August 23, 2009

Small Water Saving Initiative from my company

Dear All,
I just wanted to share a small initiative of my new company towards saving of water. This can be presented by you to your management for implementation which will help us to keep our globe safe and provide sufficient water savings for many families to upkeep their daily life.

The water is very important for all living things and currently it is of utmost importance for any coroporate house, individual, and governments to conserve water & promote conservation fo water.

In our present company the management has implemented waterless urinals in the corporate building. This is saving around 2 Litres of water / day / person. We have around 400 people working in this office which is therefore, saving around 2 x 400 x 300 = 240000 Litre / Year.

Now see the importance of this number. Each family (4 persons) need around 100 litre of water / day for hygeine & drinking. Therefore, it can serve for 2400 families for 1 year OR 40 families for 60 years of average life.



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August 15, 2009

Steam Properties - II

In the previous post I discussed about P & T correlation for saturated steam. I also mentioned that its better to use Excel add-in than using these formulae in Excel sheet. However, these are essentially required if you are using some non worksheet based computer program for the evaluation / simulation of your project.

So here I will cover the other major property that is enthalpy & latent heat for steam.

2. Enthalpy of Saturated Steam at Given Temperature

Enthalpy Hv = alphat + gamma

alphat = 0.99615 t + 1.8239 x 10^-6 * t^2 -0.13468 x 10^ (-0.036 t) + 0.13468

gamma = 597.34 - 0.555 t - 0.2389 x 10^alpha

alpha = 5.1463 - 1540 / T

Where t = temperature in °C
T = temperature in K

So if you need to find out the enthalpy at a given P, you first need to calculate saturation temperature based on methods given earlier.

3. Enthalpy of Saturated Water at Given Temperature

Hl = 0.001 t2 + 0.8663 x t

Where Hl = enthalpy of Saturated liquid at t temperature in Kcal/Kg
t = temperaure in °C

4. Latent heat of Vaporization at Given Temperature

Lambda = Hv - Hl

Hv & Hl are calculated above.


5. Saturation temperature from Pressure
This part is also required frequently, so I developed an equation based on data from steam tables.

t = A x y^5 + B x y^4 + C x y^3 + D x y^2 + E x y + F
Where
y = Log10 (Pv)

A = 0.03878244
B = 0.5246778
C = 2.7767678
D = 12.6450237
E = 63.9525883
F = 99.082168

Pv is in Kgf/cm2.

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August 07, 2009

Ammonia Vapor Pressure & Temperature Equations

In the series of physical properties calculation methods, I am now covering Ammonia P & T correlations which are frequently used in refrigeration system design calculations. Or they may be used for designing of heat pumps - yes may be I don't know at this stage if one can use ammonia for heat pump service or not.


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Saturation Pressure vs Temperature.

This equation is valid for t = -75 to 70°C. I have limited this equation to this range for better accuracy of calculated data vis a vis comparison with published data.

Pv = A x T^5 + B x T^4 + C x T^3 + D x T^2 + E x T + F

where Pv = vapor pressure in Kg/cm2
T is temperature in Kelvin.

A = -4.5864778 x 10^-11
B = 9.85099347 x 10^-8
C = - 5.7526544 x 10^-5
D = 1.4773233 x 10^-2
E = -1.7847943
F = 83.3895705

Saturation Pressure vs Temperature for t > 70°C.

Pv = 89875.92511 exp (-2706.64791 / T)

where units are same as above.

Saturation Temperature Vs Pressure for P = 0 to 34 Kg/cm2.


T = A x y^5 + B x y^4 + C x y^3 + D x y^2 + E x y + F

Where
A = 0.13493672
B = 0.380555
C = 2.13810444
D = 10.058439
E = 45.5972648
F = 239.1719874

T is in Kelvin

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July 29, 2009

Latent Heat Calculation

The most used & ever needed property is the latent heat of vaporization which is critical also for various calculations. A process engineer must understand it properly.


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This is one of the property of a pure fluid or a mixture which is never available to you when you need it most specially when you need it urgently. That time you are never able to recall the source where did you see it last time.

So don't worry now. There are few easy methods available which can give you quick & accurate estimate of this be it for pure fluid or for a liquid mixture.

The first method I am discussing is Riedel's Correlation.
This method is limited to calculating latent heat at normal boiling point only. However, this is a property which can be used for the derivation of other properties also. We will see it in future & coming posts.

Hvb = 1.093 R Tc [ Trb x {Ln(Pc)-1}/{0.930 - Trb}]

Where
Hvb = Latent heat at normal boiling point. (Remember this is an important definition)
Trb = Tb / Tc Reduced Boiling Point Ratio in Kelvin
Pc = Critical Pressure atm

Hvb Unit is Lit-Atm/Gm-mole
R = Gas Constant in Lit-Atm/Gm-mole/K

So change R value in different unit & you will get Hvb in desired unit accordingly, because the value in parantheses is unitless.
Also note that 1 Lit-Atm/Gm-mole is equal to 24.12 Cal/gm-mole

The second method I am discussing is Pitzer's Correlation.
This method is applicable for a wide range from normal boiling point to critical point.

The equation given is as below.

(Hv / R Tc) = 7.08 ( 1 - Tr)^0.354 + 10.95 * omega ( 1 - Tr)^0.456

Hv = Latent heat at t °C.
Tr = (t+273.15)/Tc
omega is accentricity factor - a std property


The third method I am discussing is Watson's Correlation.
This method is most useful for the fluid where you know the latent heat at a given temperature & want to calculate it at another temperature, or using some simulation where accurate estimate is required so instead of using basic equations you can use this escalation equation.


Hv2 = Hv1 [ (1 - Tr2) / (1 - Tr1)]^0.378

Hv1 = latent heat at T1
Hv2 = latent heat at T2

Units are same as described in first method.


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July 21, 2009

Vapor Pressure Vs Temperature

Generally we search too many things for frequently used properties like the vapor pressure or saturation temperature at given condition which is the most common among all.


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Though the famous & simplest correlation of Antoine is available but many a times we do not find correlation constants A, B & C for a given compound. Therefore, I always prefer methods which uses its own thermodynamic property e.g. critical parameters for calculation of any derived property.

So this post is giving a method which uses Tc, PC atc for calculating vapor pressure, though the method is somewhat tedious but can be used once formulated in Excel or in any other program.

The method used in this case is called Gomez - Nieto & Thodos which is based on critical properties.

The equation is

Ln (Pvr^5) = beta [ (1/ Tr^m)-1] + gamma (Tr^n - 1)

Where
Pvr is basically reduced vapor pressure i.e. Pv / Pc
Tr is as usual reduced temperature at T i.e. T/Tc

m = [0.78425 exp (0.089315 * S)] - [8.5217 / exp (0.74826 * S)]
n = 7
beta =-4.267-[221.79/{(S^2.5)(exp(0.03848 S^2.5))}]+[3.8126 / exp(2272.44 /S^3)]
gamma = a * S + b * beta

a = [(1/Trb)-1]/(1-Trb^7)
b = [(1/Trb^m)-1]/(1-Trb^7)
S = Tb * Ln (Pc)/ (Tc - Tb)

Here Pc is in atm & Pvr is in mmHg.

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July 13, 2009

Cooling Tower - Audit & Efficiency

I have written three posts & one audit report on this topic of cooling towers calculations. Recently one of our reader suggested that I should go for posting an article on how to identify different components of cooling tower performance. Therefore, I decided to provide the details on how to establish different factors contributing to the inefficiencies in case of a cooling tower.

The article & report I have earlier published are here.



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Factors affecting cooling tower & how to find out the impact of each of them.

Effect of Water Temperature
The impact of return water temperature is that the cooling approach must change by ~50% of the difference in the design & actual return water temperature. For example: If a tower is designed for 4°C approach for the cooling of water from 44°C to 34°C and actual return water temperature is 42°C then you must get an approach of 3°C instead of 4°C. The range of cooling will be 9°C against design value of 10°C. Thus, if your return water temperature is 42°C against design value of 44°C and you are getting design approach of 4°C that means your cooling tower is operating inefficiently.

Effect of Wet Bulb
Similarly, In this case, the approach of cooling will go up by ~50% of the difference between design & actual wet bulb, if wet bulb is lower than design value. However, total range of cooling will also increase in this case by ~50% of the difference in actual & design wet bulb i.e. 44°C to 33°C means range of cooling will increase from 10°C to 11°C.

Yes, one should not infer from these two examples that the thumb rules expressed here are valid for any changes in the temperatures. Instead, it is always governed by the equilibrium conditions & for larger changes one should go for proper evaluation procedure as described later in this article.

Effect of Water Loading
Cooling towers are normally supplied in standard modules called tower cells. Therefore, the cooling water is distributed equally on each cell in parallel configuration. However, in actual operation it deviates from what it should be (Average water flow/cell). This causes for example 80% water on one cell & total water flow being the same it will be 120% on the other. Even the same cell might have different water loading on both sides of distribution deck. This much deviation may result in ~5-10% rise in cooling approach.

Effect of Air Distribution
Similarly, the imbalance in air loading in each side of every cell may cause 1.5 times more negative impact as compared to effect of disturbed water loading. Thus, 20% deviation in air distribution may cause ~10-15% rise in cooling approach.

Effect of Air Short Circuiting
By now it is clear that operators has to regularly monitor the performance of their cooling tower especially at different loads & at different ambient conditions. More appropriately, the comparison of actual approach with the design approach is not a good & actual indicator of its performance

NTU Calculation
For the identification of actual differences in the performance calculation of a cooling tower & for identifying the impact of each factor, NTU calculation method is the most useful & recommended one based on my experience.

As Promised earlier in the performance calculation - II, I am giving here the method for using this NTU for prediction of performance.

Step - 1
Start with how much is approximate difference in inlet & exit temperature from cooling tower & divide them in equal parts with say an increment of 0.2°C or so.

Start from first row & column A with some approx t temperature & make diff in column G as zero. (This all is explained in above linked article on this Blog)

Since all other rows are linked with same increment of 0.2 till last row, finally you should get your inlet temperature in last row & column A. If it is less or more change it in first row to get same.

With each change all rows in column G should have zero value as difference.

Step - 2
Once you get all zero in column G and inlet temperature in last row of Column A, the first row in column A should give you the exit temperature, but wait.

Now you need to check the sum of NTUL in column K or NTUG in column N. This value should be equal to your design NTU. (Yes, design NTU is also found in similar manner)

Example, so if you want to evaluate the impact of change in wet bulb compared to design, first consider all data of design & find out NTUG in design.

Now change wet bulb & you will see that ha in column F is changed, so you need to change t in column A to make difference in column G = 0.

Finally you will find the changed temperature figure at the exit by keeping NTUG same.


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July 06, 2009

Ultimate Analysis of Biomass

Heating value & ultimate analysis of any fuel be it biomass or fossil fuel is correlated long back by Du-Long in 19th Century.


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Most recently a Mr. S. A. Channiwala 1992 thesis, The Indian Institute of Technology, Bombay) collected data on over 200 species of biomass and fitted the following equation to the data:

HHV (in kJ/g) = 0.3491C + 1.1783 H - 0.1034 O - 0.0211 A + 0.1005 S -0.0151 N

Where C is the weight fraction of carbon; H of hydrogen; O of oxygen; A of ash; S of sulfur and N of nitrogen appearing in the sltimate analysis.

This equation fitted the experimental data with an average error of 1.45%, typical of the error of most measurements. This equation permits using heat values in calculations and models of biomass processes.

However, I am giving here the table for fuels indicating their ultimate analysis & HHV.










Source: - BioMass Energy Website

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June 28, 2009

Steam Properties - I

I have been giving few posts on different properties estimation methods for various uses for process engineers. However, the most useful & most frequently used property is the steam property for which earlier I suggested using an Excel Add-In called water_97.xla.


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But, sometimes if you are using different programs other than Excel, in that case, the add-in will not work & you need to know the correlations for different properties to use in other calculators.

So I thought it would be better to share these correlations for the benefit of all readers. We will cover these properties in few parts of this post. This is the first one for P & T correlation only.


1. Saturation Pressure at Given Temperature

Log(e) (2256500/P) = [7.21379 + (alpha + beta * T + gamma * T^n) (T-483.16)^2] x [647.31 / T - 1]

T = Temperature in Deg K
alpha, beta & gamma are given as below,

Case-1
alpha = 1.152 x 10^-5
beta = -4.787 x 10^-9
gamma = 0 & n = 0 for t = 0 to 210 °C

Case-2
alpha = 1.0071 x 10^-6
beta = 1.9312 x 10^-8
gamma = 8.913 x 10^-96 & n = 32 for t = 210 to 374.15 °C

Alternatively more simpler formula is

T = (P ^ 0.25) * 100

Or

P = ( T / 100) ^ 4

Where T is in °C.

One more equation developed by me
This equation is valid from t = 0 to 374 °C


Log10 (Pv) = A t^5 + B t^4 + C t^3 + D t^2 + E t + F

Where
A = 3.482223 x 10^-13
B = - 4.890675 x 10^-10
C = 3.038026 x 10^-7
D = -1.1351158 x 10^-4
E = 0.03090855
F = -2.2016923


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June 20, 2009

HAZOP - Basic Understanding

Hazard and operability studies are a methodology for identifying and dealing with potential processes, particularly those which would create a hazardous situation or severe impairment of the process. It is commonly known as HAZOP.


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Definitions
HAZARD any operation that could possibly cause a catastrophic release of toxic, flammable or explosive chemicals or any action that could result in injury to personnel.

OPERABILITY any operation inside the design envelope that would cause a shutdown that could possible lead to a violation of environmental, health or safety regulations or negatively impact profitability.


HAZOP Process
The HAZOP focuses on specific portions of the process called “nodes”. Generally these are identified from the P&ID of the process. A process parameter is identified, say flow, and an intention is created for the node under consideration.

Then a series of guidewords is combined with the parameter “flow” to create deviations. For Example, the guideword “no” is combined with the parameter flow to give the deviation “no flow”. Then focus is on listing all the credible causes of a “no flow” deviation beginning with the cause that can result in the worst possible consequence one can think of.

Once the causes are recorded, a list is made of the consequences, safeguards and any recommendations deemed appropriate. The process is repeated for the next deviation and so on until completion of the node.



Guidewords, Selection of Parameters and Deviations
The HAZOP process creates deviations from the process design intent by combining guide words (no, more, less, etc) with process parameters resulting in a possible deviation from design intent.



SPECIFIC PARAMETERS
These include Flow, Pressure, Temperature, Level, Time, Agitation, Reaction, Start-Up/Shut-Down, Draining/Venting, Utility Failure(instrument air, power), Maintenance, Vibrations etc.

Concept of Point Of Reference
When defining nodes and performing a HAZOP on a particular node it is useful to use the concept of point of reference (POR) when evaluating deviations.

Screening for Causes of Deviations
It is necessary to be thorough in listing causes of deviations. A deviation is considered realistic if there are sufficient causes to believe the deviation can occur. However, only credible causes should be listed.

There are three basic types of causes. They are:

  • Human error which are acts of omission or commission by an operator, designer, constructor or other person creating a hazard that could possibly result in a release of hazardous or flammable material.

  • Equipment failure in which a mechanical, structural or operating failure results in the release of hazardous or flammable material.

  • External Events in which items outside the unit being reviewed affect the operation of the unit to the extent that the release of hazardous or flammable material is possible. External events include upsets on adjacent units affecting the safe operation of the unit (or node) being studied, loss of utilities, and exposure from weather and seismic activity.


By Associate Writer : Ms Nidhi Gupta

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June 13, 2009

How to Read P & ID

Often when you use P&ID you will find that each company or engineering consultant is using his own list of symbols & instrumentation diagrams. This is very confusing for young engineers in the initial age where sometime somebody is using B symbol for tanks, some use D & finally some use T.

Why it is so?


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Equipment & Instrumentation details vary with the degree of design complexity. For example, simplified or conceptual designs, often called process flow diagrams, provide less detail than fully developed piping and instrumentation diagrams (P&IDs).

Being able to understand instrumentation symbols appearing on diagrams means understanding ANSI/ISA’s S5.1-1984 (R 1992) Instrumentation symbols and identification standard. S5.1 that defines how each symbol is constructed using graphical elements, alpha and numeric identification codes, abbreviations, function blocks, and connecting lines.



Deciphering symbols
ISA S5.1 defines four graphical elements—discrete instruments, shared control/display, computer function, and programmable logic controller and groups them into three location categories (primary location, auxiliary location, and field mounted).

Discrete instruments are indicated by circular elements. Shared control/display elements are circles surrounded by a square. Computer functions are indicted by a hexagon and programmable logic controller (PLC) functions are shown as a triangle
inside a square.

Adding a single horizontal bar across any of the four graphical elements indicates the function resides in the primary location category. A double line indicates an auxiliary location, and no line places the device or function in the field. Devices
located behind a panel-board in some other inaccessible location are shown with a dashed horizontal line Letter and number combinations appear inside each graphical element and letter combinations are defined by the ISA standard.

Numbers are user assigned and schemes vary with some companies use of sequential numbering, others tie the instrument number to the process line number, and still others adopt unique and sometimes unusual numbering systems.

The first letter defines the measured or initiating variables such as Analysis (A), Flow (F), Temperature (T), etc. with succeeding letters defining readout, passive, or output functions such as Indicator (I), Record (R), Transmit (T), and so forth.

Example
Referring to the Example P&ID diagram, FT 101 represents a field-mounted flow transmitter connected via electrical signals (dotted line) to flow indicating controller FIC 101 located in a shared control/display device. A square root extraction of the input signal is applied as part of FIC 101’s functionality.



The output of FIC 101 is an electrical signal to TY 101 located in an inaccessible or
behind-the-panel-board location. The output signal from TY 101 is a pneumatic signal (line with double forward slash marks) making TY 101 an I/P (current to pneumatic transducer). TT 101 and TIC 101 are similar to FT 101 and FIC 101 but are measuring, indicating, and controlling temperature. TIC 101’s output is connected via an internal software or data link (line with bubbles) to the setpoint (SP) of FIC 101 to form a cascade control strategy.

Often P&ID’s include a cover page where common and typical terms, symbols, numbering systems, etc., are defined. On the example, Typical YIC would likely appear on the cover page and the simplified form of YIC would appear throughout the P&IDs.



Typical YIC indicates an on/off valve is controlled by a solenoid valve and is fitted with limit switches to indicate open (ZSH) and closed (ZSL) positions. All inputs and outputs are wired to a PLC that’s accessible to the operator (diamond in a square with a solid horizontal line). The letter "Y" indicates an event, state, or presence. The letter "I" depicts indication is provided, and the letter "C" means
control takes place in this device.

Adherence to ISA’s S5.1 Instrumentation Symbols and Identification standard ensures a consistent, system independent means of communicating instrumentation, control, and automation intent is developed for everyone to understand.

The article is adopted from Control Engineering magazine with some modifications for the learning of young engineers.

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June 05, 2009

Viscosity of Gaseous Mixtures

In the previous few posts, I discussed about methods for calculating viscosities of pure gases using different methods at low & high pressures. What about mixtures?

Calculating property of any mixture is more tedious than pure components. They vary significantly if components widely vary. Also its very difficult to search for applicable & trusted correlations to calculate the mixture properties.


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So in this part we will discuss the method which are applicable for gaseous mixtures at low & high pressures.

The method used in this case is called Wilke correlation which is very easy for binary mixtures and can be used for multi components mixtures also.

The equation is

vism = Sum (i = 1 to n) [ yi visi / Sum (j = 1 to n) yi phi(i,j) ]

Where
vism = viscosity of mixture
Sum (i = 1 to n) = Sigma sign for i = 1 to n which is represented here in language term due to non availability of symbols in blogger.
yi = mole graction of component i in the mixture
visi = viscosity of pure gas for i compnents
phi(i,j) = double parametric symbol given as below


phi(i,j) = 1 if i = j

phi (1,2) = [ 1 + (vis1/vis2)^0.5 * (M2/M1)^0.25 ]^2 / [8 ( 1 + M1/M2 )]^0.5

Where M1, M2 are molecular weights
Vis1, Vis2 are pure component viscosity

Therefore,

phi(2,1) = vis2/vis1 * M1/M2 * phi (1,2)

So for Binary mixture it reduces to

vism = [ y1 vis1 / (y1 + y2 phi(1,2))] + [y2 vis2 / (y2 + y1 phi(2,1))]


Viscosity of Gas Mixture at High Pressure
The correlation used is of Dean & Stiel for non polar gases mixture.

(vism - vism0) * zetam = 1.08 [ exp(1.439 Drm) - exp(-1.111 Drm ^ 1.858)]

Where
vism = viscosity of mix at high pressure
vis0 = viscosity of mix at atm pressure
zetam = for mixture as defined in the article of Viscosity at High Pressure.
Drm = Reduced density for mixture

To calculate the reduced density of mixture you need to know the critical properties of the mixture for which following general equations are used.

Tcm = Sum (i = 1 to n ) yi x Tci
Zcm = Sum (i = 1 to n ) yi x Zci
Vcm = Sum (i = 1 to n ) yi x Vci
Pcm = Zcm R Tcm / Vcm

So Drm = 1/ Vrm
& Vrm = Vm/Vcm

Yes dear you are right again the unit of viscosity here is micropoise.

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May 28, 2009

Typical Heat Transfer Coefficient - Values

Some typical heat transfer coefficients for different systems.


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May 20, 2009

Assessing ESP Performance

Electrostatic precipitators are among the most effective pollution control devices for removing particles from gases. They are used in chemical process plants including thermal power plants, cement plants and pulp & paper industries.


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These devices called ESP’s, enjoy a relatively low payback period – often 3 - 5 years – especially those that are able to recover valuable solids for sale / reuse.

Calculating collection efficiency:

ESP’s are used to treat particulate laden gases at high flow rate alone or together with Bag house Filters, Cyclones, Hydro-cyclones and Wet scrubbers and have following advantages.

  • Low pressure drop (about 2.5 cm H2O compared to 5-10 cm H2O for other methods)

  • High removal efficiency (> 97+%) even for flows with particles < 2m.

  • Handling gases with high moisture content (even 30%).

  • Easy separation and recovery of collected material when it is reused.



During ESP operation, first the airborne particles are electrically charged. The charged particles then migrate to the collector (negatively changed electrode) where they are neutralized and collected in a hopper. The high voltage system works at 25-100 KV, and a corona type discharge occurs at the negatively charged electrode.

The theoretical efficiency of an ESP can be determined by equation-1, known as Deutsch equation given as under:

Eff = 1- exp(- 2VL/RU) (1)

Where L = collector length, m
V = Particle velocity towards the electrode, usually 0.03-0.21m/s
R = Collector radius, m
U = Net gas velocity, m/s

Collector area : A = 2 Pi RL - (1a)
Volumetric flow rate Q = U R^2 - (1b)

Substituting 1a & 1b in eqaution (1), it can be re-written as:

Eff = 1- exp(- AVL/Q) - (2)

Higher collection efficiencies can be obtained by increasing the throughput velocity (V), collector area (A) or by reducing the gas flow rate (Q). However, increasing collector area is not always feasible.

To increase efficiency from 90 to 99%, the area needs to be doubled; while from 90 – 99.9%, three times area corresponding to 90% efficiency, is required.

Increase in flow rate will increase the particle loading at the outlet. In a typical ESP, reduction in the collection efficiency from 99 to 97% will triple the particle loading in the exhaust.

The Deutsch equation was later modified (equation 3) and now widely used to calculate collection efficiency with varying operating parameters.

Eff = 1- exp(- AVK/Q) - (3)

Where VK is the effective migration velocity and given by equation (4).

VK = v ln (1/1-Eff) … (4)

Where v is the effective migration velocity of particles areas across the inter electrode space, as computed by equation (5). Accordingly to electrostatic field theory,

v = [PDE^2 (1 + J L/D)]/ 36x10^7 Pi Mu (5)

Where D = Particle diameter, m
E = Electrostatic force field, V/m
J = Average free distance run by the gas molecules, as given by equation (6), m
L = Length between electrodes, cm
Mu= Absolute gas viscosity, Poise
P = A unit less parameters, given by equation (7), where C is the di-electric constant, coulombs/gm
J = 1.764 + 0.562 e-0.785 D/L - (6)
P = 3C/(C+2) - (7)

Equation (5), used for calculating effective migration velocity for particles, is based on three assumptions:
  • Particles rapidly reach to their final velocity while moving towards the negatively charged electrode.

  • For particles with a diameter similar to, or lower than, the average free distance run by gas molecules, the hydrodynamic buoyancy can be calculated using ‘Stokes law’ with ‘Cunnighum Correction’ factor.

  • The accumulated electrostatic charge nearly instantaneously reaches its maximum, limit value.



Theoretical determination of ESP efficiency though straight forward, but involves theoretical formulations for variables like V, VK and J, which is often difficult to get in industrial situations.

“Practical” efficiency can be determined by equation (8) involving volumetric flow rate and particulate content, which can be determined using an Isokinetic method.

Eff = (Cpi.Qi – Cpo.Qo)/ Cpi.Qi - (8)

Where Cp = Particulate content of the gas, mg/m3
Q = Volumetric flow rate, m3/hr
i = inlet
o = outlet.

Isokinetism correlates gas velocity with sampling velocity. 100% isokinetism means gas velocity equals sampling velocity. For practical purpose (stack sampling), 90-110% isokinetism gives fairly good value.

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May 12, 2009

Viscosity of Pure Gases at High Pressure

In the previous posts, I discussed about methods called Chapman & Enskog and also Yoon & Thodos. Both were applicable for finding out viscosities of pure gases at low pressures up to 5 atm. What about the viscosity at high pressure because gas properties change significantly with pressure and behaviour becomes more & more real deviating from ideal gas condition.

So in this part I will discuss the methods which are apllicable for gases at high pressure.

In such cases, mostly correlations are linked with reduced density i.e. based on the standard definition of reduced property is density at given condition / density at critical point.


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Also, you must remember that mostly the properties at high pressure are reported in terms of residual properties which is either consider as the differential value of property at given pressure and atmospheric pressure (vis1 - vis0) or as their ratio i.e. vis1/vis0. So in both the cases you must know vis0 at atm pressure, which you can find out in my previous articles Viscosities of Pure Gases at low pressure - Part-I and, Viscosities of Pure Gases at low pressure - Part-II.

The method is called Jossi, Stiel & Thodos method.

So for Non Polar Gas the equation is -

[(Vis1 - vis0)*zeta +1]^0.25 = 1.0230 + 0.23364 Dr + 0.58533 Dr^2 - 0.40758 Dr^3 + 0.0933 Dr^4

Here
Dr = Reduced density with its usual definition i.e Dr = D / Dc
vis0 = already given in previous post as viscosity at atm pressure.
zeta is given by the following equation.

zeta = [Tc ^ (1/6)]/ [ M ^ 0.5 * Pc ^ (2/3) ]

Its valid for 0.1 < Dr < 3.0

Now for Polar Gases there are three parts depending on Dr value. The equations from Stiel & Thodos are -

Dr <= 0.1

(Vis - vis0) x zeta = 1.656 Dr^ 1.111

0.1< Dr <= 0.9

(vis - vis0) x zeta = 0.0607 * (9.045 Dr + 0.63)^ 1.739

0.9< Dr <= 2.6

log [4 - log {mod (vis-vis0)*zeta}] = 0.6439 - 0.1005 Dr - Delta

Where Delta is as below

Delta = 0 if 0.9 < Dr < 2.2
Delta = 0.000475 (Dr^3 -10.65)^2 if 2.2 < Dr < 2.6

Yes again the unit of viscosity here is micropoise.

List of other property estimation methods on this Blog.


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May 04, 2009

LED - Energy Saving Lighting Option

LEDs are growing in the current market scenario due to their higher efficiencies, their widely versatile uses & applications and due to the current technological advancements they are now available as source for lighting media also.

Due to their various applications in the industry & now even in day to day life I thought to write some thing on this topic for the benefit of all.
Currently we have three major options for lighting purpose.

1. Incandescent Lamps
2. Fluorescent Lamps
3. Light Emitting Diodes (LED)

Incandescent lamps or old conventional light bulbs produce light by converting electric energy into heat energy thereby generating illumination of visible range. They are very easy to produce but the disadvantage is that they are very less efficient in terms of energy consumption which is generally measures as illumination / watt OR Lumens/watt or Lux.

Fluorescent lamps on the other hand work on UV light which is generated by passing current through mercury vapors. The ultraviolet light is then absorbed by a phosphor coating inside the lamp, causing it to glow, or fluoresce. They are generally costlier by 5 to 10 times compared to incandescent lamps. Their efficiency is still poor & they also involve the risk of some mercury vapors (generally very less).

LEDs work on semiconductor technology & are much more efficient than any other form. LEDs produce more light per watt compared to any other lamp or bulb. They can emit light of any type of desired color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.

Advantages of LEDs
  • LEDs can be very small.

  • LEDs are Ideal for use where frequent on-off cycling is required. Fluorescent lamps burn out much more quickly in this condition.

  • LEDs can very easily be dimmed. This is a very good property of LEDs where most of other lights may fail.

  • In contrast to most light sources, LEDs emit very less heat which may be damaging to the surroundings due to its IR nature.

  • LEDs life is more than 2 to 3 times compared to fluorescent tubes while it may be 5 times or greater compared to incandescent lamps.

  • LEDs do not contain mercury or any other harmful vapors which are unfriendly to the environment.

  • LEDs are now available in the size of more than 12W or even up to 20W.


Applications of LEDs
Decorative lights on interiors, exteriors, parks, walls etc., Night lights, walkway, stair lighting, ceiling lights, porch and landscaping lights.

Car bulbs as LEDs are now available in high power of more than 5W, may be 12W and as high as 20W also sometimes.

Underbody car LED kits come in a range of exciting colors and forms. These kits are equipped with remote controls and are capable of operating in different modes, for example flashing, strobing, chasing, phasing etc. Modern high quality underbody kits also come with sound systems and they can be set to 12 different functions. Multi-color strobe lights for cars are liked by car lovers across the globe because they look amazing. These are the latest advancement in the car parts industry and have been predicted to replace ordinary car kits in the future. They come with high-intensity, wide angle LED’s housed in strong and long-lasting tubes.

LED lighting for cars is not limited to underbody car kits. Exterior car lights are also quite popular and well-liked. These lights include bumper guard lights, third-brake lights, running lights, and multi-purpose light tube strips. In addition to these, popular manufacturers are also providing position and side marker lights for cars. All these lights are quite inexpensive and can be installed easily. High power LED modules come in various colors including cool and warm white, blue, green, red and yellow. LED multiplexing is the future of LED lighting in cars which will produce brighter, more efficient and more flexible lights.

LED light bulbs are used for a variety of purposes in homes. Recessed ceiling lights can be used for lighting up a hallway or passage. These bulbs come in three shapes: MR (Multi Reflector), Universal, and PAR type. MR and PAR types are normally used for focused or directional lighting. They are ideal to be used in night or reading lamps. Universal LED bulbs, on the other hand, can be used for general lighting purposes. These bulbs replace ordinary incandescent bulbs and range in wattage from 3.3 Watt to 15 Watt. Most of these lights are made waterproof and come in various levels of brightness.

Linear LED lights used in homes are available as light bars, rope lights, net & string lights, track lights, and signage & back lights. These types of lights can light up an entire cabinet or shelf. They are known as linear because they consist of a number of small LED’s aligned in a case. Rope lights are perfect for parties and outdoor decoration. They come in a variety of colors and look amazing stretched out on the floor. The latest rope lights come in durable glass tubing and simple electrical connections. They are very flexible and quite easy to install.

Architectural and track lights can also be used to decorate and light up the interior of an office. These lights produce enough illumination for all kinds of operations. Doorways, closets and lobbies can be lit up by using signage lights. These lights are available in the form of daisy chains and are very high power. Sign boards can be effectively constructed by using these lights which are totally waterproof and robust. LED architectural lights are available as street lights, wall washers, path lights and panel lights. LED street lights save money and energy and are very reliable.

Wall washers are extensively used in buildings and homes to illuminate an entire wall. They look beautiful from a distance and can be used to make a building stand out from the rest. The best thing about these lights is that they are not very expensive. The intensity of these lights can be controlled through a remote control along with their color and speed.

Deco, underground, lawn, ceiling, lamp, pool and many other types of LED lighting are admired by people all over the world. These lights are not only affordable but also last for many years. LED’s scatter heat quickly and therefore do not heap up like incandescent bulbs. They can be fitted anywhere because of their small size and adjustable design. Lawn lights look really pretty and they come in various colors including white, green, red and yellow.

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April 26, 2009

Low Grade Waste Heat - Importance

High gasoline prices have forced us to make painful adjustments in our day to day work life in terms of improving energy efficiencies of existing systems. The world's dramatically growing energy demands are affecting all energy prices. Coal, Uranium and natural gas prices have all risen dramatically in the past few years and will continue to grow in the future as more and more of the world's population adopts our energy-wasting lifestyle. We are straining the limited resources of our planet.

Our wasteful energy habits were formed during the many decades before 1973, when oil was less than $3.50 per barrel. At those prices energy was essentially free so we learned to ignore waste. Only 15% of the power of the gasoline you burn in your car goes to move it down the road. The rest ends up as wasted heat, uselessly heating the air. Electric cars are about 75% efficient but they lost out to gas buggies back when gasoline was an insignificant cost.

This is an article from Renewable Energy World with some changes.
In 1882, Edison's first electric power plant sold their spent steam for district heating. Efficiency of electric generation reached a peak in 1910 and has been falling ever since as regulated utilities stopped selling their waste heat. Nowadays the norm is to simply discard the extra heat. Thermal power utilities today only deliver 1/3 of the power in the fuel they burn to customers. The other 2/3is simply discharged as waste heat! This 33%, efficiency level is the same as it was in 1957!

To make matters worse, the clean air act makes it dangerous for utilities to make efficiency improvements because it invites regulators to tighten emission controls as conditions for approval. Worse yet, the clean air act regulates the percent of pollutants (PPM) not the amount per kilowatt-hour (kWh) output. Currently, if you double efficiency the amount of pollutants you are allowed will be halved. Pollution standards should be changed to an output-based standard, such as grams per megawatt-hour (MWh) to stop these terrible unintended consequences. (For more how the power monopolies cling to their power, click on each of the bullet points on this page.)
Iceland provides an excellent example of the benefits of efficient energy use. It approaches power generation as a complete ecosystem where available heat is used with about 90% overall efficiency. The hot water from its geothermal wells is first used to generate electrical power. If the waste heat were discarded, this would be less than 20% efficient. But the wastewater is instead piped to nearby factories and used for drying fruits and vegetables or to run absorption chillers in a refrigeration plant.

The hot water that exits those applications is still pretty hot so it is sold for district heating to greenhouses and apartment buildings. Next in line are the lower temperature applications like fish farming, snow melting and bathing.
By making use of all of the heat instead of discarding it as waste, the efficiency of the entire system can be 90% or more even though the power plant itself is only 20% efficient! This amazing improvement in efficiency requires nothing more than designing with an expanded awareness that considers synergies that will turn waste into profit. The model for this is all around us in nature where nothing goes to waste.

This new paradigm has been extensively developed as industrial ecology and is closely related to the concept of permaculture. It is a new way of thinking that opens awareness beyond design in isolation to consider the design as part of an interrelated ecosystem. As energy costs increase, we can use this new thinking to maintain a gentler form of our current lifestyle by simply taking advantage of the synergies we have ignored in the past. In Europe they have a $6 billion project called Lo-Bin ($3 billion already EU funded) to develop a 98% efficient geothermal power project based on these principles.

In cases where it isn't convenient to pipe hot water or steam to where it is needed, an ORC generator can convert waste heat to electricity. These generators are essentially air conditioners running in reverse: The heat boils a low boiling point liquid driving a turbine which turns a generator. With minor redesign, an air conditioner can be converted to a waste heat generator that will convert heat to electricity. Small ORC generators based on this principle are just beginning to be released to the market.

Solar thermal heating and hot water has become very popular in China where the cost of rooftop solar collectors has become very competitive. Fifty million rooftops already have solar thermal collectors and the numbers in China are growing by 26% per year. These collectors are mostly arrays of concentric glass tubes with an insulating vacuum between them. A hot water tank provides energy storage. These systems could easily be converted to also provide power generation by just adding a small ORC power generator. Mini-generators are not available yet but they could be very inexpensive high-volume products. Since home air conditioners sell for only US $0.10/watt, they could be a very economical way to generate power in the home from the excess heat when the water is already hot enough. Currently, this excess heat is simply wasted.

Combined Heat and Power (CHP) cogeneration can be done in the home with 85% efficiency. Honda has sold over 45,000 of its Freewatt micro-CHP home heater/generators in Japan. The generator uses a very quiet, natural gas powered, internal combustion engine that has the usual 20% efficiency. The unit is installed in place of your furnace and runs only when heat is needed. When it is running, it puts out 1200 watts of electrical power to run your meter backwards. The 80% "wasted heat" works just fine as a furnace to heat your home!

Most industrial plants that were designed in the days of almost free energy release most of their energy into the air as waste heat. ArcelorMittal has a steel mill in Indiana that they retrofitted to recycle wasted energy. They were able to recover about 250 MW of power, cutting the power consumption of the plant in half! This is like building a new 250-MW power plant that will never need any fuel. The cost of the construction required was less than half of what it would have cost to build a coal power plant. (Watch a video interview with Tom Casten, chairman of RED, the company the worked on this project.)

In the US we don't hear much about cogeneration or CHP but Denmark generates 55% of their electricity this way and Finland and Holland do about 40 percent. When wasted power is recovered we are saved the trouble, expense and pollution of building another power plant to generate that power. If our utilities laws can be changed so that efficiency becomes profitable, we could see a doubling of plant efficiency in just a decade. Since 69% of our greenhouse gas emissions are from heat and power, doubling efficiency could reduce our emissions by 34%. Instead of spending billions of dollars building new power plants, we should be using ecological thinking to put to use the millions of megawatts of heat we throw away every day.

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April 18, 2009

Viscosities of Pure Gases at low Pressure-2

In the previous post, I discussed about a method called Chapman & Enskog, while also mentioned about another method called Yoon & Thodos. So in this part I will discuss the second method which is also apllicable for polar & non-polar gases both but still at low pressure say till 5 atm.

First method of Chapman Enskog is available Here or at the end of this post.

Viscosity by Yoon & Thodos correlation
There are two things to remember - 1 Polar Gas, 2 Non Polar Gas.

So for Non Polar Gas the equation is -

Vis * zeta = 4.610 * Tr^0.618 - 2.04* exp(-0.449 * Tr)+ 1.94 * exp(-4.058 Tr) + 0.1

Here
Tr = Reduced temperature with its usual definition i.e Tr = T / Tc
zeta is given by the following equation.

zeta = [Tc ^ (1/6)]/ [ M ^ 0.5 * Pc ^ (2/3) ]

So its more simpler than first method of Chapman & Enskog.

Now for Polar Gases there are two parts. One is for Tr Hydrogen bonding & second is non hydrogen bonding. The equations are -

For Hydrogen bonding gases, the applicability is Tr < 2.0

Vis x zeta = (0.755 Tr - 0.055 ) * [Zc ^ (-1.25)]

For Non Hydrogen bonding gases, the applicability is Tr < 2.5

Vis x zeta = [(1.90 Tr - 0.29 )^0.8 ] * [Zc ^ (-2/3)]

Zc = Compressibility factor at critical point, usually available.
zeta = is already given above.

Yes again the unit of viscosity here is micropoise.

List of other property estimation methods on this Blog.


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April 10, 2009

Viscosities of Pure Gases at low Pressure-1

Finding or calculating almost good estimate of any physical property is very important for young engineers & therefore I emphasise a lot on these methods which are time tested and proven and gives sufficient approximations in calculated numbers. So I am writing few such post in the next coming weeks on properties or other quick methods.

I have already posted following useful posts on different properties estimation on this blog.


The two co-relatons are those of Chapman & Enskog and of Yoon & Thodos. Both of them require temperature, molecular weight, critical temperature, critical pressure of the pure gas.

First method of Chapman Enskog also require accentric factor w. Both the methods can handle polar gases also but then they need more physical constants. These correlations are valid only for low pressures may be upto 5 atm or less.

Viscosity by Chapman & Enskog correlation
There are two things to remember - 1 Polar Gas, 2 Non Polar Gas.

So for Polar Gas the equation is -

Vis = (5/16) x (pi x M R T)^0.5 / (pi x Sigma^2) / CI

Which can be simplified as below.

Vis = 26.69 x (M T)^0.5 / (Sigma^2) / CI

Here
M = Mol Wt of gas
T = Temperature of gas
Sigma is given by the following equation

Sigma = (2.3551 - 0.087 w) x Tc/Pc
Where

w = accentric factor of gas(This is a std property & available in any good reference)
Tc = Critical Temperature of Gas
Pc = Critical Pressure of Gas

CI in the above equation is given as below by Lennard Jones method


CI = ( A / TT^ B) + ( C / exp(D * TT) + E / exp(F * TT)

Where TT = (k /epsilon) * T

For TT calculation k / epsilon is available from below equation.

(epsilon/k) = (0.7915 + 0.1693 * w) * Tc


Now for Non Polar Gases the equation is -

Yes the basic equation is same, the first one you read in the beginning however CI constant evaluation changes. Now the balance method remains same except calculate CI from the following Stockmayer equation.

CI (Stockmayer) = CI (Lennard Jones) + 0.2 * delta^2 / TT

TT is already given above, & delta is polarity of the gas.


Oops! I forgot to mention that viscosity is in micropoise here.
Yoon Thodos method shall be covered in next part of this post.

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April 02, 2009

Calculate Diffusion Coefficient in Gases

Compared to other physical properties the advantage with diffusion coefficients is that they are fairly uniform for a given state. For gases the value of 10-5 m2/s gives you nearly always the correct order of magnitude. If this rough estimate isn't enough and you don't find tabulated values in the literature and you also don't want to make measurements you can try one of the prediction methods.

Quite a number of different correlations and methods have been proposed over the years but the one semi-empirical equation from Chen and Othmer (J. Chem. Eng. Data 7 (1962), 37) is preferable because

• It is Simple
• Sufficiently Accurate in most of the cases
• Availability of inputs required


According to Chen and Othmer the diffusion coefficient D1,2 for the diffusion of gas 1 in gas 2 at moderate pressures can be calculated from the following equation:

The equation is very simple to use & is given below.

D(1,2) = 6.04 x 10 ^ -9 x (T^1.81 / p) x ( (M1+M2)/M1/M2)^0.5 x (Tc1 x Tc2)^0.1405
x (Vc1 ^0.4 + Vc2 ^0.4)^2

Where
M1, M2 = Mol Wt of both components
Tc1, Tc2 = Critical Temp in K
Vc1, Vc2 = Critical Volume in Cm3/mol
P = System Pressure in bar
T = Temp in K
D1,2 = Diffusion Coeff in M2/sec

I hope it is useful for many of you.

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March 25, 2009

Oil From Sand

The oil sand industry is growing fast and so is its impact on the environment. As it happens though, the industry has been impacted by the current business recession causing a number of companies to delay their expansion plans.

Oil sand is often referred to as non-conventional oil or crude or bitumen, in order to distinguish the bitumen and the synthetic oil extracted from oil sands from the free flowing hydrocarbon mixture known as crude oil traditionally produced from oil wells.
Oil sands, tar sands or extra heavy oil is a type of bitumen deposit. The sands are naturally occurring mixture of sand, clay, water and an extremely dense and viscous form of petroleum called bitumen. They are found in large quantities particularly in Canada and Venezuela.


Process Technology:
The main drivers of technology innovations in oil sands production are a desire to reduce the cost and environmental impact.

These can be broadly divided into two categories:
- Surface Mining Operations
- In situ processes

SURFACE MINING

In a surface mining operation, oil sand is scooped up by huge shovels, trucked to a crusher, then slurried with warm water and piped to an extraction plant. There, hot water(40 – 50 deg C) is added, air and process aids are injected into the stream, & the mixture is fed into a conical primary separation vessel. Sand settles to the bottom of the vessel and the overflow is a froth that contains 60% Bitume, 30% Water and 10% Clay.

Naptha is typically mixed with the froth to lower the viscosity of the bitumen, making it easier to separate it from the water and clay by centrifuging. Finally, the bitumen and naptha are separated by distillation. The naphtha is recycled and the bitumen, whose specific gravity is 10API is upgraded by delayed or fluid coking to obtain a synthetic crude of 25-30API for refining

Tailings Treatment: Surface mining operations produce tailings that are a mixture of water, clay, sand, residual bitumen and napthenic acids. The problem is that the clay stays suspended, settling to a max of only 30-40% solids content after 3-5years. For this, gypsum is added at times to the tailings to accelerate the release of water. Another way out is CO2 injection. The injected CO2 forms carbonic acid, which changes the pH and coagulates the clay, thereby increasing the settling rate of the tailings.



IN SITU PROCESSES
1) SAGD (Steam Assisted Gravity Drainage)
This is a popular process for in situ mining. A horizontal well is drilled into the oil formation, and a second, producer well is drilled parallel to it at a lower level. Steam is injected into the upper well to liberate the bitumen, which is punped out from the producer well.



2) THAI (Toe to Heel Air Injection)
A horizomntal producer well with a slotted liner is drilled at the base end (toe) of the producer well. Steam is injected fro 2-3 months to raise the reservoir temperature to 100 deg C. Finally, air is injected at 450-550 psi, initiating a combustion front that moves along the axis of the producer well, causing oil to flow into the well.
Compared with SAGD, there are substantial savings in capital costs and water and energy.

Benefits of in situ mining are that the produced bitumen is ‘clean’, theer are no tailings and the water use is much lower than those operations based on surface mining.

Advantages:
  • Cheaper than the conventional oil.

  • Easier to produce.



Disadvantages:
  • Environmental Issues.

  • Land : Large areas of land are converted into useless lakes.

  • Water: This uses large volume of water and create huge tailings ponds that contain toxic residues.

  • Air: Emissions of carbon dioxide and hydrogen sulphide is a major issue.

  • Energy Usage

  • Transportation

  • Oil from sand is very difficult to transport. It has be mixed with naptha and other things to lower the viscosity.


Some Interesting facts about oil sand:
  • World’s largest deposit of oil and sands occur in two countries: Canada and Venezuela, both of which have reserves approximately equal to the world’s total reserves of conventional crude oil.

  • Venezuela prefers to call its oil sands “extra heavy oil” instead of Bitumen. Bitumen and extra-heavy oil are closely related types of petroleum, differing only in the degree by which they have been degraded from the original crude by bacteria or erosion.

  • Venezuelan deposits are less degraded than the Canadian deposits and are at a higher Temperature (above 50 degree C vs. freezing for northern Canada), making them easier to extract by conventional technique.

  • First Oil Sand mining project began in 1967.

  • The oil from sands is as cheap as $27 per barrel as against the $70 per barrel some time back.


By Associate Writer - Ms. Nidhi Garg

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March 17, 2009

Calculate Diffusion Coefficient in Liquids

The order of magnitude of diffusion coefficients in liquids is 10^-9 m2/s with the bulk of data in the range between 0.5 x 10^-9 m2/s and 5 x 10^-9 m2/s. Note that these data also hold for gases dissolved in liquids.

Equation of Chen and Othmer for gases is used for

• Its Simplicity
• Sufficiently Accurate
• Availability of Input Data

Correlation for liquids is that of Wilke and Chang (AIChE J. 1 (1955), 264):

The equation is very simple to use & is given below.

D(1,2) = 7.4 x 10 ^ -12 x (T / mu) x (C x M2)^0.5 x (V1)^-0.6

Where

D(1,2) = is the diffusion coeff of solute 1 in Solvent 2 (M2/s)
T = Temperature in K
mu = dynamic viscosity of solvent 2 (mPa s)
M2 = Mol Wt of solvent 2
V1 = Molar volume of solute 1 at NBP (Cm3/mol)
C = association factor of solvent 2 as below

C = 2.6 for water
C = 1.9 for methanol
C = 1.5 for Ethanol
C = 1.0 for non polar solvents.

I hope it is useful for many of you.

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March 09, 2009

Heat capacities of dissolved solids & Organic solutions - Quickest Method

In my previous article on the topic, I suggested a method called dimoplon rule for estimating the heat capacity for solution with dissolved solids.

The method was giving a detailed estimate with accurate prediction for such values of specific heat capacities.

However, I am also giving a very quick method for such evaluations with some approximation. The results from this method are accurate enough for preliminary estimates and are not too far from experimental values compared to previous method.

The Quick method is known as Vosseller method proposed by him in 1973.

Cpsolution = 1 - 0.7 x W1
Where W1 is Wt% of solids in solution.

So for our example of Sodium Carbonate solution of 20% in previous post will result in as below.

For 20% solids W1 = 0.2
So specific heat of solution shall be 1 - 0.7 x 0.2 = 0.86

Now compare this with literature data & previous detailed method.

Literature data = 0.85
Dimoplon method = 0.848
Vosseller method = 0.86

So error is still only 1.2% but the method is quite quick for initial understanding of the system & calculations.

Similarly Vosseller also proposed this kind of equation for organic solutions with water where the equation is

Cpsolution = 1 - 0.45 X W1
Here W1 is the Wt % of Organics in water. So for 20% Ethanol mixture the heat capacity can be assumed as = 1 - 0.45 x 0.2 = 0.91 which is quite close to experimental value.

Hope it will be useful for many of you in different industries.

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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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