INTEGRATION OF THE PROCESSES FCC FEED HYDROTREATMENT

Vassil Yankov, Dicho Stratiev, Atanas Yalamov

Lukoil Neftochim Bourgas, Bulgaria

Key words: cracking gasoline, Prime G

Abstract

Cracked-gasoline has 90% sulfur content of end-product – market vehicle gasoline produced at a refinery. So market gasoline production complying with standard Euro V requirements is directly related to providing cracked-gasoline with sulfur content no higher than 10 – 15 ppm. The following research presents a survey on combining both technologies used for cracked-gasoline production with close to 0 sulfur content – preliminary FCC feed hydrogenation and post cracked-gasoline treatment applying Prime G process method. It is established that post-treatment of cracked gasoline using Prime G process technique octane number loss due to unwanted cracked-gasoline olefins hydrogenation turns appears to be function of sulfur content in cracked-gasoline which is hydro- desulfurization feed. Catalyst system intermediate regeneration cycle duration used for FCC feed desulfurization is related to sulfur content in hydro -desulfurized vacuum gas oil and consequently sulfur content at cracked- gasoline obtained by hydro-desulfurized vacuum gas oil catalytic cracking process. In result of executed comparison research with different operation modes at the preliminary FCC feed hydrogenation Unit and Prime G cracked-gasoline hydrogenation Unit catalyst system intermediate regeneration cycle duration at the preliminary FCC feed hydrogenation Unit and hydro-desulfurized cracked -gasoline octane number loss were determined. This data have been used to establish the most economically effective operation of both hydrogenating units utilizing the refinery simulation module based on software product for linear programming RPMS.

Introduction

The main contributor of sulfur in the refinery finished gasoline for sale (about 90%) is actually the gasoline from the FCC. For that reason the main technologies for reducing the sulfur content in the gasolines are those referring to desulfurization of cracking gasoline or desulfurization of FCC feed  or a combination of both.

The limit for sulfur content in the cracking gasoline below 10ppm (near zero content) is a serious challenge for the units for hydrodesulfurization of FCC feed and is hard to implement without increasing the catalyst volume. In the opposite case the duration of the catalyst run between two regenerations is extremely low.

Lukoil Neftochim Bourgas, Bulgaria (LNB) is a FCC based refinery. Since 2009 the LNB has been producing near zero sulfur gasoline (NZSG) (less than 10 ppm) by applying FCC feed hydrotreating and later in 2010 by applying also Prime G FCC gasoline post treatment technology. Fifteen ppm sulfur in FCC gasoline is the cap for production of NZSG in the LNB refinery. After the Start-up of Prime G, the issue occurred regarding the optimum combination of the two technologies: hydrotreating of FCC feed and hydrotreating of cracking gasoline with the purpose to produce NZSG at minimum loss of octane number and a maximum cycle between regeneration of the FCC feed hydrotreating section. The purpose of the present (piece of) work is to study the regularities with the two processes of hydrotreating: hydrotreating of FCC feed and hydrotreating of the cracking gasoline with a view to find the optimum operation mode of the two processes, in which to obtain the maximum profit for the refinery.

Experimental section

Figures 1 and 2 present the principle process diagrams of the two processes, examined in the present work: hydrotreating of FCC feed and hydrotreating of cracking gasoline through Prime G Process. The Catalytic Cracking process.

FCC type by license from the Process Institute in Grozny, was commissioned in LNB in 1982. Its main objective is to obtain high octane components for motor gasolines, components for diesel fuels and fuel oils, as well as gases, feeds for production of high octane components for motor gasolines. FCC process covers three stages, the preliminary feed hydrotreating, cracking proper, and products fractionation. Further the article reviews only the section for pre-hydrotreating of the feed.

The Section for pre-treating of FCC feed is intended to reduce the content of sulfurous, nitric, oxygenates, organic metal compounds, polycyclic aromatic compounds, reducing, at the same time, the cocking capacity of the feed for catalytic cracking proper (FCC). The process take place on Co-Mo catalyst situated in two reactors in series. Table I presents the process conditions in the section for pre-treating of FCC feed, in which the present research is done. The typical feed for FCC is heavy vacuum gas oil (HVGO), obtained in vacuum distillation of oil residue from crude Russian Export Mixture type. Table II indicates the main physical and chemical properties of FCC feed, subject to a preliminary hydrotreating.

Prime G Process licensed by Axens was commissioned in LNB in 2010. Its main objective is to achieve ultra-low sulfur content in the cracking gasoline (below 10ppm), at minimum loss of octane number. The Unit consists of two sections, selective hydrogenation unit (SHU), and section for hydrodesulfurization (HDS) of cracking gasoline. SHU section is designed for selective hydrogenation of diolefins to olefins, converting the light mercaptans and light sulfur compounds in heavier ones and isomerization of external olefins to internal olefins. The process takes place in a reactor, on Ni-Mo catalyst. HDS section is intended to decompose the organic sulfurous, nitric, oxygen compounds, during which hydrogen sulfide, ammonia and water are formed respectively. The process takes place in a reactor, filled with two layers of Co-Mo catalyst and one layer of Ni catalyst. The temperature profile of HDS reactor is controlled, by feeding quench gasoline after the first layer. Table III presents the process conditions in Prime G Unit, applied in the present research.

Results and Discussions

It was ascertained that with post-treating of cracking gasoline by applying Prime G Process the loss of octane number, due to undesirable hydrogenation of the olefins in the cracking gasoline is actually a function of the level of hydrodesulfurization (Fig.3). Unfortunately, however it does not become clear from the data in Fig.3, what the loss of octane number will be as a function of sulfur content in the cracking gasoline, and what the sulfur content in the cracking gasoline shall be in order to minimize the loss of octane number.

Figure..4 presents data for loss of RON and MON depending on the sulfur content in the cracking gasoline. These data refer to hydrodesulfurization of cracking gasoline, obtained from non-hydrodesulfurized feed and the same one after hydrotreating at various severity of operation (case 1, 2, 5, 6, 7, 8, 10 from Table III).

Form these data it is evident that with increasing the sulfur content in the cracking gasoline the loss of RON increase considerably, while the loss of MON is considerably smaller compared to the loss of FON. In the analysis of the dependence of ΔRON on the sulfur content it is evident that the highest deviation from the dependence is in the point of co-ordinates 992:1.9. keeping in mind that the reproudcibility of the analytical method of RON is 0.7 points, then the reported value of deviation of 0.5 points, can be assumed as correct. Form the data it is evident also that, the correction of  ΔMON with the sulfur content is week. The maximum deviation form the dependence is in the point of co-ordinates 200:0.6 and is within the  reproducibility of the analytic method MON-0.9  points.