**H**ii all......!!

Hope everyone is fine, today i want to post something about heat load calculations and this will be a very simple thing.

Have seen many of the process engineers were getting troubled while performing these calculations. And i think the only reason for it is lack of knowledge on terms like Sensible heat, Latent heat, Enthalpy etc.

So here i would like to make it simple with an example later followed by a case study and i'll try to add a template for this, which would be helpful for calculating the utility requirement for a production block.

Let us start with some definitions,

*What is Sensible Heat ?*Heat intake / out exchanged from a body to system without any phase change can be termed as Sensible heat.

In simple, if we heat / cool the reaction mass in a reactor to a particular temperature without any phase change, it will fall under Sensible Heat.

*What are the units of Sensible Heat ?*Sensible heat have the units of KCal/hr or KJ/hr or KW

*How to Calculate Sensible Heat ?*I'll explain it in the below section with a case study.

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*What is Latent Heat ?*Heat required for a matter to change its phase. Simply, we'll heat our reaction mass to reflux / we'll distill out solvent from reaction mass, so it falls under latent heat.

*What are the units of Latent Heat ?*Latent Heat will have the units of KCal/Kg or KJ/Kg

*What is Enthalpy ?*We can consider enthalpy as energy

*What is Heat Capacity ?*Heat Capacity is the energy required for a substance to raise the temperature of it by 1 degree. Heat Capacity is not a fixed value, it varies with temperature.

*What are the units of Heat Capacity ?*Heat Capacity will have the units of KCa/Kg.C or KCal/mole.C

*What is Specific Heat ?*Specific Heat is the ration of Heat Capacity of a liquid to Heat Capacity of Water. But in general we'll refer Heat Capacity as Specific Heat. Theoretically, Specific heat doesn't have units, but we engineers will mention Specific heat in KCal/Kg.C or KCal/mole.C.

*How to Calculate Heat Capacity ?*For calculating Heat Capacity, we need three practical values at various temperatures. Just say, for toluene the Heat Capacity

at 0 C be 1.61 KJ/Kg.C

at 50 C be 1.8 KJ/Kg.C

at 100 C be 1.968 KJ/Kg.C.

From these known heat capacities we need to develop three equations from an empirical equation,

Cp = a + bT + cT^2,

and then we need to solve the three equations for a, b, c. Lets the values of a, b, c be 1, 2, 3 respectively.

Then the equation of Heat capacity for Toluene will be Cp = 1 + 2T + 3T^2,

We need to use it for determining at various temperatures. But usually we'll consider it between 0.2 to 0.6. If its water it should be 1 KCal/Kg.C

Now I'll explain the above discussed terms with an example.

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**Example:**

Below is the Temperature Scale of Water:

Water has three forms,

1. Solid - Ice,

2. Liquid - Solvent,

3. Vapour - Steam.

All three shall be exhibited at triple point i.e., at 0.01 C and 4.58 mmHg barometric pressure.

So, water basically exists as solid below 0 C.

Lets say our case during an intermediate manufacturing, we have water as solvent and we need to filter our reaction mass at 5 - 10 C in a centrifuge, for that we need to cool from 50 C to 5 - 10 C, so the cooling will fall into sensible heat, as there is no change of phase observed.

Next case say, during a vessel entry of reactor or during a change over cleaning for a product we need to maintain at reflux for 1 - 2 hours, then there will be a phase change as water will turn to vapour, so it shall be considered as Latent heat.

*** Basically latent heat will vary with temperature and it wont be constant.*

*If water is boiled at atmospheric pressure it will have an energy of 540 KCal/Kg,*

*if the same water is boiled at 120 C, then the latent heat will be lower than 540 KCal/Kg,*

*if the water is boiled below 100 C (say at 70 C under vacuum), the latent heat will be higher than 540 Kcal/Kg.*

The same will be the case for other solvents also,

The latent heat observed at boiling point will be latent heat of vaporisation / condensation,

Latent heat observed during freezing point shall be termed as latent heat of fusion / melting,

And now let's say we need to raise water temperature by 1 C from 39 C to 40 C, then the energy required can be called as Heat Capacity / Specific Heat.

Now Lets jump into calculation part with some real-time examples

Case Study:

Manufacturing process for an API AKK has reaction between X & Y at 50 C in a solvent, (where X need to be added to Y), then cooling to 30 C followed by addition of water at 30 C. Then layer separation followed by distillation of solvent below 70 C under vacuum. Cooling the reaction mass to 0 - 5 C, then filtration and drying.

Now i need to calculate the heat loads involved in the process,

First thing is Addition of X to Y, this needs RC1 study (Reaction Calorimetry), and i'll discuss it at end.

Second thing is Cooling of reaction mass from 50 C to 30 C,

(Lets say total reaction mass volume be 2000 L with density 0.9 gm/cc and specific heat 0.5 Kcal/Kg.C),

Sensible heat = M x Cp x dT = (V x rho) x Cp x dT = 2000 x 0.9 x 0.5 x (50 - 30) = 18000 KCal,

Cooling Capacity in TR = 18000 / 3024 = 5.95 TR.

Third one is distillation, So we need to distill out solvent below 70 C, let it be 65 C,

Initial reaction mass be at 30 C, Latent heat of vaporisation be 100 KCal/Kg (Can get it from MSDS)

**For reactor:**

Heat load = (M x Cp x dT) + (M x Æ›) = (2000 x 0.9 x 0.5 x (65 - 30)) + (2000 x 0.9 x 100)

= 31500 + 180000 = 211500 KCal

For this we need to get steam requirement, Steam required = 211500 / 540 = 391.67 Kgs

*[1 Kg of steam have 540 Kcal of enthalpy at 1 atm pressure]*

**For condenser:**

Heat load on condenser = M x Æ› = 2000 x 0.9 x 100 = 180000 KCal,

But condenser will also cool the condensed liquid, so to compensate that consider 20% of total latent heat load = 180000 x 1.2 = 216000 KCal.

Cooling Capacity required = 216000 / 3024 = 71.43 TR

Fourth one is cooling from 65 C to 5 - 10 C

Let the total mass be 500 L,

Again sensible heat = 500 x 0.9 x 0.5 x (65 - 5) = 13500 Kcal.

Cooling Capacity = 13500/3024 = 4.47 TR.

Coming to heat load due to reaction, from RC1 study we can find the,

i. Heat released/required per Kg of KSM,

ii. Specific heat,

iii. Adiabatic temperature raise (if reaction is exothermic)

Let's say, we found that 500 KJ/Kg is energy released per Kg of KSM and the batch size of RC1 study be 50 grams and the plant batch size be 500 Kgs.

So energy liberation for 500 Kgs batch size = 500 Kgs x 500 KJ / Kg = 250000 KJ = 59751.4 KCal,

Cooling Capacity required = 59751.4 / 3024 = 19.76 TR.

That's It.......!!!

To make this post understandable to all, i've made it simpler,

Hope everyone can understand this,

If any queries pl comment, Comments are most appreciated ....!!

Below is the screen of Heat Load Spreadsheet template for your reference:

Click Here to Download

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**A**bout The Author

Hi namaste my self MP I am steam engg profesional ,very good explanation also read all other posts in this lock down

ReplyDeleteCan u get above example interesting by taking actual case study like water ethanol eztropropic distillation

Thanks ��..pls keep doing

ReplyDelete