Name: steam distillation and low-boiling separation
Uploaded: 2020-03-26T07:22:28.000Z
Duration: 3 min 54 s
Channel: JoVE

Wenn ein ideales Gemisch aus zwei mischbaren Flüssigkeiten zum Sieden erhitzt wird, siedet die Lösung bei einer Temperatur zwischen den Siedepunkten der einzelnen Komponenten. Wenn diese Flüssigkeiten sehr unterschiedliche Siedepunkte haben, ist der Dampf, wenn das Gemisch zu kochen beginnt, reich an den Molekülen der flüchtigeren Komponente. Dieses Phänomen wird häufig genutzt, um Gemische durch einfache Destillation zu trennen, bei der ein Gemisch aus zwei mischbaren Flüssigkeiten erhitzt wird und der Dampf dann wieder zu Flüssigkeit kondensiert und gesammelt wird.
Da der Dampf, der reich an der flüchtigeren Komponente ist, als Destillat gesammelt wird, wird die flüssige Phase reich an den Molekülen der weniger flüchtigen Komponente. Diese Technik erfordert jedoch, dass die Lösung mindestens bis zum Siedepunkt der flüchtigeren Verbindung und oft darüber hinaus erhitzt wird.
Bei temperaturempfindlichen organischen Verbindungen könnte diese hohe Temperatur dazu führen, dass sich die organischen Moleküle in etwas anderes zersetzen. Wie können wir also diese Art von Verbindungen trennen? Lassen Sie uns zunächst einen Schritt zurückgehen.
Es sei daran erinnert, dass der Druck eines Dampfes, der sich im Gleichgewicht mit seiner kondensierten Phase befindet, als Dampfdruck bezeichnet wird. Die Bestandteile eines Gemisches von Flüssigkeiten haben jeweils einen eigenen Dampfdruck, den wir ihren Partialdruck nennen. Wir wissen, dass eine Lösung siedet, wenn der Gesamtdampfdruck der Lösung dem Atmosphärendruck entspricht. Der Gesamtdampfdruck ist gleich der Summe der Partialdrücke der Komponenten.
Für ein Gemisch mischbarer Flüssigkeiten, d.h. dass jede Kombination der Flüssigkeiten eine homogene Lösung bildet, werden die Partialdrücke aus den Dampfdrücken der reinen Verbindungen multipliziert mit ihren Molenbrüchen in der Lösung berechnet. Bei einem heterogenen Gemisch nicht mischbarer Flüssigkeiten, d.h. die Flüssigkeiten sind ineinander unlöslich, sind die Partialdrücke jedoch einfach die Dampfdrücke der reinen Verbindungen.
Da jede Komponente des heterogenen Gemisches unabhängig von den anderen Komponenten zum Gesamtdampfdruck beiträgt, siedet das Gemisch, wenn der Gesamtdampfdruck, der die Summe der Partialdrücke ist, gleich dem Atmosphärendruck ist. Dies geschieht bei einer niedrigeren Temperatur als die einzelnen Siedepunkte jeder Komponente, da der Gesamtdampfdruck mit der Temperatur viel schneller ansteigt, als man es selbst für die flüchtigste Komponente erwarten würde.
Wir können uns dieses Phänomen zunutze machen, um eine Wasserdampfdestillation durchzuführen, bei der eine temperaturempfindliche organische Verbindung isoliert wird, die sich unter hoher Hitze zersetzt und in Wasser aus nichtflüchtigen Substanzen unlöslich ist. Die Wasserdampfdestillation ähnelt einer einfachen Destillationsanlage mit einem zusätzlichen Wasserreservoir, um während des gesamten Prozesses Wasser aufzufüllen.
Während die Mischung siedet, werden sowohl das Wasser als auch die interessierende organische Verbindung verdampft. Das Wasser und die Dämpfe der organischen Verbindungen gelangen in den Kondensator, werden zu Flüssigkeit kondensiert und gesammelt. Die nicht mischbaren Flüssigkeiten werden anschließend abgetrennt. In der Mischung im Kolben verbleiben nur Wasser und nichtflüchtige Stoffe.
In diesem Labor richten Sie ein Wasserdampfdestillationsexperiment ein und führen es durch, um ätherisches Öl aus den nichtflüchtigen Bestandteilen einer Orangenschale zu extrahieren. Anschließend verwenden Sie eine Flüssig-Flüssig-Extraktion, um das ätherische Öl aus Wasser in ein organisches Lösungsmittel zu extrahieren.

wasserdampfdestillation

<h4>Wasserdampfdestillation </h4>
Die Wasserdampfdestillation ist eine Trenntechnik, die sich die Eigenschaft des niedrigen Siedepunkts nicht mischbarer Mischungen zunutze macht. Es wird hauptsächlich verwendet, um temperaturempfindliche organische Moleküle von einer nichtflüchtigen Verunreinigung zu trennen. Das organische Molekül muss in Wasser nicht mischbar sein.
Bei der Wasserdampfdestillation wird das nicht mischbare Gemisch bis zum Sieden erhitzt, wodurch sowohl das Wasser als auch die flüchtigen organischen Verbindungen destilliert werden. Das bedeutet, dass das gasförmige Gemisch nach oben zu einem Kondensator gelangt, der den Dampf dann zu Flüssigkeit kondensiert, damit er aufgefangen werden kann. Im Gegensatz zur einfachen Destillation wird bei der Wasserdampfdestillation ein Wasserreservoir verwendet, um das Wasser in der erhitzten Mischung während des gesamten Prozesses wieder aufzufüllen. Die nicht mischbare organische Komponente wird langsam zusammen mit dem Wasser destilliert, während die nichtflüchtige Komponente in der erhitzten Mischung verbleibt. Sobald die organische Komponente destilliert ist, kann sie durch Flüssig-Flüssig-Extraktion vom Wasser getrennt werden.
<h4>Dampfdruck eines Gemisches</h4>
Bei einem mischbaren Gemisch, das eine homogene Lösung bildet, ist der Dampfdruck jeder Komponente nach dem Raoult'schen Gesetz abhängig vom Dampfdruck der reinen Komponente und ihrem Molenbruch in der flüssigen Mischung.
pA = pA*xA
wobei pA der Dampfdruck einer flüssigen Komponente in einem mischbaren Flüssigkeitsgemisch ist, pA* der Dampfdruck der reinen Flüssigkeit und xA der Molenbruch dieser Flüssigkeit in dem Gemisch ist, der gleich nA/nt ist. nA die Anzahl der Mole der einzelnen Flüssigkeit in dem Gemisch ist, und nt ist die Gesamtzahl der Mole aller Flüssigkeiten in der Mischung.
Der Gesamtdampfdruck über dem mischbaren Flüssigkeitsgemisch ist gleich der Summe des Dampfpartialdrucks jeder Komponente darin, was als Daltonsches Gesetz bekannt ist. Der Dampfdruck einer Flüssigkeit steigt mit der Temperatur, da mehr Moleküle kinetische Energie gewinnen, um aus der flüssigen Phase in die Gasphase zu entweichen. In einem mischbaren Gemisch, das zwei Flüssigkeiten enthält, kann der Gesamtdruck wie folgt beschrieben werden:
P = pA + pB
wobei pA und pB die Dampfdrücke der Flüssigkeit A bzw. der Flüssigkeit B über dem Gemisch sind. P ist der Gesamtdampfdruck beider Flüssigkeiten über dem Gemisch. Die Kombination der Gleichungen beschreibt das Verhältnis zwischen dem Gesamtdampfdruck der Lösung und dem Molenbruch der einzelnen Komponenten:
P = pA*xA + pB*xB
In einem nicht mischbaren Gemisch, bei dem die Komponenten ein heterogenes Gemisch bilden, tragen die Dampfdrücke jeder Komponente unabhängig voneinander zum Gesamtdampfdruck bei. Somit ist der Gesamtdampfdruck gleich der Summe der einzelnen reinen Dampfdrücke. In einem nicht mischbaren Gemisch, das aus zwei Flüssigkeiten besteht, ist der Gesamtdruck definiert als der Dampfdruck der ersten Flüssigkeit plus der Dampfdruck der zweiten Flüssigkeit.
P = pA* + pB*
<h4>Siedepunkt eines nicht mischbaren Gemisches</h4>
Wenn eine Flüssigkeit erhitzt wird, steigt der Dampfdruck. Jede Komponente in einem Gemisch hat ihren eigenen Siedepunkt. In einem Gemisch von mischbaren Flüssigkeiten erfolgt das Sieden bei einer Temperatur zwischen den Siedepunkten der konstituierenden Flüssigkeiten.
Bei einem nicht mischbaren Gemisch erfolgt das Sieden bei einer viel niedrigeren Temperatur als die Siedepunkte der einzelnen Komponenten. Da jede einzelne Komponente unabhängig voneinander einen Beitrag leistet, wird weniger Wärme benötigt, um den Gesamtdampfdruck auf den Atmosphärendruck zu erhöhen.
Betrachten Sie zum Beispiel das nicht mischbare Gemisch aus Benzol und Wasser. Der Siedepunkt von Benzol bei normalem Atmosphärendruck liegt bei 80,1 °C und der Siedepunkt von Wasser bei normalem Atmosphärendruck bei 100 °C. Die Lösung siedet, wenn der Gesamtdampfdruck 760 mm Hg (normaler atmosphärischer Druck) erreicht. Bei 69,3 °C beträgt der Dampfdruck von Wasser 227 mm Hg und der Benzoldampfdruck 533 mm Hg, was in Summe den zum Kochen erforderlichen 760 mm Hg entspricht. Dies liegt deutlich unter dem Siedepunkt der einzelnen Komponenten.
<h4>Referenzen</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr., P.M., Townsend, J.R. (2012). Chemie und chemische Reaktivität. Belmont, Kalifornien: Brooks/Cole, Cengage Lernen.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Chemie: Die molekulare Natur von Materie und Veränderung. Boston, MA: McGraw Hill.</li>
</ol>

Wasserdampfdestillation

Wasserdampfdestillation 
Die Wasserdampfdestillation ist eine Trenntechnik, die sich die Eigenschaft des niedrigen Siedepunkts nicht mischbarer Mischungen zunutze macht. Es wird überwiegend ...

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Steam Distillation

steam distillation and low-boiling separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations.

In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser, where it cools and becomes a liquid that can be collected. Unlike simple distillation, this setup uses a water reservoir to keep replacing water in the heated mixture during the process.

The organic component is carried over slowly with the water, while the non-volatile component stays behind in the heated flask. After distillation, the organic product can be separated from the water by liquid-liquid extraction. This makes steam distillation useful for isolating sensitive organic materials from unwanted impurities.

The same low-boiling behavior can be explained by vapor pressure. For a miscible mixture, each liquid follows Raoult’s law, so its vapor pressure depends on the pure liquid vapor pressure and its mole fraction. The mole fraction is the number of moles of one liquid divided by the total number of moles in the mixture. The total vapor pressure above a miscible mixture follows Dalton’s law and equals the sum of the partial vapor pressures.

As temperature rises, vapor pressure increases because more molecules gain enough kinetic energy to escape into the gas phase. In a two-liquid miscible mixture, the total pressure can be written as P = pA + pB, or as P = pA*xA + pB*xB when Raoult’s law is included. In an immiscible mixture, the components act independently, so the total pressure is the sum of the vapor pressure of the first liquid and the vapor pressure of the second liquid.

This independent vapor-pressure behavior is why immiscible mixtures boil at lower temperatures than either pure liquid. In a miscible mixture, boiling usually happens between the boiling points of the components. In an immiscible mixture, less heat is needed to reach atmospheric pressure, so the boiling point drops well below the boiling points of the individual liquids.

A benzene-water mixture shows this effect clearly. Benzene boils at 80.1 °C, and water boils at 100 °C at normal atmospheric pressure. The mixture boils when the total vapor pressure reaches 760 mm Hg. At 69.3 °C, water contributes 227 mm Hg and benzene contributes 533 mm Hg, which adds up to 760 mm Hg. That means the mixture boils at a temperature below either pure component.

Steam Distillation and Low-Boiling Separation

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Steam Distillation and Low-Boiling Separation

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