Monitoring of Diaphragm Cell Chlor-Alkali Plants for Improved Safety and Performance
Authors: Foller Peter C. , Gu Jason 
Published: February 2020

Introduction

SenSevere LLC (Pittsburgh, PA) has recently introduced new technologies for the monitoring critical diaphragm cell chlor-alkali plant metrics. We have developed a semiconductor-based sensor for the monitoring of hydrogen in chlorine and an RF-based sensor for the monitoring of brine level. These sensors, along with reporting temperature and voltage, have been engineered in conjunction with wireless data transmission such that a cell-by-cell graphical display of cell room parameters facilitates level adjustment, scheduling of maintenance, and the avoidance of unsafe operating conditions. Lessons learned during plant trials and commercialization are reviewed.

Technology

SenSevere’s chlor-alkali developments began with adapting an academic semiconductor-based H2 sensing technology to the problem of detecting H2 in wet Cl2 over the range 0 - 5 vol.%. Circumstances such tears in diaphragms from pressure fluctuations can lead to levels of hydrogen approaching the lower explosion limit. Lower levels can also result in dangerous conditions downstream in liquefaction.

The monitoring of brine level is important for several reasons. If level is too high, brine flow will be too high resulting in low caustic strength. The steam requirements in the evaporator train will then be too high. If level is too low, high current density and low brine flow may result in cell overheating, additional loss of current to oxygen production, and, dangerously, the possibility of hydrogen crossover through unflooded portions of the diaphragm. “Seeing” through foam to establish a valid brine level has traditionally required guesswork. With the development of the hydrogen and brine monitors, and powering them from the voltage across each individual cell, simultaneous monitoring cell voltage and temperature became straightforward.

The feasibility of 2.4 GHz wireless was demonstrated for cell room service and a customized intelligence system was developed that stores, processes, and presents the data in a simple actionable format. The solution offered displays simple green/yellow/red color-coded data in order to aid in scheduling cell cut-outs and maintenance.

Lessons learned

One of the core technologies arose during the course of Ph.D. research at Carnegie-Mellon University. As such, a license was required in order to secure freedom to operate. As is common, university research is more an indicator of feasibility than a ready-made solution set to go to market. It is important in licensing to recognize specific applications and a full understanding of plant conditions will dictate additional invention and adaptation. As such, licensing discussions must be clear as to what technologies are components, rather than complete systems, and value them as such. Royalty streams should decline, if not end prior to patent expiry, as academic patents are generally starting points soon superseded.

It is important to find a plant location willing to host R&D and tolerate a flexible time line due to the number of iterations likely required. Numerous surprise practicalities will surface. Such a host plant should have a reasonably common cell room technology and be willing to share non-commercial data with other operators. Purposes must also be understood: most monitoring systems are meant to be strictly advisory. Liability issues need to be addressed.

Straightforward visualization and reduction of data to actionable conclusions has been our goal. It is important not to overwhelm the client with unneeded data simply because it is possible. Agreeing a set of averaging algorithms and thresholds is critical so as to avoid alarming of the type that might undermine system credibility.

Though in certain industries uncertainty remains over the security of wireless control systems, monitoring is often treated differently. Obtaining approval for wireless rather than 4-20 mA has proven key to minimizing disruption and moving forward.

Every diaphragm cell plant is unique. Differences in cell technology can require modifications to sensing positions and fixturing. The result is that relationships are central to working sufficiently closely to tie the data obtained to known plant events. An engineer must be assigned to solve problems and help navigate plant culture.

For small/unknown entities, it is important to form a win/win relationship with an established distributor who would benefit from new reasons to call on their customers. To this end, SenSevere formed a relationship with Industrie DeNora (Milan, Italy), a long-established supplier to the chlor-alkali industry.

It is also important to gain an understanding of how plants purchase and who makes decisions. Is it easier to access an operating budget than a capital budget? Would the plant prefer a service? Is certification necessary prior to advance testing?

Status

Key elements of the SenSevere monitoring system are now on test within four North American diaphragm cell chlor-alkali plants. Additional installations are being sought.