100
8.5 Process Safety and Health Concerns
This process involves the selective oxidation of a gaseous hydrocarbon, meaning that the primary safety concern is the possibility for explosion. As a result, great care has been taken to
ensure that all streams in the process are safely outside the flammability limits of propane. The process is controlled primarily by running under an excess of propane, making oxygen the
limiting reagent. The stream feeding the reactor contains the most oxygen in the process aside from the pure oxygen feed, and is 21.8 propane and only 4.6 oxygen, with the majority of
the stream being inert nitrogen at 61.6. The Upper Flammability Limit UFL of propane is 10.1 in air and 55 in pure oxygen. All streams are thus too oxygen-poor to allow combustion
to occur. It should be noted that all three mixers in the process M-101, M-102, and M-103 should be carefully monitored and isolated from all flames and sparks, as their contents may pass
through the flammable regime as they mix recycled propane with air and pure oxygen feeds. Another safety concern is the presence of high temperatures during the process. The
optimal reaction temperature is 750°F, and the reactor outlet temperature could rise as high as 780°F even with cooling with molten salts. This is still well under the auto-ignition temperature
of propane and propylene 878°F and 858°F, respectively. In the unlikely event that the reactor temperature begins to approach these values, a control valve upstream from the reactor V-101
could be closed to stop the flow of feed so that the reactor may cool. Still, it is recommended that a concrete shell be erected over the reactor in order to keep operators safe in the event of a fire or
explosion. Care must be taken to ensure propane does not leak from the reactor into this shell, where it may form a combustible mixture. The Occupational Safety and Health Administration
recommends using Detector Tubes manufactured by AUERMSA or Drager in order to detect concentrations greater than 200 ppm of propane in the air.
17
The process also involves storage of liquefied propane and acrylic acid to account for changes in supply of propane from the pipeline and demand for acrylic acid. It is recommended
that the propane storage tank be stored in a pressure vessel so as to prevent explosive vapor from forming. In addition, the propane storage tank must be isolated from all sparks and flames, and
operators must be careful to check for leaks using the methods described above. The acrylic acid storage tank can be designed as a fixed cone roof because acrylic acid will have a relatively low
vapor pressure at storage conditions.
17
U.S. Department of Labor. Propane. http:www.osha.govdtschemicalsamplingdataCH_264000.html
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8.6 Environmental Considerations
Acrylic acid is relatively non-toxic to bacteria and soil microorganisms. It is also miscible with water and is not expected to adsorb significantly in soil or sediment. When released into the
atmosphere, it reacts to produce hydroxyl radicals and ozone which results in quick degradation. There is no bioaccumulation of acrylic acid as it readily biodegrades in water and has an
atmospheric lifetime of less than one month.
18
Environmental concerns will be analyzed with respect to U.S. standards for pollution control. As there is no solid waste emitted from this process, the plant will meet the Federal
Hazardous and Solid Waste Amendments HWSA under the Resource Conservation and Recovery Act RCRA.
The problematic pollutants for this process in terms of air quality are CO
x
. Currently, the EPA’s Clean Air Act only regulates CO at the level of 35 ppm per hour.
19
As this process assumes that all CO is quickly oxidized to CO
2
, only trace amounts of CO may remain and this standard will easily be met. Although there is no present standard for CO
2
emissions, companies are required to obtain permits for construction and operation of their facilities from the EPA to
insure compliance.
20
Additionally, CO
2
emissions are likely to be regulated in the future. The process may need to include a carbon capture and sequestration process if the plant is found to be
above regulatory limits at a future date. The main impact on the environment is through the organic content in the wastewater
streams. A cost for wastewater treatment has been included in the financial calculations and an off-site facility is assumed.
18
World Health Organization: International Programme on Chemical Safety. 1997. Environmental Health Criteria 191: Acrylic Acid.
19
Environmental Protection Agency 2012. National Ambient Air Quality Standards NAAQS http:www.epa.govaircriteria.html
20
Haggin, P. 2012. EPAs CO2 Regulation Upheld as Unambiguously Correct. Time. http:science.time.com20120628epas-co2-regulation-upheld-as-unambiguously-correct
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