Fundamentals of Material Balances 2 🚀

In the 4th post, we have already given the introduction to Material Balances. Now we go further and continue our journey with the rest of it 🤩☄️!

Excess Reagent 🖋

In industrial reactions the components are seldom fed to the reactor in exact stoichiometric proportions. A reagent may be supplied in excess to promote the desired reaction; to maximize the use of an expensive reagent; or to ensure complete reaction of a reagent, as in combustion. 📍The fractional and percentage excess is defined by the following equation:

It is necessary to state clearly to which reagent the excess refers. This is often termed the limiting reagent.

🖍Example 4. To ensure complete combustion, 20% excess air is supplied to a furnace burning natural gas. The gas composition (by volume) is methane 95%, ethane 5%. Calculate the moles of air required per mole of fuel.

Let’s look at the solution🥴:

🖍🖍Solution:

Basis of calculation is 100 mole as the analysis is volume %.

Reactions:

1. CH4+2O2-CO2+2H2O

2. C2H6+ 1.5O2-2CO2+3H2O

Stoichiometric moles of O2 required=95*2+ 5*1.5=207.5 moles

With 20% excess, moles O2 required: 207.5*((100+20)/100)=249 moles

Moles of air (21% O2)=249*(100/21)=1185.7 moles

Air per mole fuel= 1185.7/100=11.86 moles

Conversion and Yield📉

Conversion is a measure of the fraction of the reagent that reacts. To optimise reactor design and to minimise by-product formation, the conversion of a particular reagent is often less than 100%. If more than one reactant is used, the reagent on which the conversion is based must be specified. Conversion is defined by following equation:

Sometimes figures given for conversion refer to one specific product, usually the desired product. In this instance the product must be specified as well as the reagent, this is a way to express yield.

🖍Example 5. In the manufacture of vinyl chloride (VC) by the prolysis of dichloroethane (DCE), the reactor conversion is limited to 55% to reduce carbon formation, which fouls the reactor tubes. Calculate the quantity of DCE needed to produce 5000 kg/h VC.

🖍🖍Solution:

Basis 5000 kg/h VC- it is required quantity. Reaction:

• C2H4Cl2-C2H3Cl+ HCl

mole weights DCE 99, VC 62.5

kmol/h VC produced= 5000/62.5=80

From the stoichiometric equation, 1 kmol DCE produces 1 kmol VC. Let x be DCE Feed kmol/h:

Conversion%=55=(80/x)*100

x=80/0.55=145.5 kmol/h

Yield🔑

Yield is a measure of the performance of a reactor plant. For the reactor yield is defined by:

Yield= (moles of product produced* stoichiometric factor)/ moles of reagent converted.

📌Stoichiometric factor= Stoichiometric moles of reagent required per mole of product produced

🖇See yield and selectivity in Reactor engineering: https://m.youtube.com/watch?v=IuJWc5b6qQk

Recycle Processes♻️

Processes in which a flow stream is returned (recycled) to an earlier stage in the processing sequence are frequently used. If the conversion of a valuable reagent in a reaction process is appreciably less than 100%, the unreacted material is usually separated and recycled. The return of reflux to the top of a distillation coloumn is an example of a recycle process in which there is no reaction.

🖇See more details: https://neutrium.net/unit-operations/distillation-fundamentals/

Without recycle, the material balances on a series of processing steps can be carried out sequentially, taking each unit in turn; the calculated flows out of one unit become the feeds to the next. If a recycle stream is present, then at point where the recycle is returned the flow will not known as it will depend on downstream flows not yet calculated.

Purge🧩

It is usually necessary to bleed off a portion of a recycle stream to prevent the build-up of unwanted material. For example, if reactor feed contains inert components that are not separated from the recycle in the separation units these inerts would accumulate in the recycle stream until the stream eventually consisted of entirely of inerts. Some portion of the stream would have to be purged to keep the inert level within acceptable limits. Under steady-state conditions:

• 💡Loss of inert in the purge= Rate of feed of inerts into the system

🖍Example 6. In the production of ammonia from hydrogen and nitrogen the conversion, based on either raw material, is limited to 15 percent. The ammonia produced is condensed from the reactor (converter) product stream and the unreacted material recycled. If the feed contains 0.2% argon (from nitrogen separation process), calculate the purge rate required to hold the argon in recycle stream below 5%. Percentages are by volume.

🖍🖍Solution:

By pass🤹🏻

A flow stream may be divided and some part diverted( by-passed) around some units. This procedure is often used to control stream composition or temperature. Material balance calculations on processes with by-pass streams are similar to those involving recycle, except that the stream is fed forward instead of backward. This usually makes the calculations easier than with recycle.

✅Finally, the last and general part

General Procedure for Material-Balance Problems🤹🏾‍♂️

Procedure 🎷:

• Draw a block diagram of the process; Show each significant step as a block, linked by lines and arrows to show the stream connections and flow direction. (as shown in the last solution);

• List all available data. Show on the block diagram the known flows (or quantities) and stream compositions;

• List all information required from the balance;

• Write out all the chemical reactions involved for the main products and by-products.

• Check the number of conservation equations that can be written and compare with unknowns

• Decide the basis of calculation

References 📖

1. Coulson&Richardson, Chemical Engineering Design Volume 6

2. CHOPEY, N. P. (ed.) Handbook of Chemical Engineering Calculations (McGraw-Hill, 1984).

3. FELDER, R. M. and ROUSSEAU, R. W. Elementary Principles of Chemical Processes, 6th edn (Pearson, 1995).

4. HIMMELBLAU, D. M. Basic Principles and Calculations in Chemical Engineering (Prentice-Hall, 1982).

5. RUDD, D. F., POWERS, G. J. and SIIROLA, J. J. Process Synthesis (Prentice-Hall, 1973).

6. WHITWELL, J. C. and TONER, R. K. Conservation of Mass and Energy (McGraw-Hill, 1969).

7. WILLIAMS, E. T. and JACKSON, R. C. Stoichiometry for Chemical Engineers (McGraw-Hill, 1958).

Additional Resources 🧪

🔑You can get deep insight about Process/Chemical Engineering from these sources😉:

1. https://t.me/ebookgate- Chemical Engineering E-books (Telegram Channel)

2. https://www.youtube.com/channel/UCqioh32NOJc8P7cPo3jHrbg- Piping Analysis

3. https://www.youtube.com/channel/UCQfMyugsjrVUWU0v_ZxQs2Q -Mechanics of engineered devices

4. http://chemicalengineeringguy.com/- suggests a wide range of courses in Chemical engineering (you can find free courses on topic of Aspen HYSYS, Aspen Plus)

5. https://www.youtube.com/user/LearnEngineeringTeam- suggests working principles of every engineered devices, equipment and etch.

6. https://www.youtube.com/channel/UCR0EfsRZIwA5TVDaQbTqwEQ- suggests great information about pumps, compressors with animation.

🔌Today we have already learned about whole Fundamentals of Material Balances, recycle, by-pass, excess reagent, purge now time to say goodbye👋🏻 until tomorrow and Stay tuned for more content 😉🌝✨!

✏️Note: If you need one of those books or links, you can contact me via my email or LinkedIn profile.

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