Flows and Compositions

A reflux reduction of 15% is typical. Improved control achieves this by permitting a reduction in the margin of safety that the operators use to handle changes in feed conditions. The key element is the addition of feed-forward capabiUty, which automatically handles changes in feed flow and composition. One of the reasons for increased use of features such as feed-forward control is the reduced cost of computers and online analy2ers. 

For certain simplified cases it is possible to calculate directly the number of stages required to attain a desired product composition for a given set of feed conditions. For example, if equilibrium is attained in all stages and if the underflow mass rate is constant, both the equilibrium and operating lines on a modified McCabe-Thiele diagram are straight, and it is possible to calculate direc tly the number of ideal stages required to accommodate arw rational set of terminal flows and compositions (McCabe, Smith, and Harriott, op. cit.) ... 

FIG. 20" 7 Normal closed-circuit continuous grinding system with stream flows and composition matrices, obtained hy solving material-halance equations. [Callcott, Trans. Inst. Min. Metall., 76(1), Cl-11 (1967).]... 

During this preparation stage, analysts will frequently find that there is insufficient quantity or quality of measurements to close the material balance. Analysts should make eveiy effort to measure aU stream flows and compositions for the actual test. They should not rely upon closing material balances by back-calculating missing streams. Tne material balance closure will provide a check on the vahdity of the measurements. This preparatoiy material balance will help to identify additional measurements and schedule the installation of the additional instruments. 

The measurements and flows can be adjusted to close the constraints. These adjustments can then be compared to the measurements to determine whether any are reasonable. Statistical routines or hand adjustments are possible. These adjusted flows and compositions might form the basis for the interpretation step bypassing any deeper reconciliation and rectification. This is particularly appropriate where many compositions are left unmeasured and those that are... 

Full documented formal test runs at design and normal conditions. All instruments need to be calibrated for the runs and the documentation should include more than just the primary process variables of temperature, pressure, flow, and composition... 

Equalizing basin A holding basin in which variations in flow and composition of liquid are averaged. Such basins are used to provide a flow of reasonably uniform volume and composition to a treatment unit. Also called a balancing reservoir. Estuaries Bodies of water which are located at the lower end of a river and are subject to tidal fluctuations. 

Material balances are the basis of process design. A material balance taken over the complete process will determine the quantities of raw materials required and products produced. Balances over individual process units set the process stream flows and compositions. 

Algebraic symbols are assigned to all the unknown flows and compositions. Balance equations are then written around each sub-system for the independent components (chemical species or elements). 

Three variables are specified the feed flow and composition fixes the flow of each component in the feed. 

This section is a general discussion of the techniques used for the preparation of flowsheets from manual calculations. The stream flows and compositions are calculated from material balances combined with the design equations that arise from the process and equipment design constraints. 

The extent to which the flow-sheet can be drawn up before any work is done on the detailed design of the equipment will depend on the complexity of the process and the information available. If the design is largely a duplication of an existing process, though possibly for a different capacity, the equipment performance will be known and the stream flows and compositions can be readily calculated. For new processes, and for major modifications of existing processes, it will only be possible to calculate some of... 

Some examples of how design constraints can be used to determine stream flows and compositions are given below. 

If the yields and conversions are known, the stream flows and compositions can be calculated from a material balance see Example 2.13. 

It is often possible to make a material balance round a unit independently of the heat balance. The process temperatures may be set by other process considerations, and the energy balance can then be made separately to determine the energy requirements to maintain the specified temperatures. For other processes the energy input will determine the process stream flows and compositions, and the two balances must be made simultaneously for instance, in flash distillation or partial condensation see also Example 4.1. 

The split-fraction coefficients can be estimated by considering the function of the process unit, and by making use of any constraints on the stream flows and compositions that arise from considerations of product quality, safety, phase equilibria, other thermodynamic relationships and general process and mechanical design considerations. The procedure is similar to the techniques used for the manual calculation of material balances discussed in Section 4.3. 

A feed to a column has the composition given in the table below, and is at a pressure of 14 bar and a temperature of 60°C. Calculate the flow and composition of the liquid and vapour phases. Take the equilibrium data from the Depriester charts given in Chapter 8. 

Sorel (1899) first derived and applied the basic stage equations to the analysis of binary systems. Figure 11.4a shows the flows and compositions in the top part of a column. Taking the system boundary to include the stage n and the condenser, gives the following equations ... 

Figure 11.4. Column flows and compositions (a) Above feed (b) Below feed...

Start by considering the material balance for the part of the column above the feed, the rectifying section. Figure 9.6 shows the rectifying section of a column and the flows and compositions of the liquid and vapor in the rectifying section. First, an overall balance is written for the rectifying section (assuming L and V are constant, i.e. constant molar overflow) ... 

Process safety information is compiled and made available to ah employees to facilitate the understanding and identification of hazards. This information includes block flow diagrams or process flow diagrams, process chemistry, and process limitations, such as temperatures, pressures, flows, and compositions. Consequences of process deviations are also required. This... 

The equations for modeling the 2-D rib effects require a domain where the boundary conditions in terms of gas flow and composition are specified only at the channel. At the solid rib, there is no flux of gas and liquid, but all of the electronic current must pass through it. Furthermore, the modeling domain is usually as shown in Figure 16b thus, only a half channel and rib is modeled, and symmetry conditions can be used to model the other half. Besides those noted above, the boundary conditions and equations are more-or-less the same as those discussed in section 4. 

The liver secretes about 1 L of bile daily. Bile flow and composition depend on the secretory activity of the hepatic cells that line the biliary canaliculi. As the bile flows through the biliary system of ducts, its composition can be modified in the ductules and ducts by the processes of reabsorption and secretion, especially of electrolytes and water. For example, osmotically active compounds, including bile acids, transported into the bile promote the passive movement of fluid into the duct lumen. In the gallbladder, composition of the bile is modified further through reabsorptive processes. 

Figure 14.10. Algorithm for computing flows and compositions in an extraction battery of a specified number of stages (after Henley and Seader, 1981).

Figure 15.26. Continuous and UOP simulated continuous moving bed liquid adsorption processes [Broughton, Sep. Sci. Technol. 19, 723-736 1984-1985)]. (a) Continuous moving bed liquid adsorption process flows and composition profiles, (b) UOP Sorbex simulated moving bed adsorption process.

One of the major technical problems that had to be overcome to integrate the POLYBED system with the steam reformer was the variation in tail gas flow and composition. Because of the cyclic nature of the process, tail gas is rejected by the POLYBED unit during blowdown and purge with significant flow and composition variations. The fluctuations would have made it impossible to use the tail gas for fuel and a sophisticated system was developed to balance tail gas heating value. This buffer/mixing tank system has proven to be very reliable in holding heat input variation to 1% (2). ... 

The above equations allow us to solve for Tc, 7), Tan, Tca, Nh2, Nh2o- No2 (bars have been dropped) as a function of time. As in the model of the prior section, an additional assumption is needed namely, that the Tumped internal conditions for the anode and cathode gases will be the average of the inlet and outlet values. The given parameters for this analysis are all the cell design parameters (geometry, materials, properties, etc.), the input temperatures, pressures, mass flow, and compositions of the anode and cathode gases, and the load current on the cell. Such a simple set of coupled ordinary differential equations is readily solved via Matlab-Simulink, and a sample case is presented in Section 9.5. 

At this point all the units in the flowsheet are installed and converged. The last issue is to converge the recycle stream. The initial guessed values are adjusted to be close to the calculated values of flow and composition leaving the split S1. When these two streams are fairly close, the source of the recycle stream is defined as the split SI and the recycle stream is defined as a Tear stream. The flowsheet did not converge when the default convergence method... 

In this example, the five manipulated variables are so assigned to the five controlled variables that the heat input at the reboiler and the distillate composition are fixed and therefore the bottoms flow and composition are allowed to change with the variations in feed flow or composition. 

Hengstebeck a (15) procedure extends the x-y diagram to multicomponent distillation, A multicomponent separation is treated as a binary separation between the keys, Flows and compositions are based on the two keys alone, that is,... 

This is an important concept and is generic to many chemical processes. From the viewpoint of individual units, chemical component bal ancing is not a problem because exit streams from the unit automatically adjust their flows and compositions. However, when we connect units together with recycle streams, the entire system behaves almost like a pure integrator in terms of the reactants. If additional reactant is fed into the system without changing reactor conditions to consume the reactant, this component will build up gradually within the plant because it has no place to leave the system. 

However, we see in this strategy that there is no flow controller anywhere in the recycle loop. The flows around the loop are set based upon level control in the reactor and reflux drum. Given what we said above, we expect to find that this control structure exhibits the snowball effect. By writing the various overall steady-state mass and component balances around the whole process and around the reactor and column. wre can calculate the flow of the recycle stream, at steady state, for any given fresh reactant feed flow and composition. The parameter values used in this specific numerical case are in Table 2.1. 

With the control structure in Fig. 2.6 and the base-case fresh feed flow and composition, the recycle flowrate is normally 260.5 moles/h. However, the recycle flow must decrease to 205 moles/h when the fresh feed composition is 0.80 mole fraction A. It must increase to 330 moles/h when the fresh feed compositon changes to pure A. Thus a 25 percent change in the disturbance (fresh feed composition) results in a 60 percent change in recycle flow. With this same control structure and the base-case fresh reactant feed composition, the recycle flow drops to 187 moles/h if the fresh feed flow changes to 215 moles/h. It... 

If we select temperature, we would like the reactor flow and composition to be nearly constant and we are constrained by the upper reactor temperature limit of 1300°F. If we select toluene composition, we can control it either directly or indirectly. If directly, a reactor feed composition analyzer is needed and is used to adjust either the fresh toluene feed rate or the total reactor toluene feed rate. If indirectly, the separation section is used as an analyzer for toluene. This allows us to control the total flow of toluene to the reactor (recycle plus fresh). Fresh toluene feed flow is used to control toluene inventory reflected in the recycle column overhead receiver level as an indication of the need for reactant makeup. Controlling the total toluene flow sets the reactor composition indirectly and is advantageous because it is less complicated and does not require an on-line analyzer. 

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