Optimal design of sustainable hydrogen networks

Hydrogen is widely used in modern oil refineries to remove the sulfur, nitrogen and aromatic contents of fuels. The existence of such contents would aggravate the greenhouse gas (GHG) emission of petrol fuels. The ultimate goal of massive hydrogen consumption in refineries is to cut down the GHG emission. However, current researches on hydrogen networks are focusing on reducing the cost of hydrogen consumption. The environmental impact of hydrogen consumption, especially the GHG emission, has not been considered yet. If the hydrogen supply network itself discharges too much CO₂, then the significance of the hydrogen consumption will be discounted considerably. It is of great importance to design a sustainable hydrogen network. This paper presents a systematic mathematical modeling methodology for the optimal synthesis of sustainable refinery hydrogen networks. The proposed mixed integer nonlinear programming (MINLP) model accounts for both the economic and the environmental aspect of the hydrogen network. Total annual cost (TAC) is employed to evaluate the economic efficiency of the network, while the environmental performance is assessed by the total CO₂ emission of the network. Two types of fresh fuels are investigated in the case studies. A multi-objective optimization is carried out via the Pareto front generation, which is obtained by an adaptive weighted-sum method. The economic–environmental Pareto front will allow for determining the most promising options for the reuse, purification and combustion of hydrogen streams. The numerical example has shown the proposed approach to be efficient and powerful.     

A P-Graph approach for the synthesis of hydrogen networks with pressure and impurity constraints

The increasing demand for hydrogen in refineries and petrochemical plants is challenging these facilities to minimise their hydrogen utility without incurring high capital and operating costs. As environment-related fuel specifications become more stringent, the demand for hydrogen increases, especially for the operation of hydrodesulphurisation in refineries. A P-graph model is developed in this paper for the synthesis of hydrogen networks. The model is capable of generating optimal and near-optimal solutions for the hydrogen network. The proposed methodology is computationally efficient and require minimal understanding of programming language. The developed model includes both direct recycle/reuse and regeneration schemes; and accounts for pressure and impurity constraints in the hydrogen network. In addition, the application of hydrogen header can also be handled by the P-graph model. The methodology is illustrated with three literature examples and the results obtained match those reported in literature.     

Multi-component optimisation for refinery hydrogen networks

Heavier crude oil, tighter environmental regulations and increased heavy-end upgrading in the petroleum industry are leading to the increased demand for hydrogen in oil refineries. Hence, hydrotreating and hydrocracking processes now play increasingly important roles in modern refineries. Refinery hydrogen networks are becoming more and more complicated as well. Therefore, optimisation of overall hydrogen networks is required to improve the hydrogen utilisation in oil refineries. Previous work over hydrogen management has developed methodologies for H₂ network optimisation, with a very simplistic assumption that all H₂ rich streams consist of H₂ and CH₄ only, which leads to a serious doubt of solution’s feasibility. To overcome the drawbacks in previous work, an improved modelling and optimisation approach has been developed. Light hydrocarbon production and integrated flash calculation are incorporated into a hydrogen consumer model. An optimisation framework is developed to solve the resulting NLP problem. A case study is carried out to demonstrate the effectiveness of the developed approach.     

An iterative method for designing hydrogen networks with regeneration unit

In order to release resource shortage constraints, hydrogen effective use is of great importance for refineries nowadays. Hydrogen utility consumption in a hydrogen network involving regeneration can be reduced further compared to that of the network involving reuse only. In this article, an iterative method is presented for design of hydrogen networks involving regeneration reuse. The following new insight obtained in water networks is adopted: the hydrogen network involving regeneration can be formed by adding the regenerated stream into the original hydrogen network. This will simplify the design and targeting of the networks with regeneration unit(s). The iterative method proposed can be used to design and target the networks with both fixed regenerated concentration and that with fixed hydrogen recovery ratio. An estimation method is proposed to calculate the regenerated concentration and flow rate for the regeneration unit with fixed hydrogen recovery ratio. A few literature examples are investigated with the proposed method. The results obtained in this work are comparable to that obtained in the literature. It is shown that the method proposed is simple and effective.     

Robust optimization of hydrogen network

Process integration is an effective way to reduce hydrogen utility consumption in refineries. A number of graphical and mathematical programming approaches have been proposed to synthesis the optimal network. However, as the operation of refineries encounters uncertainty with the rapidly changing market and deteriorating crude oil, existing approaches are inadequate to achieve robust hydrogen network distribution due to the uncertain factors. In this paper, robust optimization is introduced as a framework to optimize hydrogen network of refineries under uncertainty. In this framework, a number of scenarios representing possible future environments are considered. Both model robust and solution robust are explicitly incorporated into the objective function. A possible optimal network distribution which is less sensitive to the change of scenarios and has the minimum total annual cost is achieved by the tradeoff between the total annual cost and the expected error. Case studies indicate that this method is effective in dealing with hydrogen network design and planning under uncertainty in comparison to the deterministic approach and the stochastic programming method.     

Investigation of different approaches for hydrogen management in petrochemical complexes

Extensive use of hydrogen in refineries and petrochemical plants is an incentive for designing integrated hydrogen networks to utilize hydrogen more efficiently. On the other hand, hydrogen, as an important byproduct, is not properly used in some petrochemical complexes and mostly sent to the fuel system. Few works have been reported in literature for improving hydrogen network in petrochemical complexes. In this paper, however, a modified automated targeting approach has been developed and applied to petrochemical plants for estimating fresh hydrogen target. Furthermore, this paper is aimed to develop a superstructure based optimization framework to consider hydrogen plant as part of the whole network. Also, four industrial cases are studied to demonstrate the importance of hydrogen management in petrochemical complexes and proving the applicability of the new method.     

Hydrogen sulfide removal process embedded optimization of hydrogen network

Due to the trend in tighter environmental regulations on heavier crude oil processing, hydrogen has become an important strategic resource in modern refineries. Refiners have to improve the efficiency of hydrogen distribution networks to satisfy the increasing demand of hydrogen. Consequently, plenty of work has been focusing on optimizing hydrogen reuse and purification schemes, which is known as hydrogen network integration (HNI). In refineries, hydrogen purification techniques include hydrocarbon removal units and hydrogen sulfide (H₂S) removal units. Hydrocarbon removal units such as membrane separation and pressure swing adsorption (PSA) are frequently employed in the HNI study. However, the possibility of integrating H₂S removal units into HNI study has been overlooked until recently. H₂S removal units are usually modeled as mass exchangers and independently studied as mass exchange networks (MEN). In the present work, an improved modeling and optimization approach has been developed to integrate H₂S removal units into HNI. By introducing a desulfurization ratio, R_dspl,i′$R_{d{s}_{pl,i^{\prime }}}$, simplified MEN is incorporated into hydrogen distribution network. Total annual cost (TAC) is employed as the optimizing object to investigate the tradeoffs between hydrogen distribution network cost and MEN cost. Pressure constraints and impurity concentrations are considered, and cost equations are established to determine the installation of new equipments in order to synthesis an economical network. A practical case study is used to illustrate the application and effectiveness of the proposed method.     

Simulation-based optimization and design of refinery hydrogen networks with hydrogen sulfide removal

Hydrogen consumption in oil refineries increases sharply because of more and more heavy and sour crude oil processing, which also makes hydrogen sulfide a considerable contaminant in off-gases of hydrotreaters. This work presents a simulation-based optimization model for synthesis of hydrogen networks with H₂S removal. Aspen HYSYS is employed for rigorous process and thermodynamic modeling of the H₂S removal unit. The proposed model is solved using the genetic algorithm combined with the linprog and fmincon solvers in the Matlab platform. The optimal hydrogen sources and the recirculated absorbent fed into the H₂S removal unit as well as the optimal design of the hydrogen network can be determined simultaneously. A case study is performed to illustrate the application and effectiveness of the proposed model. The result shows that the introduction of H₂S removal can decrease the fresh hydrogen consumption by 43% and the total annualized cost by 17%.     

Modelling of gas network transient flows with multiple hydrogen injections and gas composition tracking

Blending hydrogen into the natural gas (NG) network could provide an efficient pathway for decarbonising the NG system through power-to-gas technologies. However, due to the presence of potentially multiple and intermittent hydrogen injection sources, the gas blended throughout the network would be neither homogenous nor at a constant mole fraction. The above features are not captured by the current transient modelling techniques. To bridge this gap, this work presents a transient analysis model that enables the tracking of gas compositions and particularly hydrogen fractions in real-world meshed networks with multiple NG sources, non-pipe elements, and multiple and intermittent hydrogen injection sources. A time-varying compressibility factor is also introduced to account for the variable gas composition across the network. Moreover, numerical techniques are adopted for improving the stability of the Eulerian numerical calculation, and a specific grid size threshold Δx_max is introduced for selecting the stable mesh grid to alleviate convection-dominated oscillations caused by the hydrogen fraction tracking. The case study based on the well-known 20-node Belgian gas network validates the effectiveness of the method in solving practical-scale problems, whereas the unsuitability of steady-state models is also discussed and highlighted. The results clearly demonstrate the effect and importance of introducing variable compressibility factor, hydrogen fraction tracking, and variable gas demand. The impacts of hydrogen blending on pressures and linepack of the network are further investigated.     

Simulation-based modeling and optimization for refinery hydrogen network integration with light hydrocarbon recovery

Purge gases from hydrocrackers and hydrotreaters and refinery off-gases are important hydrogen sources. Some of these hydrogen sources are also rich in light hydrocarbons that are valuable energy resources and chemical materials. In this work, a systematic method is proposed to integrate hydrogen networks considering light hydrocarbon recovery. This work first develops a hydrogen network superstructure with light hydrocarbon recovery. Aspen HYSYS is employed for rigorous process and thermodynamic modeling of the light hydrocarbon recovery process, and a simulation-optimization model is then developed. To solve the simulation-optimization model efficiently, the genetic algorithm is used as the global solver to determine the feed to light hydrocarbon recovery unit, and the linprog and fmincon solvers are combined to determine the optimal hydrogen network design. The application and effectiveness of the proposed method is validated through a case study. The results show that fresh hydrogen consumption decreases by 13% and the total annualized cost reduces to 72% because of light hydrocarbon recovery. This method could provide useful guides for the management of hydrogen and light hydrocarbons in refineries.     

Long-term stable hydrogen production from acetate using immobilized Rhodobacter capsulatus in a panel photobioreactor

Biological hydrogen production is attractive since renewable resources are utilized for hydrogen production. In this study, a novel panel photobioreactor (1.4 L) was constructed from Plexiglas with a network of nylon fabric support for agar immobilized bacteria complex. Two strains of Rhodobacter capsulatus DSM 1710 wild-type strain and Rhodobacter capsulatus YO3 (hup⁻, uptake hydrogenase deleted mutant) with cell concentrations of 2.5 and 5.0 mg dcw/mL agar, respectively were entrapped by 4% (w/v) of agar. The system was operated for 72–82 days in a sequential batch mode utilizing acetate as substrate at 30 °C under continuous illumination. Immobilization increased the stability of the photobioreactors by reducing the fluctuations in pH. The pH remained between 6.7 and 8.0 during the process. Both hydrogen yield and productivity were higher in immobilized photobioreactors compared to suspended culture. The highest hydrogen productivities of 0.75 mmol H₂/L/h and 1.3 mmol H₂/L/h were obtained by R. capsulatus DSM1710 and R. capsulatus YO3 respectively.     

The integration of hybrid hydrogen networks for refinery and synthetic plant of chemicals

Hydrogen is the core source to both refinery and synthetic plant of chemicals. Refinery consumes high purity hydrogen while synthetic plant of chemicals needs syngas consists of hydrogen and carbon oxides. As main hydrogen production technologies, industrial coal gasification and steam methane reforming based pathways generate H₂, CO and CO₂, which is actually the mixture of hydrogen and carbon oxides. Hence, the gases demand of refinery and synthetic plant of chemicals and their supply from hydrogen production can form hybrid hydrogen networks. On the basis of complementary reuse, this paper firstly proposes integration of hybrid hydrogen network for refinery and synthetic plant of chemicals. Superstructures of individual and hybrid hydrogen networks are employed as problem illustration and corresponding linear programming (LP) mathematical models are formulated. Practical refinery and synthetic plant of chemicals cases are employed to demonstrate its application. Compared with individual networks, the natural gas conservation case can recover 8660.4 Nm³·h^-1 hydrogen in purge gas, reduce 1386.6 Nm³·h^-1 CO₂ emission, equaling to reduction of 278.11 kmol·h^-1 natural gas feedstock and 14.8% of total gas production load; the coal conservation case can even waive the total coal consumption and extra 104.1 kmol·h^-1 natural gas, recover 8660.4 Nm³·h^-1 hydrogen in purge gas, reduce 5255.8 Nm³·h^-1 of CO₂ emission and decrease 21.2% of the total gas production load. Furthermore, economic evaluation is also placed to account for the economic advantage of hybrid network.     

A novel two-step method to design inter-plant hydrogen network

Inter-plant hydrogen network with reuse/recycle optimization is important for saving hydrogen resource. Thus, it's necessary to optimize the inter-plant hydrogen network. In this paper, a novel two-step method that combines the pinch insight with mathematical programming is developed to optimize the inter-plant hydrogen network with purification reuse/recycle. A new transhipment model for targeting inter-plant hydrogen network is built to target the hydrogen utility consumption for each refinery. After the hydrogen consumption is determined, individual plant hydrogen networks are designed separately. The mathematical model is linear to guarantee global optimal solution. Furthermore, the model can also optimize the number of inter-plant connections of the hydrogen network. Case study indicates the effectiveness of model.     

Steady state simulation for the de-bottlenecking of heat recovery networks

This work presents the use of a steady state simulator for the de-bottlenecking of heat recovery networks. It is shown how a heat exchanger network designed for fixed conditions can be de-bottlenecked when process streams undergo changes in operating conditions such as flow rate and supply temperature. A network is said to be flexible if it is capable of maintaining acceptable operation either during normal or under modified conditions. The de-bottlenecking of heat recovery networks can be considered as a special case of the design for improved flexibility. A simulation model for a single phase network of heat exchangers is presented. The model is based on the use of the thermal effectiveness (ε) parameter for heat exchangers. Any type of exchanger configuration and flow arrangement can be modeled by using the appropriate ε–number of transfer units relationships. A general methodology for improving network flexibility is proposed.     

Manipulating the hydrogen production from acetate in a microbial electrolysis cell-microbial fuel cell-coupled system

A biological hydrogen-producing system is configured through coupling an electricity-assisting microbial fuel cell (MFC) with a hydrogen-producing microbial electrolysis cell (MEC). The advantage of this biocatalyzed system is the in-situ utilization of the electric energy generated by an MFC for hydrogen production in an MEC without external power supply. In this study, it is demonstrated that the hydrogen production in such an MEC-MFC-coupled system can be manipulated through adjusting the power input on the MEC. The power input of the MEC is regulated by applying different loading resistors connected into the circuit in series. When the loading resistance changes from 10 Ω to 10 kΩ, the circuit current and volumetric hydrogen production rate varies in a range of 78 ± 12 to 9 ± 0 mA m⁻² and 2.9 ± 0.2 to 0.2 ± 0.0 mL L⁻¹ d⁻¹, respectively. The hydrogen recovery (RH₂), Coulombic efficiency (CE), and hydrogen yield (YH₂) decrease with the increase in loading resistance. Thereafter, in order to add power supply for hydrogen production in the MEC, additional one or two MFCs are introduced into this coupled system. When the MFCs are connected in series, the hydrogen production is significantly enhanced. In comparison, the parallel connection slightly reduces the hydrogen production. Connecting several MFCs in series is able to effectively increase power supply for hydrogen production, and has a potential to be used as a strategy to enhance hydrogen production in the MEC-MFC-coupled system from wastes.     

Heat integration retrofit analysis of a heat exchanger network of a fluid catalytic cracking plant

The impact of a process system on environmental pollution has both a local and global effect. The performance of the heat exchanger network (HEN) in a plant is an important aspect of energy conservation. Pinch technology and its recent extensions offer an effective and practical method for designing the HEN for new and retrofit projects.The fluid catalytic cracking (FCC) is a dominant process in oil refineries and there has been a sustained effort to improve the efficiency and yield of the unit over the years. Nevertheless, benefits and scope for improvement can still be found. The HEN of the FCC process considered here consists of a main column and a gas concentration section. Appropriate data were extracted from the existing network, using flowsheeting simulation. The stream data consists of 23 hot and 11 cold streams and cost and economic data required for the analysis were specified. The incremental area efficiency methodology was used for the targeting stage of the design and the design was carried out using the network pinch method consisting of both a diagnosis and optimisation stage. In the diagnosis stage promising designs were generated using UMIST developed sprint software. The generated design was then optimised to trade-off capital cost and energy savings. The design options were compared and evaluated and the retrofit design suggested.The existing hot utility consumption of the process was 46.055 MW with a ΔT_min of 24°C. The area efficiency of existing design was 0.805. The targeting stage using incremental area efficiency sets the minimum approach temperature at 11.5°C, thereby establishing the scope for potential energy savings. To achieve a practical project, the number of modifications is limited. The selected retrofit design has 8.955 MW saving – 74% of the whole scope. This corresponds to 27% utility cost savings with a payback period of 1.5 years. The modifications include addition of four heat exchanger units and repiping of one existing exchanger.     

Design and implementation of an energy efficient power conditioner for fuel cell generation system

The energy use of the world grows continuously and the development of a clean distributed power generation becomes environmentally important. Fuel cells are one such integral part of Renewable Energy Sources based clean energy supply; that they operate with hydrogen as fuel and water with heat as process waste. Due to the electrochemical reaction, fuel cell has the power quality of delivering low voltage with high current capability. Here an attempt is made to develop a power conditioner with a series of conversion to get a 220 V sinusoidal AC, 50 Hz single phase voltage of low distortion and fast dynamic regulation to cater load variations. A novel Polyphase Boost DC-to-DC switching converter based on parallel connection of 8 identical converters with current mode control is devised to have minimum reflected ripple current and voltage injected to fuel cell input. A full bridge converter with high frequency transformer isolation, step-up the DC voltage level from the low voltage fuel cell along with poly phase boost converter, deliver required DC to the PWM inverter, which generate AC utility power output. Recent trend of Ultra-capacitor based transient energy storage and retrieval system, to cater for the sluggish behavior of fuel cell, for load transients is incorporated. DSP and FPGA based digital real time controllers are used to realize the gating of MOSFETs and IGBTs used in the power conditioner. A 1 kW power conditioner is developed for a PAFC fuel cell system with 12 V DC nominal and their performance evaluations are satisfactory.     

Modeling the operation of hydrogen supply networks considering facility location

Hydrogen draws increasing attention as an alternative energy source. In order to provide hydrogen to various sectors such as industry, transportation on a global scale, how to produce and distribute it economically is an essential issue not to be missed. This study thereby addresses mathematical modeling of hydrogen supply networks. The proposed model is concerned with how much H₂ can be produced, where can be stored with the aim of maximizing the total net profit. Particularly the physical form of the hydrogen in the network is explicitly taken into account in terms of whether it is stored as a gas or a liquid. The applicability of the proposed model will be demonstrated by a case study of the Korean H₂ supply network with some remarks.     

Experimental evaluation of radiator control based on primary supply temperature for district heating substations

In this paper, we evaluate whether the primary supply temperature in district heating networks can be used to control radiator systems in buildings connected to district heating; with the purpose of increasing the ΔT. The primary supply temperature in district heating systems can mostly be described as a function of outdoor temperature; similarly, the radiator supply temperature in houses, offices and industries can also be described as a function of outdoor temperature. To calibrate the radiator control system to produce an ideally optimal radiator supply temperature that produces a maximized ΔT across the substation, the relationship between the primary supply temperature and outdoor temperature must be known. However, even if the relation is known there is always a deviation between the expected primary supply temperature and the actual temperature of the received distribution media. This deviation makes the radiator control system incapable of controlling the radiator supply temperature to a point that would generate a maximized ΔT.     

Plant energy saving through efficient retrofit of furnaces

Tubular process furnaces belong to energy demanding equipment in the process industry, especially in the chemical and petrochemical process plants and refineries. Several ways of energy saving in such plants usually exist. Retrofit of furnaces can be considered as one of the straightforward and efficient ways. However, operational and geometrical constraints of an existing furnace are the reasons due to which the retrofit of a furnace is a very difficult task. Therefore, the process retrofit is usually focused on heat exchanger network (HEN) retrofit considering maximum furnace duty. Nevertheless, the furnace retrofit should be considered wherever possible. In some older plants, the placement of new shells or topology changes in HENs can be expensive due to various reasons and only minimum topology modifications are usually allowed. The furnace retrofit procedure described in this paper is based on an advanced furnace integration approach using some principles of Pinch Analysis and considering furnace limitations. It can bring surprising results. This method combines principles of an effective design of both processes and equipment. An efficient methodology for furnaces retrofit, using optimization of both stack temperature and air preheating system, is applied. An advantage of this approach is demonstrated through a case study — retrofit of furnace in petrol hydrogenation refining plant for energy efficiency improvement.