تعيين ميزان انتشار و شبيه سازي پراكنش استايرن و اكريلونيتريل از واحد ABS صنعت پتروشيمي

Estimation of Emission Rate and Simiulation of Styrene and Acrilonitryle Dispersion Around an ABS plant

امروزه افزايش آلودگي هواي ناشي از رشد صنعتي در كشورهاي در حال توسعه به يكي از نگراني هاي اصلي زيست محيطي تبديل شده است. فرآيندهاي مرتبط با نفت و گاز به عنوان يكي از منابع عمده آلودگي هوا محسوب مي شوند كه نشتي از تجهيزات فرايندي سهم عمده اي در انتشار آلاينده ها و به خصوص تركيبات آلي فرار (VOCs) در اين صنايع دارند. استايرن و اكريلونيتريل به عنوان دو ماده اصلي در فرآيند واحد ABS، امكان انتشار به محيط از نشتي هاي مختلف موجود در واحد را دارند. در مرحله اول اين تحقيق، منابع اصلي انتشار اين آلاينده ها با استفاده از اطلاعات موجود در اسناد مختلف از جمله PFD، PID و كتابچه طراحي واحد ABS شناسايي شد. سپس ميزان انتشار از هر منبع با استفاده از ضرايب انتشار ارائه شده توسط سازمان حفاظت محيط زيست آمريكا (USEPA) تخمين زده شد. در نهايت، مقادير دبي انتشار بدست آمده به عنوان ورودي مدل ISCST3 و به منظور بدست آوردن الگوي پراكنش اين آلاينده ها در اطراف واحد ABS مورد استفاده قرار گرفت. نتايج نشان داد كه بيشترين غلظت آلاينده استايرن در اطراف واحد µg/m3 646 مي باشد كه كمتر از غلظت مرجع ارائه شده توسط USEPA مي باشد. بيشترين غلظت تخمين زده شده توسط مدل ISCST3 براي اكريلونيتريل نيز µg/m3 272 مي باشد كه به مراتب بيشتر از غلظت مرجع اين آلاينده مي باشد. بررسي ميزان انتشار از منابع مختلف نيز نشان داد كه بيشترين سهم آلاينده هاي بررسي شده به ترتيب مربوط به نشتي از كمپرسورها، پمپ ها و مخازن مي باشد.

Increasing pollution levels due to rapid industrialization and urbanization are now causes of major concern in industrializing countries. Petroleum and chemical processes are responsible for many emissions both into the air. Equipment leaks in chemical and petroleum processing industries are responsible for significant amount of emissions. Even if each individual leak is generally small, it is the largest source of emissions of volatile organic compounds (VOCs) from petroleum industries and chemical manufacturing facilities. Styrene and Acrylonitrile are two major components in the streams of ABS plant of Tabriz Petrochemical Complex which is expected to be released to the atmosphere through various sources such as equipment leaks and tank venting. In the first step of this study the major sources of pollutants emission in the ABS plant were identified considering the PDF and PID of the plant. Then the emission rate of each source was estimated using the emission factors presented by USEPA. An emissions factor is a representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant. Emission factors are powerful tools for policy makers as they can be used to relate emissions and concentrations. In the last step, the estimated emission rates were used as the input of Industrial Source Complex Short-Term Version 3 (ISCST3) model to predict the ground level concentration of Styrene and Acrylonitrile around the ABS plant. The ISCST3 is steady-state Gaussian plume model which can be used to assess pollutant concentrations from a wide variety of sources associated with an industrial complex. The model is generally applicable for near-field (within 10 km) impact assessment of air pollutant in meteorologically and topographically uncomplex conditions. Among the 54 pumps, 23 compressors and other equipments of the plant, 11 pumps, 8 compressors and 6 storage tanks were identified as the emission sources of considered pollutants. The emission rates of pumps and compressors were estimated using the emission factors presented in AP-42 document of USEPA. The emission estimation of Styrene and Acrylonitrile from six storage tanks has been done using USEPA standard regulatory storage tanks emission model (TANKS 4.0.9a). The emission software program TANKS is developed using emission factors presented in AP-42. The results showed that the compressors are the significant sources of considered pollutants which release about 586 g/day Styrene and 2506 g/d Acrylonitrile to the atmosphere. The emission rate of Styrene and Acrylonitrile from pumps were estimated 36 g/d and 94 g/d, respectively. The results of using TANKS model indicated that Styrene and Acrylonitrile emission rates are 7 g/d and 22 g/d, respectively. The estimated emission rates were used as the input of ISCST3 model to find the ground level concentrations of considered pollutants around ABS plant. The results showed that the maximum level of Styrene was 646 μg/m3 which is below the Reference Concentration (Rfc). In the case of Acrylonitrile the maximum level of estimated concentration was 272 μg/m3 which is higher than Rfc. The implementation of a leak detection and repair (LDAR) program or modifying/replacing leaking equipment with “leakless” components were recommended to reduce the emissions from equipment leaks of ABS plant.