A graphical method, system, and computer readable medium for determining the number of trays in a distillation column

Abstract

The present application relates to rectifying column processing technical field, especially to a kind of rectifying column tray number graphic method, system and computer readable medium, including to rectifying section make balance frame, construct rectifying section operating line equation;To stripping section make balance frame, construct stripping section operating line equation;With light component conservation in whole column material balance, by simplifying and to feed plate material and energy balance, obtain q line equation;The phase equilibrium of extractive rectification is calculated, and vapor-liquid equilibrium diagram is obtained;When satisfying each tray in tower satisfies constant molar flow and ignoring the content of extraction solvent in gas phase, vapor-liquid equilibrium diagram becomes gas-liquid equilibrium diagram;Rectifying section, stripping section operating line equation and q line equation are made with gas-liquid equilibrium diagram, and the tray number of extractive rectifying column is calculated.The present application can solve the problem of the calculation precision of important design parameters such as existing method influence extractive rectifying column solvent amount, theoretical plate number etc..

Description

Technical Field

[0001] This invention relates to the field of distillation column processing technology, and in particular to a method, system, and computer-readable medium for illustrating the number of trays in a distillation column. Background Technology

[0002] Distillation is a crucial mass transfer unit operation in chemical engineering principles, and calculating the theoretical plate number is an important aspect of distillation column design. For ideal binary systems with constant relative volatility or known phase equilibrium, the theoretical plate number and the vapor-liquid phase composition on each plate can be obtained through equations. However, for non-ideal systems, separation is difficult using ordinary distillation. In such cases, separation can be achieved by introducing an extractant to alter the relative volatility of the components to be separated.

[0003] Currently, graphical and simplified methods are commonly used in extractive distillation, but both are based on solvent removal methods. However, solvent removal methods only consider the effect of solvent on relative volatility, but do not consider the effect of solvent on liquid-vapor ratio. This will affect the calculation accuracy of important design parameters such as solvent quantity and theoretical plate number of extractive distillation column, and sometimes the calculation results will be contradictory.

[0004] In addition, existing calculation methods are not applicable to special distillation sections and cannot be better applied to extractive distillation. Summary of the Invention

[0005] To address the shortcomings of existing methods, the technical solution adopted in this invention is: a method for graphically calculating the number of trays in a distillation column, comprising the following steps:

[0006] Step 1: Construct a balance box for the rectification section and build the operating line equation for the rectification section;

[0007] Furthermore, the formula for the operating line equation of the rectification section is as follows:

[0008]

[0009] Where R is the reflux ratio, x n x represents the composition of the light components in the liquid phase descending from the nth tray in the rectification section. s x represents the composition of the solvent in the liquid phase descending from the trays in the rectification section. D It consists of light components from the distillate at the top of the column.

[0010] The operating line equation for the rectification section of the extractive distillation column is a straight line, but the slope of the equation has an additional term (1-x) compared to that of ordinary distillation. s The value (x) represents the effect of the solvent on the operating line, which can be adjusted by changing x. s The value of the slope is adjusted, thereby adjusting the operating line equation of the rectification section, and ultimately adjusting the number of trays in the distillation column.

[0011] Step 2: Construct a balance frame for the stripping section and build the operating line equation for the stripping section;

[0012] Furthermore, the operating line equation for the stripping section is:

[0013]

[0014] Where, x m-1 Let V' be the liquid phase composition descending from plate m-1 in the stripping section, V' be the vapor rising from plate m in the stripping section, L' be the amount of liquid phase descending from plate m in the stripping section, W be the distillate from the bottom of the column, and x be the liquid phase composition. w This refers to the composition of the distillate from the bottom of the column.

[0015] Step 3: Using the conservation of light components in the overall tower material balance, the q-line equation is obtained through simplification and material and energy balance of the feed plate;

[0016] Furthermore, the formula for the q-line equation is:

[0017]

[0018] Where q is the feed thermal state parameter, x F It is a volatile component in the raw material liquid.

[0019] Step 4: Calculate the phase equilibrium of extractive distillation to obtain the vapor-liquid phase equilibrium diagram;

[0020] Step 5: When the constant molar flow is satisfied for each tray in the column and the content of extraction solvent in the gas phase is ignored, the vapor-liquid phase equilibrium diagram is changed to a gas-liquid phase equilibrium diagram.

[0021] Step 6: Plot the operating line equations and q-line equations of the rectifying section and stripping section against the gas-liquid equilibrium diagram, and calculate the number of trays in the extractive distillation column.

[0022] Furthermore, the distillation column tray number diagram system includes: a memory for storing instructions executable by a processor; and a processor for executing the instructions to implement the distillation column tray number diagram method.

[0023] Furthermore, a computer-readable medium storing computer program code, which, when executed by a processor, implements a method for illustrating the number of trays in a distillation column.

[0024] The beneficial effects of this invention are:

[0025] 1. This invention uses simulation software to predict the phase equilibrium of the system, improves the operating line equations of the rectification section, the stripping section, and the q-line equation, and incorporates the operational equations into the phase diagram for graphical interpretation. This can improve the calculation accuracy of important design parameters such as solvent quantity and theoretical plate number of the extractive distillation column, and greatly reduce separation difficulty and energy consumption. Attached Figure Description

[0026] Figure 1 This is a flowchart of the method, system, and computer-readable medium for illustrating the number of trays in a distillation column according to the present invention.

[0027] Figure 2 (a) and Figure 2 (b) are diagrams of the rectifying section and the stripping section of the extractive distillation column, respectively.

[0028] Figure 3 The x, y phase diagram of the ethanol component after solvent removal;

[0029] Figure 4 This is the x,y phase diagram of the ethanol component at the actual concentration.

[0030] Figure 5 This is a diagram illustrating the extractive distillation process. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings and embodiments. The drawings are simplified schematic diagrams, which only illustrate the basic structure of the present invention in a schematic manner, and therefore only show the components related to the present invention.

[0032] To model an extractive distillation column, several assumptions are needed to facilitate problem handling:

[0033] Assumption 1: The column plates satisfy constant molar flow: that is, the amount of vapor rising from the plates in the rectifying section is V, and the amount of liquid phase falling from the plates in the rectifying section is L; the amount of vapor rising from the plates in the stripping section is V', and the amount of liquid phase falling from the plates in the stripping section is L'.

[0034] Assumption 2: The solvent has a high boiling point, the content of the extraction solvent in the gas phase is very small and negligible, and the molar content of the solvent in the liquid phase remains constant. s x represents the composition of the solvent in the liquid phase descending from the trays in the rectification section. s 'This refers to the composition of the solvent in the liquid phase descending from the trays in the stripping section;'

[0035] Assumption 3: For ease of calculation, assume that the solvent enters the column from the top;

[0036] Assumption 4: The vapor-liquid two-phase composition on the tray satisfies phase equilibrium.

[0037] like Figure 1 As shown, a method for graphically calculating the number of trays in a distillation column includes the following steps:

[0038] Step 1: Construct a balance box for the rectification section and build the operating line equation for the rectification section;

[0039] like Figure 2In (a), the stream entering the balance box contains extractant S and vapor V rising from the (n+1)th tray. The composition of vapor V is y. n+1 The stream leaving the balance frame consists of a liquid phase descending from the nth tray, containing a light component composition x. n The composition of the solvent in the liquid phase descending from the trays in the rectification section (x) s And the composition of the overhead distillate D and the corresponding light components of the overhead distillate x D Considering the reflux liquid l from the top of the column and the solvent entering the extractive distillation column, the overall material balance equation is derived as follows:

[0040] S+V=D+L (1)

[0041] From assumption 2, we can conclude that solvent material is conserved, and therefore we can obtain:

[0042] S = Lx s (2)

[0043] In addition, the light components in the distillation column also satisfy the material conservation law, that is:

[0044] Vy n+1 =Dx D +Lx n (3)

[0045] Substituting equation (2) into equation (1) yields:

[0046] V = D + L(1 - x s (4)

[0047] Equation (3) becomes:

[0048]

[0049] Material balance shows that l = L(1-x) s ), define the reflux ratio as R = l / D; transform equation (5) into:

[0050]

[0051] Further simplification yields:

[0052]

[0053] As can be seen from equation (7), the operating line equation of the rectification section of the extractive distillation column is a straight line equation, but the slope of the equation has one more term (1-x) compared with that of ordinary distillation. s This represents the effect of the solvent on the operating line, and the equation passes through two fixed points (0, x). D / (R+1)) and (x D (1-x s ),x D ),like Figure 5 Points e and b in the diagram.

[0054] Where V represents the vapor rising from the plates in the rectification section, L represents the amount of liquid phase descending from the plates in the rectification section, and x s The solvent composition in the liquid phase descending from the tray in the rectification section is given by: S = extractant, y = y. n+1 Let x be the composition of the vapor V rising from the (n+1)th tray in the rectification section. n Let x be the composition of the light components in the liquid phase descending from the nth tray in the rectification section, and let D be the distillate from the top of the column. D l represents the light component composition of the distillate from the top of the column, and l represents the reflux liquid from the top of the column.

[0055] Step 2: Construct a balance frame for the stripping section and build the operating line equation for the stripping section;

[0056] Figure 2 (b) The stream entering the balance frame is the liquid phase L' descending from the (m-1)th plate, and its composition is x. m-1 The solvent composition is x s ', Leaving the balance frame is the gas phase V' rising from the m plates, whose composition is y m and the distillate W and x from the bottom of the column w The overall material balance equation for the entire tower is:

[0057] L'=V'+W (8)

[0058] For the light components in the extractive distillation column, the material balance equation is as follows:

[0059] L'x m-1 =V'y m +Wx W (9)

[0060] Equation (9) can be rewritten as:

[0061]

[0062] This shows that the stripping section also follows a linear equation, passing through two fixed points (0, -Wx). W / V') and (x w x w ), fixed point (x) w x w )like Figure 5 Point c in the middle.

[0063] As can be seen from equations (5) and (10), the operating line equations of the extractive distillation column are consistent with those of ordinary distillation, both being linear equations, and the physical meaning of the slope of the equations is the same, representing the liquid-vapor molar ratio. However, the distillation operating line at this time includes the influence of the solvent; mainly, the liquid phase contains solvent, which affects the slope of the operating line; therefore, it needs to be distinguished from the operating line equations of ordinary distillation.

[0064] Where, x m-1 x represents the liquid phase composition descending from plate m-1 in the stripping section. s ' represents the composition of the solvent in the liquid phase descending from the tray in the stripping section, W represents the distillate from the bottom of the column, and x represents the solvent composition. w The composition of the distillate in the bottom of the column, x F V' represents the composition of the volatile components in the feed liquid, V' represents the vapor rising from the plate in the stripping section, L' represents the amount of liquid phase descending from the plate in the stripping section, and y represents the vapor volume. m The composition of the gas phase rising from plate m.

[0065] Step 3: Using the conservation of light components in the overall tower material balance, the q-line equation is calculated by simplification and material and energy balance of the feed plate;

[0066] The q-line equation is the locus equation of the intersection point of the rectifying section operating line equation and the stripping section operating line equation, which can be obtained through calculation; it is also obtained by utilizing the conservation of light components in the overall material balance of the column.

[0067] Fx F =Dx D +Wx W (11)

[0068] Substituting equation (11) into equation (10) yields:

[0069] L'x m-1 =V'y m +Fx F -Dx D (12)

[0070] Using equation (3) + equation (12), we can obtain:

[0071] Vy + L'x = Lx + V'y + Fx F (13)

[0072] Simplifying it further, we get:

[0073]

[0074] Based on the material and energy balance of the feed plate, we can obtain the following:

[0075] L'=L+qF (15)

[0076] V=V'+(1-q)F (16)

[0077] Substituting equations (15) and (16) into equation (14), we get:

[0078]

[0079] Equation (17) shows that the q-line equation for the extractive distillation process must pass through a fixed point (x). F ,x F ),like Figure 5 At point g, this conclusion is consistent with the q-line equation of ordinary distillation. Considering that the feed thermal state has a significant impact on the liquid phase load of the rectifying and stripping sections, which in turn affects the content of the extraction solvent in each section of the column, and thus affects the relative volatility of the components to be separated, causing difficulties in column design and operation, it is necessary to ensure that the molar content of the solvent in the entire extractive distillation column remains constant as much as possible to facilitate design and control. Therefore, in engineering, the feed thermal state is usually selected as saturated steam feed, i.e., q = 0.

[0080] Where F is the raw material flow rate, x F q represents the volatile component in the feed liquid, and q is the feed thermal state parameter.

[0081] Step 4: Predict the phase equilibrium of the system using Aspen software;

[0082] For binary systems with relative volatility close to 1.0 or exhibiting azeotropic properties, the introduction of a third component as an extractant becomes highly non-ideal, making calculations extremely complex. Therefore, Aspen software is used to calculate the phase equilibrium of extractive distillation. Taking an ethanol-water separation system as an example, conventional distillation cannot yield ethanol with a purity exceeding 90%. Figure 3 It is the mole fraction of the extractant ethylene glycol x s The x,y phase diagram showing the effect of ethylene glycol on the vapor-liquid equilibrium of the ethanol-water system at a value of 0.5, calculated under a pressure condition of 101.3 kPa; the NRTL thermodynamic model was used to... Figure 3 As can be seen, the azeotropic point of the ethanol-water system disappears at this point, and separation can be achieved through distillation. However, the diagram does not include the influence of the ethylene glycol component, so this phase diagram cannot be directly used to solve for the theoretical plate number of a distillation column.

[0083] Step 5: Modify the phase diagram obtained in Step 4 using Assumption 1 and Assumption 2;

[0084] Based on assumption 1 and assumption 2, the two points will... Figure 3 Gas-liquid phase equilibrium at the true concentration of ethanol Figure 4 .

[0085] Step Six: Plot the operating line equations for the rectifying and stripping sections of the column together with the attached... Figure 4In order to perform graphical calculations of the extractive distillation column;

[0086] Interpolate the operating line equations of the rectifying and stripping sections of the column. Figure 4 From Figure 5 Where line eb represents the distillation operation line and line cd represents the stripping operation line; starting from point b, a series of steps are drawn between the phase equilibrium line and the operation line until x. m Composition less than x c , where x c The composition of the bottom liquid of the tower, x m The liquid phase composition descending from plate m in the stripping section; consisting of Figure 5 The required number of theoretical plates is 8, with the feed plate positioned at the 6th plate. Considering that the vapor phase at the top of the column contains a small amount of extraction solvent, an additional 2 to 5 theoretical plates are needed to remove the solvent. The angle of the diagonal line of the graph is constant at 45°. The larger the angle between of and line ob, the fewer theoretical plates are required.

[0087] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for graphically calculating the number of trays in a distillation column, characterized in that, Includes the following steps: Step 1: Construct a balance box for the rectification section and build the operating line equation for the rectification section; The formula for the operating line equation in the rectification section is: ； in, R For reflux ratio, x n The composition of the light components in the liquid phase descending from the nth tray in the rectification section. x s This refers to the composition of the solvent in the liquid phase descending from the trays in the rectification section. x D The distillate consists of light components from the top of the column. Step 2: Construct a balance frame for the stripping section and build the operating line equation for the stripping section; The operating line equation for the stripping section is: ； in, x m-1 For the stripping section from m The liquid phase composition of the -1 plate during its decline. V’ The steam rising from the plates within the stripping section, L’ This refers to the amount of liquid phase descending from the plate within the stripping section. W The distillate from the bottom of the column, x w The composition of the distillate from the bottom of the column; Step 3: Utilizing the conservation of light components in the overall tower material balance, through simplification and material and energy balance of the feed plate, the following is obtained: q Line equation; Step 4: Calculate the phase equilibrium of extractive distillation to obtain the vapor-liquid phase equilibrium diagram; Step 5: When the constant molar flow is satisfied for each tray in the column and the content of extraction solvent in the gas phase is ignored, the vapor-liquid phase equilibrium diagram is changed to a gas-liquid phase equilibrium diagram. Step Six: Combine the operating line equations for the rectifying and stripping sections with... q Plot the linear equations and the gas-liquid equilibrium diagram to calculate the number of trays in the extractive distillation column.

2. The method for graphically determining the number of trays in a distillation column according to claim 1, characterized in that, q The formula for the equation of a line is: ； in, q These are the thermal state parameters of the feed. x F It is a volatile component in the raw material liquid.

3. A graphical system for illustrating the number of trays in a distillation column, characterized in that, include: Memory is used to store instructions that can be executed by the processor; A processor for executing instructions to implement the plate number diagram method for a distillation column as described in any one of claims 1-2.

4. A computer-readable medium storing computer program code, characterized in that, The computer program code, when executed by a processor, implements the method for plotting the number of trays in a distillation column as described in any one of claims 1-2.

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