12 replies to this topic

[asetofenon]

Dear members,


My problem is,

I am a plant engineer in the vacuum unit, the capacity of the column was increased to 4000 m3 from 3000 m3 without a top condenser revamp. So most of the times because of the condenser capacity we are loosing vacuum. I was looking for an answer if I can decrease the amount of coil steam in the heater? All the articles that I read was telling that coil steam increases the velocity of the feed in the furnace in order to prevent coking. But since my feed is increased from 3000 to 4000 M3 the velocity is already increased by 1/3 so does that mean that ı can decrease the amount of coiling steam? ( right now the amount of coil steam is never changed by operators, will it be wrong If I arrange a ratio controller so that coil steam amount is automaticall change according to the feed? And as ı said earlier is there a magic number above what m3/s we dont need any coil steam?

thank you in advance

[bmk]

Dear sir,

1.Please clarify how you have increased the capacity of column? Is it 4000 m3/hr?

2.It seems that you have increased steam pressure in the steam coil which results in increase in boil-up rate. You can therefore adjust the steam pressure so that your condenser can handle the vapour load without affecting vacuum.

3.I request you to furnish detailed process parameters with flow diagram.

--- bmkhare

email = "bmkhare@gmail.com"

[djack77494]

This is an interesting question that I have not run across in the past. Hopefully Zauberberg will offer some comments since he has a lot of experience in this area.

I can speculate, however. Hopefully it is informed speculation. I would say that the increased throughput raises the velocity and lowers the residence time in the furnace coils. Coincidentally, that is the purpose of coil steam. So I would deduce that coil steam may be lowered since you are effectively meeting the objectives via the higher throughput. Of course, the actual situation is not so simple as this, but, lacking other inputs, I would feel confident enough to proceed forward (slowly).

Doug

[Padmakar Katre]

Dear,
Answer to you question is 'Yes'. You can reduce the coil steam in heater with same 'm3/hr' quantity compensated by the heater HC throughput. The minimum velocity needed(or based on the previous total volumetric feed) in you furnace tubes would remain same. But when it comes to vacuum column Ovhd condenser (I presume it's a pre-condenser ejector configuration). Though the decreased steam in furnace coil off-loads the vacuum column overhead to ejector or pre-condenser at the same time you will need more steam in your vacuum column stripping section so we can say it will partly offload your ejector system and not much benefit. If it's a dry vacuum operation as no stripping section at the column bottom, then obviously yes, you will have benefit of this.

[asetofenon]

Thanks for the kind replies,

until I recieve the replies, during these 2 weeks I made some new research and found some new data that I want to share with you, maybe our discussion con move to other directions.

ı was just reading Norman liebermann s book ( troubleshooting to process operations) or some similar name. What he says is, the most frequent problem of vacuum columns is the condenser capacity of the top, ( which is true for my case as well) and the reason for the problem is cracking of the feed in to some non condensables and hardly condensables. According to him the hottest section in the furnace is inside the furnace not at the outlet.(I am not measuring the temperature of the fluid inside the furnace so i dont have a proof for this), coil steam is there to decrease this hottest temperature without changing the outlet temperature. ( In his figure I can see a 40 F of decrease )

What I understood from his article is decreasing the steam will increase the amount of cracked gas and will load my top condenser. I personally try to test the opposite of the argument, and increase the steam ( I decrease column bottom stripping steam with the same amount) and look for a top vacuum and condenser improvement. Unfortunately I could not increase the steam amount that mucf ( nearly 20 percent only) alltough i opened all the valves full. and I could not notice any improvement. Do you have any suggestions for the issue?

there is also another question in my mind, I am giving 100 pound steam for coil steam and motive steam to ejectors which both of them are fed from the same header. What will probably happen if I decresea the pressure or increse the pressure? ( Both the coil steam and ejectors)?

best wishes

[Zauberberg]

... is there a magic number above what m3/s we dont need any coil steam?

I am quoting this particular number from one of the articles published by Process Consulting Services: "The three factors that determine coking are -

• oil residence time;
• localized heat flux; and,
• oil mass flux rates.

Low radiant section oil residence time is less than 10 seconds. However, dry heaters can operate with more than 60 seconds residence time. Minimizing residence time is essential. Localized heat flux is determined by burner layout and design and coil layout. For instance, minimizing excess air maximizes localized heat flux. The last major factor is oil mass flux rate. Higher mass flux rate reduces the rate of coking. Ideally, mass flux at the radiant section inlet should be 450 lb/sec-ft² (2200 kg/sec-m²). Ultimately peak oil film temperature and oil residence time determine the rate of coke formation. Operating changes that decrease peak oil film temperature and oil residence time reduce the rate of coking."

Ideally, you would be looking for reduction of steam injection equivalent to the mass flow of additional hydrocarbon feed - having all other things equal. Obviously, you will be affecting all of the three variables:

- Oil residence time: reduced, which is good
- Localized heat flux: increased, due to more firing required to achieve the same COT at increased throughput, which is bad
- Oil mass flux: increased, which is good

The only way to verify the performance of your unit is to gradually reduce the coil steam injection and monitor performance of the overhead vacuum system (I believe it has to be a system with precondenser due to steam injection). Reducing steam injection is beneficial for ejector/condenser operation, and as long as you see increase of the vacuum inside the tower it means you are doing good. Once (if) you start seeing gradual loss of vacuum as the steam rate is decreased further, this would mean that cracking of oil probably takes place inside the furnace, and that you should restore the steam injection rate up to that level where the vacuum was the highest. Just organize a performance test run with the Operators, and do it slowly and in a planned way, and you can't make a mistake.

Good luck,

Attached Files

•  Vacuum Unit Coking.pdf   331KB   74 downloads
•  Vacuum Unit Design Effect on Operating Variables.pdf   105.68KB   64 downloads
•  Why Vacuum Unit Fired Heaters Coke.pdf   385.28KB   65 downloads
•  Vacuum Unit Fired Heater Coking - Avoid Unscheduled Shutdowns.pdf   206.07KB   53 downloads

[Raj.che]

Dear Zauberberg Sir,

Thanks a lot for sharing such valuable information. You are correctly pointed that higher localized heat flux will be increased (Due to increased process load) which is not good for coil design point of view, as more firing will be required to achieve the same COT.
I have a question, If coil outlet terminals of the heater are lies in the low heat flux zone, then can one decrease coil steam quantity to the lowest extent feasible keeping metal skin temperatures within allowable limits.
If not, How it will affect performance of the vacuum column and heater system.

Raj

[Zauberberg]

Raj,

Who is keeping you away from proving all the subjetcs listed below?
We are the process engineers and the world is ours unless someone proves the opposite.

Read the last post?

[asetofenon]

Dear zauberg

thank you very much for these valuable information. Do you have any suggestions about the steam pressure to the ejectors ( motive steam) and to the coils? Right now both of them are fed from the same header which is 100 pounds. What can be the results of changind the pressure to these two locaitons?

[djack77494]

asetofenon,
Why would you be interested in changing the steam pressure? Do not do this unless you're sure what you are doing. The steam ejectors are designed for a particular motive steam pressure. They may not respond well to some willy-nilly changes in motive pressure. The coil steam pressure doesn't seem very important to me since it's throttled to get into the heater anyway, but I do not think you are pursuing a meaningful (or profitable) path.

[asetofenon]

asetofenon,
Why would you be interested in changing the steam pressure? Do not do this unless you're sure what you are doing. The steam ejectors are designed for a particular motive steam pressure. They may not respond well to some willy-nilly changes in motive pressure. The coil steam pressure doesn't seem very important to me since it's throttled to get into the heater anyway, but I do not think you are pursuing a meaningful (or profitable) path.

let me describe the reason of this, and then we can discuss

last week ( the time that I decided to deal with the top system) I noticed that the motive steam valve was throttled and decreased to 4,5 kg by the operators, which is normally produced at the kettle rebilers of the plant at 7.0 kg. and its saturation temperature is around 170 C.

Since the ejector is already running at 4,5 kg at 170 C because of throttling ( Which is now superheated) Why not decrease the header pressure to 4,5 and remove the throttling. So I can produce 4,5 kg steam at my kettle reboilers which will be saturated steam and will be around 145 C somehow. So If we compare two cases

1. 4,5 kg 170 C
2. 4,5 kg 145 C

I will definetely choose the second case which is easier to condanse.

I tried to run the ejector at 7 kg but as operators told, I lost vacuum.

[djack77494]

asetofenon,
I am getting confused due to your changing between Imperial and SI units and also your imprecise use of mass units when I think you're discussing pressures. Please use psia/psig or kg/cm2[a]/kg/cm2[g] for your pressure units.

In furtherance to this discussion, an steam vacuum eductor is not a linear device in that its performance does not behave well as motive pressure is varied. They are not designed in this way, and the way they work is such that the motive steam pressure is not to be used as a control variable. Think of it as being on or off. The eductor will operate fine as long as the motive steam pressure is acceptable. But if you throttle the steam, you will reach a point where the eductor essentially stops working. This is a discontinuity, and there is NOT a smooth transition in eductor capacity as the motive pressure drops.

Edited by djack77494, 16 July 2010 - 10:46 AM.

[Zauberberg]

The answer is fairly simple: as Doug has emphasized, each particular ejector system is design for certain motive steam pressure. Lowering the pressure will reduce their performance, simply because less steam (mass flow) enters the ejector nozzle. The steam is motive fluid that pulls the vacuum. The same effect - although at much lesser extent - is superheating of steam. For constant pressure, the density of steam is reduced as temperature increases, resulting in smaller mass flow of steam.

As for the heater, Doug has answered your question. As long as the available pressure drop (header - heater) is sufficient to provide required mass flow, there is no concern if the steam supply pressure is reduced down to a certain extent.