Distillation columns are one area within a chemical plant that offer considerable potential in terms of improving energy efficiency. In the United States, there are more than 40,000 distillation columns within the refining and bulk chemical industries, and according to the US Department of Energy they consume approximately 40 % of the total energy used to operate these plants. Because they are very high users of energy they are obvious candidates for closer inspection with a view to making performance improvements that lead to greater energy efficiency.
However, reducing the amount of energy that distillation columns consume is not always straightforward. Columns will have different operating objectives and different configurations which will produce different behaviours. The operation of these processes typically involves a trade-off between energy usage and product recovery, and setting the appropriate operational targets involves careful evaluation of the relative economic value of these two factors. Another factor is the complexity of the distillation process, with multiple process parameters interacting strongly, making straightforward control difficult. An example of this occurred with a distillation column operated by Clariant at one of its sites in Germany.
Not content to limit itself to compliance with current directives, Clariant aims to minimise the environmental impact of its operations. The company has developed an extensive energy efficiency programme called eWatch, which uses analysis to identify potential savings in energy consumption, via the optimisation of machines and processes across all its global production sites. This is supplemented by training programmes for employees, to sharpen their awareness regarding energy-saving possibilities.
Reducing steam usage
As part of its eWatch programme, Clariant wanted to improve energy efficiency at its chemical facility in Frankfurt, Germany. The facility produces azo pigments, highly specialised high-performance pigments and polyolefin waxes, which are further processed into automotive and industrial paints, inks and plastics. Clariant wanted to reduce the amount of steam used by a distillation column which separates water from acetic acid, before returning the acetic acid back to the process. Key to achieving this was to optimise the performance of the column. One major obstacle to achieving this was that many of the column’s PID controllers were not able to run in automatic mode. This meant that they were being manually controlled by operators, which was not only a labour-intensive and time-consuming process, but also made it extremely difficult to optimise activities.
Clariant turned to Emerson, who is a major supplier of instrumentation at the plant and had worked with the company on other energy reduction projects. Members of Emerson’s lifecycle support team, specialising in process optimisation, were invited to investigate the distillation column to see if they could reduce energy usage. The initial approach to the problem was to conduct an energy saving study, including analysis of historical process data. This identified that the column had not been performing at optimum levels, leading to higher steam usage than necessary, loss of product due to too much acidic content in the reflux water, and subsequently higher water treatment costs due to the removal of that acid trace.
Investigation of the control loops
Emerson provided its Advanced Loop Service, during which a process optimisation team investigated the layout of all the column’s PID control loops to check that they were working properly and being controlled in the best way possible. PID control loops present an excellent opportunity for optimisation. There can be several hundred active in some chemical plants and it’s not uncommon for as many as two-thirds of these to be underperforming, due to deficiencies in design and tuning. This is a recipe for chronic process inefficiency and places a heavy burden on experienced operators who must continually control major aspects of the process manually, adjusting set-points locally and reacting to changes. A loop tuning sequence was performed as well as manual output step tests to detect problems and measure process dynamics.
In addition to this, all measurements and control valves were evaluated, to ascertain whether they were correctly designed and performing as they should. Distillation columns feature several critical control valves, such as a feed valve to control the rate of feed flowing into the column, a reflux valve to adjust the purity of the overhead product, bottom and overhead product valves that regulate the level in the bottom of the column and in the overhead receiver, pressure control valves, and a re-boil valve that controls the amount of heat put into the column by the re-boiler. The sizing and performance of these valves are crucial, as any discrepancies can affect both product purity and energy usage.
Finally, a complete appraisal of all installed instrumentation was performed, checking to ensure that the right measurement methodology was being used, every device was working correctly and installed in the correct position as well as identifying where additional measurements would help provide a greater understanding of the process leading to greater process optimisation.
Revised control strategy
After completing testing, Emerson’s team produced a comprehensive report with details about which control loops, valves and equipment were performing sub-optimally and therefore affecting process performance, process optimisation and subsequently increasing the energy use of the distillation column. The report highlighted various opportunities for improvement and a complete revised control strategy was provided which would contribute directly to reduced energy usage. Clariant implemented all the recommendations, with Emerson providing support by tuning all the PID loop controllers, enabling them to run in automatic mode.
Critically, the changes implemented meant that set-points could be controlled with an accuracy of ±0.5 % compared with ±2.5 % previously. Greater control helped to make significant improvements in terms of energy usage.
The target of Emerson’s performance-related contract had been to achieve a 2 % reduction in steam usage resulting from its Advanced Loop Service. The actual result was a 6 % saving, which was an outstanding success, especially considering how previous attempts had failed to generate any significant reductions.
Fast return on investment
Previously, the distillation column used approximately 6,1 GWh/a, amounting to approximately 1 million euros per year in energy costs. The 6 % steam usage reduction achieved amounts to 5,7 GWh/a and therefore a saving of 80,000 euros per year. Based on the target figure of 2 % steam usage reduction, Clariant had expected that it would take approximately one year to achieve a return on investment (ROI), but the actual 6 % saving meant that ROI was achieved in less than four months.
The improvements made to the distillation column also produced other significant benefits. Greater control and a more optimised distillation process enabled a reduction in product loss in the form of acetic acid content in water removed from the distillation column. Subsequently there was also a reduction in water treatment costs to remove the acid, which further improves profitability.
Switch to automatic mode
Following Emerson’s work, operators are now able to run all the distillation column’s control loops on automatic mode, rather than relying on manual control as before. This means that operators who previously had to watch and control the steam input and make manual adjustments every 30 to 60 min are now freed up to concentrate on other roles, therefore improving both worker and plant efficiency. Clariant was so satisfied with the increased energy reduction and process optimisation achieved on this two-month project that it is now looking to review the performance of other distillation columns at its other production sites.
Emerson Automation Solutions, Langenfeld