L4-5 Processes and Process Variables

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Bypass and Recycle Processes

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Objectives

At the end of this chapter, you should be able to:

• Apply material balances for processes involving bypass and recycle

• Derive and solve material balance problems

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Recycle

• One of the more common processing configurations is the material recycle structure

• These are particularly useful for reactors, where they allow better control of reactor selectivity when multiple reactions occur

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Recycle

• When we study recycle systems, we are often asked to calculate the recycle ratio.

• Usually, this is found by dividing the mass flow of the recycle stream by the mass flow of the "fresh feed" entering the system.

• In the industrial world, recycle ratios have important consequences for system performance and operating costs

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Recycle

• A common recycle structure is the reactor/separator

which is used to recover unreacted material and return it to the reactor. The "separator" may be a single piece of equipment or it may be an entire process on its own

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Bypass A related process structure is the bypass system

One use of bypass is to obtain precise control of the output stream, as when a small wet air stream bypasses a drier so that the output humidity can be regulated

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Solving Recycle and Bypass Problems

• The methods for solving recycle and bypass problems are basically the same.

• In the steady state, there is no buildup or depletion of material within the system or recycle stream of a properly designed and operated process.

• When solving, you can write balances (total material or component) around:

– the entire process structure – the mixing point – the splitter – the processing unit (inside the recycle/bypass)

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Recycle and bypass

OUTOUTININ

RecycleRecycle

ININOUTOUT

“Mixing Points”

“Splitting Points”

BypassBypass

Note:

At bypass, the components mass fractions remain the same. The ONLY difference is the mass flow rate.

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Solving Recycle and Bypass Problems

• Only three of these will be independent (the fourth is a linear combination).

• If you pick the right balances, you may be able to organize the problem for sequential solution.

• In particular, when you write the balance around the entire process system, terms describing the recycle/bypass stream do not appear; only the fresh feed and the product are required.

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Example 1

Given the process shown, find the recycle flow in pounds/hour, the production rate of potassium nitrate, and the recycle ratio

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Example 1

• You are asked to find three things: • (i) the recycle flow (labeled R on the drawing), • (ii) the production rate (labeled C on the drawing),

and• (iii) the recycle ratio, which will be calculated as

R/10000 if we don't change the basis

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Example 1

• The sketch is already done, so we need to label the variables.

• Let's call the fresh feed F. • If we look over the compositions, we'll note that

they are not consistently represented. – Two are given as %KNO3, one as %H2O, and

one as lb KNO3 per lb H2O. • We'd usually prefer all the numbers to be in

percent, so let's do the problem using percent nitrate.

• Shifting the water percentage to nitrate is easy, just subtract from 100.

• The other stream (R) requires a calculation

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Example 1

• Do we really need to calculate the mass fraction nitrate in R? Since we're asked two questions about the stream, it seems almost certain. To do that calculation, look at the composition given and choose a basis for computing the composition (we can change the basis for the rest of the problem)

• Basis : 1 lb H2O in water in stream R

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Example 1

• Step 1: Make a KNO3 balance over the entire system

• Step 2: Make a balance around the crystallizer

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Example 2

Fresh air containing 5.00 mole% water vapour is to be cooled and dehumified to a water content of 2.00 mole% H2O. A stream of fresh air is combined with a recycle stream of previously dehumidified air and passed through the cooler. The blended stream entering the unit contains 2.50 mole% H2O. In the air conditioner, some of the water in the feed stream is condensed and removed as liquid. A fraction of the dehumidified air leaving the cooler is recycled and the remainder is delivered to a room.

Taking 150 mol of dehumidified air delivered to the room as a basis of calculation, calculate the moles of fresh feed, moles of water condensed, and moles of dehumidified air recycled.

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Example 2

AIR COND0.950 mol DA/mol

0.050 mol W(v)/mol

0.975 mol DA/mol

0.025 mol W(v)/mol

0.98 mol DA/mol

0.020 mol W(v)/mol

0.98 mol DA/mol

0.020 mol W(l)/mol

0.98 mol DA/mol

0.020 mol W(v)/mol

mol 150)( 1 moln

)( 5 moln

)( 2 moln )( 4 moln

)]( [ 3 lWmoln

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Purge

• A stream bled off to remove accumulation of inert or unwanted material that might otherwise build up in the recycle stream

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Purge - Example

• Two of the main gases that can be generated under suitable conditions from in-situ coal combustion in the presence of steam are H2 and CO. After cleanup, these two gases can be combined to yield methanol according to the following equation

CO + 2H2 CH3OH

• The figure below illustrates a steady state process. All compositions are in the mole fractions. The stream flows are in moles.

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Purge - Example

Feed, F

67.3% H2

32.5% CO

0.2 CH4

REACTOR SEPARATORE

100% CH3OH

Purge, PRecycle, R

x H2

y COz CH4

You will note that some methane enters the process. A purge stream is used to maintain the CH4 concentration in the exit of the separator at no more than 3.2 mol%. The conversion of the CO in the reactor is 18%.Compute:

– Moles of recycle, CH3OH and purge– Purge gas composition

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Example 2

4500 kg/h of a solution containing 1/3 sugar( by mass) is mixed with a recycle stream containing 36.4% sugar and the combined stream is fed into an evaporator. The concentrated stream leaving the evaporator contains 49.4% sugar is fed into a crystallizer, where it is cooled ( in order to form sugar crystals) and filtered. The filter cake consists of sugar crystals and a solution with 36.4% sugar by mass and the crystals account for 95% of the total mass of the filter cake. The solution that passes through the filter, also 36.4% sugar is the recycle stream.

Calculate the rate of evaporation, rate of production of crystalline sugar, and the recycle ratio ( mass of recycle/mass of feed)

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• You have learnt– To derive and solve material balance

problems on recycle processes

Conclusions