According to Soares & McKenna 2012: 90, a small-scale, two-reactor polyethylene plant producing 200,000 tons of polyethylene with 8,000 operating hours generates heat at a rate of 3,600 kJ/kg, equaling an impressive energy amount of 25 MW to be evacuated and recovered. The enthalpy of reaction of polypropylene polymerization is about 50% less compared with polyethylene process. Thus, while heat transfer is a concern for polypropylene, it is critical for polyethylene.
In some reactors, the heat of reaction is removed by means of cooling water circulating into the reactor jackets. Some processes use external coolers, overhead condensers or internal cooling coils. Regardless of the cooling method, efficiency is required to control the desired reaction rates and to avoid any associated fouling.
The challenge of flashing
Flashing is a common phenomenon associated with water cooling, and sometimes cannot be completely avoided by the process design. It is therefore essential that the valves controlling these flows are capable of resisting flashing. When the downstream pressure of a control valve is less than the liquid's vapor pressure, part of the liquid is vaporized and remains as vapor downstream of the valve. Flow downstream of a valve is part liquid and part vapor. Flashing flow may cause mechanical difficulties, like erosion and vibration. Yet unlike cavitation, this is caused by the high velocity of the two-phase flow stream. The high velocity is due to the larger vapor volume compared with the liquid. High velocity can be very erosive. Therefore, more resistant materials should be used for such cases.
The flashing may even result in severe erosion and leakage for carbon steel valves with metal back seals and downstream piping. Metal-to-metal back seals allow slit flow that erodes a carbon steel body very rapidly.
Avoiding flashing damage
To avoid flashing damages occurring in the valve and downstream piping, there are a few things to consider. First, the valves should be installed as close as possible to the cooling tank to allow the high velocity flows to mix in the tank space instead of hitting the piping walls. Proper sizing is important to optimize the opening angle of the valve to around 40 and 60%. An eccentric plug valve, which is able to close the flow or reverse its direction, and a stainless steel body is highly recommended to avoid any flashing damages in the valve body. A tight graphite back seal eliminates the slit flow erosion. Restriction orifices are not usually needed.
Proven valve body solution
These types of valves have been successfully used, for example, in tubular LDPE process reactor water cooling tanks. There, manual flashing valves are used to control the reactor cooling circuit water flow to the cooling water tanks in severe flashing conditions.
In the customer LDPE plant case example, it was possible to return to the original design specification by replacing competitor carbon steel valves with completely new Metso eccentric plug control valves in upgraded material (stainless steel) and removing the restriction orifices. During the scheduled shutdown inspection, the new valve bodies did not show any sign of wear in locations previously found on the carbon steel bodies. Some wear was noticed on the seat retaining insert. However, it was not significant and all potential damages could be covered by keeping spare part sets available at the plant, thus ensuring the continuous operation of the reactor cooling tanks.
Oil & Gas Application Manager, Metso Flow Control