In chemical, petro-chemical and petroleum refining industries, proper working of process columns such as distillation, extraction and stripper is very important as it affects the production efficiency and product quality. This results in huge revenue losses in case of any mal-functioning. During the operational life of these columns, many problems can develop, which can lead to mechanical damage to the column internals or the problem in the process itself. The malfunctioning of columns may also lead to fire hazards and atmospheric pollution. To determine why a column does not perform up to the desired design expectations is challenging. This is because many problems in the column can produce similar symptoms.
The conventional techniques used to identify troubles in the column are: on-line tests like pressure drop, density and viscosity measurements and simulation studies based on mathematical models and hydraulic correlations. The validity of these depends upon the availability of accurate operating data on stream composition or vapour- liquid distribution data.
These studies are useful to identify the problem areas but they cannot pinpoint the exact location of the problem. Gamma scanning is a non-invasive technique used frequently for troubleshooting of distillation columns. The technique is frequently used even for predictive maintenance of column hardwares. The technique uses absorption of gamma ray emitted from radioisotopes by process fluids consisting of vapour and liquid. The technique is gradually being used to solve more and more complex problems like maldistribution in packed bed and entrainment from tray columns. In the international scenario this technique is exploited on a routine basis and offered as specialized service. Gamma scanning technique has also emerged as a reliable research tool to generate valuable performance data.
Gamma scanning of trayed or packed bed column is based on the absorption of gamma radiation by the process fluids and the column components. For investigating a column, a source of penetrating radiation, usually a gamma ray emitting radioisotope, is placed on one side of the vessel and a suitable detector on the other. The scans are made by simultaneously moving the source and detector up or down the opposite sides of the tower. The wall thickness and column diameter are taken into account when specifying the energy and intensity of the source, so that the radiation not only penetrates the walls but is also sufficiently absorbed within the in-process material. The source is usually Cobalt – 60 which is heavily shielded and its radiation beam is collimated, so that it represents minimal risk to personnel. The radiation detector is generally a thallium-activated sodium iodide crystal that produces fluorescent light when struck by radiation.
In India, gamma scanning was introduced about 7 years ago, under an MOU between the Bhabha Atomic Research Centre (BARC) and Engineers Indian Limited (EIL) as a technique to see inside of an operating distillation column. The technique has since been developed to a high degree of precision and adaptability and has been successfully applied in column troubleshooting process optimization and predictive maintenance for almost all the major chemical process industries in the country. BARC alone has successfully diagnosed over 150 different types of columns so far. BARC’s expertise in gamma scanning is being used by several public and private sector undertakings within India and Sri Lanka. These include Bharat Petroleum Corporation Ltd., Indian Oil Corporation Ltd., Gas Authority Of India Ltd., Alkyl Amines Chemicals Ltd., Patalganga, Maharashtra, National Organic Chemicals Ltd(NOCIL), Thane, Reliance Industries Ltd., Hazira,Gujarat, Reliance Petroleum Ltd., Jamnagar, Gujarat, Hindustan Petroleum Corporation Ltd., Indian Petrochemical Corporation Ltd., Gujarat Narmada Valley Fertilizers (GNFC), Bharuch, Herdillia Chemicals Ltd., Navi Mumbai, Mangalore Refineries and Petrochemicals Limited, Mangalore, Heavy Water Plant, Department of Atomic Energy and Ceylon Petroleum Corporation Ltd., Sri Lanka.
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