Difference between revisions of "Utility systems"
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Revision as of 07:59, 31 January 2014
Title: Pressure Vessels
Author: David Chen
Steward: Fengqi You
Date Presented: January 13, 2014 /Date Revised: January 14, 2014
Many chemical processes do not take place at ambient temperature or pressures. In order to reach these non-ambient conditions, utilities will have to be used to raise or lower temperatures and compress gases. (Towler, Towler/UOP) Utilities often contribute 5 to 10% of the price of a product, and may come from public or private utility companies or on-site plants. For purchased utilities, costs depend on consumption, while for company-owned utilities, there will be both capital and operating costs. They include things such as steam for heating, electricity, cooling water, refrigeration, fuels such as natural gas, wastewater treatment, waste disposal, and landfill. Steam is often the largest utility cost. Cogeneration unit can supply electricity accompanied with different steam pressures. (Seider 2010)
Steam is used both as a process fluid (feedstock, diluent to absorb heat of reaction, heating agent, and stripping agent in absorbers and adsorbers ) and utility. As a utility, it can be used in place. It can be used to drive pumps and compressors, ejectors (for producing a vacuum), heat exchangers to heat and vaporize. Steam is usually at 50,150, and 450 psig. Generating high-pressure steam is more costly. (Seider) There are many benefits to using steam: high heat of condensation, its temperature can be controlled by controlling the pressure, good heat transfer when condensing, nontoxic, nonflammable, and it is inert with respect to many chemicals. (Towler 107)
Electricity is used to power many different kinds of equipment. It has many advantages:it is efficient (> 90%), reliable, available in wide range of power, shaft speeds, designs, lifetime, convenience, cost, maintenance. It is generally used up to 200 hp, and sometimes over 10,000 Hp.
The use of electricity carries with it some hazards depending on the environment. Extra care must be taken when using electrically-powered equipment in areas which may have combustible fluids, vapors, or dust, and where liquids may be present. (Seider pg 606)
Cooling water is used to cool and/or condense streams. Cooling water is usually circulated between process heat exchangers and a cooling tower. Water is cooled during downward motion by contact with air blown upwards, which can bring the water temperature to come within ~ 5 ⁰F of air’s wet-bulb temperature.Approximately 80% of the temperature reduction is due to evaporation of the cooling water and heat transfer to the surrounding air. Water can also be cooled in spray ponds and cooling ponds. Both work by providing high area for water to exchange heat with air. Water in cooling towers is lost through drift and blowdown, and makeup is usually 1.5 to 3% of the circulating rate. If a large natural body of water is nearby, it can be used as a source of cooling water and discharged downstream. This water is usually filtered to remove salts and impurities that may lead to fouling, but it is not treated.
Process water and boiler-feed water
Process water is water that will be directly used in the process. Boiler-feed water (BFW) is used to produce steam. Both may need to be purified to prevent impurities from contaminating a process or from foul equipment. It can be used as a cooling stream when the temperature of the stream to be cooled is greater than ~300 ⁰F. Cost of BFW can be partially offset by the steam credit.
Process water that undergoes moderate pretreatment can cost ~ $0.75/1,000 gal.
Extensive treatment ~ $6.00/1,000 gal.
Sterilized for pharmaceutical processes ~ $550/1,000 gal. (Seider pg 608)
In demineralized water, minerals have been removed by ion exchange. In boiler feed water, this is to prevent salt deposition, corrosion, formation of foam, and sluicing. In process water, the ions may contaminate the process.
Cooling water can usually be used to cool a stream to ~ 100 ⁰F. Air can only cool to ~ 120 ⁰F. Air may be used in places where water is scarce or more costly to transport. To cool or condense streams below 100 ⁰F, a refrigerant is typically used. Chilled brine can also be used, but is less common.
Until 1995, CFC Freon R-12 (dichlorodifuloromethane) and HCFC Freon R-22 (chlorodifuloromethane) were commonly used refrigerants. However, the chlorine atom in the molecules caused the depletion of the ozone layer. The U.S. Clean Air Act Amendments of 1990 went into effect in 1995, and the production of these refrigerants has since stopped or been greatly reduced.
Cost estimates are based on ton-day of refrigeration, where a ton is the heat that needs to be removed to freeze 1 ton per day of water at 32 ⁰F. Substitutes have since been developed. R-134a is often used in place of R-12. According to the EPA, R-134a is not combustible at ambient conditions, and is essentially non-toxic under 400 ppm, and is not ozone-depleting. (Seider pg 607)
Nitrogen is used as an inert agent and for purging. It can be purchased in liquid form or obtained if an air separation plant is already on-site
Fuel is burned in utility facilities such as boilers, electricity generation, and cogeneration, and can be in solid, liquid, or gas form. It can also be burned to provide heating for a process or stream or to drive pumps and compressors. The fuel is usually burned with excess air to ensure complete combustion.
A way of quantifying the amount of heat generated is by using the heating values. Higher heating value (HHV) and the lower heating value (LHV) are used. The heating is the total heat evolved by complete combustion of a fuel with dry air with both at 60 ⁰F and the flue gas after combustion brought back down to ⁰F. If the water vapor in the flue gas is not condensed, we obtain the LHV. If the water vapor is condensed, the value of heat evolved is a bit higher, and this is the HHV. Heating values for solids and liquids are usually on a per-mass basis, and gases on a per-volume basis. A specified amount of heating cannot be met with the amount of fuel calculated using only the HHV. There will be heat losses, the flue gas temperature will be greater than 60 ⁰F, and water in the flue gas will typically be vapor. (Seider 608)
Most chemical processes will produce some sort of waste. Disposal occurs to the atmosphere (in the case of some gases), sewers, body of water, or a landfill. Waste may require some treatment before disposal to meet regulations. Depending on process economics, byproducts may be recovered and processed. (Seider pg 609)
(Seider pg 609)
Waste gases are commonly released to the atmosphere. Particulates and volatile pollutants that need to be removed before disposal may be present. Particle removal equipment includes: cyclones, wet scrubbers, electrostatic precipitators, and fabric-filter systems. Methods for removing inorganic and organic gaseous pollutants include: absorption, adsorption, condensation, and combustion. (Seider pg 609)
U.S. federal regulations require that solid waste be classified as hazardous or nonhazardous. Conditions for a classification of hazardous include: ignitability, corrosivity, reactivity, toxicity, or posing a substantial threat to the surrounding environment and its inhabitants. Hazardous waste must be treated on- or near-site before being removed in containers. Non-hazardous waste may be landfilled or incinerated in some cases. A typical estimate of costs for waste disposal is $0.03/lb for nonhazardous solids and $0.10/lb for hazardous solids. (Seider pg 609)
- Towler, G.P. and Sinnot, R. (2012). Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design. Elsevier.
- Seider; Seader; Lewin; Widagdo. (200\9). Plant Design and Economics for Chemical Engineers, 5th Edition. Hoboken: Wiley.
- Turton R.; Bailie, R.C.; Whiting, W.B.; Shaeiwitz J.A.; Bhattacharyya D. (2012). Analysis, Synthesis, and Design of Chemical Processes. Upper Saddle River: Prentice Hall.