HEAT EXCHANGERS

Fertilizer International 463 | November - December 2014 www.fertilizerinternational.com 33

tube sheet

Straight-tube heat exchanger (one pass tube-side)

tube sheet

shell-side fluid in

tube-side fluid out

shell-side fluid out

tube-side fluid in

shellbaffles

inle

t ple

num outlet plenum

tube bundle with straight tubes

Fig 1: A shell and tube heat exchanger

A heat exchanger is a piece of equip-ment built for efficient heat transfer from one medium to another. The

media may be separated by a solid wall to prevent mixing or they may be in direct con-tact. Heat exchangers are widely used in many chemical and fertilizer plants, natural gas processing and petroleum refineries.

There are three primary classifications of heat exchangers according to their flow arrangement. In parallel-flow heat exchang-ers, the two liquids enter the exchanger at the same end and travel in parallel to one another to the other side. In counter-flow heat exchangers, the liquids enter the exchanger from opposite ends. The coun-ter- current design is viewed as the most efficient, as it can transfer the most heat from the heat medium due to the fact that the average temperature difference along any unit length is greater. In a cross-flow heat exchanger, the liquids travel roughly perpendicularly to one another through the exchanger.

For efficiency, heat exchangers are designed to maximise the surface area of the wall between the two liquids, while mini-mising resistance to fluid flow through the exchanger. The exchanger’s performance can also be affected by the addition of fins or corrugations in one or both directions, increasing the surface area and channelling the fluid flow, as well as inducing turbulence.

Numerous types of heat exchangers are available, comprising:l Electric heatingl Double-pipe heat exchangerl Shell and tube heat exchangerl Plate heat exchangerl Plate and shell heat exchangerl Plate fin heat exchangerl Fluid heat exchangerl Waste heat recovery unitsl Phase-change heat exchanger l Direct-contact heat exchangerl Spiral heat exchanger.

Double-pipe heat exchangers are the sim-plest exchangers used in industries. They

are cheap for both design and mainte-nance and are ideal for small industries. Shell and tube heat exchangers consist of series of tubes. (Fig. 1) One set of these tubes contains the fluid that must be either heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes forms the tube bundle. Shell and tube heat exchangers are typically used for high-pressure applica-tions (with pressures greater than 30 bar and temperatures greater than 260°C). Their shape ensures that shell and tube heat exchangers are robust.

Plate heat exchangers (PHEs) are com-posed of multiple, thin and slightly sepa-rated plates that have very large surface areas and fluid flow passages for heat transfer. This stacked plate arrangement can be more effective in a given space than the shell and tube heat exchanger. Advances in gasket and brazing technol-ogy have made the plate heat exchanger increasingly practical.

Plate and shell heat exchangers com-bine plate heat exchanger with shell and tube heat exchanger technologies. The heart of the heat exchanger contains a fully welded circular plate pack made by press-ing and cutting round plates and welding them together. Nozzles carry flow in and out of the plate-pack. The fully welded plate-pack is assembled in an outer shell that creates a second flow path. Plate and shell technology offers high heat trans-fer, high pressure, high operating tem-peratures, compact size, low fouling and close approach temperatures. It avoids the need for gaskets, which provides secu-rity against leakage at high pressures and temperatures.

Plate fin heat exchangers use sand-wiched passages containing fins that increase the effectiveness of the unit. The design includes cross-flow and counter-flow coupled with various fin applications. Plate and fin heat exchangers are usually made of aluminium alloys, which provide

Advances in designThe design of heat exchangers used in ammonia and urea production has continued to advance.

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33 www.fertilizerinternational.com Fertilizer International 463 | November - December 2014

“For efficiency, heat

exchangers are

designed to maximise

the surface area of

the wall between the

two liquids, while

minimising resistance

to fluid flow through

the exchanger.

high heat transfer efficiency. The material enables the system to operate at lower temperature and reduce the weight of the equipment. Plate and fin heat exchangers are mostly used for low temperature ser-vices, such as natural gas and air sepa-ration units. They offer high heat transfer efficiency, especially in gas treatment and a larger heat transfer area, being approxi-mately five times lighter in weight than a shell and tube heat exchanger. Plate fin heat exchangers can also withstand high pressures. On the other hand, the narrow pathways can led to clogging and are dif-ficult to clean. The aluminium alloys are also susceptible to mercury liquid embat-tlement failure.

In a fluid heat exchanger, gas passes upwards through a shower of liquid, and the liquid is then taken elsewhere before being cooled. This is commonly used for cooling gases while also removing certain impurities.

A waste heat recovery unit is a heat exchanger that recovers excess heat from a hot gas stream while transferring it to a working medium, typically water or oils. The hot gas stream can be the exhaust gas from a gas turbine or a waste gas from elsewhere in the process.

Phased-change heat exchangers are used to heat a liquid to evaporate or boil it, or serve as condensers to cool a vapour and condense it to a liquid. Direct-contact heat exchangers involve heat transfer between hot and cold streams of two phases in the absence of a separating wall and can be classified as gas-liquid, immis-cible liquid-liquid, solid-liquid or solid-gas. Most direct-contact heat exchangers fall into the gas-liquid category, where heat is transferred between a gas and liquid in the form of drops, films or sprays. Such types of heat exchangers are used predominantly in air conditioning, industry hot water heat-ing, water cooling and condensing plants.

Spiral heat exchangers (SHEs) take the form of either coiled tube configurations or a pair of flat surfaces that are coiled to form the two channels in a counter-flow arrange-ment. SHEs make efficient use of space and are well suited for heat recovery, pre-heat-ing and effluent cooling applications.

Criteria for choiceThree main criteria apply when choosing the optimal design of heat exchangers:l Minimising the pressure drop (pumping

power)

l Maximising the thermal performance l Minimising entropy generation (thermo-

dynamic irreversibility).

Due to the many variables, selecting opti-mal heat exchangers is challenging. To select the appropriate heat exchanger, the plant system designers and engineers will consider the design limitations for each type of heat exchanger. Cost is often a primary consideration, but other selection criteria include:l High/low pressure limitsl Thermal performancel Temperature ranges l Product mix (liquid/liquid, particulates

or high solids/liquid)l Pressure drops across the exchanger l Fluid flow capacityl Cleanability, maintenance and repairl Construction materials.

heat exchanger installation performing an important role. These roles may include:l Process gas heating and coolingl Gas compression heat recoveryl Process waste recoveryl Steam generationl Boiler water heatingl Reactor preheatl Reaction heat recoveryl Process gas chilling with product con-

densingl Gas cooling with steam condensing l Vapour-liquid separation column reboilingl Solvent heating and coolingl Lubrication oil coolingl Refrigerant vapour cooling.

Many ammonia plants in operation today were built between 25-35 years ago, nota-bly in the FSU and United States, and present opportunities for modernisation, improving efficiency and capacity. Heat exchanger designs have been upgraded as part of the desire of producers to reduce operating costs and the overall environ-mental footprint. New designs for replace-ment heat exchangers add lasting value to the plant operation, through reduced gas pressure loss, higher throughput, greater heat transfer and combinations of these benefits, reducing production costs overall. (www.chemicalengservices.com)

Certain designs of vertical process gas waste heat recovery boilers are prone to corrosion failure of tubes when the shell side fluid is boiler feedwater at failure regions near the tube sheets. Accumula-tion of corrosion damage is slow in the early life of the equipment, but accelerates after about 10-15 years of service. Retub-ing or plugging when such failures begin is time-consuming and difficult and may pro-vide only a temporary solution. A perma-nent solution is to replace the waste heat boiler with a new design, with the boiler feedwater arranged up-flow in the tubes to eliminate the possibility of trapping corro-sive boiler dissolved solids on the outside of tubes in the lower region of the bundle shell side.

In most ammonia plants, heating and cooling of synthesis gas streams is accom-plished by the reactor effluent being used for heating reactor feed, recovering a sub-stantial part of exothermic reaction heat. Examples of such heat exchanger equip-ment include methanation and ammonia synthesis feed heating. When plants are expanded in capacity, these exchanger ser-vices often become a reliability problem,

The operating performance of heat exchangers can be affected by fouling, which occurs when impurities deposit on the heat exchanger surface. This results in decreased heat transfer effectiveness over time. Regular maintenance will ensure effective operations. Plate heat exchang-ers must be disassembled and cleaned periodically, while tubular heat exchangers can be cleaned by acid cleaning, sand-blasting, high-pressure water jet or drill rods.

Fertilizer applicationsHeat exchangers are widely employed in the fertilizer manufacturing process. They make up the largest number of equipment items in an ammonia plant, with each

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34 www.fertilizerinternational.com Fertilizer International 463 | November - December 2014

feed gas & stream

combined reformed gas

catalyst filledreformer tubes

autothermalreformereffluent

Fig 2: The KBR KRES™ systembecause of tube leaks from failures caused by shell side-induced tube vibration, result-ing in wear from contact with adjacent tubes or shell baffles. Shell side gas velocities induce tube vibration and develop into tube leaks when plant rates are pushed typically beyond 20-40% higher production through incremental expansion projects.

Compressors need inter-stage heat recovery to maximise capacity while mini-mising head and power requirements in the pumping of gases through the process equipment. Lower compressor intercooler pressure drop translates into decreased energy consumption for the compres-sor when upgrading with improved heat exchanger designs. Well-designed com-pressor intercoolers can have a useful life of 10-20 years, but occasional failures can result from higher loads caused by gradual plant expansion. Additional problems can also develop, such as reduced intercooling from fouling or mechanical leaks through the tubes, resulting in gas loss into the cooling water.

When redesigning exchangers, chang-ing shell types can provide cost-efficient solutions for achieving reduced intercooler gas pressure losses and energy savings. Crossflow designs can provide extremely low pressure drop performance compared with alternative designs.

Replacing outdated, damaged or over-loaded compressor intercooler equipment can provide additional heat removal, low-ering downstream stage power require-ments. Energy savings benefits for each inter-stage intercooler depend on individual stage loads. For refrigeration compres-sors, reducing pressure losses of existing intercoolers or adding low pressure drop intercoolers where none exist can provide economic solutions to improve compressor capacity while reducing power usage. At a time of escalating energy costs, replace-ment of damaged or under-performing inter-coolers with updated designs can improve plant efficiency, lower plant operating costs and enhance operating reliability.

Innovations and case studiesHaldor Topsøe is a leading supplier of heat exchangers for the ammonia industry. The HTER (Haldor Topsøe Exchange Reformer) has been developed for use in synthesis gas plants. In ammonia plants, this unit is operated in parallel with the primary reformer. The HTER offers the advantage of reducing the size of the primary reformer

while at the same time reducing high-pres-sure steam production. It is particularly suited for operations in large-capacity plants (particularly stand-alone ammonia plants not requiring a large steam export to a urea plant), and it can also be retrofitted as part of an ammonia plant revamp where the reforming section is a bottleneck.

The principle of the HTER is that reac-tion heat is provided by the exit gas from the secondary reformer, and the waste heat normally used for HP steam produc-tion can therefore be used for the reform-ing process down to typically 750-850°C. Operating conditions in the HTER are adjusted independently of the primary reformer in order to get the optimum per-formance of the overall reforming unit. Typically up to around 20% of the natural gas feed can in this way by-pass the pri-mary reformer. The first reference for an HTER was in a synthesis gas plant in South Africa in 2003. The HTER concept is also widely used in the design of high-capacity hydrogen plants.

KBR has developed the KRES™ (KBR Reforming Exchanger System), a proprie-tary heat exchanger-based steam reforming technology comprising a fired preheater, an autothermal reformer (ATR) and a reforming exchanger. (Fig. 2) KRES™ takes the place of a conventional primary reformer by feed-ing excess air, natural gas feed and steam to the ATR and feed and steam in parallel into the upper end of the robust, shell and

tube reforming exchanger. The compact ATR and reforming exchanger in combina-tion with the fired preheater take up much less plot space than a conventional fired steam methane reformer.

The tubes in the KBR reforming exchanger are open-ended and hang from a single tube sheet at the inlet cold end to minimise expansion problems. They are packed with a conventional reform-ing catalyst, which can be easily loaded through a removable top head. The tubes are accessible and removable as a bundle for maintenance. This simple, proprietary design has proved to be very reliable and maintenance-free in commercial opera-tions since 1994.

Heat to drive the reforming reaction is supplied by the effluent gas from the ATR, which operates in parallel with the reform-ing exchanger. To ensure adequate heat to drive the reaction, the ATR receives excess process air, typically 50% more than what is required for nitrogen balance.

In a typical KRES™ installation, the hot ATR effluent enters the lower shell side of the reforming exchanger, where it com-bines with reforming gas exiting the reform-ing tubes. This combined gas stream travels upwards through the baffled shell side of the reforming exchanger, providing heat needed for the endothermic reforming reaction occurring inside he catalyst-filled reforming tubes. In this way, heat energy that would otherwise be used to generate possibly unneeded steam in a waste heat boiler downstream of the reformer is used instead to replace fuel as the source of heat to drive the reforming reaction.

GEA PHE Systems of Germany special-ises in the provision of plate heat exchang-ers (PHEs). PHEs offer the advantage of compact size, with a higher heat-transfer performance, lower temperature gradi-ent, higher turbulence and easier mainte-nance compared with shell and tube heat exchangers. In an ammonia/urea complex, plate heat exchangers are installed in sev-eral areas, including CO2 cooling, residual gas scrubbing and other process sections.

GEA PHE recently undertook the rede-sign of the plate heat exchangers at a fertilizer plant in Egypt, following problems of fouling with the cooling water inside the CO2 coolers. (Ammonia Technical Manual, 2013) The 1,250 t/d ammonia and 1,925 t/d urea complex uses Uhde’s proprietary ammonia process. For cooling the ammonia plant CO2 prior to feeding the urea plant, three PHEs are switched in

HEAT EXCHANGERS

Fertilizer International 463 | November - December 2014 www.fertilizerinternational.com 33

CO294°C (201.2°F)1.4 bar (20.3 psia)

38°C (100.4°F) 33°C (91.4°F) 33°C (91.4°F)

35°C (95°F)

25.5 t/h(56.2 1,000-lb/h)

WBP CW326 t/h (718.7 1,000-lb/h)35°C (95°F)3.3 bar (47.86 psia)

623 t/h (1373.5 1,000-lb/h)30°C (86°F)

623 t/h (1373.5 1,000-lb/h)30°C (86°F)

BC A

CW

62°C(143°F)

40°C(104°F)

40°C(104°F)

25.5 t/h(56.2 1,000-lb/h)

46.46 t/h(102.43 1,000-lb/h)

CO2

cooling water

Fig 3: CO2 coolers in the GEA PHE cooling system

parallel, two in operation and one in stand-by. (Fig. 3) The CO2 flows into the PHEs as a wet gas mixture at 94°C and is cooled in a counter-current process to 33°C. Water at 30°C is used as the coolant. The trans-ferred heat capacity of the PHE installation is 14.5 mW (49.4 mmBtu/hour).

Because of fouling on the cooling water side, the cooling water flow rate on the CO2 coolers fell from 500 m3/hour to 300 m3/hour. This led to operational problems in the urea plant, where the CO2 feed tem-perature was rising at between 0.5-1°C per day on average, reaching 50°C after 30 days of operation. It was noted that depos-its had accumulated at an area about 20 cm from the plate inlet and selectively covered the plate surface. The deposits plugged the channels and restricted the water flow over the plate.

GEA PHE proposed two technical solu-tions to these fouling problems: redesign-ing the existing plate heat exchangers, and new plate geometries. The PHEs were origi-nally designed with large surface margins in order to meet the pressure drop limits on the CO2 side. In the first design modifica-tion, the surface area of the heat exchang-ers was reduced by removing 86 plates out of 254 plates, reducing the surface area by 34%. The average cooling water veloc-ity inside the gaps increased from 0.30 to 0.42 m/second. The rate of depos-its formed on the surface of the plates

decreased as a result of the increase in the shear stress, in turn leading to a fall in the calculated surface temperature from 72 to 69°C and aiding the decreased rate of solid deposition. The operation time for the cooler increased from 30 days to 43 days before cleaning. However, even with the modification, the CO2 outlet tempera-ture started to increase after about 23 days of operation, and deposits continued to accumulate on the cooling water side, which eventually led to a reduction in the cooling water flow rate.

After cleaning the CO2 coolers, the PHE installation was further modified, reverting to the original configuration of 254 plates, and one plate cooler was put into opera-tion instead of two coolers. In this new arrangement, the full plant capacity of CO2 and cooling water went through one cooler instead of two. Good results were achieved initially, although after one month, the CO2 outlet temperature rose from 30°C to 34-35°C, subsequently rising further to 50°C.

After these two trials, GEA PHE then installed a new NT (New Technology) PHE with computer-modelled geometry in paral-lel with the existing two coolers. The NT series sets new economic standards and the OptiWave plate design requires less heat transfer surface for the same perfor-mance. The new plate design offers higher gap velocities (shear stress) due to bet-

ter fluid distribution over the plates and smaller gap size.

In conventional plates, more fluid flows from the inlet in the nearest channels, while the fluid velocity over the plate’s width decreases. This uneven distribution is due to the higher pressure drop in the longer flow channel. The optimised fluid distribution channels of the NT series lead to balanced velocity over the whole plate width and equal distribution of the medium. The flow channels in the distribution and collection area of the NT-plates vary in their width and were optimised, based on com-putational fluid dynamics (CFD). The chan-nels located further away from the inlet hole have a larger diameter than those closer to the inlet hole. This leads to the highest heat exchange rates being achieved with the lowest pressure drop.

The even flow distribution over the channels with the NT plates ensured that fewer deposits were accumulated. The NT unit was put into operation in parallel with the two old conventional plates units. As a result, most of the cooling water flowed to them. The average cooling water veloc-ity inside the gaps increased to 0.52 m/second, while the rate of deposits formed on the surface of the plates decreased. The calculated surface temperature conse-quently fell to 69°C, which aided the lower rate of solid deposition. The unit with the NT plates was designed, in principle, to run in parallel with one of the conventional-plate units and not to run in parallel with both. After installing the PHE, the units with conventional plates could be cleaned every six months and the units with NT plates every eight months.

Alfa Laval has supplied more than 50 fertilizer plants with PHEs that replaced shell and tube installations. One ammonia plant in North Africa has installed semi-welded PHEs as ammonia condensers. Through the increased sub-cooling of the ammonia, the company is also saving large amounts of energy in the refrigera-tion section of the plant. Another Alfa Laval installation is in an ammonia/urea facility in Malaysia. This uses Alfa Laval Compa-bloc heat exchangers for its CO2 removal system, reducing investment costs and recovering more than 5 mW of energy. A urea plant in Ukraine switched to Compa-bloc heat exchangers to serve as a hydro-lyser interchanger and reboiler in the waste water treatment system. The payback was less than a year, due to steam savings in the reboiler.

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33 www.fertilizerinternational.com Fertilizer International 433 | November - December 2014

and the shell-side flow path is wasteful on pressure drop, limiting maximum thermal effectiveness and encouraging dead spots where fouling may occur. The twisted tube exchanger design was originally developed in the 1980s. It eliminates the baffles entirely by arranging for the tubes to sup-port themselves. The tubes are formed into an oval cross-section with a superimposed twist. This is done in a special, single-step process which ensures that the wall thick-ness remains constant. The advantages of the twisted tube design include:l Higher thermal-hydraulic performance:

replacement of the zigzag flow with a more unidirectional flow on the shell side gives a much higher heat transfer coefficient per unit of pressure drop, typically being 40% higher.

l Higher thermal effectivenessl Lower fouling and cleanabilityl Avoidance of vibration.

The US company Tranter has designed a range of shell and plate heat exchang-ers for use in ammonia and urea plants, offering a smaller footprint, lower costs and simplified mechanical cleanability and better leak resistance. Designed for oper-ating pressures of up to 900°C, Tranter’s welded plate heat exchangers offer high performance under extreme conditions. The Tranter Supermax SPW heat exchanger incorporates the benefits of plate and frame exchangers, without gaskets. The unit is compact, requiring only 30-50% of the space of an equivalent shell and tube heat exchanger. Because of the advanced plate welding technology, no filler material is used. The SPW can be installed horizon-tally or vertically. For condensing, evapo-rating and boiling applications, horizontal installation is recommended.

Schoeller-Bleckmann Nitec (SBN) of Austria are specialist manufacturers of pressure vessels for the fertilizer industry, particularly for ammonia and urea plants. SBN provides a wide range of equipment for ammonia plants, including primary and secondary reformers, heat exchangers for various process stages and internals for ammonia synthesis converters, as well as heat exchangers for the high-pressure synthesis section. Depending on the spe-cific conditions, either monowall or multi-layer construction can be used. SBN also supplies urea plants with high corrosion-resistant material clad elements designed for urea synthesis, including heat exchang-ers, reactors and columns. n

BFW

steam

gas

1

2

3

4

gas

1. Ferritic tubes are used, which are not sensitive to stress corrosion, contrary to incology tubes.2. Unique patented hot/cold tube arrangement which results in tubesheet temperatures below from where nitriding starts3. Hydraulically expanded tubes avoid crevice corrosion.4. Hot incoming gas is guided through internal gas chamber directly to tube inlet ends, no special protection of combined gas inlet/ outlet chamber against nitriding and hydrogen embrittlement is necessary.

Fig 4: The Borsig process heat exchanger

Borsig Process Heat Exchanger GmbH is a leading supplier of pressure vessels, heat exchangers and other systems for use in the fertilizer industry. The Borsig Pro-cess Heat Exchanger hot/cold tubesheet design for synthesis loop waste heat boil-ers has been widely applied in waste heat recovery systems in ammonia plants. The Borsig design incorporates ferritic tubes, which are not sensitive to stress corro-sion cracking. (Fig. 4) U-tubes with hot and cold ends are alternatively arranged, while the hot shank is surrounded by cold shanks. One advantage of this design is that the tubesheet and the hot-end tube wall temperature inside the tubesheet can be kept below 380°C, thus avoiding nitriding. As a result, the inlet ends of the

tubes inside the tubesheet as well as the whole tubesheet itself are at gas outlet temperature.

Compared with conventional tube arrangements, Borsig’s unique hot/cold tube arrangement achieves an even temper-ature distribution across the tubesheet thick-ness, which is below nitriding temperature.

SKW Piesteritz is the largest producer of ammonia and urea in Germany, operating plants designed by M.W. Kellogg and engi-neered by Toyo Engineering Corporation. The plants were commissioned in 1973 and 1975. Capacity was enhanced to 1,650 t/a ammonia in 1989. In more recent years, problems arose with the high-pressure heat exchangers in the form of leaking tubes and cracking. Pitting corrosion due to caustic reaction resulted in tube leakage. (Ammonia Technical Manual, 2011)

The only known process for sealing the leaking tubes was to install tube plugs with a thread. These plugs were welded into the tubes with preheating and post heat treat-ment. This repair process is lengthy and welded plugs can suffer from circumferen-tial cracks after the units returned to ser-vice and were exposed to the temperature cycles of the waste heat boiler.

The heat exchanger at the SKW Piester-itz site comprises a fixed tube vertical heat exchanger with 1,101 tubes. SKW approached EST Group to find a solution to plug the heat exchanger without welding and within a shorter time-frame with equal or improved reliability. EST Group offered the Pop-A-Plug

® heat exchanger tube plugging

system, which features internally-serrated plugs designed to maintain a leak-tight seal under extreme thermal and pressure cycling. The Pop-A-Plug

® system was installed using

a controlled force, protecting against dam-age to the tube sheet ligaments and the adjacent tube sheet joints. The system took just minutes to install and enabled the life of the heat exchanger to be extended while reducing operating costs.

The system is available in a wide range of materials and can be matched to the tube or tube sheet it is installed in. Match-ing the material eliminates differences in thermal expansion rates and ensures that a perfect seal is maintained during tem-perature cycles experienced by the heat exchanger.

Koch Heat Transfer Co. supplies Twisted Tube

® heat exchangers. While con-

ventional shell and tube exchangers have an excellent record of acceptance and functionality, they have some limitations,