Process analysers for blending applications
18 December 2012
In modern refineries, process analysers provide consistently reliable measurement results within the shortest possible time. These results represent the actual process status and are the basis for any kind of process control and optimisation.
Process analysers have become an integral part of process control in modern refineries since the early 1970s, not only for final blending, but also upstream in the various process plants. These analysers are not only used to provide status information about the process, but also valuable and necessary information that is required by modern process control and process optimisation packages.
There are several reasons for refineries to use process analysers. They fulfil environmental regulations (e.g. in terms of reduction of sulphur, aromatics and vapor pressure), help respond to any given market situation (in terms of the open market for imported products, stagnating sales due to more efficient automobile engines and market shifts between product classes), and also help respond with flexibility to the frequent changing of mixing recipes.
Refineries have identified the rationalisation of their processes as a significant contribution towards mastering the challenges they face, with automated test procedures using process analysers as an important part of it. This allows the gathering of specific properties of hydrocarbon products 24 hours a day and to integrate this information into their process control and optimisation level. The main aims are to minimise waste, to maximise process efficiency and to guarantee the high quality of the products.
In the past, the use of process analysers was reduced to direct process control, e.g. to check if the actual blend meets the required specifications. Now, the product blending in refineries is increasingly complex. This is, on the one hand, due to the fact that the number of quality parameters to be met has significantly increased (e.g. sulphur content, aromatics), and on the other, due to the changing market situation and other economic variables.
The behavior of product blending in refineries is rather non-linear, so that it is more often very difficult or even impossible to reach the quality targets within the economical constraints without on-line adaptation of the blend model / recipe during the blend. To do this, only process analysers are able to provide the necessary status information within the shortest possible time.
The goals for any investment in process control and optimisation packages are amongst others product qualities adapted to market needs, overall quality (no out-of-spec products, consistently high quality) and optimisation (flexibility, optimal conformity with applicable standards).
However, only full integration of instrumentation engineering and the consequent use of process analysers make the implementation of such complex process control and process optimisation packages possible. The payback from the investment principally results from higher productivity, reduced giveaway, rapid adaptation to market conditions, profit-optimal blending, lower manufacturing costs, lower labour costs and lower energy costs.
Blending in tank farms for sales purposes
In some refineries, the products from the different process units, which are each different in their qualities and compositions, are stored in intermediate storage tanks for further use. Depending on the required final product for sales, the specific final blending has to be carried out either into storage tanks or directly by “in-line” blending into a pipeline or vessel, for example. To get approval to sell the final product a number of quality parameters have to be observed and met. One of these is usually the boiling characteristic of the final product, which has to be carried out according to the ASTM D86 standard.
To minimise laboratory analysis during the blending process, refineries established the following procedure: Before starting the blending process a reference measurement is carried out with Protofuel. This Protofuel is used for the validation and approval of the analyser results. If the analyser is validated successfully, the blending starts. During the complete process the results of the analyser are used to monitor and control the blending. At the end of the blending process, the analyser has to perform a further validation cycle with Protofuel. If the validation with Protofuel shows results within the given specification, the final product is approved and released for sale.
The precondition for the performance of such a procedure requires the use of appropriate and reliable analysers, which are compliant to applicable standards, such as the redesigned Distillation Process Analyser DPA-4.
Based on an established standardised and robust design, this analyser is suitable for installation in hazardous areas and is available with different certificates. The analyser is equipped with state of the art communication interfaces and optionally available with customised sampling conditioning systems and chiller units.
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