Magne Hillestad

Long term dynamic optimization of a catalytic reactor system

Abstract The purpose of this work is to find an optimal operating strategy for a reactor system with a slowly deactivating catalyst. The process studied is Lurgis methanol synthesis, but the methods also applies to other fixed bed reactor systems. The catalyst deactivates irreversibility and must be replaced after some years. The catalyst life time is considered to be known. The problem is formulated as a dynamic optimization problem with time-varying coolant temperature as the free variable. A sequential method is used to solve the dynamic optimization problem.

Self-Optimizing and Control Structure Design for a CO2 Capturing Plant

In this thesis, the systematic plantwide procedure of Skogestad (2004) is applied to two processes; 1- Post-combustion CO2 capturing processes, 2- Natural gas to liquid hydrocarbons (GTL) plants, in order to design economically efficient control structures, which keep the processes nearoptimum when disturbances occur. Because of the large magnitude of energy consumption in both these processes, optimal operation is of great importance. The self-optimizing concept, which is the heart of the plantwide procedure is used to select the right controlled variables in different operational regions, which when they are kept constant, indirectly give the operation close to optimum. The optimal is to reconfigure the self-optimizing control loops when the process is entered into a new active constraint region, but we try to arrive at a simple/single control structure, which does not need switching, where a reasonable loss in operating economic objective function is accepted. The CO2 capturing process studied here is an amine absorption/stripping system. The chosen objective function for this process is first to minimize the energy requirement while fixed CO2 recovery of 90% is met. This leads to one unconstrained degree of freedom. Maximum gain rule is applied and a temperature close to the top of the stripper is found as the best controlled variable. Further, we introduce penalty on CO2 amount released to the atmosphere, and this results in two unconstrained degrees of freedom. CO2 recovery and a temperature close to the top of the stripper are found as the best individual controlled variables in low feedrate. In higher flue gas flowrates, stripper heat input saturates and the self-optimizing method is repeated to select the right controlled variable for the remaining degree of freedom. We validate the propose control structures using dynamic simulations, where 5 different alternatives including decentralized control loops and multivariable controller are studied. We finally achieve a simple control structure, which handles a wide range of change in throughput and keeps the process close to optimum without the need for switching the control loops or updating the controlled variables setpoints by a costly real time optimizer. The GTL process modeled in this thesis includes an auto-thermal reformer (ATR) for synthesis gas production and a slurry bubble column reactor (SBCR) for the Fischer-Tropsch (FT) reactions. The FT products distribution is determined using a well-known Anderson- Schultz- Flory (ASF) model, where carbon component in CO (consumption rate is found based on the proposed rate by Iglesia et al.) is distributed to a range of hydrocarbons. ASF is a function of chain growth probability and the chain growth is a function of H2/CO ratio. We study different scenarios for chain growth and we arrive at a suitable model for optimal operation studies. The optimal operation is considered in two modes of operation. In mode I, natural gas feedrate is assumed given and in mode II, natural gas feedrate is also a degree of freedom. After optimization, in both modes, there are three unconstrained degrees of freedom. The best individual self-optimizing controlled variables are found and since the worst-case loss value is rather notable, combination of measurements is done, which reduces the loss significantly. Mode II happens when oxygen flowrate capacity reaches the maximum and we show that operation in mode II in this case is in snowballing region where operation should be avoided. Operation at maximum oxygen flowrate capacity is where maximum practical profit can be achieved.

Staging of the Fischer–Tropsch reactor with an iron based catalyst

The Fischer–Tropsch reactor is sectioned into stages based on the systematic method given by Hillestad (2010). The design functions are optimized to maximize the concentration of C11+ at the end of reactor path. The decision variables are fluid mixing, hydrogen distribution, heat transfer area distribution, coolant temperature, and catalyst concentration. With the path temperature constrained by 250 °C, staging of the reactor will increase the concentration of C11+. For a three-stage reactor, the concentration is increased by 2.50% compared to a single-stage reactor. The optimal mixing structure is plug flow to have the maximum possible conversion. A case study is conducted to separate and distribute hydrogen along the reactor path. This will reduce H2/CO at the beginning of the path and increase chain growth probability. The results show that for a three-stage reactor, the concentration of C11+ is increased by 15.93% compared to single-stage reactor.

Modeling and optimization of a reactor system with deactivating catalyst

Abstract The scope of this work is to find an optimal operating strategy for a fixed bed reactor system with a slowly deactivating catalyst. Catalyst deactivation occurs practically in all fixed bed reactors, and optimal operation of reactor systems undergoing catalyst deactivation is an important economic issue. The process studied is Lurgis methanol synthesis, but the method also applies to other fixed bed reactor systems. An economic objective has been maximized with respect to coolant temperature and recycle ratio over a fixed time horizon by control vector parameterization and a sequential optimization method. An optimal operation strategy resulting in 1,800,000 USD increase in profit over four years compared to a reference case is found.

A multifluid-PBE model for simulation of mass transfer limited processes operated in bubble columns

Abstract Modeling of reactive dispersed flows with interfacial mass transfer limitations require an accurate description of the interfacial area, mass transfer coefficient and the driving force. The driving force is given by the difference in species composition between the continuous and dispersed phases and thus depends on bubble size. This paper shows the extension of the multifluid-PBE model to reactive and non-isothermal flows with novel transport equations for species mass and temperature which are continuous functions of bubble size. The model is demonstrated by simulating the Fischer-Tropsch synthesis operated in a slurry bubble column at industrial conditions. The simulation results show different composition and velocity for the smallest and largest bubbles. The temperature profile was independent of bubble size due to efficient heat exchange. The proposed model is particularly useful in investigating the effects of bubble size on strongly mass transfer limited processes operated in the heterogeneous flow regime.

Integrated reactor staging and plant optimization of a Biomass-To-Liquid technology

Abstract In this work an industrial Biomass-To-Liquid (BTL) plant simulation and optimization are presented. Biomass is first gasified with oxygen and steam, and the produced syngas is fed to a multi-tubular fixed bed reactor for Fischer-Tropsch (FT) synthesis, obtaining a distribution of hydrocarbons with different molecular weight. A simplified model for the biomass gasification section is implemented in HYSYS® V8.4, while the Fischer-Tropsch reactor is simulated using MATLAB® R2013a. The kinetic parameters of the FT reaction have been determined by using a non-liner regression performed with the experimenta data obtained with a bench-scale FT-rig. The model developed for the Fischer-Tropsch reactor takes into account the catalytic pellets effectiveness factor and the eventual formation of a liquid phase in each point along reactor’s axial coordinate. The whole BTL plant is simulated connecting MATLAB and HYSYS. Moreover, the staging of the Fischer-Tropsch reactor is studied performing a techno-economic analysis of three different plant configurations and evaluating the corresponding pay-back time.

Spinning Cellulose Hollow Fibers Using 1-Ethyl-3-methylimidazolium Acetate–Dimethylsulfoxide Co-Solvent

The mixture of the ionic liquid 1-ethyl-3-methylimidazolium acetate (EmimAc) and dimethylsulfoxide (DMSO) was employed to dissolve microcrystalline cellulose (MCC). A 10 wt % cellulose dope solution was prepared for spinning cellulose hollow fibers (CHFs) under a mild temperature of 50 °C by a dry⁻wet spinning method. The defect-free CHFs were obtained with an average diameter and thickness of 270 and 38 µm, respectively. Both the XRD and FTIR characterization confirmed that a crystalline structure transition from cellulose I (MCC) to cellulose II (regenerated CHFs) occurred during the cellulose dissolution in ionic liquids and spinning processes. The thermogravimetric analysis (TGA) indicated that regenerated CHFs presented a similar pyrolysis behavior with deacetylated cellulose acetate during pyrolysis process. This study provided a suitable way to directly fabricate hollow fiber carbon membranes using cellulose hollow fiber precursors spun from cellulose/(EmimAc + DMSO)/H₂O ternary system.