91Ó°ÊÓ

A drug (D) is produced in a three-stage extraction from the leaves of a tropical plant. About 1000 kg of leaf is required to produce 1 kg of the drug. The extraction solvent (S) is a mixture containing 16.5 wt\% ethanol (E) and the balance water (W). The following process is carried out to extract the drug and recover the solvent. 1\. A mixing tank is charged with \(3300 \mathrm{kg}\) of \(\mathrm{S}\) and \(620 \mathrm{kg}\) of leaf. The mixer contents are stirred for several hours, during which a portion of the drug contained in the leaf goes into solution. The contents of the mixer are then discharged through a filter. The liquid filtrate, which carries over roughly \(1 \%\) of the leaf fed to the mixer, is pumped to a holding tank, and the solid cake (spent leaf and entrained liquid) is sent to a second mixer. The entrained liquid has the same composition as the filtrate and a mass equal to \(15 \%\) of the mass of liquid charged to the mixer. The extracted drug has a negligible effect on the total mass and volume of the spent leaf and the filtrate. 2\. The second mixer is charged with the spent leaf from the first mixer and with the filtrate from the previous batch in the third mixer. The leaf is extracted for several more hours, and the contents of the mixer are then discharged to a second filter. The filtrate, which contains \(1 \%\) of the leaf fed to the second mixer, is pumped to the same holding tank that received the filtrate from the first mixer, and the solid cake- -spent leaf and entrained liquid - is sent to the third mixer. The entrained liquid mass is \(15 \%\) of the mass of liquid charged to the second mixer. 3\. The third mixer is charged with the spent leaf from the second mixer and with \(2720 \mathrm{kg}\) of solvent \(\mathrm{S}\). The mixer contents are filtered; the filtrate, which contains \(1 \%\) of the leaf fed to the third mixer, is recycled to the second mixer; and the solid cake is discarded. As before, the mass of the entrained liquid in the solid cake is \(15 \%\) of the mass of liquid charged to the mixer. 4\. The contents of the filtrate holding tank are filtered to remove the carried-over spent leaf, and the wet cake is pressed to recover entrained liquid, which is combined with the filtrate. A negligible amount of liquid remains in the wet cake. The filtrate, which contains \(\mathrm{D}, \mathrm{E},\) and \(\mathrm{W},\) is pumped to an extraction unit (another mixer). 5\. In the extraction unit, the alcohol-water-drug solution is contacted with another solvent (F), which is almost but not completely immiscible with ethanol and water. Essentially all of the drug (D) is extracted into the second solvent, from which it is eventually separated by a process of no concern in this problem. Some ethanol but no water is also contained in the extract. The solution from which the drug has been extracted (the raffinate) contains \(13.0 \mathrm{wt} \% \mathrm{E}, 1.5 \% \mathrm{F},\) and \(85.5 \%\) W. It is fed to a stripping column for recovery of the ethanol. 6\. The feeds to the stripping column are the solution just described and steam. The two streams are fed in a ratio such that the overhead product stream from the column contains \(20.0 \mathrm{wt} \% \mathrm{E}\) and \(2.6 \% \mathrm{F},\) and the bottom product stream contains \(1.3 \mathrm{wt} \% \mathrm{E}\) and the balance \(\mathrm{W}\). Draw and label a flowchart of the process, taking as a basis one batch of leaf processed. Then calculate (a) the masses of the components of the filtrate holding tank. (b) the masses of the components \(D\) and \(E\) in the extract stream leaving the extraction unit. (c) the mass of steam fed to the stripping column, and the masses of the column overhead and bottoms products.

Step by step solution

01

Draw a flow chart

Start the process by clearly drawing a flowchart for the entire process, making sure to label all the streams (fluent, spent leaf, solvent etc.) and their conditions as stated in the problem. Keep a clear focus on mass flow rates and the composition of each stream that is given in the problem.

02

First mixer

In the first mixer, \(620 \mathrm{kg}\) of leaf and \(3300 \mathrm{kg}\) of solvent S are mixed. The filtrate consists of \(1 \%\) of waste leaf and \(15 \%\) of entrained liquid, which is mass equal to \(15 \%\) of liquid charged to the mixer (0.15*3300 = 495 Kg). Thus the filtrate has a total mass of \(495 + 6.2 = 501.2 \mathrm{kg}\). This mass of filtrate is transferred to a second mixer.

03

Second mixer

In the second mixer, the filtrate is combined with the spent leaf from the first mixer. The solid cake consists of \(15 \%\) of entrained liquid, which is \(15 \%\) of liquid in the second mixer (0.15*501.2 = 75.18 kg). Thus the liquid filtrate leaving the second mixer has a mass of \(501.2 - 75.18 = 426.02 \mathrm{kg}\) and is transferred to the third mixer.

04

Third mixer

The third mixer is filled with the filtrate from the second mixer and \(2720 \mathrm{kg}\) of Solvent S. Entrained liquid here is \(15 \%\) of the total liquid (429.182*0.15 = 64.377 Kg). So, the filtrate leaving the third mixer to the holding tank has a mass of \(429.182 - 64.377 = 364.805 \mathrm{kg}\). The residue from the third mixer is discarded.

05

Extraction Unit

In the extraction unit, filtrate (which contains D, E, and W) comes into contact with solvent F. Nearly all the drug is transferred to F, along with some of E. Mass conservation applies here to track the components in the feed, extract and raffinate streams. This allows the determination of D and E masses in the extract.

06

Stripping column

The bottom product from the stripping column has a mass being the difference of the feed to the column and the overhead product. The mass of the steam fed can be determined from the mass balance of E and F in the feed, overhead and bottom product streams.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Mass Balance

The foundation of chemical engineering and process design is the mass balance principle. It dictates that matter cannot disappear or be created; it can only be transferred from one form to another within a process. Understanding the movement and conservation of mass through different stages of production is crucial for determining the efficiency and economics of a process. In the given exercise, we use the mass balance principle to trace the components of the drug extraction process: the solvent (S), the ethanol (E), the water (W), and the drug (D).

For every stage in the process, calculations can be conducted to ensure that the input mass equals the output mass, accounting for any losses or accumulations in the system. For instance, in the first mixer, the mass balance is used to calculate the amount of filtrate, which then feeds into the second mixer. With each transition from one stage to the next, the mass balance equations provide the necessary insights to calculate the component masses at each step, from mixing to solvent recovery.

Filtration Process

Filtration is a vital separation technique in both the laboratory and industry, used to separate solids from liquids by passing a mixture through a medium that allows only the liquid to pass through. In this problem, filtration is used multiple times to separate the leaves containing the drug from the extraction solvent. During the process, a certain amount of leaf is carried over in the filtrate, a situation that is handled in subsequent steps through additional filtrations.

It's essential to understand the efficiency of the filtration process, which involves evaluating the percentage of solids removed and the clarity of the filtrate. In the context of this drug extraction process, the exercise highlights the capture of a portion of the drug into the solution during stirring and how some leaf is entrained in the subsequent filtrate. To improve upon this process, engineers might focus on optimizing the ratio of leaves to solvent for better extraction yields and reducing solvent loss.

Solvent Extraction

Solvent extraction is a separation process by which compounds are transferred from a liquid phase to a solvent in which they are more soluble. It's often used to refine or concentrate substances from a mixture by selective removal or partitioning of the desired component. In this exercise, the drug (D) is transferred from a water-ethanol mixture to solvent (F), with careful attention paid to the relative solubilities and partition coefficients.

The solvent extraction step is crucial for isolating the drug efficiently, minimizing the loss of ethanol (E) and completely leaving out the water (W) in the raffinate. This step helps purify the drug from the rest of the mixture. The optimization of this process is vital, as it directly influences the purity and yield of the drug. By understanding the interactions between the components and the solvents, chemical engineers can design solvent extraction steps that maximize efficiency and reduce costs.

Flowchart Diagramming

Flowchart diagramming is a powerful tool for visualizing process flow and serves as a blueprint for understanding and designing chemical processes. In the context of chemical engineering, a flowchart helps to clarify the sequence of operations, the stream compositions, and the mass transfers between stages.

For students and professionals alike, constructing a clear and logical flowchart is the first step in process analysis and design. As seen in the exercise, a well-crafted flowchart provides a visual representation of each step in the drug extraction process, including mixing, filtration, and solvent extraction stages. Flowcharts also aid in identifying areas where improvements can be made, such as streamlining processes to reduce waste or energy consumption. By mastering flowchart diagramming, chemical engineers can improve their ability to design, analyze, and optimize processes.