Selection of materials for chemical engineering equipment
This color indicates a link available via Clarkson University (off-campus access)
The selection of materials of construction for chemical engineering equipment is not a trivial matter. The choice of material influences the safety, reliability, lifetime, and cost of the equipment. Many criteria must be considered, and many types of materials exist – although the vendors of equipment normally have narrowed the availability considerably. While an entire course could be devoted to this topic, we aim here only to provide sufficient information for preliminary design decisions.
Types of materials
There are many classes of materials:
· Metals. The most common class of materials for chemical engineering equipment because they generally are easy to fabricate, have high strength, and are resistance to fracture.
· Glasses. Typically rich in silica (SiO2). The most common material for chemical laboratory equipment, but usually considered too prone to fracture for large-scale plant equipment. Highly resistant to corrosion, except by fluorides and strong bases.
· Ceramics. A variety of compounds, usually oxides. Can be very strong to very high temperatures, generally very resistant to chemical attack, but difficult to fabricate and usually brittle.
· Polymers. Use in piping, valves and equipment is increasing, particularly as strength and temperature stability are increased.
· Carbon. Carbon comes in many forms (google for more details of each):
· Composites. Any mixtures of two or more solids, but generally refers to high-strength but brittle fibers dispersed in a softer ductile matrix. The oldest man-made example is glass fibers in polymer (see “fiberglass”). The fibers may be weaved or wound on a form before the matrix is applied. Fiberglass-reinforced polymer may be used for storage tanks for water and some solvents. However, in general, fiber-reinforced composites are not used in chemical engineering equipment.
· Linings and coatings. Many chemical resistant materials are not suitable for large equipment, e.g., because they are difficult to form to a desired shape, cannot be welded, are either too brittle or too soft, or are too expensive. Often, however, advantage can be taken of their chemical resistance by coating on another material, such as carbon steel. Examples are glass-lined piping and vessels; polymer-coated gaskets, o-rings, pipes and vessels.
Criteria for selection
Following are the primary criteria for materials selection. The weighting of these is somewhat flexible, although those influencing safety trump all others.
· Strength. The material must be sufficiently strong to withstand indefinitely the pressure difference between the inside of the equipment and the exterior. Lower strength can be compensated somewhat by use of thicker walls. Ashby charts of mechanical properties, from wood to ceramics
· Ease of fabrication: ductility, weldability, castability. Metals reign supreme here.
· Resistance to mechanical and thermal shock: A sudden blow or a continuously applied stress can cause a brittle material to fail catastrophically, i.e. fracture. A sudden change in temperature can induce a stress sufficient to damage some materials. Ductility is the ability of a material to deform without failing, e.g. by cracks or fracture. When brittle materials such as glass are chosen, care must be taken in the plant to avoid situations that might cause damage. For example, don’t drop a wrench into a glass-lined steel vessel used for hot sulfuric acid!
· Tendency to form sparks: Because leaks do sometimes develop, when a combustible gas is processed in a unit one must avoid sparks. For this reason, in such a unit constructed of steel, brass tools are supplied to maintenance personnel.
· Corrosion and chemical resistance: “Corrosion” generally refers to attack of metals by aqueous systems. The attack may take several forms, dependent on the particular conditions and metal: general penetration, pitting, and even cracking. While non-metals don’t corrode in this sense, they may be attacked in other ways. Thus, for example, silica glasses are dissolved by fluorides and strong alkalis. A solvent may dissolve a polymer or cause it to swell or crack. Although there are large amounts of data available, for new situations laboratory testing is desirable. At high temperatures, materials can react with a wide variety of compounds, including hydrogen and those containing sulfur, nitrogen, chlorine and even carbon.
· Oxidation resistance: The exterior of some materials exposed to air will oxidize, particularly as temperature is increased. Some materials, such as metals, form solid oxides, while others such as graphite form gaseous oxides. For materials such as aluminum and stainless steel, the resulting oxide forms a coherent film that protects the material. In other materials, such as carbon steel at high temperature, the oxide is not protective and damage continues until the material fails. Table 28-34 in Perry's Chemical Engineers’ Handbook, 8th edition gives the temperatures above which ferrous alloys experience excessive scaling in air. The minimum such temperature is 565oC for carbon steels.
· Chemical compatibility: While unusual, one must be alert to the possibility that a material or its oxide can catalyze a dangerous reaction. For example, maleic acid reacts with iron to form iron maleate, which is pyrophoric, i.e., capable of spontaneous ignition when exposed to air. Ethylene oxide vapor in contact with high surface area metal oxides, such as the gamma form of iron oxide, can undergo exothermic reactions (“disproportionation”) that can raise the temperature above the decomposition temperature of ethylene oxide.
· Temperature stability: Temperature influences all of the factors above, generally decreasing strength, increasing ductility, and increasing the rate of chemical reactions. Considering strength alone, upper temperature limits may be set for materials. Very low temperatures (“cryogenic”) can cause normally ductile materials to become dangerously brittle.
· Costs: Typically a variety of suitable materials can be identified for a particular application. The sensible thing then is to choose that with the lowest total cost, not just the cost of the bulk material but including also the cost of fabrication and installation. In economic evaluations of equipment, such as in CAPCOST, one multiplies the base module cost of the equipment fabricated from carbon steel by a materials factor, which is the ratio of the cost for the equipment fabricated from a different material to that fabricated from carbon steel. If this materials factor is unknown, it may be estimated by the following: [($/kg)material/($/kg)steel](rmaterial/rsteel)(ssteel/smaterial), where r is density and s is the allowable stress (typically on the order of ½ of the yield strength).
Disclaimer: The material on these pages is intended for instructional purposes by Clarkson University students only. Neither Clarkson University nor Professor Wilcox are responsible for problems caused by using this information.
Last updated April 4, 2013. Comments and corrections should be sent to Professor William R. Wilcox