January 2020

Process Optimization

Optimization study in hydrocracker unit using simulation model

Hydrocrackers are key refinery units that convert heavy feed components into valuable lighter products. The profitability of these units is directly related to this conversion level. Conversion reactions take place in a nearly pure hydrogen environment, with the help of selective catalysts, under high pressure and high temperature. The products of the unit are separated in the fractionator section. Unconverted oil taken from the fractionator bottoms reveals the conversion performance of the unit.

Hydrocrackers are key refinery units that convert heavy feed components into valuable lighter products. The profitability of these units is directly related to this conversion level. Conversion reactions take place in a nearly pure hydrogen (H2) environment, with the help of selective catalysts, under high pressure and high temperature. The products of the unit are separated in the fractionator section. Unconverted oil taken from the fractionator bottoms reveals the conversion performance of the unit.

These processes require high energy consumption, such as fuel gas, steam, electricity and cooling water. Therefore, utility minimization should be a primary goal. However, it is also important to consider the separation performance of the fractionator because it affects the conversion target of the unit reactor section. Optimization of utilities while keeping separation quality constant should be the target for these units.

For this case study, a hydrocracker unit model (one-stage and once-through) was prepared and validated with site data, using a commercial simulation program. After this step, optimization studies were performed for energy efficiency, column product yields and product specifications. The results of the optimization studies were incorporated into the fractionator and feed furnace parameters—e.g., the flowrate of internal reflux from the fractionator to the stripper, and the feed furnace temperature and fractionator stripping steam rates. Changing these parameters affected product specifications (e.g., the gap between diesel and fractionator bottoms), as well as energy consumption.

Purpose of study

A heat integration project was implemented to decrease the overall energy consumption of the hydrocracker at Tüpraş’ İzmit refinery. With this project, additional heat exchangers and a preflash drum were installed in the unit.

The role of the preflash drum in this project is to separate light-ends products from the stripper bottoms stream. The flashed stream is the feed to the fractionator furnace. The preflash decreases the furnace duty by flashing light ends, but deteriorates the stripping effect of the column.

After startup with a new configuration, all operating parameters were revised to find the optimum unit operation for energy and yield. At that point, simulation tools were utilized to perform an optimization study for the new process conditions.

Main parameters used in the optimization study

In addition to the product profitability, refinery energy consumption is becoming a key point for future productivity. New investments concentrate on pinch study by improving heat transfer resources with additional heat exchangers, rather than using fuel/steam sources. The optimization study for the 30-yr-old hydrocracker unit was conducted after a heat integration project in the unit.

One of the main performance parameters in hydrocracker units is conversion. This is calculated as the percentage of feed converted to diesel plus products, and the reactor temperatures are controlled according to the target conversion in the unit. Therefore, the gap between the diesel product and the fractionator bottoms directly affects the reactor temperatures. If some diesel remains in the fractionator bottoms, then the reactor temperatures are increased to maintain the same fractionator bottoms product flow. In that case, the reactor operates at unnecessarily higher temperatures. As a result, the catalyst deactivation rate increases indirectly, and middle distillate yield decreases slightly.

Maximizing diesel cut is one of the most important parameters for increasing hydrocracker profitability. The first step of the optimization study was to minimize the fractionator’s separation performance gap. Therefore, the fractionator feed furnace outlet temperature was increased up to 10°C. A test run was performed after the heat integration project at the optimized fractionator furnace condition.

Operation parameters with the most promising potential for better yields and minimum energy consumption are studied via a simulation model. The following is a brief list of related operating parameters and their possible effects:

  • Fractionator furnace outlet temperature: Fractionator separation performance is directly related to column inlet temperature. The optimization of the fractionator furnace is a key parameter, particularly for minimizing the diesel cut in the fractionator bottoms product.
  • Fractionator bottoms recycle flowrate: Recycle flow is supplied from the fractionator column bottom to the stripper column bottom in unit design. The aim of this recycle is to ensure a minimum flow on the four arms of the fractionator furnace. Optimization of the recycle flowrate is crucial for stripper operation, fractionator performance and furnace load.
  • Stripping steam in the fractionator and stripper: The aim of this stripping steam is to ensure optimum separation in the column.

Simulation of hydrocracker model

The hydrocracker unit was modeled with a commercial simulation program that has the capability to model reactors, heat exchangers, furnaces, columns, pipe segments and other equipment.

The first step of the simulation of a hydrocracker is the selection of an appropriate thermodynamics property package. The Peng-Robinson fluid package was selected to estimate hydrocarbon properties, and the ASME steam table was chosen to estimate water and steam properties. The simulation model was created with operational data. A unit process flow diagram serves as the basis for the simulation flowsheet.

The hydrocracker simulation model has the following main equipment:

  • High-/low-pressure separators
  • Fractionator column
  • Stripper column
  • Debutanizer column
  • Naphtha splitter column
  • Heat exchangers (as a network)
  • Furnace
  • Preflash drum
  • Waste heat boiler (WHB) systems.

The aim of the model is optimization

In this simulation case study, the reactor outlet back blend is used, instead of a detailed reactor model. At the beginning of the detailed analysis, the base model of the unit is completed and used for the modifications to optimize the system.

Simulation model validation. Simulation models are validated according to the unit’s design data or field data, before using them in refinery projects. The main steps during validation are summarized:

  1. Generation of the model using field data: Field data (distillation, flowrates, operational data, etc.) and equipment process data sheets are used to configure the model. The simulation is done according to two different data sets having a mass balance between 98% and 102%. Several parameters are fixed in the model, according to field data:
  • Reactor outlet temperature
  • Column and preflash drum pressure profiles
  • Fractionator and stripper columns’ stripping steam rates
  • Fractionator reflux rate to the stripper column
  • Fractionator inlet (furnace outlet) temperature.
  1. Validation of the model results: The simulation results are validated according to plant data and based on predefined validation limits. Critical parameters for the validation of the hydrocracker model include:
  • Product flowrates [LPG, light naphtha (LN), heavy naphtha (HN), kerosene and diesel]
  • Product specifications for LN, HN, kerosene and diesel (ASTM D86 T5–T95), and LPG (C5 limit)
  • Fractionator furnace fuel gas consumption
  • Stripper, fractionator, debutanizer and naphtha splitter operational parameters:
    • Pumparound rates for the fractionator
    • Reboiler duty for the debutanizer and the naphtha splitter
    • Temperature profile (condenser, top, bottom and product draws)
  • Heat exchanger network temperature profile (heat exchangers inlet/outlet temperatures; and reactor, furnace and column inlet temperatures).
  1. Acceptance of the deviations from actual data: When the model is compared with field data, the results are acceptable if the difference is ±5%. These criteria are based on possible measurement errors and general refinery practice.

The percentage differences between the plant data and simulation output are compared in Table 1. The validation is complete, as the differences are ±5%.

Optimization study

A study was performed based on the validated model of the unit. Energy consumption of the unit was minimized by reducing fuel consumption and increasing steam production during this study. Furthermore, the gap between diesel and the fractionator bottoms was also optimized to maximize diesel production in an efficient and economical way.

Different limits play a role in defining optimization ranges, such as furnace burner limits, metallurgical limits or column capabilities, etc. The selected optimization ranges for each operating parameter are listed:

  • The gap between the fractionator bottoms and diesel affects the economic feasibility of the unit and will be positive. The gap value varies by ±10°C.
  • The furnace outlet temperature/fractionator feed temperature should be kept as low as possible to reduce fuel consumption (but maintain a positive gap). The aim of the increase in the furnace outlet temperature/fractionator feed temperature is improving the gap between the fractionator bottoms and the diesel products. The furnace outlet temperature/fractionator feed temperature is changed by ±10°C.
  • Stripping steam consumption in the fractionator column must be kept as low as possible to reduce energy consumption, but product specifications must achieve desirable values. The aim of the increase in stripping steam consumption in the fractionator column is to improve the gap between the fractionator bottoms and the diesel products. Stripping steam consumption is adjusted by ±40 tpd.
  • In the stripper column, stripping steam consumption should be optimized to reduce the H2S amount in the overhead products. The aim of the increase in the stripping steam consumption in the stripper column is to reduce H2S content in the overhead products. Stripping steam consumption fluctuates by ±50%.
  • Internal reflux from the fractionator to the stripper column should be optimized to reduce energy consumption of the fractionator furnace and enhance the gap between diesel and the fractionator bottoms.
    The aims of the reduction in internal reflux from the fractionator to the stripper column are to improve the
    gap between the fractionator bottoms and the diesel products, and to reduce the furnace duty. Internal reflux from the fractionator to the stripper column is adjusted by ±20 m3/hr.

The optimum energy study is accomplished with minimum energy consumption, without any violation of product specifications. The optimum gap study relies on maximum product yields without neglecting energy optimization.

Implementation of model results in the field

As the first steps of the optimization study, the furnace outlet temperature was increased up to 5%, and the column diesel pumparound was adjusted to increase the gap between the fractionator bottoms and diesel. The separation index in the fractionator increased as the furnace outlet temperature increased, and diesel product flow expanded up to 5%.

Fig. 1 shows the gaps between diesel and fractionator bottoms flow over the period of one month, as the outlet temperature was increased. With a rise in the furnace outlet temperature of 4.5%, the difference between the fractionator bottoms (5%) and the diesel (95%) improved from –3°C to 13°C.

Fig. 1. Furnace outlet temperature and gap graph.
Fig. 1. Furnace outlet temperature and gap graph.

Within the scope of optimization, the bottom circulation flow from the fractionator bottoms line to the stripper column was reduced to 55%. The resulting gap improvement between the fractionator bottoms distillation (5%) and the diesel distillation (95%) is shown, in comparison with the decreasing trend displayed in Fig. 2.

Fig. 2. Gap between fractionator bottoms (5%) and diesel distillation (95%).
Fig. 2. Gap between fractionator bottoms (5%) and diesel distillation (95%).

One of the parameters to be achieved in the site execution was fuel savings in the fractionator furnace. With the reduction in the bottoms circulation flow, the furnace flow decreased by an average of 9%, and the energy savings was 0.59 Gcal/hr.

As the third step of the optimization study, the amount of fractionator stripping steam increased by 15%, enabling the furnace outlet temperature to drop by 1.3%. Fig. 3 shows the fuel, furnace outlet temperature and steam consumption trends before and after the study. The consequent energy savings was 0.24 Gcal/hr.

Fig. 3. Stripping steam and fuel flow.
Fig. 3. Stripping steam and fuel flow.

Takeaway

Operational parameters were optimized according to simulation outputs, and no requirement for any investment or modification is foreseen. With the implementation of the optimization parameters, energy efficiency and product specifications have improved. Reduction of utility consumption by the unit contributed to the sustainability of hydrocracker units in Tüpraş’ İzmit refinery.

The main optimizations from the study included:

  • Increasing the amount of stripping steam improved the separation index in the fractionator
  • Increasing the furnace outlet temperature improved the separation index in the column
  • Increasing the amount of stripping steam in the stripper overcame the high H2S content of the product
  • Decreasing the internal reflux from the fractionator to the stripper column improved the separation index in the fractionator and decreased the fractionator furnace duty
  • Increasing the gap between the diesel product and the fractionator bottoms indirectly slowed the deactivation rate of the catalyst.

In this study, the effect of each operating parameter, both individually and collectively, was analyzed. Looking at the system as a whole, some parameters have a snowball effect, while some have a diminishing effect. Before making any optimization in the unit, it is better to check the overall effects. The combined effects of the parameters are summarized in Table 2. A summary of the optimization study, compared to the base case for the unit, is shown in Table 3.

As seen in Table 2 and Table 3, the actual results retrieved from the field are consistent with the simulation results in terms of gap and savings for the optimum gap case. The optimization tool can be used to improve the energy efficiency and/or product/yield efficiencies via simulation models of process units. These provide easily procurable and effective solutions. Broadening the vision around the main process units of a refinery could also offer opportunities for productivity improvements. HP

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