Chapter 5:Material Balance for

Multi-Unit Operations

AE 205 Materials and Energy Balances

Asst. Prof. Dr. Tippabust Eksangsri

Process Flowsheet (Flow Chart)

1. Process Information: key properties data, market conditions, technicalinformation, etc., are collected.

2. Input/Output Diagram: All major inputs and outputs streams and theirstoichiometric balances are illustrated, according to the potential chemicalpathways.

3. Functions Diagram: All the major functions of the process and materialsflow to and from those functions are specified. The functions include reactionboxes, separation boxes, finishing boxes, etc.

4. Operations Diagram: Detail technologies used such as types of reactor,operating conditions, etc. are specified.

5. Process Flowsheet: Details for each function are described, ready to use.

Process Flowsheet Development

Sodium Dodecylbenzene Sulfonate Surfactant Manufacturing

Input/Output Diagram

Function Diagram

Operation Diagram

Process Flowsheet

MB for Multiple Units Process

Example 1: on MB for Multi-units Process

Example 2: on MB for Multi-units Process

Recycle System

• The stream containing the recycled material is knownas a recycle stream.

• Because of relatively high cost of industrial feeds,recycle of unused reactants/feed is a commonpractice.

• Recovery of catalyst, circulation of working fluids,dilution of process streams, control of processvariables, etc., are all reasons for applications ofrecycle system.

• Recycle ratio = mass of recycle/mass of fresh feed

Example 3: Evaporative Crystallizer

Forty-five hundred kilograms per hour of a solution that is one-third K2CrO4

by mass is joined by a recycle stream containing 36.4% K2CrO4, and thecombined stream is fed into an evaporator. The concentrated stream leavingthe evaporator contains 49.4% : this stream is fed into a crystallizer in which itis cooled (causing crystals of K2CrO4 to come out of solution) and thenfiltered.

The filter cake consists of crystal and a solution that contains 36.4% K2CrO4 bymass; the crystals account for 95% of total mass of filter cake. The solutionthat passes through the filter, also 36.4% K2CrO4, is the recycled stream.

Calculate the rate of evaporation, the rate of production of crystalline K2CrO4,the feed rates that the evaporator and the crystallizer must be designed, andthe recycle ratio

The flowchart of steady state process to recover crystalline potassium chromate (K2CrO4) from an aqueous solution of this salt is shown below.

Recycle with Chemical Reaction

Example 4: Cyclohexane Production

Cyclohexane can be made from the reaction of benzene andhydrogen gas.

C6H6 + 3H2 C2H12

After the reaction in the reactor, the gross product is separatedto spit the net product and the recycle stream. Determine theratio of the recycle stream to the fresh feed stream if the overallconversion through the reactor is 95% and the single-passconversion through the reactor is 20%. Assume that 20% excessof hydrogen is used in the fresh feed, and that the compositionof recycle stream is 22.74mol% benzene and 78.6mol%hydrogen.

Example 5: Fructose Production from Glucose

Immobilized glucose isomerase is used as a catalyst in producing fructose from glucose in a fixed-bed reactor (water is a solvent).

For the system shown below, what percent conversion of glucose results on one pass through the reactor when a ratio of the exit stream to the recycle stream in mass units is equal to 8.33?

Fixed-bed Reactor

Recycle

Feed Product

40% glucose in water

4% fructose

Purging

A problem may arise in processes that involve recycling. Supposea material that enters with the fresh feed or is produced in areaction remains entirely in a recycle stream, rather than beingcarried out in a process product. If nothing were done about thissituation, the substance would continuously enter the processand would have no way of leaving; it would therefore steadilyaccumulate, making the attainment of steady state impossible.To prevent this buildup. A portion of the recycle stream must bewithdrawn as a purge stream to rid the process of the substancein question.

Example 6: on Purging in Ethylene Oxide Production

Example 7: Recycle and Purge in the synthesis of Methanol

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

CO2 + 3H2 CH3OH + H2O

The fresh feed to the process contains hydrogen, carbon dioxide, and 0.4mole% inerts. The reactor effluent passes to a condenser that removesessentially all of the methanol and water formed and none of the reactants orinerts. The unused reactants and inerts are recycled to the reactor. To avoidbuildup of the inerts in the system, a purge stream is withdrawn from therecycle.

The feed to the reactor contains 28.0 mole% CO2, 70.0mole% H2, and 2.0 mole% inerts. The single-pass e conversion of hydrogen is 60%. Calculate the molar flow rates and molar compositions of the fresh feed, the total feed to the reactor, the recycle stream, and the purge stream for a production rate of 155 kmol CH3OH /hr.

Recycle and Purge in the synthesis of Methanol

Bypass

Bypass is a fraction of feed stream that is diverted around the unit and combined with the output streams of such a unit. By varying the fraction of the feed that is bypassed, we can vary the composition and properties of the product.

A stream containing 5.15wt% chromium is contained in the wastewater froma metal finishing plant. The wastewater stream is fed to a treatment unit thatremoves 95% of the chromium in the feed and recycles it to the plant. Theresidual liquid stream leaving the treatment unit is sent to a waste lagoon.The treatment unit has a maximum capacity of 4500 kg wastewater/hr. If thewastewater leaves the finishing plant at a rate higher than the capacity of thetreatment unit, the excess by passes the unit and combines with the residualliquid leaving the unit, and the combined stream goes to the waste lagoon.

Wastewater leaves the finishing plant at a rate of 6,000 kg/hr. Calculate theflow rate of liquid to the waste lagoon, and the mass fraction of Cr in thisliquid.

Example 8: Bypassing wastewater to waste Lagoon