15 replies to this topic

[raj1]

Actually v have twin cell furnaces in our plant ethylene plant which is using ethane feedstock. V r facing increase in skin temperatures in cell B side radiation coils. Which restrict our furnace throughput and runlength. Actually , the main cause for the issue is non-uniform heat distribution in cell B side. V cant able to solve dis as v have burner limitations. Can i suggest them to increase the dilution steam for the particular coils which is being coked. If so how it affect the skin temperatures. Does my radiation coils skin temperatures decrease with increase in steam flow to particular coils with affecting my furnace ethane conversion and ethylene yield? 

[PingPong]

This is really a question for the company (licensor) that designed and installed your heater. You should discuss with them the possible cause(s) of the non-uniform heat distribution.

 

We have no idea what your heater looks like, but nevertheless some remarks:

 

If Cell A has the same burners and layout then why is there only a problem in Cell B?

Maybe replace (some of) the burners in Cell B?

Or interchange them with (some of) the burners in Cell A?

 

Maybe Cell B is more subject to wind and rain than Cell A, affecting flue gas circulation and thereby causing non-uniform heat distribution.

 

I don't see how the skin temperatures will go down by using more dilution steam, to the contrary. Coke that is already there is not going away simply because you increase dilution steam.

For the same ethane feed through the coils more steam means more absorbed duty required, and therefor more fired duty and consequently more heat flux, resulting in an increase in skin temperature.

More steam could only reduce the speed at which further coking occurs.

 

To reduce skin temperature you would need to:

solve the non-uniform distribution problem,

or reduce the fired duty by reducing the feed throughput of Cell B, if that is possible independent of throughput of Cell A.

 

The best way to solve a problem is always to remove the cause, not by trying to fight the symptoms.

Edited by PingPong, 20 May 2016 - 04:51 AM.

[raj1]

Pingpong. Thanks for ur reply. Your answer looks valuable. The best way to solve the problem is addressing the real cause. Basically v r running neck to neck with downstream. At current condition, v cant down our furnace and try burner switching all. Moreover, v cant reduce the feed in furnace too. V r in a situation to push furnace as much as possible. Basically, its "conversion" control and maintaining 70% conversion in each cell by varying fuel gas pressure. when i increase dilution stream in particular coil. partial pressure of hydrocarbon will decrease which favours higher ethylene yield. Now i ll be getting margin to decrease my conversion to match ethylene yield . This will reduce my fuel gas flow and thereby heat flux. Does it works? 

[PingPong]

when i increase dilution stream in particular coil. partial pressure of hydrocarbon will decrease which favours higher ethylene yield. Now i ll be getting margin to decrease my conversion to match ethylene yield. This will reduce my fuel gas flow and thereby heat flux..

I don't quite understand what you mean.

 

Note that more dilution steam decreases hydrocarbon partial pressures, but it also reduces residence time.

 

You want to decrease ethane conversion but at the same time keep ethylene yield the same?

That is only possible if you also increase ethane feed.

[raj1]

I have attached one sheet...u ll understand wt i try to mean...If its wrong...explain me

Attached Files

•  partial pr.jpg   131.19KB   6 downloads

[Sagar Nawander]

Raj,

 

What is the feedstock of the plant to produce Ethylene ??

 

I am surprised at how the Cell B side only has coking problem and not Cell A side, provided that both have same layout..

 

Your point to raise dilution steam looks fine as it will decrease the hydrocarbon partial pressure, which enhances the selectivity towards Ethylene.

 

Note that adding extra dilution steam will raise the energy number of the plant.

Edited by Sagar Nawander, 22 May 2016 - 12:36 AM.

[PingPong]

I have attached one sheet...u ll understand wt i try to mean...If its wrong...explain me

I am familiar with the theory of ethylene production.

 

when i increase dilution stream in particular coil. partial pressure of hydrocarbon will decrease which favours higher ethylene yield. Now i ll be getting margin to decrease my conversion to match ethylene yield. This will reduce my fuel gas flow and thereby heat flux..

It is still not clear what you want to achieve here. Only increasing dilution steam is not going to decrease coil skin temperature.

[raj1]

Hi sagar...as i mentioned earlier...feedstock is pure ethane...Am also surprised how cell B side alone getting coke when cell A & cell B have same layout..surprising...and cell B side coking takes place especially in 2 twin cell furnaces out of 3 twin cell furnaces. 

Edited by raj1, 23 May 2016 - 06:57 AM.

[raj1]

I agree with u sagar..That adding dilution steam increase energy no.V r in a situation to push furnace as long as possible till its SOR atleast...Dont know how cell B side alone getting coked?

[raj1]

Ping pong...Actually. i agree with u... dilution steam never help in decreasing coil skin temperatures...If i want to decrease coil skin temperatures while furnace is in operation... i have to decrease coking rate or i have to decrease heat input...at running condition...i cant decrease coking inside coils..so s there any solution? How cell B side alone getting coked? 

[raj1]

V have separate TLE for each cell. Does TLE mechanical design change coking rate of particular cell? Am not clear...How it ll affect

[PingPong]

It is impossible for outsiders like me or Sagar to know what caused the high skin temperatures in Cell B.

 

You need to look at historic data since the last decoking to see when the skin temperatures in Cell B started deviate from those in Cell A, and ask yourself:

What happened then?

An upset in Cell B?

An incompetent operator at the controls?

 

Are you sure that Cell B was as well decoked as Cell A?

 

Do the burners in Cell A and Cell B get exactly the same fuel composition?

 

Do the burners in Cell A and Cell B operate on the same amount of excess air?

 

Look at all the instrument readings (pressures, temperatures, fuel flowrates, oxygen analysers, ......) on the A and B sides, and note any differences. Look also at historic data since the last decoke.

 

Do Cell A and Cell B get exactly the same ethane feed composition?

Or does Cell B operate mainly on recycle ethane that contains some olefins?

 

V have separate TLE for each cell. Does TLE mechanical design change coking rate of particular cell?

Is there a difference in TLE design for Cell A and B? A different TLE type can have a different pressure drop causing a different coil outlet pressure (COP) which has an impact on reactions.

 

The above are just some suggestions, you can use your imagination to add a few more.

[raj1]

Thanks ping pong..one more thing..this cell B side rise are happening everytime when furnace has been taken online after decoke..in all cycles...after decoke there is no much variation in CIP.which indicates..decoking is proper...as u said...there is much diff in excess oxygen of individual cell..anyway ..thanks for suggestion..i ll keep u posted

[Sagar Nawander]

Raj,

 

Ethane Cracker has it own significance of cracking which defers in coke formation as compare to cracking Naphtha. The coke formed in Ethane cracker is amorphous coke and its very hard compare to coke formed during Naphtha cracking. It's still very hard to suggest the root cause problem of why this is happening in Cell B and not Cell A. Anyway can you please specify the Coil Outlet Pressure and Temperature ?

 

Thanks.

Edited by Sagar Nawander, 05 June 2016 - 10:35 AM.

[raj1]

In SOR , Coil outlet pressure and temperature in cell B side are around 14-17 psig and 1495°F. In EOR condition , coil outlet pressure is around 18-21 psig and interestingly COT is at 1455°F( for the same conversion). Interesting thing is, decrease in COT and increase in TLE pressure drop on cell B side are in line.

Edited by raj1, 08 June 2016 - 11:24 AM.

[raj1]

Dear all, 

Some of the observation i have found,following are listed below for furnace cell B side coking issue

 

1) For the same individual draft pressure (Cell A & B side individual draft), oxygen content in cell B side is higher as comparing cell A side which is reflected in high fuel gas consumption in cell B side (fuel gas controllers and pressures are also in higher side) than cell A side.

2) After furnace decoke and TLE cleaning, both cell A & B side CIP's & COP's are starting at the same value. However, there is rise in CIP's & COP's (linear rise) during the run in cell B side even though there is not much significant rise in pressure drop across each individual coils in both cell A and B side, appears possibly with carry over of loose cokes to TLE in furnace cell B side (cross checked with rise in TLE pressure drop and outlet temperature).

3) For the same ethane conversion in cell A & B side, ethylene yield in cell A & B side remains same in SOR condition. However there is drop in ethylene yield during the run in cell B side (coincide with rise in TLE B side pressure drop).

 

Suppose high oxygen content leads to high fuel gas consumption is the issue with cell B side. The same phenomena is not happening in other furnace which have the same B side coking issue. Although, fuel gas consumption and oxygen content in cell A side is higher than B side in those furnaces (for the same feed and its composition).

 

Can anyone suggest me possible causes?

Edited by raj1, 14 June 2016 - 12:06 PM.