Process operating limits

Determine in-process operating limits to guarantee acceptable finished product and yield. 

Replication of some or all design conditions is especially critical at this stage. Replication allows us to develop sound estimates of process variation. These variance estimates serve several functions. First, they form the basis for predicting how reproducible the process will be in the plant—they predict the lot-to-lot variation. Variance estimates can also tell us about unstable regions in the reaction space. Are there regions that the plant should avoid because the process cannot be well controlled there Finally, variance estimates will be vital to our establishment of process operating limits. 

Two main operational variables that differentiate the flotation of finely dispersed coUoids and precipitates in water treatment from the flotation of minerals is the need for quiescent pulp conditions (low turbulence) and the need for very fine bubble sizes in the former. This is accompHshed by the use of electroflotation and dissolved air flotation instead of mechanically generated bubbles which is common in mineral flotation practice. Electroflotation is a technique where fine gas bubbles (hydrogen and oxygen) are generated in the pulp by the appHcation of electricity to electrodes. These very fine bubbles are more suited to the flotation of very fine particles encountered in water treatment. Its industrial usage is not widespread. Dissolved air flotation is similar to vacuum flotation. Air-saturated slurries are subjected to vacuum for the generation of bubbles. The process finds limited appHcation in water treatment and in paper pulp effluent purification. The need to mn it batchwise renders it less versatile. 

Montecatini Process. This partial combustion process operates at higher pressure, 405—608 kPa (4—6 atm), than the BASF and SBA processes. The burner dimensions are proportionately smaller. Because of the higher pressure, the danger of premature ignition of the methane—oxygen mixture is higher so that 2 vol % of steam is added to the gas mixture to alter the flammabiUty limits. 

Catalytic dewaxiag (32) is a hydrocrackiag process operated at elevated temperatures (280—400°C) and pressures, 2,070—10,350 kPa (300—1500 psi). However, the conditions for a specific dewaxiag operatioa depead oa the aature of the feedstock and the product pour poiat required. The catalyst employed for the process is a mordenite-type catalyst that has the correct pore stmcture to be selective for normal paraffin cracking. Platinum on the catalyst serves to hydrogenate the reactive iatermediates so that further paraffin degradation is limited to the initial thermal reactions. 

There is, however, only a limited quantity of by-product power available, and for large process operations the demand for power is usually far greater than the simple steam cycle can produce. Many steam system design decisions fall back to the question of how to raise the ratio of by-product power to process heat. One simple approach is to limit the turbines that are used to extract power to large sizes, where high efficiency can be obtained. 

Flash points, lower and upper flammability limits, and autoignition temperatures are the three properties used to indicate safe operating limits of temperature when processing organic materials. Prediction methods are somewhat erratic, but, together with comparisons with reliable experimental values for families or similar compounds, they are valuable in setting a conservative value for each of the properties. The DIPPR compilation includes evaluated values for over 1000 common organics. Detailed examples of most of the methods discussed are available in Danner and Daubert."... 

In the use of temperature measurement for control of the separation in a distillation column, repeatability is crucial but accuracy is not. Composition control for the overhead product would be based on a measurement of the temperature on one of the trays in the rectifying section. A target would be provided for this temperature. However, at periodic intervals, a sample of the overhead product is analyzed in the laboratory and the information provided to the process operator. Should this analysis be outside acceptable limits, the operator would adjust the set point for the temperature. This procedure effectively compensates for an inaccurate temperature measurement however, the success of this approach requires good repeatability from the temperature measurement. 

Other Process Constraints Typical of these constraints are composition requirements, process temperature limits, desired recoveries, and yields. These are frequently the focus of operators. Violation of these constraints and an inability to set operating conditions that meet these constraints are frequently the motivation for the unit analysis. 

Frequently a piece of equipment is used in different processes during its lifecycle. This could result in process conditions that exceed the safe operating limits of the equipment. Equipment inspection may provide a poor prediction of the equipment s useful life and reliability, due to the change of material handled or change in process chemistry over the life of equipment. Batch operations are also characterized by frequent start-up and shut-down of equipment. This can lead to accelerated equipment aging and may lead to equipment failure. This chapter presents issues and concerns related to the safe design, operation, and maintenance of various pieces of equipment in batch reaction systems, and provides potential solutions. 

Use of materials sensitive to shock, high temperature or high pressure. If the material is inadvertently exposed to an unsuitable condition, or if the process moves out of the safe operating limits, it could result in a loss of containment. 

The standard operating procedures demand great attention as they reflect personnel safety issues, safe operating limits and quality considerations. They should be written simply and clearly. The level of detail is determined by the training and experience of the operations staff but should also take into account the hazards inherent in the process. 

Normal operations Any process operations intended to be performed between startup and shutdown to support continued operation within safe upper and lower operating limits. 

The above process is limited to simple shapes whose principal dimension is not more than four times, and preferably less than twice, that of the next largest dimension. More intricate shapes must be made by machining or in some instances by a coining operation which involves stamping a sintered moulding of the same weight and approximate dimensions as the finished part at 320°C. 

You will have to investigate and apply trial and error field tests to resolve some of these problems. When new equipment are installed, it is wise to spend time during a shake-down and start-up period to explore the operational limitations of the process and train operators on how to handle these types of problems. 

It is inherently safer to develop processes with wide safe operating limits that are less sensitive to variations in critical safety operating parameters, as shown in Figure 4.3. Sometimes this type of process is referred to as a forgiving or robust process. If a process must be controlled within a very small temperature band in order to avoid... 

There are few chemical plants that are so forgiving that a control system or a safety interlock system is not required. Process engineers provide controls to assure product yield and quality and maintain safe operating conditions. This type of control system is a BPCS. The BPCS acts to alarm and moderate a high or low operating condition specified by the normal operating limits within the never exceed critical limits. The SIS is provided to shut down or otherwise place the process in a safe state if the BPCS fails to maintain safe operating conditions. A BPCS should not be used as the sole source of a process safety shutdown. 

Process operators may take limited action in danger areas if they (1) have informed the incident command structure of the emergency. (2) have adequate PPE. (3) have adequate training in procedures they are to perform, and (4) employ the buddy system... 

Process technology information will be a part of the process safety information package and should include employer-established criteria for maximum inventory levels for process chemicals limits beyond which would be considered upset conditions and a qualitative estimate of the consequences or results of deviation that could occur if operating beyond the established process limits. Employers are encouraged to use diagrams that will help users understand the process. 

Normal operating pressure Anticipated process operating pressure used to determine pipe diameter requirements and pressure drop limitations for various operating conditions. 

It could be argued that the presence of enhanced protection systems could lead to a plant being operated to its operational limits in order to obtain better yields in the expectation that, if the process entered a dangerous state, it would be tripped automatically. However, the loss of availability that could arise from such a strategy would discourage this type of behavior... 

Figure 1-26. Partial presentation of piping materials specifications for a specific process service. By permission, Borden Chemicals and Plastics, Operating Limited Partnership. (Figure continued on next page)...

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