Advanced Engineering Separations

Performance indicators for sequencing designs

- Total vapour load - Total energy demand/heat duties - Operating cost of the column - Capital cost of the column - Total annualized cost

Factors of solvent selection When feed is: - Aqueous solution: Use organic solvent - Organic: Use water

*- High selectivity for the solute relative to the carrier (Minimizes the need to recover carrier from solvent) - High capacity for dissolving the solute (Minimizes the solvent to feed ratio) - Minimal solubility in the carrier - A volatility sufficiently difference from the solute that recovery of the solvent can be achieved by distillation - Availability at a relatively low cost* - Stability to maximize the solvent life & minimize solvent makeup requirement - Inertness to permit use of common materials of construction -Low viscosity to promote phase separation, minimize pressure drop & provide a high solute mass transfer rate - Non-toxic & non-flammable characteristics to facilitate its safe use - Moderate interfacial tension to balance the ease of dispersion & promotion of phase separation - Large difference in density relative to the carrier to achieve a high capacity in the extractor - Compatibility with the solute and carrier to avoid contamination - Lack of tendency to form a stable scum layer at the phase interface - Desirable wetting characteristics with respect to extractor internals

Why relative volatility is taken as an average of the top & bottom compositions

*- Provides a better representation of the system than a single value - Too much effort to calculate local values - Pressure changes in the column so an average is used because it is dependent on the saturated vapour phase* - The differences in their compositions is the largest - The values of their compositions are known

Benefits of liquid-liquid extraction over distillation

*- Separation of mixtures according to chemical type rather than relative volatility - Separation of close-melting/close-boiling liquids where solubility differences can be exploited - Separation of mixtures that form azeotropes - Recovery of heat-sensitive materials where extraction may be less expensive than vacuum distillation* - Dissolved/complexed inorganic substances in organic/aqueous solutions - Removal of a contaminant present in small concentrations - A high-boiling component present in relatively small quantities in an aqueous waste stream

Disadvantages of MSA (Mass separating agent) Advantages of MSA (Mass separating agent)

- An additional separator is needed to recover the MSA - MSA will be lost in the process so MSA makeup is required - Incomplete recovery of MSA may lead to potential contamination of product - MSA separation processes are more complex to design MSA allows for some separations to occur that ant occur with a ESA

Underwood Equation Assumptions

- Constant molar overflow - Constant relative volatilities - Infinite number of stages - No pinch points of non key components

Fenske Equation Assumptions

- Constant molar overflow - Constant relative volatilities - Total reflux

Short-cut distillation method Assumptions

- Constant relative volatility (for ideal mixtures) - Constant molar overflow

Simple distillation columns heuristic rules

- Do the most difficult separation last - Favour the direct sequence - Remove large fraction components first - Favour near equimolar splits between top & bottom products (Evenly distribute mass load in subsequent columns)

Total condenser (Bubble point - Saturated Liquid) Partial condenser (Dew point - Saturated Vapour)

- Does not represent an equilibrium stage since all the vapour that enters it leaves at the same temperature as a liquid & liquid is taken as the top product - Q = V * λvap where V = (R+1)D -Easy to pump, store & increase the pressure of a liquid product - Represents an equilibrium stage since some of the vapour that enters it condenses into a liquid while the rest remains as a vapour (because it is only cooled to the dew point which is at a higher temperature & not the bubble point) & vapour is taken as the top product - Q = L * λvap where L = RD - Lower condenser duty since only part of the top product condenses - May be able to avoid expensive refrigeration as cooling water can be used for large temperature differences

Why a saturated liquid feed is preferred

- Easier to control column pressure by pumping liquid to the correct pressure - More difficult & expensive to pump gas using compressors to the correct pressure so a saturated gas feed isn't preferred - Superheated/subcooled feeds can't participate in the separation process until thermal equilibrium is achieved

Liquid-Liquid extraction design considerations

- Initial feed flowrate, composition, temperature & pressure - Type of stage configuration (one/two) - Choice of solvent (Good solubility with solute & immiscible with carrier) - Operating pressure/temperature of the column - Desired degree of recovery of one/more solutes for one-section cascades - Solvent flow rate for one-section cascades & reflux rate for two-section cascades - Number of equilibrium stages

Differences between simple distillation & thermally coupled sequencing

- Operating lines: Rectifying operating line generally has a smaller slope in thermally coupled sequences - Middle boiling component distribution: Uniform in thermally coupled sequences - Configuration: Removed reboiler and/or condenser in thermally coupled sequences - Total vapour load: Smaller in thermally coupled sequences - Energy consumption & capital cost: Pre-fractionator arrangement requires 30% less energy - Feed composition requirement: Side rectifier & side stripper arrangement require more than 30% middle boiling component

Distillation operating parameters

- Operating pressure (Temperature of condenser & reboiler will be fixed by operating pressure of the distillation column) - Reflux ratio (High reflux ratio = Low number of stages but high amount of utilities since a higher flowrate is recycled back into the column = Low capital but high operating cost) - Feed condition - Feed location (Should best match with the feed stage in terms of composition & temperature since poor matches result in higher energy duties & disruptions in the material profile) - Type of condenser

Reactive distillation

- Reaction occurs in the liquid phase - The products formed should have a higher boiling point than the reactants

Separation methods

- Simple distillation: Differences in volatility - vapour pressure & boiling points - Distillation with solvent: Increases the relative volatilities between the species, reducing number of trays, allows the crossing of distillation boundaries - Crystallisation: Differences in melting points - Adsorption: Differences in dipole moment - Centrifugation: Differences in radius of gyration - Liquid-Liquid Extraction: Differences in solubility parameters - Filtration: Differences in molecular properties (e.g. molecular weight, shape, van der waals volume & area) -Electrophoresis: Differences in electric charge & diffusivity Example: Separating mixture of ethanol & water - Ethanol & water can be separated by simple distillation due to their large differences in relative volatilities/boiling points up to a certain limit. This is because they form an azeotrope at about 96wt% ethanol which simple distillation can't separate at this composition since both compositions will have the same boiling points. Thus, alternative methods like extractive distillation & liquid-liquid extraction are required to achieve purities > 96 mol%

Purpose of pre-concentrator column

- Takes feed mixture of A & B and performs simple distillation in order for the distillate composition to reach that of the binary azeotrope A-B - Reduces amount of material passing into subsequent columns & also reducing amount of solvent required (Cost of system reduces) Have to determine whether it is worth including into distillation sequences in terms of cost

Fugacity

A measure of the tendency of a component of a liquid mixture to vaporise from the mixture

Dividing wall column

Advantages: - Reduces capital cost by removing condenser & reboiler and being combined in a single shell structure Disadvantages: - Sometimes difficult to cope with differences in temperature & pressure operated in the combined two columns Restrictions: - Both the operating pressure & number of trays for the two divided columns should be the same or similar (although a different number of trays can be on either side of the dividing wall due to different spacings, this is only true within reason)

Residue vs Distillation curves

Distillation curves (For continuous distillations): Point towards top products/low boiling components, based on total reflux Residue curves (For batch distillations): Point towards bottom products/higher boiling components, based on no reflux Although there is little difference between the two, residue curves are the better choice since they are simpler to calculate & provide a good initial approximation of the distillation

Liquid-liquid extraction equipment

Equipment similar to that used for absorption, stripping & distillation is used for solvent extraction when: -Liquid viscosities are low -Differences in phase densities are high Centrifugal & mechanically agitated equipment are preferred The carrier & solvent can either be the dispersed phase (phase that gets mixed in as droplets) or the continuous phase (phase that the droplets get mixed into) Mixer-settlers: - Low head room for equipment needed - Easy scale up and high efficiency needed - Wide ratio of feed-to-solvent flow rates needed Spray towers - Used when there is an irreversible reaction occurring in one of the phases (e.g. waste acid neutralization) Packed columns - Used when few stages are needed due to backmixing which results in higher HETP Columns with mechanically assisted agitation - Used when interfacial tension is high, density difference between liquid phases is low & liquid viscosities are high (when gravitation forces are inadequate for phase dispersal and turbulence creation since it increases the interfacial area per volume and decreases mass-transfer resistances)

Adding entrainer during extractive distillation

For minimum-boiling azeotrope: - Add entrainer near top of the column For maximum-boiling azeotrope: - Add entrainer with the feed into the column Entrainer should always have a lower volatility than the key components

Complex distillation columns heuristic rules

For side stream columns: Vapour (Bottom of the column) - B > 50% - C < 5% - αBC >> αAB Liquid (Top of the column) - B > 50% - A < 5% - αAB >> αBC For side-rectifier, - B > 30% - A > C For side-stripper, - B > 30% - C > A For pre-fractionator, - B is a large feed fraction - αAB = αBC - Flowrate of A = C

Heterogeneous vs Homogeneous azeotropic distillation

Heterogeneous - A binary/ternary azeotrope is needed for the separation - The products dont have to lie in the same distillation region since the heterogeneous azeotrope allows the separation to move between distillation boundaries - Minimum boiling azeotropes are preferred Homogeneous - The products have to lie in the same distillation region - At least one maximum-boiling azeotrope is needed

Composition profiles in simple distillation vs pre-fractionation arrangements

In simple distillation (direct sequence): - The composition of the stage at the feed location doesn't match that of the feed entering the column due to remixing where the concentrations of the middle boiling component need to be redistributed. - This results in greater heat duties because of an increase in remixing and vapour load. - Heating/cooling is provided indirectly through a heat exchanger. In pre-fractionation arrangements: - There is an uniform distribution of middle boiling component which gives greater freedom to match the composition of the feed entering the column with that of the stage at the feed location. - This results in lower heat duties due to a decrease in remixing and vapour load. - Heating/cooling is provided directly by a process stream from the downstream column. Although thermally coupled arrangements require less energy, they run at higher temperatures so operating costs may be reduced depending on the type of utilities used and their costs.

Effect of pressure on distillation

In some cases, as the pressure of a separation in a distillation column increases: - Relative volatility decreases - Minimum reflux ratio increases - Minimum number of stages increases - Temperature of the condenser & reboiler increases All of which indicate a more difficult separation. However, the latent heat of vapourisation of the separation could decrease significantly as the pressure increases which may reduce the overall operating costs despite running the separation in a larger distillation column at a higher pressure. Thus, it is always worth investigating the effect that the pressure of the distillation column has on the separation.

Removing components with large feed fractions first

Keeps the inventory/amount of material in subsequent columns to a minimum

Properties of homogeneous (1 liquid & vapour phase in equilibrium) & heterogeneous azeotropes (2 liquid & vapour phase in equilibrium)

Minimum-boiling homogeneous azeotrope: - γ > 1 - Its boiling point is slightly lower than that of the two components - Prefer to be as pure components rather than a mixture - Obtained in the top product Maximum-boiling homogeneous azeotrope - γ < 1 - Its boiling point is slightly higher than that of the two components - Prefer to be as a mixture rather than pure components - Obtained in the bottom product Heterogeneous azeotrope - γ >> 1 - Formed due to the phase splitting of a minimum boiling azeotrope into 2 liquid phases It is sometimes possible to shift the equilibrium by changing the pressure sufficiently to 'break' the azeotrope. However, this might not always be the optimal solution since it may not be practical to operate under vacuum & some systems don't have sensitivity in azeotropes so pressure has no effect on their compositions. In this case, a third component (solvent) will be added to by-pass azeotropes.

Properties of species

Molecular properties - Molecular weight - Van der Waals weight, area - Molecular shape - Dipole moment (Related to shape & size of molecules) - Polarizability - Dielectric constant - Electric charge - Radius of gyration (Related to when there is a magnetic field present) Thermodynamic & Transport properties: - Vapour pressure - Solubility - Adsorptivity - Diffusivity

Liquid-Liquid extraction designs

Multistage system (e.g. single cascade systems) allows the raffinate composition to be specified Two-section cascade system allows both the raffinate and extract composition to be specified Dual solvent systems are used when it is more difficult to obtain the desired compositions due to complex liquid-liquid extractions (where one solvent preferentially dissolves the solute & the other preferentially dissolves the carrier)

Separation techniques

Phase creation (e.g. Evaporation, Crystallisation, Distillation) - Involves the creation of a second phase that is immiscible with the feed - Achieved through energy transfer or pressure reduction - Suitable for mixtures that tend to vapourise Phase addition (e.g. Extractive distillation, Liquid-Liquid extraction) - Involves the development of a second immiscible phase to separate homogeneous single phase mixtures - Achieved through either an energy or mass separating agent Phase by barrier (e.g. Filtration, Osmosis) - Involves the use of microporous (based on diffusion rates) and non-porous membrane (based on solubilities) as semipermeable barriers Phase by solid agent (e.g. Adsorption, Chromatography) - Involves the use of solid mass separating agents that are normally in the form of a granular material Phase by force field/gradient (e.g. Centrifugation, Gravity Settling, Electrolysis) - Involves the use of external fields that take advantage of differing degrees of response of molecules & ions to force fields The separation method depends on the nature of the mixture: - If the mixture is homogeneous (i.e. single phase), another phase will have to be created to separate it - If the mixture is heterogeneous (i.e. multi phase), the separation can be carried out by exploiting the differences in properties between the phases Thus, heterogeneous separations should always be carried out first before homogeneous and that mechanical separations (e.g. centrifuge, pressure reduction, electric/magnetic field) are preferable when two or more immiscible phases exist since it is cheaper.

When selecting utility temperature in reboiler

Typically want a 10C difference between the material being rebiller and the utility

Azeotrope separation techniques (Enhanced distillation)

Used when either α < 1.1 or an azeotrope forms: Extractive distillation (Changing α) - Uses a high-boiling solvent that increases the α between key components without forming any azeotropes Pressure-swing distillation (Changing P) - Separates a mixture that forms a pressure-sensitive azeotrope by utilizing two columns in sequence at difference pressures Homogeneous azeotropic distillation (Changing α) - Uses an entrainer than forms either a homogeneous maximum-boiling or minimum-boiling azeotrope with one of the feed components - Recycles the azeotrope & one of the other components instead of the pure entrainer Heterogeneous azeotropic distillation (Add secondary separation mechanism) - Uses an entrainer that forms a minimum boiling ternary heterogeneous azeotrope with one of the feed components that splits into two liquid phases in the overhead condenser Reactive distillation (Changing species) - A chemical that reacts selectively and reversibly with one of the feed components is added & the reaction product is later distilled from the non-reacting components