SERVICIOS Y EQUIPAMIENTOS

This Module outlines the experience of using compact heat exchangers in various sectors of the process industry. For more detailed application information, enquirers should contact equipment suppliers.

Today compact heat exchanger designs are used for many applications in process industries, as summarised in Table 3.3.1. Of course, applications are process specific with different exchanger designs being used for different processes. Further information on the generic applications is given in Section 3.3.2.

Process Industry

Sector Distillation Evaporation Reactor Separations Compressors Refrigeration

Mechanical

✔ - Compact heat exchanger application track-record

✘ - Compact heat exchangers generally not suitable

Table 3.3.1 - Compact Heat Exchanger Applications in Process Industry Sectors

3.3.1.1 Chemicals and Petrochemicals

The chemicals and petrochemicals sector uses the full range of compact heat exchangers across its wide range of processes.

They are used in certain ‘standard’ products, for example compact cores in kettle reboilers and plate-fin dephlegmators, used for partially condensing/purifying fluids in applications such as ethylene recovery. There are also ‘one-off’ uses, for example application of the printed circuit heat exchanger as a nitric acid plant tail-gas heater and as a compact economiser.

Although the conditions under which many of these processes operate would appear to limit the use of compact designs, there are numerous instances where compact heat exchangers have replaced more conventional equipment. Other compact unit operations, reactors in particular, also employ compact heat exchanger technology.

3.3.1.2 Pharmaceuticals

The pharmaceutical industry uses batch processes extensively, and compact heat exchangers often form part of the plant. A typical application would involve a plate and frame heat exchanger on a heat-cool-chill process, transferring heat indirectly to reaction masses. Batch plant needs to be rapidly and thoroughly cleaned with each product change. It is therefore preferable to use heat exchangers with no dead spots where product can accumulate.

3.3.1.3 Cryogenics

Providing nitrogen, oxygen and argon is an energy-intensive operation that needs heat exchangers capable of multi-stream operation at very low temperatures with close approach temperatures. The capital and operating costs associated with thermodynamic irreversibilities increase as the processing temperature falls, so one of the prime aims of heat transfer in cryogenics is to minimise these irreversibilities.

Plate-fin heat exchangers can meet these requirements and minimise heat gains from the environment. Their ability to accommodate many process streams and their robust aluminium construction makes them even more attractive, and they have found widespread application in cryogenics.

It is important to prevent the freezing of components, such as carbon dioxide and water, and to limit corrosion by mercury, acid gases etc. The former can largely be achieved by removing the relevant components upstream using molecular sieves, while particulates can be filtered out, typically using physical filters of 100 µm or less. However, older plant tends to rely solely on filters, using reversing (freezing/unfreezing) to remove solidified components.

The exchangers used in such plant can ultimately fail as a result of corrosion. Pre-treatment is a successful way of overcoming these problems, the only reservation being that any perlite insulation dust present upstream of the heat exchanger can affect performance during construction/start-up.

3.3.1.4 Food and Drink

The food and drink sector pioneered the use of the plate and frame heat exchangers. Plate and frame designs are used increasingly in evaporators and are an integral component of

‘compact’ evaporators, which are normally of the mechanical vapour compression type (see Section 3.3.2.2). They are also used in edible oil processing.

Figure 3.3.1 – Paraflow Plate Heat Exchanger (Courtesy of APV)

3.3.1.5 Paper and Board

Opportunities for using compact heat exchangers in the paper and board sector are limited, unless radical process changes allow new uses in the future. However, it is currently feasible to use plate and frame exchangers to recover heat from the dryer when a spray condenser is used as the primary heat exchanger. Compact heat exchangers may also be used in combined heat and power plants.

Experiments using 65% solids black liquor from a New Zealand pulp mill have demonstrated that plate and frame heat exchangers can operate significantly longer than tubular heat exchangers⁽¹⁾.

Fluidised bed heat exchangers have proved valuable in applications where fouling would prevent the use of heat exchangers with small channels, for example in processing white water that contains paper stock. Experience in Scandinavia and the USA suggests that wide-gap plate and frame exchangers can be cost-effective, and spiral heat exchangers can also be used in this application.

3.3.1.6 Textiles and Fabric Care

One of the relatively few applications of compact heat exchangers in the material production sector is in the spinning of acrylic fibre from an extremely viscous dope.

Conventional cooling equipment is unsatisfactory in this environment because of the wide variation in viscosity over the temperature range involved. A much closer temperature control can be achieved using plate and frame exchangers, and their greater efficiency allows

Plate and frame exchangers have become established over the last decade for use on most liquid effluents in textile dyeing and finishing, and in fabric care. Their close approach temperatures and the easy access for cleaning (gasketed types) makes them very cost-effective. Filters are commonly fitted upstream to remove gross contamination.

3.3.1.7 Oil and Gas Processing

Plate and frame heat exchangers are used both onshore and offshore for low-pressure duties, while brazed-plate units can be employed at higher pressures. Welded plate heat exchangers are used extensively in gas dehydration regeneration systems (rich triethylene glycol / lean triethylene glycol exchangers). They are also used in gas desulphurisation regeneration systems for amine/amine heat transfer and can even replace shell and tube units for crude oil heating. Where the safety of gasketed plate and frame heat exchangers might be a concern, welded compact heat exchangers would be a logical choice.

Printed circuit heat exchangers and diffusion-bonded plate-fin heat exchangers are used offshore for high pressure and export gas cooling where gas pressures can exceed 400 bar and sea water is used for cooling. Compressor aftercoolers based on printed circuit heat exchangers are in use up to 400 bar, and other offshore printed circuit heat exchanger applications are listed in Table 3.3.2.

Thermal Duty Pressure titanium for direct sea water

cooling

2 off 0.173 MW 400 Coolers HP fuel gas compressor aftercoolers

6 off to 29 MW 45 – 200 Coolers and heaters Major project exchangers - substantial deck cost reduction 2 off to 3 MW 35 Gas/gas/gas-liquid High pressure, counter-flow,

multi-stream, multiphase Table 3.3.2 - Offshore Printed Circuit Heat Exchanger Applications

For corrosion resistance under these conditions, the heat exchangers are normally made of titanium. For example, titanium plate-fin exchangers are used on a North Sea gas production platform to provide direct seawater cooling of product. These units will have a duty of 12.1 MW and operate at 125 bar.

In the future, the manufacturers of both printed circuit heat exchangers and superplastically formed plate-fin heat exchangers foresee units operating at more than 500 bar with temperatures of up to 400^oC. Duplex stainless steel printed circuit heat exchangers are already available and plate-fin exchangers of the same material will be available in the future.

There are possible new uses in hydrocarbon dewpoint control, condensate recovery, etc., while other applications include liquefied natural gas sub-cooling and cryogenic distillation, for example for nitrogen removal.

Extra protection needed when operating offshore can be expensive and can necessitate extensive modifications during retrofits or when new regulations are brought into effect.

For example, where a shell and tube heat exchanger is used, a relief system is necessary to cope with a possible tube rupture i.e. the high-pressure system discharges into a low pressure one. Relief provision for a conventional heat exchanger would have required extensive modification; however, because a printed circuit heat exchanger with a much smaller hold-up capacity was used, it was possible to provide only the relief needed in the case of fire or other emergency.

When used offshore, plant, including heat exchangers, requires mandatory external protection systems, including fireproof containment structures. So compact heat exchangers have lower inventories, require less containment and therefore are cheaper.

Compact heat exchangers are not yet widely used in onshore refineries. One company reported that only 1 to 2% of refinery heat exchangers are compact heat exchangers, typically welded plate units on catalytic reformers, whereas there are typically 400 -500 shell and tube units in a refinery.

3.3.1.8 General Heating/Cooling

Plate and frame heat exchangers are often used to exchange heat between treated cycle water and river, or sea, water as process cooling.

In addition, the use of compact heat exchanger designs in cogeneration and district heating/cooling systems is becoming a significant market.

Figure 3.3.2 – Brazed Plate Heat Exchanger Heating Tap Water in a Combi Gas Boiler

Figure 3.3.3 – Plate and Frame Exchanger in a 10 MW Heat Distribution Substation (Courtesy of APV)