operation of a bench-scale

8/3/2019 Operation of a Bench-scale

http://slidepdf.com/reader/full/operation-of-a-bench-scale 1/5

Computers& Chemical

ELSEVIER Computers and Chemical Engineering 24 (2000) 4955499

Engineering

Operation of a bench-scale

column (HIDiC):

www.elsevier.com/locate/compchemeng

ideal heat integrated distillation

an experimental study

K. Naito a, M. Nakaiwa a,*, K. Huang a, A. Endo a, K. Aso b, T. Nakanishi b,

T. Nakamura c, H. Noda d, T. Takamatsu e

aNat ional Institut e of Mat erials and Chemical Research, Tsuku ba 305-8565, Japan

b Kim ura Ch emical Co ., Hyogo 660-8567, Japan

’ Maruz en Petrochem ical Co., Chiba 290-8503, Japan

’ Kansa i chemical Engineering Co., Hyogo 660-0053, Japan

e Instit ute of Industrial Technology, Kansai University , Suita 564-8680, Japan

Abstract

Experimental study of an ideal heat integrated distillation column (HIDiC) is introduced in this work. It is found that the ideal

HIDiC can be operated very smoothly, with no special difficulties compared with its conventional counterparts. The higher energy

efficiency of the ideal HIDiC is confirmed by the bench-scale experiments. Reflux-free and/or reboil-free operations of the ideal

HIDiC are also demonstrated to be feasible by the experiments. 0 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Distillation; Process dynamics; Control configuration; Startup; Process operation

1. Introduction

Distillation column has long been known as an en-

ergy-intensive but not a high energy efficient process. It

absorbs heat from a high temperature heat source at

the bottom and simply discharges the heat to a low

temperature heat sink at the top. For the purpose of

enhancing the process energy efficiency, process integra-

tion appears to be the most effective method and has

already found wide applications in distillation pro-

cesses. These include heat integration, i.e. heat integra-

tion between a condenser and a reboiler of the same or

different columns, and mass integration, e.g. complex

arrangements of distillation columns, such as the Pet-

lyuk configuration. For a binary distillation column it

is even feasible to conduct heat integration between its

rectifying and stripping sections (Mah, Nicholas &

Wodnik, 1977). Recently Takamatsu, Nakaiwa, Huang,

Noda, Nakanishi and Aso (1997) further claimed that

by such a heat integration, a reboiler and condenser

are, in principle, not necessary for distillation processes.

It means that separation of binary mixtures can be

achieved even when the reflux ratio and/or the reboil

* Corresponding author. Fax: + 81-298-544660.

ratio are zero. Hence, operating costs can be sharply

reduced. We are currently investigating the design and

operation of such a distillation column, which is named

the ideal heat integrated distillation column (HIDiC) in

our work.

Owing to the heat integration between the rectifying

and the stripping sections, the operation of the process

seems to be much more difficult than conventional

distillation columns. Although the simulation study for

the operation of the process indicated that the ideal

HIDiC can be operated quite well, with no special

difficulties compared with conventional distillation

columns (Nakaiwa, Huang, Owa, Akiya, Nakane &

Takamatsu, 1998; Nakaiwa, Huang, Endo, Owa,

Akiya, Nakane & Takamatsu, 1999), it is still necessary

to further examine the operation feasibility by a series

of experiments. Up to now, there has been no experi-

mental study reported for the ideal HIDiC, although

the concept was proposed in the late 1970s. The de-

tailed experimental plan includes two steps. First of all,

a lab-scale ideal HIDiC was established in the Kimura

Chem. Plant Co. in 1997 and was used to study the

internal design for the ideal HIDIC. It was found that

by proper internal design the influences of heat transfer

towards mass transfer can be neglected. Moreover, the

009%1354/00/S - see front matter 0 2000 Elsevier Science Ltd. All rights reserved

PII: s0098-1354(00)00513-5



K. Naito et al. /Computers and Chemical Engineering 24 (2000) 495-499

Reducing valve

Fig. 1. A representative scheme of the ideal HIDiC.

x 1Fig. 2. Illustration of principle on a x-y diagram.

I I

Fig. 3. Layout for the bench-scale HIDiC plant.

heat integration between the rectifying and the strip-

ping sections can work sufficiently as a source for the

internal liquid and vapor flows. Secondly, a bench-scale

plant was established in the Maruzen Petrochemical

Corporation and is being used to check the process

operation feasibility.

The objective of this paper is to examine the process

operating feasibility through experimental tests. Theexperimental study will cover process startup operation

and normal operations with and without external reflux

and reboil flows. Economical evaluations of the ideal

HIDiC are also made compared with a conventional

distillation column. Advantages for reflux-free and/or

reboil-free operations are indicated.

2. Principle and configuration of the Ideal HIDiC

The ideal HIDiC is such a process that its stripping

section and rectifying section are separated into two

columns, while connected through the heat integration

between them (Fig. 1). To accomplish internal heat

transfer from the rectifying section to the stripping

section, the rectifying section is operated at a higher

pressure and a higher temperature than the stripping

section. For adjusting the pressures a compressor and a

reducing valve have to be installed between the two

sections. Owing to the heat integration, a certain

amount of heat is transferred from the rectifying sec-

tion to the stripping section and generates the reflux

flow for the rectifying section and vapor flow for the

stripping section. Thus, the heat duties of condenser

and reboiler are reduced. By proper process design,even reflux-free and/or reboil-free operations can be

achieved. As a result, the energy consumption could be

reduced. Fig. 2 shows the design principle of the ideal

HIDiC on a x-y diagram.

When trim-condenser and trim-reboiler are in use,

the control configuration can be the same as conven-

tional distillation columns, namely, the sensitive stage

temperatures in the rectifying and stripping sections are

controlled by external reflux and reboil flows, respec-

tively. When trim-condenser and trim-reboiler are not

in use, the process can be controlled by the pressure

difference, pr -ps, between the rectifying and the strip-ping sections and feed thermal condition, q.

3. Bench-scale experimental

The layout for the bench-scale plant is shown in Fig.

3. A binary mixture of benzene-toluene is to be sepa-

rated by the ideal HIDiC. Feed is introduced to the

process at a constant flow rate. Several temperature and

pressure sensors are installed along the length of the

ideal HIDiC. The levels of reflux drum and reboiler are



K. Nai to et al. /Comput ers and Chemical Engineering 24 (2lWlI) 495-499 491

(a) (b) w

Fig. 4. A general structure of HIDiC.

maintained by the top and bottom product flows, re-

spectively. The top and bottom products are then

mixed together and recycled back to the feed tank.The bench-scale ideal HIDiC is about 20 m in height

and 254 mm in diameter. The structure of a vertical

shell and tube heat exchanger is adopted as a general

configuration for the ideal HIDiC. Fig. 4c illustrates a

detailed arrangement of the ideal HIDiC. The tube side

works as the rectifying section and the shell side as the

stripping section. The tube wall acts as effective heat

transfer area. Structured packing, MC PACK, is

adopted for the internals of the ideal HIDiC. In the

rectifying section, because the vapor is condensed grad-

ually along the tube within the heat exchange, the

vapor flow rate decreases as it goes from the bottom tothe top. On the contrary, in the stripping section,

because the liquid evaporates gradually along the shell

within the heat exchange, the vapor flow rate increases

as it goes from the bottom to the top. The ideal cross

section of HIDiC should take a shape as shown in Fig.

4a, because the cross section area of the distillation

column should be proportional to the vapor flow rate.

However, it is extremely difficult to manufacture such a

column in practice, then it is modified to a form as

shown in Fig. 4b. As can be seen, the tube size of the

bench-scale ideal HIDiC changes twice from the top to

the bottom. Their diameters are 140, 165 and 190 mm

and their corresponding lengths are about 3, 8 and 5 m,

respectively.

4. Experimental results

4.1. Sta rtup procedure and operation

During process startup, the inverse heat transfer

from the stripping to the rectifying sections must be

avoided, because it can lead not only to consumption of

extra energy, but also risks of potential operation prob-

lems. Therefore, it is very crucial to raise a certain level

of pressure difference between the two sections as soon

as possible after startup operation begins. To enhance

the pressure difference one needs to start the overheadtrim-condenser at a proper time later than the bottom

trim-reboiler.

Based on the operating characteristics of the ideal

HIDiC, we devise a startup operation procedure as

indicated in Table 1.

With this procedure, it was found that no special

difficulties were encountered in the startup operation.

Generally 10 h are needed for the process to reach the

normal steady state, although further improvement of

startup operation seems to be possible. Fig. 5 gives a

typical transient response of the ideal HIDiC.

4.2. Steady state runs

Steady state operations of the ideal HIDiC are ob-

tained after startup operation. Table 2 shows the nor-

mal operations of the ideal HIDiC and Table 3 gives

some representative values of the process. More than

100 h of continuous operation of the ideal HIDiC is

performed and no special difficulties are encountered

Table 1

Startup operation procedure of the ideal HIDiC

Phase Procedure

(i)(ii)

(iii)

(iv)

(v)

(vi)

(vii)

The ideal HIDiC is empty.

Feed is introduced into the feed stage.

Liquid reaches the bottom stage and the trim-reboiler starts to increase its holdup.

The trim-reboiler has a certain specified volume of liquid and level control system begins to work. Heat into the reboiler is

introduced at this time, and the compressor begins to work.

The pressure in the rectifying section reaches a pre-specified level and the trim-condenser starts to work. The pressure is maintained

by the trim-condenser duty and level control starts to work. The process is run at total reflux until the vapor flow rate from the

overhead plate equals to a specified value.

The distillate product is drawn out and the overhead and bottom composition controllers are switched on. Meanwhile, the reflux

and reboil rates are reduced to zero gradually.

Continuous operation starts.



498 K. Naito et al. /Computers and Chemical Engineering 24 (2000) 495-499

90

bo’;60

-Bottom (Rec. Sec.)

-Bottom (Str. Sec.)

0 3 6 9 12

t b-dFig. 5. Transient responses of the ideal HIDiC during startup opera-

tion.

Table 2

Results for steady state runs of bench-scale HIDiC plant

Items Values

Pressure of rectifying section

Pressure of stripping section

Feed flow rate

Distillation rate

Feed composition

(Benzene)

(Toluene)

Feed thermal condition

0.26 MPa

0.13 MPa

3.2 kmol h-’

1.6 kmol hh’

0.5

0.5

1.0

during the experiment. Although further experiments

should be carried out, the already obtained results are

sufficient to make sure that the process can be operated

very smoothly just as its conventional counterparts.

Reflux-free and/or reboil-free operations of the ideal

HIDiC are also confirmed by more than 10 h continu-

ous steady state runs. It indicates that the heat integra-

tion between the rectifying and the stripping sections

can work effectively to generate necessary internal liq-

uid and vapor flows. It lays the basis for the further

study of the process operation when external distur-

Table 3Steady state values of the ideal HIDiC

Table 4

Comparisons between the ideal HIDiC and a conventional distillation

column

Items

Conventional

(R = 7.0)

Energy consumption Comparison

(kW) (%)

73.9 100

Ideal HIDiC(R = 0.0)

44.1 60

HIDiC (R = 0.3) 45.7 62

bances occur, such as feed flow rate or composition

changes.

4.3. Reduction of energy consumption

In terms of the steady state operation condition, the

energy consumption can be examined. Table 4 com-

pares the operating profits between a conventional dis-tillation column and the ideal HIDiC. The tabulated

data are obtained by simulation, The conversion coeffi-

cient from electric power to heat energy for compressor

is assumed to be 3. The conventional distillation

column is assumed to have the same number of stages

as the ideal HIDiC. The comparisons clearly demon-

strate the advantages of the ideal HIDiC. The ideal

HIDiC is about 40% more energy saving than the

conventional distillation column when external reflux is

equal to 0.0 kmol hh ‘, and the HIDiC is 38% more

energy saving when external reflux is equal to 0.3 kmol

h - ‘. As can be seen, there appears almost no differencein energy consumption between the ideal HIDiC and

the HIDiC, because the major part of energy consump-

tion is from the compressor. Moreover, the HIDiC can

be expected to be more energy efficient through heat

recycle of the overhead vapor flow and optimization of

external reflux flow and pressure difference between the

rectifying and the stripping sections. Future work will

be done to calculate the actual energy consumption for

the bench-scale HIDiC plant by taking into account the

actual heat loss.

Items Composition (mol%)

t (h) 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0

Top (benzene)

(R = 0.0)

(R = 0.3)

Bottom (toluene)

(R = 0.0)

(R = 0.3)

99.92 99.94 99.92 99.92 99.94 99.97 99.92 99.92 99.92

91.36 97.34 99.92 99.95 99.92 99.94 99.94 99.92 99.92

80.40 80.40 98.19 98.26 98.95 97.53 97.53 99.54 99.54

87.46 87.49 91.41 91.41 99.71 99.71 99.72 99.72 99.68



K. Nait o et al. /Comput ers and Chemical Engineering 24 (2000) 495-499 499

5. onclusions

Bench-scale experiments have been carried out to

evaluate the operation feasibility of the ideal HIDiC.

Although further experimental data should be collected,

it is now understood that the ideal HIDiC can be

operated as smoothly as conventional distillation

columns. In addition, both the top and bottom productspecifications can be met, and the following conclusions

have been reached.

Reflux-free and/or reboil-free operation can be

achieved smoothly by the ideal HIDiC. It indicates that

the heat integration between the rectifying and the

stripping sections can work effectively to generate nec-

essary internal liquid and vapor flows.

Experimental data shows that the ideal HIDiC is

really more energy efficient than conventional distilla-

tion columns. For the benzene-toluene binary mixture,

more than 40% reduction of energy consumption can

be achieved.

Acknowledgements

This work is supported by the New-Energy and

Industry Technology Development Organization

(NEDO) through the Energy Conservation Center of

Japan, and their support is hereby acknowledged.

Appendix A. Nomenclature

pr-pS pressure difference between rectifying and

stripping sections, MPa

:

feed thermal condition

reflux flow rate, kmol h-’

t time, s

T temperature, KX mole fraction of liquid

Y mole fraction of vapor

References

Mah, R. S. H., Nicholas, J. J., & Wodnik, R. B. (1977). Distilla-

tion with secondary reflux and vaporization. American Institute

of Chemical Engineering Journal, 23, 651-658.

Nakaiwa, M., Huang, K., Owa, M., Akiya, T., Nakane, T., &

Takamatsu, T. (1998). Operating an ideal heat integrated distil-

lation column (HIDiC) with different control algorithms. Com-

put ers & Chemi cal Engineering, ,922, S389-S393.Nakaiwa, M., Huang, K., Endo, A., Owa, M., Akiya, T., Nakane,

T., & Takamatsu, T. (1999). Evaluating control structures for a

general heat integrated distillation column (general HIDiC).

Comput ers & Chemical Engineering, S23, 851-854.

Takamatsu, T., Nakaiwa, M., Huang, K., Noda, H., Nakanishi, T.,

& Aso, K. (1997). Simulation-oriented development of a new

heat integrated distillation column and its characteristics for

energy saving. Comput ers and Chemical Engineering, 21, 243-

247.

operation of a bench-scale

Documents