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Water Technologies & Solutions case study

butadiene unit run length extended with popcorn polymer inhibitor and monitoring programbackground A Western European N-methylpyrrolidone (NMP) process butadiene plant failed to achieve its targeted run length of 6 years for the columns and exchangers in the purification section.

Polybutadiene, in the form of “popcorn polymer,” was forcing unscheduled plant shutdowns every 1 to 2 years for manual cleaning. The fouling material is a highly cross-linked butadiene polymer, called “popcorn polymer,” due to its very rapid and expansive nature when formed. Each shutdown resulted in a month or more of extremely costly production loss, in addition to increased maintenance costs and safety concerns. At the same time, the finishing column overhead condensers required cleaning and passivation every 3 months.

Under normal operating conditions in the tower and the overhead condensers, oxygen intrusion leads to peroxide formation, which in turn can catalyze and create polymer. From a fouling perspective, its formation can dramatically diminish unit performance, but it can also cause significant damage to equipment.

At this plant, TBC (Tertiary Butyl Catechol) was being injected in different parts of the finishing section to help prevent rapid growth of popcorn polymer.

solution In 2010, SUEZ began injecting Butaclean* 4610, a polymerization inhibitor, in conjunction with the existing TBC treatment program. The inhibitor was fed to the vapor line overhead of the de-propynizer column to prevent popcorn polymer formation in the upper part of the column and in the condensers. Additionally, the

treatment program also helped reduce polymer formation in the finishing column.

SUEZ monitored the performance parameters that would indicate fouling of the de-propynizer and finishing columns using Multiple Regression Analysis (MRA) and Statistical Process Control (SPC). MRA/SPC helped monitor the fouling tendency on a weekly basis by modeling the column’s delta Pressure (dP) as a function of independent parameters (time, flowrates, pressures, temperatures, etc.), normalized to start-up conditions.

The monitoring and its frequency helped SUEZ and the plant operators evaluate the efficiency of the treatment, and provided ample warning when fouling began to increase, due to oxygen entering the system.

results A prescheduled regulatory shutdown occurred 1 year after initiating the SUEZ treatment program. At that time, both columns were found to be very clean after 15 months of run. Since the restart of the unit in November 2010 and 3 years of treatment, no unplanned shutdown has occurred and the program is ongoing.

One year after restarting the unit, the MRA/SPC assisted monitoring program identified, in real time, a slight increase in the dP trend. To address the issue, it was decided to temporarily increase the inhibitor dosage rate to provide extra inhibition and passivate any polymers that might form. This corrective action helped avoid a costly throughput reduction, or worse, a unit shutdown.

MRA/SPC confirmed that the dP of the de-propynizer column was under control and remained very stable at less than 5 mbar per year.

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passivation Working closely with a cleaning company, SUEZ developed a complete passivation treatment program designed to protect the columns, reboilers and condensers.

The passivation process combines Butaclean 4603, a metal passivator, with Butaclean 4604, a multifunctional polymerization inhibitor, capable of neutralizing active polymer seeds and scavenging oxygen. This passivation process is now standard procedure before each restart of the entire unit or section of the unit.

In addition to passivation, proper monitoring, and appropriate inhibitor selection, SUEZ also recommends the continuous flushing of dead legs on transfer lines. Measures should be taken to reduce dead spaces in vessels (stagnant areas and underneath column trays near the walls) to avoid oxygen contamination and subsequent polymerization as well as reduce the possibility of stagnant butadiene accumulation.

conclusion Controlling popcorn polymer formation is a very critical matter in a butadiene unit. Its formation can be very quick and can cause a shutdown after significant disruption of unit performance. Consequently, reliable monitoring of operating parameters in real time, combined with appropriate chemical selection and the ability to adjust treatment programs quickly in response to performance deviations is essential to the efficient, safe, and profitable operation of the butadiene unit.