RESIDUE CURVE MAPPING

Transcription

1 ()pltnuzalloll o/ Desqpi & ~a11c>n in Reactuie Dlsl!llallon 65 CHAPfER V RESIDUE C URVE MAPPING RESIDUE CURVE MAPPING Application11 of residue curve mapping to RD processes have recently been reported, but n generalized and systematic approach is still missing for the case of reactive feeds outside the conventional composition ranges. This chapter is addressing these design aspects. The reaction invariant space, defined in terms of transformed composition variables, is divided into sub-regions characterized by separation boundaries. A reasibihty analysis of the RD process i11 performed based on the location of the reacting mixture and initial separation sequences are generated according to the feed transformed-composition. The practical conclusion or this chapter is that by using RCM technique one can virtually generate RD trains over the entire possible feed composition space.

2 ()pclmlzolloll of Des>gn & 0ptra11on 111 ReaCllue Vi.s!dlallon 66 '5.0. INTRODUCTION The art of process design involves finding process configurations, operating conditions and,.;zc of equipment that will allow an economical, safe and environmental responsible operation, only by specifying the stale of the feeds and the targets on the output streams of a system (Almeida-Rivera et al., 2004a; Doherty and Buzad, 1992; Buzad and Doherty, 1995). This task is not trivial and requires the development of specialized design tools (e.g. computational methods, algorithms and procedures). At early stages of the design activity (e.g. conceptual phase) the chemical process designer faces a lru-gcly influencing question: is the processing route feasible? The impact of preliminary design and feasibility analysis on the fate of the final plant is enormous. Surprisingly, it is a common practice to allocate a relative small fraction (< 2-3%) of total budget for the conceptual phase. Due to the coarse level of detail, the design tools in the conceptual phase should be powerful enough to screen feasible design alternatives within a huge design space 5.1. INPUT-OUTPUT INFORMATION FLOW ln the case of the feasibility analysis stage, the n:quired input information involves the process basis of design. In other words, feedstock and product purities and operational boundaries should be defined. Additionally, sustainability and safety constraints should be included as inputs. The output information comprises the determination of distillation boundaries, (non-) reactive azeotropic mixtures and feasible product compositions. The domain knowledge at this design space belongs to the field of thermodynamics, kinetics and overall mass balances. An extended overview of input and output Information at this design level is given in table The generation of separation sequences is composed of process requirements related to the mode of operation, which is in turn determined using operational skills. The temporal mode of operation mode is chosen based on operational expertise in RD processing. For the sake of analysis

3 Optimizauon of Design & Operation m Reactwe Distillation 67 simplification and without restricting the validity of the approach to other temporal modes, a Table 5.1 Input-output information ror the feasibility analysis phase. Design specifications PROCESS. - set of components - set of chemical reactions - feed composition - operational pressure PRODUCT - compositions target (s) - SHE constraints Des.gn/operational variables Domain knowledge DISCRETE - thermodynamics distillation boundaries - kineocs - (non-) reactive azeotropes - overall balances CONTINOUS : - product feasible composit1ons Continuous operation is chosen. The output information at this design s pace includes the type of task required for the design specifications (i.e. separation and/ or reaction), the type of column internal (i.e. tray or packing), the number of units and their connectivity. The relevant domain knowledge at this design space corresponds to operational thumb-ruled skills and basic component ba lances. An extended overview of input and output information a t this design level is given in table 5.2. Table 5.2 Input-output information for the column sequwncing phase. Design specifications Design/operational variables Domain knowledge PROCESS : DISCRETE : operational skill - mode of opeartlon type of task component balance (separatk>n and I or reactive) - number of columns - columns connectrvity CONTINOUS - feed ratio of reactant streams Making each stage of the design approach opera tional requires the use of (novel) design tools. The Residue Curve Mapping technique (RCM) is found to be of particular effectiveness in the case of RD feasibility analysis and generation of column sequencing RESIDUE CURVE MAPPING T ECHNIQUE When an entertainer is added to a binary mixture, a ternary mixture results, and it is then necessary to consider the phase equilibria for this new mixture as well as to predict the ranges of possible overhead and bottoms

4 .,...,,,n & n...rnuon in H~c1d11H' O.sr./lallon 68 OplltNZGllOrl o, ~.., "I~ compositions. Triangular diagrams arc often used to describe the equilibrium relationships for ternary mixtures. Especially important are the residue curves on the diagrams. A residue curve represents the liquid residue composition with time ns the result of a simple, one stage batch distillation. The results, when plotted on a triangular graph, are called a residue curve because the plot follows the liquid residue composition in the still. Different residue lines result from different starting compositions. A collection or these curve.- for a given ternary system is called a residue curve map. A residue curve map has the following characteristics: 1. If we assign the direction of the residue curves as being from the starting composition to the ending composition, then the arrow on each curve points from a lower boiling component or azeotropc to a higher boiling compont:nt or azeotrope. 2. The presence of azeotropes can create distillation boundaries which cannot be crossed by a residue curve. These distillation boundaries represent the residue curve on which the light or starting residue composition is a lower boiling pure component or a7.eotrope and the heavy or ending composition residue is a higher pure component or azeotrope. Any given pure component point or azeotrope will be connected to some but not all other pure component points and azcotropes on the graph. Those that are connected form distillation boundaries. These boundaries arc thermodynamic in nature. 3 These distillation boundaries partition the map into distillation regions. The nature of these regions is such that two pure components which lie in different regions cannot be separated using conventional distillation. Some definitions are in order before we explain how residue curve maps apply to enhanced distillation systems. Node Residue curves begin and end at nodes. Stable node The componenl or azcotropcs with the highest boiling point in the region. ALI the residue curves in the region point to (terminate) at this point.

5 Optimization of Design & Operalion in Reactiue Distillation 69 Unstable node The component or azeotropes with the lowest boiling point in the region. Saddle Residue curves move toward and then away from saddles. Pure components and azeotropes which have a boiling point between the stable and unstable nodes are saddles. Vapor line The vapor line is the line formed by those vapor compositions which are in equ ilibrium with the liquid compositions on the distillation boundaries. In other words, if you take all the compositions on a distillation boundary line as saturated liquid compositions; then determine the composition of the vapors which are in equilibrium with all these points; and plotted all these vapor compositions on the residue curve map, one line would be form ed for each distillation boundary. That line is called the vapor line. Vapor boil up curve The vapor boil up curve is the plot of the vapor compositions in equilibrium with any given residue curve. Binodal plot This is a constant temperature plot of all saturated liquid compositions. It is used to identify the liquid-liquid region of the mixture. Liquid boiling envelope This is the constant pressure binodal like plot of the two liquid phase region of a ternary system. In the residue map shown below,the residue curves are represented by the light black curves, The distillation boundaries are represented by the heavy curves, The triangles indicate the azeotropes, and The two phase regions (at constant pressure and at constant temperature) are outlined by th e liquid boiling envelope and the binodal plot as indicated. For this system you will notice that; The binary azeotropes are all saddles. As a result the residue lines tend to move toward them at first and then at some point they turn away from them :o the stable node. The ternary azeotrope, being a minimum boiling azeotrope, is an unstable :iode (in this case the only one in the system). This means that in any simple ::listillation, this azeotrope will always come out the top of the column.

6 Optimization of Design & Operation in Reactive Distillation 70 The pure components are all stable nodes. This means that in any simple distillation they will always come out the bottom of the column. Which component comes out the bottom depends on which region we are operating in. The liquid boiling envelope and the binodal plot are not the same. The condenser will generally operate at fixed temperatures so it is a nalyzed using the binodal plot. The top tray(s) of the column will b e at the column pressure, so it must be analyzed using the liquidboiling envelope. Please note the following properties of a residue curve map: l.based on experimental evidence, for ternary mixtures with very few exceptions, there are at most three binary azeotropes and restrictions that a pply to a ternary system where, N1 N = 3 N = B < 3 NJ + 8J = 1 or 0 the number of pure single component nodes. N2 the number of binary nodes. NJ the number of ternary nodes. 51 the number of pure component saddles. 52 the number of binary saddles. 5J the number of ternary saddles. And 2N N2-B+ 18T, = 2 2. For homogeneous azeotropes the nodes may be either stable (maximum boiling) or unstable (minimum boiling). However, h eterogeneous azeotropes can be either unstable n odes or saddles but not stable nodes (i.e. they cannot be maximum boiling). 3. The liquid temperature always increases along residue curves in the heterogeneous region.this property, coupled with fact that all singular points on heterogeneous residue curve m aps are restricted to be either nodes or saddles, means that the entire topological methodology for analyzing and constructing homogeneous residue curve maps from boiling temperature data alone extends verbatim to h eterogeneous

7 Op1tmtZC1t10n of Destgn & Operatton in React111c Dlst.Uation 71 nuxtures CONSTRUCTING A RESIDUE CURVE MAP To help in understanding residue curve maps, you can use lhe following procedure to construct a qualitatively accurate map by hand: Step 1: Lab<'I the ternary diagram, (numcs and boiling points) with the low boiling component at the top vertex; the high boiling component at the lower right vertex; and the intermediate at the lower left vertex.plot compositions of all o.zcotrope:; and label the points with their boiling points. This determines the value of 8. Step 2: Draw arrows on the edges of the triangle in the direction of increasing temperature for each pair of adjacent species (components and/or azeotropes). Step 3: Determine the type of point (node or saddle) that each pure component vertex is by using the arrows drawn in Step 2. This determines N 1 and Sl. Step 4: (for ternary azeotropes, if present): Determine lhe type of point of the ternary azcotrope (if one exists). The pomt is a node if; a) Nl + B < 4, and/or b) Excluding the pure component saddles, the ternary azeotrope has the highest, second highest, lowest, or second lowest boiling point of all the species. Otherwise, the point is a saddle. This determines N3 and S3. Step 5: (for ternary saddle only, if prescntl: Connect the ternary saddle, by straight lines, to all binary azcotropes and lo all pure component nodes. Draw arrows on the lines in the direction of increasing temperatures. Using these arrows determine the type of point of each binary azcotropes. This determines N2 and S2. If N 1 + B 6, special checks should be made. If not, the sketch is complete.

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9 ()pl1ml%llbon of Design & Operation in RMCfl"" '511/lation Of Ot OJ I 0 AcebcAc1d 1111 ~ Fig. 5.1 Residue curve map V ttl t1! 11.. ll" flt. o i ~ l'i I Fig. 5.2 E.~rimenlal Residue curve map \, 5.4. DISTILLATION CURVE MAPS An alternative representation for distillation on a ternary diagram is a distillation curve for continuous, rather than batch, distillation. The curve is most readily determined for total renux. The sequence of liquid-phase compositions, which corresponds to the operating line al total reflux, is

10 ,, ~~n (\ '"'-rnhort '" RNICll\.. D1$1tllOIJCm 74 ()JJtllOl.7..0rJon OJ ~-v...,, ft tn' n~gular diaarom. Distillation curve maps can be arbitranly plotted on ~ -.,.,. directed to increasmg or decreasing temperatures. In the former case, they closely approximate residue curve maps. S PRODUCT COMPOSIT ION R EGIONS Residue-curve maps and distillation-curve maps are used to ml:lkc preliminary estimales of regions of feasible product compositions for distillation of non-ideal ternary mixtures. The product regions are determined by superimposing a column matenat balance line on the curve map diagram. If a straight line is drawn that connects distillate and bottoms compositions, that line must pass through the feed composition at some intermediate point to satisfy overall and component material balances. For such a material balance line, the distillate and bottoms composition s must lie on the same distillation {residue) curve. Because of this, the feasible product region can be established like so: i. Find the limiling distillate composition point for the region. Draw a line from this point, through the feed composition, to the opposite side of the map. This point represents the bottoms composition w1th the lowest amount of low boiler possible for the limiting distillate composition. Call this material balance line M 1. ii. Find the limiting bottoms composition point for the region. Draw a line from this point, through the feed composition, to the opposite side of the map. This point represents the distillate composition with the lowest amount of high boiler possible for the limiting bolloms composition. Call this material balance line M2. iii. Locate and draw the distillate curve which contaln11 th e recd composition. Coll this curve DP iv. The areas on the convex side of DF, and lying between M 1 ruid DF and between M 2 and DP", are the feasible product regions. For azeotropic systems, where diswlation boundaries arc present, a feasible product region can be found for each distillation region.

11 Optimtzalion of /)es'!}n & Operation on Reactive V.Slil1Wio C ONCLUDING REMARKS Residue curve maps have shown to provide valuable insights and design assistance for nonideal systems, particularly for reactive distillation. Transforming the composition variables according to Doherty's approach allows to define a reaction invariant space of lower dimension, formed by attainable product compositions and where the conventional concepts for residue curves can be applied.