Patent ID: 7176032

Claim:
A process for maintaining a property P of a polyisobutene around a target value V during a polymerisation of isobutene monomer that is carried out continuously in a reactor having a boiling liquid reaction phase which contains the monomer and the polyisobutene being formed and is in equilibrium with a gas phase above the liquid phase, the process comprising continuously introducing, into the reactor, a catalyst and a C4 hydrocarbon feed containing the isobutene monomer, continuously withdrawing, from the reactor the boiling liquid reaction phase and subsequently subjecting, continuously, the liquid reaction phase to at least one purification step to isolate the polyisobutene produced, wherein the property P is the viscosity or the average molecular mass of the polyisobutene produced, and wherein said property P of the polyisobutene produced is maintained around the target value V: i) by measuring at least the isobutene partial pressure PiC4 in the gas phase of the reactor; ii) by calculating the isobutene concentration CiC4 R2 in the boiling liquid reaction phase from at least the previously measured value of PiC4 according to the following formula: CiC 4 R2 =PiC 4· [k HO ·F 1( Ci )· F 2( T )] −1 ; where K HO is a constant; Fi(Ci) is determined by the following equation: F 1( Ci )=1+ Σki· ( Ci−Ci mean ); where Ci is a concentration of at least one comoound i of the C4 hydrocarbon feed in the feed or in the aas phase of the reactor, ki being a (oositive or negative) constant, Ci means is a constant; and F2(T) is determined by the following equation: F 2( T )=1 A 2·( T+B 2) C2 ; where T is the polymerization temperature and A2, B2 and C2 are constants; iii) by calculating an instantaneous value Pinst of the property P according to the equation: Pinst= C·C 3 F3 ·C 4 F4 ·C 5 C5 ·C 6 F6 . where C, C3, C4, C5 and C6 are constants; F3 is determined by the equation: F 3= A 3· CiC 4 R2 ·(1+ B 3· CiC 4 R2 ) −1 where A3 and B3 are constants and CiC4 R2 is as defined above; F4 is determined by the equation: F 4= A 4· T, or F 4= A 4· T· (1+ B 4· T ) −1 where A4 and B4 are constants and T is the polymerization temperature; F5 is determined by the equation: F 5= A 5·[1+ B 5·( Cc ) n ] −1 where A5 and B5 are constants and Cc is a catalyst concentration; and F6 is determined by the equation: F 6= A 6·[1+ B 6·( Cco ) m ] −1 where A6, B6 and m are constants and Cco is a cocatalyst concentration; or according to the equation: Pinst= C·C 3 F3 ·C 4 F4 ·C 5 F5 ·C 6 F6 ·C 7 F7 where C7 is a constant and F7 is determined by the equation: F 7= A 7· Pd+B 7· Td wherein A7 and B7 are constants, Pd is a distillation pressure and Td a distillation temperature at which the polyisobutene produced is distilled; iv) optionally, by calculating a mean value Pmean of the property P by means of a dynamic model predicting the value of Pmean from the previously calculated value of Pinst; and v) by varying at least one process parameter so as to make the instantaneous value Pinst or the mean value Pmean tend towards the target value V of the property P.