[How To] Calculate Hydrogen Requirement & Consumption during Hydrogenation

Hiii all ……!!

Received a query requesting the way to calculate the hydrogen requirement and hydrogen consumption.

Hence writing today. This post includes some chemistry too.

Basically hydrogenation reactions involves addition of hydrogen and thereby increase of the raw material molecular weight by 2g/mole and sometimes the increase will be a multiple of 2g/mole. The reactions are performed in presence of catalysts like nickel, palladium, platinum etc. Sometimes the reaction will be performed in basic medium which will help in retaining the activity of catalyst.

Mostly hydrogenation involves in conversion of alkenes/alkynes to alkanes i.e., simple saturated hydrocarbon.

Before getting in depth, let’s have something to get familiar with the real content.

What is the difference between hydration and hydrogenation reaction?

Hydration reaction involves addition of water whereas hydrogenation involves addition of hydrogen

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What is the difference between addition reaction and hydrogenation?

Hydrogenation is a special type of addition reaction, where addition of dihydrogen happens.

What is the difference between hydrogenation and reduction reaction?

Reduction	Hydrogenation
Decrease in oxygen atoms of a compound	Addition of hydrogen to a compound
Arises in compounds having higher oxidation number	Arises in unsaturated compounds
Doesn’t requires a catalyst	Requires a catalyst

What are alkanes, alkenes, alkynes?

Alkanes, Alkenes, Alkynes are organic compounds which shall be differentiated based on the carbon bondings. Alkanes have single bonded carbons, Alkenes have double bonded carbons, Alkynes have triple bonded carbons. These three can be differentiated based on the chemical formulae i.e., alkanes will have a syntax of Cn H2n+2, Alkenes will have Cn H2n, Alkynes will have CnH2n-2.

Let’s say there are 3 carbons, then if its alkane then there will be 2n+2 hydrogens i.e., 2 (3) + 2 = 8,

If its alkene, then there will be 2n hydrogens i.e., 2 (3) = 6,

If its alkyne then there will be 2n-2 hydrogens i.e., 2 (3) – 2 = 4.

** This syntax would be applicable to aliphatic alkanes / alkenes / alkynes.

What is ideal gas law?

PV = nRT.

What is water capacity of a cylinder?

Water capacity indicates the net volume of cylinder measured by filling it with water, simply volume of the cylinder.

What can be the hydrogen weight in a cylinder?

Usually there will be approx. 150 Kg/cm2g pressure build inside a cylinder and the water capacity is 46.7 Litres (gained from Andhra Sugars Catalogue), Storage at RT conditions.

P = 150 Kg/cm2g = 145.17 atm, V = 46.7 Litres, T = 273.15 K (Say), R = 0.082 atm.L / mol.K.

n = P V / R T = (145.17 x 46.7) / (0.082 x 273.15) = 302.68 moles

Weight = 302.68 x 2 = 605.35 grams.

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What can be hydrogen volume compressed in a cylinder?

Molecular weight = 2 g/mole,

1 mole of gas occupies 22.414 Litres volume at STP, Density = 2 / 22.414 = 0.089 g/L (or) Kg/m3.

Volume in cylinder = 605.35/0.089 = 6801.72 Litres

Now, let’s get into topic and I’ll explain you with a case study, this is going to be quite short.

Let’s suppose we have to hydrogenate 100 Kgs of Cyclohexene to Cyclohexane with Palladium carbon as catalyst at 3.5 Kg/cm2 pressure and 50 °C, Hydrogenator capacity be 0.5 KL and the lines assembly connected will attribute to additional ~100 Litres. Now we need to calculate the hydrogen requirement and the hydrogen consumption.

The reaction would go as C6H10 + H2 ----> C6H12

Now for calculating the hydrogen requirement, we need to perform the material balance:

	Cyclohexene	Hydrogen		Cyclohexane
Mol. Formula	C6H10	H2		C6H12
Mol. Wt.	82.14	2		84.16
Qty. (Kgs)	100	2.434		102.42
Moles (Kmoles)	1.217	1.217		1.217

So, our theoretical requirement would be 2.44 Kgs.

Some of you might get a doubt about performing material balance, I’ll explain this in short,

i.        Firstly we calculate the Moles of Cyclohexene i.e., wt/mol. wt. = 100/82.14 = 1.217 Kmoles

ii.      So, we need 1.217 Kmoles of hydrogen for reaction and 1.217 Kmoles of product will be formed,

iii.   Now place the 1.217 in moles section for Hydrogen (Reactant 2) and Cyclohexane (Product),

iv.      Now multiply the 1.217 Kmoles of hydrogen with hydrogen mol. wt. i.e., 2 and 1.217 Kmoles of Cyclohexane with its mol. wt. i.e., 84.16, you will get those.

Getting back into topic, we got our requirement i.e., 2.44 Kgs of hydrogen,

Now considering the plant case i.e., the pressure that we have to build during hydrogenation will be the additional quantity that we have to count.

Total system volume is 500 L hydrogenator + 100 L piping and auxiliaries = 600 L,

Let’s say the reaction mass would be ~250 L, so the overhead space would be 600 – 250 = 350 L.

In the remaining 350 L of overhead volume, we need to build up 3.5 Kg/cm2.g (3.387 atm) pressure,

So the requirement would be 3.387 x 350 / (0.082 x (50+273.15)) = 44.7368 moles

= 44.74 x 2 = 89.473 grams

Total requirement would be 2.44 + (89.473/1000) = 2.53 Kgs,

Net Usable hydrogen quantity in each cylinder:

Now again we need to calculate the net usable hydrogen gas from a cylinder i.e., as per the standard cylinder, the pressure would be ~150 Kgs/cm2.g, but as per the process we have to maintain the pressure of 3.5 Kg/cm2. So when the gas is pressurized from the cylinder into the system, the operating pressure of system will become datum pressure of cylinder, i.e., the 3.5 Kg/cm2 pressure will be left over in cylinder after usage.

Net usable pressure = 150 – 3.5 = 146.5 Kg/cm2 (or) 141.788 atm, T = 273.15 K, V = 46.7 Litres,

Moles = 141.788 x 46.7 / (273.15 x 0.082) = 295.625 moles = 295.625 x 2 = 591.25 grams

No. of cylinders requirement would be 2.53 x 1000 / 591.25 = 4.279 ~5 cylinders.

That’s it……….!!!

Hope you have understood it well, all we have done is using Ideal gas law by considering Hydrogen as ideal gas.

Feel free to comment, Comments are most appreciated……!!!

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About The Author

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Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a tech geek by passion, and a chemical process engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
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