reciprocating verses centrifugal compressors
reciprocating verses centrifugal compressors
(OP)
please explain me in NHT whenever we increse the pressure of product seperator recycle gas flow(tn/Hr) increses.
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reciprocating verses centrifugal compressors
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RE: reciprocating verses centrifugal compressors
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
RE: reciprocating verses centrifugal compressors
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)
.
RE: reciprocating verses centrifugal compressors
Is the meaning of NHT naphtha hydrotreater, as a unit to be used prior to the catalytic reformer ?
RE: reciprocating verses centrifugal compressors
RE: reciprocating verses centrifugal compressors
Beside what JJPellin has said, please consider the possible effect of a pressure ratio decrease on the volumetric efficiency of the reciprocating unit.
RE: reciprocating verses centrifugal compressors
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
RE: reciprocating verses centrifugal compressors
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 ?
RE: reciprocating verses centrifugal compressors
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
RE: reciprocating verses centrifugal compressors
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.
RE: reciprocating verses centrifugal compressors
I am talking of the total pressure of separator drum which decides the system pressure or reactor pressure.
cheers
atm
RE: reciprocating verses centrifugal compressors
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 340oC) 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.