Friday, July 23, 2010

Basic Laboratory Skills

All scientific studies involve some aspect of practical work. It is therefore essential to be able to observe and to record information accurately. In the context of environmental analyses, it should be borne in mind that not all practical work is carried out in the laboratory. Indeed it could be argued that the most important aspects of the whole practical programme are done outside the laboratory in the field, as this is the place where the actual sampling of environmental matrices (air, water, soil, etc.) takes place. It is still common practice, however, to transport the acquired sample back to the laboratory for analysis, so knowledge and implementation of the storage conditions and containers to be used are important.

No laboratory work should be carried out without due regard to safety, both for yourself and for the people around you. While the Health and Safety at Work Act (1974) provides the main framework for health and safety, it is the Control of Substances Hazardous to Health (COSHH) regulations of 1994 and 1996 that impose strict legal requirements for risk assessment wherever chemicals are used.
 
Within this context, the use of the terms hazard and risk are very important. A hazardous substance is one that has the ability to cause harm, whereas risk is about the likelihood that the substance may cause harm. Risk is often associated with the quantity of material being used. For example, a large volume of a flammable substance obviously poses a greater risk than a very small quantity. Your laboratory will operate its own safety scheme, so ensure that you are aware of what it is and follow it.
The basic rules for laboratory work (and, as appropriate, for associated work outside the laboratory using chemicals) are as follows:
• Always wear appropriate protective clothing. Typically, this involves a clean laboratory coat fastened up, eye protection in the form of safety glasses or goggles, appropriate footwear (open-toed sandals or similar are inappropriate) and ensure that long hair is tied back. In some circumstances, it may be necessary to put on gloves, e.g. when using strong acids.
• Never smoke, eat or drink in the laboratory.
• Never work alone in a laboratory.
• Make yourself familiar with the fire regulations in your laboratory and building.
• Be aware of the accident/emergency procedures in your laboratory and building.
• Never mouth pipettes – use appropriate devices for transferring liquids.
• Only use/take the minimum quantity of chemical required for your work.
• Use a fume cupboard for hazardous chemicals. Check that it is functioning properly before starting your work.
• Clear up spillages on and around equipment and in your own workspace as
they occur.
• Work in a logical manner.
• Think ahead and plan your work accordingly.
What is one of the first things that you should consider before starting a laboratory experiment?
Answer:
You should make yourself aware of the particular safety aspects that operate in your own laboratory. This includes the position of fire safety equipment, the methods of hazard and risk assessments for the chemicalsto be used, the use of fume cupboards, fire regulations and evacuation procedures, and the disposal arrangements for used chemicals.

Source: John R. Dean.( Methods of Enviromental Trace Analysis)

Sunday, July 11, 2010

PFHE & CWHE Comparison in LNG Plant

Main Cryogenic heat exchanger (MCHE) is one of key equipment in natural gas liquefaction (LNG) plant. MCHE used in liquefaction of natural gas having some special characteristic :
  • Intensive/excessive heat exchange (230 - 400 kW / ton LNG)
  • Complex heat transfer - Heat transfer from one (or several) very high pressure natural gas stream to one or several low pressure refrigerant streams
  • Thermal stress/shock - Very large temperature difference between inlet (40 degC) and outlet temperature (-162 degC)
  • Operate at very low temperature (-162 degC)
  • High heat transfer efficiency - Very low temperature approach (2-3 degC) to maximize heat transfer per unit area
  • Involve phase change and risk of phase separation and proper distribution
  • High risk of leakage and safety related issue
  • High risk of blockage/plugging
  • Lightweight & easy transportation



Two type of compact MCHE widely used in LNG plant. There are Plate-Fin Heat Exchanger and Coil (Spiral) Wound Heat Exchanger. Below images are typical CWHE and PFHE.


CWHE

CWHE is coils/tubes wound in spiral around a mandrel and all coils / tubes are contains within a pressure vessel. Multiple coils/tube in bundle can be flew simultaneously within the pressure vessel.



PFHE

PFHE is corrugated or serrated plate stacking on each and others to creates cross and/or counter flow paths to allow heat transfer of multiple fluids.

Although both type of HE have been widely used, CWHE and PFHE have their own special features and advantages. Below are simple comparison between both CWHE and PFHE.

FeaturesCoil Wound Heat Exchanger (CWHE) Plate-Fin Heat Exchanger (PFHE)
CompactnessCompact Extremely compact
Heat Transfer area (m²/m³) 20 - 300300 - 1400
Flow type in heat transferCross-CounterCross and/or Counter
Flow patternSingle and/or two phasesSingle and/or two phases
Flow streamsSingle or MultipleSingle or Multiple
ConfigurationSingle or multiple coil-in-vessel unitMultiple plate-fin units
Flow path8-12mm tube1-2 mm flow channel
Risk of contaminant built-upLess (smooth tube surface)More (multiple channels / cores increase crevices)
Risk of PluggingLowerHigher
Thermal Stress ResistanceHigher (tube robustness & flexibility)Lower (plate fin inflexible)
Risk of Thermal StressLowerHigher
Gas/Liquid distributionLess mal-distribution (single flow channel)Higher mal-distribution (multiple unit in parallel)
Risk of Thermal ShockLowerHigher (mal-distribution lead to imbalance heat transfer)
SafetyLower (tube contains within pressurized vessel - natural gas leaks to vessel)Higher (natural gas leaks to atmosphere)
AvailabilityHigher (production continue with some tube leaks until next shutdown)Lower (immediate production shutdown when leaks occur)
TransportationReasonable easy (with multiple bundles)Easy (Multiple units)
MaterialAluminum / Stainless Steel / Carbon Steel / Others AlloyAluminum
CostHigherLower


Related Post

Free CE June 2010

Recommended :

Some of you may not received your FREE Chemical Engineering Digital magazine for past two months. If you still complete the subscription form in recent update, you may still possible to access latest issue of Chemical Engineering of that particular month. The changes would lead to more visit to Chemical Engineering website and potentially increase sales to CE. However, it could be less favorable to most of you. But you may still obtain FREE Chemical Engineering continuously. The only differences is you need to login to Chemical Engineering website.

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Some  interesting articles of CE July 2010.
  • Piping Design for Hazardous Fluid Service
  • Containing Fugitive Emissions
  • Distillation Tray Design 
  • Propelling distillation research
  • Process cooling: Just Cool It! 
  • Focus on Computer Modeling 
  • Decoding Pressure Vessel Design

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Free "Introduction to Heat Transfer" Ebook

Recommended :

It is often found necessary to transfer heat form hot to cold fluids by means of heat exchangers. there is wide variety of equipment available for this purpose, although in this Engineering Design Guide discussion is restricted to the more common types (other heat exchangers being mentioned only in passing). The very important aspect of removing heat from a primary source, such as a fired heater or the fuel elements of a nuclear reactor, also falls outside the scope of this guide. Although sufficient information is provided to enable the reader to understand and deal with simple heat transfer problems, this text to a large extend serve as an introduction to the more specialized books to which reference is made...


This FREE HEAT TRANSFER ebook discusses on several topics related to heat transfer and heat exchange. It content cover the general problem of heat exchange, analysis of heat conduction, convective heat transfer, thermal radiation heat transfer, mass transfer, etc

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Source : http://www.hts.org.uk/
Thanks to D. Butterworth

Related Post

HP July 2010

FREE Hydrocarbon Processing for JULY 2010 is available now...




Select Articles from the July 2010 Issue 

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Liquefied natural gas and North American shale gas: Room for both?
Let's take a look at the drivers affecting these commodities

Materials market update: Opportunities in a challenging economy
As companies emerge from the downturn, new capital project models will
be vital

Dynamic simulation of liquefied natural gas processes
Here's how to improve the process design and operation of your facility

Perfecting liquefied natural gas analysis techniques and methods
Following these procedures will improve measurement accuracy and reliability

Liquefied natural gas terminal with low environmental impact
Implement these steps to mitigate performance risks

Finetune boil-off gas generation from refrigeration storage facilities
Improved estimation methods accurately calculate vaporized product for liquefied natural gas transfer systems
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