Sulfur compounds are one of the most important pollutants in petroleum cuts, and removing these compounds is one of the important objectives in refining industry.

Hydrotreating processes have been well known since 1933, and a wide range of research has been conducted in this field throughout the world. Nowadays, a lot of capital investments is inclined in refining industries to produce cleaner middle distillates to meet environmental standards. On the other  hand, poisoning of valuable metal catalysts in refinery units  and deactivation of them as a result of contacts with such harmful compounds are the major reasons  that necessitates hydrotreating of fuels.

Also a toughening trend of environmental regulations and standards in most countries cause a drawback in the export of highly pollutant fuels.

It should be noted that total hydrotreating capacity in the world is about 900 million tons per year.

                          Table 1 - Distribution of Hydrotreating Plants in the World 

It is estimated that there are more than 1000 hydrotreating units in the world with catalyst consumption of about 30,000 tons per year. The cost of consumed catalyst for a 70,000 BPD Hydrotreating unit is about 8.8 million dollars. Therefore, from both economical and public health points of view, producing standard fuels is  inevitable.

Description  of HDS Process

Sulfur is one of the important pollutants in fossil fuels. Sulfur contents in crude oil may be categorized to the following groups:

1- Free Elemental Sulfur
2- Mercaptans & Tiols (R-SH)
3- Hydrogen Sulfide
4- Sulfides
5- Disulfides (R-S-S-R')
6- Poly Sulfides (R-S_n-R')
7- Thiophenes and their derivatives such as BT and DBT

It is possible to recycle gas from separation section to reaction section as this gas contains a significant amount of excess hydrogen in its composition.

Following diagrams show a simplified flow sheet of a hydrodesulfurization process.



 



Advantages of HDS
 

• Sulfur & nitrogen removal to less that 10 ppm

• Complete removal of metal compounds from feedstock

• Reduction of environmental pollutants

• Increase in catalysts age and reduction in poisoning of valuable metal catalyst

• Reduction in corrosion of process equipment

• Easy treatment of waste water

• Simple operation of process unit

  Process Simulation

  Nowadays, utilization of simulation software for development of reactor design and scale up to industrial capacities, is the most desired method to reduce costs of technology development. Using simulation software makes it possible to study more carefully various scenarios of process such as investigation of increasing and decreasing capacity of operating units and finally design and optimize  a HDS unit. With this simulation software we were able to study reactor dimensions, catalyst properties and various parameters such as temperature, pressure, flow rate, and feed composition effects. In this regard, in order to prepare an ultra-deep hydrodesulfurization software, modeling & simulation of HDS process have been performed in modeling & control department of Engineering & Process Development Division at RIPI.

  This software is easy to use and with the aid of a built-in iteration program at different conditions, it is possible to predict a complete collection of process performance in various states in a short period of time. Having done the above tasks, a decrease in operation costs (water, Energy) as well as further reduction in excess costs and more flexibility in the process design may be achieved.  Also, since a perfect process design solely depends on equipment and mechanical design, piping and instrumentation, it is possible to use the obtained simulation data in different conditions for further improvements in designing such systems.

   

  

  
  Modeling of a HDS Reactor

  A software is necessary for linking between industrial and pilot data to study the effects of changes in physical condition and dimension of reactor, catalyst properties, and feed composition. As a trickle bed reactor is the heart of a HDS process, thus reactor modeling has been done for dibenzothiophene component in gas oil.

  Then model equations were developed for kinetics, hydrodynamics, mass and heat transfer  and  then they are solved for reactor by using kinetics parameters and phase physical properties with fourth order Runge-Kutta numerical method.

  Also for effectiveness factor calculation in the catalyst, mass balance equations of the catalyst  have been written and orthogonal collocation method with the aid of modified Powel-Dogleg numerical method, has been used to solve nonlinear equations obtained in the catalyst pieces.

   

   

  Dynamics & Process Control

  Dynamic simulation is widely used in the chemical process engineering, in order to predict and analyse dynamic behavior of the chemical process; so that it can help us to design a better process. Hydrodesulfurization units can never operate at a steady state condition actually; and they always have a dynamic nature. Because of importance of the deep HDS units in refineries, modeling and simulation study of TBR process for petroleum cuts hydrodesulfurization, in the dynamic state has a significant importance to process and control engineers for the following reasons:

  1) Designing new plants using this model as a tool

  2) Possible   automation and optimization of process control

  3) Possible Process optimization with regard to some limitations such as reactor materials, hazop and catalyst deactivation.

                    4) Predicting dynamic behavior of the reactor in start-up and shut-down or inlet feed fluctuations, then designing a control system for adjusting process according to above factors.

                    5) Designing the required modifications in the process according to probable changes in the feed quality or product quality.

  6)       Training operators as a tool.

  Also, in order to improve product quality for improving the market situation in the international competition, dynamic optimization of process is necessary.

  For this purpose, real time optimization can be applied the whole plant, and prepare a software to achieve this goal, so that economic benefits of the process can be increased by evaluation and continuous changes in process condition. Hence, in this regard RIPI with its vast amount of experience in process control and modeling, has taken a step forward and extended its activities with relying on its personnel capabilities.

  
  Kinetics and Catalyst Deactivation

  Since complete knowledge of HDS process is necessary to research and study on HDS reaction kinetics, therefore, design parameters, optimum process condition and catalyst performance can only be achieved using a suitable kinetics model.
  For this purpose, many researches have been conducted at RIPI to investigate HDS reaction kinetics.

  
  

  A systematic study was performed to determine HDS reaction kinetics with dibenzothiophene component by using a commercial CoMo/ catalyst, and operating parameters of the process were optimized. Experiments were carried out in a trickle bed micro reactor.

  

  Pore diffusion and film diffusion resistance effects were evaluated in the working operation range. Finally HDS reaction kinetics was determined by Power Law model using thiophene and dibenzothiphene as sulfur compounds.

  

  Catalyst deactivation is a complex phenomenon. Activity and selectivity of catalyst change with time. This phenomenon occurs by three factors including heat, physical, and chemical effects.

  Factors that cause catalyst deactivation are summarized in table (2).

  Mechanical	Sedimentary	Chemical	Heat
  Heat shock	Coke formation	Poisoning (irreversible adsorption or surface reaction)	Hardening and deposits
  Erosion & scratch		Inhibitors (chemicals that retard or stop reaction)	Combination & Alloy creation
  Physical breakage		Change or deforming of catalyst surface	Support deformation
  		Physical /chemical block of catalyst pores	Reaction of metal support
  			Oxidation
  			Metal volatility

   

  

  
  HDS Pilot Test

  In a joint venture project with Japan's NOC corporation, pilot tests were performed  using a commercial catalyst (NHS-204) for gas oil fuel. First, required feed for tests runs, were supplied by Shiraz (12% LGO, 88% LVGO) and Tehran refineries (Isomax gas oil). Tests were performed after set up preparation, catalyst sulfidation  and determination of optimum operating condition . Then total sulfur, density, viscosity, nitrogen content, pour point and cetane index were determined. Excellent results were obtained from long time tests (5 months). Total sulfur of less than 10 ppm was obtained by using Ultra deep desulfurization pilot tests. Also, pilot tests have been performed for South Pars gas condensates as feed and KF-757 commercial catalyst.

   

  List of Completed Projects:  

  • Simulation and modeling of hydrodesulfurization process (HDS)

  • Conceptual design and technology transfer for hydrodesulfurization process super pilot

  • Basic design for HDS pilot with 20 BPD capacity

  • Basic design review and detailed design for HDS pilot

  • HDS pilot test for Shiraz refinery gas oil feedstock and study  its kinetics

  • Feasibility study and conceptual design for hydrodesulfurization (HDS) unit of Shiraz refinery

  • Pilot tests of HDS process for Assaluyeh's gas condensate

  • Optimization of energy consumption in HDS process

  • Feasibility study and conceptual design for a HDS unit of Assaluyeh's gas condensates

  • Basic design for a HDS super pilot and an amine unit for Kermanshah's pentane feed with capacity of 200 BPD

  • Investigation and selection of HDS catalyst suppliers and licensors

  • Studying and investigation of dynamics of hydrodesulfurization

With reference to recent investigations on hydrodesulfurization reactions, it has been learned that sulfur removal from mercaptans, sulfides, and disulfides are easily done and free sulfur hydrocarbon and H₂S gas are produced as a result , while thiophenes and particularly benzothiophenic and dibenzothiphenic derivatives are very difficult to desufurize and if deep hydrodesulfurization of diesel fuel is concerned, removing  benzothiohenic and dibenzothiophenic compounds become very important. Most of  industrial hydrodesulfurization methods are similar, and there are only minor  differences in their details. A typical hydrodesulfurization plant has two main sections: