CENCE Introductory Module MATERIAL BALANCE

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KKA 4106 Toxic and Hazardous Waste
EngineeringLecture 6
Dr Robiah Yunus Dept. of Chemical and
Environmental Eng. Universiti Putra Malaysia
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Physical-Chemical Processes

• Includes technologies that can be used for
• hazardous waste treatment
• soil remediation
• Each section includes
• description of technology
• theory
• design

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Physical-Chemical Processes

• What does it mean?
• Chemicals are used to alter and treat pollution
  conditions/ contaminants and to make easier for
  removal process.
• Depends on
• Chemical properties
• Reactivity
• Flammability / Combustibility
• Corrosivity
• Compatibility with other wastes

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Physical-Chemical Processes

• Neutralization (pH adjustment)
• Chemical Oxidation (Redox)
• Air Stripping
• Soil Vapor Extraction
• Carbon Adsorption
• Air Stripping
• Steam Stripping
• Supercritical Fluid Extraction

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Neutralization

• Process of adjusting pH of highly acid or
  alkaline wastes to near neutrality.
• Acidic and alkaline wastewater greatest volume
  in industrial wastewater
• Highly acidic alkaline effluent destroy
  aquatic lives , pose hazards to human, drain,
  sewer structures
• pH for biological system for optimum activity
  6.5 8.5

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Neutralization- Problems

• pH value increases rapidly once the majority of
  acid neutralized
• Only applicable to waste that hazardous due to
  acidity or alkalinity
• Others, only as preliminary step to other
  treatments or ultimate disposal
• General Acid-Base Neutralization
• H OH- ? H2O

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Neutralization Reagents
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Chemical Oxidation (Redox)

• Process of detoxifying waste by adding an
  oxidizing agent to chemically transform waste
  components.
• Can be converted to CO2 H2O or an intermediate
  lesser toxic product.
• Capable of destroying organic (VOC, mercaptans,
  phenols) inorganic (cyanide) molecules.
• Ultraviolet (UV) usually added accelerate to
  the process

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Chemical Oxidation (Redox)

• Widely practiced in plating industry (destroy
  cyanide) pesticide industry (compounds not
  readily detoxified)
• Chemical oxidation oxidation of hazardous
  material (in aqueous solution) by oxidizing
  agents.
• Chemical reduction applied to certain heavy
  metals that are hazardous in oxidized state but
  not in reduced state.

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Chemical Oxidation

• Example Oxidation / Reduction Process
• 4Fe2 O2 10H2O ? 4Fe(OH)3 8H
• 2SO2 O2 2H2O ? 2H2SO4

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Chemical Oxidation

• Principle oxidants ozone, hydrogen peroxide,
  chlorine. The chemicals that are reduced are the
  contaminants.
• Oxidation-reduction reactions occur in pairs to
  comprise an overall REDOX reaction. Oxidizing
  agents are non-specific and will react with any
  reducing agents present in the waste stream.

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Oxidizing Agents
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Chemical OxidationProcess Description

• Completely mixed or plug flow reactor
• Mixing can be provided by
• mechanical agitation
• pressure drop
• bubbling air
• Hydrogen peroxide and UV, ultraviolet light are
  generally used together.
• Common redox reactions - the reduction of
  hexavalent chromium to trivalent chromium using
  sulfur dioxide and ferrous sulfate.

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Chemical Oxidation Example

• Given A waste stream of 5000 gal/day containing
  86 mg/l of hexavalent chromium.
• Find The stoichiometric amount of ferrous
  sulfate required to reduce it to trivalent
  chromium.
• 2CrO3 6FeSO4 6H2SO4 3Fe2(SO4)3 Cr2(SO4)3
  6H2O
• From cover of book, atomic weights Cr 51.996
  O 15.994 Fe 55.847 S 32.06
• From the equation 6 moles of ferrous sulfate are
  required to reduce 2 moles of hexavalent chromium
  or a ratio of 6/23.

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Chemical OxidationProcess Description

• The power of an oxidizing or reducing agents is
  measured by its electrode potential. An
  indication of how a reaction will proceed can be
  determined from the free energy considerations
• G? -nFE? - RTlnK
• It is possible to measure Oxidation Reduction
  Potential (ORP) directly by means of a galvanic
  cell made up of a gold or platinum anode and a
  reference electrode, the cathode.
• Nernst equation
• E E? - (RT/nf)ln Q

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Chemical OxidationDesign

• Ozone is a blue gas with a pungent order and the
  most powerful oxidant available. Ozone has a very
  high free energy which indicates that the
  oxidation reaction may proceed to completion.
  Ozone dissociates to oxygen very rapidly and must
  be generated on site.
• Hydrogen peroxide is effective in oxidizing
  organic in soil through in situ treatment. As
  with ozone, hydrogen peroxide is greatly enhance
  when used with UV.

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Chemical OxidationDesign

• Given Hexachlorobiphenyl
• Find the time required for 60 removal using
  ozone with and without UV.
• 60 removal means 40 remaining or C/Co.4
• For no UV, Timenot doable
• With UV, Time 50 min, UV make a significant
  difference
• Given It is desired to use hydrogen peroxide
  UV to reduce chloroform to 90.
• Find Time required.
• 90 removal means 10 or .1 remaining
• _at_ .1 on the graph for chloroform, time 150
  minutes

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Chemical OxidationChlorine

• Chlorine and its various compounds are used
  extensively in water and waste water treatment
  and is the principal chemical involved in
  disinfection.
• When combined with organic material, chlorine
  forms THM, TriHaloMethanes, which are
  carcinogenic.
• Chlorine is evaporated to a gas and mixed with
  water to provide a hypochlourous acid (HOCl)
  solution.

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Air Stripping

• Air stripping is a mass transfer process that
  enhances the volatilization of compounds from
  water by passing air through water to improve the
  transfer between the air and water phases.
• One of the most common remediation methods for
  VOCs.
• Suited for low concentrations lt 200 mg/l.

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Air Stripping

• Types of processes
• Packed towers,
• tray towers
• spray systems,
• diffused aeration
• mechanical aeration
• Packed towers are generally used for remediating
  ground water.

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Air StrippingProcess Description

• Process consists of counter-current flow of water
  and air through a packing material. The packing
  material provides a high surface area for VOC
  transfer from the liquid to the gaseous phase.
• Typical packing material consists of plastic
  shapes with high surface to volume ratios,
  specific volume.
• R H'(Qa/Qw)
• where, R stripping factor

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Air StrippingTheory

• Two film theory.
• Bulk film to liquid film
• Liquid film to air film
• Air film to bulk air
• Sherwood Holloway equation
• KLa a x DL x (305L/m)1-n(m/rDL)0.5
• Where, Z (depth of column) HTU x NTU
• HTU L/MwKLa
• NTU (R/R1)ln (Cin/Cout)(R-1) 1/R

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Air StrippingPacking

• Different packing shapes are available.

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Air StrippingDesign

• Stripping towers have diameters of 0.5-3m and
  heights of 1-15m.
• The air-to-water ratio is 5 to several hundred
  and is controlled by pressure drop and flooding
  considerations.
• Distribution plates should be placed every
  5-diameters to avoid channeling around the wall
  as opposed to the packing media
• It may be necessary to clean up the off-gas with
  activated carbon.
• The pressure drop in the tower should be between
  0.25-0.5 inches H2O/ft of tower to avoid
  flooding.

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Soil Vapor Extraction

• Soil Vapor Extraction (SVE) is a remedial
  technique to remove VOCs from soil in the vadose
  zone or from stockpiled, excavated soil.
• The vadose zone is the zone above the GWT.

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Soil Vapor Extraction

• SVE consists of passing an air stream through the
  soil, thereby transferring the contaminants fro
  the soil matrix to the air.
• SVE systems can be enhanced
• Install ground water extraction pumps to increase
  the vadose zone and perhaps simultaneously treat
  ground water.
• Impermeable barrier over the surface to minimize
  short-circuiting
• Install air recharge wells.
• Install wells into the ground water.

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Soil Vapor ExtractionTheory

• The removal of VOCs from the vadose zone can be
  modeled as a 5-step process
• Gases desorb from the soil particles.
• Transfer to the soil water.
• Volatize to the soil gas
• Gas migrates to surface
• Released to atmosphere

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Soil Vapor ExtractionTheory

• The movement of contaminants in the soil gas
  through the soil media can be described by two
  processes
• Advection. Movement with bulk airflow through the
  soil media and best describes the flow through
  permeable soils with the unsaturated zone.
• Diffusion. Movement through the soil media via
  concentration gradients. Diffusion tends to
  control in soils of low permeability.

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Soil Vapor ExtractionTheory

• Provided the leak is of sufficient quantity, the
  VOC contaminants will migrate downward through
  the unsaturated zone, leaving globules, films and
  small droplets of the released material.
• Low density contaminants will accumulate in the
  capillary fringe or float on the ground water
  surface.
• Dense contaminants will pass through the ground
  water until encountering a impermeable layer.

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Soil Vapor ExtractionTheory

• A release of contaminants will result in residual
  contamination of the soil pores. This residual
  material in the unsaturated zone is the target
  contamination for cleanup via SVE.
• Diffusion may be the rate limiting step for mass
  transfer.
• Current practice is to utilize empirical models
  to select the most appropriate mechanical system
  and then to use field data to refine system
  design.

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Soil Vapor ExtractionTheory

• Movement of VOCs through the soil is controlled
  in part by diffusion and Fick's Law
• J -DvdC/dz
• The partition coefficient refers to the
  preference of contaminant for soil or water. A
  higher Kp indicates that a contaminant is more
  likely to remain on the soil and not be
  transmitted through soil moisture movement and is
  given by
• Kp X/C

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Soil Vapor ExtractionExample
Given Ethylene Dibromide and hexane. Based on H,
which is a more likely candidate for SVE. Assume
T20C, TC273.220273.2 293K Ethylene
Dibromide, From app. A, p.1046 A5.70 B3.24 x
103 H exp A-B/T H exp5.70-3.24x103/293.2
exp5.70-11.05 4.748 x 10-3 atm.m3/mol
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Soil Vapor ExtractionExample

• Hexane, A25.3
• B7.53 x 103
• H exp A-B/T
• H exp25.3-7.53x103/293.2
• exp-0.382 e-0.382 .682 atm.m3/mol
• Since hexane, .682 atm.m3/mol gt Ethylene
  Dibromide, 4.748 x 10-3 atm.m3/mol, hexane would
  be best suited for vadose treatment by SVE.

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Soil Vapor ExtractionExample

• Hartley equation estimates the volatilization of
  chemicals from soil
• J
• The second term in the denominator, indicates the
  resistance to volatilization due to thermal
  elements and may be neglected for compounds
  significantly less volatile than water.

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Soil Vapor ExtractionDesign

• SVE main benefit is that it is an in situ method,
  thus does not require the removal and
  transportation of the hazardous waste.
• SVE can remediate soil beneath structures does
  not require reagents and employs conventional
  equipment, labor and materials.
• SVE is not appropriate for
• low-permeability soil
• low vapor pressure contaminants
• high ground water table

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Soil Vapor ExtractionSystem

• Infrastructure
• vapor extraction wells, 6-11' deep
• Piping
• monitoring wells
• gauges and valves
• impermeable cover
• vent wells

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Soil Vapor ExtractionSystem

• Equipment
• vacuum/blower unit, 0.5-30" Hg.
• moisture knockout drum
• off-gas treatment
• Three variables control performance
• well spacing (critical)
• air flow rate
• subsurface pressure

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Carbon Adsorption

• Adsorption is a process in which a soluble
  contaminant is removed from water by contact with
  a solid surface typically activated carbon
  usually in granulated form (GAC).
• The activated carbon is placed in cylindrical
  vessel, contaminated water enters the top,
  contacts the carbon and exits the bottom.

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Carbon Adsorption

• Ancillary considerations include a way to
  regenerate the carbon which is done thermally.
• Extensively used in water and wastewater systems
  for the removal of non-biodegradable organics and
  as a polishing step.

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Carbon AdsorptionTheory

• Sorption occurs when a component moves from one
  phase to another across some boundary. In
  adsorption the process takes place at a surface.
• Movement of an organic molecule to a surface
  involves 4 transport phenomena
• bulk fluid transport
• film transport
• pore diffusion
• actual physical attachment

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Carbon AdsorptionTheory

• Driving forces that control adsorption
• chemical affinity between the pollutant and the
  activated carbon.
• electrical attraction
• van der Waal's forces
• hydrophobic nature of the organic

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Carbon AdsorptionTheory

• A plot of the amount of contaminant adsorbed per
  unit mass of carbon, X/M, against the
  concentration of contaminant in the bulk fluid,
  C, is an adsorption isotherm
• The Freundlich isotherm is an empirical model
  mathematically expressed as
• X/M KCf1/n

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Carbon AdsorptionDesign

• The design of adsorption units requires column
  tests that simulate the actual operation of full
  scale units.
• In the lab, 2-inch diameter columns are filled
  with carbon and the contaminated ground water is
  run through the columns. The effluent is
  monitored for the contaminants of interest.
• The adsorption zone is where adsorption takes
  place. Breakthrough is the point where a
  specified amount of the influent is detected in
  the effluent usually 5-10.

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Steam Stripping

• The differences between steam stripping and air
  stripping
• steam not air is the stripping gas
• the stripping gas, steam, is infinitely soluble
  in water
• much higher temperatures are used
• the organics in the water are recovered as a
  separate liquid phase

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Steam Stripping

• Based on distillation. The heated feed water is
  fed to the tank and flows down where it
  encounters the steam which is flowing upward,
  counter-current to the organics. The organics
  volatize and are carried upward the mixture is
  condensed and since the organics are
  supersaturated they separate and are disposed
  of.

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Steam Stripping

• Steam stripping is the purging of contaminants
  from ground water by the use of steam. Capable of
  reducing VOCs to very low concentrations.
• The heated feed water is fed to the tank and
  flows down where it encounters the steam which is
  flowing upward, counter-current to the organics.
  The organics volatize and are carried upward the
  mixture is condensed and since the organics are
  supersaturated they separate and are disposed
  of.

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Steam Stripping

• The differences between steam stripping and air
  stripping
• steam not air is the stripping gas
• the stripping gas, steam, is infinitely soluble
  in water
• much higher temperatures are used
• the organics in the water are recovered as a
  separate liquid phase
• EPA established Best Available Technology
  Economically Achievable (BATEA) for Organic
  Chemical, Plastics and Synthetic Fibers.

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Steam StrippingDesign considerations

• The strippalitity of the organics
• Simple for a single organic, but for a mixture of
  organics, a computerized process simulator to
  assess the thermodynamics of the interactions
  between the various organics is required.
• Rule of thumb. Any priority pollutant that is
  analyzed by direct injection on a gas
  chromatograph can be considered a good candidate
  for high-efficiency stream stripping.
• Rule of thumb. Any compound with a boiling point
  lt150C is a good candidate.

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Steam StrippingDesign considerations

• Whether the organics will form a separate organic
  phase in the overhead decanter
• If one organic compound has a low solubility
  limit (lt1) then there is a basis to expect a
  phase separation at the decanter.
• Some organics are infinitely soluble in water and
  will not form a separate organic phase in the
  decanter and are not good candidates for stream
  stripping.
• Only one sparingly soluble organic need be
  present to make steam stripping feasible. As a
  general rule, the organics will preferentially
  partition to the organic phase created by the
  single sparingly soluble organic.

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Steam StrippingDesign considerations

• Mechanical design
• random packing
• valve trays
• sieve trays

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Supercritical fluids (SCF)

• The contaminated stream is introduced into the
  extraction vessel, heated and pressurized. The
  contaminant dissolves in the SCF which is then
  expanded which lowers the solubility of the
  organic contaminant resulting in separation of
  the organic contaminant from the extracting fluid
  

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Supercritical fluids (SCF)Theory

• The critical point is a temperature/pressure
  where the material exhibits properties between a
  liquid and gas densities approaching the liquid
  phase, dffusivities and viscosities approaching
  the gas phase.
• As a result of these properties, organic
  compounds are highly soluble in SCFs and can
  easily transfer to the SCF from their original
  medium.

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Membrane Processes

• Membrane which is a solid matrix or swollen gel
  refers to a barrier to flow which will allow the
  passage of water, ions or small molecules. Not a
  conventional, gravity, filtration process as the
  driving force may be electrostatic or high
  pressure.
• The membranes are subject to fouling and in
  hazardous waste management, they are limited to
  extremely toxic materials that can not be removed
  by cost-effective technologies.
• Processes include electrodialysis, reverse
  osmosis and ultrafiltration

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Membrane ProcessesProcess Description

• Electrodialysis.
• Consists of the separation of ionic species from
  water by applying a direct-current electric field
• By alternating cation and anion exchange membrane
  between two electrodes, alternate dilute and
  concentrated cells are created.
• The membranes are about .5mm thick and the
  spacers are 1mm thick. Operated at 40-60psi and
  90 of the feed is turned into product water, the
  remainder is concentrate.

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Membrane ProcessesProcess Description

• Reverse Osmosis
• In reverse osmosis, a solvent is separated from a
  solution by applying a pressure greater than the
  osmotic pressure, thus forcing the solvent
  through a semi permeable membrane.
• The membrane will allow the water but not the
  salt to pass.
• In reverse osmosis, a pressure is applied to
  force the salt, solvent through the membrane,
  thus leaving product water.

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Membrane ProcessesProcess Description

• Ultrafiltration
• Ultrafiltration separates solutes from a solvent
  on the basis of molecular size and shape by
  passing the solution through a membrane module
  where a pressure difference is maintained across
  the membrane.
• Water molecules pass through, heavy molecules are
  retained on the filter.

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Membrane ProcessesProcess Description

• Ultrafiltration
• Fouling is avoided by high velocities which in
  turn yields a low efficacy requiring multiple
  passes.
• Molecules of molecular weight greater than 500
  and less than 500,000 can be separated.
• Heavier molecules can be separated by
  conventional filtration.
• The lower size reflects the opening size in
  commercially available membrane.

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Membrane ProcessesTheory

• Electrodialysis.
• Faraday's Law yield required current
• I (FQN/n) x (E1/E2) eq.9-69 units p.537
• Voltage, Ohm's Law
• E IR
• Power
• P I2R
• Current density, CD, is the current passing
  through a unit areas of membrane, amp/m2.

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Membrane ProcessesTheory

• Reverse Osmosis.
• Osmotic pressure, solute rejections and flows
  are of interest. Osmotic pressure is determined
  by the Van't Hoff equation
• ? ?cNCsRT
• The flow is
• Jw Wp x (P -?? )

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Membrane ProcessesExamples

• Manganous nitrate, Mn(NO3)2, salt solution at
  8400 mg/l at 30C. .87, The wastewater has a flow
  rate of 100gpm. A vendor gives the following
  data
• Wp2.0 x 10-6 gmol/cm2.sec.atm
• Area of a bundle 500ft2
• 65 recovery rate
• Optimal pressure across the membrane 625 psi
• Find
• 1.) The Osmotic Pressure.
• 2.) Flow through the membrane, Jw
• 3.) Number of bundles required

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Membrane ProcessesSolutions

• 1.) Reverse osmosis
• N 3 (Mn(NO3)2, Mn1, NO32)
• Molecular weight
• Mn24.305x123.305 N 14x 228 O16x696
• MW for Mn(NO3)2147.3 g/gmol
• Cs 8400 mg/l 8.4 g/l / 147.3 g/gmol
• Cs .0570 gmol/l
• K C 273.2 30 273.2 303.2
• ? ?cNCsRT
• .87 x 3 x .0570gmol/l x .082 atm.l/gmol.K x
  303.2K
• 3.698 atmospheres x 14.7psi/atm. 54.35psi

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Membrane ProcessesSolutions

• 2.) Flow, Jw
• Jw Wp x (P - ) units p.540
• Jw 2.0 x 10-6 gmol/cm2.sec.atm x (625-54.35)
  (1atm/14.7psi)
• Jw 7.76 x 10-5 gmol/cm2.sec
• 3. ) Number of bundles for 100gpm
• Q 100gpm x 3.79l/gal x 1min/60sec x 1000g/l x
  1gmol/18g (AWof water) x cm2.sec/ 7.76 x 10-5
  gmol
• Q 4.52 x 106 cm2 452 m2of membrane
• Each bundle contains 500 ft2.
• No. of bundles 452 m2 x Bundle/ft2 x
  (3.28)ft2/m2
• No. of bundles 9.73 use 10 bundles