Chapter 2: Separation Factor and Molecular Properties

Unit Operation Design

Dr. S.H. Esmaeili-Faraj Molecular weight Molecular volume Properties of Chemical reaction equilibrium pure Acidic Molecular shape groups substances Basic groups Ionization depends Molecular charge (e.g. -COOH and -NH2 in proteins) on pH Dipole moment & polarizability

Groups like O-H and C=O in molecules are polar. The molecule Such groups present in an asymmetrical fashion exhibits a finite within a molecule dipole moment Polar molecules interact more strongly with other molecules than nonpolar molecules of the same size do.

Polar substance, e.g., water has a lower vapor pressure than a nonpolar species of about the same molecular weight, e.g., methane.

The tendency for a dipole to be induced in a molecule of that substance Depends upon the size of molecule and the mobility of electrons Electrons in aromatic rings and, to a lesser extent, olefinic bonds are more mobile than the electrons in single covalent bonds and therefore impart a greater polarizability. Greater More Lower vapor solubility in polarizable pressure molecule polar solvent Interaction with mass separating Properties of pure substances agent or barier

Dipole moment Chemical Molecular Molecular Molecular Molecular Molecular Dipole Moment & Separation process & reaction size & weight volume shape charge polarizability polarizability equilibrium shape

Distillation 2 3 4 2 - - - - Crystallization 4 2 2 3 2 - - - Clathration - - - - - 3 1 3 Solvent extraction & - - - - - 2 3 2 absorption Ordinary adsorption - - - - - 2 2 2

Adsorption with ------1 3 molecular sieves Dialysis; gel - 2 3 - - - 1 3 permeation Ultrafiltration - - 4 - - - 1 - Gaseous diffusion 1 ------Sweep diffusion 2 2 ------Ultracentrifugation 1 ------Electrophoresi 2 3 3 - 1 - - - Electrodialysis - - - - 1 - 2 - Ion exchange - - - - - 1 2 - Key: 1. primary effect; necessary for any separation 2. primary effect 3. secondary effect (perhaps through another property) 4. small effect - No effect Morphological analysis

Systemically analysis of methods and alternatives for doing special case and then, review of combination of these alternatives. Producing phethalic Gasoline producing anhydride & other blending & terephethalic acid & phthalates as a conversion to dimethyl plasticizer for PVC other isomers terephethalate property O-xylene M-xylene P-xylene Ethyl benzene

Percent(%) at 1000K in equilibrium mixture 23 43 19 15

Boiling point(K) 417.3 412.6 411.8 409.6

Freezing point(k) 248.1 225.4 286.6 178.4

Dipole moment (10^-18esu) 0.62 0.36 0 ------

Polarizability(10^-31m3 ) 141 141.8 142 ------

Molecular weight 106.16 106.16 106.16 106.16

Density, at 298k (g/cm^3 ) 0.8802 0.8642 0.8610 0.8670

λ,at boiling point(cal/g) 82.9 82.0 81.2 81.0 = =1.16 Dipole moment of p-xylene = 0 =1.02

p-xylene Narrow molecule Choosing a method on the basis of molecular shape m-xylene Nearly spherical difference • same molecular weights Separation processes based on Mw

• different in dipole moment between ortho & meta Distillation(R=15,N≥100)

• para & meta separation distillation Separation process based on molecular shape and Reactivity. Crystallization Membrane

Adductive Adsorption crystallization

Chemical Clathration reaction

Extractive HF-BF3 crystallization The difference in molecular shapes has two effects:

1. p-xylene molecules can stack together more readily into a crystal structure because of their symmetrical shape, and as a result p-xylene has a much higher freezing point(286.6 k)

2. The difference in shape between p- and m-xylene means that m-xylene molecules cannot fit easily into the p-xylene crystal structure in the solid phase. Thus, the solid phase formed by partial freezing of a mixture of the two isomers contains essentially pure p-xylene. 100% Ortho

30.5%

Para meta100% 100% • p-xylene manufacture by crystallization & isomerization.

Recycled isomerized xylene Benzene Toluene Light gas feed 99% Para x crystallization s y 5% ethyl benzene product t l 23% p-xylene a e 50% m-xylene Mixed xylene(9% Para) 22% 0-xylene b n e i s l isomerization p i l z i e t r t e O-xylene r product • addition of CCl4 as separation agent. refrigeration temperature. but Recovery of p-xylene. additional stage for para xylene separation.

• addition of nickel[(4-methylpyridine)4 (SCN)2 ] as a separation agent.

Recovery of 92% of the p-xylene in a single stage. • addition of normal-pentane . Shifting eutectic point.

• αp-o = 2.0 ; αp-m = 1.3

• low density polyethylene membrane.

• disadvantages Difficulty of staging

Difficulty of provision of driving force for p-xylene crossing from membrane. • molecular sieves

• more than 22 new industrial units (1968-1978) Benzene product Toluene product Non aromatics Clay B T sulfolane O Light treating E Platformate N L

S P L naphtha I plat former THDA T T O-xylene E poduct R P-xylene product Heavy platformate

Clay PAREX isomer treating Heavy aromatic Xylene column Heavies splitter Sulfonation with cold & m- and o-isomers are sulfonated, concentrated while p-isomer is unchanged.

Treatment with Elimination of and excess & formation of sodium salts of the sulfonates.

Evaporative crystallization O-xylene Sodium sulfonate precipitates first. • only laboratory method. Ethyl benzene D P-xylene D I I S < .03% m-xylene S T T I I l E l L X L A HF-BF3 T A T R D T I Feed A E I O C C N Mixed O O-xylene xylene T O N P O D O R HF-BF3 E S recycle C E O R isomerization M P m- O xylene S 99.5% E Recycled isomerized xylene r • removal between 60% to 99.5% of the water Berry: fruit present. juices

Juice stability. 2-3 hr @ 328k <1min @ 367k great economy in transportation & storage costs. 1s @ 389k

• common process Multi-effect Reduction of steam evaporation costs.

thermal sensitivity

• problems build up semisolid layer next to the surface in evaporators

escaping of flavor and aroma compound Improvement of evaporation process

• solution

Use of other separation process

reduction of boiling point

• vacuum evaporation reduction of residence time(A/V )

preheating by injection of steam 1.addition of fresh juice to the concentrate

2.obtaining layer material from peels , cores , & adding it to concentrate Prevention of escaping aroma 3.Separation of aroma from the water vapor & returning to concentrate

4.Use another process

• first technique Product contains 10% of volatile compounds.

• second technique Most successful process , difference between flavoring material in peels & juice. • third technique Essence recovery process

Initial Receiving Means of creating phase of phase for water chemical potential Example water difference Liquid Vapor Pressure(vacuum) Flash evaporation

Temperature Drying with (feed superheat) superheated steam Composition(carrier) Air drying

Immiscible liquid Composition(solvent) Extraction

solid Temperature(freeze) Freeze concentration Composition Clathration (precipitate)

Composition(adsorb) Solid desiccant Initial Receiving phase Means of creating phase of for water chemical potential Example water difference solid Vapor Pressure(vacuum) Ordinary freeze-drying

Composition(carrier) Carrier-gas freeze – drying

Immiscible liquid Composition (solvent) ------Initial Receiving Means of creating phase of phase for chemical potential Example water water difference liquid Vapor Pressure(vacuum) Pervaporation with compression Temperature (feed superheat) ------Composition (carrier)

liquid Pressure Pervaporation (pressurized feed) with carrier Reverse osmosis

Composition Direct osmosis (added solute)

Composition(solvent) Perstraction Initial Receiving Means of creating phase of phase for chemical potential Example water water difference

Liquid ------Screening Pulp removal

Density difference Centrifugation before evaporation solid ------Screening Grinding & screening for frozen juices

Density difference Grinding & flotation in liquid of intermediate density for frozen juices Flash (or partial) evaporation is the partial vapor that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device.

If the throttling valve or device is located at the entry into a pressure vessel so that the flash evaporation occurs within the vessel, then the vessel is often referred to as a flash drum.

Ref: www.wikipedia.org If the saturated liquid is a single-component liquid (for example, liquid propane or liquid ammonia), a part of the liquid immediately "flashes" into vapor. Both the vapor and the residual liquid are cooled to the saturation temperature of the liquid at the reduced pressure. This is often referred to as "auto-refrigeration" and is the basis of most conventional vapor compression refrigeration systems.

If the saturated liquid is a multi-component liquid (for example, a mixture of propane, isobutane and normal butane), the flashed vapor is richer in the more volatile components than is the remaining liquid.

Ref: www.wikipedia.org  A close loop pneumatic conveying type  Transport steam is superheated indirectly via a tubular heat exchanger or electrical heating  Only 5-60 seconds residence time, For some materials a second superheater is necessary to achieve the required dryness  Separation of transport steam and the dry material in a high efficiency cyclone  Reuse of the generated excess steam with a pressure of 1-5 bar, directly or after a re- SuperheatedSuperheated Steam Steam Dryer Dryer (SSD (SSD™) ™) boiler generating clean steam withwith pre pre-heater-heater and and backmixing secondary superheater Ref: http://www.barr-rosin.com/ Advantages Disadvantages

Much faster, Lower net energy More complex equipment consumption and more cost- effective than air drying and than hot-air drying system; freeze drying leak proofing needed; No free oxygen in superheated feed/discharge can be steam, so reduction the difficult decomposition of easily oxidized nutrients such as vitamin C Better product quality, Safe operation, no fire/explosion hazard; no oxidation The simplest and commercially used method for transforming a wide variety of liquid food products into powder form is spray drying.

Spray drying takes place in two stages: 1) Constant rate 2) Falling rate

Ref: Mani, S., S. Jaya and H. Das, 2002 ASAE/CSAE North-Central Intersectional Meeting, CANADA and figure from www.wikipedia.org Feed in

Air in

Product out

Ref: Luis Cisneros-Zevallos, Orange Fruit Processing, Department of Horticultural Sciences,Texas A&M University  In the constant rate period, evaporation takes place at the surface of the particle and the evaporation rate is controlled by the diffusion rate of the vapor through the surrounding air film. The primary driving force is the temperature differences between the surrounding air and the temperature of the particle, which can be considered as the wet bulb temperature of the inlet air.

 In the constant rate period, the diffusion rate of the water through the particle is capable of being greater than the evaporation rate.

Ref: Mani, S., S. Jaya and H. Das, 2002 ASAE/CSAE North-Central Intersectional Meeting, CANADA  Decaffeination of coffee and tea  Separation of essential oils (flavors and fragrances)  Recovery of aroma compounds from orange essential oil by liquid- liquid extraction with concentrated aqueous essence as solvent  Freeze concentration is a process for the separation of soluble solids from an aqueous phase by freezing the water.

 There are two basic methods for freeze concentration: 1) Suspension freeze concentration 2) Film freeze concentration

 These names refer to the mechanism of ice crystal formation employed. Recrystalliz er

Wash Colum n

Removing Heat

Ref: www.gea-messo-pt.com  A typical installation for suspension freeze concentration consists of:

(i) Scraped surface heat exchangers to generate ice nuclei. (ii) Recrystallisers, to increase ice growth and partially control the Gibbs–Thompson effect (iii) A system for separation of the ice from the juice, usually by wash columns, operated at elevated pressures.

 This system is used industrially and has high investment costs.  This process is based on ﬁlm crystallisation, which consists of the formation of a single crystal that grows layer by layer from the solution to be concentrated. The crystal growth (direction of the dendrites) tends to be parallel and opposite to the direction of heat transfer.

 There are no industrial applications for this process.

 Compared to the suspension freeze concentration process, the film freeze concentration process is expected to have lower equipment costs and less energy consumption, yet it will result in greater amounts of impurities in the ice.  Compounds form molecular scale cages of definite size  Forming a crystline latice structure around a molecule  Chemicals which have a high affinity for water vapor or moisture called desiccants.  Solid desiccants (silica gel, activated alumina, activated carbon, natural and synthetic zeolites (molecular sieves), clays, ion exchange resins, some synthetic polymers and …) adsorb water vapor physically or chemically.  Solid-based systems inherently present more design and operating problems than liquid-based systems because handling solids is more difficult. Two approaches can be used: moving (or fluidized) beds for the solids or fixed beds with periodic gas flow switching.  Cyclic operation (PSA, TSA, …)

Ref: M. Carabasa et al., Journal of Food Engineeting 37(1998) 25-41  Including four stages:  Freezing the material, then . Pretreatment: {treating the product prior to freezing} reducing the . Freezing: pressure to allow the {dry ice, methanol, or liquid nitrogen} frozen water in the  Larger crystals: freeze slowly, material to sublimate  Smaller crystals: freeze rapidly directly from the . primary drying: {Sublimation of water} solid phase to the (Remove 95% of water content) gas phase Low pressure & Heat . secondary drying: {Remove unfrozen water } Lower pressure & more Heat A simplified freeze-drying machine  Is the only membrane Vacuum process where phase Pervaporation transition occurs.

 At least the heat of Gas carrier vaporization have to be Pervaporation supply.

 The mass transport is Temperature achieved lowering the difference activity of the permeating Pervaporation component on the permeate side by: gas carrier, vacuum Schematic draws of pervaporation processes or temperature difference.  The driving force is the partial pressure difference of the permeate between the feed and permeate streams.

 The permeate pressure has to be lower than the saturation pressure of the permeant to achieve the separation. ( )

Pervaporation involves a sequence of three steps:

• Selective sorption

• Selective diffusion through the membrane

• Desorption into a vapor phase on the permeate side a membrane technical filtration by applying pressure to the solution on one side of a selective membrane. porous cellulose acetate membrane material Advantages Disadvantages

 high quality products due to  high operating pressures low temperature operation (10 to 200 bar)  maintenance of nutritional, aroma and flavor  lower concentration level compounds

 lower energy consumption  as a first stage process  use of compact installations coupled with other  easy operation processes like osmotic  much more efficient with evaporation energy, time, space, yield, product quality and cost Ref: www.waterworld.com  Direct osmosis concentration (DOC) is another membrane process capable of concentrating fruit juice at low temperatures and low pressures, thereby maintaining original ﬂavor and color characteristics of the fruit.

 The principle uses an osmotic agent (OA) solution to establish an osmotic pressure gradient across a semi- permeable membrane and thus remove water from a single strength fruit juice.

Ref: B. Jiao, A. Cassano, E. Drioli, Journal of food engineering 63 (2004) 303. Ref: B. Jiao, A. Cassano, E. Drioli, journal of food engineering 63 (2004) 303.  An osmotic agent is generally a solid highly soluble in water, hygroscopic, non-toxic, inert toward the ﬂavor, odor and color of the food stuf, and which does not pass through the membrane. Generally, the higher the concentration of the dissolved solids and the lower the molecular weight of the dissolved solids, the higher the osmotic pressure.

 The most frequently employed constituents are: sodium chloride, sucrose or glycerol, cane molasses or corn syrup.

 The OA solutions must have an osmotic pressure greater than that of the concentrated fruit juice.

Ref: B. Jiao, A. Cassano, E. Drioli, journal of food engineering 63 (2004) 303.  Technical aspects: - Low temperatures, low pressures - No fouling problems, constant permeate ﬂux in time - Modularity, easy scale-up - Possibility to treat solutions with high level of suspended solids - Possibility of using modules in series, the same unit can concentrate several diﬀerent products

 Economical aspects: - Low cost of membrane replacement  Technical aspects: - New technology that requires an evaluation at industrial level, ﬂexibility to be evaluated. - The long life of the membrane to be evaluated. - Relatively low permeation (1.8–2.5 l/m2.hr)

 Economical aspects: - High investment costs - High energy consumption  permeation through a membrane and subsequently extraction with a solvent  Mechanism: solution-diffusion under the influence of a concentration gradient  no direct contact between two phases c

d Figure a. b., c. and d: Fruit pulper screw finisher, paddle pulper, paddle pulper with coarse screen, paddle pulper with brushes for soft fruit.