How to select fluid package in Aspen Hysys

 

Table below provide a rough guide on selection of fluid package.

However, the guide doesn't provide a detail description in selection of   activity coefficient.

Source: Elliot, Liraj : Prentiice Hall , 1999

Non-intrusive type pig detector

 

Generally, pig detector is group by intrusive and non-intrusive type. For non-intrusive type pig, there is no mechanical part that required to be installed inside the pipeline.

Henceforth, it has a number of benefits:

1. Any type of PIG can be detected in both directions.

2. There are no mechanical moving parts resulting in low maintenance.

3. Non-intrusive design benefits include:

- No wetted parts

- No pipe pressure drop

- Easy to install (no tappings or welding required)

- No shutdown required for installation

4. Finally, it is easy to retrofit for existing installations with no need for modifications to the piping or PIG

The non-intrusive pig detector is an acoustic device. When a PIG travels through the pipe, the friction between the PIG and the pipe will generates a characteristic noise. This noise contains information that can be interpreted to gain more knowledge on the situation inside the pipe. The noise is detected and can be transformed into digital signal.

Image courtesy from Roxar Pig Detector catalogue. Emerson Process Management.

Typical process in refinery

• Fractionation (distillation) is the separation of crude oil in atmospheric and vacuum distillation towers into groups of hydrocarbon compounds of differing boiling-point ranges called "fractions" or "cuts."
• Conversion Processes change the size and/or structure of hydrocarbon molecules. These processes include:
  • Decomposition (dividing) by thermal and catalytic cracking;
  • Unification (combining) through alkylation and polymerization; and
  • Alteration (rearranging) with isomerization and catalytic reforming.
• Treatment Processes are intended to prepare hydrocarbon streams for additional processing and to prepare finished products. Treatment may include the removal or separation of aromatics and naphthenes as well as impurities and undesirable contaminants. Treatment may involve chemical or physical separation such as dissolving, absorption, or precipitation using a variety and combination of processes including desalting, drying, hydrodesulfurizing, solvent refining, sweetening, solvent extraction, and solvent dewaxing.
• Formulating and Blending is the process of mixing and combining hydrocarbon fractions, additives, and other components to produce finished products with specific performance properties.
• Other Refining Operations include: light-ends recovery, sour-water stripping, solid waste and wastewater treatment, process-water treatment and cooling, storage and handling, product movement, hydrogen production, acid and tail-gas treatment and sulfur recovery.
• Auxiliary Operations and Facilities include: steam and power generation, process and fire water systems, flares and relief systems, furnaces and heaters, pumps and valves, supply of steam, air, nitrogen, and other plant gases, alarms and sensors, noise and pollution controls, sampling, testing, and inspecting and laboratory, control room, maintenance, and administrative facilities.

 

Problem In Plotting Txy Diagram In Hysys 8.4

 

A strange condition where the Equilibrium Unit Operation Extension fail to generate neither Txy plot nor xy plot.

The Plot and update plot tab seems unresponsive.

Your Comment is highly appreciated.

 

Controller Action: Direct and Reverse

Controller Action: Direct and Reverse
There are two types of controller action: Direct action and reverse action.
What type of change in the controller output (increase or decrease) is required to bring the process variable in line with the control setpoint?” If an improper action setting is selected, the controller will respond in a manner opposite the intended response (increasing temperature, rather than decreasing it, for example).
Direct action causes the output value to change in the same direction as the change in PV (increase in PV à increase in controller output).
The control valve will fail in the closed position.
The error is initially positive (PV - SP>0). The positive error is counteracted by an increased controller output; therefore, this controller is direct-acting.

Reverse action causes the output value to change in the opposite direction as the change in PV (increase in PV à decrease in controller output).
The control valve will fail in the open position.
The error is initially positive (PV - SP>0). The positive error is counteracted by a decreased controller output; therefore, this controller is reverse-acting.

 
Remember:
PV ­+ , MV + ­ = Direct Control
PV +,  ­MV -   = Reverse Control
 
Good reference of Controller action can be found in:

1. http://www.controlglobal.com/articles/2014/controllers-direct-vs-reverse-acting-control/

Great Hysys Training material

Internet is always a great place to learn. One of the interesting manual available to learn hysys especially for young engineers is:

HYSYS: an introduction to chemical engineering simulation for UTM Degree++ program

The book is written by Abd Hamid, Mohd Kamaruddin for University Technology Malaysia/
Below is the extract obtain from UTM website
Abd Hamid, Mohd Kamaruddin (2007) HYSYS: an introduction to chemical engineering simulation for UTM Degree++ program. Manual. Universiti Teknologi Malaysia. (Unpublished)

                               Click the link to download the tutorial manual.
 
 
Also click Aspen and Hysys tag  for more information.

 

Abstract

HYSYS is a powerful engineering simulation tool, has been uniquely created with respect to the program architecture, interface design, engineering capabilities, and interactive operation. The integrated steady state and dynamic modeling capabilities, where the same model can be evaluated from either perspective with full sharing of process information, represent a significant advancement in the engineering software industry. The various components that comprise HYSYS provide an extremely powerful approach to steady state modeling. At a fundamental level, the comprehensive selection of operations and property methods allows you to model a wide range of processes with confidence. Perhaps even more important is how the HYSYS approach to modeling maximizes your return on simulation time through increased process understanding. To comprehend why HYSYS is such a powerful engineering simulation tool, you need look no further than its strong thermodynamic foundation. The inherent flexibility contributed through its design, combined with the unparalleled accuracy and robustness provided by its property package calculations leads to the presentation of a more realistic model. HYSYS is widely used in universities and colleges in introductory and advanced courses especially in chemical engineering. In industry the software is used in research, development, modeling and design. HYSYS serves as the engineering platform for modeling processes from Upsteam, through Gas Processing and Cryogenic facilities, to Refining and Chemicals processes. There are several key aspects of HYSYS which have been designed specifically to maximize the engineer’s efficiency in using simulation technology. Usability and efficiency are two obvious attributes, which HYSYS has and continues to excel at. The single model concept is key not only to the individual engineer’s efficiency, but to the efficiency of an organization. Books about HYSYS are sometimes difficult to find. HYSYS has been used for research and development in universities and colleges for many years. In the last few years, however, HYSYS is being introduced to universities and colleges students as the first (and sometimes the only) computer simulator they learn. For these students there is a need for a book that teaches HYSYS assuming no prior experience in computer simulation.

Cause of thermal expansion inside cavity:

 
Cause of thermal expansion inside cavity:
For the floating type/soft seat ball valve, and liquid fluid
1) In case of temperature of fluid and atmosphere has some increasing(for example""water: 30°C"", depends on fluid), and valve not operated for the time of increasing temperature,"
2) In case of valve position is "Full Open" for the time of increasing temperature,
3) In case of valve position is ""Full Close"" for the time of increasing temperature and differential pressure between upstream and downstream is below 0.98MPa,
4) In case of operation frequency is little and ball seat is new,
The above conditions are happened more than 2subjects, fluid inside body cavity(inside pocket) is increased pressure by thermal expansion. This condition is called "Thermal Expansion Inside Cavity".
 
Thermal Expansion inside Cavity is happened other problems as followings.
1) Operation torque is increased suddenly, and happened operation problem for on-off valve and manual valve.
2)There is possibility to get damage for seal parts.
3)There is a possibility to get damage for valve body. (especially for cast iron body.)

 
In order to avoid thermal expansion in cavity:
1)Making a hole (bypass) body cavity to valve port. (For the time of increasing temperature and valve position ""Full Open"", body cavity and port is same pressure condition.) Standard Spec. for HF5 model."
2) In case of increased temperature, valve position "full open" and fluid pressurizing direction 1-way,
1. make a relief groove on ball seat at upstream side.
2. make a hole on ball face at upstream side pass to port.
3) In case of increasing tempertaure, valve position ""full close"" and fluid pressurizing direction non-fixed, valve should be used Trunnion type ball valve."


 
Reference:
http://www.hisaka.co.jp/english/valve/techDoc/techDoc08.html

Diaphragm seals in chemical/ petrochemical / offshore plant /facilities

Diaphragm seals, also known as chemical or remote seals, are used for pressure measurements when the process medium must not come in contact with pressurized parts of the measuring instrument.

Example of diaphragm seals used in measurement of dp across a filter.
Image source: http://www.aplisens.de/produkty/pdf/APRE-2200.pdf
Diaphragms are used in:

1. The medium is corrosive. Diaphragm seals protect the pressure measuring element (e.g., the interior of the Bourdon tube) against corrosive medium.
2. The medium is highly viscous and fibrous. Diaphragm seals prevent measuring problems due to dead spaces and constrictions in the bores of the measuring instrument (pressure channels, Bourdon tubes)
3. The medium has a tendency towards crystallization or polymerization. Diaphragm seals will stop the crystallization/ polymerization from reaching the measuring instrument.
4. The medium has a very high temperature. Diaphragm seals and capillaries minimize the effect of high temperatures, reducing high temperature errors in the display of the measuring instrument and averting damage due to heat exceeding the upper limits for the thermal loading of the instrument components.
5. The pressure measuring point is in an awkward position, inhibiting the installation of the measuring instrument or prohibiting the accurate reading of the display. By using a diaphragm seal and a capillary, the measuring instrument can be conveniently installed in a location where it can be easily viewed.
6. Hygienic standards require special requirements. Diaphragm seals remove dead space in the measuring instrument and fittings.
7. The medium is toxic or harmful to the environment. The design of the diaphragm seal helps prevent leakage into the environment.

Advantages of diaphragm seal:

1. Longer service life of the measuring assembly;
2. Lower mounting costs
3. Elimination of maintenance.

Common error during design and installation of diaphragm seal
1)        Failure to use Low Volume Nipples:
Always use a low volume nipple with high quality threads when a nipple is required to connect the diaphragm seal to the instrument. This will help eliminate temperature induced errors and reduce the possibility of fill fluid leakage.
2)        Fill Fluid Vaporization:
The fill fluid can vaporize and destroy the diaphragm seal system if the process or ambient temperatures exceed the capabilities of the fill fluid. The potential for problems increases with high operating temperatures at low pressure ranges. Always ensure the fill fluid will work within the pressure and temperature range of the application.
3)        Improper Filling:
Overall performance of a diaphragm seal system can be dramatically affected by improper filling of the system. The diaphragm may bulge outward or the static pressure exerted by the fill fluid on the measuring instrument may induce gross measurement errors if the system is overfilled. The system may experience a lack of response or non-linear reading if the system is under filled.
4)        Improperly Sized Diaphragm:
The diaphragm seal may not be capable of driving the measuring instrument if the diaphragm is too small. There may be problems with instrument resolution while measuring small pressure changes and the system may be susceptible to temperature errors caused by contraction and expansion of the fill fluid.
5)        Slow Response Time:
Longer capillary lines were used than were necessary for the application, consideration was not given to ambient temperature effects, incorrect fill fluid was specified, or incorrect capillary internal diameter was used. Always consult vendor for assistance in determining the response time of a diaphragm seal system in an application.
6)        Unequal Capillary Lines:
Unequal capillary lines are not recommended for differential pressure instruments since the system may be susceptible to zero shifts resulting from fill fluid expansion and contraction.
 
Reference:

1. http://www.aplisens.de/produkty/pdf/APRE-2200.pdf

Proper location of level-measurement nozzle

 

DETAILED DESIGN of a vessel includes determining the proper locations for level gauge/transmitter nozzles.

Figure above shows what happens when a gauge glass is connected to a vessel containing a vapour and two liquid phases. Assume that equal amounts of a liquid with Sg = 1.0, e.g. water, and a liquid with Sg = 0.5, perhaps oil, gradually flow into the vessel. Assume further that the span of the gauge glass is four feet, beginning one foot from the bottom of the vessel.

As the level of the oil rises, it flows into the glass. As both liquids rise further, water begins to enter the bottom of the glass. This is the state shown in vessel A. Up to this point, the glass shows a true indication of the level of propane in the vessel. Once water enters the glass, the oil is cut off. A constant plug, one foot thick, floats on top of the water. Its level no longer bears any obvious relationship to the actual level in the vessel. This is state shown in vessel B. The only relationship between the vessel and the glass is that the hydrostatic pressure is the same for both at the point where the glass taps into the vessel. A gauge glass is really nothing more than a manometer.

Once the level of the oil rises above the upper tap, it flows into the glass and the two interface levels adjust to the same elevation, as shown in vessel C. The gauge will continue to read correctly as long as its lower tap is in the water and the upper tap is in propane. If either fluid is withdrawn so that the upper tap is in the vapour space, the glass will once again read falsely.

This same analysis applies to any type of level indication based on density. Remember that a DP transmitter only gives a single reading, i.e. differential pressure. Therefore only a single quantity can be inferred. If the instrument is affected by only two fluids, it can yield the correct interface level between the two. If there are more than two distinct phases within the span of the two taps, it will give a reading based on the average densities of all the fluids within its span.

Capacitance or nuclear level transmitters will give similar results in multiphase situations, based on the average dielectric or nuclear absorption constants, respectively.

Question:

How can the process controls engineer be assured that the level readings are meaningful if even a gauge glass can't be trusted?

1. Make the entire vessel out of glass. But, this isn't usually practical.
2. Every section of a gauge glass must have separate taps into the vessel so that each pair of taps has no "hidden" phase floating in between. Either that, or accept the fact that until the interface reaches its "normal" range, gauge glasses and transmitters will read falsely.
3. For proper location of externally mounted level measurement nozzles, ensure that at least one nozzle is located in the top liquid phase and at least one nozzle is located in the bottom liquid phase.

 

Reference:

1. http://www.driedger.ca/ce6_v&t/CE6_V&T.html

1. Chemicalprocessing.com , Best Practices for Level Measurement

Error in bridles level measurement

Why bridles pipe are not accurate when measuring liquid with different density. 

A simple calculation to show error in level measurement by bridles.
Pv + ρoil g H1 + ρwater g H2 = Pv + ρoil g H1* (1)
Pv + ρoil g H1 + ρwater g (H2+ Htap) = Pv + ρoil g(H1* + H2*) + ρwater g Hwater (2)
Equalizing the formula:
ρoil H2* = ρwater H2*
Clearly it is a contradiction since both oil and water density could not be the same, under same temperature and pressure.


Reference:

1. Shinskey, F. G.; Process-Control Systems, McGraw-Hill Book Company.

1. Driedger, W. C., " CONTROLLING VESSELS and TANKS"; Hydrocarbon Processing , 1995.
2. Chemicalprocessing.com , Best Practices for Level Measurement

Proper location of level-measurement nozzle

del.icio.us Tags: Instrument,Level,Process Design,EPCC

DETAILED DESIGN of a vessel includes determining the proper locations for level gauge/transmitter nozzles.

Figure above shows what happens when a gauge glass is connected to a vessel containing a vapour and two liquid phases. Assume that equal amounts of a liquid with Sg = 1.0, e.g. water, and a liquid with Sg = 0.5, perhaps oil, gradually flow into the vessel. Assume further that the span of the gauge glass is four feet, beginning one foot from the bottom of the vessel.
As the level of the oil rises, it flows into the glass. As both liquids rise further, water begins to enter the bottom of the glass. This is the state shown in vessel A. Up to this point, the glass shows a true indication of the level of propane in the vessel. Once water enters the glass, the oil is cut off. A constant plug, one foot thick, floats on top of the water. Its level no longer bears any obvious relationship to the actual level in the vessel. This is state shown in vessel B. The only relationship between the vessel and the glass is that the hydrostatic pressure is the same for both at the point where the glass taps into the vessel. A gauge glass is really nothing more than a manometer.
Once the level of the oil rises above the upper tap, it flows into the glass and the two interface levels adjust to the same elevation, as shown in vessel C. The gauge will continue to read correctly as long as its lower tap is in the water and the upper tap is in propane. If either fluid is withdrawn so that the upper tap is in the vapour space, the glass will once again read falsely.
This same analysis applies to any type of level indication based on density. Remember that a DP transmitter only gives a single reading, i.e. differential pressure. Therefore only a single quantity can be inferred. If the instrument is affected by only two fluids, it can yield the correct interface level between the two. If there are more than two distinct phases within the span of the two taps, it will give a reading based on the average densities of all the fluids within its span.
Capacitance or nuclear level transmitters will give similar results in multiphase situations, based on the average dielectric or nuclear absorption constants, respectively.
Question:
How can the process controls engineer be assured that the level readings are meaningful if even a gauge glass can't be trusted?

1. Make the entire vessel out of glass. But, this isn't usually practical.
2. Every section of a gauge glass must have separate taps into the vessel so that each pair of taps has no "hidden" phase floating in between. Either that, or accept the fact that until the interface reaches its "normal" range, gauge glasses and transmitters will read falsely.
3. For proper location of externally mounted level measurement nozzles, ensure that at least one nozzle is located in the top liquid phase and at least one nozzle is located in the bottom liquid phase.

Reference:

1. http://www.driedger.ca/ce6_v&t/CE6_V&T.html

1. Chemicalprocessing.com , Best Practices for Level Measurement

Type of Liquid Level Sensor AND ITS APPLICATION

 
 Technorati Tags: Level,Instrument,Process Design

LEVEL MEASUREMENT, which is the detection of the phase split between vapor/liquid, liquid/ liquid, vapor/solid and even liquid/solid, is a key parameter in the operation and control of modern industrial processes. Failure to measure level reliably has resulted in some of the most serious industrial accidents, including those at the Buncefield, U.K., fuel storage depot and BP’s Texas City refinery.
Type of level measurement includes:

Hydrostatic
This continuous indirect method measures the pressure due to liquid level and density plus over-pressure. The sensor measures the difference between this pressure and a reference one, normally atmospheric; so, it’s not well suited for vacuum and pressure service. Instruments come in flanged-mount­ed or rod-insertion styles, the latter not being recom­mended for turbulent conditions. Typical accuracies claimed are ±0.2% of reading but this depends on process fluid properties and conditions.

Float displacer
Suitable for point or continuous applications, it measures the change in buoyancy via a torque tube, lever or servo arrangement. The continu­ous measuring range is set by the displacer length im­mersed in the tank’s external cage, which is preferable for noisy applications, or servo mechanism. The point method uses a float, with the range being limited by the length of the float arm.

Nucleonic
Good for point or continuous duties, this non-contact method, which is independent of fluid density and viscosity, measures the signal strength of a radioactive source beamed across a vessel and has typical ranges of 0.24 m to 3.36 m. Accura­cies generally claimed are ±2% of reading. It’s the preferred method for monitoring level in flash vessels and reboilers under all temperature and pressure conditions.

Radar
Applicable to point or continuous applications, it measures the travel time of an impulse reflected from the liquid surface. Interfer­ence echoes from tank internals, and agitators are suppressed and signals can be characterized to give liquid volume. The sensor doesn’t contact the liquid but is exposed to headspace conditions, which don’t affect the measurement. Reflectivity requires the liquid dielectric constant, εR, to be at least 1.4 (hydrocarbons are 1.9–4.0, organic solvents are 4.0–10 and conductive liquids are over 10). Adjusting the antenna and signal conditions allows tailoring to the particular process, with guided radar used for low εR and turbulent conditions. The method can handle custody transfer because of its claimed accuracy of ±0.5mm.

Capacitance
For point or continuous service, it suits liquids that can act as dielectrics. Sensitivity increases with the difference in dielectric constants, δεR, between the liquid and the vapor space or between the two liquids. Special designs, involving coated and twin probes, are used when δεR is under 1.0, conductivities exceed 100 ʮmho, or to overcome probe build-up effects, and when vessel material is non-conducting. Typical accuracies claimed are ±0.25% of span. However, fluid properties affect mea­surements, so the method isn’t suitable for changing conditions. Maximum conditions are 200°C at 100 bar and 400°C at 10 bar.

Ultrasonic.
Suitable for point or continuous use, it is based on the time-of-flight principle. A sensor emits and detects ultrasonic pulses that are reflected from the surface of the liquid. The method is non-invasive, with some types being non-contact, and isn’t affected by εR, conductivity, density or humidity. Maximum conditions are 150°C at 4 bar.
Load cells.
Appropriate for point and continuous applications, such devices, which can be based on strain gauge or piezoelectric technology, measure the weight of the process vessel plus contents. Individual load cell accuracy of 0.03% of full scale is achiev­able but overall performance depends on correct installation practices to exclude external forces due to associated piping and equipment. For vessels with jackets, agitation and complex piping, it’s difficult to obtain an acceptable accuracy. When the container can be totally isolated, as in final dispensing and filling applications, precision weighing can be achieved.

Tuning fork
This method can detect point liquid level but isn’t suitable for viscous and fouling applica­tions. Maximum conditions are 280°C at 100 bar.

Conductivity
Good for finding point level, it requires a liquid conductivity exceeding 0.1 ʮmho and frequently is used on utility and effluent pump control systems.
Typical compara­tive costs
From lowest to highest, are: conductivity - capacitance - tuning fork - hydrostatic - displacer - ultrasonic - load cell - radar - nucleonic.

APPLICATION CONSIDERATIONS

1. Selection also must consider both the process and its control.

1. Process. It’s essential to understand the physical property variations of the process fluids and the phase changes that may occur within the process during normal and abnormal conditions.

1. Boilers, flash vessels and distillation column bottoms involve boiling liquids, resulting in noisy levels. Displacers in external cages frequently are used on steam generators and flash vessels, provided the process fluids are of low viscosity and relatively clean.

1. Non-contact nucleonic method will prove most reliable for distillation column bottoms, where reproducibility is more important than absolute accuracy. While expensive, it can be more than justified given its value in providing stable column operation and in preventing reboiler fouling due to loss of level.

1. Avoid the use of impulse lines in level systems if the process pressure varies and there’s a tendency for solids’ formation due to freezing, precipitation or polymerization. Purging the lines with inert gas or process compatible fluids will have limited suc­cess and is high maintenance.

1. Nucleonic level detection provides a powerful tool to perform on-line process diagnostics. Typical applications include monitoring level profiles in tray towers, distribution in packed beds, locating level build-up and blockages in vessels, and general flow studies.

Float, lead and Lag

Introduction:
When it comes to project activity management, activity sequencing is one of the main tasks. Among many other parameters, float is one of the key concepts used in project scheduling.
Float can be used to facilitate the freedom for a particular task. Let's have a look at the float in detail.
Float:
When it comes to each activity in the project, there are four parameters for each related to the timelines. Those are defined as:

• Earliest start time (ES) - The earliest time, an activity can start once the previous dependent activities are over.
• Earliest finish time (EF) - This would be ES + activity duration.
• Latest finish time (LF) - The latest time an activity can finish without delaying the project.
• Latest start time (LS) - This would be LF - activity duration.
The float time for an activity is the time between the earliest (ES) and the latest (LS) start time or between the earliest (EF) and latest (LF) finish times. During the float time, an activity can be delayed without delaying the project finish date. In an illustration, this is how it looks:

Leads and Lags:
Leads and Lags are types of float. Let's take an example to understand this.
In project management, there are four types of dependencies:

• Finish to Start (FS) - Later task does not start until the previous task is finished
• Finish to Finish (FF) - Later task does not finish until the previous task is finished
• Start to Start (SS) - Later task does not start until the previous task starts
• Start to Finish (SF) - Later task does not finish before previous task starts
Take the scenario of building two identical walls of the same house using the same material. Let's say, building the first wall is task A and building the second one is task B. The engineer wants to delay task B for two days. This is due to the fact that the material used for both A and B are a new type, so the engineer wants to learn from A and then apply if there is anything to B. Therefore, the two tasks A and B have a SS relationship.
The time between the start dates of the two tasks can be defined as a lag (2 days in this case).

If the relationship between task A and B was Finish to Start (FS), then the 'lead' can be illustrated as:

Task B started prior to Task A with a 'lead.'
Conclusion
For a project manager, the concepts of float, lead and lag make a lot of meaning and sense. These aspects of tasks are important in order to calculate project timeline variations and eventually the project completion time.
For Process Engineer, the basic concepts of scheduling is vital in ensuring that all process design related activities is carried in a planned matter without delaying the schedule.

PTS –Petronas Technical Standard- Sizing of PRV at Vaporised Liquid - Fire Case with Hysys

PTS 80.45.10.11. has recommended following procedure to select a suitable PRV in vaporised liquid - fire case, by aid of UNISIM/HYSYS as per different applications;
1) If a vessel contains two separate liquid phases,e.g. hydrocarbon and water, all the fire heat input shall be applied to one phase and then to the other phase. The case that results in the highest orifice area shall be used. (it means, orifice area is estimated based on that phase with lower latent heat, and relief temperature is reported based on that phase with higher bubble point.)
2) If a vessel contains a mixture of fluids, the relieving fluid properties shall be calculated by assuming that 5 % by weight of the original mixture has already flashed. This shall be done as follows:
a) Evaluate all the following properties at the maximum allowable accumulated pressure.
b) Flash off the lightest 5% wt.
c) Use the remaining 95% wt to determine the average properties.
d) Determine the latent heat of vaporization of the remaining 95% wt by:
- Finding the bubble point specific enthalpy of the liquid in J/kg (Btu/lb).
- Finding the dew point enthalpy of the vapour in J/kg (Btu/lb).
- Subtracting the liquid enthalpy from the vapour enthalpy to find the latent heat of vaporisation in J/kg (Btu/lb).
e) Find the average molecular weight, compressibility, and temperatures associated with the vapour at Maximum Allowable Accumulated Pressure and use these properties for the PRV vapour sizing.
NOTE: The latent heat, temperature, and molecular weight of the liquid that is initially vaporized will be lower than the average values. A rigorous (dynamic) analysis of one mixture has illustrated that the simplified approach of using average values is conservative.
3) For a column's fire relief, the PRV shall be sized assuming two compositions as follows:
a) First composition based on the tray below the top tray.
NOTE: For modelling purposes, this is the third theoretical stage composition or reflux composition if the third theoretical stage data is not readily available.
b) Second composition based on the column bottom.
c) The column's PRV shall be sized for both compositions and the larger size of the two shall be used. Do not apply the 5 % by weight rule since the removal of light materials is already taken into account by using the composition in the tray below the top tray.