Instructor: Dr. Ilgin Paker Yikici

Fall 2015

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Define a multiunit process and its main features.

Apply the degrees of freedom analysis and material balance to solve multiunit process problems.

Properly construct a process flowchart and label all known quantities and unknown quantities for a multiunit process involving recycle, bypass, makeup, and purge streams.

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A multiunit process is simply a process that contains more than one unit.

The process consists of one mixing point, two unit operations, and one splitter.

In the process, one can define five subsystems delimited by a boundary: the overall process system (S1), mixing point (S2), and unit one (S3), unit two (S4), and splitter (S5).

The procedure for material balance calculations on single-unit processes can be readily extended to the solution of multiunit processes

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The material balance is applied to each subsystem constituting the multiunit process being investigated and is expressed as

Accumulation = (input − output) + (generation − consumption)

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The proper analysis of a multiunit process problem calls for the isolation of subsystems and application of the material balance to several subsystems of the process.

The goal, again, is to determine all unknown process variables.

For each system, it is strongly recommended that you perform a degrees of freedom analysis to find out if the problem at hand is solvable.

Accordingly, you start by solving the material balance equations of the system that has the number of degrees of freedom equal to 0, that is, NDF = 0.

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Perform a degrees of freedom analysis for the process whose flowchart is shown below:

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The process consists of four subsystems: the overall process, column 1, fixing point, and column 2. For a stream to be fully specified, its flow rate and composition of all of its components must be known.

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Consider the following chemical reaction taking place in a reactor:

A → B

Since it is rare for any chemical reaction to proceed to completion, some of A will remain in the product stream

This is not an ideal situation as some unreacted A is left in the product stream and the final product is not very pure in B.

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There are several reasons for using a recycle stream, including recovery and reuse of unconsumed reactants, recovery of catalyst, dilution of a process stream, control of a process variable, and circulation of a working fluid.

Component A reacts to form a product B in an incomplete reaction. The remaining amount of A in the product stream is separated and recycled back to the reactor.

When solving for the unknown process variables, write material balances (total material or components) around the entire process, the mixing point, the reactor, and the separator.

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Pick the right subsystem (NDF = 0) to organize the problem for a sequential solution. In particular, the balance around the entire process system, terms describing the recycle (or bypass, defined next) stream do not appear in the equations; only the fresh feed and the product stream are considered.

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Bypass is defined as a fraction of the feed to a process unit that is diverted around the unit and combined with the output stream, thus affecting the composition and properties of the product.

Processes involving bypass streams are treated in exactly the same manner as processes carrying recycle streams.

The flow sheet is drawn and labeled, and, in this example, material balances are written down around the splitter, the process unit, or the mixing point downstream of the process unit.

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A purge stream is a small stream bled off from a recycle loop to prevent buildup of inert or impurities in the system.

Often, the purge flow rate is so much smaller than the recycle flow rate that it can be neglected in the steady state overall material balance of the process.

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A makeup stream is required to replace losses to leaks, carryover, etc., within the recycle loop.

Proper sizing and location of makeup and purge streams can prevent many problems within process plants.

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Two separation columns in sequence are used to separate the components of a feed consisting of 30% benzene (B), 55% toluene (T), and 15% xylene (X). The analysis of the overhead stream from the first column yields 94.4% B, 4.54% T, and 1.06% X. The second column is designed to recover 92% of the toluene in the original feed in its overhead stream at a composition of 0.946. The bottoms are intended to contain 92.6% of xylene in the original feed at a composition of 0.776. Calculate the compositions of all streams when feed is 100 kg/h.

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Fresh feed containing 55 wt% A and 45 wt% B flowing at 100 kg/h enters a separator that removes a portion of pure component A only as a bottom product. The top product stream of the separator contains 10 wt% of component A and the balance is B. A small part of the separator product stream is recycled and joined in the fresh feed stream. The other portion is purged. The separator is designed to remove exactly two-thirds of component A fed to it (not the fresh feed). The recycle loop is used to achieve this goal. Compute all unknown stream flow rates and compositions.

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The raw feed to a sulfur removal process contains 15 wt% CO2, 5% H2S, and 1.5% CO, and the balance is CH4. The original absorber design placed a maximum flow rate limit of 80 kg/h. The product stream of the whole process contains 1% H2S, 0.3% CO, and the balance is carbon dioxide and methane. Any feed flow rate in excess of 80 kg/h is bypassed. The absorber absorbs hydrogen sulfide and carbon monoxide only. If the fresh feed to the unit is 100 kg/h, find the flow rates of the product streams.

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Hexane (H) and pentane (P) are separated in a distillation column with a reflux ratio of 0.60 (reflux ratio = Reflux/Distillate = R/D). If the feed contains 50 wt% hexane, the distillate is 5wt% hexane, and the bottom stream is 96wt% hexane, determine the distillate, bottom, recycle, and overhead flow rates on the basis that the mass flow rate of the feed stream is 100 kg/h.

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Fresh feed stream flowing at 100 kg/h contains 20% by weight KNO3 (K) in H2O (W). The fresh feed stream is combined with a recycle stream and is fed to an evaporator. The concentrated liquid solution exited the evaporator contains 50% KNO3 is fed to a crystallizer. The crystals obtained from the crystallizer are 96% KNO3 and 4% water. The liquid from the crystallizer constitutes the recycle stream and contains 0.6 kg KNO3 per 1.0 kg of H2O. Calculate all stream flow rate values and compositions.

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A fresh feed stream contains liquid mixture containing 35 mol% toluene (T), 27 mol% xylene (X), and the remainder benzene (B) is fed to a distillation column. The fresh feed molar flow rate is 100 kmol/h. The bottom product contains 97 mol% X and no B, and 93% of the xylene in the feed is recovered in this stream. The overhead product is fed to a second column. The overhead product from the second column contains 5 mol% T and no X, and 96% of the benzene fed to the system is recovered in this stream. Draw and completely label a flow sheet of the process. Calculate (a) the composition of the bottoms stream from the second column, and (b) the percentage of toluene contained in the process feed that emerges in the bottom product from the second column.

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