Environmental technologyfor

Cleaner Production

Mårten EricsonResearch engineer

Content• Introduction to cleaner production • Ion exchange- How it works, mechanisms & generic case

- Applications

• Adsorption• Absorption

• Catalytic reduction

• Condensation

• Membrane techniques• Summary - What have we learned

Cleaner production• Environmental technology is a tool for Cleaner

Production

• Cleaner Production strategies: • Raw material

• Process

• Equipment

• Process control

• Management

• Separation and extraction

• Product design

• Internal/external

Things to concider for an engineer to solve a environmental problem

• Current status – total flows, concentrations, amounts, running conditions

• What should be separated? – Particles, solubles, in water or air?

• What to do with the separated ”product”

• Efficiency

• Stability of method

• Space requirements

• Economy

• Maintenance

Kidney

Pollutants

Processstage x

Processstage y

Processstage x

Processstage x

Kidney

Pollutants

Different unit operation can be used for separation of certain components in order to prolong the usage time of a processsolution - kidney function

Separation operations for cleaner production solutions

Common unit operations for the separation stage are e.g.Ion exchange, RO, UF, Stripping a.o

Separation operations for cleaner production solutions

Processstage x

Separation stage

Recycling of a component

Common unit operations for the separation stages are:

Ion exchangeEvaporationMembrane processes, e.g. RO and UFExtractionStripping

Different unit operation can be used for separation of certain components from a process flow in order to recycle them intothe process - recovery function

Process stage

Separation stage, e.g. adsorption, UF,

RO a.o.

Specific com-pounds to be handled as waste

Sludge

Waste water treatment stages

Effluent

Separation operations for cleaner production solutions

Different unit operation can be used for separation of certain components in a wastewater flow from a process in order toprotect for instance the biological stage in the external waste-water treatment plant from toxic substances

Ion exchange

• Ion exchange definition: Exchange of ions between two electrolytes or between an electrolyte solution and a complex.

• What is an ion?

• When can we use ion exchanger (to be answered later)

Me2+

An-

R-H+

H+

An-H+

An-

2 R–H + Me2+ R2–Me + 2H+

R2–Me + 2H+2 R–H + Me2+

Ion exchange reaction:

R-2Me2+

Me2+

An-

Ion exchangeIon exchange RegenerationRegeneration

Low conc.High conc.

Cationresin

Regeneration reaction:

Classification of synthetic ion exchange resins

Type of Functional Ion to exchange resin group 1. Strong acid -SO3

-H+ Cations in general cation resin 2. Weak acid -COO-H+ -’’- -’’- , espec. cation resin Ca2+, Mg2+, Na+

Cs+ & multi-valent cations

3. Strong base Quaternary Anions, espec. fr. anion resin amine weak acids (CN-, CO3

2-, SiO32-)

4. Weak base Primary, secon- Anions to strong anion resin dary and ter- acids (SO4

2-, Cl-, tiary amine NO3

-, CrO42-,

HPO42-)

5. Chelating Cations, espec. resins heavy metals

O-H+

Typical exchange capacities for synthetic resins are 2 - 10 eq/kg resin

Selectivity for ions - a strong acid cation resin and a strong base

anion resin Cations Anions Pb2+ 9,9 NO3

- 3,0-4,0 Ca2+ 5,2 Cl-

1,0 Ni2+ 3,9 HCO3

- 0,4 Mg2+ 3,3 SO4

2- 0,15 Na+ 2,0 F- 0,1 H+ 1,3 OH- 0,06 Li+ 1.0 CO3

- 0,03

Notice - the relative selectivity to different ions is depending on which ion exchange resin that is in use.

Decreasing Decreasing selectivityselectivity

Important parameters to concider

• When can we use ion exchange?

• Load

• Concentration

• Contaminants – particles, other metals?

Applications

• Applications in biochemistry, chemistry

• Metal plating – chromating (Cr3+, Cr2O72-,

CrO42-)

• Wastewater containing NH4+ (nitrogen)

Ion

exchanger

Product

RinseEconomy rinse

Water

Process bath

Using ion exchange in order to increase the recovery of metals

from an economy rinse

Concentrate

Drag out

Water

H+

To wastewater treatment

Ion

exchanger

To waste water treatment

Rinse 1

Water

Process bath

Using ion exchange as a kidney in order to clean the rinsing water

Drag out

Rinse 2

To waste water treatment

Me2+

H+

Product

Ion exchanging as a polishing method after a chemical metal precipitation stage

The ion exchanger will give a very clean water. Since the ion exchanger is in use as a polishing stage the ion exchanger doesn´t have to be regenerated so often.

Effluent

OH-

Waste water containing metals

PrecipitationPrecipitation

Flocculating agent

Sludge

SettlingSettling

Ion exchangeIon exchange

FlocculationFlocculation

Movie 1

Absorption• Definition: The process by which one

substance, such as a solid or liquid, takes up another substance, such as a liquid or gas, through minute pores or spaces between its molecules. A paper towel takes up water, and water takes up carbon dioxide, by absorption.

Physical absorption

• Physical absorption involving such factors as solubility and vapor-pressure relationships

• Examples: Acetone can be recovered from an acetone–air mixture by passing the gas stream into water in which the acetone dissolves while the air passes out

• Ammonia may be removed from an ammonia–air mixture by absorption in water

• Particles can be removed from a particle-air mixture by absorption in water

Chemical absorption

• Chemical absorption involving chemical reactions between the absorbed substance and the absorbing medium

• Examples: Oxides of nitrogen can absorbed in water to give nitric acid

• Carbon dioxide is absorbed in a solution of sodium hydroxide

• Removal of SOx using CaO/CaCO3 slurry or Na2SO3

Design of equipment

• In considering the design of equipment to achieve gas absorption, the main requirement is that the gas should be brought into intimate contact with the liquid, and the effectiveness of the equipment will largely be determined by the success with which it promotes contact between the two phases.

Equipment

Equipment

Spray scrubber Spray scrubber with rotating air flow

Counter cross flow spray scrubber

Equipment

Venturi scrubber Cascade scrubber

Adsorption

• Adsorption definition: adhesion of molecules to a solid surface

• Two types of adsorption: physical /chemical

Chemisorption

Chemisorption is characterized by strong interaction between adsorbate and substrate surface (chemical bond between reactant and surface)

Binding energy: 1-10 eV

Physisorption

Physisorption is characterized by mainly Van der Waals bonds between adsorbate and substrate surface

Binding energy: 10-100 meV

Desorption/Regeneration

• Chemical desorption- Using an acid

- Using a base- Using an organic solvent

• Thermal regeneration- The carbon is heated in an oven and the adsorbate is driven

off as gas – the adsorbate is oxidized and destroyed

Thermodynamics

Spontaneous: ΔG < 0Non-spontaneous: ΔG > 0

Δ H (enthalpy): heat content of a systemΔ S (entropy): measure of how organized/disorganized a system is

Adsorption = exothermicHow will the temperature affect the adsorption?

ΔG = Δ H - T Δ S

About adsorbents

• Adsorbents used today:- Activated carbon

- Zeolites

- Polymeric adsorbents

• Tomorrow?- Super activated carbon (>3000 m2/g)

- Magnetic adsorbents

Specific surface area: 500-1500 m2/g

Capacity: 100-200 g/kg

Activated carbon is used for wastewater treatment and the substances should have the following properties:

- High molecular weight

- Low solubility in water

- Low polarity

- Low temperatureNotice: when adsorption of many substances in a water the

adsorption capacity of any individual compound is lower than if this compound is alone in the water. But the total adsorption may be higher

Activated carbon

• Activated carbon- High adsorption efficieny, even when the substance has a low

concentration in the water

- High adsorption capacity

- Difficult to regenerate- Flat breaktrough curve

• Polymeric adsorbents:- Lower adsorption capacity

- Easy to regenerate

- Low adsorption efficiency at low concentrations

- Steep breakthrough curve

• Conclusion: - Activated carbon – polishing method- Polymeric adsorbent – recovery

Characteristic comparison

Adsorbent Specific surface area

(m2/g)

Pore volume (cm3/g)

Mean pore diameter (Å)

Relative cost

Activated carbon (granular) 700-1300 1 30-59 1Activated carbon (powdered) 800-1800 1 40-60 3

Zeolite 700 0.3 3-10 5

Polymeric (PS, DVB) 350 0.4 90 7

Polymeric (acrylate esther) 450 0.4 80 7

Adsorption

Important parameters to concider:

• Partition coefficient (distribution coefficient)

• Concentration

• Flows

• Temperature

• Polarity

Liquid containing organic substances at low concentrations!

Applications I

• Domestic water cleaning – to remove substances givin water a bad taste or odour

• Municipal wastewater treatment (when a high cleaning efficient is necessary)

• Industrial wastewater treatment especially to get a toxicity reduction

• Process internal cleaning

• Wastewater treamtent with the PACT-process (activated sludge + activated carbon)

Important to remember!

• Adsorption is usually a polishing method and is not used to recover substances!

Movie 2

Condensation

• Condensation is the change in the phase of matter from the gaseous phase into liquid droplets or solid grains of the same element/ chemical species.

• Condensation commonly occurs when a vapor is cooled and/or compressed to its saturation limit (dew point) when the molecular density in the gas phase reaches its maximal threshold.

Equipment

• Heat exchangers (tubes)

• Scrubbing with water

Applications

• Separation of water soluble Hg in flue gases

• Lots of different salts will go out with the condensed water

• Energy!!! Lots of energy in water vapour

• Recovery/separation of solvents with high boiling point (why high boiling point?)

Catalytic reduction

• Reduction of compounds – many toxic compound can be transformed to less toxic for example NOx N2

• Oxidation of HC, CO (catalyst in cars most common) CO2 & H2O

• NOx - where, what, when

SNCR

• SNCR – selective non catalytic reduction

• Use ammonia (NH3) for the reduction of NOx

• Directly spray NH3 into the furnace

• Important reactions can be described with these formulas

4NO + 4NH3 + O24N2 + H2O

6NO2 + 8NH3 7N2 + 12H2O

SCR• SCR – selective catalytic reduction

• Chemical reactions in a reactor with a catalyst (TiO2/V2O5)

SNCR vs SCR

• Investment

• Cost

• Reduction %

• Pollution/de-activation

• Placement

• Running conditions

Introduction to membrane filtration

• Oldest separation technique? Separation technique – sieving or diffusion

• Many applications

Permeate

Feed water

Semipermeable membrane

Retentate

Microfiltration (MF)

• Separation mechanism: Sieving

• Separates: Particles with diameter 0.2-10 µm

• Pressure: 0.01-0.1 MPa

Applications

• Last stage after chemical precipitation of waste water from surface coating industry.

Ultrafiltration (UF)

• Separation mechanism: Sieving

• Separates: Particles with diameter 0.001-0.1 µm

• Pressure: 0.2-1.5 MPa

Ultrafiltration

• Ultrafiltration for good purification of waste water. Can also be used for pre-concentration and then as a ”recovery function”

Applications

• Treatment of alkaline degreasing bath (kidney)

• Treatment of oil emulsions

• Electrodip painting industry

UF for alkaline degreasing

• Using an UF in order to clean a degreasing bath results in a longer life time for the bath (4-5 times longer). That means:

- Decreased chemical consumption- Decreased water consumption

- Decreased waste production (40-50 times lower)

Movie 3

Nanofiltration (NF)

• Separation mechanism: Sieving + membrane diffusion

• Separates: Molecules with diameter 0.001-0.01 µm

• Pressure: 2-4 MPa

Nanofiltration (NF)

• Nanofiltration ranges somewhere between ultrafiltration and reverse osmosis

• Relative new technology

• Lower pressure as compared with RO which reduces the operation cost significantly

• However, problem with fouling

Applications

• Used for removal of contaminants from water

• Desalination of water.

Reverse osmosis (RO)

• Separation mechanism: Membrane diffusion

• Separates: Molecules with diameter 0.0001-0.002 µm

• Pressure: 2-10 MPa

Reverse osmosis

Reverse osmosis

Applications

• Surface coating industry – preconcentration of cromic acid bath

• Chemical or galvanic industry that works with Ni, Cu, Zn etc… use RO instead of IE

• Desalination

• Polishing method for ultra-pure water

• Leechate water from landfills

Important!

• RO is mainly a cleaning technology NOT for pre-concentration. This is because the osmotic pressure over the membrane is very large if the concentration gradient is large.

• In the example with cromic acid is the level of pre-concentration not that high…

Electrodialysis (ED)

• Similar to electrolysis. In principle, two membranes (cationic and anionic specific) that only let positive or negative charged ions pass through. The ions are drawn to two electrodes.

• Is a pre-concentration method

• Limitations: working best at removing low molecular weight ionic components

Applications

• Desalination and production of salt (economically favorable if not ultra pure water is required)

• Can chose ion selective membranes so that one can separate several cationic/anionic ions (not 100% selective though)

• Acid retardation. ED also take the acid which are in complex thus a better method compared to ion exchange

Running conditions• Velocity over membrane surface- Increased velocity -> higher flux

• Pre-treatment- Better pre-treatment minimize the clogging of filter

• Temperature- For most liquids does the flux increase with higher temp. (viscocity)

• Pressure- The flux increase linear to the pressure up to a certain level

• Concentration- The flux decreases with increasing concentration

Membrane properties

• Cut off – The molecule weight of the smallest material rejected by the membrane (how “thick” is the membrane and the pores)

• NaCl retention – Describes the removing properties of a RO membrane (how much is going through)

• Flux – Volume or mass rate of transfer through a membrane: RO = 50 l/m2,h. UF= 200-250 l/m2, h

Membrane properties

• Temperature – New types of materials in the membranes that can handle temperatures above 100 degrees celcius

• pH – Today there are membranes that works at all pH (1-14)

Comparison

Process Operating pressure, kPa

Energy consumption, kWh/m3

Microfiltration 100 0.4Ultrafiltration 525 3Nanofiltration 875 5.3

Reverse osmosis I 1575 10.2

Reverse osmosis II 2800 18.2

Summary – what have we learned

• Ion exchange – how it works, mechanisms, generic case and applications

• Absorption - how it works, mechanisms, generic case and applications

• Adsorption - how it works, mechanisms, generic case and applications

Summary – what have we learned

• Different membrane techniques and when to use them

• Membrane properties and how to affect the flux

• SCR/SNCR

Summary – what have we learned

Further reading• Coulson & Richardson Vol 2. Particle Technology and

Separation Processes (membrane techniques, absorption, adsorption, ion exchange)

• Atkins/de Paula, Physical chemistry (for understanding the theory behind adsorption, RO etc)

• Per Olof Persson et al. Chapter 2-6 from the "Environmental Technology - strategies and technical solutions for a sustainable environmental protection". Can be ordered through Industrial Ecology, KTH.