Boiling Reactor – Reactive Distillation in Polymerizing Ethylene
Publication Date: 2004-Jul-20
The IP.com Prior Art Database
In a solution polymerization of ethylene removing excess heat of polymerization is important. This could be done using a boiling reactor where part of the solvent boils to remove excess heat. This could be accomplished in a reactor with two interconnected compartments. The reactor contents are circulated in such a way that boiling occurs in the larger compartment circulating the contents upward. The cold recycled condensate and fresh feed are fed into the smaller compartment causing the contents to sink. An agitator promotes the circulation between the reactors. Or the reactor could be operated under conditions where two liquid phases can coexist. The heavier polymer rich phase would collect at the reactor bottom, will not circulate through the reactor and will be withdrawn and fed into the polymer recovery part of the process. The polymerization will occur in the solvent rich phase and the polymer will separate and replenish the polymer rich phase. The solvent rich phase will recalculate and boil and thus the heat of polymerization would be removed.
In the solution process for polymerization of ethylene the reaction is carried out in an adiabatic reactor, the solvent serves as a heat sink for the heat of polymerization. The temperature difference between the reactor inlet and the outlet is directly proportional to the concentration of the polymer. The raise is typically 11 - 12°C for each weight % of the polymer in the solution. The reactor inlet temperature depends on the temperature of the cooling water available in the plant and it is typically 25 - 35°C. Refrigeration can be used to lower the inlet temperature; however there is considerable cost associated with it. The maximum outlet temperature is limited by the stability of the polymerization catalyst and by its capability to produce polymer of useful molecular weight at high temperatures. The practical limit of the present processes is about 210°C. Catalysts containing Vanadium can be used at higher temperatures, however the catalyst residues must be removed in an expensive de-ashing step. Consequently the polymer concentration in the reactor would not exceed some 17 weight %, i.e. for each kilogram of polymer 4.5 - 5 kg of solvent must be flashed off. This makes the polymer – solvent separation energy intensive and requires large and costly equipment.
One strategy for increasing the polymer concentration is to remove some heat during the polymerization. This strategy, however, presents some challenges: The temperature of the cooling surface must not drop below about 130°C to prevent precipitation. This leads to relatively low temperature difference between the cooling medium and the polymer solution. Furthermore, the polymer solution has low thermal conductivity and high viscosity. Consequently the heat exchanger must be quite large. The other challenge is the heat exchanger design. A multi-tube heat exchanger in cooling duty is inherently unstable; if the flow decreases in one of the tubes, the temperature of the solution in that tube will also decrease, the viscosity will increase and this will slow the flow in this tube even further. Various ways to manage this problem have been developed, the use high-pressure drop restricting orifices at the tube inlets for example. Another design of a cooler is based on a static mixer – heat exchanger which provides cooling and radial mixing. A heat exchanger of this type is manufactured by Sulzer (Switzerland), for example.
In either case the temperature gradients within the reactor and the cooler cause the polymer to be produced under different conditions and its uniformity may be thus compromised. This problem can be alleviated by rapid circulation of the solution in the reactor – heater loop. A pump of large capacity is required in this scheme. DOW – US Patent 5,997,251, patents such process.
Quite a different strategy to remove the heat of polymerization is to boil o...