Sunday, August 29, 2010

Quick Way to Estimate BOG

Earlier post "How Boil-Off-Gas (BOG) is Generated" has discussed several ways can result Boil-Off-Gas generation. They are listed below :
  1. vaporized vapor due to barometric pressure decrease
  2. vaporized vapor due to ambient temperature increase
  3. cryogenic fluid rundown piping
  4. cryogenic fluid circulation / loading line
  5. ship / truck loading arm
  6. cryogenic fluid storage tank
  7. cryogenic fluid rundown pump
  8. cryogenic fluid in-tank pump
  9. flashed non-condensable gasses
  10. negative Joule-Thompson effect
  11. "hot" rundown cryogenic liquid into "cold" cryogenic liquid 
  12. cooling of loading arm
  13. cooling of ship / truck

This post will discuss quick way to estimate BOG flow.

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Atmospheric pressure at sea level is 101.325 kPa abs. Atmospheric pressure is reduced with increase in altitude. For example, at elevation of 1,000 meter, the atmospheric pressure can be as low as 89.81 kPa abs. Cryogenic storage may be designed to operate between 50-70 mbar gauge. If the cryogenic storage tank is at beach (sea level), the operating pressure in the tank is approximately 106.325 - 108.325 kPa abs. If this cryogenic storage is at 1,000 meter, the operating pressure in the tank is approximately 94.81 - 96.81 kPa abs. Lower operating pressure in tank can results higher vaporization and more BOG is generated. Therefore, it is always a good practice to use absolute pressure whenever dealing with cryogenic storage tank. Correct pressure modeling in process simulator is extremely important in finding quantity of BOG generated.

Heat leaks into cryogenic fluid can be via rundown / circulation piping, loading arm & storage tank. Proper selection, installation and maintenance of insulation is one of the key factor in minimizing heat leaks into cryogenic system, hence BOG generation. Besides insulation, other external factors such as wind speed, solar radiation, ambient temperature, sand conductivity and etc, affect heat leak. However, these factors are hard to be managed. Heat leaks into system can be calculated by considering heat conduction, convection and radiation. However, this type calculation involve a lot of uncertainties, assumption and rather complicated. Based on past experiences, an approximate method using vaporization coefficient in determining BOG generation due to heat leaks via storage tank, may be considered during conceptual phase.


Vaporization coefficient (k)  may range from 0.04% to 0.06% for LNG whilst 0.06% to 0.1% for Propane, Butane and LPG. One may take note that above are typical for large storage tank e.g. 160,000m3. Higher k factor should be used for smaller storage. For example, 60,000m3, k of 0.08 - 0.1% may be considered.



Above equation is applicable to storage tank which is low surface area-to-volume ratio. However, piping with very low volume and high surface area may experience higher heat input comparatively. Following equation may be used to estimate BOG generated due to piping.

Average heat flux subject to piping diameter. In general, kp of 25 -35 W/m2 may be considered.

Energy is transferred to pump to move quantity of liquid. Part of the energy will loss due to deficiency. and results BOG generation. Following equation may be considered to estimate BOG generated due to pump deficiency.


Pump efficiency can be range from 55% - 75% for common centrifugal pump.

Cryogenic liquid produced from main plant and transfer to cryogenic liquid storage tank. Inflow liquid will displaced vapor and add-on to BOG generation. Following equation may be used.


Other factors result generation of flashed vapor or BOG generation such as present of non-condensable gasses, negative Joule-Thompson effect and "hot" rundown cryogenic liquid into "cold" cryogenic liquid, will possibly be modeled in process simulator.


Cooling of loading arm and tank in ship / truck may generate substantial amount of vapor initially and reduce as loading arm and tank is cooled. This BOG generation may required dynamic simulation which will not be presented in this post.

Monday, August 23, 2010

Brazed Aluminium Heat Exchanger (BAHX) Standard

A brazed aluminium plate-fin heat exchanger consists of a block (core) of alternating layers (passages) of corrugated fins. The layers are separated from each other by parting sheets and sealed along the edges by means of side bars, and are provided with inlet and outlet ports for the streams. The block is bounded by cap sheets at the top and bottom. An illustration of a multi-stream plate-fin heat exchanger is shown below image.




The Standards of the Brazed Aluminium Plate-Fin Heat Exchanger Manufacturers' Association (ALPEMA) is the result of the work by a technical committee of all the Members to meet the objective of the Association to promote the quality and safe use of this type of heat exchanger. The Standards contain all relevant information for the specification, procurement, and use of Brazed Aluminium Plate-Fin Heat Exchangers. The First Edition was published in 1994, has proved extremely successful and popular. Changes in the industry, experience with using the Standards and feedback from users has resulted released of Second Edition. Now the Standards of the Brazed Aluminium Plate-Fin Heat Exchanger Manufacturers' Association (ALPEMA) is available FREE for download.


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How Boil-Off-Gas (BOG) is Generated

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Liquefied Petroleum Gas (LPG) contains mainly Propane (C3) and Butane (i-C4 & n-C4), Liquid Ethylene (C2=) and Liquefied Natural Gas (LNG) contains mainly Methane will evaporate at ambient condition e.g. 20 degC @ 101.325 kPag.

LPG, Liquid Ethylene and LNG can be stored in refrigerated vessel at its bubble point and atmospheric pressure. Their bubble point can be as low as -40, -104 and -162 degC are commonly known as cryogenic temperature and fluid as cryogenic fluid.

Heat leaks into the cryogenic fluid will results vaporization and lead to Boil-Off-Gas (BOG) generation. Other than heat leak, there are other scenarios can lead to BOG generation :
  1. vaporized vapor due to barometric pressure decrease
  2. vaporized vapor due to ambient temperature increase
  3. cryogenic fluid rundown piping
  4. cryogenic fluid circulation / loading line
  5. ship / truck loading arm
  6. cryogenic fluid storage tank
  7. cryogenic fluid rundown pump
  8. cryogenic fluid in-tank pump
  9. flashed non-condensable gasses
  10. negative Joule-Thompson effect
  11. "hot" rundown LNG into "cold" LNG 
  12. cooling of loading arm
  13. cooling of ship / truck



Vaporized vapor due to barometric pressure decrease & ambient temperature increase
Environment pressure and temperature change affects BOG generation. Maximum BOG generation during summer, noon and high elevation (with low barometric pressure). On the other hand, minimum BOG generation during winter, mid-night and near sea side (high barometric pressure). 

Heat leaks into Cryogenic fluid rundown piping, circulation / loading line, ship / truck loading arm & storage tank
Ambient heat leaks cryogenic fluid will be limited by insulation layer. Heat  leaks into subjects to insulation thickness, thermal conductivity, installation quality, etc. Higher insulation thickness, lower thermal conductivity, high installation quality and etc maintain good heat insulation and reduce BOG generation.

Heat generated by rundown pump & in-tank pumpand leaks into Cryogenic fluid
Pumps is required for transferring cryogenic liquid from production plant to storage tank and from storage tank to ship/truck. Pump will absorb power to move cryogenic fluid and any deficiency will generate heat and it will transfer into cryogenic fluid. Pump heat leaks subject to pump capacity, develop head and efficiency. Larger pump, higher head and lower efficiency lead to excess heat generation and leaks into cryogenic fluid. 

Flashing of non-condensable gasses
Present of inert / non-condensable gasses such as nitrogen, carbon monoxide in cryogenic fluid may flash in the storage tank.

Negative Joule-Thompson effect
Another phenomenon is negative Joule Thompson (negative JT) where pressure decrease in rundown line lead to higher temperature. Typical gas is Hydrogen. 

"Hot" rundown into "cold" cryogenic fluid 
Hot cryogenic fluid from one train with hotter temperature which carries heat and rundown into cryogenic  tank with colder temperature can results vaporization.

Cooling of loading arm & tank in ship / truck
Loading arm is heated to ambient temperature when it is unrest for long time. Cryogenic tank in ship / truck is heated by ambient when it is returned with empty tank. All loading arm and tank in ship/truck will needs cooling prior to storage. Large amount of BOG is generated during cooling time.

Coming topic will discuss quick way to estimate BOG rate.

CE Aug 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.
Free article from Chemical Engineering for August 2010.

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Synthetic natural gas
Technology developed 40 years ago to convert coal into substitute natural gas is making a comeback -  processes to make Bio-SNG are approaching


FAYF - Heat Transfer Fluid - Filtration
Indirect heating of processes by organic thermal-liquid fluids offers highly reliable operation, and the heat transfer systems are generally treated as low-maintenance utilities. Occasionally, the heat transfer fluid can become contaminated, resulting in the formation of sludge particles, or other sources of dirt can in-filtrate the system. This contamination can cause operational problems. The solid particulates can cause shaft-seal leakage in the circulation pump, valve stem wear, plugging of flow passages and sometimes fouling of heat exchange surfaces. After contamination, the fluid can sometimes be cleaned by in-system, side-stream filtration. For seriously fouled systems requiring more extensive cleaning, the heat transfer fluid can sometimes be cold filtered outside of the system. Side stream filtration may also enhance the performance of pump  suction strainers on startup of a system.

Oh What a Relief it is!
Improvements in pressure relief devices provide advanced process protection

Foolproofing Regulatory Document Generation
Software helps ensure that you always have the right data in the right format

pH Measurement And Control
When measured correctly, pH can be an invaluable tool for both product and process control

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HP Aug 2010

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




Select Articles from the August 2010 Issue

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Prevent electric erosion in variable-frequency drive bearings
Here are the reasons and remedial actions 

Valve design reduces costs and increases safety for US refineries
The goals were achieved by using alloys with superior corrosion resistance

Pump aftermarket offers solutions for abrasive services
Upgrades substantially increased MTBR

How the inertia number points to compressor system design challenges
It facilitates predicting compressor system performance

Gas refineries can benefit from installing a flare gas recovery system
Take a look at these environmental and economic paybacks

Estimating tank calibration uncertaintyUse these calculations for a specific tank calibration

Thursday, August 12, 2010

Design guideline for subsea oil systems

Design and operating guidelines for subsea oil systems have been developed to ensure the control of hydrates, wax, and other solids, which may impede flow. System designs are primarily driven by the need to avoid the formation of a hydrate plug in any portion of the system. Remediation of hydrate plugs may require system shut-in for weeks or even months. Design and operation guidelines for wax management are also well developed. Asphaltenes present a new challenge to subsea system design and operation. A number of projects now under development (Europa, Macaroni) are likely to experience some asphaltene deposition in flowlines and wellbores. Strategies have been developed to manage asphaltenes, but have not yet been tested in the field. The design and operating guidelines for control of solids in subsea oil systems are a product of the flow assurance process.

by S. E. LORIMER & B. T. ELLISON, Shell Deepwater Development Inc.

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Related Topic

Access to Facts At Your Fingertips (FAYF)

Chemical Engineering magazine allows subscriber to access to their monthly  issue for current month. Free subscription to Chemical Engineering for qualify subscriber already provided since 2008. You may only view current issue. If you need to view past issue, you may have to subscribe full version.  Fact At Your Fingertips (FAYF) is one of the simple factsheet publish monthly which provide brief description, compilation of simple equation and tips for a special chosen topic. Following is a typical image of FAYF.


Following is the listing of FAYF since April 2007.
  • July 2010: Conservation economics: Carbon pricing impacts
  • June 2010: Distillation Tray Design
  • May 2010: Burner Operating Characteristics
  • April 2010: Measurement guide for replacement seals
  • March 2010: Steam Tracer Lines and Traps
  • February 2010: Positive Displacement Pumps
  • January 2010: Low-Pressure Measurement for Control Valves
  • December 2009: Creating Installed Gain Graphs for Control Valves
  • November 2009: Aboveground and underground storage tanks
  • October 2009: Chemical Resistance of Thermoplastics
  • September 2009: Heat Transfer: System Design II
  • August 2009: Adsorption
  • Juy 2009: Flowmeter Selection
  • June 2009: Specialty Metals
  • May 2009: Choosing a Control System
  • April 2009: Energy Efficiency in Steam Systems
  • March 2009: Membrane Configurations
  • February 2009: Pipe Sizing
  • January 2009: Column Internals
  • December 2008: Fluid Flow
  • November 2008: Alternative Fuels
  • October 2008: Heat Transfer
  • September 2008: Crystallization 
  • August 2008: Valves
  • July 2008: Vacuum Processing
  • June 2008: Humidity Control
  • May 2008: Acid Handling
  • April 2008: Tower Packing
  • March 2008: Membranes
  • February 2008: Pressure Relief
  • January 2008: Centrifuging
  • December 2007: Sealing Systems
  • November 2007: Pump Selection and Specification
  • October 2007: Pristine Processing
  • September 2007: Heat Transfer
  • August 2007: Materials of Construction
  • July 2007: Fuel Selection
  • June 2007 (1): Solvent Selection
  • June 2007 (2): Controlling Crystal Growth
  • May 2007: Hazardous Area Classification
  • April 2007: Reaction Engineering
If you are subscriber to Chemical Engineering, you may download all above FAYF. You may try you luck to apply for Free subscription by clicking here.

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Sunday, August 8, 2010

Compression Prediction Gap & Highlights

Earlier post "Compression Prediction - Compressor Vendor, GPSA & HYSYS" has presented equation in determining Polytropic head, polytropic exponent, gas horse power and compressor discharge temperature.

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From data presented, two main findings are (i) GPSA method may be used but shall keep in mind GPSA potentially overpredicted discharge temperature. This potential results over conservative design and excessive cooling required. (ii) HYSYS prediction is using rigorous method which adjusting prediction rigorously and within a good range of prediction. This post will look at the influence of compressibility  (z) on predicted temperature difference for several ordinary components.

Check Case Basis
Following tabulating the basis of compression calculation using GPSA and HYSYS.
1) Component used : Methane (C1), Ethane (C2), Propane (C3) and Iso-Butane (i-C4)
2) Suction temperature fixed at 45 degC for all calculation
3) Suction pressure range : 1, 3, 4.5,  5, 10, 20, 50 barg
4a) Discharge pressure for Methane (C1) case : 4, 8, 15, 30, 55 & 150 barg
4b) Discharge pressure for Ethane (C2) case : 4, 8, 15, 30, 55 & 150 barg
4c) Discharge pressure for Propane (C3) case : 4, 8, 15 & 30 barg
4d) Discharge pressure for Is-Butane (I-C4) case : 4, 8 & 15 barg
5) Polytorpic efficiency artificial set at 73% for all cases



Results
Calculation results shows that
i) GPSA predicted discharge temperature (Td) consistently higher than HYSYS rigorous prediction.
ii) As discharge pressure (Pd) increase, discharge compressibility (Zd)  decrease consistently.
iii) As compressibility decrease (Zd) , the discharge temperature difference / gap (dT) increase significantly. See following chart.


iv) Considering discharge temperature difference / gap (dT) of 10 degC as limit, the compressibility is limited to 0.90 for discharge pressure 8 barg (and below).
v) Considering discharge temperature difference / gap (dT) of 10 degC as limit, the compressibility is limited to 0.96 for discharge pressure 15 barg (and below).
v) As discharge pressure increase above 15 barg, discharge temperature difference / gap (dT) will possibly higher than of 10 degC limit.

Highlights
i) Pressure lower than 15 barg, the GPSA and HYSYS prediction are considered acceptable.
ii) Once pressure higher than 15 barg, GPSA can severely overpredicts discharge temperature. Shall consider to use rigorous compression calculation (like HYSYS).

Related Topic

Process Design of Turboexpander Based Nitrogen Liquefier

Hampson and Linde patented efficient air liquefiers with self-intensive or regenerative cooling of the high pressure air by the colder low pressure expanded air in long lengths of coiled heat exchanger. In this simple way, the complications of cascade precoolers employing liquid ethylene and other liquid cryogens were removed and removal of moving parts at low temperature. The cooling being produced by Joule-Thomson (JT) expansion through a nozzle or valve.




Georges Claude, in 1902 produced a piston expansion engine working at the low temperatures required for the liquefaction of air. The increase in cooling effect over the Joule-Thomson nozzle expansion of the Linde-Hampson designs. The expansion through an expansion valve is an irreversible process. energy is removed from the gas stream by allowing it to do some work in an expansion engine or expander.


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The process is based on a suitable modified Claude cycle which minimizes the umber of heat exchangers and also takes care to accommodate the in house developed turbo xpander. The process design is carried out using the standard calculation procedure and is validated by using process simulation software, Aspen Hysys. parametric analysis is carried out to access the role of different component efficiencies in predicting overall system efficiency at the design and off design conditions. In this analysis, the available turbo expander efficiency is considered to evaluate the feasible heat exchanger efficiency in order to optimize the plant efficiency. The thermodynamic parameters (temperature, pressure, pinch point temperature) are evaluated to obtain the optimum mass fraction through turbo expander for maximum liquid yield. This investigation not only gives the analysis of nitrogen liquefier, but also it will act as a basic frame work for any liquefier and helium liquefier in particular as a future mission.

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Sunday, August 1, 2010

HYSYS OLGA Link User Guide

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This user guide details all the procedures you need to work with the OLGA Link extension which will help you learn how to use OLGA Link efficiently, this manual thoroughly describes the views and capabilities of the OLGA Link as well as outlining the procedural steps needed for running the extension. The basics of building a simple OLGA Link model is explored in the tutorial (example) problem. The case is presented as a logical sequence of steps that outline the basic procedures needed to build an OLGA Link case. This guide also outlines the relevant parameters for defining the entire extension and its environment. Each view is defined on a page-by-page basis to give you a complete understanding of the data requirements for the components and the capabilities of the extension.

The OLGA Link User Guide does not detail HYSYS procedures and assumes that you are familiar with the HYSYS environment and conventions. If you require more information on working with HYSYS, please refer to the HYSYS Manuals. Here you will find all the information you require to set up a case and work efficiently within the simulation environment. Throughout this document, when describing OLGA keywords that are required in the *.inp file for your OLGA model, capital letters will be used for the complete keyword. For example BOUNDARY represents the keyword and specification of a boundary node and its relevant boundary conditions in the OLGA model. Throughout this document (and when you are using distributed computing with one computer for HYSYS and the OLGA Link, and another computer for the OLGA software), you will see the reference to the HYSYS PC (local computer) and the OLGA PC (remote computer).

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Compression Prediction - Compressor Vendor, GPSA & HYSYS

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Compressor is commonly used to compress gas and vapor to higher delivery pressure. Energy is supplied to the compressor to develop compression head. Part of the energy is lost when energy is transferred via shaft and part of energy lost due to compression activity. Energy lost via shaft will convert to vibration and noise. Energy lost due to compression activity (instead of carry out compression work) will turn to fluid internal energy of fluid. As fluid internal energy is increased, temperature of fluid will rise. How much energy is lost to compression activity ? How much internal energy is increased and how fluid temperature is increased ? All this relates to one well known parameter in compression field, Polytropic efficiency.

There are two paths compression is carried out :

1. isentropic reversible path - a process during which there is no heat added to or removed from the system
and the entropy remains constant, pvk = constant
2. polytropic reversible path - a process in which changes in gas characteristics during compression are considered, pvn = constant

One shall take note that most compressors operate along a polytropic path but approaches the isentropic. Most compressor will use polytropic efficiency to account for true behavior.
Compression following polytropic path,



Polytropic head 

where
Zavg = Average compressibility factor
Ts = Suction temperature (degK)
M = Molecular weight
n = polytropic exponent
Pd = Discharge pressure (bara)
Ps = Suction pressure (bara)

Polytropic exponent (n) can be calculated base on following equation


where
k = isentropic exponent
np = Polytropic efficiency

Gas Horse Power,


where
W = gas flowrate (kg/h)

Compressor discharge temperature


where
Td = Discharge temperature (K)
Ts = Suction temperature (K)


Above equations were extracted from GPSA section 13.

Recent compression studies using several cases to find compressor gas horse power and discharge temperature with specific polytropic efficiency. The studies have used
  • GPSA method (as tabulated above) 
  • HYSYS 
to estimate compressor gas horse power and discharge temperature. Results from several international compressor suppliers.
CaseItems Supplier GPSA HYSYS
1aDischarge temperature(degC) 117.6117.5 117.2

Gas Horse Power (kW)2696.02678.42690.1





1bDischarge temperature(degC) 121.2121.0 120.6

Gas Horse Power (kW)2877.02858.42871.4





2aDischarge temperature(degC) 117.2117.5 116.5

Gas Horse Power (kW)10828.01074010651.4





2bDischarge temperature(degC) 88.088.0 87.4

Gas Horse Power (kW)4628.04575.14532.6





3aDischarge temperature(degC) 124.1140.9 124.7

Gas Horse Power (kW)8966.09147.89039.0





3bDischarge temperature(degC) 85.0117.9 86.1

Gas Horse Power (kW)3682.03798.03736.6





4aDischarge temperature(degC) 122.5139.5 125.8

Gas Horse Power (kW)9090.49210.79162.0





4bDischarge temperature(degC) 86.3120.7 87.1

Gas Horse Power (kW)3829.63926.73859.7





5aDischarge temperature(degC) 123.2142.3 125.6

Gas Horse Power (kW)9104.09262.29149.5





5bDischarge temperature(degC) 95.7121.0 87.2

Gas Horse Power (kW)4293.03937.83844.0






Several observations :
i) HYSYS consistently predict discharge temperature similar to compressor supplier results.
ii) GPSA overpredict discharge temperature for several cases.
iii) HYSYS & GPSA predict gas horse power proximity to compressor supplier results with HYSYS in better prediction.


Above results give us some indication that
i) GPSA method may be used but shall keep in mind GPSA potentially overpredicted discharge temperature. This potential results over conservative design and excessive cooling required.
ii) HYSYS prediction is using rigorous method which adjusting prediction rigorously and within a good range of prediction.

Related Topic

CE July 2010

Chemical Engineering Digital Issue for July 2010...


Aging Relief Systems - Are they working properly
Common problems,  cures and tips  to make sure your pressure relief valves  operate properly  when needed
Coal is current champion of China chemical industry
As the discussion on peak oil continues, industry increases its efforts to unlock new resources. Biomass may well become an attractive and renewable alternative in the long term, but the concept of biorefineries still requires some massive research and development efforts before it can be employed competitively on an industrial scale. Industries worldwide therefore turn to known and readily available alternatives, and coal seems to be the current champion. 
 
Controlling Acoustic Coupling
Furnace pulsation is a problem caused by the coupling between heat release from a burner and acoustic waves of the hosting heater. Enhancing natural damping  of the heater is a practical and attractive solution
 
Optimize Shift Scheduling using Pinch Analysis
This technique, already proven in countless heat-integration and waste-minimization operations, can also be applied to human resources management
 
Using Inserts to address solids flow problems
It might seem counter intuitive at first, but inserting an obstacle in the flow path often results in improved flow characteristics 

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