Martin John Atkins

Thermocline management of stratified tanks for heat storage

Stratified tanks are useful for maximising the thermal energy efficiency of non-continuous and semi-continuous processes. Liquid at two or more dissimilar temperatures is stored within the same tank to provide a buffer for variations in heating and cooling loads. Control of the thermocline between the hot and cold fluid regions is needed to minimise thermocline growth and maximise operation of the storage tank. An experimental programme using a scale model of an industrial stratified tank (aspect ratio 3.5) and Perspex tank (aspect ratio 8.2) is reported. The behaviour and growth of the hot-cold thermocline under various operating conditions is presented. A siphoning method to re-establish the thermocline without interrupting the use of the tank is tested. Siphoning of the thermocline region from either 20%, 50% or 80% of the tank height is an effective strategy for uninterrupted interface re-establishment. However, the rate and position of siphoning and the load balance of the exit streams are critical variables for minimising the time for effective re-establishment of the two temperature zones.

Area targeting and storage temperature selection for heat recovery loops

Inter-plant heat integration across a large site can be achieved using a Heat Recovery Loop (HRL). In this paper the relationship between HRL storage temperatures, heating and cooling utility savings (heat recovery) and total HRL exchanger area is investigated. A methodology for designing a HRL based on a ΔTmin approach is compared to three global optimisation approaches where heat exchangers are constrained to have either the same Number of Heat Transfer Units (NTU), Log-Mean Temperature Difference (LMTD) or no constraints (actual global optimum). Analysis is performed using time averaged flow rate and temperature data. Attention is given to understanding the actual temperature driving force of the HRL heat exchangers compared to the apparent driving force as indicated by the composite curves. The cold storage temperature is also varied to minimise the total heat exchanger area. Results for the same heat recovery level show that the ΔTmin approach is effective at minimising total area to within 5 % of the unconstrained global optimisation approach. The study also demonstrates the efficiency of the ΔTmin approach to HRL design compared to the other methods which require considerable computational resources.

Total Site Methodology as a Tool for Planning and Strategic Decisions

A Total Site (TS) is defined as a set of processes (industrial plants, residential, business and agriculture units) linked through the central utility system. The utility system incorporates a number of operating units such as boilers, steam turbines, gas turbines and letdown stations. Many sites are using the TS system representation. Heat Integration at TS level has been well developed and successfully implemented. However, sites typically develop with time and even minor changes/extensions can affect TS heat recovery significantly. It is beneficial to plan their strategic development in advance, to increase or at least not to decrease the rate of heat recovery when integration of additional processes takes place. Even when this has not been done at the initial stage, the TS methodology can still be used as a tool for the strategic planning decision making. This work illustrates how the TS methodology can contribute to the strategic development and the extension planning of already existing TS. The aim is to reveal the potentials for Heat Integration, when new units or processes are considered for the inclusion in the TS. Moreover, some operating parameters (e.g. temperature or capacity) of the unit can be proposed to achieve the best possible heat recovery. The degrees of freedom for TS changes can be on two levels: (i) only adding an operating unit to the current utility system (the Total Site Profiles remain the same) or (ii) changing of the TS by including more processes (the Total Site Profiles are changed). The first group of changes includes the integration of heat engines to produce electricity utilising heat at higher temperature and returning it to the system at lower temperature, which is still acceptable for the heat recovery and simultaneously for the electricity production. The second group of changes is more complex. For evaluating these changes a plus/ minus principle is developed allowing the most beneficial integration of new units to the TS. Combinations of both types of changes are also considered.

Ensuring Cost-effective Heat Exchanger Network Design for Non-Continuous Processes

The variation in stream conditions over time inevitably adds significant complexity to the task of integrating non-continuous processes. The Time Averaging Method (TAM), where stream conditions are simply averaged across the entire time cycle, leads to unrealistic energy targets for direct heat recovery and consequently to Heat Exchanger Network (HEN) designs that are in fact suboptimal. This realisation led to the development of the Time Slice Method (TSM) that instead considers each time interval separately, and can be used to reach accurate targets and to design the appropriate HEN to maximise heat recovery. However, in practise the HENs often require excessive exchanger surface area, which renders them unfeasible when capital costs are taken in to account. An extension of the TSM that reduces the required overall exchanger surface area and systematically distributes it across the stream matches is proposed. The methodology is summarised with the help of a simple case study and further improvement opportunities are discussed.

Minimising Energy Use in Milk Powder Production Using Process Integration Techniques

Spray drying of milk powder is an energy intensive process and there remains a significant opportunity to reduce energy consumption by applying process integration principles. The ability to optimally integrate the drying process with the other processing steps has the potential to improve the overall efficiency of the entire process, especially when exhaust heat recovery is considered. However, achieving the minimum energy targets established using pinch analysis results in heat exchanger networks that, while theoretically feasible, are impracticable, unrealistic, contain large number of units, and ultimately uneconomic. Integration schemes that are acceptable from an operational point of view are examined in this paper. The use of evaporated water is an important factor to achieve both energy and water reductions. The economics of additional heat recovery seem favourable and exhaust heat recovery is economically justifiable on its own merits, although milk powder deposition should be minimised by selecting an appropriate target temperature for the exhaust air. This will restrict the amount of heat recovery but minimise operational risk from heat exchanger fouling. The thermodynamic constraints caused by the operating temperatures of the dryer and the poor economics exclude the use of heat pumps for exhaust heat recovery in the short to medium term.

Pinch Analysis of an Industrial Milk Evaporator with Vapour Recompression Technologies

The present study focuses on applying Pinch Analysis to an industrial milk evaporator case study. Modern milk evaporators are typically integrated using both mechanical and thermal vapour recompression technologies as the primary means for attaining a high level of energy efficiency. A significant step change in energy efficiency for milk evaporators is achieved in this study by modifying the set-up of the concentration processing pathway in combination with an improved heat exchanger network design. To effectively perform the Pinch Analysis, a validated mass and energy balance model of the milk evaporator case study has been implemented in an Excel spreadsheet from which appropriate stream data may be extracted. In particular the Grand Composite Curve plays a critical role in identifying where vapour recompression units, which are a type of heat pump, may be applied to reduce thermal energy use by as much as 67 %, which represents an annual utility cost saving between 640 – 820 k

Importance of Understanding Variable and Transient Energy Demand in Large Multi-product Industrial Plants for Process Integration

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Pinch Analysis Techniques for Carbon Emissions Reduction in the New Zealand Industrial Process Heat Sector

Two case studies are presented to highlight the value of understanding variable and transient energy demand of a plant as a precursor to developing Process Integration (PI) and energy cost reduction solutions. A dairy factory example illustrates how heat recovery combined with thennal storage is vital for smoothing out variable energy demand of dissimilar processes coming offline for regular cleaning. A batch pulp mill example illustrates how large steam swings occur even with evenly scheduled batch cycles and that process modifications through dynamic analysis and modelling can lead to reduced peak steam swings and reduced supplementary gas boiler costs.

Application of P-graph Techniques for Efficient Use of Wood Processing Residues in Biorefineries

Options for reducing industrial process heat greenhouse gas (GHG) emissions in New Zealand are investigated using the Carbon Emissions Pinch Analysis (CEPA) and Energy Return on Energy Invested (EROI) analysis methods. Renewable sources like geothermal, biomass, biogas from animal waste and heat pumps from renewable electricity are investigated. Results indicate that some regions of New Zealand are well placed to make significant reductions to process heat GHG emissions through shifting from fossil fuel heating to renewable heating without a large increase in energy expended or cost. Reducing GHG emissions below 1990 levels can be achieved by using wood waste and biomass in place of coal (33.3 PJ) and biogas from animal waste in place of natural gas (12.1 PJ) where high temperature heating is required (>90 °C), and renewable electricity driven heat pumps for low temperature heating (<90 °C) in dairy and meat processing industries (7.0 PJ). The expected increase in expended energy is 20 %. Over all the Central North Island of New Zealand has a significant degree of renewable and natural resource convergence and hence is a prime region for creating low carbon emission industries requiring process heat based on renewable energy and agricultural and forestry waste.

A derivative method for minimising total cost in heat exchanger networks through optimal area allocation

It is anticipated that demand for chemicals and fuel derived from sustainably grown bio-mass will increase over the coming decades. Forest and wood processing residues and waste are likely to become a significant feedstock to large scale biorefineries to produce both renewable fuels and chemicals. Maximising the economic value of these residues whilst simultaneously minimising the environmental impact of the manufactured product is an important task in process and product selection and design. Multiple processing and product pathways exist and it is often unclear what the best options are without detailed assessment or preliminary design. The P-graph framework was used to examine the economically feasibility of utilising five types of wood processing residues: wood chip, pulp logs, saw dust, and landing and cutover residues. Twenty different products were considered, based on three main production platforms or routes, sugars, pyrolysis, and gasification. Kraft pulp production and energy products were also considered as viable options for residues. Only six of the products considered were found to be profitable with the most economically viable uses being kraft pulp production and boiler fuel. Products included in the feasible solutions and the source of residues are all finely balanced, and slight changes in feedstock cost, product price, and operational and capital costs can cause major changes to the feasible structures. When heat integration for using Total Site was incorporated into the P-graph there was no economic benefit for the routes and scale of production considered here.