Moisture Estimation in Recycle Gas 3

[thiru4vino]

Dear All!

Recently I started working on a semi-regenerative catalytic naphtha reforming unit.

There i am told by my seniors that the moisture in the recycle gas would be 4 - 5 times that of the moisture in the feed naphtha...

When i pestered for the reasons, they could not reason it out but insisted ont the same ratio, when we checked up with the laboratory, we got results that almost support the ratio...

can anybody explain how this ratio comes up?
thanks for everything you do.

cheers
atm

 

[25362]

Water entering the unit with the feed will pass over the catalyst in the reactors and will be in equilibrium between the recycle gas and the product liquid in the separator vessel.

The amount of water passing over the catalyst will be the sum of water in the feed and in the recycle gas.
We all know what harm too much water can do.

Can you provide some water ppm figures to get an idea ?

 

[thiru4vino]

Yeah! First lemme thank you for the response..

Our Feed moisture content is regularly checked by our QC people and is found to be 25 - 50 ppm. I beleive it is on
weight basis.

We have an online moisture analyser which is not working since recent months...We used to check the moisture during the test runs by another instrument/analyser by quality control people..Presently it is also under repair...so people are going by the thumb rule...hence my seniors are telling me that there will be 125 - 200 ppm of moisture by vol basis in the recycle gas...

If it is true or false, i want to prove this by calculation with your help...

thanks once again...

atm

 

[25362]

Can you tell me whether you are using a halide as acidic-promoter? If the answer is affirmative, the water content of the feed is a bit on the high side, except if it contains more than 2 ppm chlorides, see also

http://www.criterioncatalysts.com/english/PDF/NPRA_2002AM_62.pdf

 

[thiru4vino]

Yes! we are using carbon-tetrachloride as acidic-promoter. We get our chloride results around 0.8 to 1.1 ppm...
is it tell you something??

Teach me if anything i should know..
thanks & cheers

atm

 

[25362]

Here are some thoughts which may or may not be applicable to your particular case.

1. Coming back to the moisture content of the recycle gas. The only reason I can find for a high concentration of water in the recycle gas is that at the reaction temperatures -because of the vapor pressure- the gas becomes enriched (probably saturated) in moisture which again redissolves in the reformate upon cooling.

Whether equilibrium is reached or not in the high-pressure separator isn't clear to me. Redissolving is always a slower process than evolution, thus equilibrium would need some residence time to take place. On equilibrium at the usual P,T conditions prevailing in the separator, the high moisture ppm level in the gas couldn't be explained. The only explanation I thought of, is the in the mechanism of re-dissolution and lack of equilibrium.

2. Water content of the feed should be about 10-20 ppm to prevent rapid stripping of the chloride from the promoted catalyst.

3. Is your naphtha straight-run or it contains also fractions from thermal processes and FCC, which may contain sufficient nitrogen to form ammonia and neutralize the chloride acid functionality ? Besides, ammonium chloride can deposit on the stabilizer downstream, reducing efficiency and may even result in blockages.

4. If the chloride content of the feed were much higher than 2 ppm, you'll need to add water to remove any excess that could induce hydrocracking activity, maintaining lower reaction temperatures while this operation is carried out.

5. A sudden excess of water in the feed can also promote, in the short-term, hydrocracking with a higher C1-C4 hydrocarbon production and a commensurate reduction in hydrogen make and in yield of reformate. This would oblige to increase the recycle gas rate to maintain hydrogen partial pressure and to reduce production of coke precursors. In the long-term, hydrocracking will be reduced.

6. The catalyst should, however, contain some water. It has ben reported that a high demethylation activity develops when the water content of the feed is lower than 10 ppm.

Let me have your comments.

 

[25362]

I totally forgot about the binary azeotropes of water with C5 and C6. Since these hydrocarbons appear in the separator vapors, they will chase out the water from the reformate as azeotropes. Thus the moisture entering with the feed will leave the unit mostly with the net gas produced. The moisture concentration in the recycle gas would naturally be the same as in the net gas offtake.

Depending on the yield of gas of the unit and its molecular weight you'll probably find that the ratio of the ppm (vol) moisture in the gas [÷] ppm wt moisture in the feed will be not far from the value reported by your co-workers.

 

[25362]

I can help you to carry out an example if so wish.

 

[thiru4vino]

Hi 25362!

Thanks for the details, which I am trying to understand.
I wish to state these details to you so that if you can help me if i pose a doubt again after my understanding.

Ours is a straigt run naphtha of low sulphur crude, splitted into 60-90 cut, and hydrotreated. You have raised a question in your reply about the equilibrium and I am also informed that our product separator is an equilibrium separator, am i right if that means the vapor and liquid separated there are in equilibrium with each other? How can i ensure that, since the liquid and vapor are having different compositions..


I have another doubt that you mentioned the formation of binary azeotropes of water with C5 and C6, are they paraffins or aromatics, I sincerely beleive that we are not getting pentanes or hexanes or benzene or higher aromatics in our off-gas, it could consist of hydrogen, methane and ethane and some propane i suppose.. I am saying this because we are separating some liquid in this range in reformate stabilizer and sending it to MS pools...
SOrry if i am wrong and i am eager to learn what is inside this??? can you suggest me where i can find the details of these azeotrops??

can you also provide me an example of how to calculate the ratio of moisture content as you mentioned??

Thanks for everything you do! and really a great job you are doing..

cheers

atm

 

[25362]

1. Azeotropes

The binary azeotropes at atmospheric pressure are:

C5: BP 34.6^oC, water 0.054 mol fraction
C6: BP 61.6^oC, water 0.221 mol fraction
Benzene: BP 69.3^oC, water 0.295 mol fraction

All these azeotropes have BP lower than their components, therefore if those HC's appear in the separator gas, as they in fact do in small proportions, the vapor should contain the azeotropes.

2. Equilibrium separating drum.

Equilibrium concentrations are estimated by a conventional flash vaporization method using L = F/[1+K(V/L)] in about three iterations on values of V/L.

3. Moisture balance exercise example.

Assuming 15% w/w gas produced of molecular weight = 12.

30 ppm w/w water in the feed passing (~totally) to the gas, becomes 200 ppm w/w.
For a gas of MW=12 this means: (200)(12)/(18) = 133 ppm v/v.

The ratio: 133/30= 4.4 !! In full accordance with your seniors' statement.
You may conclude by yourself what steps are needed to reduce moisture in the recycle gas.

4. Process variables

Feed chemical characteristics (P/N/A split), moisture content, lower boiling fractions containing C5, not amenable to reforming, operating severity (to get a given RON of reformate), all affect the above results. The true boiling point (TBP) of the feed would indicate how much (unreacting) C5 is present.

Please note that more paraffinic stocks depend more on dehydrocyclization, hydrocracking and isomerization reactions, needing more severe conditions resulting in lower reformate yields than the naphthenic stocks.

Gasoline % vol yields as function of vol % naphthenics (N) in feed, vary approximately as follows:

RON 25% N 50% N

90 77 88
95 72 85
100 63 81

 

[thiru4vino]

Thanks a ton, 25362!

I will certainly understand the details you put over and come back to you, if I have doubts, if any...

Cheers

ATM