PCB Work at Penn State University
First manufactured commercially in 1929, due to their unique chemical and thermal stability, Polychlorinated Biphenyls (PCBs) quickly gained widespread use in a number of industries, including gas and electric. Their use continued for more than four decades before their toxic nature was established. Although their production was banned by the U.S. Congress by the Toxic Substance Control Act (TSCA) of 1976, because of their inherent thermal and chemical stability, they persist in the environment. It has been established that, due to past use of PCB-based oil as lubricants in compressors in thousands of gas compressor stations across the country, PCBs entered the natural gas transmission and distribution systems.
What are PCBs?
PCBs refer to a group of aromatic compounds and there are 209 distinct PCB congeners, depending on the number and position of chlorine atoms on the biphenyl. Another subdivision of PCBs is in terms of their degree of chlorination where each group is referred to as a homologue; there are ten homologues of PCBs. The PCB compounds used are known by the trade name of Aroclor in the U.S. and each of these is a mixture of several homologues. Their characterization is therefore very complex; this and their other physico-chemical characteristics makes migration and transport of PCBs very complicated.
What is the problem?
Thousands of miles of gas pipelines are deemed to be contaminated by PCBs. These components may enter the environment through spills and leaks around gas pipelines and compressor stations. In spite of the cessation of use of this product, PCBs have persisted in the pipeline systems, thus necessitating a remediation plan. This plan must be based on a comprehensive understanding of the mechanisms governing the transport and distribution of PCBs in gas pipeline systems. This should be in the form of a predictive computer simulator, since sampling of the entire pipeline system would be cost prohibitive and impractical.
What is the solution?
It must be emphasized that in gas pipelines, PCBs are being transported in the presence of gas condensates, a multiphase dynamic environment. This makes their migration characterization very challenging. Under the sponsorship of Gas Research Institute, Adewumi and his students and associates are currently studying the migration characteristics of PCBs in gas pipelines using multiphase hydrodynamic approach. The understanding gained from this effort will help devise remediation and preventive strategies that would ensure that these toxic substances do not end up in homes and factories. Research efforts in this area target the development of a predictive model that the gas industry would use in devising operational as well as remediation strategies that would help to resolve this problem.
Some Current Research Results
The current research efforts on this front consist of two main areas, namely phase behavior and hydrodynamics. The focus of the research so far in 1995 is developing PCB clean-up scenarios for natural gas pipelines. One method of clean-up that has been investigated is injecting a solvent into the pipeline to remove the PCB from the pipeline. Several solvents were selected and tested with the computer models. Some of these results are presented below.
This graph shows the phase behavior of a natural gas system contaminated with 1 ppm PCB and with 100 gal/MMSCF of acetone injected into the system. The quality lines show the concentrations of PCB in the liquid phase. (Since PCB exists predominantly in the liquid phase, we consider only PCB concentration in the liquid phase.) The dahsed red box represents the "operating region" for a typical pipeline, or in other words, the temperatures and pressures that might be found in a natural gas transmission pipeline.
We are interested in the relative shift in the quality lines effected by the solvent. If we can achieve a shift to the right in the quality lines by adding solvent, then the PCB becomes more concentrated in the liquid. Hence, if the liquid is removed from the pipeline, then more PCB will be also be removed. As you can see from this graph, the quality lines do not shift significantly with a fairly high quantity of acetone added. Next, we will see the effect of another solvent, terpinolene, on the phase behavior of the PCB-natural gas system.
In this graph, one can clearly see that there is a dramatic shift in the quality lines within the operating region. By running the phase behavior model for various solvent injection scenarios, we can determine which solvents would hold more potential for clean-up purposes. Hence, in this part of the research, the phase behavior model serves as a screening tool. Our findings indicate that terpinolene would be the best solvent to test in an actual pipeline.
The hydrodynamic model was used to determine the effect of solvent injection on PCB concentration within a natural gas transmission pipeline. A 12,000-foot pipeline segment was tested with several injection rates of terpinolene. The following results confirm that terpinolene effectively reduces the PCB concentration in the liquid within the pipeline.
The hydrodynamic model was modified to account for the PCB adsoprtion/desorption to the pipe wall. The final step is to run the modified hydrodynamic model in order to determine how long it will take to clean the pipeline. The initial PCB distribution in the pipeline was determined from the previous runs of the hydrodynamic model. The graph shown below indicates that at a terpinolene injection rate of 50 gal/MMSCF, the pipeline is effectively cleaned after 4-1/2 hours. The blue curve shows the initial PCB concentration distrubution.
The next step in this research project is to go to the field to conduct some tests. Field testing will allow us to validate and tune the multiphase hydrodynamic model. After the model has been validated and appropriately tuned, clean-up protocols can be recommended. We are optimistic that problem of PCB-contaminated natural gas pipelines will soon be resolved.
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