Process location and layout decisions

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Author: Ellen Zhuang [2016]

Stewards: Daniel Garcia, and Fengqi You

Contents

Introduction

Location is one of the first decisions in the design of a new chemical plant. It impacts the profitability of the project and the scope for future expansion. If the project is a new facility, a suitable site must be found and an optimal layout of the site and process units must be planned. If the project adds to an existing site, the impact of the new addition on the existing plant must be considered. The plant also needs to accommodate for the nearby infrastructure, the services that it requires, and its environmental impacts.

Location Selection Factors

A plant's site is chosen based on several factors. These include:1,2

1. Raw material supply: The source and price of raw materials is one of the most important factors that determine the location of a plant. Facilities that produce chemicals in bulk are usually located close to the source of raw material if the costs of shipping the product is less than the costs of shipping the feed. For example, ethylene production is growing in the Middle East since cheap ethane from natural gas is readily available. Oil refineries tend to be located near areas with high population since it is expensive to transport the oil.

2. Location with respect to market: If the plant produces high-volume and low-cost products, such as cement and fertilizer, it may be better to situate the plant closer to the primary market since transportation cost is a large fraction of the sales price. If the product is low-volume and high-cost, like pharmaceuticals, then the benefits of being closer to the primary market may not be there.

3. Transport facilities: Facilities should be close to at least two major forms of transportation, whether that be road, rail, waterway, and/or seaport.

4. Availability of labor: Skilled workers are usually brought to the plant from outside the area. There should be a local pool of unskilled labor that can be trained to operate the plant, and of skilled craft workers to maintain the process units. Local labor laws, trade union customs, restrictive practices for recruitment and training should also be taken into consideration.

5. Availability of utilities: Processes that require a substantial amount of cooling water is usually located near water sources, such as rivers or wells. Those that need large quantities of power, such as electrochemical ones, are typically close to cheap power sources.

6. Availability of suitable land: The ideal land is flat, well-drained, with suitable load-bearing characteristics. Further considerations have to be made if the land is reclaimed land near the ocean in earthquake zones.

7. Environmental impact: Depending on the location, it may be more difficult and costly to dispose of wastes. During the project design phase, experts are typically consulted to learn more about an area's local regulations.

8. Local community considerations: The plant location should impost no additional risk to the local residents. For example, they should be downwind of the residential areas. Building a new plant can also create jobs in that area. Local communities also need to be able to accommodate the plant personnelles.

9. Climate: The climate of the area may affect costs. For example, plants in cold areas need more insulation and special heating. Facilities in earthquake areas need to be seismically sound.

10. Political and strategic considerations: Government sometimes gives capital grants, tax concessions, and other incentives to encourage plants to be built in specific areas. Companies can also globalize and take advantage of areas with preferential tariff agreements.

Site Layout

The process units and buildings need to be arranged in such a way that allows for the most economical flow of materials and people. Furthermore, dangerous processes need to be a safe distance from other buildings, and the layout should be planned to allow for future expansion.

Process units are usually laid out first in an arrangement that allows for smooth flow of materials between the process steps. The distance between equipment is usually at least 30 m. Next, the location of the principal ancillary buildings are sited as to minimize the time that it takes the workers to travel between buildings. Administrative offices and laboratories are located away from hazardous processes. Control rooms are next to the processing equipment. Utility buildings are located as to minimize piping between the process units. Storage is placed between the loading and unloading facilities and next to the process units that they serve. Tanks containing hazardous material are placed at least 70 m from the plant. An example of a typical site plan is shown below.1

A typical site layout.[1]


Plant Layout

The main factors that are considered when planning the layout of the plant are listed below.1

1. Economic considerations (construction and operating costs): Construction costs can be minimized by arranging process units and buildings that minimize pipes between equipment, the amount of structural steel work, etc. However, this layout may conflict with the layout that gives the optimal operation and maintenance.

2. Process requirements: Examples of process considerations that must be taken into account is the elevation of the base of columns to give enough net positive suction head to a pump.

3. Operation convenience: Process units that are attended to frequently should be placed with convenient access. Valves, heads, and sample heads should be placed where operators can easily access. If the plant anticipates replacement of equipment, space must be allowed for removal and installation.

4. Maintenance convenience: Equipment that requires maintenance should be in a location with easy access, and should have sufficient space for the maintenance tasks. For example, shell-and-tube exchangers need space so that tube bundles can be removed for cleaning and repair.

5. Future expansion: The layout should be planned to conveniently allow for future expansion of processes. Pipe racks should have space for future piping, and pipes should be oversized to allow for more flow in the future.

6. Modular Construction: Modular construction is where sections of the plant is constructed outside of the plant, and then transported to the site by road or sea. Advantages include improved quality control, reduced construction costs, less requirements for skilled labor on site. Tradeoffs are more flanged connections and possible problems with onsite assembly.

7. Safety: Escape routes for workers need to be in place at each level in process buildings. Blast walls must isolate equipment that pose hazards to confine potential explosions.

First, a conceptual flowsheet for the process is developed. The types of equipment and their connections with each other is described in a process flow diagram (PFD). Before the PFD is translated into detailed piping and instrumentation diagrams (P&ID) the layout of the process units must be planned.2,3 Scale drawings are made to show the relationships between storage space and process equipment based on the flow of materials and people, and on future expansion. Three-dimensional visualization are the layouts are then carried out with cardboard cutouts of the equipment outlines or rectangular and cylindrical blocks. When a layout of the major process units has been decided, drawings of the plan and elevation are made, and design of the structural steelwork and foundations are done. Computer-aided design has also become increasingly popular.1,2,4

Environmental Considerations

Laws that protect the environment restrict the waste that plants can emit in order to preserve air, land, and water quality. States, provinces, and municipals usually have additional laws that are stricter than national ones. Some of the main environmental legislations in North America are:

  • The National Environmental Policy Act of 1969 (NEPA)
  • The Clean Air Act (CAA, 1970)
  • The Federal Water Pollution Control Act (The Clean Water Act, 1972)
  • The Safe Drinking Water Act (SDWA, 1974)
  • The Resource Conservation and Recovery Act (RCRA, 1976)
  • The Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or Superfund, 1980)
  • The Superfund Amendments and Reauthorization Act (SARA, 1986)
  • The Pollution Prevention Act (PPA, 1990)
  • The Oil Pollution Act of 1990 (OPA, 1990)
  • The Department of the Environment Act (E-10)
  • The Canadian Environmental Protection Act (CEPA, C-15.31, 1999)
  • The Canada Water Act (C-11)

Wastes comprise mainly of by-products, unused reactants, and off-specification product produced by misoperation. Leaking seals and flanges, and spills and discharges also emit waste. Material is also discharged in emergency situations. Instead of considering how to treat or manage waste, designers should start by tackling the source and find ways to minimize the production of waste.

The process of designing waste management systems are: (1) source reduction, (2) waste stream recycle, (3) waste treatment to reduce environmental impact, and (4) disposal that is legally sound. Source reduction can be achieved by reducing the concentration of impurities in the feed, protecting catalysts and adsorbents from contaminants, eliminating the use of extraneous materials, increasing recovery from separation, and improving the quality of the fuel by switching to cleaner-burning fuel. Unused feed can be recycled, and off-specification products can be reprocessed. In integral processes, the waste of one process is used as the feed for another. By-products can also be sold to another company for use as raw material. Tighter control systems, alarm, and interlocks can reduce misoperation of process unit.1

Conclusion

The location and layout of a plant can greatly impact its economic and operational success. Factors such as cost minimization, distribution, room for expansion, and safety of the plant operators and local community play important roles in the site decision. The location should be obtained before design of the details of the process.

References

[1] G.P. Towler, R. Sinnott, Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design, Elsevier, 2012.

[2] M.S. Peters, K.D. Timmerhaus, Plant Design and Economics for Chemical Engineers, 5th Ed., McGraw-Hill: New York, 2003.

[3] L.T. Biegler, I.E. Grossmann, A.W. Westerberg, Systematic Methods of Chemical Process Design, Prentice-Hall: Upper Saddle River, 1997.

[4] R.T. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, Analysis, Synthesis, and Design of Chemical Processes, Prentice Hall: Upper Saddle River, 2003.