process simulator effective in de-ethanizer tower revamp - oil & gas journal

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03/25/2002

Revamping an existing plant to increase production with minimum investment is important in today's process industries.Revamping a process plant is more complex than building a new plant. Revamps require a thorough knowledge andexpertise of detailed material and energy balances.

A process simulator is a powerful tool for identifying bottlenecks to achieve higher capacity.1 For some pieces ofequipment like pumps, compressors, heat exchangers, etc., revamping is relatively straightforward. For example,replacing the impeller on a centrifugal pump or installing an additional pump in parallel to achieve higher capacity.

A distillation column, on the other hand, is a major cost component and simply replacing it during a revamp is not

acceptable. Existing column performance must be understood and analyzed in a distillation column revamp.2

Process simulators are an extremely useful tool to perform revamps with considerable ease. A simulator has anextensive thermodynamic model database, unit operations library, and pure component databank for modeling theprocess.

This article presents a case history of using a process simulator as a tool for revamping an existing distillation columnto establish additional throughput without a major hardware change.

The column was revamped for additional throughput by combining feed precooling and providing a side reboiler. Thisconcept requires minimal hardware changes for columns excessively loaded at stripping section. The key to asuccessful revamp lies in setting the appropriate location and side reboiler duty.

This exercise demonstrates how a process simulator can help analyze column profiles and locate the side reboiler.

Distillation revamp

A revamp should increase column throughput. The new throughput will increase column traffic and, consequently, limitthe reboiler and condenser. There are various revamping options available to overcome these constraints.

A simple and generally used approach is to use structured packing or high efficiency trays to handle additional columntraffic and provide additional surface area to overcome the reboiler and condenser bottlenecks. A more careful analysisof the existing column may give insight on alternate revamping strategies that will lead to an optimum design.

Hydraulic loads in the stripping and rectification sections may vary substantially. If a column is typically hydraulicallyloaded more in the stripping section and less in the rectification section, adding heat at an intermediate tray betweenthe feed and reboiler can help redistribute the load; column throughput can be enhanced without replacing internals.

Side reboiler

Using a side reboiler is useful for columns loaded relatively heavily in the stripping section.3 The heat load between therectification and stripping sections can be balanced by varying the feed temperature. Feed preheating decreasesreboiler duty and increases condenser duty while feed precooling has the opposite effect. Component physicalproperties influence the sensitivity of heat load variations with respect to feed temperature.

With a side reboiler:

Feed is precooled.Condenser heat load decreases.Vapor-liquid traffic in the rectifying section decreases.Vapor-liquid traffic in the stripping section increases.Reboiler heat load increases.

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A side reboiler placed between the feed point and bottom tray reduces the tray load below the point where heat isadded. It will also, to some extent, increase the tray load above the heat addition point.

With top section traffic reduced due to feed precooling and bottom section traffic reduced due to a side reboiler, columnthroughput can be increased so that column traffic, condenser heat duty, and reboiler heat duty remain the same.

When using this approach for a distillation revamp, the trick is to specify the side reboiler location and fix the heataddition rate that will give maximum overall capacity increase. This is a trial-and-error procedure with no simpleformula.

Problem definition

In an LPG recovery scheme, ethaneis removed from the main feedstream in a de-ethanizer columnfollowed by a LPG column. LPG isthe overhead product in the LPGcolumn and the bottoms are mainlyC5+. Fig. 1 shows the flow scheme.

A column with the feed streamshown in Table 1 was considered.The revamp study was performedon the de-ethanizer column. Thesespecifications for overhead andbottom streams were fixed:

Ethane with 0.5 wt % propane in the overhead.Bottom stream with 0.5 wt % ethane.

The simulation used single-pass Glitsch valve trays inthe rectifying section and two-pass Glitsch valve traysin the stripping section.

The distillation column had 40 theoretical stagesexcluding the condenser and reboiler. Feed entered atTray 20. This was the optimum feed location.

The de-ethanizer was swaged below the feed; the topsection was 0.95 m diameter and the bottom sectionwas 1.7 m diameter, with a tray spacing of 0.5 m in bothsections.

Column top pressure was 30 kg/sq cm, feed rate was40,000 kg/hr, and feed temperature was 65C. The simulation used the Peng-Robinson thermodynamic method. Afterachieving the required specifications, further studies were performed with this model as the base case.

Tower simulation

A detailed analysis of the base case revealed thatthe column was heavily loaded in stripping section(Fig. 2). This was due to various factors.

Latent heat in the reboiler vapor is substantially lowerthan the overhead. Liquid molecular weightincreases in the stripping section up to the bottomtray, resulting in lower vapor velocity and trayloading. With higher fluid temperature, however,liquid density is lower resulting in higher tray loading.The net overall result is increased tray loadingbetween the feed and column bottom.

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The column profile for this simulation case is tabulated in Table 2.Flooding at the feed tray was 51%, and it steadily increased towardsthe bottom. The bottom tray flooding was 79.8%, the recommendedmaximum for the column.

The top section was not hydraulically loaded (Fig. 2). The mass flowrate of liquid in the rectifying section was 12,000 to 16,000 kg/hrwhereas the mass flow rate of liquid in stripping section was 60,000to 75,000 kg/hr.

A further increase in throughput was not possible due to floodinglimitations. The loading pattern, however, shows that if we could uniformly balance hydraulic load in the strippingsection, it would be possible to revamp the column such that traffic in the stripping section's bottom trays would besimilar to traffic before the revamp. Supplying intermediate heat in the stripping section at a suitable location wouldachieve this goal.

Feed precooling or preheating before it enters the column can also alter vapor-liquid traffic. However, feed preheat orprecool will not help in altering the relative load in a section. Feed precooling accompanied with a side reboiler can,however, substantially increase column throughput.

Feed was precooled to 40C. to lower the condenser duty andcolumn rectification section loading. Table 3 shows the results.

Condenser dutywas 22% lower(Table 4)compared to thebase case.There was acorrespondingincrease of15.5% in thereboiler duty.

Fig. 3 shows that stripping section loads increase with flooding,reaching an unacceptable value of 91.6%. Flooding on the top traywas 57.5%.

Adding an intermediate heat source below the feed tray will reducethe traffic below it.

Intermittent heat addition

The heat duty from a side reboiler must be determined by trial-and-error. A first estimate of the side reboiler duty usesthe net differential duty between the base case and revamp case. Since the revamp case, however, was itself afunction of side reboiler duty and location, it was extremely difficult to arrive at an initial estimate.

We observed that the total duty required for the revamp case was not equal to the base case plus a differential duty. Itwas a little less if heat was added at an intermittent location and was sensitive to where the heat was added.

For this case, the side reboiler added 1 millionkcal/hr. A study was performed to evaluate thereduction in reboiler duty by varying the side reboilerlocation. Fig. 4 shows the results. There is nosignificant reduction in reboiler duty when the sidereboiler is placed in the rectifying section. So, furtherstudies varied the side reboiler location in just thestripping section.

The side reboiler was tried at Tray 25, 30, and 35.The condenser and reboiler duties, and columnflooding at each stage were studied for each run.Fig. 5 shows the results.

Adding heat at an intermediate location decreasedthe loading below the heat addition tray andincreased the loading above it. As the heat additionlocation was lowered, loading on the trays above theheat addition point was not as great.

Fig. 5 shows that maximum flooding in the strippingsection reaches 82% when heat was added on Tray35. This is more than the maximum flooding in thebase case; so this heat addition point was not


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accepted.

When heat was added on Tray 30, maximumflooding in stripping section was 74.9%, lower thanmaximum flooding for the base case.

In the rectifying section, maximum flooding was59.6%, lower than maximum flooding for the basecase by 25.1%.

When heat was added at Tray 25, maximum floodingin the stripping section was 71%. Maximum floodingwas 70% in the rectifying section, 12% lower thanthe maximum flooding for the base case.

Adding heat at Tray 25 gave more margin for flooding than Tray 30, when compared to the base case. Tray 25 waschosen as the optimum head addition point.

Revamp case

After side reboiler duty and location were finalized, column runs were performed to evaluate additional throughput. Thefeed rate was gradually increased and column parameters were analyzed.

At a throughput of 47,500 kg/hr, column hydraulic performance and duties were fairly close to the base case. Floodingin the column was compared to the base case.

Fig. 6 shows column flooding before and after therevamp.

Since intermediate heat is added on Tray 25, themaximum flooding point in the bottom section (Tray24) is 80%. This matches closely with maximumflooding of 79.8% in bottom section (Tray 39) for thebase case.

Maximum flooding in the revamped top section is75.3%, well below the maximum flooding of 79.5%for the base case.

Condenser duty for the base case was 1.055 millionkcal/hr and the condenser duty after the revampremains at 1.06 million kcal/hr (Table 4). Reboilerduty before and after the revamp was 2.51 and 2.55million kcal/hr, respectively.

The study revealed that there was no need tochange the existing reboiler and condenser surfaces after the revamp. The column traffic limiting points were alsomaintained so that the internals did not have to be changed after the revamp.

Precooling the feed from 65 to 40 C. and adding 1 million kcal/hr of heat using a side reboiler at Tray 25 increaseddistillation column throughput by 18% without affecting product specifications.

Role of the simulator

We performed a similar exercise with different combinations of heat duty and heat addition points. We observed thatthe overall column throughput increase was less than 18% for all other cases.

This type of study is practically impossible to perform without the use of a process simulator.4-6

This case study is a classic example that stresses the importance of process simulators for revamping distillationcolumns or other complex operations within a process plant.

References

Sloley, A., and Fraser, A.C.S., "Consider modeling tools to revamp existing process units," HydrocarbonProcessing, June 2000, pp. 57-63.

1.

Fair, J.R., and Seibert, A.F., "Understand distillation-column debottlenecking options," Chemical EngineeringProgress, June 1996, pp. 42-48.

2.

Nye, J.O., Herzog, K., and Cheaney, S., "Use a side reboiler to increase tower capacity," HydrocarbonProcessing, September 1999, pp 51-56.

3.

Kister, H.Z., "Troubleshoot distillation simulations," Chemical Engineering Progress, June 1995, pp. 63-75.4.Kister, H.Z., "Distillation design," McGraw-Hill, New York, 1992.5.Dimian, A., "Use process simulation to improve plant operations," Chemical Engineering Progress, September1994, pp. 58-66.

6.

The authors

Atul Choudhari is a senior process engineer at Kvaerner Powergas India Ltd., Mumbai, India. Hisexperience of more than 9 years includes flowsheet simulations and process optimization forpetrochemicals, fine chemicals, and hydrocarbon processes. Choudhari also has experience in variousdebottlenecking and distillation revamping projects. He received his BS in chemical engineering fromMarathwada University, Aurangabad, India.

Pradnya Gune is a principal process engineer at Kvaerner Powergas India Ltd., Mumbai, India. She hasmore than 10 years of process engineering experience in the field of petrochemicals and fine chemicals.Gune previously worked for Reliance Industries Ltd. She is a graduate engineer from the University ofMumbai, Department of Chemical Technology.

S. K. Saxena is a senior engineering manager (process), head of process for KvaernerPowergas India Ltd., Mumbai, India. He has also worked for Synthetics & Chemicals Ltd.,Humphrey & Glasgow Ltd., Engineers India Ltd., Bechtel Group Inc., and PetrofacInternational Ltd. in various capacities. Saxena has more than 25 years experience in


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process design, troubleshooting, and operations. His experience covers refining,petrochemicals, and fine chemicals. He has a masters in chemical engineering (design).

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process simulator effective in de-ethanizer tower revamp - oil & gas journal

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