Plant efficiency by interrogating mixing performance in reactor tanks
Reactor vessels, whether they be anaerobic digesters (AD) or other water treatment reactor tanks, tend to deviate from ideal mixing conditions over time. This can have a huge impact on plant performance resulting in a range of issues. A lithium tracer test is regarded as one of the most effective techniques to interrogate mixing efficiency so that smooth operations can be maintained.
Mixing efficiency is a common metric to interrogate plant performance against contractual requirements during commissioning and at regular intervals during plant operations.
Factors such as gradual grit deposition in the reactor tanks can reduce mixing efficiency leading to reduced active volume, dead zones and short circuiting which can all have a direct impact on plant performance, safety, failure to comply with permits and lower revenue generation. When left unmonitored, grit accumulation can account for active volume reductions of up to 8-10% per annum, resulting in a rapid drop off in plant performance and energy output.
Operators can choose from methods going – from rudimentary thermal imaging – to dyes – to more comprehensive tracer testing to interrogate the mixing efficiencies and active volume of their reactor vessels. While thermal imaging is a quick test to highlight issues, it struggles to provide a complete and accurate assessment especially in the case of insulated tanks. Tracer testing, on the other hand, while being more resource intensive than the former, provides detailed operational parameters within the reactor vessels. Performed at regular intervals the tracer tests provide valuable insight and can help operators take proactive, informed decisions to maximize revenues, prevent major failure events and keep within permits.
A Lithium tracer test is regarded as one of the most effective technique to interrogate mixing efficiency, buildup of grit and sediment, hydraulic retention time and short-circuiting in reactor tanks either of CSTRs or plug flow configurations. Activated sludge treatment plants are typically plug flow while AD reactors are CSTR.
Factors such as gradual grit deposition in the reactor tanks can reduce mixing efficiency leading to reduced active volume, dead zones and short circuiting which can all have a direct impact on plant performance, safety, failure to comply with permits and lower revenue generation. When left unmonitored, grit accumulation can account for active volume reductions of up to 8-10% per annum, resulting in a rapid drop off in plant performance and energy output.
Operators can choose from methods going – from rudimentary thermal imaging – to dyes – to more comprehensive tracer testing to interrogate the mixing efficiencies and active volume of their reactor vessels. While thermal imaging is a quick test to highlight issues, it struggles to provide a complete and accurate assessment especially in the case of insulated tanks. Tracer testing, on the other hand, while being more resource intensive than the former, provides detailed operational parameters within the reactor vessels. Performed at regular intervals the tracer tests provide valuable insight and can help operators take proactive, informed decisions to maximize revenues, prevent major failure events and keep within permits.
A Lithium tracer test is regarded as one of the most effective technique to interrogate mixing efficiency, buildup of grit and sediment, hydraulic retention time and short-circuiting in reactor tanks either of CSTRs or plug flow configurations. Activated sludge treatment plants are typically plug flow while AD reactors are CSTR.
The question is: how widespread is the use of regular tracer testing across the AD industry in particular, as part of preventative maintenance and not reactive maintenance?
While restricted operational and plant maintenance budgets may be a barrier to frequent testing and preventative maintenance, the benefits of a regular testing programme arguably outweigh the testing costs by enhancing plant efficiency and revenue generation from biogas production as well as enhancing safety and avoiding fines for out-of-compliance site permits, or complete plant failure. Furthermore, tracer testing is substantially less expensive that draining a tank for planned maintenance, especially if that maintenance isn’t required. There are also growing regulatory drivers as part of updated permits, whereby all AD facilities operating under an Environmental Permit will need to carryout regular 'tank capacity and sediment assessment'.
Scheduled tracer testing needs to be part of preventative maintenance and asset optimization schemes as it is a key element of resilience strategies.
Scheduled tracer testing needs to be part of preventative maintenance and asset optimization schemes as it is a key element of resilience strategies.
What is a Tracer Test and how is it performed?
Tracer tests are performed by introducing a tracer which can be a dye, radioactive compound or a salt solution into the inlet of a reactor, followed by measuring the concentration of the tracer in the effluent at regular intervals. The difference in concentration profile for an ideal mix condition to that experimentally determined, provides an insight into active volume, dead zones and short circuiting within the reactor / tank. The most common tracer used in the industry is lithium chloride which is safe to human health at the concentrations of use, and accordingly safe to handle and transport. Lithium also has high solubility in water and has the added advantage of not interacting or being consumed by the process it is interrogating.
We deploy lithium tracer tests for clients during commissioning/handover of assets as well as during normal operations, helping them optimize performance and build robust resilience strategies. It is a clever test, perfected by our experts with far reaching implications for revenues and profitability of our clients.
Dr Mikael Khan , Aqua Enviro Solutions Architect, SUEZ