Chemical & Process Technology

Earlier post "How Boil-Off-Gas (BOG) is Generated" has discussed several ways can result Boil-Off-Gas generation. They are listed below :

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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, k_p 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.

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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 :

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). 

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.

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. 

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

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.

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

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.

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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.

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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.

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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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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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, pv^k = 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,

where
Z_avg = Average compressibility factor
T_s = Suction temperature (degK)
M = Molecular weight
n = polytropic exponent
P_d = Discharge pressure (bara)
P_s = Suction pressure (bara)

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

where
k = isentropic exponent
n_p = Polytropic efficiency

Gas Horse Power,

where
W = gas flowrate (kg/h)


where
T_d = Discharge temperature (K)
T_s = 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.

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).

Common problems,  cures and tips  to make sure your pressure relief valves  operate properly  when needed

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. 

 

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

 

This technique, already proven in countless heat-integration and waste-minimization operations, can also be applied to human resources management

 

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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