Stan Kirsch outlines the upgrading materials of construction needed for the plant or process to reach better operational reliability and economic performance.
Upgrading of materials of construction can strike the proper balance between risks and costs. From the first day a new plant or process is put into service, the battle against corrosion begins. This study provides the case history of corrosion in a chlorine scrubber system, a system which removes various chlorinated compounds from a process gas stream.
In many cases the corrosion rates for various pieces of process equipment are manageable. The choice of materials is often based on a simplified economic analysis of the materials cost versus estimated equipment life. However, in many situations, once the plant or process is commissioned and operating, new information about the process becomes available that may not have been accessible to the design team or was not initially taken in to account. In light of this fresh data, the corrosion rates, which were once considered tolerable and manageable, are no longer acceptable. Ideally, all of the corrosion data needed and the experience of others would be available and used by the project design team.
In many cases, the materials of construction will need upgrading for the plant or process to reach a desirable level of operational reliability and therefore a satisfactory level of economic performance. If the process or plant does not have a large buffer or green zone between it and population centres or public areas, then successfully upgrading the materials of construction is even more critical and the consequences of not upgrading are potential civil or criminal penalties along with continued economic losses due to unplanned outages.
Water scrubber recirculation pumps
In the first part of this process (Fig.1) a gas with various chlorinated compounds enters into the bottom of the water scrubber (8 x 20ft), which uses a water spray to clean the gas. The primary function of the water scrubber is to remove any silicon tetrachloride. It also removes solid particles that might come in with the gas. Silicon tetrachloride reacts with the water to produce silicon dioxide (silica) and hydrochloric acid.
SiCl4 + 2H2O ------> SiO2 + 4HCl
The liquid exiting the water scrubber typically operates in a pH range of 1 to 2 at ambient temperature and is very abrasive due to the silica particles formed.
The original type 316 stainless steel water scrubber recirculation pumps did not last long. The pump's wet end assemblies were replaced about every 2 to 3 months due to loss of flow and discharge head.
The first material upgrade was to a Monel alloy wet end pump assembly, which did not last any longer than the stainless steel. A second upgrade to a Karbate (graphite) wet end pump assembly did not endure any longer. Next, the wet end assembly was upgraded to Alloy 20, which also failed after 2 to 3 months. None of these materials survived longer than 2 to 3 months due to the liquid's severe corrosive and abrasive nature.
The problem was eventually resolved by using an air oxidised zirconium grade 702 cast, wet end pump assembly. It was installed 17 years ago and is still in service today. It is expected to last another 17 years.
Why has zirconium worked so well where other materials failed? The key is in one of zirconium's unique properties. Zirconium naturally forms an adherent, self-healing protective zirconium oxide film that protects it from chemical attack. However, this protective oxide film can be further enhanced by heat treating zirconium in an air atmosphere at 550°C for four to six hours. This heat treatment forms a thick black oxide film on the zirconium surface that is ceramic like and gives the zirconium excellent abrasion and erosion resistant properties. This black oxide layer is equivalent to sapphire in hardness (Mohs hardness of 9).
Heat treated zirconium grade 702 solved this erosion and corrosion problem and illustrates how the right materials selection allowed this chlorine scrubber system to achieve improved operational reliability and lower maintenance costs without placing plant personnel, the community or the environment at risk.
In the next step of this process (Fig.1), the gas enters into the bottom of the first packed caustic scrubber, caustic scrubber-A (10 x 30ft). Here the other chlorinated compounds are reacted out with a 20percent, sodium hydroxide (caustic) solution. This results in the chlorine compounds forming oxoacids and oxoanions such as sodium hypochlorite (ClO-), chlorite (ClO2-), chlorate (ClO3-) and perchlorate (ClO4-) ions. The liquid exiting the caustic scrubber is usually controlled at a pH of 9 at ambient temperature. Occasionally, the pH will swing and cycle between 2 to 11 due to instrument problems and process variances.
Caustic scrubber-A is made of fibreglass with a high density Teflon (polytetraflouroethylene (PTFE)) liner to protect the fibreglass against attack from the oxoacids and oxoanions. The original liquid pumps were type 316 stainless steel, the original liquid distribution trays and packing supports were polyvinylchloride (PVC).
After one year in service, the original liquid pumps, liquid distribution trays and packing supports all showed signs of advanced corrosion and deterioration. The pump impellers were severely thinned and reduced in diameter and the PVC parts were flaking, brittle and beginning to sag. The pumps and distribution trays and packing supports were approaching failure. A demonstrated service life of one year was determined to be unacceptable for this system.
In this strong oxidising chloride environment, zirconium was chosen again to solve the problem. Zirconium has the ability to go from a very acidic environment to a very alkaline environment better than other corrosion resistant materials, such as titanium, tantalum, graphite, glass and PTFE. The caustic scrubber liquid pumps, distribution trays and packing supports were replaced with zirconium grade 702. In this case, the zirconium required no special oxide or heat treatment before being used. The zirconium has been in this service for 16 years is expected to last at least another 16 years.
Caustic scrubber blowers
As shown in the Fig.1, the gas from caustic scrubber-A then flows into the bottom of the second packed caustic scrubber, caustic scrubber-B, for a final polish before being discharged to the atmosphere. Caustic scrubber-B is identical in design to the caustic scrubber-A, except that it is not PTFE lined, since the level of oxoacids and oxoanions formed here is considered small. The polished gas then enters a blower and is sent to the vent stack. The original blower wheels (closed radial) were made of rubber coated mild steel.
The rubber coatings failed about every three to four months, causing high vibrations that resulted in shutting down the blower for repairs. After the original rubber coatings a Kynar coating [polyvinylidene fluoride (PVDF)] was tried but also failed in the same time frame. The mode of failure was always the same; the coating peeled off the metal wheel. Next, an Alloy 20 wheel was tried and was unsuccessful, due to stress cracking and generalised corrosion.
The blower wheel failures were attributed to entrained liquid droplets from caustic scrubber-B's overheads attacking the blower wheel. A knock out drum was proposed but rejected due to the high estimated costs. Its installation would require major structural and piping modifications due to the area's limited space restrictions.
The blower wheel problem was answered by using zirconium grade 702 plates to fabricate new blower wheels. Again, the zirconium grade 702 required no special oxide or heat treatment before use in this service and has been in service for 15 years with no significant signs of corrosion.
The overwhelming majority of the original materials of construction chosen for this chlorine scrubber system (fibreglass, PVC, PTFE, rubber liners and coatings) have proven to be very reliable and cost efficient. The only exceptions were the water scrubber's recirculation pumps, the caustic scrubber's liquid pumps, liquid distribution trays, packing supports and blower wheels, which in this case were upgraded to zirconium grade 702.
The higher initial cost of the zirconium has more than paid for itself many times over by increasing operational reliability and reducing maintenance costs and downtime, while reducing the risk of exposure of plant personnel, the community and the environment to the chlorinated compounds this system treats.
Stan Kirsch is Senior Engineer, Technical Services, ATI Wah Chang, in Albany, USA. " target="_blank">www.wahchang.com