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Minimalization Principle of Inherently Safer Design


To minimize is to reduce the quantity of material or energy contained in a manufacturing, process or plant. We often think of process minimization as resulting from the application of innovative new technology to a chemical process, for example, tubular reactors with static mixing elements, centrifugal distillation techniques, or innovative, high surface area heat exchangers. However, we must not forget that much can be accomplished in process inventory reduction simply by applying good engineering design principles with more conventional technology. Application of reliability-centered maintenance techniques can also increase the inherent safety of a plant by reducing plant downtime, thus reducing the need for intermediate inventory and storage. This in-process storage or surge capacity may be required to allow portions of the plant to continue to operate while other parts of the plant are shut down because equipment requires maintenance. Improving the reliability of critical pieces of equipment may eliminate or significantly reduce the need for in-process storage of hazardous chemical intermediates.

When designing a plant, every piece of process equipment should be specified as large enough to do its job and not larger than it is needed. We should minimize the size of all raw material and in-process intermediate storage tanks, and question the need for all in-process inventories, particularly of hazardous materials. Minimizing the size of equipment not only enhances inherent process safety, but it can often save money.

If we can eliminate equipment from a manufacturing process, we do not have to design, purchase, operate, or maintain that equipment, thus saving money. Equipment which is eliminated also cannot leak or otherwise release hazardous material or energy into the surrounding environment. The true art of the engineer is to determine how to accomplish a given task with a minimum of equipment, and with the required equipment of the smallest size.

The term "process intensification" is used synonymously with "minimization." "Process intensification" is also often used more specifically to describe new technologies which reduce the size of unit operations equipment, particularly reactors. Innovative process intensification techniques are receiving more and more attention. Interesting possibilities for a range of unit operations, including reaction, gas-liquid contacting, liquid-liquid separation, heat exchange, distillation, and separation are often reviewed during an international conference on process intensification.

Reactors

Reactors can represent a large portion of the risk in a chemical process. A complete understanding of reaction mechanism and kinetics is essential to the optimal design of a reactor system. This includes both the chemical reactions and mechanisms, as well as physical factors such as mass transfer, heat transfer, and mixing. A reactor may be large because the chemical reaction is slow. However, in many cases the chemical reaction actually occurs very quickly, but it appears to be slow due to inadequate mixing and contacting of the reactants. Innovative reactor designs which improve mixing may result in much smaller reactors. Such designs are usually cheaper to build and operate, as well as being safer due to smaller inventory. In many cases, improved product quality and yield also result from better and more uniform contacting of reactants. With a thorough understanding of the reaction, the designer can identify reactor configurations that maximize yield and minimize size, resulting in a more economical process, reducing generation of by-products and waste, and increasing inherent safety by reducing the reactor size and inventories of all materials.

Continuous Stirred Tank Reactors

A continuous stirred tank reactor is usually much smaller than a batch reactor for a specific production rate. In addition to reduced inventory, using a continuous stirred tank reactor usually results in other benefits which enhance safety, reduce costs, and improve the product quality. For example:

  • Mixing in the smaller reactor is generally better. Improved mixing may improve product uniformity and reduce by-product formation.
  • Controlling temperature is easier and the risk of thermal run­away is reduced. Greater heat transfer surface per unit of reactor volume is provided by a smaller reactor.
  • Containing a runaway reaction is more practical by building a smaller but stronger reactor rated for higher pressure.

In considering the relative safety of batch and continuous processing, it is important to fully understand any differences in chemistry and processing conditions, which may outweigh the benefits of reduced size of a continuous reactor.

Tubular Reactors

Tubular reactors often offer the greatest potential for inventory reduction. They are usually extremely simple in design, containing no moving parts and a minimum number of joints and connections. A relatively slow reaction can be completed in a long tubular reactor if mixing is adequate. There are many devices available for providing mixing in tubular reactors, including jet mixers, eductors, and static mixers.

It is generally desirable to minimize the diameter of a tubular reactor, because the leak rate in case of a tube failure is proportional to its cross-sectional area. For exothermic reactions, heat transfer will also be more efficient with a smaller tubular reactor. However, these advantages must be balanced against the higher pressure drop due to flow through smaller reactor tubes.

Loop Reactors

A loop reactor is a continuous steel tube or pipe which connects the outlet of a circulation pump to its inlet (Figure 1). Reactants are fed into the loop, where the reaction occurs, and product is withdrawn from the loop. Loop reactors have been used in place of batch stirred tank reactors in a variety of applications including chlorination, ethoxylation, hydrogenation, and polymerization. A loop reactor is typically much smaller than a batch reactor producing the same amount of product.

Figure 1 - A loop reactor production system


Reactive Distillation

The combination of several unit operations into a single piece of equipment can eliminate equipment and simplify a process. Combining a number of process operations into a single device increases the complexity of that device, but it also reduces the number of vessels or other pieces of equipment required for the process. Careful evaluation of the options with respect to all hazards is necessary to select the inherently safer overall option.

Reactive distillation is a technique for combining a number of process operations in a single device. Inventory is reduced and auxiliary equipment such as reboilers, condensers, pumps, and heat exchangers are eliminated. Figure 2 shows the conventional design, and Figure 3 shows 'the reactive distillation design. Industry has reported significant reductions in both capital investment and operating cost for the reactive distillation process.

Figure 2 - Conventional process for methyl acetate



Figure 3 - Reactive distillation methyl acetate process


Storage and Material Transfer

Raw material and in-process storage tanks and pipelines often represent a major portion of the risk of a chemical plant. Attention to the design of storage and transfer equipment can reduce hazardous material inventory.

Storage tanks for raw materials and intermediates are often much larger than really necessary, usually because this makes it "easier" to operate the plant. The operating staff can pay less attention to ordering raw materials on time, or can accept downtime in a downstream processing unit because upstream production can be kept in storage until the downstream unit is back on line. This convenience in operation can come at a significant cost in the risk of loss of containment of the hazardous materials being stored. The process design engineers and operations staff must jointly determine the need for all intermediate hazardous material storage, and minimize quantities where appropriate.

Similarly, hazardous raw material storage should also be minimized, with greater attention being given to “just in time" supply. Inventory reduction lowers inventory costs, while increasing inherent safety. In determining appropriate raw material inventories, the entire raw material supply chain must be considered. Will the supplying facility have to increase inventories to provide “just in time" service, and will this represent a greater risk than a larger inventory at the user facility? Will the raw material be stockpiled in a local storage facility, or in parked railroad cars or tank trucks, perhaps at a greater risk than on-site storage in a well-designed facility? How much additional burden will “just-in-time" delivery place on operating staff? Will unplanned shutdowns due to running out of raw materials increase risks?

The reduction in inventory resulting from greater attention to plant operations and design of unit interactions can be substantial. For examples:

  • An acrylonitrile plant eliminated 500,000pounds of in-process storage of hydrogen cyanide by accepting a shutdown of the entire unit when the product purification area shut down. This forced the plant staff to solve the problems which caused the purification area shutdowns.
  • Another acrylonitrile plant supplied by-product hydrogen cyanide to various other units. An inventory of 350,000 pounds of hydrogen cyanide was eliminated by having the other units draw directly from the acrylonitrile plant. This required considerable work to resolve many issues related to acrylonitrile purity and unit scheduling.
  • A central bulk chlorine system with large storage tanks and extensive piping was replaced with a number of small cylinder facilities local to the individual chlorine users. Total inventory of chlorine was reduced by over 100,000 pounds. This is another example of conflicting inherent safety strategies. Use of the central bulk chlorine system reduces the need for operators to connect and disconnect chlorine cylinders, but with the disadvantage of a large inventory which could be released if a leak occurs. The use of a number of local cylinder facilities results in a greater likelihood of a leak because of the necessity to connect and disconnect the cylinders more frequently-but the maximum size of the leak will be limited to the inventory in one cylinder.



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