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Application NoteDecember 2016
Corrosion Management
Cosasco ● www.cosasco.com ● [email protected]
Controlling Corrosion Upsets in Phosgene Productionwith Microcor® Online Systems
Corrosion Management
Cosasco ● www.cosasco.com ● [email protected]
Application NoteDecember 2016
Background
Phosgene is a relatively easily manufactured through highly toxic gas (chemically COCl2) which is not an end product in itself. The gas is used primarily for the production of urethane and Toluene Di Iso-cyanate (T.D.I.). T.D.I. is a product subsequently used in the manufacture of foam for upholstery, car seats and similar. The toxicity of the phosgene means that it is produced as necessary at the facility where it is required for further use, in preference to storing or transporting the highly toxic gas.
Phosgene gas forms highly corrosive hydrochloric acid when hydrated. Microcor® rapid response online corrosion monitoring system and Electrical Resistance (ER) probes provide a very successful and fast responding system for control of corrosion upsets in these systems resulting from any mixing of water and phosgene.
Phosgene Production Process
Chlorine and carbon dioxide gases are passed through a catalyst bed to produce phosgene gas. The exothermic reaction produces a phosgene outlet temperature which is typically around 180°F necessitating an outlet cooler. The heat exchanger reduces the phosgene gas temperature producing liquid phosgene (see Figure 1). On a typical installation 5 or 6 of these systems may be operating in parallel with water as the cooling media in the heat exchangers; any leakage in the exchanger results in mixing of the phosgene and the cooling water producing highly corrosive conditions.
Heat Exchanger Leak Detection
The corrosion rate of carbon steel in dry phosgene is zero. In cooling water alone it is around 5 mpy. However, with phosgene and cooling water mixed by leakage the corrosion rate may exceed 1,000 mpy. At this corrosion rate, a Microcor Wireless/Wired ER system will indicate the leakage within minutes.
The corrosion rate level of 20 mpy is typically set as an alarm level for leakage detection.
CARBONDIOXIDE
CO2
CHLORINECI2
CATALYSTBED
EXOTHERMICREACTION
PHOSGENE
1800F AMBIENT
COOLINGWATER
MATERIAL OFCONSTUCTIONSTAINLESS STEEL
MATERIAL OFCONSTUCTIONCARBON STEEL
HEAT EXCHANGER
MATERIAL OF
STAINLESS STEEL
MATERIAL OF
CARBON STEEL
CARBONDIOXIDE
CO2
CATALYSTBED
EXOTHERMICREACTION
Ambient
HEAT EXCHANGER
COOLINGWATER
PHOSGENE
1800F
CHLORINECI2
Figure 1 Schematic of Phosgene Process
CONSTRUCTION CONSTRUCTION
Corrosion Management
Cosasco ● www.cosasco.com ● [email protected]
Application NoteDecember 2016
Probe Mounting and Safety
Permissible probe mounting configurations vary from plant to plant. Some regulations permit use of retractable probes with stuffing boxes, while others will only allow fixed flanged connections (see Figure 4). Standard probes of this type have been used in many applications. However in line with our philosophy of providing high integrity equipment, we are now offering probes with double glass seals at the connector end of the probe for this type of lethal service (see Figure 3). As deterioration of the measurement element occurs process fluid may penetrate the interior of the probe. With this design two sets of seals remain between the process and the exterior of the probe. If desired the cavity between these seals may be pressurized with a suitable fluid above process pressure or vented to a safe detection area. Glass seals provide good sealing capability for acidic environments except for hydrofluoric acid (they should not be used for toxic environments with a pH greater than 9.0).
Figure 3 - Double Back Glass Sealed Connector
Figure 2 - Microcor Online Wireless System
Figure 4 - M2520 Microcor Fixed Probe
Corrosion Management
Cosasco ● www.cosasco.com ● [email protected]
Application NoteDecember 2016
Corrosion Monitoring of System Fed by Phosgene The production plants fed by phosgene, such as urethane and T.D.I. production facilities may well provide greater potential for ER probes than the phosgene plant itself. An existing customer manufacturing T.D.I. at one of the largest USA facilities (production approximately 2 million pounds per year) has two probes on his phosgene plant but a further 25 probes on the T.D.I. production section of the plant.
The plant comprises reactor vessels, distillation columns and heat exchangers. While the reactor vessels are nickel-lined and there are a number of monel pipes but the majority of the plant is carbon steel.
At the front end of the plant the problems derive from corrosive nature of phosgene and other solvents if they become hydrated. Reactor vessels with steam heated jackets present another potential corrosion problem since a leak would permit the higher pressure steam to leak into the reactor causing a highly corrosive condition. Probes positioned in the reactor outlets provide detection for such leaks.
On the distillation columns the outlet streams pass through overhead condensers cooled by water. Generally the process pressure is greater than the cooling water pressure so that any leaks would be into the cooling water. Hence in these cases probes are placed in the cooling water outlet. Alternatively where process side pressures are less than that of the cooling water, the probes would be installed on the process outlet of the exchanger.
In some cases, carbon steel probes are also used in the nickel and monel areas of the plant to provide a fast response to changes in corrosive conditions, provided the conditions are not too severe so that probe life is unacceptably short. In these cases optimization of probe spans may be required.
Corrosion Management
Cosasco ● www.cosasco.com ● [email protected]
Application NoteDecember 2016
Summary
Corrosion monitoring in the above type of plant is an excellent example of corrosion monitoring for upset conditions, caused here primarily by internal process leaks.
The Microcor ER system can provide fast effective detection not accomplished as easily or as quickly by any other method, thereby preventing expensive and dangerous plant damage.
Read more about Microcor Online Wireless/Wired technologies and corrosion monitoring systems.
Microcor Wireless Corrosion Monitoring System
Microcor Wired Online System