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Distillation — Grokipedia Fact-checked by Grok 2 months ago Distillation Ara Eve Leo Sal 1x Distillation is a physical separation process used to purify liquids or separate components of a liquid mixture by exploiting differences in their boiling points or volatilities, involving the evaporation of more volatile components followed by their condensation into a purer liquid form. [1] [2] This technique relies on the principle that components with lower boiling points vaporize at lower temperatures, allowing selective separation when the mixture is heated and the vapors are cooled and collected separately from the residue. [3] [4] Originating in ancient Mesopotamia around 3500 BCE and advancing through Alexandrian, Islamic, and European innovations, distillation has evolved into a cornerstone of chemical engineering . It is widely used in laboratories for purifying compounds and industrially for processes like petroleum refining, beverage production, water desalination, and air separation . [4] [5] [6] Fundamentals Definition and Principles Distillation is a physical separation method that exploits differences in the volatility of components in a liquid mixture to isolate them based on their boiling points. [7] In this process, a mixture is heated to produce vapor enriched in the more volatile (lower boiling point ) components, which can then be separated from less volatile ones remaining in the liquid phase. [6] The distillation process relies on selective evaporation and condensation driven by differences in component volatilities. Upon heating the homogeneous liquid mixture, the component with the lower boiling point (higher volatility) vaporizes preferentially. The resulting vapors, enriched in the more volatile component, are separated from the liquid, cooled in a condenser, and collected as a purified or enriched liquid distillate. Several main types of distillation are employed depending on the properties of the mixture: Simple distillation : Suitable for mixtures with substantial differences in boiling points (typically greater than 50 °C). It employs a basic setup with a distillation flask, direct condenser, and receiver. Fractional distillation : Used for mixtures with closer boiling points. A fractionating column facilitates multiple vaporization-condensation cycles, significantly improving separation efficiency. Vacuum distillation : Conducted under reduced pressure to lower boiling points, making it ideal for heat-sensitive compounds such as essential oils. Steam distillation : Applied to immiscible substances that are sensitive to heat; steam carries the volatile component, facilitating separation without high temperatures. Molecular distillation : Performed at very low pressures for high-viscosity liquids or large molecules, minimizing thermal decomposition. Typical laboratory equipment includes a distillation flask (or round-bottom flask) to hold and heat the mixture, a thermometer to monitor temperature, a fractionating column (for fractional distillation), a condenser (such as Liebig or Graham types), and a receiver flask to collect the distillate. Boiling chips or stones are commonly added to promote smooth boiling and prevent superheating or bumping. The general procedure for distillation involves the following steps: Introduce the mixture into the distillation flask and add boiling chips if necessary. Heat the mixture gradually until boiling begins. Vapors ascend through the apparatus, becoming enriched in the more volatile component(s). The vapors cool and condense in the condenser. The distillate is collected in the receiver, with temperature monitored to identify and separate fractions. The distillation curve (distillate composition versus temperature) provides insight into the progress and purity of collected fractions. The core principle underlying distillation is vapor-liquid equilibrium, where, upon boiling , the vapor phase becomes enriched with more volatile components compared to the liquid phase. [8] This enrichment occurs because components with higher vapor pressures evaporate preferentially, establishing a composition difference between the phases that drives the separation. [9] The basic steps involve heating the mixture to induce vaporization , collecting the vapor, and condensing it to yield a purified fraction, often repeated for greater separation efficiency. [8] For ideal mixtures, Raoult's law governs the behavior by stating that the partial pressure $ P_i $ of component $ i $ in the vapor is given by $ P_i = x_i P_i^\circ $, where $ x_i $ is the mole fraction in the liquid and $ P_i^\circ $ is the vapor pressure of the pure component at that temperature . [10] This law assumes no interactions between components beyond their ideal mixing. A key measure of separability is relative volatility $ \alpha $, defined as $ \alpha = \frac{y_A / x_A}{y_B / x_B} $, where $ y $ denotes vapor mole fractions; higher values of $ \alpha $ indicate easier separation of components A and B. [11] The term "distillation" derives from the Latin destillare , meaning "to drip down" or "trickle," reflecting the process of liquid dripping from a condenser. [12] Thermodynamic Basis Distillation relies on the principles of vapor-liquid equilibrium (VLE), which describes the distribution of components between the liquid and vapor phases in a mixture at equilibrium. For binary mixtures, VLE is graphically represented using T-x-y diagrams, where temperature (T) is plotted against the liquid mole fraction (x) and vapor mole fraction (y) of one component at constant pressure . These diagrams feature a bubble point curve, indicating the temperature at which the first vapor forms as liquid is heated, and a dew point curve, showing the temperature at which the first liquid condenses from vapor upon cooling; the region between these curves represents the two-phase coexistence, essential for understanding separation feasibility in distillation. [13] [14] The thermodynamic constraints on such equilibria are governed by the Gibbs phase rule, which quantifies the degrees of freedom (F) available in a system : $ F = C - P + 2 $, where C is the number of components and P is the number of phases. In a binary distillation system (C = 2) at VLE (P = 2), F = 2, meaning temperature and pressure (or one composition) can be independently specified to define the state, while compositions in both phases are interdependent; this rule ensures that equilibrium conditions are precisely determined, limiting the variability in phase behavior during separation. [15] [16] Energy requirements in distillation stem from the enthalpy of vaporization , the latent heat needed to transition a liquid to vapor, which drives the phase change and mass transfer between stages. Heat balances account for this latent heat in boiling (at the reboiler ) and condensation (at the condenser), where the energy input must overcome the enthalpy difference between liquid and vapor phases; for instance, sensible heat for temperature changes is typically minor compared to latent heat , which dominates the overall energy demand and efficiency of the process. [17] [18] Real mixtures often deviate from ideal behavior, where Raoult's law ($ y_i P = x_i P_i^\text{sat} ) h o l d s , d u e t o i n t e r m o l e c u l a r i n t e r a c t i o n s ; t h e s e a r e q u a n t i f i e d u s i n g a c t i v i t y c o e f f i c i e n t s ( ) holds, due to intermolecular interactions; these are quantified using activity coefficients ( ) h o l d s , d u e t o in t er m o l ec u l a r in t er a c t i o n s ; t h ese a re q u an t i f i e d u s in g a c t i v i t ycoe ff i c i e n t s ( \gamma_i $) in modified fugacity relations: $ y_i \phi_i^\text{V} P = x_i \gamma_i \phi_i^\text{L} P_i^\text{sat} $. Models like the van Laar equation capture positive deviations leading to azeotropes, while the Wilson equation accounts for both positive and negative deviations through local composition effects; for binary systems, the Wilson model is given by $ \ln \gamma_1 = -\ln(x_1 + A_{12} x_2) + x_2 \left( \frac{A_{12}}{x_1 + A_{12} x_2} - \frac{A_{21}}{x_2 + A_{21} x_1} \right) $, and similarly for $ \gamma_2 $, where $ A_{12} $ and $ A_{21} $ are temperature-dependent interaction parameters derived from molar volumes and energy differences, enabling prediction of non-ideal VLE curves critical for accurate distillation design. [19] [20] In distillation, each vapor-liquid equilibrium stage approaches spontaneity with $ \Delta G = 0 $ at equilibrium (equal chemical potentials in both phases). However, the overall separation process requires energy input to overcome the positive Gibbs free energy change associated with unmixing, primarily through the enthalpy of vaporization , enabling the fractionation toward purer components. [21] [22] To achieve desired separations with minimal energy, the minimum reflux ratio is calculated using the Underwood equations, which determine the pinch condition where operating and equilibrium lines touch; for multicomponent systems assuming constant relative volatility ($ \alpha_i $), the key relation is $ \sum \frac{\alpha_i x_{D,i}}{\alpha_i - \theta} = 1 - q $, where $ x_{D,i} $ is the distillate composition, $ \theta $ is a root between adjacent volatilities, and q is the feed thermal condition, providing the theoretical lower bound on reflux to avoid excessive stages or energy use. [23] [24] Historical Development Ancient and Classical Periods The earliest known evidence of distillation-like processes appears in ancient Mesopotamia , where archaeological excavations at Tepe Gawra uncovered apparatus dating to approximately 3500 BCE. This setup consisted of a deep ceramic bowl for heating liquids, a strainer basin to hold plant materials, and a bell-shaped lid to capture and condense vapors, primarily used for extracting aromatic essences from botanicals for perfumes and medicinal preparations. Experimental replications have confirmed that this equip