Process Operating Variables

Temp ature Converaon of ammonium carbamate to urea in the absence of excess ammonia increases with temperature to a masdmum of about 50% at 170°-190°C when the pressure is sufficiently high to Ireep the reactants in the liquid state. The rate of reaction increases with temperature it is slow at 150°C and below (with stoichiometric NH3 C02 ratio) and quite rapid at 210°C. A satisfectory approach to equilibrium can be obtained in the temperature range of 180°-2(K) C in 0.3-1.0 hours or at lower temperatures with excess ammoiua. Corrosion difficulties increase with temperature, and a range of 180°-210°C is generally accepted as optimum for most processes. 

Ptessure - At constant temperature, converaon increases with pressure up to the critical point which is the point at which the vapor phase is substantially eliminated and the reactants are in the liqiid state. A further increase in pressure is not benefidal. Tlie critical temperature is a complex function of the temperature and composition of the reactor s contents. For example, at 150°C a pressure of about 100 atm might be near opti-mum tor a stoichidme lric NH3-.C02 ratio, but at this temperature the rate of reaction is unacceptably slow. At the preferred temperature of 180°-210 C, pressures of 140-250 atm are commonly used. 

Mole Ratio of NH3iC02 - Excess NH3 above the stoichiometric mole ratio of 2 favors the rate of the reaction. The percentage of CO2 converted to urea is increased but, of course, the percentage of NH3 converted to urea is decreased. Because recycling of excess NH3 is relatively simple (as compared with that of. CO2, which remains as carbamate), most processes use 50% or more excess ammonia (a mole ratio of 3 1 or more). Today all processes account for a balance of both CO2 and NH3 conversion to reduce total recycling to a minimum plantwide (i.e., reactor and synthesis). 

Other Factors - The presence of water decreases conversion therefore, most processes are designed to minimize the amount of water recycled to the reactor. The presence of small amounts of oxygen decreases corrosion all processes use this method of minimizing corrosion. 

Optimum Conditions - It is generally not economical to maximize the percentage conversion in the reactor because this would require an excesdve retention time. The aim therefore is to attain maximum quantity 

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Control of the process, i.e., prevention of hazardous conditions in process operating variables by utilizing automatic control and relief systems, interlocks, alarms, etc. 

Since the characteristics of the linear polyolefin dispersions that are being studied and produced vary over wide limits, the selection of the proper centrifuge for a specific separation becomes an important consideration. The performance of a given centrifuge and the economics of its use can frequently be improved by a factor of 2 or more by a minor change in reactor conditions without detrimental effect on the product itself. The role of the pilot plant in this connection cannot be overemphasized. In many cases, pilot plant-size centrifuges have been used to monitor reactor conditions and other process operating variables with substantial economies in the final process. 

When or SPE charts exceed their control limits to signal abnormal process operation, variable contributions can be analyzed to determine which variable (s) caused the inflation of the monitoring statistic and initiated the alarm. The variables identified provide valuable information to plant personnel who are responsible for associating these process variables with process equipment or external disturbances that will influence these variables, and diagnosing the source causes for the abnormal plant behavior. The procedure and equations for developing the contribution plots was p-resented in Section 3.4. 

The relationship among process operation variables and final molecular and/or end-use properties of polymer materials is strongly non-linear, which means that the classical linear control theory is of limited use in the polymerization field. For this reason, advanced non-linear control techniques should be used in many process applications. 

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