(165f) Comparative Analysis of Different Retrofit Methods for Heat Exchanger Networks

Improving the energy efficiency of process plants through cost effective methodologies for heat exchanger networks (HENs) has received interest in recent years. To attain this, the heat transfer duties of heat exchangers within the network might be changed. Retrofit of HEN can either be performed by fixing the network structure or performing structural changes.

For a fixed network structure, there are two general ways by which retrofit can be conducted. Consider the basic equation governing heat transfer:

                            Q = UAΔT_LMF_T                                                          (1)

If the heat transfer duty Q is required to be increased at constant log mean temperature difference, ΔT_LM and correction factor, F_T, then either the value of the overall heat transfer coefficient, U or the heat transfer area, A needs to be increased.

Considering a shell-and-tube heat exchanger, for small additional heat transfer area requirement, a new tube bundle can be installed into the existing shell (or shells). If a significant amount of additional area is required, a new shell or more than one shell can be installed. However, increasing the heat transfer area can present a complex and capital intensive retrofit process.

In place of installing additional heat transfer area, the overall heat transfer coefficient can be increased i.e. heat transfer enhancement. This can be attained by changing the number of tube passes, baffle arrangements, applying tube inserts or replacing the existing plain tubes with twisted tubes on extended surfaces. Although there have been some research on the use of heat transfer enhancement for HEN retrofit^1-5, there still remains some key questions that needs to be answered. These are:

1)      What are the best heat exchangers to enhance in a HEN and in what order?

2)      Considering a shell-and-tube heat exchanger, what is the best enhancement technique to apply i.e. shell-side, tube-side, both side or replacing existing plain tubes with twisted tubes?

3)      What is the augmentation level of each enhancement based on the exchangers’ geometry?

4)      How to deal with downstream effects after enhancement?

Structural changes can be used in improving the energy performance of an existing HEN by overcoming the network pinch, which restricts energy savings.  However, the application of too many structural modifications i.e. resequencing and stream splitting and, additional area in the form of a new heat exchanger are not ideal in industrial practice. This is as a result of the high costs of civil and piping engineering work that needs to be carried out. There might also be production losses associated with the prolonged shutdown periods required to carry out the stipulated modifications. Adding a new heat exchanger might be very difficult to implement or not practical as a result of the constraint imposed by the existing HEN in terms of congestion. Therefore it is vital that the retrofit process based on structural changes should be kept to a minimum.

Another option in retrofit is to consider structural modifications and additional area with enhancement. This is because, when modifications are made to the existing network, there might be a reduction in the driving force of existing heat exchangers. Therefore, in addition to the modifications carried out, additional heat transfer area might be required in existing heat exchangers. If the increased Q, the new ΔT_LM and F_T obtained as a result of modifications carried out in the network are fixed, the relationship between the heat duty and the additional heat transfer area (ΔA) is given by:

Q = U(A_existing + ΔA) ΔT_LMF_T                                                               (2)

A benefit of heat transfer enhancement is that an enhanced heat exchanger has a higher heat transfer coefficient to exchange the same duty under smaller heat transfer area requirement. In other words, an enhanced heat exchanger with an overall heat transfer coefficient (U_E) at the same duty might be able to maintain an existing heat transfer area requirement. This relationship is given by:

Q = U_EA_existing ΔT_LMF_T                                                                              (3)

Therefore, depending on the additional area requirement, heat transfer enhancement can either be used in eliminating or reducing the additional heat transfer area required after structural modifications and additional area are performed.

The first part of this work presents a methodology for the application of heat transfer enhancement in HEN retrofit. The proposed methodology is a combination of a set of heuristic rules for the identification of the best candidate heat exchanger and location to apply enhancement and; a non-linear optimisation based methodology which tackles the issue of downstream effects after enhancement.  The optimisation strategy is based on utility path analysis to ensure the feasibility and cost effectiveness of the retrofit process.

The second part of this work presents better insights into the identification of the best structural changes that should be applied to an existing heat exchanger network to obtain decrease in energy consumption, while ensuring the network constraints are met. This is then followed by the application of enhancement with structural modifications and additional area.

The proposed retrofit methods have been applied to a simplified crude-oil preheat train with an objective of maximising the retrofit profit i.e. difference between the profit from energy saving and total cost of retrofit. Results obtained showed that the use of structural modifications and additional area alone provided a decrease in energy consumption approximately twice that of using only heat transfer enhancement. In terms of retrofit profit, the use of only enhancement had a higher retrofit profit compared to other retrofit options. However, when enhancement was considered together with structural changes and additional area, there was a sharp increase in retrofit profit.

References

1. Pan, M., Bulatov, I., Smith, R., and Kim, J.-K. (2011a). Improving energy recovery in heat exchanger network with intensified tube-side heat transfer. Chemical Engineering, 25.
2. Pan, M., Bulatov, I., Smith, R., and Kim, J. K. (2011b). Novel optimization method for retrofitting heat exchanger networks with intensified heat transfer. Computer Aided Chemical Engineering, 29, 1864-1868.
3. Pan, M., Bulatov, I., Smith, R., and Kim, J. K. (2012). Novel MILP-based iterative method for the retrofit of heat exchanger networks with intensified heat transfer. Computers and Chemical Engineering, 42, 263-276.
4. Wang, Y., Pan, M., Bulatov, I., Smith, R., and Kim, J.-K. (2012). Application of intensified heat transfer for the retrofit of heat exchanger network. Applied Energy, 89(1), 45-59.
5. Jiang, N., Shelley, J. D., Doyle, S., and Smith, R. (2014). Heat exchanger network retrofit with a fixed network structure. Applied Energy, 127, 25-33.