Distillation Column Thermal Optimization

8/19/2019 Distillation Column Thermal Optimization

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Distillation Column Thermal Optimization: EmployingSimulation Software

Applying process simulation software in distillation-column design andoperational analysis can lead to significant reductions in operational and

maintenance costs and improved column performance

Irina Rumyantseva and Ron Beck Aspen Technology

Several market conditions, including availability of light crude oilds in the U.S., low natural-gasprices and improving macroeconomics, are driving debottlenecking projects related tocolumns in both petroleum refining and chemical settings. Fortunately, the process-simulationtools that can be used to perform the front-end of these projects have advanced significantlyin the past several years.

Distillation columns present one of the most challenging design and operational challenges inmost chemical and petroleum-refining processes. ccording to the U.S. Dept. of !nergy, over

"#,### distillation columns are involved in plant operations in the chemical and petrochemicalindustries in $orth merica, and they consume appro%imately "#& of the total energy used'1(.

)he hydraulics and phase behavior of chemicals within a column are comple% and aredependent on a number of geometrical and maintenance factors, making it difficult to optimi*ecolumn performance by hand calculations. )hus, industry is employing process-simulationsoftware in order to optimi*e energy use in columns and pinpoint potential columnmodifications to implement in column design and retrofitting. +odern process-simulationsoftware has all the tools necessary to ma%imi*e column energy efficiency, reduce utilitiescost, improve thermodynamic driving forces to reduce capital investment, and aid in columndebottlenecking. !ngineers routinely use process-simulation software to perform thermal andhydraulic analyses Figure of columns '1–3(.

Figure . )hermal and hydraulic column analysis in advanced process-simulation software can be initiated with a mere check markselection

)his article focuses on applying software to perform thermal and e%ergy analyses of columnsin conjunction with pinch analysis of the entire process for optimi*ed conceptual designs andimproved plant operations. /ne of the key values of using process-simulation software incolumn design and troubleshooting is the ability to screen and evaluate a variety of process-configuration options and operating conditions rapidly. s is also described in this article,some simulators can provide integrated economic evaluation so the costs of differentalternatives can be contrasted. Further, steady state simulation can be augmented bymodeling in a dynamic mode to look in more detail at the column behavior.

Thermal and exergy analyses

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0rocess simulation can be applied when performing process design or retrofit analysis toidentify potential design modifications to improve energy efficiency and reduce energyconsumption.

)he simulation compares column performance based on user inputs to the columnperformance in thermodynamically ideal column-operating conditions, which are determined

theoretically, based on the practical near-minimum thermodynamic condition (!"TC# appro%imation '3(.

0$+)1 assumes thermodynamically reversible distillation-column operation, where reboilingand condensing loads are evenly distributed over the operating temperature range of thecolumn. )his ideal column is assumed to operate at minimum reflu%, has an infinite number of stages, and each stage has heaters and coolers with proper heat loads, so that the column-operating line coincides with the e2uilibrium line at any given stage. )his appro%imation alsoaccounts for losses and inefficiencies caused by column-design parameters, such aspressure drops, multiple side-products and so on. For each stage of the column, the softwaresimultaneously solves e2uations for e2uilibrium and operating lines for automatically selectedlight and heavy key components '1–3(.

Figure 3. n e%ample of using 1411 S-5 to compare two designoptions with different feed-stage locations6 stage 7 versus stage 84reen is the actual profile, red and blue are the ideal profiles


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Figure 7. )his 1411 )-5 illustrates the duty reduction potential fordesign option

Figure ". )his 1411 )-5 illustrates the duty reduction potential fordesign option 3. $ote the much slimmer duty reduction potential,as compared to Figure 7

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Figure 9. )his 1411 S-5 shows a sharper enthalpy change onthe reboiler side

Figure :. )his 1411 )-5 displays a large area between theactual and ideal profile above the pinch point, which helps identifyan opportunity for design modification of adding a side reboilerwith heating duty of :.9 +; to the base-case design


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Figure 8. )he 1411 )-5 after an addition of a side reboiler thatdisplays a much slimmer area between the ideal and actualenthalpy profiles

Figure


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they can be used to study how certain factors, such as momentum loss pressure drivingforce, thermal loss temperature driving loss, and chemical potential loss mass-transferdriving force, affect the losses in work potential due to the irreversibility of real processes '1–3(.

&sing thermal analysis results

!ngineers use the 1411 and e%ergy loss profiles, generated during thermal analysis, toanaly*e potential modifications to columns '1–3(.

)he 1411 are used to analy*e how energy efficiency can be improved by a number ofcolumn modifications, including changing feed location, reflu% ratio modifications, heating orcooling feed conditions, and adding a side cooler or heater. Software can produce the 1411for temperature versus enthalpy, and stage versus enthalpy with a click of a button.

!%ergy loss profiles are also easily generated to analy*e e%ergy loss at different stages or

temperatures, and these plots can be generated with a click of the button as well.

Stage-versus-enthalpy plots are useful in identifying opportunities for feed-stage locationmodifications. !ngineers can easily identify irregularities that resulted from incorrect feedplacement with the obvious projections at the feed location pinch point on the stage-versus-enthalpy 1411 Figure 3. )hese projections are a manifestation of a need for e%tra localreflu% to make-up for the unsuitable feed placement. feed can be introduced either too highup in the column, or too low in the column, resulting in a sharp enthalpy change on thecondenser or reboiler sides on the S-5 1411, respectively. )his will guide engineers inwhether it is most appropriate to place the feed stage down or up the column, in order toeliminate such distortions and reduce condenser and boiler duties. Figure 3compares twodesign options with feed stage located at stage 7 and stage 8. )he S-5 1411 shows anoticeable projection on the condenser side at the pinch point located between stages 3 and7. )he corrected S-5 1411 that resulted in moving the feed stage down the column is alsoshown in Figure 3 '2 (.

)emperatures versus enthalpy plots are useful in identifying opportunities for reflu%-ratiomodifications. )he potential for reductions in condenser and reboiler heat duties ischaracteri*ed by the hori*ontal gap between the )-5 1411 pinch point and the ordinate,which represents the surplus of heat during the separation process. =n order to reducecondenser and reboiler loads, reflu% ratio can be reduced, while increasing the number ofstages to sustain an ade2uate degree of separation. 1omparingFigure 7 and Figure" demonstrates how )-5 1411 are applied in evaluating different design options to see howreducing the reflu% ratio affects the condenser and reboiler duties. )hey illustrate howchanging the reflu% ratio from 8.8 to .7, while increasing the number of stages from 9 to 7#,

results in a 37.9 +; reduction in condenser and reboiler duties ' 2 (.

!ither S-5 or )-5 1411s can be applied in finding the appropriate range of modifications tothe feed 2uality. )-5 1411 plots will display sharp enthalpy changes either on the reboilerside or the condenser side, depending on whether the feed is e%cessively sub-cooled, oroverheated, respectively. =t is also worth noting that changes in the heat duty of pre-heaters or pre-coolers will lead to analogous changes in the column reboiler or condenser, based on thesame principle. Figure 9displays a sharper enthalpy change on the reboiler side in the S-51411 plot, which would lead an engineer to a conclusion that design could benefit fromadding a pre-heater. table of the simulation software results is then used to e%amine theeffects of adding a pre-heater, resulting in reduced reboiler duty to the temperature levels atwhich the hot utility for the reboiler and for the pre-heating the feed is re2uired to beprovided '2 (.

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!ven though feed conditioning is a more desirable way to reduce utility costs, adding a sidecondenser or a reboiler can also provide a way to accomplish this goal. )he goal of placing aside reboiler or a condenser is to allow heating or heat removal using a cheaper hot or coldutility, respectively. Side condensing or side reboiling provides an e%ternal way to modifycolumn design, and is typically used when it provides a more convenient temperature level. naly*ing the )-5 1411 plots helps identify the range for side condensing or reboiling.

!ngineers look at the area below or above the pinch point, that is, the area between the idealand enthalpy profiles. side condenser can be placed if there is a significant area below thepinch point, and a side reboiler can be used in the opposite case. Figure : illustrates a basecase. )here is a large area between the actual and ideal profile above the pinch point, whichleads to a design modification of adding a side reboiler to the base-case design. )he resultingslimmer area between the ideal and the actual profile is shown in Figure 8, where a sidereboiler with a duty of appro%imately :.9 +; was added at stage 33. )he addition of the sidereboiler not only leads to a reduction in the heat duty of the main reboiler, it also helps reducethe hot utility '2 (. 5owever, one should keep in mind the capital cost of adding a side reboileror condenser.

!%ergy loss analysis is a complementary tool that is used evaluate the design modificationsmentioned above '2 (. !%ergy loss profiles for different design options can be compared to

determine which design is optimal. Figure


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cooling water, a cheaper utility, can be used instead. !ach utility is associated with a certain454-emission contribution, and that is calculated together with utility consumption as well.)he user would just need to specify the appropriate utility costs and net carbon ta%.

E$aluate design'retrofit options

Some of the design modifications resulting from the thermal column analysis are actually notbeneficial to the process. For e%ample, as mentioned earlier, when the reflu% ratio is reducedin order to decrease loads on condensers and the reboiler and save energy, the number ofstages needs to be increased to ensure ade2uate separation, which will lead to increasedcapital costs. =n order to make 2uicker, more optimal decisions, engineers can employsoftware that can estimate the capital and operating costs of the process, using built-inrigorous as-built estimating models and si*ing and mapping algorithms that operate againstthe process parameters at a high enough level of accuracy to establish the tradeoffs betweenseveral proposed improvements, or between implementing the design modifications or not. common e%ample of using economic software in the decision process would be to decide

whether to add a side reboiler or condenser, and in that conte%t to compare the capital costsassociated with new e2uipment to the reductions in operating costs.

Operaility and profitaility

)here are a variety of changes and upsets that occur in process operations. =n order to bettercontrol column operations and both achieve the desired product mi% while minimi*ing energy,dynamic simulation is an e%cellent tool. )he comple%ity of columns means that they areinherently not at steady state and can usually achieve better operations when the dynamicsare well understood.

Some steady-state process simulators incorporate integrated dynamics-modelingenvironments, such that a steady-state process model can be used to 2uickly develop adynamic model. Dynamic modeling of columns typically achieves better control strategies,better control of products, and optimi*ation of energy use '4(.

Concluding remar)s

1olumns often are operated below optimum operating conditions due to their comple%ity andthe imperative to keep them within safe operating limits. )he design, troubleshooting anddebottlenecking of columns presents many opportunities to achieve improved control overproduct yields and reduction in energy consumption.

;hen the process model is used in conjunction with integrated pinch analysis, opportunitiesfor energy savings can be e%amined and optimal process arrangements can be identified.=ntegrated economics can further enable consideration of capital and operating costs withrespect to each design alternative. Dynamic modeling provides additional opportunities toidentify operating strategies that will incrementally improve column operations.

Aeyond the areas that have been discussed in this article, new innovations are around thecorner, in terms of detailed modeling of columns and more convenient column-dynamicstools.

Edited by Gerald Ondrey


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*eferences

. Demirel, B., Sustainable /perations for Distillation 1olumns, Chem. Eng. ProcessTech., vol. , p. ,##9, 3#7.

3. Samant, C., =. Sinclair, and 4. Ceady. =ntegrated )hermal and 5ydraulic nalysis ofDistillation 1olumns, 0roceedings from the 3"th $ational =ndustrial !nergy )echnology1onference, 5ouston, pril 3##3.

7. Dhole, . E., and innhoff, A., Distillation 1olumn )argets, Compters Chem. Eng., 8, pp.9"GH9:#, GG7.

". Arownrigg, $. and /thers, Iump Start6 Using spen 5BSBS Dynamics with 1olumns, spen)ech, 3#7.

+uthors

,rina *umyantse$a is a member of engineering product marketing at spen )echnology, =nc.3## ;heeler Eoad, Aurlington, + # 0hone6 J-8


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Distillation Column Thermal Optimization