February 28, 2009

Reverse Osmosis


Background:
Diffusion is the movement of molecules from a region of high concentration to a region of lower concentration. Osmosis is a special case of diffusion in which the molecules are of water and the concentration gradient occurs across a semipermeable membrane.

Diffusion and osmosis are thermodynamically favorable and will continue till equilibrium is reached. Osmosis can be slowed, stopped or even reversed if sufficient pressure is applied to the membrane from the ‘concentrated’ side of the membrane.


Introduction:
Reverse Osmosis is the movement of water molecules across the semipermeable membrane against the concentration gradient, from a region of low concentration to a region of higher concentration, by applying a pressure in excess of the osmotic pressure.



The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. The semipermeable membrane allows the passage of water but not ions (Na+, Ca+2, Cl-1) or larger molecules (e.g. Glucose, urea, bacteria)



Uses:

- One of the methods used for desalinating water
- Purification of ethanol ( here water is present as an undesirable impurity)
- Commercial and residential water filtration


Industrial Applications:
Reverse Osmosis systems can be used to treat boiler feed water, industrial wastewater, process water and more. A few of the major uses are :

  • BOILER FEED WATER TREATMENT - RO is used to reduce the solids content of water prior to feeding into boilers for the power generation and other industries.

  • PHARMACEUTICALS - RO is an approved treatment process for the production of United States Pharmacopeia(USP) grade water for pharmaceutical applications.

  • FOOD & BEVERAGE - Water used to process food products and to produce beverages is often treated by a RO system.

  • SEMICONDUCTORS - RO is an accepted component of a treatment process to produce ultrapure water in the semiconductor industry.

  • METAL FINISHING - RO systems have been successfully applied to a variety of metal finishing operations including several type of copper, nickel and zinc electroplating; nickel acetate seal and black dye.


Cost Benefits of RO:
RO is increasingly being adopted by power producers as a treatment method for purifying boiler feed water, makeup water and in zero-liquid discharge applications. The injection of high-purity water produced by RO technology into a gas turbine can improve operating efficiency and increase energy output by 10 percent or more.

There are other cost benefits as well. For example, the prices of acid and caustic solutions continue to rise while the prices of RO units and membrane elements continue to decrease. The primary cost for operating RO systems is electricity, and since these systems consume very little energy, operating costs are relatively low.

Operating efficiency differs between ion exchange beds and RO systems. Cation and anion resin beds must be regenerated once they reach a set exchange capacity. Their efficiency is related directly to the amount of dissolved solids that pass through the system. Conversely, the operating cost for RO does not vary with the level of dissolved solids in the feed water since the operating cost is based on flow rate.

An RO system does not require significant downtime with the exception of quarterly or semi-annual routine maintenance. And RO systems are highly automated, requiring minimal operator interaction. By contrast, during regeneration, which can take up to twelve hours, ion exchange equipment cannot be used and the plant is forced to stop water production.

With such advantages, expect to see continued growth in the use of RO technology in the industrial sector, particularly for power generation applications.

By Associate Writer - Ms. Nidhi Garg

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February 21, 2009

Heat Capacity with Dissolved Solids

You are working on some design problem where a concentrated salt solution is under consideration and you are looking for its heat capacity. In such situation, will you use it to be equal to water Or it will be significantly different.

Yes, the answer is that it will be significantly different than water depending on its concentration. The deviation goes higher & higher if concentration increases.

The discussed equations are very useful for heat transfer calculations for slurry systems as well as solid handling systems.

So how to calculate it? Find out the easy way to calculate it.

A quick estimation method was proposed by Dimoplon in 1972. The proposed expression is:

Cpsoln = W1 x Cps + W2 x Cpw

Where Cpsoln = Specific heat of soluion mix.
Cps = Specific heat of solids
Cpw = Specific heat of water
W1 = Wt% of solids
W2 = Wt% of water (Usually 1 - W1)

The equation is valid for a given T. So if T changes you need to change the values accordingly.

Following chart / table is very important for above equation.



Example
Calculate the heat capacity of a 20-wt% Na2CO3 solution at 150 °F

Solution
Step-1
Look up the heat capacity of this solid from table. If it is not available, apply Kopp's Rule, which says

Cp(Na2CO3) = 2 x Cp(Na) + 1 x Cp(C) + 3 x Cp(O)

From Table, we read the values at 150 °F (339 K). Notice that the heat capacity for oxygen is given as O2 (it's natural form). This value must be divided by two to get the heat capacity for one atom of oxygen.

So,
Cp(Na) = 28.5612 KJ/Mole/K
Cp(C) = 11.6364 KJ/Mole/K
Cp(O) = 14.0611 KJ/Mole/K

So by Kopp's Rule

Cp (Na2CO3) = 2 x 28.5612 + 11.6364 + 3 x 14.0611
= 110.9421 KJ/Mole/K

Step-2
Now since our Dimoplon equation uses only weight basis, we need to divide this figure by molecular weight of compound. So,

Cp (Na2CO3) = 110.9421 / 105.9 = 1.0476 KJ/Kg/K

Now if you convert to Kcal then it becomes = 1.0476 x 0.23886 = 0.25 Kcal/Kg/K

Step-3
Now note down the heat capacity of water at 150 F which is 0.9975 Kcal/Kg/K.

Step4
Now finally apply Dimoplon rule as,
W1 = 0.2 (20%)
W2 = 0.8 (80%)
Cpw = 0.9975
Cps = 0.25

Hence,
Cps = 0.2 x 0.25 + 0.8 x 0.9975
Cps = 0.848 Kcal/Kg / K

Result
The literature data for this system is reported as 0.850, SO there is a variation of only 0.2%. Thus Dimoplon rule gives a very good estimate of specific heat of solutions with dissolved solids.

The equations are useful for slurry systems where impacts are significant.
Also it can be used for identification of specific heats of solids, where solid handling systems are involved with heat transfer.

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February 12, 2009

Thumb Rules for Tray Towers

Similar to my previous post, here I am putting few good thumb rules for Tray towers also.

Again be careful while using these thumb rules.

  1. For ideal mixtures, relative volatility can be taken as the ratio of pure component vapor pressures.

  2. Tower operating pressure is most often determined by the cooling medium in condenser or the maximum allowable re-boiler temperature to avoid degradation of the process fluid.

  3. Perform the easiest separation first (least trays and lowest reflux) while sequancing columns

  4. If relative volatility, nor feed composition vary widely, take products off one at a time as the overhead.

  5. If the relative volatility of components do vary significantly, remove products in order of decreasing volatility.

  6. If the concentrations of the feed vary significantly but the relative volatility does not, remove products in order of decreasing concentration.

  7. The most economic reflux ratio usually is between 1.2 Rmin and 1.5 Rmin.

  8. The most economic number of trays is usually about twice the minimum number of trays.

  9. Typically, 10% more trays than are calculated are specified for a tower.

  10. Tray spacing should be from 18 to 24 inches, with accessibility in mind.

  11. Peak tray efficiencies usually occur at linear vapor velocities of 2 ft/s (0.6 m/s) at moderate pressures, or 6 ft/s (1.8 m/s) under vacuum conditions.

  12. A typical pressure drop per tray is 0.1 psi (0.007 bar)

  13. Tray efficiencies for aqueous solutions are usually in the range of 60-90% while gas absorption and stripping typically have efficiencies closer to 10-20%.

  14. The three most common types of trays are valve, sieve, and bubble cap. Bubble cap trays are typically used when the low-turn down is expected or a lower pressure drop than the valve or sieve trays can be provided.

  15. The most common weir heights are 2 and 3 inch and the weir length is typically 75% of the tray diameter.

  16. Reflux pumps should be at least 10% over designed.

  17. The optimum Kremser absorption factor is usually in the range of 1.25 to 2.00.

  18. Reflux drums are almost always horizontally mounted and designed for a 5-min holdup at half of the drum capacity.

  19. For towers that are at least 3 ft (0.9 m) in diameter, 4 ft (1.2 m) should be added to the top for vapor release and 6 ft (1.8 m) should be added to the bottom to account for the liquid level and reboiler return.

  20. Limit tower heights to 175-ft (53 m) due to wind load and foundation considerations.

  21. The length / diameter ratio of a tower should be no more than 30 and preferably below 20.

  22. A rough estimate of reboiler duty as a function of tower diameter is given by:

  23. Q = 0.5 D2 for pressure distillation.
    Q = 0.3 D2 for atmospheric distillation.
    Q = 0.15 D2 for vacuum distillation.

    Where Q is in Million Btu/hr and D is lower diameter in feet.

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February 03, 2009

Thumb Rules for Packed Towers

Here are some thumb rules for Packed Towers which are compiled from various internet sources.

These rules are just for preliminary checks & estimates and should not be followed as design criteria, because design criteria is always different from thumb rules depending on specific conditions & detailed design calculations.

  1. Packed towers almost always have lower pressure drop than comparable tray towers.

  2. Packing is often retrofitted into existing tray towers, to increase capacity or separation. Thus same size of packed towers can handle more than tray towers.

  3. For gas flow rates of 500 ft3/min (14 m3/min) use 1 in (2.5 cm) packing, for gas flows of 2000 ft3/min (57 m3/min) or more, use 2 in (5 cm) packing.

  4. Ratio of tower diameter to packing diameter should usually be less than 15.

  5. Due to the possibility of deformation, plastic packing should be limited to an unsupported depth of 10-15 ft (3-4 m) while metallic packing can withstand 20-25 ft (6-7.5 m).

  6. Liquid distributor should be placed every 5-10 tower diameters along the length for pall rings and every 20 ft (6.5 m) for other types of random packing.

  7. Packed columns should operate near 70% flooding.

  8. Height Equivalent to theoretical stages (HETS) for vapor liquid contacting is 1.3-1.8 ft for 1in pall rings and 2.5-3 ft for 2.0 in pall rings.

  9. Design pressure drop should be as follows




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