Department of Chemical Engineering

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The main problem in identification of continuous LTI systems is the lack of derivative information of the outputs. If all the derivatives are known exactly, a least squares approach is sufficient to obtain the parameter estimates. In this paper, we propose a method which can provide theoretical bounds on the error in the parameter estimates assuming only a few derivatives are known accurately. The error bounds are given for the finite data case as opposed to the asymptotic regimes considered in existing identification approaches. The method is based on transforming the differential equation into the Laplace domain to obtain a linear-in-parameter form for the ODE parameters. As the system is not well conditioned, the method of Tikhonov Regularization is applied to find an approximate solution. Since, exact derivative information is seldom known in practice, B-spline approximation is incorporated in the simulation study where the accuracy of method is demonstrated. © 2019, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.

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IFAC-PapersOnLine

Error bounds for identification of a class of continuous LTI systems

The activity of electrocatalysts critically depends on the chemical coordination around the active sites. Amorphous materials have short-range atomic ordering while their crystalline counterparts have both short and long-range ordering. Traditional synthesis of amorphous materials, involving quenching from high temperatures is unsuitable as it results in less porosity and surface area. In this context, room-temperature syntheses of high surface area amorphous materials with high activity are desirable. Here, we contrast two electrochemical synthesis procedures for generating high surface area amorphous Co3O4 at room temperature via electrochemical ion intercalation/deintercalation and surface oxidation/reduction cycles and evaluate their performance for electrocatalytic oxygen evolution reaction (OER). In the first approach, Li-ion is used for the intercalation/deintercalation (Li/D−Li) cycles in Co3O4, which leads to expansion and contraction of structure, inducing amorphization of Co3O4 by the pulverization of crystal structure in non-aqueous medium. In the second approach, rapid electrochemical surface oxidation/reduction (Ox/Red) of Co3O4 in the aqueous medium leads to the formation of a metastable amorphous structure. The OER specific activity (activity per unit electrochemical surface area) for Li/D−Li−Co3O4 is ∼3.5 times and Ox/Red- Co3O4 induced amorphization is ∼2.5 times higher than their crystalline Co3O4. The superior OER metrics of both the room-temperature amorphization techniques are rationalized via the increase in the ratio of Co2+/Co3+ obtained from the Co-2p XPS spectra. Further, the decrease in overall polarization resistance per site for the OER reaction for both amorphous samples were analyzed from the Tafel plot and electrochemical impedance spectroscopy (EIS). In Li/D−Li−Co3O4, the Li-ion intercalation in bulk Co3O4 structure generates higher bulk-oxygen vacancies leading to higher conductivity and reduction in overall charge-transport resistance for electrocatalyst. On the other hand, Ox/Red- induced amorphization is restricted to the surface or near-surface only with the formation of a small amount of metallic Co which hampers the OER. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

ChemElectroChem

Electrochemical Cycling-Induced Amorphization of Cobalt(II,III) Oxide for Stable High Surface Area Oxygen Evolution Electrocatalysts

A computational analysis based upon the kinetic, mesoscopic numerical tool is adapted for elucidating the buoyancy-driven convection in the square shaped open cavity having the partially active wall under the influence of the uniform magnetic field (B). A heater (size-half of the height of the cavity) is positioned along the vertical, no-slip wall of the cavity (while another vertical end is open to the atmosphere). Other regions of no-slip partially heated vertical wall, as well as top and bottom walls, are thermally insulated. The significance of heater location (bottom, middle and top), the angle of magnetic field (0°, 45° and 90°), Hartmann number (0–100) and Rayleigh number (103–105) on natural convection have been illustrated. The interpretation of results have been carried out by studying the streamlines, isotherms, centreline variation of temperature; velocity component profiles and Nusselt number (local, average). The dependence of the Nusselt number with the magneto-convective parameter is presented and discussed. Heat transfer rate increases with the angle of the magnetic field. © Springer Nature India Private Limited 2019.

International Journal of Applied and Computational Mathematics

MHD free convection in a partially heated open-ended square cavity: Effect of angle of magnetic field and heater location

The electrocoalescence of aqueous drops in an insulating oil is the fundamental process in the dehydration of emulsions of water in insulating oils, such as crude oils. The great difference in the conductivity of the aqueous droplets and the oil used as the suspending medium suggests that the conductivity of the oil medium should not affect the coalescence process. In this work, contrary to this expectation, we show a very strong dependence of the conductivity of the insulating oil on the mode of droplet coalescence. Experiments are reported using silicone oils, vegetable oils, and hydrocarbons, in the anchored drop setup that also facilitates the use of low viscosity oils. Our study indicates that the droplet undergoes cone-cone mode of interaction at low conductivities of the oil medium. On the other hand, when the conductivity of the oil is increased, the cone-dimple mode of coalescence is observed when the viscosity of the oil phase is higher, while a cone-spray-dimple mode is observed when oils of low viscosity are used. © 2019 Elsevier B.V.

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Modes of coalescence of aqueous anchored drops in insulating oils under an electric field

The effect of an electric field on the coalescence of two water drops suspended in an insulating oil is investigated. We report four new results. (i) The cone angle for the non-coalescence of drops can be significantly smaller (as small as ) than the value of reported by Bird et al. (Phys. Rev. Lett., vol. 103 (16), 2009, 164502). (ii) A surprising observation of the dependence of the mode of coalescence/non-coalescence on the type of insulating oil is seen. A cone-cone mode for silicone oil is observed as against cone-dimple mode for castor oil. (iii) The critical capillary number for non-coalescence decreases with increase in the conductivity of the droplet phase. (iv) Systematic experiments prove that the apparent bridge during non-coalescence is indeed transitory and not permanent, as reported elsewhere. Theoretical calculations using analytical theory and the boundary integral method explain the formation of the cone-dimple mode as well as the transitory bridge length. The numerical calculation and thereby the physical mechanism to explain the occurrence of very small non-coalescence angles as well as the dependence of the phenomenon on the conductivity of the insulating oil and the water droplets remain unexplained. © 2018 Cambridge University Press.

Journal of Fluid Mechanics

Electrocoalescence of a pair of conducting drops in an insulating oil

Refineries are increasingly compelled to play an imperative role in achieving sustainable management of water as mandated by sustainable development goals (SDGs). Existing optimization models lack the ability to addresses this challenge of obtaining optimal refinery water network configuration for problems of industrial scale due to difficulty in application of complex solution algorithms of mixed integer non-linear problem (MINLP) formulations to large dimensions. Therefore, the optimization formulation in this work exploits process engineering derived constraints to build an integrated superstructure, which results in a simplified non-linear problem (NLP), and thus reduces computational complexity. Further, to substantially minimize fresh water consumption, the proposed model incorporates novel features of a modern refinery complex viz. distributed effluent treatment plant based on total dissolved solids (TDS) levels in waste water, segregation of sour water treatment based on phenolics content in condensate stream, and minimizing cooling tower blowdown rates by optimizing the cycles of concentration in cooling tower operation. The formulation is demonstrated for optimal water routing and economic benefits of the proposed approach through a representative case study of a real refinery complex. Towards the goal of zero liquid discharge, an efficient new scheme of recycling treated water to the boiler feed water system and cooling tower make up water requirements in optimal ratios, has been suggested from the results of the proposed model. Sensitivity of water reallocation in optimal refinery water network with respect to crude quality and desalter performance is also presented through application of proposed optimization framework in the case studies. © 2019 Elsevier Ltd

Journal of Cleaner Production

Systems engineering based advanced optimization for sustainable water management in refineries

Fire hazard is one of the main risks associated to fuel storage tanks in petroleum and in the petrochemical industries. Such a hazard includes pool fires in the storage tanks or in the bunds, fire propagation from the source tank to target tanks both in absence and in presence of wind, and also the cascading/domino effect due to confined and unconfined vapour cloud explosion and or BLEVE associated with the source tank. In the present work, a radiation shield of flowing water has been introduced at a distance from the source fuel storage tank to prevent the domino effect originating from this source tank, under fire, to the target fuel storage tanks in a tank farm. A simple one dimensional model has been developed from the steady state energy balance to simulate the safe distances (i.e. rim–rim distance) between fuel storage tanks containing class-I fuel (e.g. gasoline), both in presence and absence of a water-shield under no-wind and cross-wind conditions. The model predictions have shown that the maximum safe inter-tank separation distance of 28.42 m is anticipated at a wind velocity of 6 m/s, compared to 16.34 m in no-wind condition, beyond which the centroid of the parallelepiped (a solid-flame geometry) falls outside the base of the tilted flame geometry causing flattening of flame and a very sluggish increase in the flame tilt angle as the inverse of the Richardson number in the presence of wind velocity vector increases. Furthermore, the present one dimensional mathematical model has also been extended to show that introduction of a water-shield with an appropriate thickness (δopt) is able to prevent the propagation of radiation heat flux under both no-wind and cross-wind conditions to a much lower distance close to 8.34 m between tanks (measured from the centre of the source tank), than those predicted from the existing empirical models; viz. Point Source model and Shokrie-Beyler’s model. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.

Fire Technology

Optimal Safe Layout of Fuel Storage Tanks Exposed to Pool Fire: One Dimensional Deterministic Modelling Approach

In this paper, the problem of input design for identification of continuous time output error models is considered. The input design problem is formulated as maximization of a measure of the Fisher information matrix, which defines the accuracy with which the system parameters can be estimated. The optimization problem involving the Fisher information matrix depends on the true system parameters, which are unknown. Existing methods use an iterative approach to tackle this circular dependency. In this paper, the system transfer function is represented using generalized orthonormal basis functions which makes the Fisher Information matrix independent of the true system parameters. Therefore, the input design problem reduces to solving a single optimization problem instead of a series of optimizations that are performed in existing methods. Further, in existing approaches, the computational complexity of the optimization grows with the length of the input vector to be identified. In the current paper, the input is also represented using smooth basis functions, thereby making the complexity independent of the length of the input vector. A least-squares approach for parameter estimation is given and it is shown that the estimates are unbiased for the optimal inputs. The accuracy of the proposed method is demonstrated with the help of two examples. Further, input design for a continuous stirred tank reactor model and a quadruple tank experimental set up is considered to demonstrate the practical applicability of the new method. © 2019 American Chemical Society.

Industrial and Engineering Chemistry Research

Input Design for Continuous Time Output Error Models

The three phase separator is an important unit in oil and gas production facilities to separate gas, water and condensate from the fluid (raw gas) generated from gas wells. A dynamic model of the three phase separator is essential for process optimization and control design. Since first principles modelling of the three phase separator is complex, a data based model (Wavelet Network based Nonlinear AutoRegressive eXogenous model (WN-NARX)) is used to capture the dynamics of the process. As most of the terms in the WN-NARX expansion are redundant, this identification problem is over parameterized. In order to handle this issue, a sparsity constraint on the parameter vector is considered and sparse estimation algorithms such as Orthogonal Matching Pursuit (OMP) and Least Angle Regression (LAR) are used for identification of the WA-NARX model. The application of these sparse estimation methods for identification of an industrial three phase separator process is demonstrated. © 2019 IEEE.

2019 5th Indian Control Conference, ICC 2019 - Proceedings

Comparative Study of Parsimonious NARX Models for Three Phase Separator

Electrical Impedance Tomography (EIT) can be used to obtain phase boundaries and gas holdups in multiphase flows. The main challenge in image reconstruction using EIT is the low spatial resolution. In this paper, a reconstruction algorithm using sparse optimization techniques is presented. For multiphase flows, gradients in the conductivity vector are sparse. Therefore, the reconstruction problem is formulated as identification of this sparse vector given the current-voltage measurements. A new iterative algorithm is proposed to estimate the conductivity values. The accuracy of the proposed method is demonstrated with the help of several examples and comparison with an existing technique. © 2019, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.