A computer method has been developed to calculate the minimum reflux rate for a specified distillation separation. The method makes no unnecessary simplifying assumptions of the distillation process model. The limitation of the method is that it cannot solve problems in which the minimum reflux rate is determined by a point of tangent between the operating line and the equilibrium line on a McCabe-Thiele diagram (see Figure 1) SCOPEThe calculation of minimum reflux rates for specified distillation separations has long been one of the important problems in distillation calculations. The concept of minimum reflux is quite basic but useful in the design of distillation processes. Columns can be designed quickly and with confidence if the minimum reflux rate is known. Obviously, the designed reflux rate must always be greater than the minimum rate by a certain amount. This amount should be selected with considerations based on the certainty of the vapor-liquid equilibrium model as well as comparative costs of capital vs. utilities. If the equilibrium model in use is not accurate, or if costs of utilities are relatively low, one should choose a large ratio (1.3 or larger) of actual to minimum reflux. A known minimum reflux rate can therefore reduce significantly operating, start-up, and design costs of a distillation process.Most of the well-known work published in the area of minimum reflux calculations appeared before the advent of high speed computers. Developed primarily for manual or low speed computer calculations, these methods used simplifying assumptions in order to save time. Bachelor (1957) presented perhaps the best hand calculation method to date. His method did not contain any of the above assumptions; however, it used simplifying approximations in equilibrium and heat and material balances. Erbar and Maddox (1962) published a computer method using Bachelor's method with added refinements but did not include problems of superheated or subcooled feeds. CONCLUSIONS AND SIGNIFICANCEThe method developed in this work is designed for use on a high speed computer and is rigorous in the sense that:1. The vapor-liquid equilibrium relationship may be represented by any nonideal model. 2. Any component or mixture enthalpy model may be used.3. All or any two components may be distributed between the distillate and the bottoms.4. Plate material and energy balances are used where needed.5. Overall column material and energy balances are satisfied at convergence. The solution speed obviously depends on the complexity of physical property and equilibrium models used for the problem. But, in general, this method requires more computer time than is required by a rigorous distillation simulation program.The most important and restrictive assumption used in the development of this method is that the pinch zone cannot be at a point where the operating line is tangent to the equilibrium surface (see Figure 1). Our experience indicated that the use of this method for such a problem resulted in large solution oscillations such...

Control of sustained limit‐cycle instability in crystal size distribution (CSD) was simulated for a class II (high yield) crystallizer equipped with a fines destruction system and product classifier. Control was simulated by proportional control of nuclei density using fines destruction rate as the manipulated variable. The control constant necessary to eliminate instability was theoretically predicted and agreed with the constant found via simulation. Poorer control of CSD (inability to completely eliminate limit cycles) was predicted using slurry density and slurry withdrawal rate as the measured and manipulated variables, respectively. Development of techniques for fine crystal population measurements to estimate nuclei density are necessary for implementation of the former control scheme. The suggested nuclei density control scheme is effective both in minimizing CSD transients and for elimination of instability.

No abstract

A mathematical model was developed which predicts hydrocarbon breathing losses from fixed-roof storage tanks containing crude oil. The model was able to predict within 10% error the hydrocarbon emission data taken from existing in-field fixed-roof storage tanks by Western Oil and Gas Association in 1977. Due to the success of the model, it was concluded that gas inside a fixed-roof tank is stratified and not mixed. The upper strata near the tank top is most Important in regulating emissions while the lower strata near the liquid surface plays a lesser role in determining tank emissions.

No abstract