Defined reboilers

The vertical thermosiphon reboiler is a popular unit for heating distillation column bottoms. However, it is indeed surprising how so many units have been installed with so little data available. This indicates that a lot of guessing, usually on the very conservative side, has created many uneconomical units. No well-defined understanding of the performance of these units exists. Kern s recommended procedure has been found to be quite conservative on plant scale units yet it has undoubtedly been the basis for more designs than any other single approach. For some systems at and below atmospheric pressure operation, Kern s procedure gives inconsistent results. The problem is in the evaluation of the two-phase gas-liquid pressure drop under these conditions. 

The mass balance relationships for the feed plate, the plates in the stripping section, of the column and for the reboiler must, however, be modified, owing to the continuous feed to the column and the continuous withdrawal of bottom product from the reboiler. The feed is defined by its mass flow rate, F, its composition xp and the thermal quality or q-factor, q. The column bottom product is defined by its mass flow rate, W, and composition, xw and is controlled to maintain constant liquid level in the reboiler. 

The amount of liquid vaporized in the reboiler should not be more than 80%, otherwise this will tend to lead to excessive fouling of the reboiler. For kettle reboilers, there is no recirculation. But for thermosyphon reboilers, a recirculation ratio can be defined as ... 

The optimal control of a process can be defined as a control sequence in time, which when applied to the process over a specified control interval, will cause it to operate in some optimal manner. The criterion for optimality is defined in terms of an objective function and constraints and the process is characterised by a dynamic model. The optimality criterion in batch distillation may have a number of forms, maximising a profit function, maximising the amount of product, minimising the batch time, etc. subject to any constraints on the system. The most common constraints in batch distillation are on the amount and on the purity of the product at the end of the process or at some intermediate point in time. The most common control variable of the process is the reflux ratio for a conventional column and reboil ratio for an inverted column and both for an MVC column. 

The input data defining column configurations, feed, feed composition, column holdup, etc. are given in Table 11.10. The reaction is modelled by simple rate equations (Table 11.10). The batch time is 12 hrs (ts). The objective of the study was to maximise the conversion (X) of the limiting reactant and to obtain the main product with purity of 0.7 molefraction by optimising the reboil ratio defined as V/L. The following optimisation problem (PI) was considered. Model type III was considered with chemical reaction. 

The column configurations and other parameters are defined in Table 11.11. Thirteen cases were considered using different initial charge compositions and different product purity specifications (same specifications for light and heavy product). The results are summarised in Table 11.11. The results clearly show the cases (shown in bold) when an IBD column is superior to a CBD column. The optimum reflux or reboil ratio profiles for 3 cases are shown in Figure 11.10. Refer to the original reference for further details. 

Available-Energy Analysis. Using standard techniques (7-8) an available-energy analysis was performed on the system operating at a reflux ratio of 1.2, and shows the overall Second-Law efficiency to be 12.9% (9). The relevant equations for this analysis are given in Table II. The reboiler has a Second-Law efficiency equal to AAi/AAst or 74.2%. The tower efficiency, defined as AAfr>.p/(AA], + A ) is 55.7%. Note that it is of little use to... 

In order to compare different solutions without explicitly calculating the cost, an objective function, OF, that considers product purity, catalyst amount and the sum of the energy supplied to the column in the reboiler and feed was defined, as follows ... 

Case B3 was defined after case B2, by increasing the reboil ratio in such a way that the internal liquid flow at the feed stage was identical to case B. For B3, an increase in the reboil ratio has to be met also by an increase in the reboiler duty, implying a growth in the energy demand of up to 10713 cal/s. 

The way the operating lines are actually constructed depends on how the particular column is specified. Figure 5.7 shows a demonstration column with two feeds, a liquid draw, a vapor draw, a partial condenser and a reboiler, a side cooler, and a side reboiler. Also shown is the Y-X diagram and operating lines for this column. If we assume the distillate rate and composition and the reflux ratio are known, the calculations can be started at the top. A vertical line drawn through the distillate composition intersects the diagonal at a point that defines the upper end of the top operating... 

A column having a completely defined single feed, an overhead product, a bottoms product, a condenser, a reboiler, and a fixed column pressure has three degrees of freedom if the number of trays is variable (Section 5.2). The column model depicted in Figure 5.3 meets these definitions and is used here to describe the method. Note that the condenser in this model is a partial condenser, so that the distillate is a saturated vapor. With three degrees of freedom, the column requires three specifications to define its operation. Let the condenser duty, q the distillate composition, 7, and the bottoms composition, Xg, be specified. 

Continuing with the procedure of defining equilibrium stages down the column, the pivot point is switched from D to B when the feed tray is crossed. Alternatively, the procedure could be started at the reboiler, defining stages up the column and switching from B to D as the feed tray is crossed. 

The column operable range is determined in part by the requirement that no tray be allowed to dry up, that is, liquid and vapor must exist on each tray to maintain phase equilibrium. This range may be defined by the limits over which the condenser and reboiler duties may vary. As such, the condenser and reboiler duties are considered the two independent variables required to define the column performance (Sections 3.2.3 and 5.2.1). Alternatively, other pairs of variables may be chosen as the independent variables defining the column performance and each set can vary... 

How many variables must be specified in order to define the performance of an existing single-feed, two-product column with a partial condenser (vapor distillate only), a reboiler, and a fixed pressure profile The feed rate, composition, and thermal conditions are also fixed. How would you conceptually control the column operation ... 

Each side product provides one additional independent column variable. To define the column performance, the flow rate of each side product must be known. Alternatively, a side product flow rate may be allowed to vary in order to meet a performance specification such as the concentration of a component in that product. The side product flow rate becomes a dependent variable which must be calculated to satisfy the performance specification. It has been established in Chapter 7 that a fixed-feed, fixed-configuration, fixed-pressure column with a partial condenser (having only a vapor distillate) and a reboiler has two degrees of freedom. Two variables, such as the condenser and reboiler duties, may be varied independently. Each side product adds to the column one degree of freedom. Hence, a column as defined above with S side products has 2 + S degrees of freedom. The duties and side product flow rates can each be varied independently, allowing 2 -i- S performance specifications. This conclusion can be reached by applying the description rule since each additional product rate can be controlled independently by external means. 

The column has four products, two of which are side draws. Since it also has a partial condenser and a reboiler, it has a total of 2 -I- S, or 4, degrees of freedom and, therefore, four performance specifications are required to define its operation. 

An existing column has a reboiler, no condenser, and two feeds, one of which is at the top of the column. The top feed is of fixed temperature, pressure, and composition, but its rate is variable. The other feed is of fixed rate, composition, and pressure, and flows through a heat exchanger upstream of the column so that its temperature may be controlled before entering the column. How many independent variables are required to define this process If the key components are ethane and propane, describe conceptual control loops to control the separation and performance of this process. 

A distillation column with a total condenser and a reboiler separates a stream into a distillate product, heavy residuals in the bottoms, and a main intermediate product as a side draw several trays above the reboiler. For an existing column, with a feed of fixed rate, composition, and thermal conditions, determine the number of variables required to define the column operation. Assume the column pressure is set, and no heat transfer occurs except in the condenser and reboiler. How would you control the separation in the column ... 

A column is designed for/feeds, a partial condenser with only a vapor product, a reboiler, a bottoms product, i side draws, and h pumparound heaters/coolers. Assuming the feeds are of fixed rates, compositions, and thermal conditions, determine the number of independent variables required to uniquely define the column during design, and its operation after construction. 

A stream containing benzene, toluene, and biphenyl is to be separated in a distillation column to produce purified benzene in the distillate. The separation will take place in an existing column with a total condenser, a partial reboiler, and several optional feed locations. The feed stream is of fixed flow rate, composition, and thermal conditions. The entire feed may be introduced at any one of the available feed trays, but may not be split and introduced at more than one feed tray. The condenser pressure is controlled by an inert gas flowing in and out of the reflux drum. Using column modules representation, determine the degrees of freedom for this operation, and recommend a set of specifications to define the column performance. 

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