Improving the Distillation Energy Network

Improving the distillation energy network

Energy-efficient design applied to the refit of a distillation unit was achieved through optimisation between the distillation column and heat network system

SOUN HO LEE GTC Technology KWANG GIL MIN GS Caltex Corporation

nergy costs are the largest process conditions. The following location is a section of the distilla- percentage of a hydrocarbon are common strategies that can be tion column where the composition Eplant’s operating expenditures. applied to practical energy of column internal liquid traffic is This is especially true of the distil- improvement projects. similar to feed stream composition. lation process, which requires In this case, the composition gradi- substantial energy consumption. Feed temperature ent between feed stream and Concerns over recent high costs Feed temperature is a major factor distillation internal fluids is mini- and economic pressures continually influencing the overall heat balance mised. In actual operation of the emphasise the need for efficient of a distillation column system. distillation column, feed composi- distillation design and operation Increments in the feed enthalpy can tions are often changed from the without a loss of performance. help reduce the required energy original design conditions. In cases This article illustrates how input from the reboiler at the same of significant deviation, discrepancy energy-efficient design can be degree of separation. Installing a between column internal liquid applied in a distillation unit through feed preheater is a very common composition and feed stream optimisation between the distillation process option to minimise reboiler composition can increase, which column and heat network system. heat duty. If the feed preheater can results in a non-optimum feed loca- Through a case study, a successful be integrated with other valuable tion. Therefore, evaluating feed retrofit of an aromatics distillation process streams (as a heating location is an essential step for unit is discussed. Detailed retrofit medium), overall energy efficiency successful distillation unit energy activities, including complex heat of the distillation system can be improvement. network evaluation, process simula- improved further. However, tion modelling and energy-friendly, increasing the feed temperature Inter-condensers and inter-reboilers high-performance distillation equip- does not always improve the over- Adding inter-condensers and/or ment implementation, are described. all energy efficiency of a distillation inter-reboilers can help improve unit. Excessive feed temperature overall energy efficiency. Strategies for improving the increments can cause a significant Pumparound, one of the inter- distillation energy network amount of flash of heavy key and condenser concepts, has been As continuous distillation requires non-key components at the distilla- widely applied to numerous petro- simultaneous heat input and tion column feed zone. In this case, leum multi-product fractionators. removal (thus requiring significant a higher amount of reflux stream is On the other hand, implementing energy consumption), complex heat necessary to maintain required an intermediate reboiler can reduce integration becomes more common overhead distillate purities. This the main reboiler duty. As the for modern distillation units to augmented reflux ratio thus required temperature of an inter- improve unit energy efficiency. requires a higher boil-up ratio. mediate reboiler is lower than that Since a distillation column’s degree Overall energy efficiency is eventu- of the main reboiler, this strategy of separation and enthalpy balance ally aggravated.1 Therefore, careful may allow heat integration with influence each other, it is critical to review of the feed temperature and other valuable heat sources that are evaluate and optimise the distilla- phase is critical to minimise the not as costly or not fully utilised in tion column and heat exchanger overall energy consumption of the the plant. networks together in order to distillation unit. maximise plant economics. Column operating pressure There are numerous strategies to Feed location Relaxation of the column top oper- improve the energy efficiency of Improper feed location of a distilla- ating pressure decreases the distillation processes, with the tion column can also increase the distillation column’s temperature amount of improvement through reflux/boil-up ratio and energy profile and results in a lower each strategy varied according to consumption. An ideal feed reboiler duty. It has been observed

www.eptq.com Revamps 2013 3 The function of the xylene column is to separate the feed mixture to xylene components and

heavier C9+ components. This column has two different feed sources. The reformate splitter bottom stream and the toluene column bottom stream (which belongs to the aromatic extraction Reflux ratio unit) are introduced as the xylene column bottom feed stream. The Steep Moderate Flat sensitivity sensitivity zone sensitivity zone zone bottom feed stream is split equally and charged to two different feed trays. The reformate splitter’s Theoretical stage bottom stream is treated at the clay towers to eliminate traced olefin Figure 1 Typical column efficiency vs reflux ratio curve components before charging to the xylene column. Meanwhile, the that numerous commercial distilla- analysis. A typical curve is shown deheptaniser bottom stream (from tion columns have been operated in Figure 1. This curve visualises the xylene isomerisation unit) is with lower operating pressures column efficiency sensitivity and charged as the xylene column top than their original design values. energy-saving gain. The curve can feedstock. This stream is also split However, this strategy is not appli- be categorised by three district and introduced to three different cable to columns operated under an zones: steep, moderate and flat feed trays. atmospheric pressure range. sensitivity.2 The xylene column overhead Column overhead circuit pressure Column efficiency improvement vapour stream is split into three drop and condenser temperature is usually very feasible when the parallel streams. Two vapour approaches both heavily influence reflux ratio falls into the steep streams are utilised as the heat feasibility. In addition, column sensitivity zone and at ratios source of the extract column pressure reduction expands vapour considerably in excess of the mini- reboiler and the raffinate column traffic and pushes the limits of mum reflux ratio. In this scenario, reboiler, respectively. The existing distillation equipment. even the small addition of stages, condensed xylene column overhead or an increase in distillation equip- liquid streams are returned to the Column pressure drop ment efficiency, can enhance overall xylene column receiver. The other Reducing column pressure drop column separation with significant vapour stream is supplying heat to can lower reboiler duty at the same energy reductions. the xylene column overhead steam degree of separation. The amount Improvement gain is diluted in generator, which produces #250 of reboiler duty saving relies on the moderate sensitivity zone. steam. The condensed overhead operating pressure and enthalpy Further detailed feasibility study is liquid stream is also returned to the balance. This strategy is generally necessary through economic analy- xylene column receiver. The over- feasible when the distillation sis. The magnitude of energy head distillate of the xylene column column is operated under vacuum savings is negligible when reflux is sent to the paraxylene recovery pressure range. Meanwhile, pres- ratio variation follows flat motion unit. sure drop improvement does not in the remaining zone. In the xylene column reboiler often provide noticeable energy circuit, the xylene column bottom savings in high-pressure range Case study: unit description reboiler inlet stream is first trans- distillation service. The following is a revamp case ported to the other two distillation study of a xylene mixture separa- column reboilers as heating medi- Column efficiency improvement tion unit that demonstrates ums. After providing heat to these Column efficiency improvement well-thought-out, proven design reboilers, the xylene column bottom can reduce the reflux/boil-up ratio practices and a selection of the streams are combined and intro- at a given degree of separation. correct, high-efficiency distillation duced to the furnace-type xylene This strategy can be delivered by equipment to fulfill the improve- column reboiler. increasing the number of theoreti- ment in energy efficiency. Figure 2 In the paraxylene recovery unit, cal stages and/or enhancing the illustrates the xylene mixture sepa- the xylene components from the efficiency of distillation equipment. ration unit’s configurations under xylene column are separated The feasibility can be gauged by a discussion. This schematic reveals through the adsorption process. dedicated sensitivity analysis. that the original distillation units The pre-separated extract stream Constructing a column efficiency have implemented the full heat from the adsorption process is curve with a reflux ratio is one of integration network for energy- charged to the extract column in the core tools for sensitivity efficient operation. order to separate paraxylene from

4 Revamps 2013 www.eptq.com the desorbent. At the same time, the pre-separated raffinate stream Extract from the adsorption process is column charged to the raffinate column to rectify the raffinate components Xylene (metaxylene, orthoxylene and ethyl column benzene) as a side-cut product.3 A pasteurising section is arranged at the top of the raffinate column to remove moisture from the side-cut Raffinate product.4 column Top Case study: process evaluation for feed energy efficiency improvement To achieve an additional gain in energy efficiency, a dedicated process evaluation was conducted for the unit. Column operating Bottom feed conditions were first compared to Steam the original design conditions. This generator comparison helps comprehend deviations between the original design and the actual operational environment. It was observed that Mixed the actual product purities of the xylene + xylene column were higher than C9 the aromatic rundown product requirements. Relaxing the degree Figure 2 Xylene column heat network configuration of separation of the xylene column can reduce the reflux and boil-up Pre-revamp test run and base simulation: comparison of results ratio, as well as save fuel consumption for the xylene column furnace reboiler. However, the xylene Case parameter Pre-revamp test run Simulation results Extract column column overhead vapour streams Column top temperature, °F Base +∆ 0.9°F are utilised as the raffinate and Column bottom temperature, °F Base +∆ 1.1°F extract column reboiler heating Reboiler return temperature, °F Base +∆ 0°F mediums, and contribute steam Reflux ratio (to overhead distillate), volume Base +1.1% Reflux temperature, °F Base ∆0 production in the current unit Overhead distillate rate, BPD Base ∆0 energy network. Lower reflux/ Bottom rate, BPD Base ∆0 boil-up ratios in the xylene column p-DEB impurity in overhead distillate, wt ppm Base ∆0 decrease the amount of xylene Xylene impurity in bottom, wt ppm Base ∆0 overhead vapour used as heating Raffinate column medium for the extract/raffinate Column top temperature, °F Base +∆ 6.7°F column and/or steam generation. Side cut draw temperature, °F Base -∆ 4.3°F Process simulation modelling was Column bottom temperature, °F Base -∆ 0.5°F Reboiler return temperature, °F Base -∆ 2.9°F utilised as part of the process eval- Reflux ratio (to side cut), volume Base -0.58% uation activities to quantify and Reflux temperature, °F Base ∆ 0 predict gains in energy efficiency. Overhead distillate rate, BPD Base ∆ 0 Equilibrium base simulation soft- Bottom rate, BPD Base ∆ 0 p-DEB impurity in side cut stream, wt ppm Base ∆ 0 ware was utilised for the modelling. Xylene impurity in bottom, wt ppm Base ∆ 0 Base simulation modelling was first constructed through pertinent unit Xylene column test run data. Gathered major Column top temperature, °F Base -∆ 0.9°F Column bottom temperature, °F Base ∆ 0°F process stream flow rates were Reboiler return temperature, °F Base -∆ 2.25°F verified via flow meter orifice Reflux ratio (to overhead distillate), BPD Base + 0.04% calculations. Regular stream Reflux temperature, °F Base ∆ 0°F composition analysis reported bulk Overhead distillate rate, BPD Base -0.1% Bottom rate, BPD Base -5% compositions for non-key compo- C9+ impurity in overhead distillate, wt ppm Base ∆ 0 nents such as non-aromatic and C9+ Xylene impurity in bottom, wt ppm Base ∆ 0 component groups. Preliminary simulation modelling showed that Table 1

www.eptq.com Revamps 2013 7 component assumptions for these component groups varied simula- 2.4 2.3 Pre-revamp degree of separation tion results significantly. To 2.2 improve accuracy of simulation, 2.1 Pre-revamp test run detailed component analysis was 2.0 specially arranged for the test run. 1.9 Detailed component analysis was 1.8 Revamp design utilised for rigorous simulation 1.7 volume modelling. Key component balance 1.6 closures for the extract and raffinate 1.5 columns were less than 3%. 1.4 Reconstructed feed compositions 1.3 using products were applied for the Reflux ratio (reflux/distillate), 1.2 simulation of the extract and raffi- 30 35 40 45 50 55 60 65 70 nate columns. For the xylene Theoretical stage number column, the given overall mass balance closure was off by 5%. It Figure 3 Extract column sensitivity analysis (revamp design) was found that measured feed rates were more reliable than product results were reasonably matched to streams of both of the columns are rates, and bottom product rate was the test run data. As mentioned preheated by bottom product less reliable through overall unit earlier, the xylene column bottom streams. Adding independent feed mass balance investigation. Based rate was not matched. Through preheaters was not feasible due to on this investigation, simulation various sensitivity analyses, the limited plot and poor economics. modelling for the xylene column tray efficiencies of the columns The study showed that reducing focused on matching feed and over- were quantified.5 column pressure drop using the head distillate rates. Extensive case studies were low-pressure drop nature of trays A reasonable matching reflux performed for the feasibility of does not deliver reboiler duty temperature as well as rate is criti- energy efficiency improvement in savings in both of the columns. cal to quantify reliable column the unit. The case studies focused Case study results for column internal traffic conditions. It has on energy consumptions in the efficiency improvement showed been observed that matching reflux extract and raffinate columns. that energy efficiency improvement temperature is often overlooked in Energy improvements in the other was feasible. The column efficiency simulation modelling. Reflux rate is two distillation column reboilers in curves were constructed using usually metered at a flow meter the xylene column reboiler circuit simulated reflux ratio and theoreti- located on the reflux piping. When also help reduce fuel consumption cal stage values, and base the external reflux rate is recycled in the xylene column furnace modelling and improved column back to the column, the internal reboiler, but magnitude was not efficiency points were plotted. reflux rate will vary, depending on significant. Figures 3 and 4 display these the external reflux temperature. Since both columns are operated curves. An improved column effi- Therefore, a poor matching reflux under atmospheric pressure, reduc- ciency point was predicted through temperature in the simulation will ing column operating pressure is an increased number of trays. not predict the actual internal not applicable. In addition, feed Simulated tray efficiency values of column traffic accurately. This can result in erroneous efficiency assumptions of the existing column 2.3 or provide a misleading, incorrect Pre-revamp degree of separation result. 2.2 Instrumentation for measuring 2.1 the pressure drop of the extract and Pre-revamp test run raffinate columns was not -perti 2.0 nent. Matching pressure drop was 1.9 Revamp design ignored in simulation modelling; volume instead, matching column tempera- 1.8 ture data were focused on. 1.7 Temperature profiles are more 1.6 important to predict distillation Reflux ratio (reflux/side cut), Reflux ratio (reflux/side column energy consumptions. 1.5 Table 1 summarises the base model 30 40 50 60 70 80 90 Theoretical stage number simulation results and compares test run data for the three columns. This table depicts that base model Figure 4 Raffinate column sensitivity analysis (revamp design)

www.eptq.com Revamps 2013 9 the base model were maintained Test run data comparison for the improved column efficiency case study. Extra individual tray Case parameter Pre-revamp test run Post-revamp test run efficiency improvement was not Extract column considered. Original trays were Feed rate, BPD Base +18% arranged with 600 mm (~24”) regu- Overhead distillate rate, BPD Base +36% lar tray spacing. The increased Feed temperature, °F Base +∆ 2.3°F Reflux temperature, °F Base +∆ 16.9°F number of trays was predicted Column top pressure, psi Base +∆ 1.3 psi through a reduced tray spacing Reflux ratio (to overhead distillate), volume Base -28% scenario: 450 mm (~18”) regular p-DEB impurity in overhead distillate, wt ppm Base -∆ 1 ppm tray spacing. As a higher tray count Xylene impurity in bottom, wt ppm Base +∆ 16 ppm can increase the column pressure Raffinate column drop, increased column pressure Feed rate, BPD Base +14% drop values were applied for case Side cut product rate, BPD Base +28% studies of column efficiency Feed temperature, °F Base +∆ 5.6°F Reflux temperature, °F Base +∆ 0.1°F improvement. The charts in Figures Column top pressure, psi Base +∆1.4 psi 3 and 4 show that the reflux ratios Reflux ratio (to side cut), volume Base -6% of the columns were positioned in p-DEB impurity in side cut, wt ppm Base +∆ 23 ppm the steep sensitivity zone and that Xylene impurity in bottom, wt ppm Base +∆ 4 ppm enhancing column efficiencies is Xylene column beneficial to improve energy Feed rate,1 BPD Base +30% consumption in both columns. Unit reboiler fuel consumption,2 EFO BPD/BPD Base -22% Reduced reboiler duties of the Unit reboiler fuel consumption,3 EFO BPD/BPD Base -9% 250# steam generation, lb/hr Base +26% extract and raffinate columns contribute to the xylene column Note furnace reboiler duty saving in the 1. Total feed rate. 2. Equivalent furnace fuel oil consumption rate per feed charge rate. unit energy network. 3. Simulated fuel consumption saving through the column efficiency improvement.

Column modification Based on the case study results, the Table 2 extract and raffinate columns were modified. The number of trays was tray vapour velocity that can down- ratio was not matched to the neigh- increased in both of the columns. At grade tray efficiency due to bouring three-pass tray pass open a given column shell height, a insufficient vapour/liquid contact area ratio. In order to improve flow higher number of trays requires volume. Various performance- ratio balancing, the number of short tray spacing, causing tray enhancing features of GT-Optim passes for the pasteurisation section capacity loss. To prevent column trays were added to improve tray trays was changed from three to capacity reductions, GT-Optim efficiency. Nevertheless, extra -indi two and a new chimney tray was high-performance trays were imple- vidual tray efficiency improvement installed as per pass change. mented and replaced the original was not counted for a conservative sieve trays in both of the columns. approach to revamp design. Applied Case study: The original xylene column trays tray efficiencies for the revamp post revamp operation review remained unchanged. The higher- design were the same as the sieve The pre- and post-revamp perfor- capacity nature of the GT-Optim tray efficiencies obtained through mances are summarised and tray maintains the desired column simulation modelling of the pre-re- compared in Table 2. As the over- capacity with shorter tray spacing. vamp test run. all aromatic unit capacity has been In addition, the efficiency enhance- Original trays for the pasteurisa- expanded, the column charge rates ment features of these trays can tion section of the raffinate column are also increased. Post-revamp help to maximise column were designed with a three-pass operation verifies that the reflux efficiency. Various performance- geometry. A chimney tray was ratios of the extract and raffinate enhancing features adapted in the positioned between three-pass column are reduced, and these trays improve the vapour-liquid pasteurisation section trays and reduced reflux ratios eventually contact mechanism and enhance two-pass rectification section trays. contribute to energy savings in the tray efficiency. These include It is inherently difficult to achieve a xylene column reboiler furnace. specialised, shaped downcomers, uniform liquid-to-vapour traffic Measured furnace fuel consump- liquid inlet momentum breakers, ratio in each section of the three- tion as per the feed rate is tray inlet vapour/liquid contact pass trays. Moreover, the original substantially improved. Since the initiation devices and directional pasteurisation section trays and the paraxylene recovery unit adsor- valves positioned in the tray periph- chimney tray did not equip any bent upgrade also contributes to ery area. Tray pressure drop was feature for proper flow ratio balanc- savings in furnace fuel consump- optimised to prevent a “too low” ing. Each chimney pass open area tion, the net energy-saving

www.eptq.com Revamps 2013 11 pre- and post-revamp cases. 2.4 Slightly relaxed xylene loss and 2.3 Pre-revamp degree of separation Post-revamp degree of separation improved tray efficiency through 2.2 GT-Optim trays contribute to 2.1 Pre-revamp test run achieving further reflux ratio 2.0 savings in post-revamp operating 1.9 1.8 conditions. Revamp design 1.7 It is found that the raffinate volume 1.6 column feed structure is changed in Post-revamp test run 1.5 the post-revamp operating mode. 1.4 Sensitivity analysis through simula- 1.3 tion modelling shows that the Reflux ratio (reflux/distillate), Reflux ratio (reflux/distillate), 1.2 post-revamp degree of separation 30 35 40 45 50 55 60 65 70 line is substantially shifted by the Theoretical stage number changed feed compositions, and the pre-revamp degree of separation Figure 5 Extract column sensitivity analysis (pre- and post-revamp) line is no longer applicable. In Figure 6, the post-revamp degree of the separation curve for the raffi- 2.5 nate column is significantly shifted Pre-revamp degree of separation 2.4 Pre-revamp test run by the new feed composition. Post-revamp degree of separation 2.3 Although tray efficiencies are substantially improved and higher 2.2 theoretical stages are achieved, 2.1 changed feed composition erodes 2.0 Post-revamp test run energy savings. Moreover, pre-

volume 1.9 revamp product purities cannot be 1.8 maintained in post-revamp operat- 1.7 ing conditions, therefore purities are a little relaxed. 1.6

Reflux ratio (reflux/side cut), Reflux ratio (reflux/side Revamp design The post-revamp economic eval- 1.5 uations are updated and compared 30 35 40 45 50 55 60 65 70 75 80 85 90 to the revamp target evaluations.6 Theoretical stage number The evaluations are based on 3% inflation, 10% weighted average Figure 6 Raffinate column sensitivity analysis (pre- and post-revamp) cost of capital, 15-year deprecia- tion, 1% of the total investment for contribution of the column modifi- operation simulation modelling and maintenance, 22% tax bracket and cations was simulated and is compared to the pre-revamp base year 2012 average fuel price. included in Table 2. modelling curves. Degree of sepa- Profitability indexes are expressed As product quality specifications ration lines between pre- and with regards to payback period, are a little relaxed to maximise the post-revamp operations are shown net present value (NPV) and inter- energy saving and column feed in Figures 5 and 6. The overall nal rate of return (IRR). These structures are changed, it is neces- values gained from tray efficiency indices are shown in Figures 7-9. sary to re-evaluate the performance through simulation modelling are The charts show that actual of the extract and raffinate columns compared in Table 3. revamp profitability is better than with post-revamp operating condi- In Figure 5, the extract column expected. tions. The column efficiency curves degree of separation curves have are constructed using post-revamp similar patterns between the Acknowledgment The paper is updated from an earlier presentation given at the AIChE 2013 Spring Overall tray efficiency comparison meeting’s Distillation Topical Conference/ Kister Distillation Symposium 2013, 29 April-2 Case section Pre-revamp test run Post-revamp test run May 2013, San Antonio, Texas. Extract column Rectification section, % Base +∆ 3 Stripping section, % Base +∆ 3 GT-OPTIM is a mark of GTC Technology. Raffinate column Pasteurisation section, % Base +∆ 9 Rectification section, % Base +∆ 7 Stripping section, % Base +∆ 6 References 1 Lee S H, et al, Optimize Design for Distillation Table 3 Feed, Hydrocarbon Processing, June 2011.

12 Revamps 2013 www.eptq.com 2.0 20 150

1.5 15 100

1.0 IRR, % 10 50 Pay-out, years 0.5 million US$ NPV,

0 5 0 Revamp Actual Revamp Actual Revamp Actual design target post-revamp design target post-revamp design target post-revamp

Figure 7 Profitability index – payback Figure 8 Profitability index – net present Figure 9 Profitability index – internal rate period value of return

2 Hanson D, et al, High capacity distillation 6 Largeteau D, et al, Challenges and design and troubleshooting for refining and revamps, PTQ, Autumn 2001. opportunities of 10 ppm sulphur gasoline: part aromatic applications. 3 Meyers R, Handbook of Petroleum Refining 2, PTQ, Q4 2012. Email: [email protected] Processes, McGraw-Hill Company, 1986. 4 Moczek J S, et al, Control of a distillation Kwang Gil Min is the Senior Process Engineer column for producing high-purity overheads for GS Caltex Corporation, Yeosu, Korea, and and bottom streams, I&EC Process Design and specialises in process engineering/operation Development, 1963. Soun Ho Lee is the Manager of Refining services. He has been assigned various projects 5 Kister H, et al, Sensitivity analysis is key Application for GTC Technology US LLC, Euless, in the aromatic complex including reforming, to successful DC5 simulation, Hydrocarbon Texas, and specialises in conceptual process aromatic extraction and xylenes separation Processing, October 1998. design, simulation modelling, energy-saving units. Email: [email protected]