GB2183245A - Process of preparing propylene homo- or co-polymers - Google Patents

Abstract

To control the molecular weight of a propylene homo- or co-polymer at a constant level when subjecting propylene alone, or together with another alpha -olefin, to bulk polymerization in the presence of hydrogen as a molecular weight modifier, the polymerization is performed in a reaction tank equipped with a reflux condenser, using liquid monomer as the reaction medium. The molecular weight is controlled at a desired level, by constantly measuring the quantity of heat removed from the reaction tank and constantly calculating the volume of hydrogen to be consumed, on the basis of the above-measured quantity of removed heat and the volume (X) of hydrogen consumed per unit amount of polypropylene, which volume (X) is obtained by utilizing the relation In eta =InX+A. (A being a constant and eta being the intrinsic viscosity of the propylene polymer). Hydrogen is charged into the reaction tank in quantities corresponding to variations in the calculated quantity.

Description

SPECIFICATION Process of preparing propylene homo- or co-polymers This invention relates to a process of preparing homo-polymers or co-copolymer of propylene.

More specifically, the present invention relates to a method in which a propylene homo- or copolymer of controlled molecular weight is obtained by subjecting propylene alone or a mixture of propylene and another a-olefin copolymerizable with propylene to bulk polymerization in the presence of hydrogen as a molecular weight modifier. The polymerization is carried out in a reaction tank equipped with a reflux condenser using the propylene or the mixture as a liquid medium.

It is well-known that in the polymerization of propylene in the presence of a Ziegler-Natta catalyst, the molecular weight of the resulting polypropylene can be controlled by adjusting the volume of hydrogen added during the polymerization [see, for example, J. Polymer Sci., C2, 109 (1974)]. Since there is a certain close relationship between the concentration of hydrogen in the vapor phases and the molecular weights of the resulting polypropylenes [see, for example, J.

Polymer Sci., Part Al, Vol. 8, 2717 (1970)], polypropylene is usually prepared by controlling the concentration of hydrogen in a vapor phase at a constant level so that the molecular weight of the resulting polypropylene has a desired value.

When polypropylene is prepared by bulk polymerization in a large reaction tank, it is difficult to remove the heat of polymerization if the removal of heat is effected merely through the wall of the reaction tank or by means of a heat exchanger provided inside the reaction tank. Accordingly, it has also been known to use a reflux condenser which makes use of the latent heat of a liquid medium.

When polypropylene is subjected to bulk polymerization in a reaction tank equipped with a reflux condenser, the concentration of hydrogen in a vapor phase varies significantly in accordance with the load to the reflux condenser. It is therefore necessary frequently to repeat the introduction or discharge of hydrogen into or out of the reaction tank in order to maintain the concentration of hydrogen at a constant level in the vapor phase, so as to control the molecular weight of the resulting polymer. This means that a great deal of hydrogen is discharged and moreover, a large volume of propylene is also discharged along with the hydrogen discharged, resulting in a problem that the above process is not economically preferred.

An object of this invention has been to provide a process for the preparation of a propylene homo- or co-polymer of a controlled molecular weight without loss of raw materials.

The present inventors have carried out an extensive investigation with a view toward providing a solution to the above-described problems. The investigation has now resulted in the development of a process which allows the molecular weight of polypropylene to be adjusted with good controllability and without loss of hydrogen and/or propylene.

According to one aspect of this invention, there is provided a process for the preparation of a propylene homo- or co-polymer by subjecting propylene alone, or a mixture of propylene and another a-olefin copolymerizable with propylene, as a monomer or monomer mixture to bulk polymerization at a constant temperature in the presence of hydrogen as a molecular weight modifier; the polymerization is carried out in a reaction tank equipped with a reflux condenser using the propylene or the mixture as a liquid medium too, and vapor of the medium is condensed in the reflux condenser so as to remove at least a part of the heat of polymerization.

In the process, the following procedures are followed: the amount of the monomer or monomer mixture polymerized per unit time is calculated based on the quantity of generated heat calculated as the sum of the quantity of heat removed artificially and the quantity of heat allowed to dissipate naturally, both from the reaction tank per the same unit time; the volume of hydrogen required per unit amount of homo- or co-polymer of a desired molecular weight is determined in accordance with the following equation established beforehand between the intrinsic viscosities 1 of propylene homo- or co-polymers measured as their tetralin solutions at 135"C and the volumes (X) of hydrogen consumed respectively per unit amounts of the propylene homo- or co-polymers: In =ln X+A wherein A is a constant; and the volume of hydrogen, which is to be charged into the reaction tank, is controlled in accordance with variations in the volume of hydrogen consumed in the reaction tank calculated as a value which is obtained by subtracting the volume of hydrogen, which is to be introduced along with a slurry into the reaction tank, from the sum of the product of the above-determined volume of hydrogen and the above-calculated amount of the monomer or monomer mixture and the volume of hydrogen to be discharged along with a slurry from the reaction tank.

The invention will now be described in more detail by way of example only with reference to the accompanying drawings, in which: Figure 1 shows one example of an apparatus suitable for use in the practice of the process of this invention; Figure 2 is a diagrammatic representation of the relationship between the volume of hydrogen consumed in an exemplary polymerization process at a constant temperature and the intrinsic viscosity of the resulting polymer measured as its tetralin solution; and Figure 3 is a diagrammatic representation of the relationships between the reaction time periods in Examples of the invention and the concentrations of hydrogen in the reaction vessels and the intrinsic viscosities of the resulting polymers.

The term "another cr-olefin copolymerizable with propylene" as used herein means at least one of ethylene, butene-1, hexene-1, etc. and may also be called "copolymerizable a-olefin" hereinafter. When a propylene copolymer is prepared in accordance with the process of this invention, no particular limitation is imposed on the amount of the copolymerizable a-olefin so long as the resulting polypropylene remains in a slurry state. However, the upper limit of the proportion of the copolymerizable a-olefin other than propylene in each resulting polymer may generally be about 40 wt.% or so.For the sake of convenience in describing the present invention, the term "propylene" as used in the descriptive portion of the present specification other than the Examples should be interpreted to include not only propylene alone but also a mixture of propylene and another a-olefin copolymerizable with propylene. Correspondingly, the term "polypropylene" as used in the descriptive portion of the present specification other than the Examples means not only propylene homopolymer but also the copolymer of the mixture.

For the following reasons, the process of this invention finds extremely important utility when propylene is polymerized in the presence of hydrogen as a molecular weight modifier in a reaction tank equipped with a reflux condenser.

In a reaction tank having no reflux condenser, the vapor phase and liquid phase are maintained in vapor-liquid equilibrium and moreover, the vapor phase is in a substantially even state.

Therefore, the concentration of hydrogen in the vapor phase can be accurately determined if the gas of the vapor phase is sampled and its hydrogen concentration is measured. It is hence possible to control the molecular weight of the resulting polypropylene by comparing the thusdetected hydrogen concentration with a desired hydrogen concentration by conventionally-known desired comparator means and on the basis of the results of the comparison, and automatically controlling a feed valve for controlling feed of hydrogen to the reaction tank and thus always introducing a deficient volume of hydrogen into the reaction vessel so as to maintain the concentration of hydrogen in the vapor phase substantially at a constant level.

However, the vapor phase and liquid phase are not always maintained in vapor-liquid equilibrium when a polymerization is conducted using a reaction vessel equipped with a reflux condenser. In addition, the concentration of hydrogen in the vapor phase varies considerably depending on the load to the reflux condenser along the passage of time as mentioned above. As a result, it is impossible to control the molecular weight of the resulting polypropylene if a simple automatic controlling method such as that referred to above is relied upon.

As exemplary polymerization catalysts useful in the practice of this invention, may be mentioned catalyst systems composed of conventionally-known transition metal catalysts and organometallic compounds. One of more stereoregularity improvers may also be used, alone or in combination if necessary or desirable.Although not limited specifically to the following polymerization catalysts, illustrative polymerization catalysts may include titanium trichloride obtained by reducing titanium tetrachloride with a reducing agent such as aluminum; organoaluminum or organomagnesium, those obtained by subjecting titanium trichloride to activation treatments such as its treatments with oxygen-containing organic compounds, titanium tetrachloride and the like subsequent to its grinding; and those formed of titanium trichloride or titanium tetrachloride supported on carriers such as magnesium chloride. As exemplary organometallic compounds, may be mentioned organoaluminums such as trialkylaiuminiums, dialkylaluminum halides, alkylaluminum sesquihalides and alkylaluminum dihalides and organomagnesiums such as dialkylmagnesiums.

One particular embodiment of the present invention is now described with reference to the accompanying drawings.

Fig. 1 illustrates one example of an apparatus suitable for use in the practice of the process of this invention. In Fig. 1 are shown an agitator-equipped reaction tank 1, a reflux condenser 2 in the form of a horizontal shell-and-tube heat exchanger, a jacket 3 for the reaction tank 1 and an inlet line 5 for the introduction of a slurry into the reaction tank 1. Where the reaction tank 1 is employed for single-tank polymerization or is used as the first tank upon polymerization in a plurality of tanks connected in series, the inlet line 5 is used for the introduction of a catalyst slurry. Where the reaction tank 1 is the second or subsequent tank in such a series of reaction tanks, the inlet line 5 is employed for the introduction of a reaction slurry from the preceding reaction tank.Also shown is a discharge line 6 for the removal of a slurry from the reaction tank 1, a charge line 7 for the introduction of propylene and a catalyst, a sampling line 9 for the collection of gas from the vapor region of the reaction tank 1, and a blower 18 adapted to recycle to the reaction tank 1 uncondensed gas which has not been condensed in the reflux condenser 2 and is composed principally of hydrogen gas.Also illustrated are a detector 4-1 for the flow velocity and temperature of gas at the entrance to the reflux condenser 2, another detector 4-2 for the flow velocity and temperature of a condensate returning to the reaction tank 1 subsequent to its recovery in the reflux condenser 2, a flow rate regulating valve 4-3 for hydrogen gas to be introduced into the reaction tank 1, a further detector 4-4 for the flow velocity and temperature of cooling (or heating) water leaving the jacket 3, a still further detector 4-5 for the flow rate and temperature of cooling (or heating) water to be introduced into the jacket 3, an inlet line 10-1 for the introduction of cooling water into the reflux condenser 2, and an outlet line 10-2 for the discharge of the cooling water.

The following procedure may be followed by way of example in order to calculate the amount of a monomer or monomer mixture polymerized per unit time in the reaction tank 1. Data signals a,b,c,d, which have been output from the detectors 4-1,4-2,4-4,4-5 respectively, are input to a data processor 8, where the quantity of heat generated per unit time in the reaction tank 1 at the time of output of the data signals is calculated by correcting the quantity of heat removed per the same unit time from the reaction tank 1, which has been calculated from the data signals a,b,c,d, in accordance with the quantity of dissipated heat which has been calculated based on the overall structure of the polymerization system and its operational conditions.Since the relationship between polymerized amount of the monomer or monomer mixture are reaction heat can be known from the composition of the thus-polymerized monomer or monomer mixture in the manner known per se in the art, the above-mentioned generated heat is converted further at the data processor 8 into the amount of the monomer or monomer mixture polymerized per unit time in the reaction tank 1.

Incidentally, the relationship between the molecular weight of polypropylene of a desired composition and the volume of hydrogen required for the preparation of the polypropylene can be shown as in Fig. 2, namely, can be expressed by the following equation: In Il=ln X+a wherein q: the intrinsic viscosity of the polypropylene as measured in the form of a tetralin solution of 135"C; X: the volume of hydrogen consumed per unit amount of the polypropylene; and A: constant.

It is therefore possible to determine the volume of hydrogen required per unit amount of feed propylene by storing beforehand the above relational expression as an equation in the data processor 8 and then inputting a desired polypropylene molecular weight in the data processor 8.

In the above-described manner, the volume of hydrogen required in the reaction tank 1 is hence calculated at the data processor 8 as the product of the amount of the polymerized monomer or monomer mixture, which has been calculated in advance, and the vdlume of hydrogen required for the unit amount of the feed polypropylene. When the present invention is applied to such a reaction system that a plurality of tanks are connected in series to conduct a continuous polymerization therein and the molecular weight of the resulting polymer is increased successively from one tank to the next tank, hydrogen is dissolved in both slurries which are introduced through the line 5 and discharged through the line 6 respectively and therefore, the hydrogen is introduced and discharged along with the the former and latter slurries respectively.

It is hence necessary to input information on the volumes on the hydrogen in the data processor 8 and to perform a correction on the basis of the information. Namely, an operation is performed at the data processor 8 so that the volume of hydrogen introduced along with the slurry into the reaction tank 1 is subtracted from the sum of the product of the above-obtained volume of hydrogen required per unit amount of polypropylene and the amount of polymerized monomer and the volume of hydrogen discharged along with the slurry from the reaction tank 1. Results of the operation is output as a signal e from the data processor 8.It is therefore possible to control the volume of hydrogen to be introduced into the reaction tank 1, that is, to conduct the reaction while maintaining the concentration of hydrogen in the vapor phase in the reaction tank 1 substantially at a constant level, and hence to prepare polypropylene of a uniform molecular weight, provided that the opening degree of the hydrogen gas flow rate regulating valve 4-3 is adjusted in accordance with variations of the value of the signal e.

For the measurement and calculation of the heat of the polymerization reaction, the quantity of heat removed at the reflux condenser 2 is calculated based on the data signals a,b output from the detectors 4-1, 4-2 in the above embodiment. It is also feasible to calculate the quantity of heat removed at the reflux condenser 2 by detecting the temperatures and flow velocities of the cooling medium for the reflux condenser 2 at the inlet 10-1 and outlet 10-2 respectively and then inputting the thus-obtained data signals to the data processor 8 instead of the abovedescribed signals a,b.

On the other hand, when a single-tank polymerization process is effected in the abovedescribed reaction tank 1 or a polymerization process is conducted by connecting in series a plurality of reaction tanks, each, of the same type as the reaction tank 1, each of the reaction tanks has already been filled with a great deal of propylene not only as a liquid medium but also as a reaction raw material at the start-up time of the reaction. It is therefore impossible to obtain a polymer of a desired molecular weight even if hydrogen is fed in accordance with the present invention, namely, in a volume corresponding to the amount of polymerized propylene which is calculated based on the measured and calculated quantity of heat of the polymerization reaction.While taking into consideration the volume of hydrogen to be dissolved in the liquid propylene filled in each reaction tank at the start-up time and the volume of the vapor phase above the liquid medium, it is thus necessary to charge at once hydrogen in a volume corresponding to the liquid propylene at the beginning so that the polymerization reaction is conducted. The molecular weight of the resulting polypropylene is then measured and compared with a desired value. Based on the results of the comparison, a small amount of hydrogen or propylene is additionally charged in the reaction tank. The above-described fine correction procedure is repeated until the molecular weight of the resulting polypropylene reaches the desired value.The reaction is thereafter allowed to proceed further in accordance with this invention, whereby polypropylene of a constant molecular weight can be prepared.

It is a reaction tank equipped with a reflux condenser that can be used in the practice of the present invention. No particular limitation is imposed on the heat-removing capacity of the reflux condenser. The present invention is particularly effective in a steady state, that is, when it is applied to a reaction tank the temperature of which is controlled by the removal of heat through the reflux condenser while the present invention is being practised.

The present invention is extremely valuable from the industrial viewpoint because polypropylene of a constant molecular weight can be obtained with not only high efficiency but also good controllability by using a reaction tank equipped with a reflux condenser and conducting bulk polymerization of propylene in the presence of hydrogen as a molecular weight modifier in accordance with the process of this invention.

The invention is illustrated further with reference to the following non-limiting examples.

EXAMPLES: Continuous bulk polymerization of liquid propylene was conducted at 70"C in the presence of a catalyst composed of titanium trichloride and diethylalumin chloride in a reaction tank having the structure shown in Fig. 1 and an internal capacity of 40 m3 while using the liquid propylene as a medium.

Upon initiation of the polymerization, 3000 kg of propylene and 35 Nm3 of hydrogen were first charged in the reaction tank. Warm water was caused to flow through the jacket so as to heat the medium up to 70"C. The polymerization reaction was then initiated while charging the catalyst and propylene at constant feed velocities (titanium trichloride: 1.0 kg/hr; diethylaluminum chloride: 16 kg/hr, propylene: 10000 kg/hr). During the reaction, the reaction slurry was sampled from the reaction tank and the molecular weight of the resultant polypropylene was measured. The thus-measured molecular weight was compared with a predetermined value.The molecular weight of the resulting polypropylene was adjusted substantially to the predetermined value by repeating several times a fine correction procedure in which a small amount of hydrogen was charged in the reaction tank on the basis of the results of the above comparison.

About 30 minutes were spent until the predetermined value was reached.

Continuous bulk polymerization of propylene was then conducted in accordance with the process of this invention. Namely, propylene, titanium trichloride and diethylaluminum chloride were charged at constant feed velocities, namely, at 6000 kg/hr, 0.8 kg/hr and 8 kg/hr respectively into the reaction tank. At the same time, a slurry was charged out at about 6000 kg/hr from the reaction tank so as to maintain the level of the slurry constant in the reaction tank. During this polymerization, data signals a,b,c,d were input from the detectors 4-1,4-2,4-4,4-5 into the data processor, which performed both correction of a present quantity of dissipated heat and numeral conversion to the amount of the monomier polymerized per unit time. Furthermore, an intrinsic viscosity 1.73 corresponding to a desired molecular weight of polypropylene as the final product as measured in the form of its tetralin solution of 135"C was input to the data processor. The intrinsic viscosity was then converted in accordance with the above-described equation to the volume of hydrogen required for consumption per unit amount of polypropylene of the desired molecular weight. The product of the volume of hydrogen and the above-determind amount of the polymerized monomer was then calculated. A further correction was performed in view of the volume of hydrogen discharged along with the slurry from the reaction tank. As a consequence, the volume of hydrogen consumed in the reaction tank is output as the data signal e from the data processor.The continuous reaction was carried out while controlilng the volume of hydrogen, which is to be charged into the reaction tank, in accordance with variations in the data signal e.

Fig. 3 diagrammatically illustrates time-dependent variations in the concentration (vol.%) of hydrogen in the vapor phase sampled out through the line 9 shown in Fig. 1 as well as timedependent variations in the intrinsic viscosity of polypropylene in the slurry discharged through the line 6 also depicted in Fig. 1. As envisaged readily from Fig. 3, the concentration of hydrogen in the vapor phase varied but the intrinsic viscosity, namely, the molecular weight was controlled constant.

By the way, the total quantity of heat removed through the jacket and reflux condenser in a steady state, namely, while the process of this invention is practised was 860 Mcal/hr on average, of which about 65% was accounted for on average by the reflux condenser.

Claims (8)

1. A process for the preparation of a propylene homo- or co-polymer by subjecting propylene alone, or a mixture of propylene and another ez-olefin copolymerizable with propylene, as a monomer or monomer mixture to bulk polymerization at a constant temperature in the presence of hydrogen as a molecular weight modifier, the polymerization being performed in a reaction tank equipped with a reflux condenser using the propylene or mixture itself as a liquid medium and condensing vapor of the medium in the reflux condenser so as to remove at least a part of the heat of polymerization, the process further involving:: calculating the amount of the monomer or monomer mixture polymerized per unit time based on the quantity of generated heat calculated as the sum of the quantity of heat removed artificially and the quantity of heat allowed to dissipate naturally, both from the reaction tank per the same unit time; determining the volume of hydrogen required per unit amount of a homo- or co-polymer of a desired molecular weight in accordance with the following equation established beforehand between the intrinsic viscosities Q of propylene homo- or co-polymers measured as their tetralin solutions at 135"C and the volumes (X) of hydrogen consumed respectively per unit amounts of the propylene homo- or co-polymers: In )1=on X+A wherein A is a constant; and controlling the volume of hydrogen, which is to be charged into the reaction tank, in accordance with variations in the volume of hydrogen consumed in the reaction tank calculated as a value which is obtained by subtracting the volume of hydrogen, which is to be introduced along with a slurry into the reaction tank, from the sum of the product of the above-determined volume of hydrogen and the above-calculated amount of the monomer or monomer mixture and the volume of hydrogen to be discharged along with a slurry from the reaction tank.

2. The process according to claim 1, wherein propylene is used as a sole monomer.

3. The process according to claim 1 or claim 2, wherein the polymerization is performed on a continuous basis.

4. The process according to claim 1, 2 or 3, wherein the polymerization is performed in the presence of a catalyst comprising titanium trichloride and diethylaluminum chloride.

5. The process according to claim 1 and according to the Examples herein.

6. A process for the preparation of homo-polymers or co-polymers of propylene, substantially as herein described by way of example with reference to the accompanying drawings.

7. Homo-polymers or co-polymers of propylene when prepared by the process claimed in any of claims 1 to 6.

8. Apparatus for performing the process according to any of claims 1 to 6, constructed and arranged to operate substantially as herein described by way of example with reference to the accompanying drawings.