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It has already been proven that HIDiC can be superior in terms of energy savings when compared to other thermally coupled and conventional distillation columns. In an attempt to broaden the application of the ideal integration concept the economical and operational feasibility of the i-HIDiC scheme has been explored for the use in separating components of a close-boiling multicomponent mixture. It was found to be possible to employ two ideal HIDiCs to separate a hypothetical close boiling ternary mixture and two options of a direct and indirect sequence have been considered just as with its conventional equivalent.
It has been previously found that it possible to achieve 30% to 50% energy savings for the separation of two close-boiling mixtures using a HIDiC (Iwakabe et al, 2006.) However it was then found that the ideal HIDiC system is even more thermodynamically efficient than a conventional distillation system (Huang et al, 2007.) Huang et al. (2007) found a process that was conducted with minimization of the total annual cost in mind. They analysed the closed-loop controllability for the ternary mixture separation using the i-HIDiC and the intensified i-HIDiC. Upon comparison it was shown that the intensified i-HIDiC showed worse closed loop control performance with large overshoots and a longer settling out time due to the positive feedback mechanism that is involved within the intensified structure.
3.2 - Heat-integrated Extractive Distillation
It is not possible to separate a binary mixture which has a very low value of relative volatility as the two components will evaporate at almost the same temperature and at a similar rate. For such cases extractive distillation can be utilised where a third components called solvent (which is a high boiling and relatively non-volatile component) is added in order to alter the relative volatility of the original feed components.
It has previously been investigated as to the effectiveness and operation feasibility of several energy-integrated extractive distillation technologies including the divided-wall column, Petlyuk column and heat-integrated extractive distillation scheme (Abushwireb et al, 2007.) The work included a comparison between energy-integrated extractive distillation columns and conventional extractive distillation technique based on the recovery of aromatics from pyrolysis gasoline using a solvent called N-methylpyrolidone. The optimum design was found through using a minimal total annual cost as the objective function. The conclusion of the study was that the designed extractive distillation schemes should meet all expectations in terms of energy consumption and purity of cuts. It was shown that the heat-integrated extractive distillation configuration is the preferred option ahead of the Petlyuk column, divided-wall column and conventional column.
3.3 - Separating Close-boiling Mixtures using Heat Integrated Pressure-swing Distillation
Three commonly used techniques for fractionating a binary close-boiling mixture are azeotropic distillation, extractive distillation and pressure swing distillation (PSD.) The first two techniques require a third component called a solvent that enhances the relative volatility of the components that are to be separated. This can lead to certain drawbacks such as the solvent never being completely removed thus adding impurity to the products, the cost of solvent recovery, the loss of solvent and potential environmental concerns (Treybal, 1980.)
These potential issues with using a solvent have allowed the PSD approach to emerge as an attractive alternative option. An important prerequisite for the use of a PSD column is that the azeotrope separate has to be pressure sensitive. Here you have a low pressure (LP) distillation column and high pressure (HP) distillation column that are combined to avoid the azeotropic point. The inclusion of the HP and LP columns in the PSD configuration allows for the possibility of heat integration to be explored. Two appropriate types of energy integration for PSD processes were shown by K. Huang et al. (2008.) The first is the condenser/reboiler type heat integration where the condenser of the HP distillation column is integrated with the reboiler of the LP distillation process. The other option is the stripping/rectifying section type heat integration where the stripping section of the LP distillation unit is coupled with the HP distillation unit's rectifying section. It was found that for separating close-boiling mixtures the best option is the latter while for other types of mixtures the reverse is actually true. However it was clear that both types of heat integrated PSD column have potential for large energy savings when separating close-boiling mixtures.
Yu et al. (2012) also developed a new method for separating methyal/methanol using PSD. There it was found that the fully heat-integrated pressure swing distillation process had lower costs due its energy saving capabilities.
3.4 - Heat integrated Cryogenic Distillation
Cryogenic distillation columns will generally operate at extremely low temperatures. An example of this the process of separating air into its basic components where the process will run at about 100K (Mandler, JA.et al. 1989.) This temperature is low enough that oxygen and nitrogen will be in their liquid state and can consequently be separated in the column.
The cryogenic separation unit has a highly costly installation arranged with the condenser if the overhead vapour is meant to covert to liquid phase as the overhead vapour is enriched with more volatile component which has a very low boiling point. The heat integration principle can be used by coupling the reboiler and condenser in the cryogenic distillation unit in order to reduce this high energy cost. The energy that is expelled in the condenser can then be utilised in the reboiler.
A heat integrated cryogenic distillation column (HICDiC) that is constructed with two smaller columns, with one kept above the other, within a single distillation shell was proposed as a solution (Roffet et al. 2000.) The high pressure column and low pressure column are the lower and upper parts of the distillation tower respectively. In order to elevate the pressure compressors are employed. The integrated reboiler-condenser unit is positioned in the bottom of the LP column and just above the HP column. The difference in pressure leads to a difference in boiling points which becomes the heat transfer driving force in the integrated reboiler-condenser system. The vapour stream leaving the HP column will condense in the condenser and the resulting liberated heat is then used in the coupled reboiler in order to generate the flow of vapour in the LP column. In this set up the reboiler will behave the same as any normal tray.
Full-size image (14 K)
Figure - A schematic representation of the reboiler-condenser system in a HICDiC structure. Jana, 2009)
3.5 - Reactive Dividing Wall Column
Due to many potential advantages such as reduction in equipment size, decreased energy consumption and improvements in process safety and efficiency process intensification has become an important area for research in chemical engineering and in other related disciplines. Two important examples are reactive distillation and energy distillation columns; however they represent two different ways of integration.
A new integrated process that combines reactive distillation and the dividing wall column was introduced by Mueller and Kenig (2007.) This process is known as the "reactive dividing wall column." Any one side of the wall or both sides can be considered as the reactive zone here. The unit is shown in Figure 11. The structure displayed in this figure gives three high-purity product streams in a single column so it was suggested by Mueller and Kenig to consider reactive systems with more than two products (eg. Consecutive and side reactions) where each must be obtained a pure fraction, reactive systems with non-reacting components and with desired separation of both products and inert components and reactive systems that have an excess of a reagent that should be separated with sufficient purity prior to recycling.
Full-size image (11 K)
Figure - A schematic representation of a reactive dividing wall distillation column.(Jana, 2009)
3.6 - Heat integration in naphtha reforming
The demand of automobiles for high-octane gasoline has encouraged the use of catalytic reforming operations. Around 30-40% of the US's gasoline requirements are furnished by this process (Gary et al, 1994). There also exists a special need to restrict the aromatic contents of gasolines. Naphtha reformate is extracted for aromatic compounds and the aromatics are separated into virtually pure compounds. A series of binary-like conventional distillation units are used for this separation.
The application of a fully thermally coupled distillation column (FTCDC) for fractionation process in naphtha reforming plant has been researched (Lee et all, 2004). Here the first two columns of the aromatic separation process in the reforming plant were replaced with a FTCDC which is essentially a two column Petyluk structure. It was also shown the FTCDC will provide an energy saving of 13% and the investment cost reduction of 4% was comparable with a traditional two-column process.
3.7 - Heat integration in a crude distillation unit
The greenhouse gas carbon dioxide contributes to about 66% of the enhanced greenhouse effects. Fossil fuel combustion has been shown to be responsible for about 98% of the total carbon dioxide emission in the US in 1999 and also 95% of the UK's emissions in 2000. To change this and to meet the regulations as agreed in the Kyoto Protocol, the chemical process industries need capital intensive technology to decrease greenhouse gas emissions.
Energy consumption is distillation is heavily linked to carbon dioxide gases produced in the atmosphere. A link has been shown between the increase in demand for energy and the increase in carbon dioxide emissions. Crude fractionation units are the cause of the most carbon dioxide emissions of all distillation processes (Jana, 2009.)
Figure 12 displays sources of carbon dioxide emissions from various utility systems of a standard crude distillation unit (CDU.) Full-size image (26 K)
Figure - Sources of CO2 emissions from a CDU (HEN: heat exchanger network, BFW: boiler feed water) (Jana, 2009.)
The energy efficiency of crude oil distillation units can be improved in a number of ways. One way is to reduce the heat load on the furnace by installing intermediate reboilers in crude towers. A good amount of energy can be saved in the furnace by using preflash or prefractionate units to existing crude distillation towers. Using more trays in existing CDUs and strippers and boilers in stripping columns , minimising flue gas emissions from utility systems through changing fuels or utility system design, improving hear recovery and chemical treatment of flue gases will increase energy efficiency. Operational costs can be reduced through the integration of a gas turbine with an existing refinery site which will reduce flue gas emissions. Existing crude oil installations could have energy savings of up to 21% in energy and 22% in emissions if process conditions are optimised while by integrating a gas turbine with the crude tower the total emissions can be reduced by a further 48% (Gadalla et al, 2005.)
3.8 - An Improved Crude Oil Atmospheric Distillation Process for Energy Integration
Benali et al. (2012) performed a study where it was shown on thermodynamic grounds that introducing a flash in the preheating train of an atmospheric oil distillation process, along with an appropriate introduction of the resulting vapour into the column, can potentially lead to substantial energy savings, by reducing the column irreversibilities, the duty of the preheating furnace and by doing some pre-fractionation.
This idea was then expanded by showing how this can be done while keeping the throughput and the product characteristics unchanged. It was show that by placing several flashes after the heat exchangers and feeding the corresponding vapour streams to the appropriate trays of the column that pump around flows and the heat brought to the preheating train are reduced. The introduction of an additional heat exchanger compensates for the resulting heat deficit by using low level heat recuperated from the distillation products and/or import from other processes.
The use of residual heat reduces the furnace duty by essentially an equivalent amount. The experimental results from the study show that the recovery of the heat contained in the partially conditioned end products and other residual fluids can reach 9.3MW. This is more than 75% of the deficit generated which by itself amounts to a saving of 16% of the furnace duty. Also, if an additional 3.1MW of residual heat is recuperated at some other point in the refinery, then savings could reach levels as high as 21% (Benali et al, 2012.)
Figure 13 is a non-unique example of the type of complete preheating flowsheet for this process.
Figure - New preheating train using preflashes and residual heat recovery exchangers. (Benali et al, 2012)Full-size image (66 K)
The energy saving potential is strongly constrained by the temperature and consequently the composition profile in the column and also by the equilibrium of the heating and cooling duties of the preheating train. Through further analysis it is hoped that these two constraints may be satisfied. Attempts will also be made to use low temperature effluents and get the full potential profit from the saving of fuel and the consequent reduction of greenhouse gases. Due to the fact that flash drums are relatively inexpensive and the process modifications are only slight, this process would potentially be suitable for new installations and even the revamping of plants.
3.9 - James N. Sorensen's Heat Integrated Distillation column
James N. Sorensen patented a heat integrated distillation column that provides heat at each theoretical stage of distillation in a heating portion and coolant at each theoretical stage of distillation in a cooling portion.
The distillation column comprises an enclosure having an undistilled feed input, an upper reflux cooling portion which is contained within the enclosure having multiple cooling channels arranged side-by-side with multiple fractionating structures and a lower heating portion contained within the enclosure having multiple heating channels arranged alternating side-by-side with multiple second fractionating structures. There is cooling means for providing coolant to the plurality of cooling channels and heating means for providing a fluid heating medium to the plurality of heating channels. The heating means comprises at least one reboiler while the cooling means comprises at least one cooling fluid. At least a portion of the fractionating structures comprise a structural packing and an adiabatic structural packing. This is also the same for the second fractionating structures.
No information was available regarding the performance of this process.
3.10 - Nathan Kirk Powell et al. 's Process for Heat Integration for Ethanol Production and Purification Process
The production of ethanol production from the hydrogenation of acetic acid requires energy to drive the hydrogenation reaction and the purification of the crude ethanol product. Nathan Kirk Powell et al patented a heat integration process to recover heat from one part of the production process to be used within the process which improves efficiencies and reduces costs. No information was available regarding the performance of this process.
3.11 - Kenneth Kai Wong et al.'s Integrated heat exchanger system for producing carbon dioxide
Kenneth Kai Wong et al patented a system for producing carbon dioxide wherein the carbon dioxide feed fluid is first processed in a cooling section of an integrated heat exchanger before being purified in a column, and wherein column bottom fluid operate within one of an evaporating section and desuperheating section of the heat exchanger and refrigerant fluid operates within the other of the evaporating section and desuperheating section of the heat exchanger. No information was available regarding the performance of this system.
3.12 - Masaru Nakaiwa et al.'s Heat Integrated Distillation Apparatus
Masaru Nakaiwa et al patented a heat integrated distillation apparatus in which energy efficiency and a degree of freedom in design is claimed to be higher than a normal distillation column, and in which maintenance of the apparatus is simple.
The heat integrated distillation apparatus displayed in figure 14 includes: rectifying column (1), stripping column (2) located higher than rectifying column (1), first pipe (23) for connecting top part (2c) of the stripping column with bottom part (1a) of the rectifying column and compressor (4) that compresses vapour from top part (2c) of the stripping column to feed the compressed vapour to bottom part to (1a) of the rectifying column.
The heat integrated distillation apparatus also includes: a heat exchanger (8) located at the predetermined stage of rectifying column (1), a liquid withdrawal unit (2d) located at a predetermined stage of stripping column (2) and configured to remove some liquids from the predetermined stage to the outside of the column, a second pipe (24) for introducing the liquid from liquid withdrawal unit 2d to heat exchanger (8) via second pipe (24) and fluids flowing from heat exchanger (8) to a stage directly below liquid withdrawal unit (2d.)
Figure - Masaru Nakaiwa et al 's Heat Integrated Distillation Apparatus
3.13 - Rakesh Agrawal et al Inter-column Heat Integration for Multi-Column Distillation System
Rakesh Agrawal et al.'s patented design relates to an improvement in a process for the separation of a multi-component stream comprising component A, B and C where A is the most volatile and C is the least volatile. A multi-component feed is introduced to a multicolumn distillation system comprising a first or main distillation column and a side column wherein at least a light component A is separated from a heavier component C in the main distillation column. The lighter component A is removed as an overhead fraction and the heavier component C is removed as a bottoms fraction.
The improvement for enhanced recovery of component B in the side column comprises withdrawing a liquid fraction from the main distillation column at a point in-between the overhead and feed and introducing that liquid fraction to an upper portion of the side column. Lighter components are withdrawn as an overhead from the side column and returned to an optimal location in the distillation system which is typically the main distillation column. A vapour fraction is also withdrawn from the main distillation column at a point in-between the bottoms and feed and vapour fraction is introduced to a lower portion of the side column. A liquid fraction is withdrawn as bottoms and is returned to the main distillation column. Thermal integration of the side column is affected by removing a portion from the stripping section of the side column and vaporising this fraction against the vapour fraction obtained from the main distillation column.
There is however no information readily available with regard to the performance of this system.
3.14- Johannes de Graauw et al.'s Heat integrated distillation column
Johannes de Graauw et al. patented a heat integrated distillation column. It includes a cylindrical shell having an upper and a lower end and at least one first inner volume and at least one second inner volume in the shell and being in heat exchanging contact with each other through a wall separating the volumes. The heat integrated distillation column has the capacity to exchange heat through the wall from the first volume into the second volume, whereby the inside of the heat exchanging means is in open connection with the first volume. This is what should allow for the saving of energy.
There is however no information readily available with regard to the performance of this system.
3.15 - Kazumasa Aso et al's Heat Integrated Distillation Column
Kazumasa Aso et al. patented the system displayed in figure 15 where a monotube or multitube (2) is coupled to a body shell (1) via tube plates (3a) and (3b )at both ends, so that a tube interior (4) and a tube exterior (5) of the monotube or multitube (2) are isolated from each other. A difference is made in operating pressure between the tube interior (4) and the tube exterior (5), so that one of the tube interior (4) and the tube exterior (5) is used as a lower-pressure column and the other is used as a higher-pressure column. A wall of the tube is used as a heat transfer surface, so that heat is transferred from the higher pressure side (higher temperature side) to the lower pressure side (lower temperature side). Monotubes or multitubes (2) having different diameters are connected to each other via a reducer (20,) so that a monotube or multitube (2) whose diameters are varied stepwise is disposed between the tube plates (3a) and (3b) at the upper and lower ends, thereby increasing the column cross-sectional area as moving from the top to the bottom of the column in the enriching section and decreasing the cross sectional area as moving from the top to the bottom of the column in the stripping section (tube exterior.) There is however no information readily available with regard to the performance of this system.
Figure - Kazumasa Aso et al's Heat Integrated Distillation Column
4- Future Research
Previously a representation of the current knowledge regarding heat integration in distillation has been provided. What follows is suggestions regarding what future research in the field should focus on in order to develop the technology further.
4.1 - Experimental Testing
Research of heat integrated technology has been going on for many years now. Despite this, there are still too few real-time tests that have taken place. In order for the HIDiC technology to be commercialised then the promising theoretical predictions must be confirmed through various experimentation. These experiments should test for the actual energy saved, the feasibility of the operation, control performance and the cost of running and set-up. Through this economical and operational examination is will be possible to decide for each case whether the various technologies are viable or not. If viable it must be decided whether existing convention distillation can be modified to incorporate the technology or if they would have to be replaced in which case new technology would be limited to new plants and systems only.
4.2 - Effective Process Models
Many researchers have cited the need for the development of rigorous mathematical models for heat integrated distillation columns. This would be useful for accurately predicting the process characteristics including certain imprecisely known process parameters, the column dynamics and the model-based controllers. Once a simulated model is ready it is important that is results are experimentally confirmed from various realistic scenarios in order to validate its competency and acceptable future use as a tool.
4.3 - Optimal Design Configurations
The main consequence of taking advantage of the energy savings generally provided by the HIDiC in comparison to most conventional and even some non-conventional distillation columns is the increased capital investment due to the increased complexity of the column design. In order to help compensate for this it is necessary to optimally design the HIDiC configuration in a way that will minimise the total annual cost. The typical payback period is assumed as 3 years in the costs estimation for a HIDiC structure (Jana, 2009) so emphasis should be made on the improving the design so that this payback period can be decreased. The result of solving these two problems would be a set-up for which the economic viability is further increased.
4.4 - Optimal Parameters
Finding the optimal parameters for operation is an important stage when designing a HIDiC scheme. Methods used in order to do this include solving steady state optimisation problems although this may not always result in a good enough performance at transient state and can even result in closed- loop instability (Liu et al, 2000) Therefore, either an advanced control policy is needed in order to improve the operation stability or the dynamic optimisation problem has to be solved. At this moment in time it does not appear that it has been possible to solve the dynamic optimisation problem for finding the optimal parameters for an operation nor does it seem there was any published work that presented advanced nonlinear control of HIDiCs. This emphasises the need for future research in this field.
4.5 - Multiple steady states
The difficulty regarding control and operation increases due to the existence of multiple steady states. However it is an area of research that has received little focus until recently by, amongst others, Hasebe and his research group (2007.) Such analysis would provide important information which would help when deciding upon operating conditions, control scheme and process design. It is also essential that, for a process that contains multiple steady states, special care is taken during the start up of the column in order to get it close to the desired steady state.
4.6 - Further Applications of the Heat Integration Concept
Although it is clear that the principles of heat integration have been applied successfully into many distillation operation further research should continue regarding the development of more thermally coupled distillation columns.
5 - Conclusion
An overview of some of the heat integrated distillation technology currently available and examples of current commercial process has been provided. The common features of the various forms of this technology tends to be that energy and cost savings are possible but at the added implications of difficulty in terms of operation, control and in determining the optimal design. The added complexity of these systems also increases the initial investment cost.
Despite the fact that the concept of HIDiC was first introduced decades ago, it is still essentially in a primary stage of research and is not extensively used in industry. There is little large scale evidence that has been produced to back up theoretical claims. In order to improve the heat integration technology and to move forwards several fields of necessary future research have been suggested. Due to the importance of reducing energy consumption and waste in the future hopefully this technology can be developed to the point where it is an industrial standard and many other types of distillation and other processes can too incorporate the concept and technology of heat integration.