Using Separations in Chemical Processing

reactor separator

1

2

3

4

6

5

rawmaterials

productsrecycle 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/hmode: vacuum, atmospheric or pressure up to 3 bartemperature: 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 CyprusMultistage flash distillation

A steam stripping column removes H2S and CO2 to regenerate the amine

Absorption and stripping

A column filled with an amine solution is used to absorb H2S and CO2 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 HgI2 for use in semiconductor manufacturing, as well as in detectors for X-ray and g-ray imaging

Crystallization

Multiple-Effect Crystallizer for Sodium Sulfate (Na2SO4) 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 50-90 % of the capital investment in a chemical plant

• Separations can also represent 40-70 % of operating costs

• Purity requirements depend on market tolerance– High separations costs tolerated for high value-

added products

Examples1. 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. SemiconductorsSiO2 SiCl4 Si

Metallurgical grade (97%) for alloying with steel and Al: $1/kgSolar grade for photovoltaics (99.99 %): $80/kg

4. Water treatmentIndustrial wastewater vs. potable waterSome impurities ok (Ca2+); others not (Hg2+)

Process Diagram for Ethylene hydration: C2H4 + H2O C2H5OH

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 drumhorizontal drum

valve

heat exchangers condensor

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

vaporthermal equilibrium: Tliq = Tvap

mechanical equilibrium: Pliq = Pvap

chemical equilibrium: mliq = mvap

(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 Ki = 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

vaporfeed

flow rate FTF, PF

composition zi

vapor product

flow rate VT, Pcomposition yi

liquid product

flow rate LT, Pcomposition xi

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., mi

L = miV ≠ mi

F

• if > 1 chemical species present, then xi ≠ yi

• therefore separation has occurred• vapor-liquid equilibrium (VLE) limits the

amount of separation that can be achieved

Cascade of equilibrium stagesWhat 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 1F

V

Lstage 2

V2

L2

stage 2

V3

L3

1

1

Better Alternative: Counter-current cascade

1

F V1

L1 V2

2

L2 V3

3

L3

• replace by

• replace by

1

F V1

L1 V2

2

L2 V3

3

L3

Variable temperature cascadeT1 > T2 > T3

Variable pressure cascadeP1 > P2 > P3

compressor

weir

perforated tray 3

2

1

F V1

L1

V2

L2

V3

L3

An even better alternativeIntegrate the heat exchangers:allow contact between condensing vapor and vaporizing liquid streams

downcom

er

• integrated column is isobaric and non-isothermal• promotes mixing of liquid and vapor phases

vapor

liquid

1

F V1

L1 V2

2

L2 V3

3

L3

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 timetheoretical stage: assume equilibrium is achievedactual 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.