reciprocating verses centrifugal compressors 1

[thiru4vino]

please explain me in NHT whenever we increse the pressure of product seperator recycle gas flow(tn/Hr) increses.

 

[thiru4vino]

Hello All!

Let me explain the question a bit clearer..
In NHT, we have a reciprocating compressor to maintain recycle gas flow. It takes suctiion from the product separator, which is at a pressure of 28kg/cm2(g).
When we increase this pressure to say, 28.5 kg/cm2(g), we observe that the recycle gas also increases, say from 2.0 tph to 2.2 tph.

But my mind says it is opposite to what should be happening bcos, when the pr is increased, relatively more of the lighter ends will be condensed, and so the resulting gas will also be of lower density, so naturally the flow in TPH will come down..but the reverse is happening...why so?
thanks for everything..
cheers
atm

 

[mbeychok]

Chillboy:

You wrote the subject of your posting as being "reciprocating versus centrifugal compressors". Would you please explain why you think that the type of compressor has anything to do with question you asked about the amount of recycle gas flow?

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[25362]

Is the meaning of NHT naphtha hydrotreater, as a unit to be used prior to the catalytic reformer ?

 

[JJPellin]

A reciprocating compressor moves a constant volumetric flow rate when run at a constant speed. For your recycle compressor, the discharge pressure should be constant. As you increase the suction pressure, the mass flow rate will increase because the density of the gas at the inlet will increase. Even though you may be condensing more light ends in the separator, the gas density should still increase. If I understand the process correctly, I would be surprised if you really were knocking out much more light ends as you increase the pressure. All the light stuff should already be gone regardless of the slight change in separator pressure. It is also possible that the method you are using to "measure" mass flow rate is not able to accurately compensate for changes in gas density, pressure and temperature. This is really a calculated value rather than a true measurements. You need to know the assumptions that are used to do that calculation.

 

[25362]

Beside what JJPellin has said, please consider the possible effect of a pressure ratio decrease on the volumetric efficiency of the reciprocating unit.

 

[thiru4vino]

25362! NHT is the Naphtha Hydro Desulphurisation Unit prior to Reformer.

mbeychok!
Actually I was asking one of my junior colleague to post the question using my user id. There was a mis-communication. Sorry for that.

We have a centrifugal compressor for reformer recycle gas and reciprocating compressor for NHT and KHDS plants.

We observe that in reformer unit, when our recycle gas flow in TPH, comes down, we use to increase the pressure say, from 18.5 kg/cm2(g) to 18.8 kg/cm2(g). And after say half an hour period, the recycle gas is observed to improve.
We reasoned out that when we increase the system pressure, the lighter hydrocarbons that escape from the product separator will come to the recycle circuit and thus the density of the gas increases and since the suction valve of the compressor is always kept wide open, volumteric flow increaes, the flow in TPH will increase.

But in case of reciprocating compressors, I am not sure we can reason out similarly.

That's y the title was posted as Reciprocating Vs
Centrifugal Compressors, with some wrong communication..

I am terribly sorry in title snag..

I will go through what Jphellin has posted and will come back to you all for more clarification..
thanks

atm

 

[25362]

Chillboy,

Speaking about your experience on the NRU, when you say system pressure do you mean the separating drum pressure ? Do you control this pressure with a spillback suction pressure control ?

 

[thiru4vino]

yeah i meant separator drum pressure only. we do not have spillback suction pressure control.


one of my colleague suggested that at higher pressures, NHT reactions will be more severe and hence higher dense gas and hgher mass flow.

your inputs are welcome.

cheers
atm

 

[25362]

Please explain how do you control/vary the separating drums' pressures in the reformer and in the NHT. The drums are located at the suction of the compressors.

Concerning the NHT, you spoke of changing the pressure at the drum, not the reactors, as apparently your colleague does. Please clarify whether you're speaking of total pressure or hydrogen partial pressure.

 

[thiru4vino]

We vary the pressure by controlling the venting to Fuel Gas header.

I am talking of the total pressure of separator drum which decides the system pressure or reactor pressure.


cheers
atm

 

[25362]

Chillboy, let's concentrate on pressure effects. For HDS, HDN, and HDO reactions to take place to the extent of 99%+ the important factor is the hydrogen partial pressure. The purity of the recycle and the make-up gas influence the total pressure selected.

NHT units operate in the range of 15-25 bars to give H₂ sufficient partial pressures. The apparent large difference between these limits being -mainly- due to the degree of vaporization of the feed.

The choice of operating pressure must be made with care, to ensure the NHT operates under (heteroatoms') hydrogenating conditions, but it mustn't be so high that excessive hydrogen is consumed in the hydrogenation of aromatic rings.

Now, consider that the hydrocarbons formed as a result of removing sulfur as H₂S, oxygen as water, and nitrogen as ammonia, boil at temperatures lower than the original compounds. Some of them will increase the MW of the recycle gas, others may require the product to be stripped to preserve boiling range specs.

Temperatures also vary. In their normal range, high H₂ partial pressures will increase the degree of hydrogenation. Too high temperatures (above 340^oC) can result in the formation of olefins which may recombine with H₂S to form mercaptans. If accompanied by lower H₂ p.p. they will promote naphthene dehydrogenation reactions.

Process conditions are chosen to minimize side reactions such as dehydrogenation, hydrocracking, and aromatic saturation.

Since gas rates for NHT are, in general, relatively low, a once-through gas flow can be used. For heavier fractions it is normal to have a recycle gas compressor to assist in maintaining the desired hydrogen p.p. in the reactor.

For KHT, and in general, as the feedstock boiling range increases, from naphtha to residue, the sulfur compounds become more complex and require higher partial (therefore total) pressures to react with hydrogen to form H₂S and to prevent rapid catalyst de-activation by carbon deposition.