Monday, August 23, 2010

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
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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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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.

TIPS
If you are subscriber, you may access previous digital releases. Learn more in "How to Access Previous Chemical Engineering Digital Issue".

If you yet to be subscriber of Chemical Engineering, requested your FREE subscription via this link (click HERE). Prior to fill-up the form, read "Tips on Succession in FREE Subscription".

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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).

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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.

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