Uğur Akman

Simultaneous flexibility targeting and synthesis of minimum-utility heat-exchanger networks with superstructure-based MILP formulation

The synthesis of Heat-Exchanger-Network (HEN) structures consuming minimum total utility and possessing a desired operational flexibility is accomplished via a non-iterative, superstructure-based, simultaneous-MILP formulation. The formulation presumes that the feasible region in the space of uncertain input parameters is convex. Thus, the optimal solution is explored based on the vertices of the polyhedral uncertainty region in the space of source-stream temperatures. The optimal value of the objective function is the lower bound of the total utility consumption for the HEN satisfying the targeted flexibility. When the target-flexibility value is set to zero, the optimal value of the objective function becomes equal to the minimum total-utility level as predicted by the application of the Pinch method to the HEN problem. The formulation is also used, with minor modifications, to compute the flexibility indexes of the synthesized HEN structures. The flexibility index of the HEN structure gives the upper bound for the target flexibility corresponding to the minimum level of the total utility consumption. Using a well-known HEN superstructure and a HEN problem, the formulation is solved successively, for increasing values of the targeted flexibility, to reveal the necessary structural modifications and their corresponding minimum-utility-consumption levels.

Dynamics and optimal control of flexible heat-exchanger networks

A distributed-parameter model of multi-tube, single-pass heat exchangers are used to construct the dynamic model of a heat-exchanger network (HEN). Based on the control-vector parameterization technique, open-loop (time-dependent bypass manipulation) and closed-loop (target-temperature dependent bypass manipulation) centralized optimal-control schemes are developed and tested. The parameterizations are done in such a way that the manipulated variables approach and eventually match the values obtained from static optimization which refers to an algebraic model of the HEN. Manipulation of the bypasses linearly in time up to their optimal values results in a quite satisfactory dynamic response of the HEN and eliminates temporary violations of the target-temperature control-precision constraints.

Index tracking with constraint aggregation

A generic index tracking (IT) problem is formulated and solved using a constraint aggregation (CA) technique for the first time. This technique makes the number of constraints independent of the cardinal of the universe of assets and enables the handling of large IT problems with one constraint only.

Semi-batch deterpenation of origanum oil by dense carbon dioxide

Abstract Semi-batch deterpenation of origanum oil (Origanum Munituflorum) by dense (sub/supercritical) CO2 was studied in an unpacked column with two temperature zones which were applied to columns upper and lower sections. Experimental results at combinations of 70 and 85 bar pressure and 38 and 55°C temperature levels were reported. The results were discussed in terms of 14 major components, and the monoterpene (MT) and non-monoterpene (NMT) fractions of the oil. The oil was obtained from the top of the column at four different deterpenation time-cuts (integral samples of 0–15, 15–25, 25–35, and 35–45 min time intervals). In addition, the effects of the amount of oil feed charged to the extraction vessel, the solvent-to-oil ratio, and the presence of glass beads (0.5 cm) in the column as packing material were demonstrated. The best separation between the MTs and NMTs was achieved at 70 bar with 38°C lower-section and 55°C upper-section temperatures. At these conditions, the MTs are preferentially separated (with relative-distribution-ratio values greater than 2.0) from NMTs (with relative-distribution-ratio values less than 0.5), almost independent of the deterpenation time (flat maximum occurs in the 15–25 min deterpenation time-cut). It is concluded that any occurrence of single-phase state in the column deteriorates the effectiveness of the separation. Lower densities (higher temperatures) at the upper section of the column enhance the separation by causing internal reflux. Statistical analyses of the data and a response-surface analysis based on empirical models are presented. The suggested processing scheme that employs selective CO2 extraction and rectification has the potential to be used in essential-oil refining technology.

Centralized and decentralized control of retrofit heat-exchanger networks

A method for optimal retrofit design of heat-exchanger networks that do not posses control variables in their original structures, is presented. Bypasses, as control variables, are placed in order to satisfy hard/soft target-temperature constraint specifications with minimum increase in exchanger areas. Alternative bypass-placement configurations are compared in terms of static flexibility, cost of retrofit design, and resiliency (Morari Resiliency Index). Pairings among the bypasses and controlled target temperatures are selected for the most favorable configuration according to Relative Gain Array criterion. Final control configuration is tested under centralized and decentralized feedback control schemes. Centralized scheme satisfies hard and soft target-temperature constraints and provides smooth response.

Equilibrium distributions of key components of spearmint oil in sub/supercritical carbon dioxide

Effects of temperature (at 35, 45 or 55°C) and pressure (10–110 atm) on the relative distribution coefficients of the twelve key components of spearmint oil (essential oil ofMentha cardiaca; Scotch spearmint) at equilibrium in dense CO2 were investigated under conditions ranging from subcritical to supercritical regions. Effects of vapor pressure, molecular weight and polarity of the key components on their equilibrium distributions in sub/supercritical CO2 are discussed. At 35°C, all key components of spearmint oil are equally soluble in dense CO2 within the 12–102 atm pressure region. At 45 and 55°C, the key components are equally soluble for pressures greater than about 60 atm. However, around either 45°C/27 atm or 55°C/35 atm conditions, the relative distribution coefficients of all monoterpene hydrocarbons and of isomenthone (an oxygenated monoterpene) exhibit maxima, which are due to significantly higher vapor pressures of these components and significantly lower solvating power of the dense-gas solvent at these particular temperatures and pressures. Vapor-pressure effects, coupled with the decrease in solvating power, dominate the effects of polarity and molecular mass of the key components. Deterpenation of spearmint oil with dense CO2 is possible around either 45°C/27 atm or 55°C/35 atm, where the monoterpene hydrocarbons tend to concentrate in the CO2-rich phase.

Semi-batch packed-column deterpenation of origanum oil by dense carbon dioxide

Abstract The study aims at the separation of the undesired, light monoterpenes (MTs) of origanum oil from more valuable and heavier non-monoterpenes (NMTs). The scope of this work is the analysis of fundamental aspects of semi-batch, packed-column deterpenation process of origanum oil by dense CO2 at 70/85 bar pressure and 38/55°C temperature levels. Effects of operating pressure, temperature zones, presence of packing, and entrainer (ethanol) use are investigated. GC/MS analyses of 14 major components constituting the bulk (86.4%) of the oil samples obtained from the top of the packed column in four different deterpenation time-cuts show that the resulting deterpenated product is rich in MTs (48–89% MTs, 52–11% NMTs). The oil remaining in the extraction vessel is rich in high-molecular-weight NMTs (15–24% MTs, 85–76% NMTs). Thus, the quality of origanum oil is improved as compared to the oil charged (36.44% MTs, 63.56% NMTs). Separation deteriorates with increasing deterpenation time. Enhancement of separation with packing is observed at the first time-cut. No significant entrainer effect (1.25, 2.5, and 5% ethanol in the charge) is noticed. Statistical analyses of the data and a response-surface analysis based on empirical models are presented. Suggested processing scheme that employs selective CO2 extraction and rectification has the potential for use in essential-oil refining technology.

Nonlinear model predictive control of retrofit heat-exchanger networks

Abstract Nonlinear Model Predictive Control (NMPC) of a Heat-Exchanger Network (HEN) is studied. For this purpose, an optimal retrofit design of the HEN is accomplished, and thus, bypasses as control variables are placed in order to satisfy steady-state hard/soft target-temperature constraints. The NMPC scheme, in which the nonlinear distributed-parameter HEN model is solved sequentially by referring to an algebraic steady-state-optimization model, satisfies the temporal and steady-state hard/soft constraints imposed on the target temperatures of the retrofit HEN.

Operability of an autothermal reactor linked to a flexible heat-exchanger network

Operability of an autothermal fixed-bed catalytic reactor within a Heat-Exchanger Network (HEN) is investigated. A retrofit HEN is designed by considering the critical temperature deviations that cause run-away behavior for the reactor. The flexible retrofit design is of minimum cost (area) and keeps both the hot-spot temperature (maximum temperature rise) in the reactor within a strict safety margin and the overall conversion within a strict feasibility margin against a set of possible disturbance directions. The flexible retrofit HEN enables creation of favorable disturbance-propagation paths via optimal manipulation of bypass-stream flow rates, and thus, increases the operability and controllability of the autothermal reactor within the HEN.

Analysis of Innovation and Energy Profiles in the Turkish Manufacturing Sector

We present Turkey’s manufacturing-sector innovation data and, for the first time, analyze likely relationships among GDP growth, sectoral innovation intensities, energy consumptions, and energy-saving potentials. We detect a power-law-like relationship between the projected energy-saving potentials and realized energy consumptions of the manufacturing-sector groups. We observe that the energy consumptions of the sectors do not change significantly despite varying innovation levels during transitions from economic crisis and recovery periods. We conclude that the Turkey’s manufacturing sectors’ energy consumptions are insensitive to their innovation levels, or their innovation activities are not energy-efficiency- and energy-saving-oriented, reflecting Turkey’s past supply-oriented energy policy. The leader innovating sectors are, nevertheless, expected to contribute more to Turkey’s energy-saving and energyefficiency policies if their innovation potentials can be directed to achieve higher energy savings and energy efficiencies via government incentives within the agenda of the recent energy-efficiency and R&D laws.