Retrofitting of the Heat Exchanger Network with Steam Generation in a Crude Oil Distillation Unit

Steam generation through hot streams has an important impact on the utility consumption of a crude oil distillation unit. Retrofitting of the heat exchanger network with steam generation in a crude oil distillation unit is studied with regard to efficient energy usage. The grand composite curve is employed to provide insights into the steam generation problem, and a mixed‐integer linear programming model, presented previously for heat integration through hot discharges/feeds and steam generation, is used to obtain the optimal parameters for steam generation. Three heuristic rules are then proposed to determine suitable hot streams for steam generation. Finally, the heat exchanger network is modified based on pinch technology. After the retrofit, the hot and cold utility decrease.     

Site-wide low-grade heat recovery with a new cogeneration targeting method

One of the key performance indicators for designing site utility systems is cogeneration potential for the site. A new method has been developed to estimate cogeneration potential of site utility systems by a combination of bottom-up and top-down procedures, which allows systematic optimization of steam levels in the design of site utility configurations. A case study is used to illustrate the usefulness of the new cogeneration targeting method and benefits of optimizing steam levels for reducing the overall energy consumptions for the site. Techno-economic analysis has been carried out to improve heat recovery of low-grade waste heat in process industries, by addressing a wide range of low-grade heat recovery technologies, including heat pumping, organic Rankine cycles, energy recovery from exhaust gases, absorption refrigeration and boiler feed water heating. Simulation models have been built for the evaluation of site-wide impact associated with the introduction of each design option in industrial energy systems in the context of process integration. Integration of heat upgrading technologies within the total site has been demonstrated with a case study for the retrofit scenario.     

Experiences from using heat integration software to determine retrofit opportunities within a refinery process

Heat integration techniques can be used to optimize the energy requirement for both new and retrofit plant designs. Software tools for identifying retrofit options are becoming available. This paper reports our experiences from using heat exchanger network (HEN) optimization software for a retrofit case study of an oil refinery process. The HEN optimization software was used to automate the search for the most beneficial retrofit designs following the twostage process proposed by Asante and Zhu. The software provided three potential retrofit designs. Results from this analysis were used as the basis of a rigorous mass and energy balance simulation of the plant. The simulation corroborated the energy savings, but there were some important differences. The simulation required 20% more heat exchange area. Furthermore, the retrofit design involving one topology change was shown to be less economic than an alternative design. These differences are discussed and a revised methodology is proposed.     

Integration of a Gas Fired Steam Power Plant with a Total Site Utility Using a New Cogeneration Targeting Procedure

A steam power plant can work as a dual purpose plant for simultaneous production of steam and elec-trical power. In this paper we seek the optimum integration of a steam power plant as a source and a site utility sys-tem as a sink of steam and power. Estimation for the cogeneration potential prior to the design of a central utility system for site utility systems is vital to the targets for site fuel demand as well as heat and power production. In this regard, a new cogeneration targeting procedure is proposed for integration of a steam power plant and a site utility consisting of a process plant. The new methodology seeks the optimal integration based on a new cogenera-tion targeting scheme. In addition, a modified site utility grand composite curve （SUGCC） diagram is proposed and compared to the original SUGCC. A gas fired steam power plant and a process site utility is considered in a case study. The applicability of the developed procedure is tested against other design methods （STAR？ and Thermoflex software） through a case study. The proposed method gives comparable results, and the targeting method is used for optimal integration of steam levels. Identifying optimal conditions of steam levels for integration is important in the design of utility systems, as the selection of steam levels in a steam power plant and site utility for integration greatly influences the potential for cogeneration and energy recovery. The integration of steam levels of the steam power plant and the site utility system in the case study demonstrates the usefulness of the method for reducing the overall energy consumption for the site.     

Studies on simultaneous energy and water minimisation—Part I: Systems with no water re-use

This paper addresses the simultaneous management of energy and water. A new systematic methodology has been developed for targeting and design that simultaneously minimises the requirements of energy and water. Using this new approach, the design of a water system for maximum energy recovery can be achieved, taking into account the mixing opportunities offered by water networks, while maintaining the water quality to processes in terms of contamination. Direct and indirect energy recovery are analysed and a strategy developed to decrease the number of heat transfer units based on the generation of separate systems and non-isothermal stream mixing. Initially, the analysis is restricted to no water re-use.     

Numerical optimization of heat recovery steam cycles: Mathematical model, two-stage algorithm and applications

Most of the advanced integrated energy systems need a heat recovery steam cycle (HRSC), either fired or unfired, that recovers the waste heat from gas turbines and process units in order to generate electric power and supply mechanical power to compressors, heat to endothermic processes, and steam to external users. The key feature of such HRSCs is the integration between the heat recovery steam generator (HRSG) and the external heat exchangers. This paper presents a rigorous mathematical programming model, a linear approximation, and a two-stage algorithm for optimizing the design of integrated HRSGs and HRSCs, simultaneously considering the HRSG together with the heat recovery steam network and the intensive steam cycle variables. A detailed application of the methodology is described for an integrated gasification combined cycle plant with CO₂ capture and results for other interesting plants are reported. A significant efficiency gain is obtained with respect to usual practice designs.     

Investigating the trade-off between operating revenue and CO2 emissions from crude oil distillation using a blend of two crudes

Blending of different crude types is frequently used in petroleum refineries to improve their profitability and products yields. However, energy consumption and consequential CO₂ emissions strongly depend on the types of crude being processed. The trade-off between CO₂ emissions and economic objectives, such as net revenue, is investigated for cases of different crude blends using the multi-objective optimization approach. The first objective is the minimization of CO₂ emissions whilst the second objective is maximizing the net revenue from the crude distillation unit (CDU). A rigorous model is used to estimate CO₂ emissions from different sources within the CDU. This emissions model incorporates pinch analysis for heat integration, to optimize the distribution of utilities related to emissions. Blends of two crudes, 36 API and 27.7 API, are used as feedstock to a rigorous CDU model of the atmospheric crude tower, vacuum tower and heat exchanger network. Lighter crude blends recorded higher CO₂ emissions and net revenue compared with the heavier blend due to the greater distilled fraction. However, CO₂ emissions did not vary linearly with the fraction of each crude, as the heat exchanger network also influenced the degree of heat recovery and consequently the level of CO₂ emissions. The multi-objective solutions show the influence of all 13 of the process variables on the objectives.     

Process simulation of natural gas steam reforming: Fuel distribution optimisation in the furnace

This paper describes a two-step method to simulate the natural gas steam reforming for hydrogen production. The first step is to calculate reforming tube length and fuel distribution with equilibrium approach associated with heat transfer. The second step is to calculate and validate reforming performance with kinetic model. A short-cut simulation of hydrogen plant has also been performed to calculate inputs for the reformer model, such as total flow rate and composition of mixed fuel burning in the furnace chamber. Heat transfer, especially radiative heat transfer, is the key role in the steam reforming technology, due to the high heat fluxes involved. For this reason, energy modelling of the furnace chamber has been performed. The simulation evaluates the most important design variables, as tubes height, maximum tube-wall temperature, and tube pressure drop. The heat flux profile can be selected to have suitable metal temperatures to lengthen the reformer tube life. The model calculates the design parameters for reforming tube and fuel distribution among burners.     

Modeling and simulation of energy distribution systems in a petrochemical plant

A systematic method for analysis and design of plant-wide energy distribution systems is proposed to minimize the net cost of providing energy to the plant. The method is based on the steady-state modeling and simulation of steam generation process and steam distribution network. Modeling of steam generation process and steam distribution network were performed based on actual plant operation data. Heuristic operational knowledges are incorporated in the modeling of steam distribution network. Newton’s iteration method and a simple linear programming algorithm were employed in the simulation. The letdown amount from superheated high-pressure steam (SS) header and the amount of SS produced at the boiler showed good agreement with those of actual operational data.     

An innovative trigeneration system using biogas as renewable energy

One of the ways to decrease the global primary energy consumption and the corresponding greenhouse gas emissions is the application of the combined cooling, heating and power generation technologies, known as trigeneration system. In this research an innovative trigeneration system, composed by an absorption heat pump, a mechanical compression heat pump, a steam plant, and a heat recovery plant is developed. The low temperature heat produced by absorption chiller is sent to a mechanical compression heat pump, that receives process water at low temperature from the heat recovery plant and bring it to higher temperatures. The trigeneration system is fed by biogas, a renewable energy. A design and a simulation of the system are developed by Chem Cad 6.3? software. The plant produces 925 kW of electrical energy, 2523 kW of thermal energy and 473 kW of cooling energy, by the combustion of 3280 kW of biogas. Primary energy rate(P.E.R.) is equal 1.04 and a sensitivity analysis is carried out to evaluate the effect of cooling capacity, produced electrical energy and process water temperature. The first has a negative effect, while other parameters have a positive effect on P.E.R. Compared to a cogeneration system, the trigeneration plant produces the 28% higher of power and the 40%lower of carbon dioxide emissions. An economic analysis shows that the plant is economically feasible only considering economic incentives obtained by the use of heat pumps and steam plant at high efficiency. Saving 6431 t·a~(-1) corresponding to 658000 EUR·a~(-1) of incentives, the plant has a net present value(N.P.V.) and a pay back period(P.B.P.) respectively equal to 371000 EUR and 4 year. Future works should optimize the process considering cost and energetic efficiency as the two objective functions.     

Electrical efficiencies of methane fired, high- and low- temperature fuel cell power plants

The important system difference between power plants based on low temperature and high temperature fuel cells is that gas reforming and shift conversion is thermally decoupled from the cell in low temperature cell power plants whereas the gas process steps are performed at close to the elevated fuel cell temperatures in high temperature fuel cell power plants. This article elucidates the consequences: assuming equal electrical efficiencies for the respective cells (50%) it is shown that thermal decoupling leads to energy and exergy losses and sizably lower electrical system efficiencies because heat for the generation of the process steam necessitates the combustion of methane. Also hydrogen losses in the step for preferential oxidation of carbon monoxide (Selox process) and several heat transfer steps add to the lower efficiency of low temperature systems. Low temperature fuel cell power plants need 15–17% more fuel than high temperature fuel cell power plants for the same amount of electric energy. The theoretical comparison of an adiabatic LT and HT fuel cell process reveals that, with postulated electrical cell efficiencies of 50%, the theoretical electrical efficiency of the LT process is 6–7% points lower than that for the HT-process (35 vs. 41%). For exergy efficiencies also taking into account rejected heats, the numbers read 43 and 58%.     

SOFC fuelled by methane without coking: optimization of electrochemical performance

In recent years, fuel cell technology has attracted considerable attention from several fields of scientific research as fuel cells produce electric energy with high efficiency, emit little noise, and are non-polluting. Solid oxide fuel cells (SOFCs) are particularly important for stationary applications due to their high operating temperature (1,073–1,273 K). Methane appears to be a fuel of great interest for SOFC systems because it can be directly converted into hydrogen by direct internal reforming (DIR) within the SOFC anode. Unfortunately, internal steam reforming in SOFC leads to inhomogeneous temperature distributions which can result in mechanical failure of the cermet anode. Moreover this concept requires a large amount of steam in the fed gas. To avoid these problems, gradual internal reforming (GIR) can be used. GIR is based on local coupling between steam reforming and hydrogen oxidation. The steam required for the reforming reaction is obtained by the hydrogen oxidation. However, with GIR, Boudouard and cracking reactions can involve a risk of carbon formation. To cope with carbon formation a new cell configuration of SOFC electrolyte support was studied. This configuration combined a catalyst layer (0.1%Ir–CeO₂) with a classical anode, allowing GIR without coking. In order to optimise the process a SOFC model has been developed, using the CFD-Ace+ software package, and including a thin electrolyte. The impact of a thin electrolyte on previous conclusions has been assessed. As predicted, electrochemical performances are higher and carbon formation is always avoided. However a sharp decrease in the electrochemical performances appears at high current densities due to steam clogging.     

Phase behavior of gelatin in the presence of pectin in water-acid medium

Summary Phase separation of alkaline gelatin in water-acid solutions in the presence of low etherified pectin (ED 38%) were investigated. The effects of the pectin weight fraction in pectin/gelatin mixture (q_o) as well as two conditions of complex formation, namely, mixing of the binary biopolymer-solvent systems at pH 3.5 (‘mixing conditions’), or preparation of the ternary gelatin-pectin-water systems at pH 7.5 and their subsequent acidification up to pH 3.5 (‘titration conditions’), on phase equilibrium and macrostructure of the concentrated complex phase were established using phase analysis, and optical microscopy. At q_o<0.5 the aggregative phase separation was observed in both conditions of complex formation leading to the almost complete concentration of both biopolymers in the bottom phase at q_o=0.3 (‘mixing conditions’) and at q_o=0.5 (‘titration conditions’). At q_o>0.5 unusual three phase separation took place in the ‘mixing conditions’, leading to formation of supernatant (phase 1), complex coacervate (phase 2) and concentrated pectin solution (phase 3). Possible mechanism of such phenomenon was discussed in term of segregative and aggregative phase separations.     

Design and optimization of biomass power plant

Among the renewable energy sources, biomass offers some benefits due to its low cost and presumed zero-carbon emission when compared with fossil fuels. However, the moisture content of biomass is often high that lowers its heating value, reduces the combustion temperature and causes operational problems. Because of these, when burning biomass for power generation, biomass is often dried prior to the combustion. To lower the drying cost or to maximize the power output of a biomass power plant, proper heat integration in between the steam power plant and the drying process has to be considered. In this work, heat integration studies are performed to a biomass power plant that burns empty fruit bunches (EFB) as fuel. Composite curves of all studied cases are plotted to visualize the intensity and to identify opportunities of heat integration among the drying and power generation systems. A multi-stage drying process is proposed that employs steam and waste-heat from the power plant and the drying process, respectively. Results of this study show that with proper drying and heat integration, the overall efficiency of a biomass power plant can be significantly improved.     

Analysis of flow pattern and heat transfer in direct contact condensation

In direct contact condensation (DCC) phenomenon, whenever steam (vapor) is injected with very high velocity in sub-cooled water, the momentum and the energy of the steam is transferred to the surrounding liquid, leading to generation of flow pattern, turbulent in nature. The turbulent flow pattern enhances the heat transfer coefficient at the interface of steam jet and water (vapor-liquid interface) as well as at the immersed surfaces (solid-liquid interface). The flow and the temperature pattern in DCC system have been measured using hot film anemometer (HFA). The values of heat transfer coefficient at the vapor-liquid and solid-liquid interface were estimated using the CCA module of the HFA. The nozzle diameter (d₀) was varied in the range of 1-2 mm and the nozzle upstream pressure in the range of 0.3-0.35 MPa (corresponding velocities in the nozzle were 286-304 m/s). The time series of velocity and temperature at the interface were analyzed to get the rates of surface renewal. A comparison has been presented between the predicted and the experimental values of heat transfer coefficient.     

Thermally stabilized combustion

Thermally stabilized combustion has a number of unique characteristics which permit the generation of steam or other forms of process energy from the heat of combustion of a gaseous or clean liquid fuel in remarkably compact, integrated apparatus while truly minimizing the concentrations of NO_x, CO and unburnt fuel in the effluent. These characteristics, which have been identified by a long-range program of research, are described and the advantages and limitations of this process are discussed.     

Fuel consumption rate in a heat-powered unit analyzed as a function of the temperature and consumption ratio of the air

An analysis of fuel consumption for a heat-powered unit in the processing of ceramic materials is given. The heat consumption rate is analyzed as a function of the temperature of the air supplied to the unit’s reactor for combustion of fuel and for generating the working medium, and of the consumption ratio α₂ of the air in the spent working medium whose temperature is independent of α₂. __________ Translated from Novye Ogneupory, No. 1, pp. 33–34, January, 2006.     

Minimizing investment cost for multi-period heat exchanger network retrofit by matching heat transfer areas with different strategies

Multi-period heat exchanger network (HEN) retrofit is usually performed by targeting and matching heat trans-fer areas. In this paper, based on the reverse order matching method we proposed previously, three strategies of matching heat transfer areas are proposed to minimize the investment cost for the retrofit of HEN in multi-period, in which replacement of heat exchangers, addition of heat exchangers and addition of heat transfer areas are performed. We demonstrate the procedures through three scenarios, including maximum number of substituted heat exchangers after retrofit, minimum additional heat transfer areas in the retrofitted HEN, and minimum investment cost for retrofit. The strategies are extended to a single period HEN retrofit problem. The results of multi-period and single period HEN retrofit problems indicate the effectiveness of the strategies. More-over, these results are better than those reported in literature. The strategies are simple and easy to implement, which are of great benefit to large-scale HEN retrofit in practice.     

Performance analysis of integrated biomass gasification fuel cell (BGFC) and biomass gasification combined cycle (BGCC) systems

Biomass gasification processes are more commonly integrated to gas turbine based combined heat and power (CHP) generation systems. However, efficiency can be greatly enhanced by the use of more advanced power generation technology such as solid oxide fuel cells (SOFC). The key objective of this work is to develop systematic site-wide process integration strategies, based on detailed process simulation in Aspen Plus, in view to improve heat recovery including waste heat, energy efficiency and cleaner operation, of biomass gasification fuel cell (BGFC) systems. The BGFC system considers integration of the exhaust gas as a source of steam and unreacted fuel from the SOFC to the steam gasifier, utilising biomass volatilised gases and tars, which is separately carried out from the combustion of the remaining char of the biomass in the presence of depleted air from the SOFC. The high grade process heat is utilised into direct heating of the process streams, e.g. heating of the syngas feed to the SOFC after cooling, condensation and ultra-cleaning with the Rectisol^® process, using the hot product gas from the steam gasifier and heating of air to the SOFC using exhaust gas from the char combustor. The medium to low grade process heat is extracted into excess steam and hot water generation from the BGFC site. This study presents a comprehensive comparison of energetic and emission performances between BGFC and biomass gasification combined cycle (BGCC) systems, based on a 4th generation biomass waste resource, straws. The former integrated system provides as much as twice the power, than the latter. Furthermore, the performance of the integrated BGFC system is thoroughly analysed for a range of power generations, ～100–997 kW. Increasing power generation from a BGFC system decreases its power generation efficiency (69–63%), while increasing CHP generation efficiency (80–85%).     

Numerical optimization of steam cycles and steam generators designs for coal to FT plants

In this work, a recently developed optimization methodology for the design of heat recovery steam cycles (HRSCs) and steam generators (HRSGs) is applied to the design of steam cycles for two interesting coal to Fischer–Tropsch (FT) fuels plants: a FT synthesis process with high recycle fraction of the unconverted FT gases (CTL-RC-CCS) to maximize the production of liquid fuels, and a FT synthesis process with once-through reactor (CTL-OT-CCS). The analysis reveals that designing efficient HRSCs for the once-through FT plant is relatively straightforward, while designing the HRSC for plant CTL-RC-CCS is very challenging because the recoverable thermal power is concentrated at low temperatures (i.e., below 260 °C) and only a small fraction can be used to superheat steam. In addition, it is necessary to establish whether it is advantageous to burn the FT off-gases in a boiler or in a gas-steam turbine combined cycle. For this reason, a detailed boiler model is developed and included into the optimization methodology. As a consequence of the improved heat integration, the electric efficiency of the two plants is increased by about 2 percentage points with respect to the solutions previously published.