November 2022

Process Optimization

Implement advanced level control techniques to improve crude distillation unit stabilizer performance

The performance of refinery operations depends on the stable operation of the plant.

Mandal, K. K., Nayara Energy Ltd.

The performance of refinery operations depends on the stable operation of the plant. The primary crude distillation unit (CDU) generates a lighter component from its overhead and processes it into a stabilizer to separate liquified petroleum gas (LPG) and naphtha components. Therefore, any operational improvement leads to improved refinery margins by improving the product quality and optimizing the reboiler heat duty.

The capacity of a particular CDU was 18 MMtpy, and it processes different types of crude with sulfur levels of 1 wt%–3 wt%. The production of LPG and naphtha varies depending on the type of crude processed. The overhead vapor from the CDU is cooled and routed to the overhead receiver vessel. However, the condensed liquid from the overhead vessel is partly routed to the column top as a reflux to maintain the temperature, and the balance liquid is sent to the re-contacting column to feed the stabilizer.

The overhead gases (e.g., uncondensed material from the overhead vessel top) were routed through a two-stage wet gas compressor, where the gases were pressurized up to 9.0 kg/cm2–9.5 kg/cm2; after cooling, the overhead gases were routed to the re-contacting column. Some parts of saturated gases from other units, such as vacuum gasoil (VGO) and diesel hydrotreating (DHDT) units, were also routed to the re-contacting column along with the compressed gas. Finally, the liquid hydrocarbon from the re-contacting vessel bottom was routed to a stabilizer for the separation of naphtha and LPG. The main feed to the stabilizer was from the CDU overhead via the re-contacting column based on the vessel level control in level flow cascade mode.

The stabilizer consists of a column, an overhead vessel and a reboiler at the bottom to maintain the stabilizer’s bottom temperature. Parts of the cooled liquid were routed to the column top as a reflux to maintain the top temperature. The balanced cooled product (e.g., LPG from the stabilizer-overhead vessel) was routed to an amine column for the removal of hydrogen sulfide (H2S) before sending it to the downstream unit to remove mercaptan.

The stabilizer bottom product stabilized naphtha, which is then routed to the naphtha hydrotreater (NHT) unit for further processing. A level indication was provided with a control valve in the rundown line to maintain a steady level in the overhead vessel and column bottom. Unstabilized naphtha generated from diesel hydrodesulfurization (DHDS), DHDT and VGO mild hydrocracking (MHC) was also directed to the stabilizer feed pump suction to get processed in the CDU stabilizer.

Typically in refineries, advanced process control (APC) is implemented for stabilizer operation, product yield and quality optimization; however, due to continuous disturbance in its operation, APC was not implemented in this stabilizer. Another reason for not implementing APC is because unit operations were more than the design capacity, resulting in fewer margins in operating parameters such as reflux flows, limitations in gas compressors and cooling systems.

Problems

The CDU stabilizer performance was unsatisfactory and was operating with a significant amount of disruption, such as a disturbance in LPG quality (mainly LPG weathering), amine carryover with LPG and hydrocarbon carryover with an amine. An additional knock-out drum (KOD) was provided after amine treatment to arrest the amine carryover with LPG. The LPG rundown flow control was not working smoothly in auto cascade mode, so some was kept in manual control mode for less flow variation.

Analysis of feed to the stabilizer

The incoming stream flow was set for 24 hr of operation, and the product outgoing stream flow was critically analyzed to identify the problems and improve the stabilizer performance feed.

Approximately 80%–85% of the CDU stabilizer feed was from CDU overhead and unestablished naphtha, and about 15%–20% of total feed was from other units like wild naphtha from DHDS, DHDT and VGO MHC stripper. Wild naphtha flow from DHDS, DHDT and unstabilized naphtha flow from VGO were analyzed (upper half of FIG. 1).

FIG. 1. The feed flow trend from DHDS, DHDT and VGO MHC units.

Primarily, the CDU, overhead naphtha (80%–85%) and compressed unsaturated gas were routed to the re-contacting column from the main CDU fractionator and the subsequent column bottom was routed to the stabilizer as feed. The feed flow from the CDU overhead and the re-contacting column was critically analyzed, and a wide flow variation was noticed for the CDU overhead liquid flow (upper half of FIG. 2).

FIG. 2. The level and feed flow from the CDU overhead trends.

Wild and unstabilized naphtha from DHDT, DHDS and VGO MHC flow variation was continuous, causing trouble in stabilizer operation. Feed coming to the re-contracting column from the CDU overhead was a wide flow variation with continuous disturbances.

In the crude unit, blend changeover is a common requirement and is done every 2 d–3 d. It is also observed that higher levels of disturbance occur during this blend changeover. The disturbance from re-contacting the column to the stabilizer was less than the CDU overhead feed flow.

Analysis of product flow from the stabilizer

The rundown product from the stabilizer bottom and overhead vessel with the downstream LPG, an amine extraction column and LPG KOD level were analyzed critically with the flow.

The stabilizer bottom level and flow were stable with a marginal variation. However, the overhead vessel LPG level and flow variation were wide, resulting in amine carryover with LPG (upper half of FIG. 3).

FIG. 3. The stabilizer overhead vessel product flow, level and control valve opening trends.

Implementation of target opening nonlinear (TON) control

The feed and product flow fluctuations were marginally higher in the normal distributed control system (DCS), as stated above in the flow trends. It was decided to implement TON control for performance improvements of selected level control where flow deviation was on the extreme side, such as the stripper overhead in the DHDS and VGO MHC units and the stabilizer overhead in the DHDT unit for a reduction in wild naphtha and unstabilized flow variation. TON control was also considered for the CDU overhead flow to the re-contacting column level and stabilizer overhead level control to reduce flow variation.

TON control is a correlation-based control that works based on the level difference, set level and actual level. The correlation used for level control is (Eq. 1):

CVOP = (LA – LS) × M + TOP                                         (1)

The calculated control valve output to the respective flow control valve (CVOP) = actual level (LA) – set level (LS) × M + target output (TOP). The control configuration can be selected as: 1) for normal DCS mode or 2) for TON control mode.

This modification can be carried out in the DCS with the help of an instrument engineer. Apart from the selection of DCS or TON, three fields exist in DCS: target output, level set point and factor M. Target output is the flow control valve’s average opening; the level set point is the desired level to be maintained (actual level indication value will be taken from DCS); and M is a factor to be set once as per operating practice and flow variation with a level that is very similar to controller gain—usually, it will vary from 1–2.5.

Benefits

After implementation of TON control in the above levels, the flow variation was found to be significantly less than the DCS. With this reduction in flow variation, the performance of the stabilizer was found to be stable, and the product quality concerning LPG weathering was steady. Due to a steady LPG flow after the implementation of TON control, the amine carryover problem with the LPG was eliminated. Flow trend from DHDS, DHDT and VGO wild and unstabilized naphtha coming to the stabilizer after TON control is shown in the lower half of FIG. 1. The level and flow trend for the CDU overhead coming to the re-contacting column is shown in the lower half of FIG. 2. The level, flow and control valve opening trend for the stabilizer overhead going to the amine extraction column is shown in the bottom half of FIG. 3.

Additional level TON control was implemented, such as a re-contacting column, stabilizer bottom level and amine KOD to further the improvement of the saturated gas unit.

Takeaway

A CDU’s stabilizer is a critical section of any refinery. The steady unit performance will reduce product quality giveaway and increase profitability. The steady LPG flow to the downsteam amine section will eliminate amine carryover along with LPG product. TON control has been implemented in DHDS, DHDT and the VGO MHC stripper and stabilizer overhead section, the CDU overhead, re-contacting column bottom, stabilizer overhead and bottom level control at the author’s company. HP

LITERATURE CITED

  1. Mandal, K. K., “New level control techniques,” Hydrocarbon Processing, October 2004.
  2. Mandal, K. K., et.al., “Improve desalter control,” Hydrocarbon Processing, April 2005.
  3. Mandal, K. K., “Search for advanced technique of desalter control—TON control for interface level,” Hydrocarbon Processing, January 2021.
  4. Mandal, K. K., “Implementation of advanced level control techniques in refinery operation,” Hydrocarbon Processing, September 2021.

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