United Arab Emirates University

Faculty of Engineering

Chemical & Petroleum Engineering Department

This lecture aims at introducing more practical examples on This lecture aims at introducing more practical examples on balances on reactive systems. balances on reactive systems. Upon completion this lecture, you should be able to do the Upon completion this lecture, you should be able to do the followings:followings: List the balance methods used in reactive systemsList the balance methods used in reactive systems Write the balance equation of each balance method used in Write the balance equation of each balance method used in reactive systems reactive systems Define recycle streamsDefine recycle streamsDiscuss the importance of recycle streamsDiscuss the importance of recycle streams Define single pass, and overall conversionsDefine single pass, and overall conversionsDefine and identify purge streamsDefine and identify purge streamsExplain the importance of purge streamsExplain the importance of purge streamsGiven a process description that has recycle and purge streams, (a) Given a process description that has recycle and purge streams, (a) draw and fully label a flowchart; (b) choose a convenient basis of draw and fully label a flowchart; (b) choose a convenient basis of calculation; (c) write the balance equations needed; and (d) perform calculation; (c) write the balance equations needed; and (d) perform the calculationsthe calculations

Balances on Reactive Systems

Molecular Balance Method

Atomic Balance Method

Extent of Reaction Method

Extent of Reaction Method

• Molecular Balance Method: Input +Generation = Output + Consumption + Accumulation

• Extent of Reaction Method:(ni)out = (ni)in +jijj

• Atomic Balance Method:Input = Output

• Consider the process shown below, can you define a recycle stream?

Reactor Separator

Recycle Stream

Chemical reactions usually do not proceed to completion in the reactors, and thus, the reactor effluent will have some amounts of the reactants. To discard these reactants will cost you money. We can separate these reactants from the product and Recycle them to the reactor so that they can be used again. The stream that is recycled to the reactor is called a

There are other reasons for using recycle streams in chemical processes, such as

Recovery of the catalyst Control of process variables (e.g., T, P)

As just discussed, unreacted reactants may be separated and recycled to the reactor. Then, two types of conversion are important for such cases

1. Overall conversion (OC) =

2. Single pass conversion (SPC) =

• Consider the process shown below, calculate the overall and single pass conversions of reactant A.

• Solution:

conversion example

Any Conclusions?????????

50 moles of A

20 moles of A

25 moles of A

• Consider the process shown below, can you define a purge stream?

Purge StreamRecycle Stream

A purge stream is a portion of the recycle stream that is withdrawn from it.

Why Purging?

In some cases when we are using recycling and one material that is entered with the fresh feed or is produced in a reaction remains entirely in the recycle stream, this material would continuously enter the process and would have no way of leaving, it would therefore accumulate, making the attainment of steady state impossible. To prevent this accumulation, a portion of the recycle stream must be withdrawn as a purge stream.

Notice that the purge stream will have the same composition as the recycle stream.

The Al Jubail (AR RAZI 4) methanol plantSee the web-sites: http://www.chemicalstechnology.com/projects/aljubail/http://www.chemicals-technology.com/projects/starchem/www.nhtsa.dot.gov/cars/rules/rulings/ CAFE/alternativefuels/availability2.htm www.mathjmendl.org/AltFuels/default.htmhttp://en.wikipedia.org/wiki/Fuel_cell

Methanol is produced in the reaction of carbon dioxide and hydrogen:

CO2 + 3 H2 CH3OH + H2O

The fresh feed to the process contains hydrogen, carbon dioxide, and 0.400 mole% inert (I). The reactor effluent passes to a condenser that removes essentially all of the methanol and water formed and none of the reactants or inert. The later substances are recycled to the reactor. To avoid buildup of the inerts in the system, a purge stream is withdrawn from the recycle. The feed to the reactor contains 28.0 mol% CO2 , 70.0 mol% H2 and 2.0 mol% inerts. The single-pass conversion of hydrogen is 60%. For

a methanol production rate of 155 kmol/h, Calculate: the molar flow rate and molar compositions of i) the fresh feed; ii) the total feed to the reactor; iii) the recycle stream, and the purge stream.

Solution:Methanol Production

The general balance equation is given by:

Input + Generation = Output + Consumption + Accumulation

Atomic Balance, Extent of Reaction, and Molecular Balance are the common methods used in carrying out Balances on reactive systems.

A recycle stream is used to send unused raw materials emerging from a process unit back to the unit.

A purge stream is used to prevent the accumulation of inert materials in the reactor when using recycling.