Heat Transfer in Chemical Processes
It is essential that heat transfer systems for chemical processes are designed to maximize efficiency. Because the heat transfer step in many chemical processes is energy intensive, a failure to focus on efficiency can drive up costs unnecessarily.
The transfer of heat from one fluid to another is an essential component of all chemical processes. Whether it is to cool down a chemical after it has been formed during an exothermic reaction, or to heat components before starting a reaction to make a final product, the thermal processing operation is core to the chemical process.
Understanding how to design an effective, efficient heat transfer system is a key aspect of cost-effective manufacturing of most chemicals today. This understanding includes knowledge of key process variables such as:
- The fluids’ physical characteristics and chemical makeup.
- Flow rates.
- System temperatures and pressures.
- Allowed pressure drop.
When the corrosivity of the chemicals makes it necessary to use higher alloys (e.g., corrosion-resistant alloys), it is essential that the system be designed for maximum efficiency to ensure that the overall cost of the heat transfer equipment is minimized. This is true no matter what type of heat exchanger design is used.
Shell-and-Tube Exchangers Widely Used for Chemical Processes
Shell-and-tube exchangers are the most widely used type of heat transfer equipment in the chemical processing industry (CPI) because of their flexibility in design and ability to handle fluids with varying levels of solids. They consist of two parts: the shell and the tubes.
The shell is basically a small pressure vessel that must withstand the corrosiveness of one of the fluids and contain the system pressure. It provides no heat transfer and often is insulated from the ambient environment.
The tubes’ side of the exchanger is composed of:
- Tubesheets and baffles to maximize heat transfer.
- Inlet and outlet bonnets.
FIGURE 1. The Tubular Exchanger Manufacturers Association has designations for the various types of shell-and-tube exchangers. From this designation, engineers are able to plan the design of the exchanger.
All tube-side components must be corrosion resistant to one or both of the fluids. In addition, the tube side must contain whatever pressure is on that side of the process while not collapsing due to the pressure on the shell side. Given such an operating environment, it is no surprise that both thermal and mechanical design tools are used to design an effective system.
Numerous software tools are used to determine the thermal design. The mechanical design is produced to ASME Code in the United States and other national codes in other areas of the world. A heat transfer expert is necessary for the thermal design while a professional engineer knowledgeable of the code specified is needed for the mechanical design.
The Tubular Exchanger Manufacturers Association (TEMA), a trade association of manufacturers of shell-and-tube heat exchangers, has designations for the various types of shell-and-tube exchangers (figure 1). From this designation, engineers are able to plan the design of the exchanger. For instance, one common chemical processing industry design is a Type BEM. This designation indicates the design is a one-pass exchanger (shell type E), which means fluids only go through the exchanger once. Also, the exchanger has a bonnet (B) and a fixed tubesheet (M) on each end.
The number and lengths of tubes as well as the diameter of the shell will be determined by the heat transfer requirement. (In some cases, the size may need to be constricted to fit into a defined space in a chemical plant.) The actual design and size can be determined by the end user (chemical company) or be recommended by the fabricator after an analysis of the overall system requirements.
In the chemical processing industry, many processes require exchangers that are fabricated from highly corrosion-resistant materials. While steam or cold water may be used as one fluid (to heat or cool) in many applications, there are also process requirements that depend on heat transfer from one highly corrosive fluid to a different corrosive fluid. With steam or water as one fluid on the shell side, only the tube side (bonnets, tubes and tubesheets) need to be a highly corrosion-resistant material. With a design that incorporates transferring heat from one corrosive to another, all components that the fluids touch on both the shell and tube sides need to be manufactured of a corrosion-resistant alloy.
FIGURE 2. Corrosion-resistant alloys commonly used in the chemical processing industries include austenitic stainless steels (300 series), duplex stainless steels, nickel alloys, titanium, zirconium and tantalum. Examples include a titanium tube bundle (left) and nickel-alloy heat exchanger (right).
Materials of Construction Ensure Suitability for Process Fluids
Corrosion-resistant alloys commonly used in the chemical processing industry include austenitic stainless steels (300 series), duplex stainless steels, nickel alloys, titanium, zirconium and tantalum. Each of these metals and alloys has corrosion resistance to certain chemicals and can be used in chemical plants for long life.
The materials of construction used normally are determined by the chemical company — often after consulting with a metallurgist working for the fabricator or a metals supplier. Knowledge of corrosion and an understanding of alloys and availability are critical in determining which alloy to use. The overall goal is to ensure a cost-effective system with a long service life.
An essential consideration when determining the alloy that will be used is to ensure that all aspects of the fluid chemistry are known. Often, as companies tweak processes, the corrosivity of the fluids change, and alloys that were used in the past may not be as effective today. Changing to a new metal or alloy may be the most cost-effective solution and could lengthen the service life of the equipment. This is where a metallurgist who has a strong background working with corrosion-resistant materials is needed to determine what metal or alloy would be optimal.
For example, a plant using river water for cooling changed its heat exchanger design to specify a duplex stainless steel alloy because of assumed cost savings and the potential for longer equipment life. However, the river water chemistry changed, and the duplex stainless steel alloy developed microbiologically induced corrosion (MIC) that significantly shortened the life of the exchanger. The company had to replace the duplex equipment. Ultimately, after another review of the conditions, the company opted for titanium for the entire exchanger because titanium is immune to MIC and could also handle the process fluid.
Plate-and-frame exchangers sometimes are used in certain areas of the chemical processing industry as an alternative to shell-and-tube exchangers. The plate-and-frame designs require less space than a shell-and-tube exchanger fabricated for similar heat transfer, but they also have some limitations regarding which process fluids and conditions in which they can be used.
Plate-and-frame exchangers are available mostly with stainless steel or titanium plates. They can be designed by an in-house plate-and-frame exchanger expert at the company that supplies the equipment. Heat transfer is determined by the overall dimensions of the plates as well as the number of plates in the exchanger.
The plate-and-frame exchangers are composed of two end plates, which are designed to hold the plates together, and a number of heat transfer plates between the end plates. Gaskets are required to separate the two fluids passing through the system on each of the internal plates. Ports are fabricated into each plate and one or both of the end plates.
FIGURE 3. Plate-and-frame heat exchangers are sometimes used in certain areas of the chemical processing industry as an alternative to shell-and-tube exchangers. Plate-and-frame exchangers require less space than a shell-and-tube design fabricated for similar heat transfer, but they also have limitations regarding the types of process fluids and conditions in which they can be used.
The use of plate-and-frame exchangers is limited by the temperature and pressure that the gaskets can withstand; typically, this is less than 365°F (185°C) or 360 psi pressure. These systems also are limited to fluids that have few or no solids in them because the channels on the internal plates are narrow and can plug easily.
Advantages provided by plate-and-frame exchangers include the ability to have an experienced equipment fabricator add internal plates at any time to increase the heat transfer, and the overall ease of cleaning the exchanger.
It is worth noting that some other types of heat transfer equipment are used in the chemical processing industry. These alternatives, however, are used to a much smaller extent than shell-and-tube or plate-and-frame exchangers. Some common alternatives include internal coils, external coils, half-pipe coils on the outside of vessels, block exchangers and welded-plate exchangers.
In conclusion, the majority of the heat transfer needs in the chemical processing industry are accomplished using shell-and-tube exchangers in a range of construction materials. Plate-and-frame exchangers are used to a lesser extent and are limited in their applicability. Materials used in shell-and-tube exchangers range from stainless steel to nickel alloys, titanium and even zirconium and tantalum. Using a well-known, reliable fabricator to assist in the thermal and mechanical design as well as participate in the materials-selection process to ensure alloys can handle the corrosion in the system is recommended. Involving a qualified fabricator with these types of expertise will help ensure that the end result is a cost-effective, long-lasting heat transfer system.