Distillation and Stripping purification plants for JUNO liquid scintillator (2024)

3.2.1 Working principle of the distillation column

The fractional distillation is a chemical technique widely used to separate or extract from a liquid mixture some of its components, called fractions. Taking advantage of the different boiling points of the fractions, the mixture is heated up until selective boiling and subsequent condensation of the desired component occurs. The most volatile elements concentrate in higher percentage in the evaporated phase, while the less volatile ones remain in the liquid.

In our plant, the distillation process is carried out in a 7 m-high, 2000 mm-wide custom-made distillation column. In the bottom part, a reboiler, heat up by hot oil, is directly connected to the column to boil the LAB and to produce purified vapours, which are then collected and liquified at the top of the column by a condenser.

The column is equipped with 6 sieve trays (dual flow trays) with $\sim$3500 holes (12 mm in diameter), where a layer of liquid can build-up, ensuring an intimate contact with the upward stream of vapours. The purification process is driven by both heat and mass transfers between the gas and liquid phases at each tray, thus forming multiple equilibrium conditions along the height of the column. The vapor and liquid fluxes should be limited to the design operating range, so that a suitable contact surface and time at each stage can be ensured.

The distillation of LAB is performed at 210-220°C in partial vacuum (60 mbar at column bottom), in order to reduce the boiling temperature, avoiding risks of thermal degradation and increasing the separation capability with respect to heavy impurities.

The vacuum level inside the column is kept constant by 4 vacuum pumps (VP) connected in the upper part, after the condenser. Pressure is the key operating parameter that drives all the others inside the column: by setting the pressure values at the bottom (60 mbar at the reboiler) and top (5 mbar at the condenser) parts, the first one determines the boiling temperature of the liquid, which is constant during phase transitions, while the latter is adjusted by VPs through an automatic regulating valve. The pressure difference between them corresponds to the total height of liquid accumulated on the trays: considering 55 mbar of pressure difference and a LAB density of 720 kg/m³ at 200°C average temperature, the height is approximately 78 cm, thus implying a liquid layer of $\sim$13 cm on each tray. So, in this design, the liquid is held on the perforated tray by the pressure of the lower stage, in a dynamic equilibrium between evaporation and condensation. If pressure changes abruptly, the layers could break and the liquid would fall to the bottom of the column. Since the layer height on the trays affects the global purification efficiency of the distillation, the pressure values inside the column must be carefully controlled to avoid any destabilization of it.

Among various options, sieve trays have been chosen because of their simple, but effective layout, since they have no moving parts, can be easily cleaned, and can be mounted inside the column without welding. A picture of them is shown in Figure1.

3.2.2 Layout of the full-scale plant

The plant is designed to operate at 7 m³/h of nominal flow rate. A sketch of its flow diagram is reported in Figure2, while the main parameters are listed in Table2. The LAB entering the plant is collected in a 20 m³ vertical feed tank and pumped to the distillation column to start the purification. The liquid is fed at mid-height, above the third tray, after being pre-heated by two counter-flow heat exchangers at about 180°C, thus avoiding destabilizations of the column temperature profile. In the first exchanger, the inlet cold LAB is heated-up by the distilled hot condensate, allowing to recover heat and save energy (in the order of 400 kW_th); the second one, supplied with hot oil, makes the final temperature adjustments of the column feed. The liquid, falling by gravity, is collected in the bottom part of the column vessel and boiled by the tube-bundle reboiler, thus generating LAB vapours. High-boiling and low-volatility impurities that accumulate in the unevaporated liquid phase remain in the bottom part and are regularly discarded in a waste bottom tank every 30 minutes, up to 2$\%$ of the plant nominal flow rate (7 m³/h).

With the experience of the pilot plant, the number and type of sensors used to monitor the level of liquid inside the column have been increased, for redundancy: a safe and efficient operation requires the level to be kept within a specific range, to avoid both flooding phenomena or lowering the level below the reboiler. Three level switches, one differential pressure transmitter and one guided-wave level radar are installed in the lower part of the column, while one differential pressure transmitter measures the layers of liquid on the trays.

Purified LAB vapours, that rise up towards the column head, are extracted and liquified by a two-phase condenser, supplied with water at ambient temperature from a cooling-tower. A fraction, up to 50$\%$, of the distilled liquid stream is immediately reintroduced into the column as reflux ratio, hence further increasing the purification efficiency. The remaining part is conveyed by gravity to the 20 m³ product tank, after being cooled down to ambient temperature. For technical needs and plant size optimization, the product tank is arranged horizontally and is kept at the same pressure of the column head. This forced us to place the circulation pump of LAB product inside a 4,5 m-deep well, dug into the ground floor, to ensure a suitable NPSH (Net Positive Suction Head) at the pump inlet nozzle, to avoid running the pump dry and to prevent cavitation phenomena. The pump pushes the purified liquid through a set of 50 nm filters, with the purpose of retaining any dust or microparticles that may have been washed off the surfaces.

Finally, the distilled LAB can be either sent forward to the Mixing plant or circulated back to the inlet feed tank for further purification in internal loop mode.

The plant is equipped with 4 vacuum roots pumps, to keep a constant pressure of 5 mbar at the column head and inside the product and the bottom tanks. The VPs are preceded by a vent condenser, which removes any LAB droplet from the incondensable gases sucked by the pumps. Since in the pilot plant it has been observed that some LAB vapours could anyway reach the VP, especially during the start-up phase, in the full-scale plant a small pre-vent condenser has been added on the vacuum line directly near the column.

Each equipment is supplied with a continuous stream of nitrogen (N₂) as blanketing to avoid any oxidation.

The distillation plant is now installed in the Over Ground LS building at JUNO site (Figure3). It has been built and pre-assembled in Polaris’s workshop in 6 skids, plus one vertical and one horizontal tanks, and then shipped to China by sea. Due to its huge dimensions, the installation has been performed from the roof of the building with a 120-ton truck crane, implying challenging engineering solutions.

3.3.1 Working principle of the stripping column

It is a gas-liquid separation process where gases dissolved in a liquid phase are extracted by desorption mechanisms exploiting a stream of pure stripping gas. This purification technique relies on Henry’s law, which states the dependency of the amount of a gas i in a liquid being proportional to its partial pressure (i.e. left side of equation1):

$x_{i}$ and $y_{i}$ are the molar fraction of $i$ in liquid and gas phases respectively, $p_{t}$ the total pressure and $K_{H}$ the Henry’s constant in atm units. The latter depends on temperature, hence changing this operating parameter can affect the purification efficiency.

The process is based on mass transfer of the contaminant $i$ in liquid phase passing to the vapour phase, that is the stripping gas. In an $x_{i}$-$y_{i}$ graph, the flow rates of gas ($G$) and liquid ($L$) streams determine the process operating line, with slope $L/G$, and the so-called stripping factor $S$:

This quantity represents the removal rate of the contaminant between two equilibrium stages.

Our plant is equipped with a 9 m-high custom-made vertical stripping column filled with unstructured packing, i.e. AISI316 metallic Pall rings of 13 mm diameter (specific interface area $a$ = 430 m²/m³; see Figure4), with the purpose of spreading the liquid in thin films or drops, thus increasing the contact surface between the phases. The mass transfer directly depends upon the interface surface exposed between gas and liquid phases: the larger this parameter, the higher will be the purification efficiency. Ultrasonic bath cleaning was adopted to cleanse the Pall rings before assembling and filling the column.

The process is performed in counter-current flow mode, feeding the LS from the top and the gas from the bottom. An adjustable mixture of high-purity nitrogen (HP-N₂, up to 50 Nm³/h) and ultra-pure water steam (UPW, up to 30 kg/h) is supplied as stripping gas. At top and middle height, the column features a distributor tray, which restores a uniform spatial distribution of liquid flux inside.

Also this plant is designed to operate in partial vacuum, at about 250-300 mbar, and up to 90°C, so that the LS viscosity is reduced and the global efficiency is increased, as can be inferred from equation2.

In the Daya Bay tests, the stripping process was performed at 90°C at 300 mbar and had a purification efficiency for ²²²Rn of 95,8$\%$ on a 115 L/h LS flux with a stream of 1 Nm³/h HP-N₂. The dimensions of the column have been scaled up to design the final plant for JUNO, but the height has been further increased of 50$\%$. Based on simulations, the efficiency for the full-scale plant is foreseen to be about 99$\%$ for a LS flow rate of 7000 L/h, with 30 Nm³/h HP-N₂ as stripping gas.

3.3.2 Layout of the full-scale plant

The design and layout of the plant are based on a nominal LS flow rate of 7 m³/h. The flow diagram and the operating parameters are reported in Figure5 and Table3, respectively.

The LS, delivered by the Water Extraction plant, is collected into the 20 m³ vertical feed tank and pumped through a first set of 50 nm filters, to prevent dust from previous stages or transfer pipelines to pollute the internal surfaces and the column packing. The scintillator is heated-up to the process temperature, before being fed at the column head. Similarly to the distillation plant, we take advantage of heat recovery from stripped hot scintillator in a first heat exchanger, while a second one adjust the temperature to the target value.

The falling liquid is contacted and stripped by the upward stream of gas mixture. The HP-N₂ is supplied by a dedicated plant, exploiting purification through low-temperature adsorption technology to remove radioisotopes from the gas. This is important because their residual content sets the purification limit that can be reached in the stripping process.

The water steam (30 kg/h maximum) is produced at 65°C at about 250 mbar by an oil-based evaporator, starting from ultra-pure water (>18 MΩ$\cdot$cm). A calibrated orifice of 1 cm in diameter controls the flux injected into the column. As soon as the vapors enter the column, which is kept at higher temperature, they become superheated, hence preventing condensation.

After being stripped, the LS is cooled down to 21°C and stored into the 20 m³ vertical product tank. The final step is the filtration through another set of 50 nm filters, that restrain any dust or metallic particulate that could have been release by the Pall rings or other surfaces. The purified LS can be either sent to the Central Detector of JUNO for its filling procedure or recirculated back to the inlet feed tank for internal operation, in case of need.

A set of 4 vacuum roots pumps is connected at the column head to keep the pressure constant at 250 mbar and suck the exhausted stripping gas out of the column. Condensable vapours, as water steam, are liquified and drained by a vent condenser, while the non-condensable ones, like nitrogen, are expelled by the pumps themselves.

The stripping plant is installed in the Underground LS Hall at JUNO site. It is composed of 2 vertical tanks and 3 skids, with 5 walkable floors in total. Each unit has been transported to the underground laboratory though the 1,5 km slope tunnel and assembled using the overhead crane installed on the LS Hall roof. A picture of the stripping plant is shown in Figure6.

3.4.1 Cleanliness and air tightness requirements

Several additional precautions have been adopted in order to achieve the desired LS purity and avoid spoiling it.

Selected 316L stainless steel was chosen as material to manufacture all the main components of the plants and all parts contacted with the scintillator. In addition, a special cleaning procedure was studied and implemented to treat the surfaces and prevent oxidation phenomena: a mirror finishing of the steel through electropolishing below 0,8 $\mu$m; pickling and degreasing of the welds; a mild passivation to create an inert layer, using a solution of nitric acid (25$\%$) and demineralized water (75$\%$); rinsing with water until conductivity and pH values were restored; compressed air-drying process with dehumidifier and coalescing filters and final storage under N₂ atmosphere. Several quality controls were performed to certify the cleaning procedure, such as particle counting test, endoscopic inspection, and white wipe test. A final rinsing with high purity water was performed at JUNO site, after plant installation and skids assembling. The rinsing water has been analysed via particle counting to fulfil MIL STD 1246C – Class <50 cleanliness level[19].

To maintain the required level also during run phase, 50 nm pore size pre-wetted filters have been installed on the scintillator pipelines, with the aim of preventing circulation of dust and metallic particulate that could be present inside the plants, despite the special cleaning procedure adopted. To allow maintenance operations on filters without forcing a stop of the plant, every filtering stage is composed by two independent filter holders, which allocates six 20 inch-filter cartridges each. In the distillation plant, the filtration is carried out as final step on the outlet line, while for the stripping both on the inlet (before the stripping column, to avoid the dirty being trapped into the unstructured packing) and outlet lines.

Since radioactive gases and oxygen can spoil the LS, all the plant must be air-tight proof. The layout of the flanges has been designed with a double o-ring protection (Viton gaskets for flange exposed to temperatures <150°C, Kalrez gaskets for higher temperatures). Each component, flange or moving part have been tested with a helium leak detector to check the sealing. JUNO requirements set the thresholds of <10^-8 mbar·L/s for single leak rate and <10^-6 mbar·L/s as overall integral leak rate. Everything was certified to be within specifics. Moreover, the flanges are continuously purged by a constant flux of HP-N₂, to further reduce any possible air leakage from outside.

A procedure of vacuum pumping down to mbar level and nitrogen purging above 1,2 bar was repeated three times, to remove air before filling the plant with the scintillator. During operation phase, all tanks and the columns are kept under HP-N₂ blanketing, in overpressure (compared to atmospheric pressure) whenever feasible.

3.4.2 Mechanical aspects

All main components and pipelines have been pre-assembled and mounted inside skids before shipping them to China. Orbital welding or TIG welding were adopted whenever possible, while interconnections between skids or equipment have been fitted with flanges sealed with o-rings. For ¼” sampling ports, VCR fittings with metal gaskets were chosen.

All main pipelines for LS circulation have been equipped with full-bore ball valves, while pneumatic regulating valves have been installed in crucial points to remotely control the most important plant parameters, such as feed and product fluxes and the pressure in the column.

To improve the thermal efficiency, Rockwool is adopted as insulating material for hot pipelines and equipment, while Armaflex for the cold ones.

Each plant is provided with 4 vacuum multi-stage roots pumps, mounted in parallel and located in the upper floors of the plant, near the main column, to keep the design vacuum level during the purification process. The 3 magnetic driven centrifugal pumps (CDR pompe s.r.l. company) for the circulation of the scintillator inside the plant are positioned on the ground floor (or even beneath it, in the distillation plant) to ensure stability and a proper NPSH.

Both plants can be operated either in internal loop mode, during self-cleaning, commissioning and start-up operations, where the liquid is recirculated back to the input feed tank, or in production mode, where the processed scintillator is sent forward to the next stage of the purification sequence.

3.4.3 DCS and automation

The plants are controlled by a reliable and safe Distributed Control System (DCS), which sets the operating parameters, monitors their trend, and actuates an alarm $\&$ interlock system in case of out-of-range values or emergency.

It is based on a Siemens PLC, model ET200S, installed in a control panel along with the I/O modules, and it is supervised by a local pc with a dedicated SCADA software with user interface (UI). The PLC communicates with the pc through a MODBUS serial line, while field signals from instruments and sensors are transmitted to control panel though 4-20 mA wiring protocol.

The PLC has three main logic areas: the main control functions, always active to monitor the operating parameters and adjust accordingly the automatic valves and items to reach the setpoint value; the automatic sequences, that are set by the operator to be run independently and automatically by the system under certain conditions; the alarms and interlocks, which set the operating and safety ranges for each variable, provide acoustic and visual alarms if the ranges are exceeded and inhibit some functionalities in case of emergency, until shutdown in the worst cases. The interlocks can be software based, the most common one, where the inhibition is activated by the SCADA software with logic commands (but can also be bypassed in case of need), or hard-wired based, used only for critical items, where the electric signal is physically interrupted by opening the circuit.

The supervision system relies on a user-friendly panel with different pages, that allows an easy and efficient monitoring of the plant. The Run page displays a scheme of the plant, with all the field variables measured in real-time. The second page collects a list of all alarm signals that have occurred, both past and ongoing. In the last page, several graphical trends for the main operating parameters are reported: they show the history of values measured during time for each variable and they are crucial to understand the dynamic behaviour of the plant.

3.4.4 Safety

The plants have been studied to fulfil several international regulations and certifications and they have received the EC declaration of conformity.

Skid frames and structural loading diagrams have been designed to meet both European and Chinese safety regulations. For JUNO experimental site, all equipment must comply with the National Standard of People’s Republic of China Code of Seismic Design of Buildings GB 50011-2001. During calculations, grade VII seismic fortification intensity with a 0.10g horizontal acceleration was considered.

All tanks, vessel, columns, and main components have been certified according to PED - Group1 – Category IV – Module G classification of PED directive for pressurized equipment. After the construction phase, they have undergone both overpressure and vacuum tests. Rupture disks are installed on all tanks and the two columns and are designed to break at 3,5 bar_g, while a pressure safety valve is mounted on the HP-N₂ feed line. The plants have also been licensed by the Chinese authority SELO (China Special Equipment Licensing Office).

For fire protection, all electrical equipment is provided with certification for ATEX area classification, in Class 1 Zone 2 T2. Actually, in the distillation plant, the LAB is heat-up to temperatures above its flash point.

Security lights, active also in case of power outage, have been installed to illuminate any access points and paths.

A detailed Hazop (HAZard and OPerability) analysis for industrial risk assessment was carried out in collaboration with a team of international experts. The major hazards arising from normal and abnormal operation were identified, together with their occurrence probability and severity. Each functional block of the plant was isolated and examined for possible deviations from the design intention. For the hazards, preventive and remedying actions were defined.