Difference between Conversion and Yield

It is important to differentiate between conversion and yield. Sometimes, these terms are mixed up. So, in this post, I want to share the difference between conversion and yield. I also will show you three examples of these terms.

Conversion is to do with reactants (reagents). On the other hand, yield is to do with products.

Conversion

Conversion is a measure of the fraction of the reagent (reactant) that reacts. The conversion of a particular reagent is often less than 100% to optimize reactor design and to minimize formation of by-product.

If more than one reactant is used, the reagent on which the conversion is based must be specified.

Conversion is defined by the following equation:

This definition gives the total conversion of specific reagent to all products. 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 reagent/reactant. This is a really a way of expressing yield.

Yield

Yield is a measure of the performance of a reactor or plant. Several different definitions of yield are used, and it is important to clearly state the basis of any yield figures. This is often not done when yield figures are quoted in the literature, and the judgement must be used to decide what was intended.

For a reactor, the yield is defined by:

There is term “reaction yield” (chemical yield), which includes only chemical losses to side product. And there is also “overall reactor yield” which includes physical losses.

Plant yield is a measure of the overall performance of the plant and includes all chemical and physical losses. Plant yield is expressed by this formula:

When more than one reagent is used, or product produced, it is essential that product and reagent to which the yield figure refers is clearly stated.

Example 1 (Related to Conversion)

In the manufacture of vinyl chloride (VC, C₂H₃Cl) by pyrolysis of dichloroethane (DCE, C₂H₄Cl₂), the reactor conversion is limited to 50% to reduce carbon formation. Calculate the quantity of DCE needed to produce 5000 kg/h VC.

Solution

Reaction:

C₂H₄Cl₂ → C₂H₃Cl + HCl

Mol of C₂H₃Cl = mass flow rate/molecular weight = 5000/62.5 = 80 kmol/h

From the stoichiometric equation, 1 kmol C₂H₄Cl₂ produces 1 kmol C₂H₃Cl. Therefore, required quantity of C₂H₄Cl₂ = 80 / 50% = 160 kmol/h

Example 2 (Related to Yield and Conversion)

In the production of ethanol by hydrolysis of ethylene, diethyl ether is produced as a by-product. A typical feed stream composition is: 56% ethylene, 5% inert, 40% water; and product stream: 52.26% ethylene, 5.49% ethanol, 0.16% ether, 36.81% water, and 5.28% inert. Calculate the yield of ethanol and ether based on ethylene.

Reactions:

C₂H₄ + H₂O → C₂H₅OH

2C₂H₅OH → (C₂H₅)₂O + H₂O

Basis: 100 mols feed.

As the inert will not react or converted, the amount will be the same at input and output.

Feed stream:

• C₂H₄ = 56 mol
• Inert = 5 mol
• H₂O = 39 mol

Product:

• Inert = 5 mol (the same as inlet)
• C₂H₄ = (52.26% / 5.28%) × 5 mol = 49 mol
• C₂H₅OH = (5.49% / 5.28%) × 5 mol = 20 mol
• (C₂H₅)₂O = (0.16% / 5.28%) × 5 mol = 15 mol
• H₂O = (36.81% / 5.28%) × 5 mol =9 mol

Amount of C₂H₄ reacted or converted = amount of C₂H₄ in feed stream – amount of C₂H₄ in product stream = 56 – 49.49 = 6.51 mol

Yield calculation of C₂H₅OH based on C₂H₄:

• Yield of C₂H₅OH based on C₂H₄ = mols of C₂H₅OH produced × stoichiometric factor/mols of C₂H₄ reacted or converted
• Stoichiometric factor: 1 mol C₂H₄ required per mol C₂H₅OH produced
• Therefore, yield of C₂H₅OH based on C₂H₄ = 5.20 × 1 / 6.51 = 8%

Yield calculation of (C₂H₅)₂O based on C₂H₄:

• Yield of (C₂H₅)₂O based on C₂H₄ = mols of (C₂H₅)₂O produced × stoichiometric factor/mols of C₂H₄ reacted or converted
• Stoichiometric factor: 1 mol C₂H₄ required per mol (C₂H₅)₂O produced
• Therefore, yield of (C₂H₅)₂O based on C₂H₄ = 0.15 × 2 / 6.51 = 65%

Conversion of C₂H₄ to all products = (amount of C₂H₄ in feed – amount of C₂H₄ in product) / amount of C₂H₄ in feed = (56 – 49.49) / 56 = 11.63%

Example 3 (Related to Overall Yield and Chemical Yield)

In the chlorination of ethylene to produce dichloroethane (DCE), the conversion of ethylene is reported as 99%. If 93 mol of DCE are produced per 100 mol of ethylene fed, calculate the overall yield and the reactor (reaction) yield based on ethylene. The unreacted ethylene is not recovered.

Solution

Reaction:

C₂H₄ + Cl₂ → C₂H₄Cl₂

Stoichiometric factor is 1 (1 mol of C₂H₄ required for 1 mol of C₂H₄Cl₂ produced).

There is difference between overall yield and chemical yield.

• Overall yield (including physical losses) = mol C₂H₄Cl₂ produced × stoichiometric factor / mol C₂H₄ fed = 94 x 1 / 100 = 94%
• Chemical yield (reaction yield) = mol C₂H₄Cl₂ produced × stoichiometric factor / mol C₂H₄ converted = 94 x 1 / 99 = 94.95%

I hope this quite long post is useful for you 😊