Optimum heat storage design for heat integrated multipurpose batch plants

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Heat integration to minimise energy usage in multipurpose batch plants has been in published literature for more than two decades. In most present methods, time is fixed a priori through a known schedule, which leads to suboptimal results. The method presented in this paper treats time as a variable, thereby leading to improved results. Both direct and indirect heat integration are considered together with optimisation of heat storage size and initial temperature of heat storage medium. The resulting model exhibits MINLP structure, which implies that global optimality cannot generally be guaranteed. However, a procedure is presented that seeks to find a globally optimal solution, even for nonlinear problems. Heatlosses from the heat storage vessel due to idling are also considered. This work is an extension of MILP model of Majozi [1], which was more suited to multiproduct rather than multipurpose batch facilities. Optimising the size of the heat storage vessel as well as the initial temperature of the heat storage fluid decreased the requirement for external hot utility for an industrial case study by 33% compared to using known parameters.pm201

A MILP model for energy optimization in multipurpose batch plants using heat storage

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.The concept of heat integration in batch chemical plants has been in literature for more than a decade. Heat integration in batch plants can be effceted in 2 ways, i.e. direct and indirect heat integration. Direct heat integration is encountered when both the source and the sink processes have to be active over a common time interval, assuming that the thermal driving forces allow. On the other hand, indirect heat integration allows heat integration of processes regardless of the time interval, as long as the source process takes place before the sink process so as to store energy or heat for later use. The thermal driving forces, nonetheless, must still be obeyed even in this type of heat integration. It is, therefore, evident from the foregoing statements that direct heat integration is more constrained than indirect heat integration. Presented in this paper is a mathematically rigorous technique for optimization of energy use through the exploitation of heat storage in heat integrated multipurpose batch plants. Storage of heat is effected through the use of a heat transfer fluid. The resultant mathematical formulation exhibits a mixed integer linear programming (MILP) stucture, which yields a globally optimal solution for a predefined storage size.vk201

Synthesis and optimization of cooling water systems with multiple cooling towers

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Cooling water systems are generally designed with a set of heat exchangers arranged in parallel. This arrangement results in higher cooling water flowrate and low cooling water return temperature thus reducing cooling tower efficiency. Previous research on cooling water systems has focused mainly on heat exchanger network thus excluding the interaction between heat exchanger network and the cooling towers. This paper presents a technique for synthesis and optimization of cooling water systems which incorporates the performances of the cooling towers involved. The study focuses mainly on cooling systems consisting of multiple cooling towers that supply a common set of heat exchangers. The heat exchanger network is synthesized using the mathematical optimization technique. This technique is based on superstructure in which all opportunities for cooling water reuse are explored. The cooling tower model is used to predict the thermal performance of the cooling towers. Two case studies are presented to illustrate the performance of the proposed technique. The first case results in nonlinear programming (NLP) formulation and the second case yields mixed integer nonlinear programming (MINLP) problem. In both cases the cooling towers operating capacity is debottlenecked without compromising the heat duties of the associated heat exchangers [1].mp201

Effect of flow pattern in superstructure-based optimisation of fixed-site carrier membrane gas separation during post-combustion CO2 capture

The authors would like to acknowledge the University of the Witwatersrand Research Office for support.Membrane-based gas separation continues to be an area of interest that is being explored for various applications and efforts are being made to enable large-scale implementation and commercialisation. Works on techno-economic studies in areas such as carbon capture, natural gas sweetening, and biogas upgrading has been reported. Various simulation studies have reported the effect of the membrane flow pattern on permeate recovery and purity. The simulation studies in this area have been limited to single-stage and two-stage membrane processes, while many of these studies considered polymer membranes, facilitated transport has barely been investigated. In addition, optimisation studies that compared different flow patterns in the membrane module have been few. The facilitation of gas permeation decreases as pressure is increased due to carrier saturation. However, an increased pressure increases the driving force, and a trade-off should be achieved. The different membrane flow patterns also have inherent driving force potential. In this work, a superstructurebased model that also embeds a fixed site carrier permeation membrane has been developed for CO2 capture from a coal-fired power plant and three scenarios based on the different flow patterns, i.e., co-current, countercurrent and crossflow, were analysed to determine the effect of the flow pattern in the membrane module. The main objective of the optimisation was to minimise the cost of capture. The counter-current flow pattern resulted in the lowest cost of capture as it resulted in the most energy-efficient process system. The co-current flowbased optimisation results in configuration result in an 18 % increase in cost compared to the counter-current flow pattern optimisation run due to a 29 % increase in energy consumption. The crossflow pattern optimisation results in a 9 % increase in the annualised cost of capture compared to the counter-current flow.http://www.aidic.it/cetam2023Chemical Engineerin

Steam system synthesis using process integration techniques: a graphical approach for multiple steam levels

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Steam is commonly used as the hot utility in the processing industry. The common method of designing the hot utility heat exchanger network (HEN) is to place all of the heat exchangers in a parallel configuration, and to utilize the latent heat of saturated steam. Recent work has shown how process integration and the use of hot condensate can minimize the flowrate of steam through the hot utility. This leads to debottlenecking of the boiler in retrofit designs, or the ability to purchase a smaller and cheaper boiler in a grassroots design. The purpose of this work stems from two main observations. Firstly, the work in published literature has been limited largely to only a single steam level. Many plants have more than one level of steam available, especially if a portion of high level steam is used to operate a turbine which produces exhaust steam at a lower level. Secondly, most modern process integration is conducted as a black-box design using mathematical models. Not all engineers who might want to apply these techniques have access to the expensive solvers and computers required to solve these models. The purpose of this study was therefore to develop a graphical technique that will allow one to design a HEN for minimum steam flowrate in the presence of multiple steam levels. This will be useful both as an educational tool, and to enable engineers with limited access to facilities to apply these techniques using basic drawing packages. The methodology used to apply these techniques involves constructing a limiting feasible utility curve of the cold process streams, and then systematically shifting a number of utility lines to fulfill the energy requirements. In an illustrative example of a grassroots design, application of this synthesis method resulted in a 24% reduction in steam flowrate, a 13% reduction in the capital cost of the steam system and an 8% reduction in the energy demanded from the boiler by the process.mp201

Industrial heat utilisation through water management

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.The focus of this paper is on the dependence between water and energy in industry and the way these resources can be managed in an integrated and more sustainable manner. The fundamental methodology supporting the concept of simultaneous management of water and energy is the process systems approach guided by deep understanding of the simultaneous mass and heat transfer, considering phase and pressure changes. Special attention in this case is paid to the utilisation of the latent heat of water evaporation and condensation (allowing for water and heat recycling). The paper takes a new view to water solutions management especially when processes experience difficulties for direct heat recovery. The paper also highlights the link between water management, heat recovery, process efficiency improvement and capacity de-bottlenecking, which bring additional positive impact of proposed methodologies. The advantages of efficiency improvement, water saving and improved environmental impact of proposed solutions are analysed and demonstrated on an industrial case.pm201

Internet access constrains science development and training at South African universities

The original publication is available at http://www.sajs.co.za/As a group of young scientists whose future careers depend increasingly on access to Internet resources, we are extremely concerned by the apparent lack of understanding of the stranglehold that current approaches to Internet pricing are placing on the development of science in South Africa.Lette

Cake compressibility analysis of BPOME from a hybrid adsorption microfiltration process

This study investigates the utility of a hybrid adsorption-membrane process for cake compressibility evaluation of biotreated palm oil mill effluent. A low-cost empty fruit bunch (EFB) based powdered activated carbon (PAC) was employed for the upstream adsorption process with operation conditions of 60 g/L PAC dose, 68 min mixing time, and 200 rpm mixing speed to reduce the feed-water strength and alleviate probable fouling of the membranes. Two polyethersulfone microfiltration (MF) membranes of 0.1 and 0.2 lm pore sizes were investigated under constant transmembrane pressures (TMP) of 40, 80, and 120 kPa. The compressibility factors (z), which was obtained from the slopes of power plots (function of specific cake resistance (a) and pressure gradient) were evaluated. The z values of 0.32 and 0.52, respectively obtained, for the 0.1 and 0.2 lm MF membranes provided compressible and stable z values as observed from their power plots. Besides, these membranes were found suitable for the measurement of z since the results are in consonance with the established principle of cake compressibility. Moreover, the upstream adsorption mitigated the clogging of the membranes which ultimately led to moderate resistances and cake compressibility. These are indications that with the secondary cake filtration, a sustainable flux can be achieved during BPOME filtration. The membranes exhibited close to 100% restoration after cleaning