts.
1. An example of crude oil distillation (ADU) with subsequent vacuum distillation of residuum (VDU) can be seen in http://www.simtronics.com/catalog/spm/spm2700.htm. Neither of the two distillation columns has a reboiler (usual); so steam injection helps light component stripping, as mentioned in above posts and the example (Process Description).
2. Nevertheless there is another significant reason to add steam into VDU. Vacuum is created in the VDU column to avoid thermal decomposition of residuum components (starting at, say, 770 oF) by lowering their boiling point; but vacuum is economically feasible up to a point, say to an absolute pressure of 30-40 mm Hg (α). Nelson explains it as below (α).
"The dry process of vacuum distillation, which utilizes no steam, has many theoretical advantages. Most important among these are the much smaller tower and condensing equipment required. Nevertheless, the absolute pressure that is required for dry processing of most petroleum oils amounts to less than 10 mm, and such a vacuum cannot be produced at a reasonable cost in most large-scale vacuum equipment"
(remark: 10 mm means 10 mm Hg).
3. It is noted that water and oil (γ) are practically immiscible in a mixture. Total vapor pressure is sum of vapor pressures of pure water and oil. There is equilibrium between vapor - liquid of oil in VDU column, but only H2O vapor.
Suppose (α) gas pressure of 30 mm Hg (absolute). Oil vapor pressure = 10 mm Hg at operating temperature, steam is added at a theoretical rate of 2 kmol / kmol of oil to result in 30 mm Hg total pressure.
That is roughly 36 g steam / 500 g residuum (β), about 7% w/w. Nelson (α) reports 0.3 - 1.0 lb steam / gal residuum, roughly 4 - 13 % w/w.
4. According to above, equipment (ejectors) creating a pressure of 30 mm Hg (abs) in the column cannot create 10 mm Hg (abs) in case of no steam addition. It seems so, e.g. http://www.venturipumps.com/ejector.htm, but clarifications would be appreciated.

Note (a): W L Nelson, "Petroleum Refinery Engineering", 4th edition, McGraw-Hill - 1958, where more data is presented.
Data can be also found in R.N. Watkins, "Petroleum Refinery Distillation", 2nd edition, Gulf - 1979.
Note (β) : For residuum MW = 500 is assumed. See http://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/30_3_CHICAGO_09-85_0365.pdf.
Note (γ): oil is actually made of numerous components, one (avg) considered here for simplicity, enough for conclusions.

Edited by kkala, 22 December 2012 - 03:28 PM.