CHALLENGES ASSOCIATED WITH SEPARATIONS IN PRODUCTION OF HYDROGEN USING THERMOCHEMICAL CYCLES

INTRODUCTION: Hydrogen is clean energy carrier and promising alternative to the conventional fossil fuel. It has a highest energy per unit mass of 120MJ/kg .To obtain similar amount of energy would require 2.5kg of natural gas ,2.75kg of oil and 3.7-4.5kg of coal. The most widely used method for production of hydrogen is steam methane reforming produces CO and CO2,which are green house gases. Other sustainable and green methods have been developed in the near past, one of which is production of H2 by thermochemical water splitting cycles. These cycles are promising water splitting alternatives that can be linked with nuclear reactors to thermally decompose H2O through a series of intermediate reactions. Two of the most common thermochemical cycles ,Cu-Cl and I-S have been majorly concentrated upon in this study.

THE Cu-Cl thermochemical cycle: The 5 step Cu-Cl cycle adapted form (Orhan et al .,2010) is shown below. FIG1

The 5 step cycles consists of copper chlorination(hydrogen production), disproportionation (electrolytic), drying, hydrolysis and decomposition (oxygen production step). These intermediate reaction steps interact.

The I-S thermochemical cycle: The S-I cycle consists of three chemical reactions whose net reactant is water net products are hydrogen and oxygen. All other chemicals are recycled. The S-I process requires efficient source of heat. Its temperature requirement is higher than the Cu-Cl cycle and can produce large quantities of hydrogen from nuclear energy efficiently. Figure describing I-S cycle is given below.

FIG2

MAJOR SEPARATION ISSUES RELATED TO CU-Cl CYCLE: 1)The separation of H2O/HCl ,which is required in the hydrolysis step, which is as follows H2O(g) +Cucl2(s) = Cu2OCl2(s)+2HCl(g) .As this reaction does not undergo complete conversion, the unreacted H2O (which is in excess) forms an azeotropic solution with HCl(g) ,whose separation cannot be done by conventional distillation alone.

2)The separation of H2(g)/HCl(g) mixture, which is required at the hydrogen generation step of

the Cu-Cl Cycle.

H2O/HCl azeotrope separation: Water and HCl form a maximum boiling azeotrope at 1atm pressure which mean that the boiling point of azeotrope is higher than both water and HCl. Maximum boiling azeotropes are quite uncommon in nature .They have negative deviations from ideality which means that its activity coefficient γi is negative .The reason for this non ideality is the hydrogen bonding interaction associated with the H2O and HCl.

H2O/HCl phase behaviour: The H2O/HCl phase diagram is depicted below.

FIG3

METHODS OF SEPARATION OF AZEOTROPES:

1.Extractive distillation:

Here a large amount of relatively highboiling solvent is used to alter liquid phase activity coefficients so that relative volatility of key components becomes more favourable for separation. Solvent enters the column a few trays below the top, and exits from the bottom without forming azeotropes . If the Column feed is an azeotrope ,the solvent breaks it.

2.Salt distillation:

It's a variation of extractive distillation in which relative volatility α of key components is altered by adding to the top reflux a suitable , non-volatile ionic salt which stays in the

liquid phase as it separates as it passes down the column.

3.Pressure swing distillation:

It separates a mixture that forms a pressure sensitive azeotrope, by utilizing 2 columns at different pressures. By changing the pressure the phase diagram, and the azeotropic composition changes which helps in crossing the initial azeotropic composition for obtaining higher purity.The pressure variation is oscillatory, meaning we increase ,decrease and again increase so on .

4.Homogenous Azeotropic distillation:

It's a method of separating a mixture by adding an entrainer that forms a homogenous minimum or maximum boiling azeotrope with fee component(s).Where the entrainer added

depends on whether the azeotrope is removed from top or bottom of the column.

5.Hetrogenous Azeotropic Distillation: A minimum boiling azeotrope is formed by the entrainer.The azeotrope splits into 2 liquid phases inthe over head condenser.One liquid phase is sent back as reflux, the other is sent to the another separation step or is a product.

6.Reactive distillation :A chemical that reacts selectively with one or more feed constituents is added ,and the reaction product is then distilled from the non reacting components.This reaction is later reversed to recover the separating agent and reacting component.Reactive distillation also refers to

chemical reaction and distillation conducted simultaneously in the same

apparatus.

RESIDUE /DISTILLATION CURVE ANALYSIS OF SYSTEMS FORMING AZEOTROPE:

For ordinary multicomponent distillation ,the determination of feasible distillation sequences, design of distillation column is pretty straightforward, where as enhanced methods of separation for azeotropes are fairly complicated and the determination of feasible distillation design and column design is substantially more difficult. Many a times rigorous calculations fail because of liquid solution non idealities. For making the analysis simple a very important tool is of graphical techniques developed by Doherty and co-workers and by stichmlair and co-workers. These graphical techniques are described as follows.

RESIDUE CURVE :When an entrainer is added to a binary mixture forming an azeotrope the result is a ternary mixture, so it is very important to consider the phase equilibrium for predicting the possible overhead and bottoms composition. Triangular diagrams are often used to describe equilibrium for ternary mixtures. One of the most important aspect of ternary diagrams are the residue curves. These curves represent liquid residue composition as a function of time as a result of a simple batch distillation of a ternary mixture. They are derived mathematically as a result of component mass balance done around the still, assuming that the liquid is perfectly mixed and is at its bubble point. The equation (1) governing the residue curve is as follows.

.......... (1)

where

xi=mole fraction of a component

i in W moles of a perfectly mixed liquid residue in the still

yi=mole fraction of component i in the vapour leaving the still instantaneously which is in equilibrium with xi.

The results of this equation when plotted on a triangular graph are residue curves because the plot follows with time the liquid residue composition in the still. Note that we are not

plotting vapour composition in the plotting of residue curve. A collection of the residue

curves at a fixed pressure is called a residue curve map. Plotting the residue curve by hand is a tedious and a laborious task which requires a lot of time so we use process simulators such as aspen plus or chemcad can be used .

A residue curve map has the following characteristics:

1.The direction in which the arrow points starts from the initial composition in the still to the final composition as time progresses which is it starts from a low boiling component or an azeotrope to a higher boiling component or an azeotrope.

2.There can be distillation boundaries in some cases which a residue curve cannot cross. The distillation boundary itself is a residue curve dividing the ternary plot into 2 regions. These boundaries are thermodynamic in nature

Some important terminology in residue curve analysis :

1.Node: Residue curves begin and end at nodes.

2.Stable node: The component or azeotrope with the highest boiling point in the region. All the residue curves in the region point to (terminate) at this point.

3.Unstable node: The component or azeotrope with the lowest boiling point in the region

4.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.

FIG4

FIG5

DISTILLATION CURVES: 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 reflux. The sequence of liquid-phase compositions, which corresponds to the operating line at totalreflux, is plotted on a triangular diagram. Distillation curve maps can be arbitrarily directed to increasing or decreasing temperatures. Inthe former case, they closely approximate residue curve maps.The mathematical governing relations for distillation curve are

..................(2)

this is the equation of the operating line in a distillation column (both rectification and

stripping section) at total reflux.

..................(3)

this is the equilibrium relationship between the vapour and liquid leaving a tray. To solve these equations an initial assumtion of xi,1 is made and by using above equations in succesion xi,2 , xi,3 ... etc can be determined . Plotting a distillation curve also is a laborious task but takes lesser effort and time when compared to residue curve.

RELATION BETWEEN RESIDUE CURVES AND DISTILLATION CURVES :

It is found that residue curve maps and distillation curve map of a ternary system are approximately coincident , so one can be used in place of another (interchangeably).

FIG6

PRODUCT OF COMPOSITION REGIONS AND FEASIBLITY ANALYSIS:

Residue-curve maps and distillation-curve maps are used to make preliminary estimates of regions of feasible product compositions for distillation of non-ideal ternary mixtures. The product regions are determined by superimposing a column material 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 componentmaterial balances. For such a material balance line, the distillate and bottoms compositions must lie on the same distillation (residue) curve. Because of this, the feasible product region can be established like so:1) Find the limiting 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 with the lowest amount of low boiler possible for the limiting distillatecomposition. Call this material balance line M1.2) 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 bottomscomposition. Call this material balance line M2.3). Locate and draw the distillate curve which contains the feed composition. Call this curve DF.4) The areas on the convex side of DF, and lying between M1 and DF and between M2 and DF, are the feasible product regions.For azeotropic systems, where distillation boundaries are present, a feasible product region canbe found for each distillation region.

SEPARATION OF H2(g)/HCl(g) gaseous mixture:

The most common method for separating a mixure of gas is by absorption or adsorption either in trayed towers or packed towers.The other methods also include membrane separation and use of molecular sieves.

Absorption methods(physical and chemical): Absorption methods are gas-liquid separations where absorbents are used to remove soluble components from H2/HCl mixture.The equipment consists of an adsorption column in which soluble components are trapped in an

absorbent in which the absorbed components are released at a lower pressure or high temperature .It can be of 2 kinds physical and chemical.

Physical absorption process in which the solublity of gaseous components generally obeys Henry's law ,are mostly applied to separations of H2 from feed gas. The advantage of this is that regeneration of absorbents is very easy and product hydrogen of this leaving the

absorption column is near the feed pressure.

Chemical absorption is suitable for complete removal of specific impurities from crude hydrogen and recovery of H2 from that at its relatively low pressure or low concentration in comparison with Physical absorption. This chemical absorption method is used mainly for separation of acidic substances such as H2S,HCN,carbonyl sulphide , HCl from raw H2 gas. Hot alkaline or amine solutions are used as the absorbing agents to remove the acidic impurities.

Adsorption methods: Adsorption separation methods utilize the preferential adsorption of some constituent species other than hydrogen mainly due to difference in adsorption equilibrium .Representative adsorbents are silica gel, activated carbon, activated aluminium and molecular sieves. Most separation units are operated in dual mode and molecular sieves. Most separation units are operated in dual mode of adsorption by passing the feed hydrogen gas and regeneration by means of heating, pressure, reduction, gas purge or a combination of these methods. In 1960's a new separation technology , the so called "pressure swing adsorption PSA process", was developed . A PSA unit consists of 4-12 columns containing specific adsorbents. The columns are connected through valves in order to perform pressurisation and depressurisation by sophisticated charge and discharge of H2 gases. Each column is operated in sequential cycle ,which consists of adsorption of impurities, equilibriation of pressures ,desorption of impurities ,purge of impurities and pressurisation.PSA offers several features in H2 in a product gas and other constituents in a purge gas, separation to highly pure hydrogen at a moderate hydrogen recovery, product hydrogen pressure is near the feed pressure , minor influence on the design of the separation unit and the purity and recovery of product H2 by variations in comparison in

feed hydrogen and low operating cost. Owing to these advantages PSA process of various types are widespread for recovery of H2.

Membrane separation process:

Here a membrane acts as a semi-permeable barrier and separation occurs by the membrane controlling the rate of movement of various molecules between 2 liquid phases,2 gas phases or liquid and a gas phase. The 2 fluid phases are usually miscible and the membrane barrier prevents actual, ordinary hydrodynamic flow.

Membrane process for H2/HCl is gas diffusion where the rates of gas diffusion depend on the pore sizes and the molecular weights. We may have molecular ,transition and knudsen

diffusion region depending on the relative pore and gas molecule. The type of membrane

used for gas separation are flat membranes and spiral wound membranes.

SEPARATIONS REQUIRED IN THE I-S CYCLE:

1.At the Bunsen reaction section: HI is separated from H2SO4 and H2O by distillation or by

liquid/liquid gravimetric separation.

2.At the H2SO4 decomposition section: The water ,SO2 and residual H2SO4 must be separated from the oxygen by product by condensation.

3.At the HI decomposition section: I2 and any accompanying water or SO2 are separated by condensation, and the hydrogen product remains as a gas .

USE OF EXTRACTIVE AND REACTIVE DISTILLATION IN EXTRACTION OF H2O AND HI FROM HIX:

Extractive distillation utilizes concentrated phosphoric acid for the extraction of HI and H2O. In addition, H3PO4 breaks azeotrope between HI and H2O,thus permitting the distillation of HI from the acid complex followed by decomposition.

Reactive distillation is in theory simpler process than extractive distillation, but it has yet to be demonstrated experimentally. There are 2 key differences between reactive and extractive distillation. First , unlike the extractive process, the HIx azeotrope is not broken, so the composition in both the liquid and vapour phases is the same. Second the reactive process must be conducted under pressure.

SIMULATIONS CARRIED OUT TO DETERMINE PHASE BEHAVIOUR AND AZEOTROPIC COMPOSITION OF HCL-WATER:

HCL vapor mole fraction vs HCl liquid mole fraction:

Temperature vs liquid mole fraction of HCL:

OVERVIEW OF AZEOTROPIC DISTILLATION: An azeotrope can be separated by extractive distillation, using a solvent that is higher boiling than the feed componentsand does not form any azeotropes. Alternatively, the separation can be made by homogeneous azeotropic distillation, using an entrainer not subject to such restrictions. Like extractive distillation, a sequence of two or three columns is used. Alternatively, the sequence is a hybrid system thatincludes operations other than distillation, such as solvent extraction. The conditions that a potential entrainer must satisfy have been studied by Doherty and Caldarola ; Stichlmair,

Fair, and Bravo ; Foucher, Doherty, and Malone ;Stichmlair and Herguijuela ; Fidkowski, Malone, and Doherty ; Wahnschafft and Westerberg ; and Laroche, Bekiaris, Andersen, and Morari . If it is assumed that a distillation boundary, if any, of a residue curve map is straight or cannot be crossed, the conditions ofDoherty and Caldarola apply. These are based on the rule thatfor entrainer E, the two components, A and B, to be separated, or any product azeotrope, must lie in the same distillation region of the residue-curve map. Thus, a distillation

boundary cannot be connected to the A–B azeotrope. Furthermore, A or B, but not both, must be a saddle. FIG7

The maps suitable for a sequence that includes homogeneous azeotropic distillation together with ordinary distillation are classified into the five groups illustrated in Figure 7 , 8 where each group includes applicable residue-curve maps and the sequence of separation columns used to separate A from B and recycle the entrainer. For all groups, the residue-curve map is drawn, with the lowest-boiling component, L, at the top vertex; the intermediate-boiling component, I, at the bottom-left vertex; and the highest-boiling component, H, at the bottom-right vertex. Component A is the lower-boiling binary component and B the higher. For the first three groups, A and B form a minimum-boiling azeotrope; for the other two groups, they form a maximum-boiling azeotrope.In Group 1, the intermediate boiler, I, is E, which forms no azeotropes with A and/or B. As shown in Figure 7, this case, like extractive distillation, involves no distillationboundary. Both sequences assume that fresh feed F, of A and B, as fed to Column 1, is close to the azeotropic composition. This feed may be distillate from a previous column used toproduce the azeotrope from the original A and B mixture. Either the direct sequence or the indirect sequence may be used. In the former, Column 2 is fed by the bottoms fromColumn 1 and the entrainer is recovered as distillate from Column 2 and recycled to Column 1. In the latter, Column 2

is fed by the distillate from Column 1 and the entrainer isrecovered as bottoms from Column 2 and recycled to Column 1. Although both sequences show entrainer combined with fresh feed before Column 1, fresh feed and recycledentrainer can be fed to different trays to enhance the separation .In Group 2, low boiler L is E, which forms a maximum boiling azeotrope with A. Entrainer E may also form a minimum-boiling azeotrope with B, and/or a minimum-boiling (unstable node) ternary azeotrope. Thus, in Figure 8, any of the five residue-curve maps may apply. In all cases, adistillation boundary exists, which is directed from the maximum- boiling azeotrope of A–E to pure B, the high boiler. A feasible indirect or direct sequence is restricted to the subtriangle bounded by the vertices of pure components A, B, and the binary A–E azeotrope. An example of an indirect sequence is included in Figure 11.26b. Here, the A–E azeotrope is recycled to Column 1 from the bottoms of Column 2.

Alternatively, as in Figure 8 for Group 3, A and E may be switched to make A the low boiler and E the intermediate boiler, which again forms a maximum-boiling azeotropewith A. All sequences for Group 3 are confined to the same subtriangle as for Group 2.Groups 4 and 5, in Figures 8 , are similar to Groups 2 and 3. However, A and B now form a maximum boiling azeotrope. In Group 4, the entrainer is the intermediateboiler, which forms a minimum-boiling azeotrope with B. The entrainer may also form a maximum-boiling azeotrope with A, and/or a maximum-boiling (stable node) ternary azeotrope. A feasible sequence is restricted to the subtriangle formed byvertices A, B, and the B–E azeotrope. In the sequence, the distillate from Column 2, which is the minimum-boiling B–E azeotrope, is mixed with fresh feed to Column 1, which producesa distillate of pure A. The bottoms from Column 1 has a composition such that when fed to Column 2, a bottoms of pure B can be produced. Although a direct sequence isshown, the indirect sequence can also be used.

FIG8

Alternatively, as shown in Figure 8 for Group 5, B and E may beswitched to make E the high boiler. In the sequence shown, asin that of Figure 8 the bottoms from Column 1 is suchthat, when fed to Column 2, a bottoms of pure B can be produced.The other conditions and sequences are the same asfor Group 4. The distillation boundaries for the hypothetical ternarysystems in Figure 8 are shown as straight lines. When adistillation boundary is curved, it may be crossed, providedthat both the distillate and bottoms products lie on the sameside of the boundary. It is often difficult to find an entrainer for a sequenceinvolving homogeneous azeotropic distillation and ordinarydistillation. However, azeotropic distillation can also be incorporatedinto a hybrid sequence involving separation operations other than distillation. In that case, some of the restrictions for the entrainer and resulting residue-curve mapmay not apply. For example, the separation of the close boilingand minimum-azeotrope-forming system of benzeneand cyclohexane using acetone as the entrainer violates therestrictions for a distillation-only sequence because the ternarysystem involves only two minimum-boiling binary azeotropes.However, the separation can be made by the sequence shownin Figure 9 which involves: (1) homogeneous azeotropicdistillation with acetone entrainer to produce a bottoms productof nearly pure benzene and a distillate close in compositionto the minimum-boiling binary azeotrope of acetone andcyclohexane; (2) solvent extraction of distillate with water togive a raffinate of cyclohexane and an extract of acetone andwater; and (3) ordinary distillation of extract to recover acetonefor recycle. In Example 11.6 of Seader henley 3rd edition , the azeotropic distillationis subject to product-composition-region restrictions.

FIG9

CRITERIA OF SELECTION OF ENTRAINERS FOR AZEOTROPIC DISTILLATION USING RESIDUE CURVE MAP TECHNOLOGY:

Distillation is the most widely used separation process in chemical industries. In a typical chemical plant , distillation column and their support facility can cost about one-third of

total capital cost and more than half of the total energy consumption. So many aspects of distillation effects the economics of distillation process , one of the aspect of our interest is the selection of entrainers in any azeotropic distillation process of separation of azeotropes.

An azeotrope can be either homogenous, containing one liquid phase, or heterogeneous, consisting of two liquid phases. Heterogeneous azeotropes can easily be separated using a decanter coupled with one or more distillation columns, which exploits both vapour-liquid and liquid -liquid equilibrium driving forces. Homogenous azeotropic compositions that are pressure sensitive can be separated using pressure-swing distillation, which utilizes 2 or more distillation columns operating at different pressure together with appropriate recycle strategies to achieve desired separation. However if the change in azeotrope composition is small, the pressure-swing distillation sequence will have very large recycle flow rates, resulting in an uneconomical process. In all other cases the only way to separate homogenous azeotropic mixtures via distillation is by adding an extraneous component, referred to as an entrainer or mass separating agent, to facilitate the separation. Entrainers are also used to enable the separation of non- azeotropic mixtures where the direct separation is either not feasible due to due to process constraints (eg, due to the presence of a severe tangent pinch, as in the case of acetic acid and water.). An entrainer facilitates the separation of an azeotropic mixture by selectively altering the relative volatilities of the components in question, thereby breaking the azeotrope. The entrainer is specific to the mixture in question eg, benzene is a feasible entrainer for separating ethanol and water, but not for separating ethanol and methyl ethyl ketone- so there are so universal entrainers.

Because the choice of an entrainer determines the separation sequence ( the number and order of columns and decanters, and how they are interconnected ), and hence the overall economics of the process, the entrainer selection is a crucial step in the synthesis and conceptual design of azeotropic distillation processes. Entrainers are commonly selected based on prior experience with the same or similar process. This approach rarely identifies

novel entrainers that could have a dramatic impact in the economics of the entire process.

Use of residue curve maps in selection of entrainers: (ref- selecting entrainers for azeotropic distillation, Vivek julka et al (1)

One methodology for determining entrainer feasibility utilizes residue curve map technology. A residue curve map is a geometric representation of the vapour liquid equilibrium (VLE) phase behaviour of multicomponent mixtures, highlighting, in particular, those properties that impact distillation directly. It represents a collection of residue curves or trajectories of liquid-phase compositions(mole or mass fractions) as they change compositions(mole or mass fractions ) as they change with time.

The presence of a distillation boundary such that the residue curves in each region go in different directions i.e. towards different components. A distillation boundary represents an impassable boundary for distillation (although it may be crossed using other separation

techniques, such as liquid-liquid phase separation ,solid liquid phase separation techniques etc and the distillate and bottoms product for each column must lie in the same distillation region. Consequently the presence , location and structure of these boundaries are crucial in determining the multicomponent distillation feasibility.

RCM technology has been extensively tested and verified in industry. Residue curve map have been used to systematically synthesize distillation sequences to achieve a desired, separation to determine novel separation sequences, and to identify and evaluate

entrainers.

Procedure to evaluate entrainer feasibility:

1) First compute the RCM for the system consisting of the azeotropic components to be separated and the candidate entrainer.

2) Next check the phase behaviour of the system to determine whether a liquid-liquid phase envelope exists. If one does exist a liquid-liquid phase region for the system components can be generated at a certain temperature (eg. the temperature of the heterogeneous azeotrope , at the temperature where the liquid-liquid region is the largest or at a desired operating temperature). The tie lines indicate the 2 phase compositions.

3) Then draw an envelope over the RCM composition space, superimposing the composition space, superimposing the composition scale .

4)The candidate entrainer is feasible if either (a)the entrainer does not divide the components to be separated into different distillation region, or (b) the entrainer induces a liquid-liquid phase separation and there exists a liquid-liquid equilibrium tie line crossing the distillation boundary.

5)Once the feasible entrainer is identified, the corresponding separation sequences can be synthesized from the structure of the RCM.

The workflow for screening entrainers:

Various commercial computer tools are available for generating curve maps from a thermodynamic physical property model for the system. However, in the early stages of process structures are generally made such detailed thermodynamic models for the system are generally not available and can be very expensive to develop.

Therefore, a step by step approach in which the likely separation boundaries and feasible separation sequences are first sketched is recommended. For entrainer selection the approach begins by sketching the structure of RCM ( and liquid-liquid phase equilibrium, if necessary) from only the azeotropic temperature , and approximating composition and solublity data using the methodology previously described.

To screen entrainers and determine the sequence for separating a mixture of components A and B:

1.Compile a list of candidate entrainers. Some criteria for compiling this list include:(a)components that are already present in the process, especially reactants.

(b)compounds that are present on the plant site (so no new chemical is introduced in the plant's waste treatment unit).

(c)water (since it forms heterogeneous azeotropes with many components, the separation is easier , although the use of water may increase the load on the waste water treatment facility.)

(d)entrainers used for the same or similar components .

(e) commonly available chemicals.

2.Prepare an RCM for each entrainer:(a) If a detailed thermodynamic physical property method is available, compute the RCM for the system of A,B and the entrainer. If only a partial model can be modelled using UNIFAC, provided the properties can be modelled using UNIFAC, provided predictions are in agreement with azeotropic data.

(b)If no physical-property model is available, sketch the structure of the RCM from available azeotropic temperature, composition(approximate) and solublity (approximate) data.

(c) If neither a physical property model nor azeotrope data are available the required information can either be estimated using group contribution methods or an educated guess. these can be verified experimentally.

3.From the structure of the RCM, determine if the candidate entrainer is feasible for separating components A and B lie in the same distillation region or the entrainer introduces a liquid-liquid tie-line that crosses the distillation boundary dividing components to be separated into different distillation regions.

4.Synthesize all the corresponding separation sequences- the no of distillation column and decanters and their interconnections, from the structure of RCM by mass balance. Do this for each feasible entrainer.

5.Identify the entrainer feasibility conditions for the most promising candidate entrainers if their feasibility was determined from either azeotropic data or estimated using group contribution techniques. Subsequently, experimentally verify any of the conditions whose validity may be in doubt. Once these conditions have been verified , a detailed thermodynamic physical-property model for the mixture can be developed from experimental data.

6. Design, simulate and optimize the separation sequences.

CASE STUDY FOR SELECTION OF ENTRAINER (HCL-WATER):

Here we take 2 entrainers CaCl2 and MgCl2 which have been widely cited in literature and apply the above given methodology for validation of their feasibility as entrainers in azeotropic distillation of HCL and water mixture. First we plot their binodal/ residue curve maps using the chemcad(version6.3) as the process simulator. The K value was chosen to be UNIFAC and H value as LATE.

As we see in both cases that the HCL and water lie in same distillation region both CaCl2 and MgCl2 are both feasible entrainers for homogenous azeotropic distillation. Homogenous because there is no formation of 2 liquid phases.

PRESSURE SWING DISTILLATION:

If a binary azeotrope disappears at some pressure, or changescomposition by 5 mol% or more over a moderate range ofpressure, consideration should be given to using two ordinarydistillation columns operating in series at different pressures.This process is referred to as pressure-swing distillation.Knapp and Doherty list pressure-sensitive, binaryazeotropes, mainly from the compilation of Horsley .The effect of pressure on temperature and composition oftwo minimum-boiling azeotropes is shown in Figure 10.The mole fraction of ethanol in the ethanol–water azeotropeincreases from 0.8943 at 760 torr to more than 0.9835 at90 torr. Not shown in Figure b is that the azeotrope disappearsat below 70 torr. A more dramatic change in compositionwith pressure is seen in for the ethanol–benzene system, which forms a minimum-boiling azeotropeat 44.8 mol% ethanol and 1 atm. Applications of pressure swingdistillation, first noted by Lewis in a 1928 patent,include separations of the minimum-boiling azeotrope oftetrahydrofuran–water and maximum-boiling azeotropes of

hydrochloric acid–water and formic acid–water.

Van Winkle describes a minimum-boiling azeotropefor A and B, with the T–y–x curves shown in Figure a.As pressure is decreased from P2 to P1, the azeotropic compositionmoves toward a smaller percentage of A. An operablepressure-swing sequence is shown in Figure b.The total feed, F1, to Column 1, operating at lower pressureP1, is the sum of fresh feed, F, which is richer in A than theazeotrope, and recycled distillate, D2, whose composition isclose to that of the azeotrope at pressure P2. D2 and, consequently,F1 are both richer in A than the azeotrope at P1. Thebottoms, B1, leaving Column 1 is almost pure A. Distillate,D1, which is slightly richer in A than the azeotrope but lessrich in A than the azeotrope at P2, is fed to Column 2, wherethe bottoms, B2, is almost pure B. Robinson and Gilliland discuss the separation of ethanol and water, where thefresh feed is less rich in ethanol than the azeotrope. For thatcase, products are still removed as bottoms, but nearly pure Bis taken from the first column and A from the second.Pressure-swing distillation can also be used to separateless-common maximum-boiling binary azeotropes. Asequence is shown in Figure10 c, where both products arewithdrawn as distillates, rather than as bottoms. In this case,the azeotrope becomes richer in A as pressure is decreased.

FIG10

CASE STUDY 1 - APPLICATION OF PRESSURE SWING DISTILLATION TO HCL WATER SYSTEM:

METHODOLOGY:

After some trials of binary phase diagram of HCL-water system at different pressures , it was found that the system is pressure sensitive , So we can carry out pressure swing distillation for separation of the mixture. As cited in literature (Seader henley) that for economic

feasibility of pressure swing distillation the 2 columns should be operated at pressures such that there is at least 4-5 mol% shift in azeotropic composition. Such possible values of pressures were estimated by many trials on the software (CHEMCAD6.3) and it found that 1atm and 20atm are 2 such pressure values. Another set of values were 0.01atm and 10atm. The phase diagrams plotted by the software are shown below for 1atm and 20atm using thermodynamic setting (K value) as PPAQ.

As we can see that at 1atm the azeotropic composition is 0.11 and it changes by 0.04 (4mol%) to 0.07 at 20atm, thus now it's possible to design the pressure swing distillation operation for such pressure values. An attempt to design pressure swing distillation was made on the process simulator chemcad6.3 and was successful, the flow sheet and screen shots were taken for reference.

FLOWSHEET:

1ST DISTILLATION COLUMN SPECIFICATIONS:

TEMPERATURE PROFILE OF COLUMN2:

TEMPERATURE PROFILE FOR COLUMN1:

CONDENSER HEAT CURVE:

STREAM SPECIFICATIONS :

SPECIFICATIONS FOR 2ND COLUMN:

RESULTS FOR PRODUCT STREAMS:

COST ESTIMATION CALCULATION:

CASE STUDY-2 SIMULATION STUDY OF PRESSURE SWING DISTILLATION COLUMNS OPERATING AT 0.01ATM AND 10 ATM:

The reason for doing this simulation was that the earlier simulation the value used for the 2nd distillation column used a value of 20 atm which is on the higher side and such value may not be easily achieved so a reasonable amount of 10 atm was chosen and to create a

difference of 5 mol% in the azeotropic composition the value of pressure for the 1st column

was found suitable was 0.01 atm. Similar to the earlier case study simulations was carried out for many input specifications and the following results are for the case in which the simulation converged.

PHASE DIAGRAM OF HCL- WATER AT 0.01ATM AND 10ATM: The selected K value was PPAQ and was plotted in chemcad 6.3

FLOWSHEET:

INPUT SPECIFICATIONS:

COLUMN 1 SPECIFICATIONS:

COLUMN 2 SPECIFICATIONS:

RESULTS OF 1ST COLUMN:

RESULTS OF 2ND COLUMN:

CONCLUSION: Although there are many techniques for carrying out separation of azeotropes, designing of all of them is not feasible always with the help of a process simulator especially those techniques which involve addition of third component as calculations become tedious and the simulator (chemcad) was unable to converge even after several trials, so pressure swing distillation of HCL-water was chosen and an attempt was made to design and carry out cost estimation of both the distillation columns. It was found that distillation columns operating at 1atm and 20atm along with above shown specifications made the calculations converge and hcl and water were obtained as almost

pure products. The cost of 1st distillation was found to be $90227 and 2nd column was found to be $79230.4 and the specifications for height, diameter etc were taken which were most commonly used in any chemical industry. In the second case study the pressures chosen were 0.01atm and 10atm and the top products were pure hcl and water.

BIBILOGRAPHY:

1. Seader, J. D., and Henley, Ernest J. (1998). Separation Process Principles. New York: Wiley.

2. Progress of international program on hydrogenproduction with the copper chlorine cycleG.F. Naterer a,*, S. Suppiah b,1, L. Stolberg b,1, M. Lewis c,2, S. Ahmed c,2,Z. Wangd,3, M.A. Rosen d,3, I. Dincer d,3, K. Gabriel d,3, E. Secnik ,International journal of hydrogen energy, Elsevier.

3.CHEMSTATIONS website.

4. Treybal (1980). Mass-Transfer Operations (3rd Edition ed.). McGraw-Hill.

5. Perry, Robert H. and Green, Don W. (1984). Perry's Chemical Engineers' Handbook (6th Edition ed.). McGraw-Hill

6.Selecting entrainers for azeotropic distillation Vivek julka, Madhura Chilpunkar ,Lionel O' Young ,Clear water bay technology INC.