Divided Wall Distillation Column

Why & What is it..???

• Energy used for continuous distillation processes comprises approximately 40% of total energy use in chemical process industry .This leads us to conclude following: distillation is still the most common method for separation of liquid mixtures and it is very energy intensive. Together with a fact that many successful separation systems were invented by experience alone, this makes distillation, which is considered to be the most mature among separations technologies, still an amenable subject for process intensification.

• Most impressive development in this respect is implementation of so called dividing wall column (DWC), which not only leads to energy saving but also to capital saving.

What is it..??? Contd..

• State of the art separation of hydrocarbon mixtures into three different fractions is mostly realized by a separate column arrangement.

• The selection of the optimum sequence is made considering all required aspects, such as quantity of individual fractions, relative volatilities between the components of the desired fractions, thermal stability of the components, available utilities – e.g. steam temperature levels, etc. – in order to have the lowest possible investment and operating cost.

• Mainly three types of arrangement are made which are :-

(1) Direct Sequence Column.

(2) Indirect Sequence Column.

(3) Thermally Coupled Column.

• The thermally coupled column does not require any additional heating or cooling source for the second column.

Thermally Coupled Column- A Solution ??

• A further step in development of the thermally coupled column is realized by integration of the separation task within one column shell only, thus offering considerably lower investment and also saving operating costs. If thermally coupled columns are integrated into one single shell, it is referred to as a divided wall column (DWC) or partitioned wall column, as shown in the figure.

How it works..??

• A partition wall in the middle section of the column separates the main column and the side column. In this fully thermally coupled column realized as a DWC the multicomponent feed enters the main column, where a cut between low- and high-boiling components takes place. The middle-boiling components of fraction B distribute to the top of the partition wall together with the low boiling components of fraction A (AB) as well as to the bottom of the partition wall, but along with the high-boiling components of fraction C (BC). Thus, as a major advantage of this configuration, the components of fraction C do not enter the side column at the top and the components of fraction A do not enter the side column at the bottom. The mixture of low and middle boiling components (AB) is separated in the upper column section of the main column and the same applies to the high boiling and middle boiling components (BC) which are separated in the lower section of the main column.

• It can be seen that neither low boiling components of fraction A can pass to the bottom part of the side column, nor high boiling components of fraction C can pass to the upper part of the side column. Thus, contamination of middle boiling fraction B can be avoided.

How it is beneficial over other methods..??

• In the conventional case, the concentration of middle boiling components B in the first column increases below the feed, as the concentration of low boiling components of fraction A decrease. However, further down the column, the concentration of the middle boiling components B decreases again as the concentration of the high boiling components increase. This remixing, which cannot be avoided in conventional fractionation when a feedstock contains more than two components, is a source of inefficiency.

Contd..

• By using divided-wall column fractionation, this remixing effect can be avoided as fraction A and fraction C are separated at the top and the bottom of the main column, respectively, whereas fraction B is withdrawn in the middle of the side column, where the concentration of B is maximum.

• Furthermore, any quality and yield for the fractions A, B and C, which are thermodynamically possible, can be reached in a DWC as in a conventional sequence of two fractionation columns.

• The divided-wall column consists in principle of six column sections, all of which have to be controlled by only one external reflux and only one reboiler system.

A Case Study on Benzene removal..

• Flow Diagram..

Mathematical Model..

• Consider the separation of the ternary mixture containing benzene, toluene & o-xylene. Benzene & o-xylene are obtained as top & bottom products respectively.

Contd..

• The volume holdup of condenser/reflux drum has been assumed constant. For the rectification section, prefractionator, main column, and stripping section:

• Mass balance for component i ,

=Vj+1 yj+1,i + Lj-1 xj-1,i -Vj yj,i –Lj xj,i +Fj zj,i –Sj xj,i , where y=(Mj xj,i) & j=1:n1 &

i=1:nc .

Summation Equations: =1 ; .

• Energy Balance :

= Vj+1HV j+1 + Lj-1HL j-1 – VjHV j – LjHL j +FjHF j – SjHL j , where y=( HL,j Mj ).

• Equilibrium Relationship : yj,i=Kj,I xj,I . Where,

Contd..

For the condenser :

• Material Balance:

= V1y1,i+ L0xD,i- DxD,i , where y=( M0xD,i ).

• Energy Balance:

= V1HV,1- L0HL,0- DHD- qC , where y= ( M0HD ).

Summation Equation: =1, where f=xD,i .

• For the Reboiler :

Material Balance: =Ln4xn4,i-Vn4+1yn4+1,i-wxn4+1,i ,where y=( MR xw,i ).

Contd..

• Energy Balance:

= Ln4HL,n4- HV,n4+1- wHL,n4+1+ qR , where y=( Mn4+1 HL,n4+1 ).

• Equilibrium Relationship:

yn4+1,i = Kn4+1,i * xw,I

Summation Equations:

)=1 & )=1 , where f= xn4+1,i & g= yn4+1,i .

Vapour Mixing & Liquid Splitting..

• At the intersection of rectifying section (section 1) with prefractionator (Section 2) and main column (Section 3):

Vapour Mixing: Vn1+1(1)=V1(2)+V1(3).

[Vn1+1 yn1+1,i](1)=V1 y1,i(2)+ V1 y1,i(3).

Liquid Splitting: L0(2)=Ln1(1) , L0=[(1-)Ln1](1) ,

where =Liquid Splitting factor.

x0,i(2)=xn1,i(1) , x0,i(3)=xn1,i(1).

Contd..

• At the intersection of Section 2 and 3 with Section 4 (Stripping Section):

Vapour Splitting: Vn2+1(2)= βV1(4) ,

Vn2+1(3)=(1-β)V1(4).

where β=Vapour Splitting factor.

yn2+1,i(2) = y1,i(4) ;

yn2+1,i(3) = y1,i(4) .

Liquid Mixing: L0(4) = Ln2(2) + Ln2(3)

Ln2(2) xn2,i(2) + Ln2(3) xn2,i(3) = L0(4) x0,i(4).

Parameters to Look out for..

• Various parameters that can be determined by simulation method are:

Reflux Ratio.Effect of location of feed stage.Effect of liquid & vapour splitting factor.Effect of bottom flow rate.Effect of location of side stage.Dynamic response of the DWC i.e a graphical plot representing the

time taken to split the composition into it’s constituents & their respective composition.

Energy consumption & savings along with optimal cost for the tower operations.

Applications of DWC..

• Divided-wall columns are capable of separating a feed mixture into three high purity fractions. • Due to the investment and energy advantages of this

configuration a wide field of applications, especially for fractionation tasks in the petro-chemical and refinery services, are of interest because of the usually big throughputs of such process units. • One typical application for the DWC is the reduction of the

benzene content in motor gasoline. In order to reduce vehicle emissions most industrial countries issued fuel quality directives, which limit the benzene content of motor gasoline to <1 per cent by volume.

Contd..

• Revamp options

Another main advantage of the DWC technology is the simple implementation in revamp projects. As can be imagined, an existing column may be reconstructed to a DWC by modifying virtually only the internals of the column. • Two main revamp scenarios are possible:

1. Modification of internals without column shell replacement. Here only that part of the existing column is modified where the partition wall is to

be installed.

2. Modification of internals with partial column shell replacement. For smaller column diameters this method is very attractive. Exactly the section of the column shell where the partition wall is to be installed is replaced by a new section.

Pygas Splitting Options : An Application..

Contd..

• In Option A the pyrolysis gasoline is first selectively hydrogenated. Then the C5–fraction and a C7+fraction are separated in a DWC from a C6 heart cut. The C5–fraction is either mixed with the C7+fraction and used as gasoline blend stock or it can be recycled. The C6 heart cut is fully hydro-treated in order to remove sulfur and to saturate remaining olefins.

• Option B shows a configuration where products of higher value are gained, at the price of increased hydrogen consumption. The pyrolysis gasoline is first selectively and fully hydrogenated to saturate di-olefins/olefins and to remove sulfur. The full range hydro-treated hydrocarbons are fractionated in a DWC into three products. The top product is a completely saturated C5–fraction. The bottom sulfur free C7+fraction serves as high quality gasoline blend-stock.

Conclusion..

• Compared with conventional fractionation, where two column systems are required for the separation of three fractions with high purity requirements, the DWC technology only needs one column system with one overhead condensing system and one re-boiling system. Thus, considerable investment costs and plot space can be saved. As remixing of the medium boiling components in the column can be avoided by using DWC technology, further energy cost can be saved and reduce the operating costs to a minimum.

• The advanced but simple control concept ensures a robust operating behavior even under critical and fast-changing feedstock conditions. Existing conventional columns can be easily revamped to a DWC column by adding a partition wall or exchanging the middle section of an existing column in an acceptable period of time.