Using Separations in Chemical Processing reactor separator raw materials products recycle stream

Where are separations needed? Purification of reactor feeds Purification of products for sale Purification of waste for safe disposal

Separations as Unit Operations The specific design of the separator depends on the chemical composition of the feed, and the desired purity of the product However, the general design principals are independent of the chemistry

A multi-purpose distillation column for mineral oils and chemicals 40 trays with multiple feed entry options, capacity: up to 45 mt/h mode: vacuum, atmospheric or pressure up to 3 bar temperature: up to 320°C At an oil refinery, fractional distillation columns separate hydrocarbons into separate streams, cuts or fractions Column distillation

Flash vaporization Flash drum for hydrocarbon vapor recovery Water desalination plant in Cyprus Multistage flash distillation

A steam stripping column removes H 2 S and CO 2 to regenerate the amine Absorption and stripping A column filled with an amine solution is used to absorb H 2 S and CO 2 from “sour” natural gas

Liquid-liquid extraction Mixer-settlers used for continuous, counter-current liquid-liquid extraction of rare-earth ions

Leaching Cyanide leaching of gold ore, Nevada

Sublimation Sublimation of HgI 2 for use in semiconductor manufacturing, as well as in detectors for X-ray and  -ray imaging

Crystallization Multiple-Effect Crystallizer for Sodium Sulfate (Na 2 SO 4 ) Refining Crystallizer for Salt (NaCl)

Chromatography Chromatography columns

Membrane filtration Water Treatment Plant. Each white vessel contains seven spiral-wound membrane units.

Why is good design important for separations? Separations equipment can be % of the capital investment in a chemical plant Separations can also represent % of operating costs Purity requirements depend on market tolerance – High separations costs tolerated for high value- added products

Examples 1.Petroleum refining crude oil  gasoline, diesel, jet fuel, fuel oil, waxes, coke, asphalt 2.Pharmaceuticals sub-ppm level of metal catalyst required for human consumption 3.Semiconductors SiO 2  SiCl 4  Si Metallurgical grade (97%) for alloying with steel and Al: $1/kg Solar grade for photovoltaics (99.99 %): $80/kg 4.Water treatment Industrial wastewater vs. potable water Some impurities ok (Ca 2+ ); others not (Hg 2+ )

Process Diagram for Ethylene hydration: C 2 H 4 + H 2 O  C 2 H 5 OH

Why do separations cost a lot? “unmixing” causes reduction in entropy this is not spontaneous achieve by adding an external separating agent – Energy (distillation) – Material (e.g. extraction) – Barrier (e.g. membrane) – Gradient (e.g. electrophoresis)

Table 1. Separation Unit Operations based on Phase Creation or Addition column with trays (stages) vertical drum horizontal drum valve heat exchangerscondensor reboilers

Table 1, cont. Separation Unit Operations Based on Phase Creation or Addition heater

Table 1, cont. Separation Unit Operations Based on Phase Creation or Addition

Table 2. Separation Unit Operations based on a Solid Separating Agent

Table 3. Separation Unit Operations Based on the Presence of a Barrier

Table 4. Separation Unit Operations Based on an Applied Field or Gradient

Equilibrium-staged separations Make use of thermodynamics to achieve spontaneous separation But thermodynamics also dictates the limits of the separation

Definitions of equilibrium liquid vapor thermal equilibrium: T liq = T vap mechanical equilibrium: P liq = P vap chemical equilibrium:  liq =  vap (chemical potential) Equilibrium is dynamic: molecules continue to vaporize and condense, but at equal rates, so there is no net change in either phase. Rate of approach to equilibrium depends on: (1) rate of mass transfer proportional to (a) mass transfer coefficients K i = f(T), and (b) interfacial area (2) concentration gradient becomes very small as equilibrium is approached, ∞ time required to achieve

Consider a single equilibrium stage 25 liquid vapor feed flow rate F T F, P F composition z i vapor product flow rate V T, P composition y i liquid product flow rate L T, P composition x i T, P V and L are in equilibrium with each other; they are streams leaving the same equilibrium stage. V and L are not in equilibrium with F, i.e.,  i L =  i V ≠  i F if > 1 chemical species present, then x i ≠ y i therefore separation has occurred vapor-liquid equilibrium (VLE) limits the amount of separation that can be achieved

Cascade of equilibrium stages What if we need more separation than one equilibrium stage can provide? Feed one of the two product streams (e.g., L) to another equilibrium stage Creates many vapor streams with different compositions If we combine (mix) them, we destroy some of the separation we created If we discard them, our yield is low. stage 1 F V L stage 2 V2V2 L2L2 V3V3 L3L3 1 1

Better Alternative: Counter-current cascade 1 1 F V1V1 L1L1 V2V2 2 2 L2L2 V3V3 3 3 L3L3 replace by 1 1 F V1V1 L1L1 V2V2 2 2 L2L2 V3V3 3 3 L3L3 Variable temperature cascade T 1 > T 2 > T 3 Variable pressure cascade P 1 > P 2 > P 3 compressor

weir perforated tray F V1V1 L1L1 V2V2 L2L2 V3V3 L3L3 An even better alternative Integrate the heat exchangers: allow contact between condensing vapor and vaporizing liquid streams downcomer integrated column is isobaric and non-isothermal promotes mixing of liquid and vapor phases vapor liquid 1 1 F V1V1 L1L1 V2V2 2 2 L2L2 V3V3 3 3 L3L3

Thermodynamic considerations Perfect separation requires an infinite number of equilibrium stages The engineer specifies the number of stages required for an acceptable degree of separation Equilibrium is not achieved on each stage in a finite time theoretical stage: assume equilibrium is achieved actual stage: equilibrium is not achieved (< 100 % efficiency) We always need more than the theoretical number of stages to achieve the desired separation The engineer’s role is to decide how many more

General design procedure for equilibrium-staged separations 1.Obtain relevant equilibrium data (where?) 2.Determine no. of theoretical stages required 3.Determine no. of actual stages required (requires knowledge of stage efficiency) 4.Size equipment, based on expected flow rates F, V, L * * Focus of this course.