Squeeze More From Your Process

Process intensification can enhance distillation, heat transfer and other operations

By Rocky C. Costello, R. C. Costello & Associates, Inc.

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For one installation, use of the exchangers led to an 80% reduction in surface area and a 60% drop in horsepower. The plant previously shut down for six days each month for heat exchanger cleaning. After installation of the self-cleaning heat exchangers, the plant ran 30 months without a shutdown. This changeover essentially debottlenecked the plant.

Another contender in intensified heat transfer technology is Alfa Laval Inc., Richmond, Va., with its Compabloc welded plate exchangers (Figure 7), which already have proven themselves in numerous installations. These units feature high surface area per unit volume compared to traditional shell-and-tube heat exchangers. So, they take up considerably less floor space. They also reportedly boast three to five times greater efficiency than shell-and-tube units.

Figure 7. Traditional advantages of plate design are augmented by the elimination of gaskets. Source: Alfa Laval.

The exchangers resemble traditional plate-and-frame units but instead utilize welded construction, thus eliminating the gaskets typically found between the plates. They have a wide range of operating pressures and temperatures up to a maximum pressure rating of 450 psig and 650ºF, and can be supplied in a variety of materials including exotic alloys.

Other features of these exchangers include: total accessibility on both sides; temperature cross capability; and bolted construction for easy access.

Heatric, Houston, produces diffusion-bonded heat exchangers. These units are constructed from flat metal plates into which fluid flow channels are either chemically etched or pressed. The configuration of the channels on the plates for each fluid is determined by the temperature and pressure-drop constraints and heat exchange duty. The channels can be of unlimited variety and complexity. Configurations can be counterflow, crossflow, co-flow or a combination of these to suit the application.

Plates are stacked and diffusion-bonded together to form strong, compact all-metal heat exchange blocks containing the fluid flow passages. (Diffusion-bonding is a solid-state joining process that entails pressing metal surfaces together at temperatures below the melting point, thereby promoting grain growth between the surfaces. Under carefully controlled conditions, diffusion-bonded joints reach parent metal strength.) The blocks then are welded together to form the complete heat exchange core.

Finally, fluid headers and nozzles are welded to the core to direct the fluids to the appropriate sets of passages. No gaskets or braze material — potential sources of leakage, fluid incompatibility and temperature limitations — are required for exchanger assembly. Mechanical design is normally to ASME VIII Division 1. Other design codes can be employed as required.

The majority of diffusion-bonded heat exchangers are constructed from 300-series stainless steel. Various other metals such as nickel alloys and titanium are compatible with the diffusion-bonding process.

Figure 8. These units feature mini-channels that provide high surface/volume ratio. Source: Chart Energy & Chemicals.

Mini-channels feature in the brazed aluminum heat exchangers that Chart Energy and Chemicals, The Woodlands, Texas, has been manufacturing for more than 25 years (Figure 8). These units boast surface areas per unit volume of 300 to 450 ft2/ft3 (1,000 to 1,500 m2/m3), which is six to 10 times higher than that of a comparable shell-and-tube heat exchanger. They can have up to 10 inlets and outlets — offering the potential to combine numerous heat exchanger services into a single compact unit and thus cut costs for engineering, installation, insulation, testing, etc.

The units allow temperature approaches as low as 2°F (1°C), resulting in lower compressor horsepower requirements and lower plant operating costs.

The heat exchangers have been used in cryogenic applications because the 100% aluminum construction can withstand temperatures as low as -452°F (-269°C) and pressures as high as 1,750 psig (120 bar g) or more. In addition the high thermal conductivity of aluminum coupled with the enhanced heat transfer performance of plate-fin construction reportedly produces as much as 20 times greater heat transfer performance (UA) than shell-and-tube exchangers of equal size. This higher efficiency can translate into a heat exchanger costing approximately 25% to 50% less and weighing 95% less than a stainless steel shell-and-tube exchanger.

The value of PI

Novel process-intensified unit operations create a new palette of tools for process engineers to design lower cost plants. Savings are achieved by putting less iron on the ground. Additionally, these less expensive plants can simplify production by reducing the number of unit operations, boost product purity and cut operating costs — clearly words that corporate management wants to hear. Process intensification isn’t futuristic or pie-in-sky but is here now.

Rocky Costello is president of R. C. Costello & Associates, Inc., Redondo Beach, Calif. E-mail him at rcc@rccostello.com.

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