Ethylene Partial Pressure

Polymerization conditions may vary over a wide range, where polymerization temperatiues vary from 60-110 C and total reactor pressure from 100-350 psig. Comonomers are usually limited to 1 -butene or 1-hexene to control polymer density, but 1-octene may be used if a particular catalyst exhibits a relatively high reactivity with 1-octene such as metallocene catalyst systems. Resin production rates are controlled by primarily two variables catalyst feed rate and ethylene partial pressure. In a modern gas-phase process, operating in what Univation designates as supercondens-ing mode, production rates for many grades of polyethylene of at least 125,000 Ibs/hr are reached. 

There are limits as to the ethylene partial pressure that may be used to increase reactor production rates. For example, an increase in ethylene partial pressure above a certain amount may significantly reduce polymer particle morphology, leading to low resin settled bulk density (post reactor) and low fluidized bulk density within the reactor. Low resin bulk density will reduce conveying rates of granular polyethylene as the resin is transferred to another production step. Lower fluidized bulk density will reduce the amount of granular polyethylene in the reactor, which will reduce catalyst residence time (lower catalyst productivity) at a constant production rate. Limits on comonomer feed rates may also limit the ethylene partial pressure obtainable in order to produce polyethylene with 

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The effect of Pc2H4 and Uwr on the rate of C2H4 oxidation is shown in Fig. 8.5. Increasing UWr causes a pronounced decrease in the ethylene partial pressure, pc2H4> necessary to reduce the surface Rh oxide and thus a dramatic, up to 100-fold, increase in reaction rate for intermediate PC2H4 values (p=100). 

Figure 8.79. Steady-state kinetics of C2H4 oxidation on Pt/Ce02 as a function of catalyst potential, UWR, and ethylene partial pressure (a) catalyst A, T=500°C, pO2=5.0 kPa (b) catalyst C, T=510°C, Pc2h4=4-8 kPa.71 Reprinted by permission of The Electrochemical Society.

An increase in reaction rate with ethylene partial pressure was observed, but does not follow a first-order law. An averaged formal order of 0.53 was calculated [4]. The reaction rate increases with increasing oxygen partial pressure on OAOR-modified silver with an order of 0.78 with respect to oxygen ]4]. 

Polymerization of Ethylene in the Dark by Transition Metal Benzyl Compounds in Toluene at 80°C Ethylene Partial Pressure 10 aim... 

Polymerization of Ethylene by Zirconium Alkyl Halides in Toluene at 80°C. Concentration 3.00 X 10 3 mole liter-1 Ethylene Partial Pressure 10 atm.Hydrogen Partial Pressure 10 atm (9, 16)... 

A catalyst, prepared from isoprenylaluminum and titanium tetrachloride has been used to prepare polymers with a highly irregular shape (5). The polymerization is performed at 70-80°C with an ethylene partial pressure of 0.16-0.27 M Pa. A bulk density of 0.16-0.23 gem-3 is obtained. Such materials are intended to be used as filter elements. 

Jenkins and Rideal (61) have suggested for Ni that the rate-determining step is the adsorption of H2 on a small part of the surface that is not covered with ethylene. The kinetic equation for Ni suggests that this assumption is valid. If, however, it is considered that the same assumption also should cover the reactions over Rh and Ru, difficulties arise, since the negative exponents of the ethylene partial pressures cannot be explained. One should hence make the additional assumption that coverage by ethylene may occur at low temperatures for some metals. Even so, it is possible that other assumptions will just as well explain the experimental facts. 

To complete the picture w7e should investigate if there are more dominant variables in the system. We have already touched on this issue in the section on reaction rates. There we noted that the acetic acid concentration to the reactor is not dominant. We can also argue that the ethylene partial pressure is not likely to be a dominant variable since ethylene enters the reactor in large excess. However, oxygen is the limiting component and it plays a role in the main reaction as wrell as in the side reaction. Oxygen therefore affects the economic objectives and is considered dominant. Feedback control of the oxygen concentration to the reactor is necessary if we wrant complete control of the unit. 

Pol3nnerization conditions batch reactor containing toluene at 25 °C and 1 bar ethylene partial pressure. Activity (A) Not measured. 

The chlorination reaction rate can be limited either by chemical kinetics or ethylene mass transfer, depending on ethylene partial pressure, agitation, catalyst composition, and temperature. If the ethylene partial pressure, and/or agitation rate are low, the transfer rate of ethylene to the catalyst solution will be too small to satisfy the kinetic capabilities of the catalyst, and the system will be mass-transfer limited. As either or both ethylene partial pressure and agitation are increased, the mass transfer rate will increase, and eventually ethylene can be supplied to the system at a rate equal to, or in excess of, the kinetic rate capabilities, and the reaction system will be limited kinetically from this point. The mass transfer and kinetic-limiting regimes have been delineated, and simultaneous feed operations were normally made under conditions where chlorination kinetics—not ethylene mass transfer—was controlling. 

In bench-scale tests, using hoUow-fiber membrane as support and a carrier concentration of 2 M the ethylene permeance was 4.6 X 10 barrer/cm with an ethylene partial pressure of 65 psia, while the selectivity C2H4/C2H6 was about 240. Same tests were carried out for separation of propylene from propane. The selectivity obtained was greater than 100 but this result was confirmed only at bench scale. In fact, in the large pilot system, the selectivity and flux dechned over some weeks due to loss of solvent and carrier and to the necessity of remove hydrogen from the feed gas to prevent reduction of Ag f carrier. Despite the result, this remains the first study on the use of facilitated transport membrane for gas separations on a pilot scale. 

The MW of polymer formed with the Phillips catalyst is not proportional to the ethylene concentration (more precisely, MN is not proportional to the ethylene partial pressure) [32], Doubling the ethylene concentration increases, but does not double, the MN. H transfer to chromium (the upper path in Scheme 13) would mean a linear relationship between Mm and ethylene concentration, in which MN extrapolates to zero at zero ethylene partial pressure. But it does not. In contrast, if chain transfer occurred entirely to monomer (the lower pathway in Scheme 13), then Mn should remain constant (i.e., there should be no dependence on the ethylene partial pressure). Again it does not. The actual response is neither first nor zero order, but in between, indicating that both mechanisms are in operation simultaneously. However, it is the second reaction, hydride transfer to monomer, which dominates, as on many other industrial catalysts. 

The orders of the rate of coke formation at 700 and 900°C were estimated from the measured buildup of coke on the metal coupons relative to the inlet ethylene partial pressure by the folowing equation ... 

The solubility of ethylene in aqueous silver nitrate and potassium nitrate solutions has been measured 49> at 30 °C and 0.945 atm. ethylene partial pressure to determine the zero ionic strength association constants for the following reactions... 

Kinetic data on olefin polymerization by polymer-immobilized zirconocene are scarce. It is generally accepted that homogeneous metallocene catalysts contain uniform active sites however, if they are immobilized on a polymer support, the MWD polymer production becomes broader compared with a homogeneous catalyst [103]. Kinetic analysis of gas-phase ethylene polymerization catalyzed by (CH3)2[Ind]2ZrCl2 bound at a hydroxylated copolymer of styrene with divinylbenzene and previously activated with MAO (0.17 wt.% Zr) has been carried out [104]. The influence of temperature (333 to 353 K), ethylene partial pressure (2 to 6 atm) and MAO level (molar ratio of MAO to zirconium from 2600 to 10,700) were studied. The activity of the catalyst in the gas-phase process changed from 5 to 32 kg PE (g of Zr atm h)It is possible that there are two types of active site. They are stable to temperature and deactivated by the same mechanism. A first-order reaction takes place. The propagation rate constants of two active sites show a similar dependence on temperature. 

Consistent with the preceding discussion concerning sorption and flux reductions by relatively nonin-teracting penetrants, the data shown in Fig. 20.4-11 clearly illustrate the progressive exclusion of CO] from Langmuir sorption sites in poly(mediyl methacrylate) (PMMA) as ethylene partial pressure (og) is increased in the presence of an essentially constant CO] partial pressure of T 0.13 atm. The... 

The mechanism assumes that each site is capable of binding more than one ethylene molecule and that the rate of insertion of ethylene into the growing polymer chain is dependent on the number of ethylene molecules. This mechanism is supported by the fact that second order dependencies on ethylene partial pressure are often found in homogeneous catalysts, thus indicating that two monomer molecules are involved in the mecvhanism until the rate-determining step. 

Consistent with the preceding discussion concerning sorption and flux reductions by relatively noninteracting penetrants, the data shown in Fig. 20.4-11 clearly illustrate the progressive exclusion of CO2 from Langmuir soiption sites in poly(methyl methaciylate) (PMMA) as ethylene partial pressure (Pb) increased in the presence of an essentially constant CO2 partial pressure of 3-05 7 0.13 atm. The tendency of the CO2 soiption shown in Fig. 20.4-11 to decrease monotonically with ethylene pressure provides impressive support for the competition concept on which E. . 4-17) and (20.4-18) are based. Pemreation data are not available for this stem to determine if changes in the values of Dp and Dh occur in the mixed gas situation. If offsettiiig increases in these transport coefficients do not occur in the presence of ethylene, the CO2 permeability will be depressed in the mixed gas peimeation situations. 

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