Andrew S. Morrison

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.

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

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.

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

This paper presents a novel Cost Derivative Method (CDM) for finding the optimal area allocation for a defined Heat Exchanger Network (HEN) structure and stream data, without any stream splits to achieve minimum total cost. Using the Pinch Design Method (PDM) to determine the HEN structure, the approach attempts to add, remove and shift area to exchangers where economic benefits are returned. From the derivation of the method, it is found that the slope of the e-NTU relationship for the specific heat exchanger type, in combination with the difference in exchanger inlet temperatures and the overall heat transfer coefficient, are critical to calculating the extra overall duty each incremental area element returns. The approach is able to account for differences in film coefficients, heat exchanger types, flow arrangements, exchanger cost functions, and utility pricing. Incorporated into the method is the newly defined “utility cost savings flow-on” factor, θ, which evaluates downstream effects on utility use and cost that are caused by changing the area of one exchanger. To illustrate the method, the CDM is applied to the distillation example of Gundersen (2000). After applying the new CDM, the total annual cost was reduced by 7.1 % mainly due to 24 % less HEN area for similar heat recovery. Area reduction resulted from one exchanger having a minimum approach temperature (ΔTmin) of 7.7 °C while the other recovery exchangers had larger ΔTmin values. The optimum ΔTmin for the PDM was 12.5 °C. The CDM solution was found to give a comparable minimum total area and cost to two recently published programming HEN synthesis solutions for the same problem without requiring the increased network complexity through multiple stream splits.