Energy 29 (2004) 467–489

www.elsevier.com/locate/energy

Simulation, exergy analysis and application of diabaticdistillation to a tertiary amyl methyl ether production unit

of a crude oil refinery

R. Rivero �, M. Garcia, J. Urquiza

Instituto Mexicano del Petroleo, Grupo de Exergia, Eje Central Lazaro Cardenas 152, 07730 Mexico

Abstract

This paper presents the results of a detailed exergy analysis of a tertiary amyl methyl ether (TAME)unit of a crude oil refinery and the application of diabatic distillation to the depentanizer tower of theunit. Diabatic distillation is a separation process in which heat is not only supplied to the reboiler andextracted from the condenser [as in a conventional (adiabatic) distillation column], but is also transferredinside the column. The process enables operation to approach equilibrium conditions, thus reducingexergy losses and increasing exergy effectiveness. In a TAME unit of a refinery, isoamylenes are con-verted to TAME. Before transforming the isoamylenes in the reactors, it is necessary to recover themfrom a catalytic gasoline stream by a depentanization process. The exergy losses of this depentanizationprocess represent about 70% of the total exergy losses of the unit. The results of the exergy analysis ofthe TAME unit are presented and a detailed exergy analysis of the conventional adiabatic depentanizercolumn is conducted for comparison purposes. Then, the application of diabatic distillation to the systemis evaluated by using cooling water circulating in series from tray to tray in the rectification section andby making the steam emanating from the reboiler circulate in series from tray to tray in the stripping sec-tion. The results in terms of the reduction of exergy losses, heating and cooling media flow rates, and costeffectiveness of the diabatic option for the depentanizer section of the plant are compared to the originaladiabatic system, and the effect of the diabatization on the overall exergy performance parameters of thedepentanizer section and on the whole TAME unit, are presented in this paper.# 2003 Elsevier Ltd. All rights reserved.

� Corresponding author. Tel.: +52-53-33-84-27; fax: +52-53-33-80-67.E-mail address: rrivero@imp.mx (R. Rivero).

0360-5442/$ - see front matter# 2003 Elsevier Ltd. All rights reserved.doi:10.1016/j.energy.2003.10.007

Nomenclature

A Pre-exponential factorC ConcentrationEa Activation energyEfl Effluent exergy lossesEx ExergyF Flow rateH EnthalpyIrr Irreversible exergy lossesKeq Equilibrium constantk Reaction rate constantL Liquidn NumberP PressurePex Total exergy lossesQ HeatQc Condenser heat dutyQr Reboiler heat dutyr Rate of reactionR Gas universal constantS EntropyT TemperatureV VaporX Composition

Greek symbol

e Effectiveness

Subscripts

c Concentrationalf Physicali Chemical componentMeOH Methanoln Numbernp Net produced, acceptor, sink, productns Net supplied, donor, source, fuelp Mechanicalq Chemicalr Reactionals Substantialt Thermal

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1. Introduction

The objective of this paper is to present the results of a detailed exergy analysis of a tertiaryamyl methyl ether (TAME) production unit and to evaluate the application of diabatic distil-lation to its depentanizer tower. The principles and methodology of exergy analysis and of dia-batic distillation, including its experimental demonstration for a binary mixture, have beenpresented in several papers [1–7]. Its application to industrial multicomponent distillation sys-tems is now being studied [8,9].In processes involving chemical reactions, like the production of TAME, the preparation of

the reactants prior to the reaction and the separation of products and by-products after thereaction are often the critical points in terms of energy degradation (exergy losses). Thus theplaces where important improvements in energy efficiency can be obtained.

1.1. Process description and simulation

TAME is one of the so-called oxygenates that are added to gasoline to improve octane ratingand reduce carbon monoxide and hydrocarbons emissions. The unit [10] is designed to convertto TAME most of the isoamylenes contained in the gasoline feed stream coming from the fluid-ized catalytic cracking (FCC) unit. Fig. 1 shows a simplified process flow sheet of the TAMEunit.The unit can be divided into seven sections: (1) the depentanizer section, where the gaso-

line cut is separated and isoamylenes are recovered in order to produce TAME; (2) the iso-amylenes washing section, in which contaminants for the catalyst are eliminated (the mostimportant contaminants are: sulfur, basic nitrogen, NH3, NaOH, propionitrile, acetonitrile);(3) the reaction section, which is formed by two catalytic reactors, where a conversion of85% is reached; (4) the rectification section, in which raffinate is obtained at the top of thefractionation column, while TAME is drawn out from the bottom of the column; (5) thirdreactor section, in which a conversion of 18% to TAME is reached; (6) raffinate washingsection, where unreacted isoamylenes are recovered and a stream of TAME and pentanes issent to storage, while the water–methanol mixture is sent to the methanol recovering sec-tion; and finally, (7) the methanol recovering section, in which unreacted methanol andwashing water are recovered in order to recycle them to the reaction and raffinate washingsections, respectively.

TAME Tertiary amyl methyl etherti Total inputto Total output0 Reference, dead state2MB1 2-Methyl-1-butene2MB2 2-Methyl-2-butene

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The TAME unit considered in this study is located at Pemex’s Salina Cruz refinery, with a

nominal capacity of 60,000 ton/year of TAME production. Basically, the process consists in the

etherification reaction of isoamylenes and methanol using a ionic exchange resin as catalyst, at

temperatures and pressures in the range of 58–90vC and 12–16 kg/cm2. The commercial name

of the catalyst is Amberlist 15.

Fig. 1. Simplified process flow sheet of TAME process.

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The simulation of the plant is conducted using the AspenPlus code [11]; thermodynamicproperties are evaluated using the following Aspen Property Sets: Braun K-10 (BK10), Pre-dictive-Soave-Redlich-Kwong (PSRK) and UNIFAC Liquid-Liquid (UNIF-LL).In the depentanizer section, the BK10 property set is used because the mixture to be

depentanized is formed mainly by pseudocomponents. The BK10 option set uses Braun K-10 K-value correlations, developed from the K10 charts for both real components and oilfractions. The real components include 70 hydrocarbons and light gases. The oil fractionscover boiling ranges 450–700 K. Propietary methods were developed by Aspen to coverheavier oil fractions [11]. This property set is suited for vaccum and low-pressure applica-tions (up to several atm). For calculating the liquid and vapor enthalpy of petroleum mix-tures the BK10 property set uses the Lee–Kesler model, vapor entropy is calculated withthe ideal gas model and liquid entropy is calculated with the Braun K-10 model for liquidfugacity coefficients.In other sections, the PSRK property set is used, which is an extension of the SRK

cubic equation-of-state. In the washing columns and decanters, the UNIF-LL property setis used, which is an activity coefficient model based on the group contributions methodinstead of the molecular contributions method. All these option sets are available in Aspen-Plus.In order to obtain the desired reaction product, two tubular reactor models (RPLUG) and

one stoichiometric model (RSTOIC) connected in series were used. In addition, a third tubularreactor after the TAME recovery tower is considered.In the adiabatic tubular reactors, TAME is formed specifying the length and the diameter of

the reactors, as well as the reaction kinetics [12], given by:

r ¼ ki CiC5 �CTAME

Keq0 CMeOH

� �� �(1)

where

ki ¼ Ai�exp

�EaiRT

� �(2)

k2MB1 ¼ 4:1317�1011exp � 20216

T

� �(3)

k2MB2 ¼ 5:9148�1011exp � 22018

T

� �: (4)

The secondary reactions are simulated in the stoichiometric reactor: the hydration of iso-amylenes forms teramyl alcohol, the isoamylenes dimerization forms di-isoamylene, and themethanol dehydration forms dimethyl ether and water, for a total of three stoichiometric reac-tions.The main and secondary reactions are [10]:

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. Main reaction of TAME formation:

. Secondary reaction of isoamylenes hydration:

. Secondary reaction of isoamylenes dimerization:

. Secondary reaction of methanol dehydration:

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In the simulation module all water-cooled and air-cooled coolers, as well as all reboilers haveFortran subroutines coupled to the module to determine the water, air and steam flow rates andconditions [13].

2. Exergy analysis

2.1. Exergy calculation

Exergy is defined as the amount of work obtainable when some matter is brought to a stateof thermodynamic equilibrium with the common components of the natural surroundings bymeans of reversible processes, involving interactions only with the above-mentioned componentsof nature [14].Exergy calculation is based on the determination of two thermodynamic state functions,

enthalpy and entropy [15]. In this way, if a system is considered defined by its independent vari-ables, pressure, temperature, composition, etc., and placed in a given environment ‘‘0’’, theexergy of the system is defined by the general expression:

Ex ¼ H �H0ð Þ � T0 S � S0ð Þ: (5)

The first term on the right hand side of Eq. (5) represents the total enthalpy of the system, thesecond term on the right hand side represents the total entropy of the system and the term T0 isthe environment temperature.The exergy of a substance, called substantial exergy Exs, depends on the kind of matter (on

its composition and on its temperature and pressure conditions). This exergy can be divided intotwo parts: physical exergy Exf due to the departure of temperature and pressure from the refer-ence, and chemical exergy Exq, due to the departure of composition from the reference.The physical exergy can also be divided into two terms: thermal exergy Ext, due to tempera-

ture, and mechanical exergy Exp, due to pressure, so the substantial exergy is the sum of ther-mal, mechanical and chemical exergies.In the same way, the chemical exergy can be divided into two terms: reactional exergy Exr,

resulting from the chemical reactions necessary to produce from the initial composition of thesubstance, substances existing as stable components of the dead state, e.g. the components ofthe atmosphere, the oceans and the earth’s crust; and concentrational exergy Exc, resulting fromthe processes needed to equal the chemical concentration in the dead state. The exergy of a sub-stance is then:

Exs ¼ Ext þ Exp þ Exr þ Exc: (6)

Fig. 2 shows these different terms and the conditions from which they are calculated, accordingto three different practical dead states: A (T0, P0, X), B (T0, P0, XI) and C (T0, P0, X0). Fordead state C, Szargut has produced a model of reference substances of the environment fromwhich the standard chemical exergies are obtained [14].

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2.2. Methodology of application of the exergy analysis [15]

The basis for the application of exergy analysis to a system is the block method. In thismethod, the system is assumed to be a block or black box that can have interactions with theirsurroundings of three kinds and only three: heat transfer, work transfer and mass transfer.Fig. 3 shows a block. Mass transfers accompany the input of raw materials and utilities (like

steam, chemicals, catalysis, etc.) and by the output of products, by-products, waste materials,etc. Work and heat transfers are represented by the energy needs (electricity, mechanical work,solar radiation) and the energy outputs and wastes.This approach does not imply any restriction regarding the size of the block: an industrial

process can be considered as a single block or as a group of blocks; an equipment item can beconsidered as a single block, as the portion of a block or as a group of blocks.The main objective of the exergy analysis is to find the critical points in a system where it is

necessary to apply some measures in order to reduce exergy consumption and to make energyuse more economic and efficient.

Fig. 3. Block method of exergy analysis.

Fig. 2. Substantial exergy.

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The first step in the application of an exergy analysis to a system consists in representing the

system by a series of blocks. Inputs and outputs of each block represent the main process

parameters, including exergy values for each stream participating in the block.Once the block diagram has been stated, with which the boundaries of the system are obvi-

ously defined, it is possible to conduct mass, energy (enthalpy) and entropy balances, globally

and for each block, from which the exergy balance and the exergy performance parameters are

obtained. Although the exergy analysis of the TAME unit was conducted considering 63 blocks

or pieces of equipment which receive, produce or are interconnected by 124 exergy streams, the

results presented here consider each of the seven sections as a block in order to simplify the

presentation and discussion. The simplified block diagram is shown in Fig. 4.

Fig. 4. Simplified block diagram of the TAME unit.

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Then the different performance parameters are determined; they can be classified in three cat-egories: quantitative parameters, qualitative parameters and combined parameters, such as theexergy losses, the effectiveness and the exergy improvement potential, respectively. These para-meters are described below.

2.3. Exergy losses

The exergy losses of a system can be split in two [15]. On one side are those resulting fromthe irreversibility of the processes taking place in the system (internal losses), and on the otherside, those resulting from an exergy discharge to the environment (external losses).The irreversible exergy losses are the difference between the total input exergy (Exti) and the

total output exergy (Exto). These losses can also be calculated using the Gouy–Stodola theorem:

Irr ¼X

Exinput �X

Exoutput ¼ T0DS (7)

Irr ¼ Exti � Exto: (8)

The irreversible exergy losses can also be expressed considering the system as a couple(source–sink, donor–acceptor, fuel–product), obtained from the difference between the net sup-plied exergy Exns (by the source(s), donor(s) or fuel(s)) and the net produced exergy Exnp (to thesink(s), acceptor(s) or product(s)):

Irr ¼X

DExsources �X

DExsinks ¼ T0DS (9)

Irr ¼ Exns � Exnp: (10)

The effluent exergy losses represent the ecological effects of rejecting streams to the environ-ment; they are simply the sum of all exergy streams rejected to the environment:

Efl ¼X

Exrejected to the environment: (11)

The total exergy losses in any system are then equal to the sum of irreversible and effluentlosses:

Pex ¼ Irrþ Efl: (12)

For any system composed of n blocks, the Gouy–Stodola theorem states that the overallexergy losses are the sum of the exergy losses of the n blocks.

2.4. Effectiveness

In order to determine how well the desired effect of the system is accomplished, the effective-ness is calculated as the ratio of the net produced exergy to the net supplied exergy:

e ¼ ExnpExns

: (13)

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The effectiveness can also be expressed as a function of the irreversible exergy losses sincethese are equal to the difference between both net exergies [5,15]:

e ¼ 1� Irr

Exns: (14)

The effectiveness is then the measure of the system’s capacity to produce the desired effect.In other words, it is the fraction of the net supplied exergy used by the system to perform itsfunction.

2.5. Exergy improvement potential [16]

Total exergy losses and effectiveness are the quantitative and qualitative measures of energydegradation, respectively, and they can be combined to have a more complete parameter of theperformance of the system, this parameter is called the exergy improvement potential [5,15,16].The exergy improvement potential of the system is a measure of how much and how easily thesystem could be improved for optimization purposes. This parameter is based on the definitionof thermodynamic potential for improvement proposed by Reistad [17], but incorporating anadditional term to explicitly consider the effluent exergy losses. It is obtained from the exergylosses and the effectiveness of the system:

Pot ¼ Irr 1� eð Þ þ Efl: (15)

It is formed by three contributions: the absolute potential (Irr), the relative potential (1 e)and the environmental potential (Efl). The relative potential (1� e) is a measure of the facilityto improve a system; if the effectiveness is very low the relative potential approaches itsmaximum value, so in principle the system should be improved easily; the absolute potential Irris a measure of how much the system could be improved from inside itself by attacking itsentropy production or irreversibility; the environmental potential Efl is a measure of how muchthe system could be improved from outside itself if the exergy of the effluents is used instead ofrejected to the environment.The exergy improvement potential makes it possible to determine the critical points of the

system stating a hierarchy on its components in such a way that measures be applied in the pla-ces where they will be most effective. In order to optimize a system it is necessary to increase theeffectiveness of the blocks with the higher irreversible exergy losses and to reduce the effluentexergy losses of the blocks where their utilization could be profitable.In order to optimize a system it is necessary to attack preferably the blocks with the higher

exergy improvement potential.

2.6. Exergy analysis results

A complete exergy analysis of the TAME unit has been conducted in the frame of a researchproject [8], using the general methodology presented in previous papers [5,15]. The exergy analy-sis of the plant has been conducted using the exergy analysis subroutines coupled to the AspenPlus simulation code developed by the IMP Exergy Group [8]. The dead state conditions in all

477R. Rivero et al. / Energy 29 (2004) 467–489

calculations are a temperature T0¼ 25vC a pressure P0¼ 1 atm and a composition X0 equal to

the composition used by the Szargut model for the standard chemical exergy of substances [14].The overall effectiveness of the unit is 15.59%, i.e. 84.41% of the net supplied exergy is lost by

irreversibilities. These irreversible exergy losses represent 87.68% of the total exergy losses, theremaining 12.32% are effluent exergy losses (heating air, cooling water, etc.).Fig. 5 shows the distribution of the total exergy losses of the different sections of the TAME

unit. The main exergy losses of the TAME unit (about 70% of the total) are located in thedepentanizer section, particularly in the depentanizer column (13.0%) and in its associated con-denser (22.7%) and reboiler (15.2%). The main reaction section is not the most important onefrom the exergy losses viewpoint; the reason why the main reaction section is not the mostimportant from the exergy losses viewpoint stems from the fact that in the depentanizer sectiona high energy degradation occurs between the reboiler and the condenser for separating theproducts.Fig. 6 shows the effectiveness of the sections. The most effective of which is the methanol

recovery section with 36%, followed by the depentanizer section with 23.6%.By combining the aspects measured by both the exergy losses and the effectiveness one

obtains the exergy improvement potential. Fig. 7 gives the distribution of the exergy improve-ment potential of the different sections of the TAME unit. The depentanizer section has 64.1%of the improvement potential of the TAME unit (82.2 MJe/ton of feed). The symbol ‘‘Je’’denotes joule of exergy (or electricity), whereas the symbol ‘‘J’’ refers to joule of enthalpy (totalenergy or heat).The critical section of the plant indicated by the exergy losses is confirmed to be the depen-

tanizer section. It is then very important to further analyze this section through a diabaticstudy. The highest exergy improvement potential is on the depentanizer section so the effective-ness of the unit can be improved by improving this section’s effectiveness. This is what isintended by the diabatic distillation.

Fig. 5. Exergy losses in the TAME unit.

R. Rivero et al. / Energy 29 (2004) 467–489478

2.7. Depentanizer section

The main operating conditions of the depentanizer column (base case) are shown in Table 1.In the distillation system, the condenser (see Fig. 1) EA-16 degrades 32.62% (33.73 MJe/ton

of feed) of the energy, column DA-5 degrades 18.70% (19.33 MJe/ton of feed) and reboiler EA-

13, 22.43% (23.18 MJe/ton of feed). The improvement potential in the condenser EA-16 is

44.00% of the section (26.61 MJe/ton of feed), column DA-5 21.32% (12.90 MJe/ton of feed)

and reboiler EA-13, 7.95% (4.81 MJe/ton of feed) of the improvement potential of the section.

These results are shown in Figs. 8 and 9.

Fig. 7. Exergy improvement potential in the TAME unit.

Fig. 6. Effectiveness in the TAME unit.

479R. Rivero et al. / Energy 29 (2004) 467–489

3. Application of diabatic distillation

Fig. 10 shows the main operating conditions of the depentanizer column; flow rates and heat

duties are referred to 1 ton of mixture feed.

Fig. 8. Exergy losses in the depentanizer section.

Fig. 9. Exergy improvement potential in the depentanizer section.

Table 1Depentanizer column operating conditions (base case)

Parameter

Number of stages 56Feed stage 23Reboiler heat duty (MJ/ton of feed) 301.61Condenser heat duty (MJ/ton of feed) 223.14

R. Rivero et al. / Energy 29 (2004) 467–489480

If the same amount of heat is extracted in each tray of the rectification section (including the

condenser) using cooling water flowing in series from tray to tray inside the column and simi-larly the same amount of heat is supplied to each tray of the stripping section (including the

reboiler) using, in this case, medium pressure steam flowing up towards the stripping section in

series, the amount of cooling water, the medium pressure steam and the condenser and reboilerduties are considerably reduced. The water obtained at a higher temperature can be considered

to be useful for other purposes in the process, e.g. as boiler feed water.In the totally diabatic column, the exergy losses are more uniformly distributed as shown in

Fig. 11. The standard deviation of the values of exergy losses in the trays of the diabatic columnis 0.087, whereas that of the adiabatic column is 0.368. This seems to confirm the equipartition

principle proposed by Tondeur [4] that inspired the experimental demonstration project of dia-batic distillation [5]. The relatively high exergy losses in the feed tray are due to the temperature

and composition differences between the feed and the liquid and vapor in the tray.In the totally diabatic case the cooling water flow rate is reduced from 3.83 ton per ton of

feed to 0.93 ton per ton of feed, which would result in a saving of 5,615,687 ton per year incooling water (about 561,568 US$ per year), and the reboiler heat duty would be reduced from

301.61 MJ/ton of feed to 27.21 MJ/ton of feed, resulting in a reduction of medium pressure

steam costs of about 425,724 US$ per year. The hot water would be obtained at 90.4vC which

could be used to generate the steam needed by the stripping section provided it has received the

required treatment to avoid possible salt deposition (see Table 2).The investment required to diabatize the system would have to be compensated by the econ-

omic savings in the operation costs. The savings in low pressure steam also represent savings in

Fig. 10. Adiabatic and diabatic depentanizer columns.

481R. Rivero et al. / Energy 29 (2004) 467–489

the fuel required to produce it and then a reduction in flue gas emissions of CO2, NOx, SOx CO,etc. (8,345 ton per year) as shown in Table 3.Table 4 shows the effect of converting the depentanizer tower to diabatic operation on the

overall operation of the depentanizer section. As can be seen, the use of a diabatic distillation isattractive from the point of view of all exergy performance parameters. The exergy losses of thecolumn in the diabatic case are increased by 118.1% due to heat transfer, but those of the con-denser are reduced by 99.41% and those of the reboiler are reduced by 73.35%, due to the

Table 3Utilities savings and emissions reduction

Savings/year

Medium pressure steam 42 572.4 tonCooling water 5 615 687.6 m3

CO2 emission 8,317.4 tonNOx emission 19.3 tonSOx emission 8.4 tonCO emission 0.2 ton

Table 2Comparison of the utilities requirements of the depentanizer tower

Service/ton of feed Adiabatic Diabatic

Cooling water flowrate (ton) 3.83 0.928Medium pressure steam (ton) 0.139 0.117Condenser duty (MJ) 223.14 71.95Reboiler duty (MJ) 301.61 27.21

Fig. 11. Exergy losses distribution in adiabatic and diabatic depentanizer towers.

R. Rivero et al. / Energy 29 (2004) 467–489482

reduction in heat duties, so the overall effect is a 35.33% net reduction in total exergy losses (see

Fig. 12). The effect on the performance of the whole TAME unit is also presented in Table 4.In order to operate the diabatic distillation system it is important to stress that unlike the

operation regime of a conventional adiabatic column, in the diabatic case care has to be taken

Table 4Effect of diabatic distillation on the operation of the depentanizer section and of the TAME unit

Adiabatic Diabatic

Depentanizer sectionSupplied exergy (MJe/ton. of feed) 117.45 116.43Exergy produced (MJe/ton. of feed) 27.75 42.97Effectiveness (%) 23.62 36.91Irreversible losses (MJe/ton. of feed) 89.70 73.46Effluent losses (MJe/ton. of feed) 13.69 3.15Total losses (MJe/ton. of feed) 103.39 76.61Total losses/Exergy produced 3.73 1.78Potential for improvement (MJe/ton. of feed) 82.20 49.50TAME unitSupplied exergy (MJe/ton. of feed) 154.35 159.56Exergy produced (MJe/ton. of feed) 24.06 72.10Effectiveness (%) 15.59 45.19Irreversible losses (MJe/ton. of feed) 130.29 87.45Effluent losses (MJe/ton. of feed) 18.30 8.78Total losses (MJe/ton. of feed) 148.60 96.23Total losses/Exergy produced 6.18 1.33Potential for improvement (MJe/ton. of feed) 128.28 56.71

Fig. 12. Total exergy losses in the depentanizer column.

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on the column design since the vapor and liquid flow rates inside the column vary considerablytowards the top and the bottom of the column. The liquid to vapor molar flow rate ratiodecreases towards the top and increases towards the bottom of the column. This is shown inFig. 13.If the same hydrodynamic conditions are to be maintained inside the diabatic tower, then the

diameter should be reduced towards the top and the bottom of the column. Another optioncould be to keep the diameter constant, but to modify the tower internals to maintain thehydrodynamic conditions. Fig. 14 shows the potential diameter reduction in the tower.It is important to mention that in order to keep the same number of trays in diabatizing a

column, some external reflux is needed, to obtain exactly the same composition specification ofthe products. This means that the total heat supplied to the reboiler and to all trays in the strip-ping section of the diabatic tower, is higher than the heat supplied to the reboiler of the adia-batic tower (304.12 vs. 301.61 MJ/ton of feed); similarly, the sum of the heat removed from thecondenser and from all the trays in the rectification section of the diabatic tower, is higher thanthe heat removed from the condenser of the adiabatic tower (226.71 vs. 223.14 MJ/ton of feed).Anyway, this increase in total heat duties of the diabatic column is compensated by the fact thatheating and cooling media are circulating in series from one tray to the other, so that their flowrates are much smaller than in the adiabatic column. If the total heat duties were kept constantthe diabatic column would require more trays to perform the same separation, but this is notthe case studied in this application study since it is intended to explore the diabatization notonly of new towers, but also existing and operating distillation towers.

3.1. Economic evaluation

It is very difficult to make an accurate estimation of the investment required to construct anew diabatic tower or to diabatize an existing distillation tower, since no industrial or pilotplant experience exists. This is precisely one of the main objectives of the current research pro-ject [8]. The most important aspect that would affect the capital costs would probably be the

Fig. 13. Molar L/V ratio in adiabatic and diabatic columns.

R. Rivero et al. / Energy 29 (2004) 467–489484

geometry of the simultaneous heat and mass transfer devices that will be inside the tower toprovide the vapor–liquid contact and to transfer heat to or from an external cooling or heatingmedium.An approach to this economic evaluation would be to estimate the maximum expected invest-

ment, considering that each tray is coupled to a conventional heat exchanger (with all the coststhat this would imply). In this way the economic evaluation can be performed with availablesoftware for conventional equipment.The AspenPlus simulation flow sheets of the depentanizer section for both the adiabatic and

the diabatic cases were then linked to the AspenIcarus Project Estimation software [18]. The dif-ference in project costs of the diabatic and adiabatic systems is considered the investmentrequired to diabatize the tower, which has to be compensated by the reduction in utilities con-sumption listed in Table 3.The basis for the economic evaluation of the systems is presented in Table 5.Table 6 shows the project costs of the diabatic and adiabatic depentanizer towers.As shown in Table 6, the difference in project costs of the adiabatic and diabatic towers is

considered to be the investment required to diabatize the tower: 2.3 million US$. The diabatic

Fig. 14. Size of adiabatic and diabatic columns.

485R. Rivero et al. / Energy 29 (2004) 467–489

column would have lower equipment, piping, steel and electrical costs than the adiabatic col-

umn, but higher civil, instrumentation, insulation and paint costs, so the total direct field costs

which include labor and materials costs would be lower for the diabatic column. However the

indirect costs, which are associated directly to the man-hours required for the project, are higher

for the diabatic column; so the total field costs are also higher in the diabatic case. For the

Table 6Project costs of diabatic and adiabatic depentanizer towers

Concept Adiabatic column (million US$) Diabatic column (million US$)

Equipment 2.882 1.955Piping 1.595 1.159Civil 0.169 0.310Steel 0.058 0.038Instruments 0.528 1.079Electrical 0.342 0.187Insulation 0.503 0.612Paint 0.038 0.048Direct field costs 6.116 5.387Indirect field costs 1.383 2.391Total field costs 7.499 7.778Freight 0.209 0.147Taxes and Permits 0.327 0.229Basic engineering 0.237 0.601Detail engineering 0.404 1.105Material procurement 0.149 0.531Administration 0.616 0.782Contingencies 1.699 2.011Total design, eng., procurement cost 3.642 5.405Total project cost 11.141 13.183Adjusted total project cost 12.680 15.005

Table 5Basis for the economic evaluation

Basis

Refining unit TAME of 60,000 ton/yearEvaluation type Investment increase relative to an adiabatic columnSimulation tool AspenPlusProject evaluation tool AspenIcarusProject life time 20 yearsMinimum attractive rate of return 10%Funding No without financingExchange rate 10 mxp/US$Cooling water cost 0.1 US$/m3

Medium pressure steam cost 10.0 US$/tonInflation 1% annualCarbon bonds cost 7 US$/ton.CO2

Fuel Refinery gas

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non-field costs, the diabatic column would have lower freight, taxes and permits costs, buthigher basic engineering, detail engineering, material procurement, administration and homeoffice, and contingency costs, so the total non-field costs would also be higher for the diabaticcolumn. The overall result is that the cost of the diabatic column is about 20% higher than thecost of the adiabatic column.A clearer picture of the difference in direct costs of the diabatic and adiabatic columns is

shown in Table 7, in which the investment costs of the main components of the distillation sys-tems are presented. The investment of the diabatic tower is three times higher than that of theconventional adiabatic tower due to the much more complex internals it requires, but thereduction in the investment costs of the reboiler and condenser (which are much smaller) com-pensates the higher tower investment. In the conventional adiabatic system, the tower itself rep-resents only 22% of the total investment and the condenser and reboiler represent 61%. On theopposite side, in the diabatic system the tower represents 78% of the investment, whereas thereboiler and condenser represent only 4%.Table 7 also presents the detail of labor costs for the construction of diabatic and adiabatic

columns. It is estimated that the construction of the diabatic column would cost roughly doublethat of the construction of the adiabatic column; the difference in labor costs for the construc-tion of the columns can also be interpreted as the labor cost required to diabatize an existingadiabatic column, or in other words, the labor cost to diabatize the tower is more or less thesame than the labor cost to construct a conventional adiabatic tower.Finally, the diabatization project economic evaluation is presented in Table 8, in which the

investment required to diabatize the tower should be compensated by the reduction in utilities

Table 7Direct costs and labor costs for the construction of diabatic and adiabatic depentanizer towers

Conventional adiabatic column (million US$) Diabatic column (million US$)

Direct costsColumn 1.356 4.200Reboiler 3.572 0.075Condenser 0.160 0.119Others 1.028 0.992Total 6.116 5.387Labor costs 0.883 1.722

Table 8Diabatization project economic evaluation

Without carbon bonds With carbon bonds

Investment (million US$) 2.324 2.324Net present value (million US$) 7.058 7.584Internal rate of return (%) 44.92 47.52Total income at present value (million US$) 9.383 9.909Cost effectiveness (income/investment) 4.04 4.26Payback (years) 2.1 2.0

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consumption. The net present value obtained is about 7 million dollars, with an internal rate ofreturn of more than 44%, making the investment attractive. The cost effectiveness (income/investment) of the project is estimated to be about 4, which means that it would generate 4 dol-lars per each dollar invested. However, the payback time is not as good, since the investmentwould be recovered in about 2 years.The numbers are improved a little if the reduction of CO2 emissions is given the value shown

in Table 4. Although these greenhouse gas emission reduction systems (carbon trade or carbonbonds) are not yet a generalized practice in industry, it was considered because Pemex hasimplemented an internal CO2 permit trading system [19], which could help in justifying theexecution of this kind of project [8,13].

4. Conclusions

The main improvement potential of the TAME unit is located in the depentanizer section,with 64.1% of the exergy improvement potential of the unit.If diabatic distillation is implemented in the depentanizer section, losses due to irreversibilities

are reduced by 18.10%, while losses due to effluent streams are reduced by 71.51%. The efective-ness of the depentanizer section increases from 23.62 to 36.91% after applying diabatization inthe depentanizer column.Cooling water flow rate is reduced by 75.77% equivalent to 561,568 US$ per year, while heat-

ing steam requirements are reduced by 15.83% equivalent to 425,724 US$ per year, that is tosay, there are savings of 1.0 million US$ per year due to reduction in utilities.For the depentanizer section the investment required to diabatize the tower is estimated in

about 2.3 million dollars with a payback time of about 2 years and an internal rate of return of45%, which is four times superior to the minimum attractive return rate fixed for the project.As mentioned in previous papers [9], the economic balance should justify the investment

needed for the diabatic system when the operational cost savings allow a reasonable paybackperiod. Since there is little experience in the detailed design, construction and operation ofindustrial diabatic columns, pilot plant experience is essential to allow accurate costing of equip-ment and evaluation of operation [8].

Acknowledgements

This research was conducted within the framework of the Environment and Safety ResearchProgram (PIMAS) of the Mexican Petroleum Institute (IMP). The authors whish to thank C.Rendon, C. Pineda and R. Pulido for their help in the economic evaluation.

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