Simulation studies of Naphtha Splitter unit using Aspen Plus for improved product quality

Transcription

1 Simulation studies of Naphtha Splitter unit using Aspen Plus for improved product quality Pranab K Rakshit*, AbhijeetNeog # *Corporate R&D Center, Bharat Petroleum Corporation Ltd, Greater Noida Tel: # Numaligarh Refineries Ltd * rakshitpranabk@bharatpetroleum.in 1.0 Abstract A number of Indian refineriesare in the process of enhancing their capacity by revamping the existing units and adding new units to produce diesel and gasoline products of Euro III/Euro IV specifications. Some of them have plans to replace the naphtha fuel of Gas Turbines with Natural gas. These changes will result in excess naphtha production in their refineries. One of the refineries wants to use the excess naphtha as a feedstock for naphtha cracking unit. It was observed during the review of naphtha processing facilities of the refinery that the existing streams are incapable of meeting naphtha cracker feed specifications. Hence it was decided that a naphtha splitter unit (NSU) need to be commissioned for meeting cracker feed specifications. The process design package for NSU was developed but it was observed that the design feed specifications and the actual feed specifications changed significantly with time. Refiners then had the apprehension whether the designed unit is adequate to meet the naphtha cracker feed specifications. Aspen plus distillation column models (Radfrac) were used to simulate the naphtha splitter unit. The model was validated using the design data and then the effect of feed specification changes was studied. The feed to naphtha cracker is required to have high concentration of paraffins. It was observed that all the naphtha streams in the refinery are having fairly good amount of naphthenes. Due to higher naphthene content, the paraffin content of various naphtha streams was observed to be much lower than desired. The present study focuses on analyzing the design data of Naphtha splitter unit and finding ways to increase the paraffin content of naphtha stream. Simulation studies of naphtha splitter unit were carried out using Aspen Plus separation modules. Suitable solutions for enhancing the product paraffin content are discussed in the paper. 2.0 Introduction In present scenarios, with crude oil price increasing day by day refiners across the world are forced to minimize losses and explore every possible value addition option to augment their refinery margins. Refinery produces many hydrocarbon streams with variable value. Efforts are being put to valorize these low value streams either by putting up new facilities or by employing suitable blending schemes. A prominent Indian refinery was enhancing the capacity of the existing units by revamping and adding new units to produce Diesel and Gasoline products of BS specifications. They 1

2 also had a plan to replace the naphtha fuel of GT s with Natural gas. These changes resulted in excess naphtha production in the refinery, which they wanted to sell as a feedstock for naphtha cracking units. It was observed during the review of Naphtha processing facilities of the refinery that existing units are incapable of producing naphtha which can meet naphtha cracker feed specifications Hence it was decided that a new Naphtha splitter unit (NSU) needs to be commissioned for meeting naphtha cracker feed specifications. Process package for NSU was developed but it was soon discovered that the design feed specifications and feed throughput could be significantly different from actual throughput and specifications of the refinery streams. In view of this, it was decided that simulation model will be developed to check the whether the unit is capable of meeting the desired specifications of naphtha cracker feed with changes in various naphtha streams of the refinery. 3.0 Objective: The objective of this study is to evaluate the extent of deviation in cracker feed specification that will occur if naphtha streams of varied are processed in NSU. Also to suggest possible alternatives to meet the desired cracker feed specifications. 4.0 Process Description: The proposed naphtha splitter unit (NSU) is schematically represented in Figure 1. There are two splitter columns, one () for splitting light naphtha () from Hydrocracker (HCU) and the other splitter () for splitting straight run naphtha (SRN) from Crude Distillation Unit (CDU)., bottom, Naphtha from Motor spirit Plant (MSP) and SRN from CDU blends to form the feedstock for naphtha cracker unit. The balance streams and bottom goes to MS pool. Critical Cracker feed specifications are shown in Table Naphtha splitter unit model: Simulation model of the described Naphtha splitter unit was developed using RADFRAC module of Aspen Plusto predict the composition of all streams associated with NSU. The hardware details and process parameters were taken from the process licensor s design package. PIONA analysis of all feed streams mentioned in the design package was used for defining thefeed streams of Aspen plus model. SRK equation of state was used for flash calculations.simulation studies were carried out in two steps. In the first step a design case study was done, wherein composition, flow rates and flow scheme of various streams are considered as per the design package and the product cracker feed stream results were compared with design data to validate the model. In the second step actual feed stream data and flowrates were considered to predict the composition and throughput of the cracker feed stream. Process licensors design data of cracker feed stream vis a vis product properties obtained using the simulation model is also given in Table Simulation studies: The validated model was then used for predicting the naphtha cracker feed properties with changes in the SRN and feedthroughput as well as quality. Table 2 given below highlights the actual feed composition and flowrate of SRN and. The deviations that have taken placein feed streams properties w.r.t design package can be summarized as: 2

3 As per design package, SRN splitter () is designed for processing 180 TMTPA of naphtha but in actual only 109 TMTPA of SRN is available for SRN splitter (assuming 271 TMTPA of SRN is going to MSP). Naphtha ex HCU to splitter, as per splitter design package is 51 TMTPA. Production of Naphtha ex HCU is 125 TMTPA. There is an excess of 74 TMTPA available. NSU design package considers 22.8% Naphthenes in SRN and 26.8% Naphthenes in whereas in actual SRN contains 37.8% Naphthenes and contains 33.8% Naphthenes. Due to the shortfall of SRN, make up SRN to cracker feed pool was made to zero and the total available SRN was diverted to column. The above mentioned deviations were incorporated in the simulation model and the results obtained are highlighted in Table 2. From the results given in table 2 it is clear that the cracker feed stream is not meeting the cracker feed specification of total paraffin content of 75% and also the throughput is 30-35% less compared to the design throughput of kg/hr.the results also highlights that the bottoms going to cracker pool has high concentration of naphthenes (more than 50 %). As per design package, the purpose of column was to remove lighters from and meet specs on RVP of Cracker feed pool. But in actual, the stream is not as light as perceived in design case and hence doesn t affects the RVP.Composition profile of both the splitter columns are given in figure 2. It is clearly evident from the figure that the concentration of naphthenes are higher at the bottoms. Hence adding any of the splitter bottom streams will not reduce the naphthenes content of naphtha cracker feed pool. and are the only streams which has less than 20 % vol naphthenes. Any possible solution can only come by maximizing these streams. Based on the above mentioned facts and the results highlighted in table 2 following modifications can be made to improve the product quality: Increase SRN feed to to the maximum and make up the resulting SRN shortfall in Motor spirit block(mspblk) by the bottom streams of SRNSPLT and. Take out bottom from the naphtha cracker feed pool and replace with. Replace the hydrotreater naphtha from MSPBLK to naphtha cracker feed pool by without affecting the RVP specifications. Increasing feed to 10 % above design Optimizing both the splitter operating parameters to improve product quality. The above mentioned modifications were made to the simulation model and the results obtained are highlighted in table 3. The results of naphtha cracker feed pool highlights that though the stream meets all the desired specifications but the scheme fails in meeting the desired throughput of 161 TMT by 25 %. The above scheme also leaves a substantial quantity of high naphthenic naphtha stream which needs to be disposed effectively. 5.0 Conclusion Naphtha splitter unit for producing feed for naphtha cracker has been successfully simulated using Aspen Plus based Radfrac Models. The simulation model accurately 3

4 matched the design package process data and predicted the magnitude of shortfall and deviation from specifications of cracker feed in case of feed changes, with reasonable accuracy. New process scheme were suggested to improve the naphtha cracker feed properties and were validated by the simulation model, but these schemes though improve product quality but doesn t meet the throughput requirements. Hence, it is proposed to rework the whole problem and develop a new design package for the naphtha splitter unit. Acknowledgement The authors express their sincere thanks to the management of BPCL for permitting to publish this work. References 1. PrasenjitGhosh, K. J. Hickey and S. B. Jaffe, Ind. Eng. Chem. Res., 45, , G C Anderson, R R Rosin and M A Stine, 2004 NPRA 3. Naphtha Splitter Unit Design Package Table 1: Naphtha Cracker feed specifications and Product stream parameters (design case) S. No Property Unit Spec. Design Data Simulation results 1 15 C g/ml Total Paraffins Min. Vol % Iso/Normal Max Paraffins 4 Olefins Max Vol% Aromatics Max. Vol% Naphthenes Max. Vol% Balance Balance Balance 7 IBP C FBP C RVP Psia Max Table 2: Cracker feed properties predicted by the model with alteredfeed specification SRN bottom Bottom MSPNAP Cracker feed Flow, kg/hr Paraffin Iso-paraffin Olefins Naphthenes Aromatics IBP, deg C FBP, deg C RVP

5 Table 3: Cracker feed properties predicted by the model with modified process conditions SRN bottom Bottom makeup Cracker feed Flow, kg/hr Paraffin Iso-paraffin Olefins Naphthenes Aromatics IBP, deg C FBP, deg C RVP BALANCE B6 180 TMT TOP SRN B3 SRNFEED SRNBOT 53 TMT SRNTOP 51 TMT 33 TMT 40 TMT MSPNAP BOTM 2MSP SRNMKUP 35 TMT 271 TMT MSPBLK B4 161 TMT GASCRKR Figure 1: Flowsheet of Naphtha splitter unit Mol fraction Composition profile in column Naphthenes Parraffins+Iso paraffins Stages from Mol fraction Figure 2: Composition profiles of Paraffins/Isoparaffins and Naphthenes in the splitter columns Composition profile in column Naphthenes Parraffins+Iso paraffins Stages from 5