Unified Total Site Heat Integration: Targeting, Optimisation and Network Design

Process industries in New Zealand use 214.3 PJ of process heat, of which approximately 65 % is fossil fuels. Despite increasing energy demands, depleting fossil fuel resources, and pressure to reduce Greenhouse Gas emissions, low grade heat in large-scale processing sites is still not fully utilised. This thesis presents methods to target, optimise and design more practical heat recovery systems for large industrial sites, i.e. Total Sites, and overcome technical limitations of current methods. Original contributions of this thesis to literature include novel developments and applications in six areas: i) a new Total Site Heat Integration (TSHI) targeting method – Unified Total Site Targeting (UTST) – which sets realistic targets for isothermal and non-isothermal utilities and heat recovery via the utility system; ii) a new TSHI optimisation and utility temperature selection method to optimise Total Cost of the utility system; iii) a new Utility Exchanger Network synthesis and design method based on the targets achieved by the UTST method and optimal temperatures from optimisation method; iv) a new method for calculating assisted heat transfer and shaft work to further improve TSHI cogeneration and performance; v) examination of heat transfer enhancement techniques in TSHI to achieve higher heat recovery and lower required area by substituting conventional utility mediums by nanofluids in the utility system; and vi) a spreadsheet software tool called Unified Total Site Integration to apply the developed methods to real industrial cases. The developed methods have been applied to three large industrial case studies. Results confirm that heat recovery and utility targets obtained from the UTST method were lower but more realistic to achieve in practice when compared to conventional TSHI methods. The three industrial case studies represent a wide variety of processing industries. In summary, the over-estimation of TSHI targets for the three case studies from using the conventional method compared to the new method are 0.2 % for the Södra Cell Värö Kraft Pulp Mill, 22 % for a New Zealand Dairy Factory, and 0.1 % for Petrochemical Complex. The Total Annualised Costs (TAC) for the three case studies are minimised using a new derivative based approach. Results show TAC reductions 4.6 % for Kraft Pulp Mill, 0.6 % for Dairy Factory, and 3.4 % for Petrochemical Complex case studies. In addition, sensitivity analysis for the optimisation is undertaken. The UTST method with its modified targeting procedure is demonstrated to generate simpler Utility Exchanger Network designs compared to conventional methods, which confirm the original targets are realistic and achievable. A new method for calculating assisted heat integration targets applied to an example Total Site problem increased heat recovery by 1,737 kW, which is a 21% increase in Total Site heat recovery, and increased shaft work by 80 kW. Lastly, the addition of nanoparticles to create a closed nanofluid heat recovery systems shows heat recovery from liquid-liquid heat exchangers increases of 5 % to 9 % using an intermediate fluid with 1.5 vol. % CuO/water

Assisted heat transfer and shaft work targets for increased total site heat integration

Total Site Heat Integration (TSHI) provides a valuable framework for practical integration of multiple energy users. Previous studies have introduced the idea of utilising process heat recovery pockets to assist TSHI. However, these methods are shown to be effective for only some Total Site (TS) problems. As a result, this paper presents a new method for calculating assisted heat transfer and shaft work targets for an example TS problem. Analysis results show that assisted heat transfer increases TSHI only when a process heat recovery pocket spans the TS Pinch Region. The maximum assisted TSHI can be targeted by comparing each heat recovery pocket to the Site Utility Grand Composite Curve (SUGCC) using background/foreground analysis. Where heat recovery pockets span two steam pressure levels away from the TS Pinch Region (usually above), the example shows the potential for assisted shaft work production. In this case, the source segment of the heat recovery pocket generates steam (e.g. MPS), which replaces steam that would otherwise have been extracted from a steam turbine. The sink segment of the heat recovery pocket consumes lower pressure steam (e.g. LPS), which is extracted from the turbine. If a heat recovery pocket falls outside these two situations (assuming direct inter-process integration is disallowed), the entire pocket should be recovered internal to a process

Total site heat integration: Utility selection and optimisation using cost and exergy derivative analysis

This paper presents a new Total Site Heat Integration utility temperature selection and optimisation method that can optimise both non-isothermal (e.g. hot water) and isothermal (e.g. steam) utilities. None of the existing methods addresses both non-isothermal and isothermal utility selection and optimisation incorporated in a single procedure. The optimisation affects heat recovery, the number of heat exchangers in Total Site Heat Exchanger Network, heat transfer area, exergy destruction (ED), Utility Cost (UC), Annualised Capital Cost (CC), and Total Annualised Cost (TC). Three optimisation parameters, UC, ED, and TC have been incorporated into a derivative based optimisation procedure where derivatives are minimised sequentially and iteratively based on the specified approach. The new optimisation procedure has been carried out for three different approaches as the combinations of optimisation parameters based on the created derivative map. The merits of the new method have been illustrated using three case studies. These case studies represent a diverse range of processing types and temperatures. Results for the case studies suggest the best derivative optimisation approach is to first optimise UC in combination with ED and then optimise TC. For this approach, TC reductions between 0.6 to 4.6 % for different case studies and scenarios are achieved

Heat Transfer Enhancement in Heat Recovery Loops Using Nanofluids as the Intermediate Fluid

In this paper, the effect of replacing water with various nanofluids as the heat transfer media in an industrial Heat Recovery Loop (HRL) have been modelled. Generally, nanofluids are prepared by distributing a nanoparticle through a base fluid such as water. Suspended nanoparticles slightly affect the thermal and physical properties of the base fluid. Primarily nanoparticles are added to improve the fluid’s heat transfer characteristics by increasing its Reynolds number and thermal conductivity. Results show that by applying various HRL design methods and a nanofluid as an intermediate fluid, an increase in heat recovery is possible without the need for extra heat exchanger area and infrastructure. With the addition of 1.5 vol.% CuO nanoparticles to the HRL fluid using constant temperature storage method, heat recovery from liquid-liquid heat exchangers increases between 5 % and 9 %. In the case of air-liquid exchangers, the air-side heat transfer coefficient limits the impact of using a nanofluid. In other cases, the duty available from the process stream, such as a condenser, significantly restricts the heat transfer benefit of using a nanofluid. Alternative to increasing heat recovery, results show that applying a nanofluid in the HRL design phase enables heat exchanger area to decrease significantly for liquid-liquid matches

Heat transfer enhancement for site level indirect heat recovery systems using nanofluids as the intermediate fluid

In this paper, implementation of nanofluids as a Heat Transfer Enhancement technique in Process Integration has been studied. A step by step flowchart is introduced and as a case study the effect of replacing water with various nanofluids as the heat transfer media in an industrial Heat Recovery Loop (HRL) has been modelled. Nanofluids are prepared by distributing a nanoparticle through a base fluid such as water, ethylene glycol or oils. Suspended nanoparticles slightly affect the thermal and physical properties of the base fluid. Primarily nanoparticles are added to improve the fluid’s heat transfer characteristics by increasing its Reynolds number and thermal conductivity. HRL system in a large dairy factory in New Zealand has been studied as case study. Results show that by applying various HRL design methods and a nanofluid as an intermediate fluid, an increase in heat recovery is possible without the need for extra heat exchanger area and infrastructure. 1.5 vol.% CuO/water nanofluid has been chosen as an intermediate fluid and by using a constant temperature storage control strategy, heat recovery from liquid–liquid heat exchangers increases between 5% and 9%. The air-side heat transfer coefficient limits the impact of using a nanofluid for the air–liquid exchangers. In other cases, the total available duty from the process stream, such as a condenser, significantly nullifies the heat transfer benefit of using a nano- fluid in a retrofit situation. Alternative to increasing heat recovery, results show that applying a nanofluid in the HRL design phase enables heat exchanger area to decrease significantly for liquid–liquid matches. Results show that the increase in pressure drop and friction factor effects in such a system is negligibl

Impact of hybrid heat transfer enhancement techniques in shell and tube heat exchanger design

Despite the advantages of shell and tube heat exchangers, one of their major problems is low thermal efficiency. This problem can be improved by using heat transfer enhancement techniques such as adding nanoparticles to the hot or cold fluids, and/or using tube inserts as turbulators on tube side as well as changing baffles to a helical or twisted profile on the shell side. Although all of these techniques increase the thermal efficiency; however, engineers still need a quantitative approach to assess the impact of these technologies on the shell and tube heat exchangers. This study attempts to provide a combination of such techniques to increase the impact of these improvements quantitatively. For this purpose, at first stage the thermal and hydraulic characteristics of pure fluid, Al₂O₃/water nanofluid in a plain tube equipped with and without twisted tape turbulator is evaluated based on a developed rapid design algorithm. Therefore, the impact of using enhanced techniques either in form of individual or in hybrid format and the increase of nanoparticle concentration in base fluid have been studied. The results show that using turbulators individually and in hybrid format with nanofluid can be effected on design parameters of a typical heat exchanger by reducing the required heat transfer area up to 10 %