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The Importance Of Heat Integration In Distillation Columns Engineering Essay

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The combination of high crude oil prices due to increasing energy demand and concern about pollution has led researchers to exploring the possibilities of more energy efficient and environmentally friendly process technologies. The importance of distillation as a separation technique has made making it more energy efficient a high priority. Consequently, many heat integrated design schemes have been produced through the decades that it has been investigated and many of these techniques are outlined in this report along with some current commercial schemes. However, this technology has not been fully commercialised and this is mainly due to the high initial investment costs and the complexities of the equipment design, control schemes and operation. There is also a lack of real experimental data that is needed in order to verify the many theoretical predictions that claim that large energy saving are possible. Several areas have been identified as in need of further research in the future to hopefully allow this technology to become an industrial standard and not just a theory.

Introduction

1.1 The Importance of Heat Integration in Distillation Columns

The combined threat of increasing energy demands and costs, global warming and the increased dependence upon oil imported from politically unstable parts of the world have resulted in an interest in enhancing the thermodynamic efficiency of current industrial processes. Increasing energy efficiency in chemical processes not only provides economic benefits but also it leads to reduce the emissions resulting from the process operation. Distillation is perhaps the most important and widely used separation technique in the world today as it is used for about 95% of all fluid separation in the chemical industry. In the US, about 10% of the industrial energy consumption accounts for distillation while it accounts for an estimated 3% of the world energy consumption. More than 70% of the operation costs are caused by the energy expenses (Nakaiwa et al. 2003.) It is a fact that the energy consumption in distillation and CO2 gases produced in the atmosphere are strongly related as the higher the energy demands are the larger the CO2 emissions to the atmosphere are. This is due to the energy being mostly generated through the combustion of fossil fuel. Despite its apparent importance the overall thermodynamic efficiency of a conventional distillation is only around 5-20% (Jana, 2009). Clearly, improving on this value is imperative and a top priority objective. In order to achieve this, the concept of heat integration was introduced almost 70 years ago (Jana, 2009.) The basic idea of heat integration is that the hot process streams are heat exchanged with cold process streams which results in a more economic use of resources. Consequently a whole range of heat integrated distillation schemes have been proposed.

In a conventional distillation column (Figure 1) with a feed, a top product and a bottom product, heat is added at the bottom of the stripping section. In distillation, heat is used as the separating agent. The heat is conventionally supplied at the bottom reboiler in order to evaporate a liquid mixture but is lost when liquefying the overhead vapour at the reflux condenser. The temperature of this heat corresponds to the highest temperature point in the distillation column. The temperature of the heat rejected at the top of the rectifying section corresponds to the lowest temperature point in the distillation column. Thus, distillation involves the loss of heat from a higher temperature level to a lower temperature level in order to perform the work of separation. The efficiency of distillation is reduced if the heat rejected in the rectifying section of the distillation column is not reutilized (Smith, 2005.) This is the principle from which heat integration of distillation is mainly based.

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Figure - A schematic representation of a conventional distillation column (Kiran, 2012)

1.2 - Benefits and Drawbacks of Heat Integration

The possible benefits of heat integration tend to be potential energy savings due to greater efficiency and also less waste. Unfortunately due to a number of issues the technology has yet to be commercialised. Installation of any type of heat integration will entail a higher capital investment than that of any standard distillation column due to the increased complexity of the design. Also, the amount by which the efficiency is improved by is not always substantial in certain cases and therefore it must be considered whether the perceived benefits from the greater efficiency outweigh those of the added costs. The increased complexity can also increase the difficulty of designing, operating and controlling the system. There has also been a lack of experimental data for large scale examples to verify theoretical predictions. A successful heat integrated column design would show positive energy savings at reasonable economic figures that can be effectively operated and controlled.

2. Energy-efficient distillation techniques

This section discusses some of the many heat integrated techniques that have so far been proposed with the purpose of improving the energy efficiency of separation processes.

2.1 - Pseudo-Petlyuk column

The thermally coupled distillation scheme was first patented by Brugma in 1937. The process is used for separating a ternary feed and consists of a conventional prefractionator and side stream tower. Both of these parts are equipped with a reboiler and a condenser. The unit is divided vertically by a wall through a set of trays in order to keep the feed stream and side product separated. It was Wolff and Skogestad (1995)who referred to this set up as a pseudo-petluk column. However, their research led to some concerns about serious issues during operation for high purity separations which would limit the effective use of this system in many cases (Wolff, 1995.)

2.2 - The Divided-Wall Column

The elimination of the prefractionator unit from the pseudo-Petlyk column leads to a configuration known as the divided-wall column (DWC) (Robin Smith et al, 1992.) It is displayed in figure 2. It is achieved by introducing a vertical partition into a distillation column to arrange a prefractionator and a main column inside a single shell. The advantage of this partitioned column is that a ternary mixture can be distilled into pure product streams with only one distillation structure, one condenser and one reboiler. Naturally the cost of the separation is reduced along with the number of equipment units which leads to a reduced initial investment cost.

Subsequently, further research has been undertaken with for example Agrawal (2001) discussing for multicomponent mixture separation the various types of partitioned columns and their advantages and disadvantages. However, as a result of the lack of experience in design and control, the dividing wall columns were yet to be extensively used in industry. This is changing though and there has been a rapid growth in the number of units in use. In 2004 there were 40 units used worldwide (Adrian et al, 2004)

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Figure - A schematic representation of a Petlyuk distillation column (also known as divided-wall column) (Jana, 2009).

2.3. Petlyuk column

Petlyuk et al (1965) presented a detailed theoretical study on a divided-wall column called the Petlyuk column. This reduced Petlyuk structure involves low initial investment and consumes less energy which reduces the operating costs. However, upon comparison with a conventional distillation unit the Petlyuk column has many more degrees of freedom in both design and operation which can cause difficulty when designing the column and creating a control system.

As displayed in figure 3, the two-column Petlyk configuration will commonly consist of a prefractionator connected to a distillation shell equipped with only one condenser and reboiler (Jana, 2009.). The thermal coupling in a Petyluk scheme has lead to large energy savings. Unfortunately, little progress has been made with regard to improving operation and control of the structure which hinders its usability.

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Figure - A schematic representation of a two-column Petlyuk structure. (Jana,2009)

2.4 - Multi-Effect Column

The basic idea of this method for separating multicomponent mixtures is to use the overhead vapour of the one column as the heat source in the reboiler for the next column. The columns may be heat integrated in the direction of the mass flow which is forward integration or back integration can be used with is in the opposite direction. A sample column that represents a multi-effect column with a prefractionator for a ternary mixture separation is displayed in figure 4.

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Figure - A schematic representation of a multi-effect system for ternary (A-C) feed mixture (Jana, 2009)

This integrated arrangement has been proved to provide considerable energy savings (Cheng et al, 1985.) However, the issue preventing commercialisation of the process is the operation difficulties owed to the nonlinear, multivariable and interactive nature of the process (Han et al, 1996.) More research must be undertaken to try and find appropriate solutions before there can be a more extensive use for this system and to make use of the energy saving potential.

2.5 - Heat Pump-assisted Distillation Column

The heat pump is mainly used as a way for increasing the thermal economy of a single distillation column. The heat pump-assisted distillation column or vapour recompression column (VRC) was implemented as an energy-efficient process for the chemical industries after an oil crisis in 1973 (Jana, 2009.) In the system the overhead vapour is pressurised by a compressor to the point where it can be condensed at an increased temperature which will supply the heat required in the reboiler. A schematic representation of this can be seen in figure 5.

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Figure - A schematic representation of a heat pump-assisted distillation column (Jana, 2009)

There are potentially large energy savings to be made, mainly for fractionating close boiling mixtures. This is due to the small temperature difference between the top and bottom of the column which will result in small compression ratios and consequently small compressor duties being required (De Rijke, 2007.) For a conventional distillation column attempting to fractionate the same close boiling mixture there will be a higher reflux ratio and thus larger reboiler duties would be required. The drawback for this technique is the high capital costs. Reducing the cost of running the heat pump-assisted distillation column would certainly increase its cost effectiveness and make it more viable as an option.

2.6 - Heat integrated distillation column

Using heat pump technology it is possible for separate rectifying columns and stripping columns to be heat integrated internally. This structure is a heat integrated distillation column (HIDiC.) Originally only part of the stripping and rectifying sections were integrated under the name of the SRV scheme but later column design has incorporated heat integration between the whole rectifying and stripping sections (Jana, 2009.) Figure 6 displays a typical partial energy integrated distillation scheme.

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Figure - A schematic representation of a partial HIDiC scheme (Jana, 2009))

In this configuration the stripping column operates at pressure lower than the rectifying column. A compressor and throttling valve are installed in order to adjust the pressures. The pressure differential means there will be a corresponding difference in operating temperature which allows energy to be transferred between the two columns through heat exchangers. Reflux flow for the rectifying section and vapour flow for the stripping section is generated from the heat exchanged between the rectifying hot vapour and the stripping cold liquids. This allows the reboiler heat load to be substantially reduced. Less energy is consumed the more heat that is exchanged and through appropriate process design it can be possible for reflux and/or reboil free operations to be performed.

It has been shown that the HIDiC, compared to the VRC, can lead to energy savings of about 50% (Sun et al, 2003.) However, the structure has a very complex design and requires large capital investment (Jana, 2009.)

Meanwhile it has also been found find that there are many binary feed separations where HIDiC is actually less energy efficient than simple heat pump schemes using only one or two heat transfer locations. Furthermore, it was shown that the energy efficiency of HIDiC cannot be solely decided based on the feed composition or product purities as many calculations are based. A better performance indicator is the temperature profile along the height of the rectifying section relative to the corresponding temperature profile in the stripping section (Herron ,2011)

Research is ongoing, focussing on the dynamics and the thermodynamic efficiency aspects while extensive research was undertaken by Suphanit (2011) focussing on optimal heat distribution depending on the column arrangement and number of heat exchangers.

Suphanit also produced a couple of potential schemes display in figure 7.

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Figure - Fig. 2. Internally heat-integrated distillation column (HIDiC) (a) and HIDiC constructed in a concentric column (b) (Suphanit, 2011)

The development of HIDiC has now reached the pilot plant stage in some countries such as Japan and the Netherlands. Despite this, further research, both in terms of design and hardware development issues, is still needed before this application can be fully established and accepted in commercial use while further detailed study on the economic evaluation of this column structure is needed in order to ensure its advantage over more conventional schemes (Suphanit, 2011.)

2.7 - Heat Integrated Batch Distillation Column

Batch distillation is generally known to be a less energy efficient option than its continuous counterpart. However, the batch distillation is extensively used in pharmaceutical, fine and specialty chemicals industry due to its greater flexibility where the demand and lifetime of the products can be uncertain and may vary significantly with time. Jana (2009) proposed a novel heat integrated batch distillation column (HIBDC.) The proposal was based on a binary batch distillation example that separates an equimolar ethanol/water mixture.

In comparison with the conventional batch process, the HIBDC also includes a compressor. The produced vapour in this concentric reboiler is firstly compressed and is then introduced at the bottom of the rectifier. This results in a pressure difference between the rectifier and reboiler. Consequently, energy is exchanged from the rectifier to the reboiler through the internal wall and brings the downward liquid flow for the former and upward vapour flow for the latter. This reduces the reboiler and condenser heat loads. However, an additional compressor duty is involved in the thermally coupled column.

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Figure - Schematic of a heat integrated batch distillation column (HIBDC) [D = distillate rate (kmol/h), L1 = flow rate of liquid leaving 1st tray (kmol/h), nt = top tray, Qc = condenser duty (kW), Qn = rate of internal heat transfer from nth tray (kW),R = ref reflux rate (kmol/h), VB = vapor boil-up rate (kmol/h)] (Jana, 2009)

From the investigation it was observed that the HIBDC system appears overwhelmingly superior to its conventional stand alone column providing a significant savings in energy as well as cost. The potential energy integration leads to achieving about 56.10% energy savings and 40.53% savings in total annual cost. However, a single example comparing different configurations does not indicate that the proposed method would perform equally successfully for all mixtures. Therefore it was proposed that further investigation would take place in the future to come to a full conclusion as to the future promise of this technique.

Takamatsu et al. (1998) also performed a comparative study between the heat integrated batch distillation and the conventional batch distillation that proved the superiority of the heat integrated scheme over its conventional counterpart in terms of energy efficiency. However, no more development has been found with regard to energy-efficient batch distillation.

2. 8 - Intensified Heat Integrated Ternary Distillation Column

Kiran et al. (2012) extensively investigated a novel intensified heat integrated ternary distillation column (int-HITDiC.) Their objective was to show that the int-HITDiC was superior in terms of energy consumption and economics than its general form, namely the HITDiC and the conventional standalone column. It was also investigated that the traditional HITDiC scheme shows a reasonable energy household and better economic figures than the conventional standalone column.

The int-HITDiC is a hybrid scheme which gets the advantage of both the HIDiC and VRC strategies. It was found that this kind of heat integration could help to improve the process design not only in terms of thermodynamic efficiency but also in terms of capital investment. The intensified scheme can be classified into two different structure based on the number of compressors: the single compressor int-HITDiC and the double compressor int-HITDiC. From experimentation it was found that the double compressor system provided the best performance in terms of cost and energy consumption where it produced a maximum energy saving of 59.15%. Another attraction of the proposed double compressor int-HITDiC was its least payback time of excess capital which was 3.44 years.

The performance of this proposed thermal integration techniques was measured using a ternary distillation system. A more general conclusion regarding energy and economic viewpoints could be found by extending its application to other example processes and checking for a consistent performance. An issue that should be mentioned regarding intensification is that although economic benefit is usually achieved the operability of the column tends to be reduced. Also, if the HIDiC is sensitive to disturbances then potentially the economic, safety and environmental performance may be unfavourably affected (Kiran et al, 2012.)

2.9 - Internally Heat-Integrated Reactive Distillation Processes

Internally heat-integrated distillation and reactive distillation are two promising technologies that can potentially result in considerable economical benefits. Jiao et al. (2012) conducted a study regarding internally heat-integrated reactive distillation; a technology which combines internally heat-integrated distillation and reactive distillation and is employed in order to further enhance the advantages of both technologies.

The study tested three ideal quaternary systems, that reactive distillation processes with internal heat integration have been designed to use, to find which had the best potential for decreasing the total annual cost. These systems are types IP and IIP with stoichiometric design and also type IR which has excess design. In the case of type IP which has the reaction zone located in the centre of the reactive distillation column (RDC,) M-HIRDC will provide the highest economical benefit for the endothermic and exothermic reactions, chemical equilibrium constants and various relative volatilities. Here the reaction rate in the reactive trays in the high pressure section increases while in the reactive trays located in the pressure section the reaction rate will decrease. It is desirable to use HIRDC.

The reaction zone is located at the bottom of the RDC when using type IIP . Here the process with M-HIRDC will have better economical design than that of a conventional reaction distillation process in the case of both exothermic and endothermic reactions. The M-HIRDC's reactive trays are mostly positioned in the low-pressure section. Due to low pressure and temperature values the reaction rate is also smaller. It can be concluded that there are only minimal benefits to using HIRDC.

The final system, type IR, has its reaction zone placed at the top of the RDC. This process shows the smallest total annual cost for the endothermic and exothermic reactions. The reactive trays are situated in the HP section and due to the increased temperature and pressure values the reaction rate is also increased. Thus, HIRDC is again a desirable operation. In conclusion, when the reaction zone is situated at the top of the column the lowest total annual cost will be found for the RDC.

2.10 - Externally Heat-Integrated Double Distillation Column

Liu et al. (2011) investigated the potential of externally heat-integrated double distillation columns (EHIDDiC.) In terms of the separation of an ideal binary mixture of hypothetical components A and B, the synthesis and design of the EHIDDiC were studied with the assumption of a constant pressure elevation between the low-pressure (LP) to the high-pressure (HP) distillation columns that are involved.

It was found employing between one and three external heat exchangers results in a reasonable design option for the EHIDDiC. When a number of external heat exchangers greater than three were employed the process configuration has to be carefully determined as the increase in number of stages externally heat-integrated may not actually be beneficial to the system performance. This is due to the strong mass and heat couple between the LP and HP distillation columns that are involved and reflects the unique feature of the EHIDDiC.

To reduce capital investment, the total external heat exchange areas should be installed through as small a number of heat exchangers as possible. The extreme situation would be the employment of a single external heat exchanger which would need knowledge in arranging the total heat heat transfer areas between the HP and LP distillation columns involved. These findings are of great significance both to process synthesis and design. A novel decentralised control scheme was also proposed for use for EHIDDiC operation. (Liu et al, 2011.)

Huang et al. (2011) investigated three different configurations for externally heat-integrated double distillation column's performances for separating a binary mixture of ethylene and ethane. The configurations were a symmetrical EHIDDiC (S-EHIDDiC), an asymmetrical EHIDDiC (A-EHIDDiC), and a simplified asymmetrical EHIDDiC (SA-EHIDDiC), which were compared with respect to aspects related to process design and controllability. It was found that the A-EHIDDiC and SA-EHIDDiC were both superior to the S-EHIDDiC in terms of thermodynamic efficiency as well as in terms of process dynamics and controllability. Upon comparing the A -EHIDDiC and SA-EHIDDiC, the latter showed similar behaviour with the former in terms of process design and controllability. These results demonstrated that the asymmetrical configuration should generally be favoured over the symmetrical one for the development of the EHIDDiC (Huang et al, 2011.)

2.11 - The structured heat integrated distillation column

Krikken et al's (2012) recent investigation into a structured heat integrated distillation column showed that a plate-packing configuration using structured packing gave a superior performance in comparison with the HIDIC based on the plate-fin heat exchanger. Further experimentation showed that the mass transfer and heat transfer efficiency increased significantly with increasing throughput. However, this was accompanied by an increasing pressure drop per stage. By simulating an industrial scale plate-packing unit it was found that an even better performance is possible through increasing the volumetric thermal load by further optimisation of the internals.

The principle of a S-HIDiC is shown in figure 9. Here the rectifier and the stripper are alternatively stacked in a "sandwich" of layers which creates a high surface area for the heat and mass transfer while maintaining a high voidage.

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Figure - Flow scheme of an S-HIDiC.(Krikken et al, 2012)

Internals are used inside the layers to optimize the HIDIC performance. In the plate-packing HIDiC, which was developed and tested in this study, both heat and mass transfer are in balance at an acceptable pressure drop. This result of this is a column design providing substantial cost and energy savings.

It could be possible to optimise the column configuration even further by decreasing the number of heat integrated stages and by increasing the volumetric thermal load but research is ongoing with regard to this. It is also important to note that the results obtained were purely based on one experience with conventional packed columns so further optimisation of the performance through adjustment of the internals is required. It was also noted that in order to achieve this development of design models would be useful (Krikken et al, 2012.)

2.12 - Other Noteworthy Techniques

Other techniques worth mentioning but are not explored in detail here are the inter-coupled column, concentric HIDiC, the fractionating heat-exchanger (all outlined by Jana, 2009,) control systems for heat integrated distillation systems with a multicomponent stream (Amidpour et al. 2012) and membrane distillation system using heat exchanger networks (Lu et al. 2012.)

3 - Industrial Applications

3.1 - Using i-HIDiC to Separate a Close-boiling Mixture

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.

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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.

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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.


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