AE 205 Materials and Energy Balances Asst. Prof. Dr. Tippabust Eksangsri Chapter 5: Material Balance for Multi-Unit Operations Process Flowsheet (Flow Chart) 1. Process Information: key properties data, market conditions, technical information, etc., are collected. 2. Input/Output Diagram: All major inputs and outputs streams and their stoichiometric balances are illustrated, according to the potential chemical pathways. 3. Functions Diagram: All the major functions of the process and materials flow to and from those functions are specified. The functions include reaction boxes, 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 known as a recycle stream. • Because of relatively high cost of industrial feeds, recycle of unused reactants/feed is a common practice. • Recovery of catalyst, circulation of working fluids, dilution of process streams, control of process variables, etc., are all reasons for applications of recycle 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 the combined stream is fed into an evaporator. The concentrated stream leaving the evaporator contains 49.4% : this stream is fed into a crystallizer in which it is cooled (causing crystals of K2CrO4 to come out of solution) and then filtered. The filter cake consists of crystal and a solution that contains 36.4% K2CrO4 by mass; the crystals account for 95% of total mass of filter cake. The solution that 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, and the 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 and hydrogen gas. C6H6 + 3H2 C2H12 After the reaction in the reactor, the gross product is separated to spit the net product and the recycle stream. Determine the ratio of the recycle stream to the fresh feed stream if the overall conversion through the reactor is 95% and the single-pass conversion through the reactor is 20%. Assume that 20% excess of hydrogen is used in the fresh feed, and that the composition of 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? Recycle Feed 40% glucose in water 4% fructose Fixed-bed Reactor Product Purging A problem may arise in processes that involve recycling. Suppose a material that enters with the fresh feed or is produced in a reaction remains entirely in a recycle stream, rather than being carried out in a process product. If nothing were done about this situation, the substance would continuously enter the process and would have no way of leaving; it would therefore steadily accumulate, making the attainment of steady state impossible. To prevent this buildup. A portion of the recycle stream must be withdrawn as a purge stream to rid the process of the substance in 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 0.4 mole% inerts. The reactor effluent passes to a condenser that removes essentially all of the methanol and water formed and none of the reactants or inerts. The unused reactants and inerts 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 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 from a metal finishing plant. The wastewater stream is fed to a treatment unit that removes 95% of the chromium in the feed and recycles it to the plant. The residual 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 the wastewater leaves the finishing plant at a rate higher than the capacity of the treatment unit, the excess by passes the unit and combines with the residual liquid 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 the flow rate of liquid to the waste lagoon, and the mass fraction of Cr in this liquid. Example 8: Bypassing wastewater to waste Lagoon
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