Heuristics‐Separation System

// Items to be considered for separation system.

1. Possible phase conditions of reactor effluent

2. Common separation units
   • Distillation
   • Absorption
   • Adsorption
   • Liquid‐liquid extraction
   • Leaching
   • Crystallization
   • Evaporation
   • Membrane systems
   • Filtration, Centrifugation, Settling

// General guidelines for choosing separation units, Table 12.1 Turton et al.

• Use distillation as a first choice for separation of fluids when purity of both products is required.
• Use gas absorption to remove one trace component from a gas stream.
• Consider adsorption to remove trace impurities from gas or liquid streams.
• Consider pressure‐swing adsorption to purify gas streams, especially when one of the components has a
  cryogenic boiling point.
• Consider membranes to separate gases of cryogenic boiling point and relatively small flow rates.
• Choose an alternative to distillation if the boiling points are very close or if the heats of vaporization are very
  high.
• Consider extraction as a choice to purify a liquid from another liquid.
  8 Use crystallization to separate two solids or to purify a solid from a liquid solution.
• Use evaporation to concentrate a solution of a solid in a liquid.
• Use centrifugation to concentrate a solid from a slurry.
• Use filtration to remove a solid in almost dry form from a slurry.
• Use screening to separate solids of different particle size.
• Use float/sink to separate solids of different density from a mixture of pure particles.
• Consider reverse osmosis to purify a liquid from a solution of dissolved solids.
• Use leaching to remove a solid from a solid mixture.

Avoid indirect separations but if required, remove the additive soon after it is used.

// Distillation and Gas Absorption Towers

Vapour‐liquid separation operations

Packed towers favored when:

• Lower pressure drop desired
• Under vacuum operations
• Handle larger loading

Tray towers favored for systems with:

• Large diameter columns (columns with 3 ft or larger diameter)
• When the feed contains solids
• High liquid‐to gas ratios
• Corrosive & foaming systems

// Heuristics for towers (Distillation and gas absorption), Table 11.13 Turton et al.

• Distillation is usually the most economical method for separating liquids, superior to extraction, absorption
  crystallization, or others.
• For ideal mixtures, relative volatility is the ratio of vapor pressures α12= P1/ Pi.
• Tower operating pressure is most often determined by the temperature of the condensing media, 38‐50 °C
  (100‐120 °F) if cooling water is used or by the maximum allowable reboiler
  temperature to avoid chemical decomposition/ degradation.
• Sequencing of columns for separating multicomponent mixtures:
  • Perform the easiest separation first, that is, the one least demanding of trays and reflux, and leave the
    most difficult to the last.
  • When neither relative volatility nor feed composition vary widely, remove components one by one as
    overhead products.
  • When the adjacent ordered components in the feed vary widely in relative volatility, sequence the splits in
    order of decreasing volatility.
  • When the concentrations in the feed vary widely but the relative volatilities do not, remove the
    components in order of decreasing concentration.
• Economical optimum reflux ratio is in the range of 1.2‐1.5 times the minimum reflux ratio, Rmin.
• The economically optimum number of theoretical trays is near twice the minimum value Nmin.
• The minimum number of trays is found with the Fenske‐Underwood equation Nmin = lnl[x/(1‐x))ovhd/[x/(1‐x)]btmsl/lnα.
• Minimum reflux for binary or pseudobinary mixtures is given by the following when separation is essentially complete (xD≈ 1) and D/F is the ratio of overhead product to feed rate:
• RminD/F = 1/(α‐1), when feed is at the bubble point (Rmin + 1) D/F = α/(α‐1), when feed is at the dew point.
• A safety factor of 10% of the number of trays calculated by the best means is advisable.
• Reflux pumps are made at least 10% oversize.
• The optimum value of the Kremser absorption factor A= (UmV) is in the range of 1.25 to 2.0.
• Reflux drums usually are horizontal, with a liquid holdup of 5 min half full. A take‐off pot for a second liquid phase, such as water in hydrocarbon systems, is sized for a linear velocity of that phase of 1.3 m/s (0.5 ft/sec), minimum diameter is 0.4 m (16 in).
• For towers about 0.9 m (3 ft dia), add 1.2 m (4 ft) at the top for vapor disengagement and 1.8 m (6 ft) at bottom for liquid level and reboiler return.
• Limit the tower height to about 53 m (175 ft) max. because of wind load and foundation considerations. An additional criterion is that L/D be less than 30 (20 <L/D<: 30 often will require special design).

// Heuristics for tray in towers (Distillation, absorption), Table 11.14 Turton et al

• For reasons of accessibility, tray spacings are made 0.5‐0.6 m (20‐24 in).
• Peak efficiency of trays is at values of the vapor factor F_S = uρ^0.5 in the range of 1.2‐1.5 m/s (kg/m³)0.5 [1‐1.2 ft/s{lb /ft³}^0.5] . This range of FS establishes the diameter of the tower. Roughly, linear velocities are 0.6 m/s (2 ft/s)
  at moderate pressures and 1.8 m/s (6 ft/s) in vacuum.
• Pressure drop per tray is on the order of 7.6 cm (3 in) of water or 0.007 bar (0.1 psi).
• Tray efficiencies for distillation of light hydrocarbons and aqueous solutions are 60‐90%; for gas absorption and
  stripping, 10‐20%.
• Sieve trays have holes 0.6‐0.7 cm (0.2‐0.5 in) dia., area being 10% of the active cross section.
• Valve trays have holes 3.8 cm (1.5 in) dia. each provided with a liftable cap, 130‐150 caps/m2 (12‐ 14 caps/ft2) of active cross section. Valve trays are usually cheaper than sieve trays.
• Bubblecap trays are used only when a liquid level must be maintained at low turndown ratio; they can be designed for lower pressure drop than either sieve or valve trays.
• Weir heights are 5 cm (2 in), weir lengths are about 75% of tray diameter, liquid rate‐a maximum of 1.2 m³/min m of weir (8 gpm/in of weir); multipass arrangements are used at higher liquid rates.

// Heuristics for packed towers, Table 11.15 Turton et al.

• Structured and random packings are suitable for packed towers less than 0.9 m (3 ft) when low pressure drop is required.
• Replacing trays with packing allows greater throughput and separation in existing tower shells.
• For gas rates of 14.2 m³/min (500 ft³/min), use 2.5 cm (1 in) packing; for 56.6 m³/min (2000 ft³/min) or more,
  use 5 cm (2 in) packing.
• Ratio of tower diameter I packing diameter should be > 15/1.
• Because of deformability, plastic packing is limited to 3‐4 m (10‐15 ft) and metal to 6.0‐7.6 m (20‐25 ft)
  unsupported depth.
• Liquid distributors are required every 5‐10 tower diameters with pall rings and at least every 6.5 m (20 ft) for other types of dumped packing.
• Number of liquid distributors should be >32‐55/m₂ (3‐5/ft₂) in towers greater that 0.9 m (3 ft) diameter and more numerous in smaller columns.
• Packed tower should operate near 70% of flooding (evaluated from Sherwood and Lobo correlation)
• Height equivalent to theoretical stage (HETS) for vapor‐liquid contacting is 0.4‐0.56 m (1.3‐1.8 ft) for 2.5 cm (1in) pall rings and 0.76‐0.9 m. (2.5‐3.0 ft) for 5 cm (2 in) pall rings.

// Multiple‐effect evaporator

• Used, for example, to produce fresh water from seawater. Seawater boiled and
  freshwater vapour recovered.
• Other applications: prior to crystallization in order to concentrate the solution specially for inorganic chemicals.
• Benefit in multiple stages due to recovery of heat from one stage to supply energy to next stage, reducing total energy costs.
• Use forward feed, for small number of effects and/or when the liquid feed is hot.
• Use backward feed, for large number of effects and/or cold feeds.

// Liquid‐Liquid extraction

• Used to separate liquid mixtures via second liquid solvent (separating
  agent)
• One of the liquids is drawn from solution, preferentially mixing with
  separating agent

// Heuristics for liquid‐liquid extraction, Table 11.16 Turton et al.

• The dispersed phase should be the one that has the higher volumetric flow rate except in equipment subject to back‐mixing where it should be the one with the smaller volumetric rate. It should be the phase that wets material of construction less well. Because the holdup of continuous phase is greater, that phase should be made up of the less expensive or less hazardous material.
• There are no known commercial applications of reflux to extraction processes, although the theory is favorable.
• Mixer‐settler arrangements are limited to at most five stages. Mixing is accomplished with rotating impellers or circulation pumps. Settlers are designed on the assumption that droplet sizes are about 150 μm dia. In open vessels, residence times of 30‐60 min or superficial velocities of 0.15‐0.46 m/min (0.5‐1.5 ft/min) are provided in settlers. Extraction stage efficiencies commonly are taken as 80%.
• Spray towers as tall as 6‐12 m (20‐40 ft) cannot be depended on to function as more than a single stage.
• Packed towers are employed when 5‐10 stages suffice. Pall rings 2.5‐3.8 cm (1‐1.5 in) size are best. Dispersed phase loading should not exceed 10.2 m³/min m² (25 gal/min ft2). HETS of 1.5‐3.0 m {5‐10 ft) may be realized. The dispersed phase must be redistributed every 1.5‐2.1 m (5‐7 ft). Packed towers are not satisfactory when the surface tension is more than 10 dyne/cm.
• Sieve tray towers have holes of only 3‐8 mm dia. Velocities through the holes are kept below 0.24 m/ s (0.8 ft/sec) to avoid formation of small drops. Redispersion of either phase at each tray can be designed for. Tray spacings are 15.2‐60 cm (6 to 24 in). Tray efficiencies are in the range of 20‐30%.
• Pulsed packed and sieve tray towers may operate at frequencies of 90 cycles/min. and amplitudes of 6‐25 mm. In large diameter towers, HETS of about 1 m have been observed. Surface tensions as high as 30‐40 dyne/cm have no adverse effect.
• Reciprocating tray towers can have holes 1.5 cm (9 / 16 in) dia., 50‐60% open area, stroke length 1.9 cm (0.75 in), 100‐150 strokes/min, plate spacing normally 5 cm (2 in) but in the range of 2.5‐15 cm (1‐6 in). In a 76 cm (30 in) diameter tower, HETS is 50‐65 cm (20‐25 in) and throughput is 13.7 m³/min m² (2000 gal/h ft²). Power requirements are much less than that of pulsed towers.
• Rotating disk contactors or other rotary agitated towers realize HETS in the range of 0.1‐0.5 m (0.33‐1.64 ft). The especially efficient Kuhni with perforated disks of 40% free cross section has HETS of 0.2 m (0.66 ft) and a capacity of 50 m3/m2 h {164 ft³/ft² h).

// Additional general considerations

• Product specifications for all products
  • Concentration (purity) of the desired product
  • Maximum impurity for specific contaminants
  • Physical properties such as: color, odor, and specific gravity

• Heat sensitivity of all products
  • Decompose or polymerize at elevated temperatures

• Toxic or hazardous products, by‐products and impurities
• Additional purification or further processing may be required

// Column Sequencing

The general rule is that a stream containing N components requires N – 1 separators. Assuming each separator has one inlet stream and two outlet streams.

// Guidelines for sequencing separation units, Table 12.2 from Turton et al.

• Remove the largest product stream first. This makes all of the subsequent separation units smaller.
• For distillation, remove the product with the highest heat of vaporization first, if possible. This reduces the heating/cooling duties of subsequent units.
• Do not recombine separated streams. (This may seem obvious, but it is often disobeyed.)
• Do the easy separations first.
• Do not waste raw materials, and do not overpurify streams based on their uses.
• Remove hazardous or corrosive materials first.

It is helpful to list components in order of their boiling points to determine the separation sequence.

// Sequencing of distillation column

As the number of components, N, to be separated increases, so does the number of alternative sequences.

The number of alternative sequences, S, is given by:

The sequence usually occurs in order of decreasing relative volatility, α:

• K_i is the light component (most volatile of the components in mixture)
• K_j is the heavy component (least volatile of the components in mixture)

Heuristics: the relative volatility between the two selected key components for the separation in each column should be >1.1

// Example

• a. Purify a three‐component feed into three pure components. What are the
  sequencing alternatives?
• b. If component C is water, which alternative is favorable?

// Example

• a. Purify a four‐component feed.

From Seider et al., Product & Process Design Principles.

• b. Consider the following separation process. Use heuristics to determine a good sequence of distillation units.

// Phase Separation Systems

Sometimes a phase separation system is used before the actual separation
system.

• The reactor effluent may be heterogeneous mixture (two or more phases) or homogeneous mixture (single‐phase).
• It is sometimes advantages to change the temperature and/or pressure to obtain a partial separation of the components by forming a heterogeneous mixture of two or more phases.

// Types of phase separation units

// Heuristics for process vessels (drums), Table 11.6 Turton et al.

• Drums are relatively small vessels that provide surge capacity or separation of entrained phases.
• Liquid drums are usually horizontal.
• Gas/liquid phase separators are usually vertical.
• Optimum length/ diameter = 3, but the range 2.5 to 5 is common.
• Holdup time is 5 min for half‐full reflux drums and gas/liquid separators, 5‐10 min for a product feeding
  another tower.
• In drums feeding a furnace, 30 min for half‐full drum is allowed.
• Knockout drums placed ahead of compressors should hold no less than 10 times the liquid volume passing
  per minute.
• Liquid/ liquid separations are designed for settling velocity of 0.085‐0.127 cm/s (2‐3 in/ min).
• Gas velocity in gas/liquid separators, u = k √[ρl/ρv‐1] m/s (ft/s) k = 0.11 (0.35) for systems with mesh deentrainer and k = 0.0305 (0.1) without mesh deentrainer.
• Entrainment removal of 99% is attained with 10.2‐30.5 cm (4‐12 in) mesh pad thickness; 15.25 cm (6 in) thickness is popular.
• For vertical pads, the value of the coefficient in Step 9 is reduced by a factor of 2/3.
• Good performance can be expected at velocities of 30‐100% of those calculated with the given k; 75% is popular.
• Disengaging spaces of 15.2‐45.7 cm (6‐18 in) ahead of the pad and 30.5 cm (12 in) above the pad are suitable.
• Cyclone separators can be designed for 95% collection at 5 μm particles, but usually only droplets greater than 50 μm need be removed.