Energy conserving effects of dividing wall column☆

Jing Fang,Hanmei Zhao,Jianchao Qi,Chunli Li*,Junjie Qi,Jiajia Guo

School of Chemical Engineering,Hebei University of Technology,Tianjin 300130,China

Keywords:Dividing wall column Energy-saving Heat transfer Back-mixing Thermodynamic efficiency

ABSTRACT The energy-conserving performance of dividing wall column(DWC)is discussed in this paper.The heat transfer through the dividing wall is considered and the results are compared with that of common heat insulation dividing wall column(HIDWC).Based on the thermodynamic analysis of heat transfer dividing wall column(HTDWC)and HIDWC,both computer simulation and experiments are employed to analyze the energyconserving situation.Mixtures of n-hexane,n-heptane and n-octane are chosen as the example for separation.The results show that the energy consumption of HTDWC is 50.3%less than that of conventional distillation column,while it is 46.4%less than that of HIDWC.It indicates that DWC is efficient on separating threecomponent mixtures and HTDWC can save more energy than HIDWC.Thus it is necessary to consider the heat transfer while applying DWC to industry.

1.Introduction

Distillation is one ofthe mostimportantand popular technologies for separation of multicomponent mixtures in chemical and petrochemical industries.However,high energy-consumption and relatively low thermodynamic efficiency severely limit the development of distillation[1].Therefore,reducing the energy consumption is an urging motivation for studying distillation column[2–6].On this premise,a series of complex distillation column sequences have been designed and widely used in industry,such as column with side rectifier or side stripper,thermally coupled distillation column,and dividing wall column(DWC,Fig.1).Typically a dividing wall column operated under optimum conditions can save about 30%energy compared with a conventional column sequence[7].Furthermore,it could be used within a single shell by installation of a divided wall to separate the prefractionation from the main column section[5,8,9],so the system can save up to 30%on capital investment compared to conventional equipment[10–12].

DWCcan be applied to separate many systems,such as hydrocarbons,alcohols,aldehydes,ketones,and esters.However,due to the complexity in calculation and control,ittook a long time to be applied.DWC was first used in industry in 1985.At present,most of the operational DWCs belong to BASF company,such as the DWC used to recycle 1-hexene from gasoline mixture of Sasol company in South Africa,which is also the highest DWC in the world with a height of 107 m and a diameter of 5 m[13].With the increase of energy cost,Sulzer,Koch-Gitsch,Kellogg,Linde,Uhde,Sumitorm and some other companies have also developed and applied DWC techniques.The number of DWC in industrial production is increased to nearly 300 in 2010 from less than 20 in 2000.The production practice manifests that the application of DWC has achieved great economic benefit.

In the recent study of DWC,most researchers ignore the heat transfer between the prefractionation and the middle section of the main column for simplification.However,the heat transfer is usually significant due to the temperature difference between two sides of the wall.Hence,it is important to know the effect of heat transfer on the energy-saving situation of DWC[13,14].In this study,the elimination of back-mixing in DWC is validated,which is important for energysaving.Then the in fluence of heat transfer through the wall on the energy efficiency of DWC is investigated by changing the composition of the feed.

2.The Structure of Dividing Wall Column

Fig.1.Heat insulation dividing wall column(HIDWC)(a)and heat transfer dividing wall column(b).

Conventionally,the separation ofa three-componentmixture requires a sequential system with at least two columns,while DWC can accomplish this task in one column.Generally a DWC looks like a common distillation column with a side-draw.The column has a verticalseparating wall with certain length,which distinguishes the “pre-fractionating”region[e.g.the left part in Fig.1(a)]and the “main column”region[e.g.the right part in Fig.1(b)][15–19].The feed in the prefractionator is a mixture of three components,A,B and C,where A is the lightest component and C is the heaviest component based on the boiling points.In a DWC,the prefractionator first performs a sharp split between A and C,while allowing B to distribute in both streams.The streams of AB and BC from the two ends of the prefractionator enter the main column,where components A and B are separated in the upper portion while components B and C are separated in the lower portion.Finally products A and C are collected at the top and the bottom of DWC separately,while product B gathers in the central column with the maximum concentration.In the process,the reflux liquid from the condenser and the vapor from the reboiler of the main column split on both sides of the separating wall,offering energy for the prefractionator region.Thus the bottom reboiler and overhead condenser in prefractionator can be omitted.The main advantages of DWC are lower capital investment,smaller operation area and lowerenergy consumption compared with conventionalseparation sequences.For instance,a distillation column and some ancillary equipment,such as reboilers,condensers,overhead reflux pumps and piping,can be omitted in DWC[16,19–21].If the heat transfer across the dividing wall is negligible,DWC is thermodynamically equivalent to the Petlyuk column,and the corresponding structure is called heatinsulation dividing wall column(HIDWC)as shown in Fig.1(b).If the heat transfer is considered,the structure is named heat transfer dividing wall column(HTDWC)as shown in Fig.1(a).Both types of DWC are analyzed in this paper.

3.Thermodynamic Properties

3.1.Thermodynamic properties of conventional separation sequences

There are two conventional separation sequences for threecomponent mixtures as shown in Fig.2.In the direct sequence,column 1 is used to produce the lightest component A at the top,while the mixture of components B and C is separated in column 2[Fig.2(a)].In the indirectsequence,component C is collected atthe bottomofcolumn 1,and components A and B are concentrated at the top and bottom of column 2 separately[Fig.2(b)].The concentration of each product is higher than 98%by the two steps of separation.

A distillation column can generally be described as an energy converter.Energy supply is usually necessary in order to separate a mixture to its pure components.The energy is supplied by feed materials and reboiler,while the energy is removed with products and condenser[22].The thermodynamic analysis to DWC is based on the first and the second laws of thermodynamics,

The energy loss(WLoss)inside the column is caused by the irreversible mass transfer and heat transfer processes,

where Wminis the minimum work to separate the mixture given by the following equation

Fig.2.Conventional separation sequence for three-component mixtures.

The thermodynamic efficiency(ηCSS)of conventional separation sequences can be described as

3.2.Thermodynamic properties of the heat insulation dividing wall column

In a dividing wall column there is only one condenser and one reboiler while the wall is completely insulated.The heat transfer is easily shown as follows

The thermodynamic efficiency(ηHIDWC)of HIDWC can be expressed as

3.3.Thermodynamic properties of the heat transfer dividing wall column

HIDWC has similar structure with HTDWC,but with different dividing wall.The heat transfer across the wall is considered in HIDWC due to the temperature difference between the two sides of the wall.Apartofheatis recycled as a resultofthe heattransferthrough the wall,so

The thermodynamic efficiency(ηHTDWC)of HTDWC is given by

However,by analyzing the structure of HTDWC(Fig.3),it is found that not all heat transfer can improve the thermodynamic efficiency.If part of the heat transfer(QBTH)can improve the thermodynamic ef ficiency while the other part of the heat transfer(QATH)would reduce it,the heat transfer in this work is

The thermodynamic efficiency[Eqs.(5),(9),(11)]shows the ratio of the minimum separation work to the actual input energy.For a certain task,the minimum separation work(ΔH-T0ΔS)only relates to the initial and final states of the system,which is a constant once the feed and separation requirements are given.Therefore,higher thermodynamic efficiency means smaller energy loss(WLoss),in other words,less input effective energy and more energy saving.As Wminis the same in Eqs.(5),(9)and(11),the thermodynamic efficiency is only related to the duty of the reboiler.

Fig.3.The positive heat transfer between column sections.

As shown in Fig.3,parts I,III,and V are the rectifying sections and parts II,IV,and VI are the stripping sections.It is well known that supplying heat to stripping section is good for distillation process,so the positive heat transfer improving the thermodynamic efficiency across the dividing wall is from sections IIIto VIin Fig.3(a)and sections Vto IIin Fig.3(b),while the adverse heattransfer from sections IIto Vin Fig.3(a)and VI to III in Fig.3(b)is negative.

When the positive heat transfer is larger than the negative one,the following equations are appropriate

Then it can be concluded that HTDWC has a larger thermodynamic efficiency than HIDWC:

Similarly,when the negative heat transfer is larger than the positive one,QTHis negative

Therefore,the thermodynamic efficiency of the two types of DWC is as follows

Practically,if the operating conditions and external environmentare constant,it is the feed composition that decides the heat transfer condition,and thereby affects the thermodynamic efficiency of HTDWC.Thus we study the effectof feed composition on the thermodynamic efficiency and compare the performance of HIDWC and HTDWC with that of conventional separation sequences.

4.Experiments and Simulation

Fig.4.Schematic diagram of experimental devices(a)heat insulation dividing wall column；(b)heat transfer dividing wall column.

Table 1 Parameters of apparatus and experiments

A typical DWC in experiment is shown in Fig.4 with the specific device parameters and experimental conditions in Table 1.A 2000 ml three-necked flask serves as the bottom reboiler and a glass condenser serves as the top condenser.In addition,a liquid dispenser is installed near the top of the column to control the liquid amount into the prefractionator and the main separation regions.The column is insulated with asbestos.

In this experiment,three components(A:C6H14,B:C7H16,C:C8H18)are mixed as the feed.The feed flow rate is 30 kmol·h-1at 20 °C and 1 atm.In the conventional distillation column sequences,columns 1 and 2 contain 20 and 30 theoreticalplates,respectively.Forcomparison,10 theoretical plates are in the common stripping section and common rectifying section in the dividing wall column,which are parts of the 50 theoretical plates of the main column,while the prefractionator column contain 30 theoretical plates.The operating pressure is at 101 kPa and the target purity of the product is more than 98%(mole fraction).The conditions for experiments and simulation are shown in Table 2.By changing the composition in the feed,the performance of DWC on energy-saving is investigated.

5.Results and Discussion

5.1.Temperature distribution in DWC

The temperature distribution in HTDWC and HIDWC is shown in Fig.5.The temperature on two sides of the dividing wall is different,so the heat transfers from the hot drawing side to the cold feed side.Referring to literature[16–18,23–25],appropriate models of HTDWC and HIDWC are established based on the experimental data.The simulation results are also illustrated in Fig.5,which are in good agreement with the experimental results,indicating the exactitude of the models.

5.2.Analysis of the concentration distribution

The concentration distributions of A,B and C in the direct sequence,indirectsequence,HIDWC,and HTDWC are shown in Fig.6.In the direct sequence,the concentration of component A gradually decreases whilethe concentration of component B initially increases then drops from the top to the bottom in column 1.Component B reaches the maximum concentration somewhere close to the bottom.This phenomenon is known as back-mixing,which is common in multicomponent separation process.Here back-mixing wastes the energy used to separate component B in column 1,so the directsequence is not energy efficient.In the indirect sequence,back-mixing also causes the energy loss,while the situation is improved in both HTDWC and HIDWC since component B is concentrated in the central column and the back-mixing is avoided.The back-mixing effect in a conventional distillation sequence is usually associated with high energy consumption,which has been discussed in literature.Here it is illustrated how the back-mixing wastes the energy.It can be concluded that DWC enhances the energy-saving by eliminating back-mixing.

Table 2 Parameters of simulation

Fig.6.Concentration distribution.

Fig.7.Comparison of energy loss.

5.3.Analysis of feed compositions

Based on the initial conditions,the thermodynamic efficiency is calculated with Eqs.(1)–(12)at different feed compositions and the results are shown in Figs.7–9.The direct sequence(DS)presents less energy loss and higher thermodynamic efficiency than the indirect sequence(IS),so the direct sequence is chosen as a benchmark in the heat transfer study of HIDWC and HTDWC.

As shown in Figs.7–9,as the concentration of the lightest component A in the feed mixture increases from 0.5:1:1 to 3:1:1,the energy loss in DWC increases and the thermodynamic efficiency decreases.When the proportion of component A is above 40%,DWC does not save energy and its energy loss is even higher than that of conventional separation sequences,lowering the thermodynamic efficiency.In these cases,the energy loss caused by back-mixing of middle component is not crucial,so the advantage of DWC by avoiding back-mixing is not obvious.In addition,HTDWC presents more energy loss and less thermodynamic efficiency than HIDWC under these conditions,which also indicates that the heat transfer through the wall does have in fluence on the thermodynamic efficiency.And the energy consumption of HTDWC is higher than that of the conventional separation sequences since the negative heat transfer in HTDWC is larger than the positive heat transfer.Thus DWC is not suitable to the feed with the lightest component as the major composition.It is also found that HTDWC consumes more energy than HIDWC as the concentration of the lightest component increases.

Fig.8.Comparison of thermodynamic efficiency.

Fig.9.Comparison ofreboiler duty basis in the second conventionalseparation sequences.

As the concentration of the middle component in the feed increases from 1:0.5:1 to 1:3:1,the energy loss in DWC gradually decreases and the thermodynamic efficiency increases as shown in Figs.7 and 8.When the middle component is the major composition of the feed,the back-mixing is usually crucial on energy consumption.DWC is prominent in energy-saving because it can minimize the back-mixing and give more positive heat transfer.At the ratio of A:B:C=1:3:1,the energy consumption of HIDWC and HTDWC is saved 46.37%and 50.25%,respectively,compared to that of conventional separation sequences.Better performance of HTDWC than HIDWC on energysaving is resulted from the thermal conductivity of dividing wall,so effective heat transfer across the wall can save energy.The results in Figs.7–9 show that HTDWC is more effective than HIDWC on energy saving,thermodynamic efficiency and reboiler duty in most feed compositions.Although the energy loss of conventional separation sequences is higher than DWC,the difference becomes smaller as the feed concentration of the heaviest component increases from 1:1:0.5 to 1:1:3,as shown in the right part of Fig.7.The thermodynamic efficiency and reboiler duty are closer and closer,so that the energy loss caused by back-mixing decreases gradually with the increase of the heaviest component.Therefore,the limited energy-saving is barely caused by the positive heat transfer through the wall in HTDWC.It means that a HIDWC would be worse when the heaviest component is the major composition in the feed.When the lightest component dominates in the feed,the negative heat transfer would play an important role in reducing energy saving in HTDWC.Thus HTDWC is not suitable to these cases.

With the change of mixture components,DWC shows better performance than conventional separation sequences in general,especially HTDWC.However,in some mixtures,DWC is limited,e.g.,HIDWC is not suitable when the heaviest component is prominent and HTDWC cannot work with too much lightest component.

6.Conclusions

Through the experimentaland simulation results,itcan be concluded that DWC is an energy-efficient distillation structure in two aspects:elimination of the energy loss caused by back-mixing of middle component and effective heat transfer through the dividing wall.When separating three-component mixtures,DWC is more suitable for those containing more middle component.Particularly,HIDWC is not suitable for mixtures with more heaviest component,while HTDWC is not effective for mixtures containing more lightest component.

Nomenclature

A,B,C lightest,middle,and heaviest components

B bottom flowrate,kmol·h-1

D distillate flowrate,kmol·h-1

F feed rate,kmol·h-1

ΔH enthalpy,kJ

Q heat flow,kJ

S side-draw flowrate,kmol·h-1

ΔS entropy,kJ

T temperature,°C

W work,kJ

η thermodynamic efficiency

Subscripts

ATH adverse heat transfer

BTH beneficial heat transfer

CON condensate

i the number of column

Loss energy loss

max maximum

min minimum

REB reboiler

TH heat transfer