WATER PURIFICATION BY DESALINATION METHODS AND

PRESSURE DROP CALCULATIONS

INTRODUCTIONThe scarcity of fresh water resources and the need for additional

water supplies is already critical in many arid regions of the world and will be increasingly important in the future. It is very likely that the water issue will be considered, like fossil energy resources, to be one of the determining factors of world stability. Thus, it is of utmost importance to fabricate methods to use sea water as drinking water so as to fulfill the rising demand of water supply. This can be done by water desalination methods which remove the salt content and other unwanted compounds from water thus making it suitable for various applications.

DESALINATION It is a process that removes or separates salts from saline water to give fresh

water, at the expense of energy.Depending upon the type or form of energy used, Desalination Processes

can be broadly classified into two groups:

1. Thermal Desalination

2. Membrane Desalination

THERMAL DESALINATIONThermal desalination processes involves heating of saline

water to its boiling point to produce water vapor, this pure vapor is condensed to produce fresh water.

The three types of thermal desalination units used commercially are:

1. Multistage flash (MSF)

2. Multiple effect distillation (MED)

3. Low Temperature Evaporation (LTE)

Multi-Stage Flash (MSF) Distillation

Multi-Effect Distillation (MED)

Low Temperature Evaporation

MEMBRANE DESALINATION Membrane processes use a semi permeable membrane to move water across the

membrane from the salt solution to produce fresh water on the other side of the membrane.

Membrane desalination is classified depending on the driving force.

Process Size of materials retained

Driving force

Microfiltration 0.1-10.0 microns molecules

Pressure difference

Ultrafiltration 5-100 nm molecules Pressure difference(1 - 4 bar)

Nanofiltration 0.5 - 5 nm molecules (mostly charged species)

Pressure difference(5 - 15 bar)

Reverse Osmosis < 1 nm molecules Pressure difference(10 - 60 bar)

MICROFILTRATIONMicrofiltration is a process of separating material of colloidal size and

larger than true solutions. The MF membranes are made from natural or synthetic polymers such as

cellulose nitrate or acetate, polyvinylidene difluoride (PVDF), polyamides, polysulfone, polycarbonate, polypropylene, PTFE etc. The inorganic materials such as metal oxides (alumina), glass, zirconia coated carbon etc. are also used for manufacturing the MF membranes.

Applications of MF are:

1. Food & beverages

2. Chemical industry

3. Microelectronics industry

4. Fermentation

ULTRAFILTRATIONUltrafiltration is most commonly used to separate a solution that has a

mixture of some desirable components and some that are not desirable. Rejected species include sugars, biomolecules, polymers and colloidal particles.

Applications of MF are:

1. Oil emulsion waste treatment

2. Treatment of whey in dairy industries

3. Concentration of biological macromolecules

4. Electrocoat paint recovery

5. Concentration of textile sizing

6. Concentration of heat sensitive proteins for food additives

7. Concentration of gelatin 

NANOFILTRATIONThe separation mechanism of NF involves size exclusion as well as

electrostatic interaction. In NF, organic molecules with molecular wt. greater than 200-400 are rejected.

Membranes used for NF are of cellulosic acetate and aromatic polyamide type.

Applications of NF are:

1. Concentration of sugars, divalent salts, bacteria, proteins, particles, dyes and other constituents that have a molecular weight greater than 1000 daltons.

2. Removal of color and total organic carbon (TOC) from surface water

3. Removal of hardness from well water

4. Overall reduction of total dissolved solids (TDS)

REVERSE OSMOSISRO membranes give 96%-99% NaCl rejection. Greater than 95-99% of

inorganic salts and charged organics will also be rejected by the membrane due to charge repulsion established at the membrane surface.

RO membranes are made of polymers, cellulosic acetate and aromatic polyamide types.

Applications:

1. Potable water from sea or brackish water

2. Ultra pure water for food processing and electronic industries

3. Pharmaceutical grade water

4. Water for chemical, pulp & paper industry

5. Waste treatment etc.

6. Municipal and industrial waste treatment

DEVELOPMENT OF MEMBRANESUltrafiltration: Chemicals Used:

18-20% (w/w) in N-Methyl pyrrolidone (NMP) solvent

Ultrafiltration membrane is basically used for pre-treatment of water so as to reduce the process cost and it is also used as a support for nanofiltration and reverse osmosis membranes.

Nanofiltration:

Chemicals Used:

1.Solution made of poly(ether)sulfone (PES)

2.Solution of polyamide (PA)

The nanofiltration membranes should possess good selectivity, good rejection ability and good flux. The parameters involved in the development of the membrane should be optimized so as to obtain a good quality membrane.

Reverse Osmosis:

Chemicals Used:

1. 2% amine solution (MPD)

2. Organic solution of acid chloride (TMC)Comparison between Membranes:

Microfiltration, Ultrafiltration

Nanofiltration, Reverse Osmosis

Energy required for the operation is less.

Energy required for the operation is very high.

They have a good selectivity. They only separate suspended solids.

They do not have a good selectivity. They separate suspended as well as dissolved solids.

MEMBRANE PREPARATION Membrane preparation was done using the immersion precipitation technique. Dry polysulfone beads were taken in air tight bottles and then a specific amount of DMF was

added to dissolve the polymer. The same method was applied the PVDF powder. A piece of fabric was kept on the glass plate and the casting solution was spread on it evenly. The entire assembly was immediately immersed in a room temperature gelling bath made by

using Ultra Filtered water. The membranes were stored in laboratory refrigerator maintained at 5oC.

Membrane Polymer Polymer conc. (% w/w)

Solvent(DMF) (% w/w) Gelling medium used

PSf1 Polysulfone 18% 82% Ultra filtered water

PSf2 Polysulfone 18% 82% 2% v/v DMF in ultrafiltered water

PVDF1 Polyvinylidene fluoride 15% 85% Ultrafiltered water

PVDF2 Polyvinylidene fluoride 15% 85% 2% v/v DMF in ultrafiltered water

RESULTSExperiment PSf1 PSf2 PVDF1 PVdF2

(1) Pure water permeability Pure water flux

1333.33 LMD 1000 LMD 2000 LMD 1571 LMD

(2) PEO rejection Product flux % Rejection

975 LMD66.2%

850 LMD67.2%

625LMD87.2%

500 LMD94.66%

Experiment PSf2 PVDF2

(1) BSA Product flux % rejection

875 LMD75.56%

875 LMD81.21%

/(2) Sodium AlginateProduct flux % rejection

1000 LMD85.36%

100 LMD86.58%

Renewable Energy based Desalination Conventional sources of energy are depleting fast and hence there is an

urgent need to find renewable sources of energy. Desalination can also be carried with the help of renewable sources of energy such as solar energy. One of the methods is Solar Reverse Osmosis. This setup can also be done in those areas where access to grid electricity is not possible. Also, renewable energy based methods are pollution free and environmental friendly.

Solar Reverse Osmosis Unit:• POWER PACK• PRE-TREATMENT• REVERSE OSMOSIS MEMBRANE• POST TREATMENT

PIPE PRESSURE DROP CALCULATIONS

Factors affecting pressure drop calculations:1. Friction between the fluid and the wall of the pipe

2. Friction loss as the fluid passes through any pipe fittings, bends, valves, or components

3. Pressure loss due to a change in elevation of the fluid (if the pipe is not horizontal)

4. Friction between adjacent layers of the fluid itself

FORMULAE USED Tube id is given by, Tube id (D) = (tube od – (2*thickness)) The velocity (v) of fluid is estimated based on internal cross sectional area of pipe

which is shown as follow. v = (mass flow rate/(ρ*cross sectional area)) Then, Reynold's number is: Reynolds number (Re)=((ρ*v*D)/µ) Viscosity depends upon the temperature of the liquid and is calculated from the

correlation. The friction factor (f) is: f = 1/ ( 16*(Log(((e/D)/3.7)+(5.74/(Re^0.9))^2)) From the friction factor, pressure drop (pd1) due to friction is calculated. Pd1=dp = (2 * f * l * ρ * (v2)) / D The pipe fitting is selected and its corresponding k value is selected. Therefore, pressure head(hf) due to pipe fittings is, hf= =(k*((v)^2))/2*g pressure drop(pd2)=(ρ*g*hf) Total pressure drop (Tpd) across the piping system is as follows: Tpd = (pd1+pd2)

Pressure drop calculations in MS Excel A program has been developed in MS Excel

for estimation of pressure drop based on above methodology.

FUTURE WORKDesigning of Low Temperature Evaporator

( LTE )for Desalination of Sea Water

Calculation of Heat Transfer Area of the various stages of LTE and optimising the design with the given flow rate of product water.