Process Control

Performance of large-scale helium refrigerators subjected to pulsed heat load from fusion devices

by Rohan Dutta

Rohan Dutta, Parthasarathi Ghosh, Kanchan Chowdhury
Cryogenic Engineering Centre, Indian Institute of Technology Kharagpur, India
Presented in 24th International Cryogenic Engineering Conference-International Cryogenic Materials Conference 2012, Fukuoka, Japan, May 14-18, 2012

Immediate effects of pulsed heat load from fusion devices in helium refrigerators are wide variations of mass flow... more Immediate effects of pulsed heat load from fusion devices in helium refrigerators are wide variations of mass flow rate of low pressure stream returning to cold-box of the refrigerators. In this paper, using quasi steady and dynamic simulations in Aspen HYSYS® a four expander based modified Claude cycle has been analyzed to identify the critical equipment that may be affected due to such flow rate fluctuations at the return stream and their transient performance. Suitable techniques for mitigation of fluctuation of return stream have been explored from the open literature in order to find out operability of these plants.

Successful Risk Assessment May Not Always Lead To Successful Risk Control: A Systematic Literature Review of Risk Control after Root Cause Analysis

by Alan Card

Alan J. Card, James Ward, P. John Clarkson. Successful Risk Assessment May Not Always Lead To Successful Risk Control: A Systematic Literature Review of Risk Control after Root Cause Analysis. Journal of Healthcare Risk Management. 2012;31(3):6-12.

Root cause analysis is perhaps the most widely used tool in healthcare risk management, but does it actually lead to... more Root cause analysis is perhaps the most widely used tool in healthcare risk management, but does it actually lead to successful risk control? Are there categories of risk control that are more likely to be effective? And do healthcare risk managers have the tools they need to support the risk control process? This systematic review examines how the healthcare sector translates risk analysis to risk control action plans and examines how to do better. It suggests that the hierarchy of risk controls should inform risk control action planning and that new tools should be developed to improve the risk control process.

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Wiley's copyright policies do not allow me to post the published version of the paper, but they do allow me to send out individual copies. So if you don't have access to the Journal of Healthcare Risk Management and would like a copy of this article, please email me at:
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Dynamics and control of slugging in oil production

by Florent Di Meglio

Ph.D. Thesis, defended on July, 4th, 2011

Slugging is an undesirable multiphase flow regime occurring on oil production facilities.
This manuscript studies... more Slugging is an undesirable multiphase flow regime occurring on oil production facilities.
This manuscript studies the dynamics of this intermittent phenomenon in view of suppressing it by feedback actuation of the outlet valve. We propose control solutions applicable in a broad range of industrial settings. After a quantitative description of the physical characteristics of slugging, we propose a model for two-phase (gas-liquid) flow with distributed parameters reproducing the phenomenon. The model takes the form of a hyperbolic system of transport equations, for which we propose a numerical solving scheme. Besides, we proceed to a stability analysis by constructing a strict Lyapunov function for the mixed initial-boundary value problem. Then, we present a reduced-order model which reproduces the pressure and flow rate oscillations of slugging. After a dynamical analysis of this model, we describe how to calibrate its parameters so that its behavior corresponds to that of a given slugging system. Finally, we propose feedback control laws, based on the analysis of the reduced-order model, in two distinct industrial setups: whether a bottom pressure sensor is available or not. The performances of these solutions are compared with the state-of-the art methods in each situation. The conclusion is that a bottom pressure sensor is not systematically required to stabilize the flow. When one is available, the control law we propose yields better stabilization properties than the solution commonly used in the industry, which should improve the oil recovery process.

A Refinerys Distributed Process Control System

by Dimitrios VENTZAS

IASTED International Symposium Computers and their Applications for Development, Taormina, Iatly, Septenber 2-5, 1985

The paper describes the design philosophy taken into consideration for a refinery's distributed process control... more The paper describes the design philosophy taken into consideration for a refinery's distributed process control system. It ensures reliability, availability and maintenability. The entire DPCS incudes 9 control centers. The architecture of the field control station is discussed. The memory copy and data transfer ensures smooth transfer to the back-up controller. Redundancy is enhanced by customer additional requirements, such as communication bus redundancy, operational grouping and duplication of displays. The signals interface to the DPCS and the man-machine interface are presented. Industrial environment realistic problems are incuded.

Hadjiandreou M.M., Conejeros R., Vassiliadis V.S. (2007). Towards a long-term model construction for the dynamic simulation of HIV infection, Mathematical Biosciences and Engineering 4, 489-504.

by Marios Hadjiandreou

This study involves the mathematical modelling of long-term HIV dynamics. The proposed model is able to predict the... more This study involves the mathematical modelling of long-term HIV dynamics. The proposed model is able to predict the entire trajectory of the disease: initial viremia in the early weeks of the infection, latency, and progression to AIDS; a range spanning approximately ten years. The model outcomes were compared to clinical data and significant agreement was achieved. The formulated model considers all important population compartments including macrophages, latently-infected CD4+ T-cells, and cytotoxic T-lymphocytes (CTLs), an attempt which in many respects is novel in the area of HIV modelling. The ranges of the model parameters and initial conditions were obtained from literature, and their values were determined in this work directly by fitting published clinical data. Furthermore, the simulation results emphasize the importance of macrophages in HIV infection and progression to AIDS and show a clear correlation between the level of CTLs and HIV progression. The ability of the model to correlate analytical data gives credibility to its predictions, a fact that will be exploited in future research in modelling immunological and pharmacological avenues of treatment.

Hadjiandreou M.M., Conejeros R., Vassiliadis V.S., Wilson D.I. (2008). Long-term HIV dynamics: Mathematical modeling and optimal control, Proceedings of The 2008 International Conference on Bioinformatics and Computational Biology, Las Vegas, Nevada, USA, 2, 444-450.

by Marios Hadjiandreou

This study involves the mathematical modeling of long-term HIV dynamics and the investigation of optimal treatment... more This study involves the mathematical modeling of long-term HIV dynamics and the investigation of optimal treatment strategies. The model duplicates literaturereported clinical data with good agreement and is able to predict the entire trajectory of the disease. The model is extended to account for therapy and the emergence of drugresistant virus and used to investigate how continuous therapy and Structured Treatment Interruptions (STIs) can be implemented in an optimal manner to extend the lifeexpectancy of an HIV-infected patient to desirable times, while at the same time minimizing drug-related side-effects. Whereas the former fails when treating patients that have developed strong drug resistance, STIs prove to be very promising. This is because the optimal schedule of ON and OFF treatment allows for the interplay between the drugsensitive and drug-resistant virus and prevents them from growing in an uncontrolled manner. As a result, uninfected CD4+ T-cells are maintained at relatively high values at all times.

Keywords: Drug efficacy, resistance, therapy, STIs.

Hadjiandreou M.M., Conejeros R., Wilson D.I. (2009). HIV treatment on a case-by-case basis. World Academy of Science, Engineering, and Technology 56, 130-139.

by Marios Hadjiandreou

Hadjiandreou M.M., Conejeros R., Wilson D.I. (2009). HIV treatment planning on a case-by-case basis. International Journal of Biological and Life Sciences. 6, 148-157.

by Marios Hadjiandreou

This study presents a mathematical modeling approach to the planning of HIV therapies on an individual basis. The... more This study presents a mathematical modeling approach to the planning of HIV therapies on an individual basis. The model replicates clinical data from typical-progressors to AIDS for all stages of the disease with good agreement. Clinical data from rapid-progressors and long-term non-progressors is also matched by estimation of immune system parameters only. The ability of the model to reproduce these phenomena validates the formulation, a fact which is exploited in the investigation of effective therapies. The therapy investigation suggests that, unlike continuous therapy, structured treatment interruptions (STIs) are able to control the increase in both the drug-sensitive and drug-resistant virus population and, hence, prevent the ultimate progression from HIV to AIDS. The optimization results further suggest that even patients characterised by the same progression type can respond very differently to the same treatment and that the latter should be designed on a case-by-case basis. Such a methodology is presented here.

Keywords - AIDS; chemotherapy; mathematical modeling; optimal control; progression.

Hadjiandreou M.M., Conejeros R., Wilson D.I. (2009). Long-term HIV dynamics subject to continuous therapy and structured treatment interruptions, Chemical Engineering Science 64, 1600-1617.

by Marios Hadjiandreou

This study involves the mathematical modelling of long-term HIV dynamics and the investigation of optimal treatment... more This study involves the mathematical modelling of long-term HIV dynamics and the investigation of optimal treatment strategies. In our previous work, we produced a model which replicates literature-reported clinical data from untreated patients with good agreement and is able to predict the entire trajectory of the disease. Here, we extend the model to account for therapy and the emergence of virus resistant to antiretroviral drugs. We compare the new model with clinical data and use it to investigate the effect of continuous and interrupted (structured treatment interruptions, STI) therapy. For the former, there exist optimal combinations of reverse transcriptase inhibitor (RTI) and protease inhibitor (PI) drug efficacies for which both the wild-type (drug-sensitive) virus is depleted and the time at which mutated (drug-resistant) virus becomes dominant is extended. The simulation results also suggest that ‘PI-based’ drug regimes work better than ‘RTI-based’ ones. For STIs, there exists an optimised schedule of ON and OFF treatment by which the interplay between drug-sensitive and drug-resistant virus does not allow either of them to grow in an uncontrolled manner and deplete CD4+ T-cells (the main target of HIV: they ‘orchestrate’ the immune response). Furthermore, the schedule minimises the impact of side-effects that may arise during therapy. The results show that an optimised schedule, facilitating the interplay between the two virus strains, is the key to the successful implementation of STIs, which have so far been unsuccessful in extending survival-time considerably. Whereas continuous therapy fails when treating patients that have developed strong drug resistance, STIs prove to be very promising. The simulation and optimisation results indicate that although complete eradication of the virus may not be possible, controlling it over a considerable length of time is feasible.

Keywords: Mathematical modelling; Optimal control; Chemotherapy; Drug resistance

Hadjiandreou M.M., Conejeros R., Wilson D.I. (2009). Planning of patient-specific drug-specific optimal HIV treatment strategies. Chemical Engineering Science 64, 4024-4039.

by Marios Hadjiandreou

In this study, we present a mathematical modelling and optimal control approach to formulate patient-specific... more In this study, we present a mathematical modelling and optimal control approach to formulate patient-specific drug-specific treatment strategies for HIV-infected patients. Central to this is the fact that no two individuals respond to infection and treatment in quite the same way, and that different drugs are associated with varied efficacies in the body as well as with different side-effects. We hereby present a methodology which allows optimal planning on a case-by-case basis, unlike previous work in the field which formulated treatment protocols for general use by considering drug efficacies only. Investigation of optimal strategies by using models which consider the pharmacokinetic behaviour of drugs as well as their side-effects by using a well-documented chart may be considered a novel contribution and allows for the optimum administration of all drugs depending on their degree of toxicity as well as their effectiveness in the body. The formulated model is able to replicate clinical data from different progressors to AIDS by estimation of immune system parameters only. The latter have been suggested to be key in determining the degree of progression and the ability of the model to reproduce this phenomenon further validates the formulation. Optimal treatment strategies are produced for different patients and we can conclude that a general treatment protocol cannot be proposed and therapy has to be designed on an individual basis.

Keywords: Mathematical modelling; Optimal control; Chemotherapy; Side-effect; Drug resistance; Progression