Balanced distillation equipment design Fouling resistance and efficiency requirements for distillation equipment are balanced and optimised for reliable unit performance SOUN HO LEE GTC Technology Fouling tendency is a critical issue in crude distillation units and should not be overlooked when designing crude distillation columns. Corrosion tendency can influence fouling issues as well. Since fouling resistance has an inverse relationship to efficiency in distillation equipment design, optimising equipment design between fouling resistance and efficiency requirements must be precise. Poor application know-how as well as poor equipment design often downgrade column performance and reduce unit run length. This article will discuss common fouling issues associated with crude distillation column design. Actual retrofits for crude atmospheric columns are demonstrated through two case studies. These studies examine how fouling resistance and efficiency requirements for distillation equipment are balanced and optimised through careful evaluation and design methodologies. Case study 1: crude distillation unit description and background The configuration of the crude distillation unit in this case is illustrated in Figure 1. Fractionated light and heavy streams through the crude atmospheric column and the side strippers are Off gas Crude atmospheric column Unstabilised naphtha Heavy Light Kerosene Light diesel Heavy Desalted crude Preflash drum Heavy diesel Light diesel Diesel Heavy diesel Atmospheric residue Figure 1 Case study 1: crude distillation unit configuration www.eptq.com PTQ Q1 2017 91
fractionation sections. Kerosene or diesel intermediate product yield limitation was also experienced when or diesel boiling range material composition was increased in the charged crude slate. Charge crude compositions were frequently varied during operation. Case study 1: root cause identification Figure 2 Case study 1: valve/perforation hole wearing and corrosion Figure 3 Case study 1: underside view of fouled trays combined and rundown as a single intermediate product stream. A diesel intermediate product stream is also formed from a combination of light diesel and heavy diesel streams. These crude atmospheric columns and side strippers were originally designed with conventional movable valve trays, traditionally selected in the past. The exception was the wash section which was arranged with structured packing. Three circuits are arranged at the heavy, light diesel and heavy diesel range material locations. The naphtha/ fractionation section is positioned as the crude atmospheric column top section. This column was designed without a top circuit in order to maximise fractionation between unstabilised naphtha and at a given column height.1 92 PTQ Q1 2017 This crude distillation unit faced two problems: fouling and corrosion of the distillation equipment in the unit were found during a turnaround inspection. Valve perforation hole wearing and corrosion were found in the trays for naphtha/ Fouling tendency is a critical issue in crude distillation units and should not be overlooked when designing crude distillation columns fractionation. Tray fouling was also identified in the trays for the light /heavy and heavy /light diesel Figure 2 shows that the naphtha/ fractionation trays suffered from valve/perforation hole wearing and corrosion. Some movable valve units dislodged from the tray deck. Perforation hole sizes on the tray deck were increased by wearing and corrosion actions.2 Low column top temperature required for target operation could accelerate hydrochloric acid corrosion and valve/perforation hole wearing. Significant fractionation efficiency loss between naphtha and was not recognised during the operation. The bulky fractionation nature of crude distillation service might result in fractionation efficiency being insensitive to tray weeping. However, if this valve/perforation hole wearing progresses, significant fractionation efficiency loss will be noticed through substantial weeping. Fouled trays located for the light /heavy and heavy /light diesel fractionation sections are shown in Figure 3. A tar-like substance was discovered around the periphery of the valve legs. Phosphates used for crude oil production were suspected as the root cause. Boiled phosphates may react with boiling range material and make fouling deposits. A dedicated process evaluation for or diesel yield limitation was conducted. The original column and tray drawing revealed that intermediate side product and streams were withdrawn from fractionating trays directly. The originally designed side draw configuration is illustrated in Figure 4. Flow from the crude atmospheric column to the side stripper relies on gravity flow. www.eptq.com
If the liquid head formed on the collector tray is not high enough to overcome total friction losses from the crude atmospheric column to the side stripper, the flow rate can be limited. Moreover, frothy liquid withdrawn from the fractionating tray s active area can contain vapour. The presence of vapour can limit this gravity flow. Rigorous pipe line hydraulic evaluation revealed that the gravity line hydraulics could be limited at a maximum target draw rate. 3 FT FT Pumparound Crude atmospheric column LC LT Side stripper Rundown product Figure 4 Case study 1: original design side draw configuration Freezing-point reduction, ºF 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Retrofit design prediction Number of theoretical stages gain for heavy light diesel fractionation section Figure 5 Case study 1: pre-retrofit sensitivity analysis Freezing point reduction, ºF 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 ties including the freezing point, one of the key specifications for rundown. To predict freezing point change, dedicated sensitivity analysis was conducted. 0.1ºF 3.0% Retrofit design prediction 0 10 20 30 40 50 60 70 80 90 Top simulated reflux L/V for heavy light diesel fractionation section, % Figure 6 Case study 1: pre-retrofit sensitivity analysis 0.1ºF The aforementioned high performance tray implementation could improve individual tray efficiency. Nevertheless, extra individual tray efficiency improvement was not counted to predict the retrofit Case study 1: equipment modification Based on the aforementioned process evaluation and root cause analysis, the original movable valve trays were replaced by fixed valve trays. This tray type conversion improved equipment resistance against fouling and valve/perforation hole wearing. The original fractionating trays at draw locations were converted to chimney trays to increase the liquid head for gravity flow. This chimney tray conversion also eliminates the chance of yield loss and start-up trouble through fixed valve tray implementation and increases draw liquid residence time for vapour disengagement from liquid. However, this conversion resulted in losing one tray for each fractionation section: light /heavy, heavy /light diesel and light diesel/heavy diesel fractionation. GT-Optim high performance trays with various performance-enhancing features and fixed valves were implemented for the rest of the fractionating trays. As described earlier, light and heavy streams are combined and rundown as a single intermediate product stream. Therefore, fractionation performance between light and heavy streams is not critical. The same rundown configuration of light and heavy diesel streams does not necessitate sharp fractionation between the two streams. However, fractionation performance between heavy and light diesel streams affects rundown and diesel intermediate product qualiwww.eptq.com PTQ Q1 2017 93
Case Post-retrofit Parameter Test run Test run Yield balance Crude charge, BPD Base + 12 Unstabilised naphtha, LV% Base + 6.0 Kerosene, LV% Base + 19.5 Diesel, LV% Base + 12.1 Fractionation performance Light 5% - naphtha 95%, 1 F Base + 1.0 Light diesel 5% - heavy 95%, 1 F Base - 1.0 Kerosene flash point, F Base 0 Kerosene freezing point, F Base + 0.2 Heavy /light diesel internal reflux L/V, 2,3 weight basis % Base - 15 Heat balance Heat removal - overhead condenser, 2 % of total BTU/hr 72 62 Heat removal - heavy, 2 % of total BTU/hr 4 6 Heat removal - light diesel, 2 % of total BTU/hr 7 10 Heat removal - heavy diesel, 2 % of total BTU/hr 16 21 1. ASTM D86 (LV%) 2. Simulated value 3. At the top tray of the section Table 1 heavy /light diesel fractionation performance. Case study 1: sensitivity analysis For reliable sensitivity analysis, simulation modelling was first validated with pertinent unit conditions. Simulated freezing point value was reasonably matched to actual value. The tray efficiency and internal vapour/ liquid traffic profile for each fractionation section were quantified through model validation. A constructed freezing point sensitivity curve per varied theoretical stages is plotted in Figure 5. This curve predicted that the freezing point could be increased by Freezing point reduction, ºF 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 Case study 1: performance summary Post-retrofit 15% Retrofit design prediction 0.1 F by using a chimney tray conversion scenario. Figure 6 also shows another freezing point sensitivity curve per heavy /light diesel fractionation section internal reflux L/V (liquid/ vapour) ratio. A freezing point increment of 0.1 F was predicted at a 3% lower heavy /light diesel fractionation section internal reflux L/V ratio. Undetected freezing point changes were anticipated through the sensitivity analysis. Case study 1: performance summary The pre- and post-retrofit performances are summarised and compared in Table 1. Both retrofit 8% 0.2ºF 0 10 20 30 40 50 60 70 80 90 Top simulated reflux L/V for heavy light diesel fractionation section, % Figure 7 Case study 1: pre- and post-retrofit sensitivity analysis conditions were obtained through the same operating mode, SOR (start of run) for fair comparison. Since the internal vapour/ liquid traffic and heat balances of the crude atmospheric column were not measurable operating parameters, these values were quantified through simulation modelling. Like the simulation model for the pre-retrofit case, the simulation model for the post-retrofit case was also validated with selected post-retrofit conditions. The crude charge rate was increased during the post-retrofit. The product yield balance reveals that the post-retrofit test run crude slate contained more boiling range materials compared to pre-retrofit crude slate. Kerosene and diesel yield limitation experienced in the past was eliminated. The simulated crude atmospheric column heat balance of the postretrofit case was also shifted from that of the pre-retrofit case due to a change in the crude slate composition. Laboratory test results showed that the post-retrofit freezing point was relaxed by 0.2 F. Meanwhile, simulation modelling showed that the post-retrofit internal reflux L/V ratio for the heavy /light diesel fractionation was reduced compared to the pre-retrofit value. The heavy /light diesel fractionation section performance through the post-retrofit was evaluated and compared to the pre-retrofit section performance. The post-retrofit freezing point was plotted and compared to the pre-retrofit sensitivity curve in Figure 7. Relaxing the freezing point by 0.2 F predicted an internal reflux L/V ratio reduction of 8% on the pre-retrofit sensitivity curve. A lower internal reflux ratio of 15% was simulated with the same 0.2 F freezing point relaxation at the post-retrofit conditions. Results indicated that the actual post-retrofit heavy /light diesel fractionation section efficiency was more satisfactory than the predicted retrofit efficiency value. 94 PTQ Q1 2017 www.eptq.com
Heavy /light diesel Retrofit Post-retrofit Fractionation section Test run Prediction Test run Kerosene freezing point, F Base + 0.1 + 0.2 Internal reflux L/V, 1,2 weight basis % Base - 3-15 Section efficiency, 1,3 % 55 50 65 1. Simulated value 2. At the top tray of the section 3. Overall efficiency (number of theoretical stages/number of fractionating trays) Table 2 Case study 1: fractionation efficiency comparison Table 2 shows simulated heavy /light diesel fractionation section efficiencies. As one fractionating tray was converted to a chimney tray, a lower number of theoretical stages were counted for the retrofit performance prediction. Nevertheless, the simulated theoretical stage count through the post-retrofit condition data was maintained at the same level as the pre-retrofit theoretical stage count, resulting in improved fractionation efficiency between heavy and light diesel. The aforementioned GT-Optim high performance tray implementation contributed to the fractionation efficiency improvement. Case study 2: crude distillation unit description and background The second case study also examines a crude distillation unit. Figure 8 illustrates the schematic of the unit. The crude atmospheric column in this case was designed as a fully structured packed frac- tionator excluding the bottom stripping section. Unstabilised naphtha, and atmospheric gas oil intermediate products were distillated through the crude atmospheric column and the side strippers. The unstabilised naphtha stream was further separated into LPG (liquefied petroleum gas), light naphtha, and heavy naphtha through the naphtha stabiliser and naphtha splitter. The heavy naphtha stream was directed to the reforming unit for aromatic component production. The unit utilised two circuits as well as an overhead condenser for heat removal. One circuit was positioned as the top. The other circuit was located between and atmospheric gas oil boiling range materials. A second packed bed at the top of the crude atmospheric column had the function of fractionating naphtha and boiling range materials. Crude atmospheric column Top Off gas LPG Light naphtha Naphtha/ fractionation section Heavy naphtha Bottom Kerosene Desalted crude Preflash drum Atmospheric gas oil Atmospheric residue Figure 8 Case study 2: crude distillation unit configuration 96 PTQ Q1 2017 www.eptq.com
Naphtha/ fractionation Original Modification Distributor drip point density, drip point/ft 2 16 4.6 Distributor operating range 2.5:1 1.7:1 Drip hole elevation, 1 inch 1.5 3 Distributor drip hole diameter, inch 3/16 7/16 1. From the bottom of the trough Table 3 Case study 2: distributor modification summary A particular unit limitation the refiner faced was that fractionation performance between naphtha and was substantially downgraded after a four-month operation. Substantial amounts of boiling range materials were downgraded to the naphtha stream. This downgrading not only limited the yield but also influenced the downstream reforming unit performance. The high rear end distillation point of the heavy naphtha stream adversely affected the reforming reactor catalyst activation. Case Study 2: root-cause identification Inspection during unit turnaround showed that the trough-type liquid distributor for the naphtha/ fractionation section was fouled. Several root causes of the fouling were identified through rigorous evaluation. Amine-based corrosion inhibitor was injected into the crude distillation unit. Chloride present in the column overhead may react with the inhibitor and form ammonium salt, which can foul the distributor. Formed ammonium salt particles could reside in the top bed and also migrate to the naphtha/ fractionation bed. Temperature, ºF 330 320 310 300 290 280 Findings showed that the column s inside wall cladding using Monel metallurgy was only applied to the portion where the top section was positioned. The column inside wall portion of the naphtha/ fractionation section remained as carbon steel. Rusted wall pieces from corrosion could accelerate fouling. A review of the original distillation equipment drawing revealed that the gravity flow trough-type liquid distributor for the naphtha/ fractionation section was designed with high drip point density and small drip hole size. The naphtha/ fractionation packed bed was equipped with structured packing with a 1in crimp size and a 45 inclination angle. This packing size at the given bed height was suitable to achieve 270 0 50 100 150 200 250 300 350 400 Unit run length, days Figure 9 Case study 2: performance trend naphtha end point A minimum liquid head needs to be maintained to ensure uniform liquid distribution Post-retrofit target fractionation efficiency. However, the liquid distributor drip point density originally selected was excessive for the 1in crimp size. The basic equation used to size gravity liquid distributors is: Lv H = k N HA H = Liquid height ( Head ) above round shaped hole L v = Liquid volumetric flow N = Number of drip holes HA = Hole area K = Orifice coefficient A minimum liquid head needs to be maintained to ensure uniform liquid distribution. A certain number of drip holes, which indicates drip point density, is required for the desired distribution quality. However, unnecessarily high drip point density reduces distributor drip hole size and increases a chance of fouling. Distributor operating range affects hole size because the liquid head should be maintained at the minimum rate for uniform liquid distribution. The original liquid distributor (as designed) was not properly optimised between fouling resistance and liquid distribution quality. Case study 2: distributor modification The liquid distributor modifications for the naphtha/ fractionation section are summarised in Table 3. In order to enlarge liquid distributor drip hole size, the drip point density was reduced in a new design. The new density was carefully selected by considering the commercially proven drip point density in the given size packing and application. Distributor operating range was also adjusted further to increase distributor drip hole size. The minimum end of the distributor operating range was increased. This adjusted distributor operating range does not reduce the unit operating range. The minimum rate of the liquid distributor does not have to be matched to the minimum unit charge rate. 4 Heat balance shifting through adjustments or increasing furnace coil outlet 2 98 PTQ Q1 2017 www.eptq.com
temperature can maintain the required minimum distributor rate during lower unit charge rate operation. This strategy can increase energy consumption during minimum charge rate operation. But it can assure efficient unit operation in the entire charge range and more efficient overall unit economics can be achieved. Distributor drip hole elevation from the bottom of the trough was increased to slow down distributor fouling. The measured naphtha end point and flash point trends are plotted in Figure 9 and Figure 10 respectively. Plots in red indicate values gathered during pre-modification operating periods while plots in blue represent values achieved after the modification. Stable naphtha end points and flash points were maintained for more than eight months of operation. These case studies show how fouling resistance and efficiency requirements for distillation equipment are balanced and optimised Temperature, ºF 114 112 110 108 106 104 102 100 0 50 100 150 200 250 300 350 400 Unit run length, days Figure 10 Case study 2: performance trend flash point for reliable crude distillation unit performance. This article is an updated version of a presentation given at AIChE s Spring Meeting Distillation Symposium, 11-14 Apr 2016, in Houston, TX. GT-OPTIM is a mark of GTC Technology US LLC. References 1 Lee S H, et al, Optimising crude unit design, PTQ, Q2 2009. Post-retrofit 2 Kister H Z, Distillation Operation, McGraw- Hill Company, 1990. 3 Libermann N P, Process Design for Reliable Operations, Gulf Publishing Company, 2nd Edition. 4 Bonilla J A, Don t neglect liquid distributors, Chemical Engineering Progress, Mar 1993. Soun Ho Lee is Manager of Refining Application for GTC Technology US, LLC, in Euless, Texas, specialising in process design, simulation modelling, energy saving design and troubleshooting for refining and aromatic applications. Email: Sounho@gtctech.com www.eptq.com PTQ Q1 2017 99