October 2019

Heat Transfer

Pinch analysis: A retrofit approach

Pinch analysis provides techniques for heat integration to design the heat exchanger network (HEN), not only for grassroots design but also to retrofit an existing system and optimize the process.

Raza, S. A., Penspen Intl.; Hussain, I., Oman Refineries and Petrochemical Ind. Ltd. (Orpic)

Pinch analysis provides techniques for heat integration to design the heat exchanger network (HEN), not only for grassroots design but also to retrofit an existing system and optimize the process. These techniques are interactive and user-friendly. Retrofit study is more complex as compared to grassroot design.1

This article focuses on techniques for retrofit and discusses available options to optimize the system. Generally, a retrofit may be carried out for the following reasons in an HEN:

  • To optimize energy efficiency
  • To increase throughput
  • To reduce emissions
  • Process modifications.

Previously, a retrofit was accomplished by changing the grassroots design of the network toward an ideal network. However, a challenge associated with this approach is the large number of required modifications in the existing network. A better approach is to evolve the network from its existing structure to identify only the most critical, and therefore cost-effective, changes to network structure. This means that the existing arrangement can be modified in such a way that the ideal curve can be approached.1 This can be achieved by:

  • Identifying cross pinch heat transfer
  • Selecting ΔTmin based on total cost
  • Applying network pinch technologies at either the diagnosis stage or the optimization stage.

The main purpose of the retrofit analysis is to keep the design as similar as possible to the original, which implies suggesting as few modifications as possible.

Network pinch

Linnoff and Tjoe2 proposed the first pinch retrofit method and presented the idea of area efficiency (α = Aidea/Aexisting). Alpha (α) represents how close the existing area is to the target (ideal) area. Lower values of α indicate more cross pinching heat and an unoptimized system.

Differences exist between process pinch and network pinch: Process pinch is the function of process streams only, while network pinch is the function of both process streams and HEN topology.3 However, topology modifications keep the process pinch unaffected while network pinch is changed.

The network pinch analysis is used to determine the structural elements of the network, which prohibit topology changes. The exchanger minimum approach temperature (EMAT) is the minimum approach temperature, the bottleneck in heat recovery, whereas the ΔTmin is the minimum temperature difference between hot and cold process streams only (process pinch). In retrofit studies, pinching matches are identified by the minimum value of EMAT in the HEN and are referred to as network pinch. Network pinch techniques are applied to overcome these bottlenecks.4

For retrofit, EMAT and ΔTmin are treated as the same to achieve and set energy targets. This approach is defined and adopted by Shenoy.4

After setting the targets for the heat recovery that will decide the pinch temperature, network pinch techniques are applied to carry out the retrofit to increase the heat recovery.

NETWORK PINCH TECHNOLOGIES

Pinching matches create a bottleneck in heat recovery. Network pinch techniques are applied to overcome these bottlenecks. Asante and Zhu5 proposed the following network pinch techniques in the form of topology modifications capable of overcoming this network pinch (bottlenecks):

  • Re-sequencing
  • Re-piping
  • Addition of new heat exchanger
  • Stream split.

Generally, these techniques are applied in the same sequence to overcome the network pinch heat to be moved from below the pinch to above the pinch.

Re-sequencing

Heat is moved from below to above the pinch to debottleneck the HEN. Streams remain the same, and the scope of heat recovery increases through the utility path.

Re-sequencing involves the shifting of a heat exchanger from “below the pinch region” to its new position “above the pinch region.” This is particularly important if most of the heat exchangers are provided at one certain location (i.e., at “below the pinch region”).

If this is used to heat the (cold) crude oil upstream of the crude distillation column, it will efficiently remove heat from all heat sources by expanding the heat transfer at very hot and comparatively low-temperature (also hot) streams.

This will generate a new pinching match between hot and cold streams with a reduction in their temperature approach. Efficient use of re-sequencing can result in a lower requirement for hot and cold utilities, saving capital and operating costs.

This re-sequence, shown in FIG. 1, increases heat recovery in the loop.

FIG. 1. Grid diagram showing re-sequencing of heat exchangers  from below to above the pinch.
FIG. 1. Grid diagram showing re-sequencing of heat exchangers from below to above the pinch.

 

Re-piping

Re-piping follows the same philosophy and methodology as re-sequencing, but the streams are not the same as in re-sequencing. Streams can be different depending on requirements (materials used, etc.). Re-piping provides more flexibility than re-sequencing.

Addition of new heat exchanger

If a new match is inserted such that the heat duty on the hot stream adjacent to the pinching match is decreased and replaced by the new match, then the position of the pinching match can be changed so that it is no longer pinching. As with re-sequencing, the introduction of a new heat exchanger will also generate a new pinching match and reduce the temperature approach between the hot and cold streams.

This also introduces an opportunity to exploit the utility path to reduce the utility consumption of the network until it is again pinched. The network is now pinched again, but at a lower utility consumption (FIG. 2).

FIG. 2. Grid diagram showing addition of heat exchangers and loop  for heat recovery.
FIG. 2. Grid diagram showing addition of heat exchangers and loop for heat recovery.

 

Stream splitting

By introducing a stream split (of a larger flowrate stream), two matches are pinched simultaneously. The cold stream profiles in the two pinched units can be changed to a new profile, such that one of the pinching matches is no longer pinched. This means that a utility path can be exploited to reduce the energy consumption. FIG. 3 illustrates this stream splitting.

FIG. 3. Grid diagram showing stream split with pinching match.
FIG. 3. Grid diagram showing stream split with pinching match.

 

In general, the heat is to be moved from below the pinch to above the network pinch (or vice versa, if conditions allow) to overcome the network pinch (bottleneck). After each modification in the retrofit process, an optimization is carried out to optimize the area and hot/cold utility requirements, as per set targets.6 It is recommended to adopt these approaches in sequential order to reach an optimum energy-efficient network pinch. After no more heat recovery is feasible, optimization is stopped.

Case study

The crude preheat train of a crude distillation unit of 30,000-bpd crude capacity was selected for a retrofit study to increase heat recovery. After identifying the cross-pinch heat transfer and bottlenecks in heat recovery (network pinches), the network pinch techniques were applied to increase heat recovery. FIGS. 4 and 5 show the flowsheet and grid diagram of the existing process and HEN. The existing heat transfer area of this 30,000-bpd CDU crude capacity was about 5,800 m2, whereas the ideal area of the same CDU is found as 2,000 m2. A good opportunity is found to optimize the heat exchanger train to reduce the hot and cold utility consumption.

FIG. 4. Flow diagram of existing CDU process.
FIG. 4. Flow diagram of existing CDU process.

 

FIG. 5. Grid diagram of the existing HEN of CDU showing  heat exchangers, heater and coolers.
FIG. 5. Grid diagram of the existing HEN of CDU showing heat exchangers, heater and coolers.

 

Applying network pinch techniques for retrofit based on a ΔTmin of 10°C (50°F). ΔTmin is selected based on total cost. FIGS. 6, 7 and 8 demonstrate the retrofit methodology and retrofit design after various modification options.

FIG. 6. Network pinch diagnostic techniques applied to the existing HEN.
FIG. 6. Network pinch diagnostic techniques applied to the existing HEN.

 

FIG. 7. Modified grid diagram of the HEN of CDU after retrofit.
FIG. 7. Modified grid diagram of the HEN of CDU after retrofit.

  

FIG. 8. Modified CDU process flow sheet after retrofit.
FIG. 8. Modified CDU process flow sheet after retrofit.

 

As demonstrated in TABLE 1, a significant 64% energy savings can be found using the network pinch analysis for the CDU heat exchanger train retrofit.

 

Of the three different options for network retrofit, Option B was selected based on the minimum capital, minimum modifications and shorter payback time. Option A has the same payback time, but the extent of modifications is higher than Option B, and the cost of re-piping a heat exchanger will also increase the cost of Option A.

The proposed retrofit design achieved a total energy savings of 5,500 kW—i.e., 2,750-kW hot and 2,750-kW cold utility, which is an approximate 64% savings of target utilities. The total utility cost savings obtained is $1.53 MM/yr, which is equivalent to a 23% reduction in the utility bill. The cost of re-piping and re-sequencing are counted, which can also contribute to cost and slightly increase the payback. FIGS. 7 and 8 show the grid diagrams of a modified CDU heat exchanger network and its process flowsheet after retrofit. HP

LITERATURE CITED

  1. Smith, R., Chemical Process Design & Integration, 2nd Ed., John Wiley & Sons Ltd., Chichester, UK, 2005.
  2. Linnhoff, B. and T. N. Tjoe, “Pinch technology retrofit: Setting targets for existing plant,” AICHE National Meeting, Houston, Texas, March 1985.
  3. Asante, N. D. K. and X. X. Zhu, “An automated and interactive approach for heat exchanger network retrofit,” Chemical Engineering Research and Design, 1997.
  4. Shenoy, U. V., Heat exchanger network synthesis: Process optimization by energy and resource analysis, Gulf Publishing Company, Houston, Texas, 1995.
  5. Asante, N. D. K. and X. X. Zhu, “An automated approach for heat exchanger network retrofit featuring minimal topology modifications,” Computers & Chemical Engineering, 1996.
  6. Kemp, I., Pinch analysis and process integration: A user guide on process integration for the efficient use of energy, 2nd Ed., Elsevier, Oxford, UK, 2007.

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