Dec-2020

Furnace cleaning by robots

Remotely cleaning furnace tubes slashed the cost of hydrogen production for a refiner.

YAHYA AKTAS¸, SONER OZAYLAK, SAMET S¸ANLI, METIN BECER, SULTAN ORMAN and ASLI REYHAN DINÇER
Tüpras

Article Summary

The adsorbents in a pressure swing adsorption unit, part of TüpraÅŸ Izmir refinery’s hydrogen manufacturing unit (HMU), were changed during a turnaround. After start-up, some of the adsorbents were carried by the tail gas stream from misplaced screens in one of the beds and accumulated over the steam methane reformer furnace’s convection bank tubes. This resulted in lower heat transfer, more fuel gas consumption, and increased flue gas temperatures. Within the scope of a turnaround three years later, a cleaning procedure by TüpraÅŸ’s Combustion & HC Loss Control Engineering group was applied in order to mechanically clean the tubes and therefore restore the design performance of the furnace. The aim of the study is to evaluate the performance of the cleaning operation on hydrogen production costs and on overall unit operation.

The HMU was designed to produce pure hydrogen using naphtha as feedstock, but natural gas and net gas from continuous catalytic reforming can also be used. The hearth of the unit is the steam methane reformer furnace in which there are 188 radiant tubes, each filled with catalysts promoting the endothermic reforming reactions. In the convection section of the furnace, there are six coils transferring energy to the feed and the steam streams.

To increase heat transfer and therefore to save energy in the convection section of the furnace, the decision was taken to apply mechanical cleaning. In addition and as further motivation for the project, since 1993 and the first start-up of the unit, no mechanical cleaning had been carried out in the convection tubes due to their inaccessible locations.

In order to compare the furnace’s performance before and after the cleaning procedure, a valid simulation was used for the furnace and the unit. The results can be found later in this article.

Tubetech was assigned for robotic cleaning activities in the furnace’s convection section and the work was led by the refinery’s Combustion & HC Loss Control Engineering group.

Before installation of the robots (see Figure 1), some preparations were carried out. A pool of nylon covers was established at the bottom part of the convection zone to prevent wetting of the radiant zone side refractories. Wastewater that accumulated as a pool was discharged from the furnace by means of a channel.

In addition, four new 60x45 cm² windows were cut into the metal surface so the robots could access the convection tubes. Cleaning of the tubes in the convection zone was carried out by the robots. These can deliver water pressurised to 1000 bar from their nozzles (see Figure 2). The robots’ motion over the tubes was managed via a control panel connected via cables. Two robots were installed in two different locations and the cleaning was completed in approximately 60 hours.

Diagnostics
Within the study, operational parameters before the shutdown for cleaning were regarded as the base case. According to the model’s results, the average cost of hydrogen production before and after cleaning was calculated at $943.9-919.3 $/t. The production cost of hydrogen decreased by $24.6/t after cleaning and the payback period for the project was 68 days.

After cleaning, the heat duty of the convection section increased. At the same time, the steam production rate and the steam temperature also increased. As a result, the flue gas temperature leaving the furnace decreased by about 60°C.
 
Evaluation after cleaning
Izmir hydrogen plant’s SMR furnace was evaluated with respect to operational and economic effects following cleaning in the convection zone. A simulation model was built using Aspen Hysys. After cleaning, the existing model of the unit was updated according to field data. In the model, the convection/radiant duty ratio and the fouling coefficients in the convection banks were revised to match the measured field data to the model. The effects of each of these parameters are examined in this section. Since there was a 4% difference between the inputs and outputs in the original operational data, routine unit laboratory analysis and mass balance reliability were checked and unreliable laboratory analyses were not used in the model. The simulation model is based on the following assumptions:
- In the model, the reformer outlet temperature is set according to the exit methane content. There are also thermal losses between the temperature at which the reaction takes place and the location of temperature measurement. Therefore, the exit temperature is set 25°C above the measured value to match the exit methane slip
- In the cost analysis before and after cleaning, natural gas/CCR net gas and utility prices were considered as constant and only fuel costs were changed according to their thermal content
- To compare the economic effects of cleaning on the same basis, the analysis days were selected such that unit operation is similar in terms of several operating parameters since the hydrogen production cost is very much affected by these parameters
- Feed quality and unit capacity: as the net gas feed, thus feed hydrogen content, increases, the cost of hydrogen production decreases
- SMR exit temperature: as the temperature increases, the cost of hydrogen production decreases
- Steam/carbon ratio: as this ratio decreases, hydrogen production cost decreases

Operational improvements after cleaning
The most significant improvement after cleaning was in the flue gas temperature. The average flue gas temperature was 278°C before cleaning (see Figure 3), and reduced to 220°C after cleaning. Thus, the furnace’s thermal efficiency increased by about 3%.

Conclusions
Within the scope of a planned shutdown of the SMR plant at TüpraÅŸ Izmir refinery, the aim was to increase the thermal efficiency of the convection zone of the reformer furnace by robotic cleaning performed by Tubetech. The furnace’s performance following cleaning was evaluated by a validated SMR unit simulation model using Hysys. After the convection banks were cleaned, the ratio of convection duty to total duty increased to the design level.

The flue gas temperature leaving the stack decreased by 60°C and the steam production rate rose by 20%.

Changes in utility rates (increased steam generation) resulted in a decrease in the unit cost of hydrogen production. In this study, in order to calculate the difference in the unit cost of hydrogen production independently of changing operating conditions, analysis days were selected such that the operational parameters were similar. In addition, a simulation study was performed for different steam/carbon ratio cases. The effect of changing SMR outlet temperature on cost was not evaluated. The improvement in the unit cost of hydrogen production is 24.6 $/t (see Figure 5) and the payback time for robotic cleaning costs is approximately 68 days.
In summary:
- Steam production increased by approximately 20%
- The generated steam temperature increased by 10°C-15°C
- The flue gas temperature leaving the stack decreased by 60°C
- The desulphuriser reactor furnace feed temperature increased by 40°C and the SMR furnace’s thermal efficiency increased by about 3%