FOOD ENG I NEER I NG DES I GN AND ECONOM I CS CHAPTER II GENERAL CONSIDERATIONS IN A PLANT DESIGN Plant location Plant layout Plant operation and control Utilities Structural design Storage Materials handling Waste disposal Health and safety Patents Plant Location
The geographical location of the final plant can have a strong influence on the success of an industrial enterprise.
The plant should be located where the minimum cost of production and distribution can be obtained.
The choice of the final site should first be based on a complete survey of the advantages and disadvantages of various geographical areas.
An approximate idea for the plant location should be obtained before a design project reaches the detailed estimate stage.
The source of raw materials is one of the most important factors influencing the selection because location near the raw materials source leads to reduction in transportation and storage charges.
Attention should be given to the;
The location of markets or intermediate distribution centers affects the cost of product distribution and the time required for shipping.
It should be noted that markets are needed for by-products and end products as well as for major final products.
Power and steam requirements are high in most industrial plants and the fuel is ordinarily required to supply these utilities. Consequently, power and fuel can be combined as one major factor in the choice of a plant site.
In addition, the presence and cost of electricity is an important consideration for plant location. In industrial areas the cost, voltage and availability of electricity is different than in living areas.
Excessive humidity or extremes of hot or cold weather can have a serious effect on economic operation of a plant and these factors should be examined when selecting a plant site.
The common means of transportation used by major industrial concerns are roads, highways, railroads and water. For selection careful attention should be given to “freight rates”.
In food industry, raw materials and food products are in huge amounts and not very durable. Therefore, transportation should be done with a great care and should be fast.
The process industries use large quantities of water for cooling, washing, steam generation, immobilized conveying and as a raw material. Therefore, the plant must be located where a dependable supply of water is available.
Water sources can be tab water, rivers, lakes, deep wells and artesian wells. If own sources are to be used the level of existing water, seasonal fluctuations, chemical, bacteriological content and cost for supply and purification must be considered.
The site selected for a plant should have adequate capacity and facilities for correct waste disposal. In recent years many legal restrictions have been placed on the methods for disposing of waste materials from the process industries. In choosing a palnt site, the permissible tolerance levels for various methods of waste disposal should be considered carefully and attention should be given to potential requirements for additional waste treatment facilities.
The type and supply of labor available in the vicinity of a proposed plant site must be examined. Consideration should be given to prevailing pay rates, restrictions on number of hours worked per week, competing industries that can cause dissatisfaction or high turnover rates among the workers, the ethnic distribution and variations in the skill and intelligence of workers.
Tax rates, health insurance rates and property tax rates do not change depending on position in our country. However, being a governmental policy some places are promoted for the development (as reduced tax and interest rates). In industrial regions permissions to be taken are important in cost and time delays. For the abroad enterprises local tax rates and promotions should be considered.
The characteristics of the land at a proposed plant site should be examined carefully (topography and soil structure).
The cost of land is important as well as local building costs and living conditions. Future changes may make it desirable or necessary to expand the plant facilities.
The buildings that are constructed as a result detailed land analysis, soil analysis and structural calculations are very resistant to aging as well as natural disasters like earthquakes.
Before choosing a plant site, the regional history of natural events like floods or hurricanes should be examined.
Protection from losses by fire is another important factor for selection of plant location. In case of a major fire, assistance from outside fire departments should also be available as well as fire protection systems.
The character and facilities of a comunity can have effects on the location of the plant. Cultural facilities as schools, shops, mosques, cafeterias, kindergartens, cinemas are important for a progressive community. If these facilities are not present it becomes for the plant as a necessity to provide such facilities.
new site development or addition to previously developed site
type and quantity of products to be produced
type of process and product control
operational convenience and accessibility
economic distribution of utilities and services
type of buildings and building code requirements
health and safety considerations
waste disposal problems
space available and space required
roads and railroads
possible future expansions.
+ “empty area”
Instruments are used in an industrial plant to measure process variables such as; temperature, pressure, density, viscosity, specific heat, conductivity, pH, humidity, liquid level, flow rate, chemical composition, moisture content, etc. By use of instruments having varying degrees of complexity, the values of these variables can be recorded continuously and controlled within narrow limits.
Automatic control is widely used with resulting savings in labor combined with improved ease and efficiency of operations. (which overcomes the added expense for instrumentation) This control is achieved through the use of high speed computers.
Maintenance work includes; repairs, equipment upgrading, testing, field adjustment, etc. Many of the problems involved in maintenance are caused by the original design and layout of the plant and the equipment.
In most cases the design engineer is concious only of first costs and fails to recognize that maintenance costs can easily overcome the advantages of a cheap initial installation.
Sufficient space for maintenance work on equipment and facilities must be provided in the plant layout and the engineer needs to consider maintenance requirements when making decisions on equipment.
Depending on movement of raw materials and products outside of the plant, some type of receiving and shipping facilities must be provided in the design of the plant. In those facilities cleaning and sanitation units should also be present.
Safety considerations should involve unsafe conditions (insufficient working space, inadequate aisle space, inadequate guarding of running machinery, defective equipment, inadequate lighting and ventilation, unsafe design or construction of equipment and bad floor surfaces) and unsafe acts (unsafe loading and stacking, disregard of traffic signals, carrying out repairs and adjustments on the run, operating without authority, working at unsafe speeds, using incorrect equipment, exceeding the capacity of equipment, failing to use protective clothing and practical joking)
i. Air Pollution Abatement: Air pollution control equipments can be classified into two major categories, those suitable for removing particulates and those associated with removing gaseous pollutants.
To obtain the greatest efficiency in particulate removal, particular attention must be given to particle diameter and the air velocity.
Gaseous pollutants can be removed from air streams either by absorption, adsorption, condensation or incineration.
ii. Water pollution Abatement:since waste liquid may contain dissolved gases or solids or it may be in a form of slurry, physical, chemical or biological treatment methods can be used.
The first step in any waste water treatment process is to remove large floating or suspended particles. The size of solids is wide and several separation methods are used. A common method involves the separation of coarse material using screens. (bar screens, vibrating screens, rotary drum screens etc) Screen apertures range from 25 mm down to micrometer sizes depending on the application.
This is usually followed by sedimentation or gravity settling. Sedimentation takes place in large open ponds if sufficient land area is available. Otherwise gravity sedimentation tanks are used. Entering to those cylindrical vessels the liquid stream slowly rises to the top of the tank to be removed via an overflow launder as a clarified liquid stream. Denser solids settle to the bottom as a thick sludge underflow. Slow speed scraper blades help compact the sludge and drive it to the center off-take pipe for removal. Usually residence times in these units are insufficient for anaerobic decomposition to occur.
Certain food wastes contain large amounts of oils and fats. These organic materials are immiscible with the aqueous effluent stream and float to the surface. This layer can then be mechanically skimmed from the surface of the bulk liquid. An alternative in this case is removal by aeration-floatation. With controlled aeration small bubbles will be formed which will rise through the liquid carrying grease and fine solids to the surface.
Electrodialysis has inherent limitations, working best at removing low molecular weight ionic components from a feed stream. Non-charged, higher molecular weight, and less mobile ionic species will not typically be significantly removed.
Electrodialysis systems require feed pretreatment to remove species that coat, precipitate onto, or otherwise "foul" the surface of the ion exchange membranes. This fouling decreases the efficiency of the electrodialysis system. Species of concern include calcium and magnesium hardness, suspended solids, silica, and organic compounds.
In wastewater treatment, chemical methods are generally used to remove colloidal matter, color, odor, acids, alkalies, heavy metals and oil compounds.
Coagulation is a process that removes colloids from water by the addition of chemicals which upset the stability of the system by neutralizing the colloid charge. For this purpose multivarient cations of Al or Fe.
Emulsion breaking is similar to coagulation. The emulsions are generally broken with a combination of acidic reagents and polyelectrolytes.
Precipitation method is based on the common ion effect. In this case an unwanted salt is removed from solution by adding a second soluble salt to increase one of the ion concentrations. Coagulant aids may also be needed to remove the precipitate.
Neutralization is treating acid and alkaline waste products with a strong base or acid. Even though this method may change the pH of the waste stream to the desired level it does not remove the sulfate, chloride or other ions.
Depending on the composition chemical oxidation or reduction methods can be used especially for organic materials which is hard to separate from waste water stream. ( common chemical oxidizing agents are chlorine, ozone and hydrogen peroxide)
In the presence of the ordinary bacteria found in water, many organic materials will oxidize to form carbondioxide, water, sulfates and similar materials. This process consumes the oxygen dissolved in the water and may cause a depletion of dissolved oxygen.
A measure of the ability of a waste component to consume the oxygen dissolved in water is known as the Biochemical Oxygen Demand (BOD). The BOD of polluted waters is the oxygen reported as parts per million consumed during a set period of time by bacterial action on the decomposable organic matter at 20oC.
Another measure of overall oxygen load is Chemical Oxygen Demand (COD). COD is equal to the number of milligrams of oxygen which a liter of sample will absorb from a hot, acidic solution of potassium dichromate.
In determination of BOD some incubation period is required, however, COD has the advantage of being measured in a short time.
Suspension is normally brought about by a combination of aeration and mechanical agitation. As well as the particulate material, dissolved organic matter is also adsorbed and then utilized by the aerobic microorganisms part being oxidized to carbon dioxide and water and part is assimilated to biomass. The “mixed liquor” leaving the reactor passes to a settling tank. In the tank low BOD supernatant liquor (free of suspended solids) and a thick sludge are produced. Part of the sludge which contains viable microorganisms is recycled to maintain the active microbial suspension in the aeration stage.
Sensitivity to pH makes anaerobic digestion process instable.
After digestion and settling, the reduced volume of sludge solids is usually dewatered further before disposal. The separated liquid is likely to have a high BOD and normally requires aerobic treatment before discharge to a water course.
iii. Solid Waste Disposal:solid wastes differ from air and water pollutants since the wastes remain at the point of origin until they are collected and disposed.
Process wastes can be converted into saleable products or innocuous materials that can be disposed of safely. Resource recovery is an important factor in waste disposal. Some specific chemicals may often be recovered by distillation, leaching or extraction. Also valuable solids such as metals and plastics can be recovered by magnets, electrical conductivity or hand picking. In addition to these items, hydrogenation of organics produce fuels and nitrogen or phosphorus enrichment of wastes produce fertilizers.
The controlled oxidation of solid, liquid or gaseous combustible wastes to final products of carbon dioxide, water and ash is called incineration. Since sulfur and nitrogen containing waste materials will produce their corresponding oxides, they should not be incinerated without considering their effect on air quality.
A properly designed and carefully operated incinerator can be located adjacent to aprocess plant and can be adjusted to handle a variety and quantity of wastes.
By using the heat evolved from this process in steam generation operating costs are reduced and thermal pollution control equipments are saved. Additionally, the residue is a small fraction of the original weight and volume of the waste and may be acceptable for landfill.
Pyrolysis is the heating of wastes in an air free chamber at temperatures as high as 1650 oC. Pyrolysis seems to provide several advantages over incineration. These systems encounter far fewer air pollution problems, handle larger throughputs resulting in lower capital costs, provide their own fuel and have added potential for recovering chemicals or synthesis gas.
Sanitary landfill is basically a simple technique that involves spreading and compacting solid wastes into cells that are covered each day with soil. In this method it is important that the wastes must be either or produces harmless compounds by microbial attack. In addition the possibilities of accumulation of hazardous materials which contaminate surrounding groundwater and accumulation of flammable gases produced during degradation should be eliminated.
all these problems lead the design engineer to investigate cooling systems.
The hazard and operability (HAZOP) study is a systematic technique for identifying all plant or equipment hazards and operability problems. For a HAZOP study of a specific equipment the items are;
General safety checklist for identifying process hazards involves;
Many chemicals can cause damages when come into contact with living tissues. If such chemicals are used in the process the time and amount of exposure should be determined and controlled by the design engineer.
Hazardous operations should be isolated by location in separate buildings or separated areas by brick-fire walls.
The design and construction of high pressure tanks should follow the standards and should be tested at 1.5 to 2 times the design pressure.
In order to prevent fire, smoking, welding and cutting machines, open electrical connections, heated materials and other ignition sources should be eliminated. The installation of sufficient fire alarms, temperature alarms, fire-fighting equipment and sprinkler systems are to be included in the design. Fire protection systems are classified as active and passive systems. Active systems include water sprays, foam and dry chemicals which requires action to be taken either by plant personnel or by automatic fire protection systems. Passive protection systems are designed and installed at the construction stage and remain until needed. (like fire proof insulating materials)
In the design project, health and safety of plant personnel should be considered.
To attain efficient, effective and practical noise control it is necessary to understand the individual equipment or other noise sources, their acoustic properties and characteristics. Then it should be determined that, how they interact to create the overall noise situation.
The design engineer should include noise studies in the design stage of any industrial facility. If the acoustical problems are left for field resolution, it costs roughly as much. Unnecessary costs incurred in post-construction may include the replacement of insulation, re-design of piping configuration to accommodate silencers, modification of the equipment, additional labor costs and possible down time for the plant to make necessary changes.