August 2016

Special Report: Fluid Flow and Rotating Equipment

Analyze abnormal operations of an HDS reactor loop with dynamic simulation

Hydrodesulfurization units are used in a petroleum refinery to process a variety of feeds to alter composition via the addition of hydrogen.

Hydrodesulfurization (HDS) units are used in a petroleum refinery to process a variety of feeds to alter composition via the addition of hydrogen (H2). Process objectives include reducing the sulfur and nitrogen content for subsequent downstream processing. Often, existing HDS facilities are modified for higher throughput, feed composition changes and/or increased hydrotreating severity. A revamp process study is typically undertaken to identify the changes needed to achieve these new process objectives.

For the evaluation of the HDS reactor loop, two abnormal operating conditions must be considered:

  • Heatwave to the reactor effluent equipment and piping caused by the sudden loss of feed
  • Settle-out pressure in the reactor loop after the loss of recycle gas flow.

Conventional approaches for evaluating the impact on the reactor effluent system of a heatwave can result in over-conservatism in a revamp or new design.

While conventional methods for calculating the settle-out pressure exist, if the revamped settle-out pressure exceeds the set pressure of the pressure safety valve (PSV) protecting the HDS reactor loop, then there is generally no agreed-upon method for calculating the relieving rate. Dynamic simulation is the preferred approach for analyzing these two contingencies.

Key design information is used to develop the dynamic simulation. This includes equipment design details such as tube/shell size, geometry, nozzle locations and elevations. Centrifugal pumps and compressors are modeled using the performance curves. CV data is used to model control valves, while volumes are used for piping to model the holdup.

Heatwave caused by loss of feed

In the HDS unit, a heatwave begins when the liquid feed flow is lost, resulting in a condition where the heat content in the reactor effluent—which normally transfers to the feed in the feed/effluent exchanger—is not removed. As a result, equipment and piping in contact with the reactor effluent will experience higher-than-normal temperatures. Evaluating the peak temperatures for equipment in the reactor loop with conventional methods may result in very conservative design conditions. A dynamic simulation is used to predict the transient response of the temperatures and pressures for this condition.

Fig. 1. Typical HDS reactor loop.
Fig. 1. Typical HDS reactor loop.

A dynamic simulation of a typical HDS reactor loop (Fig. 1) is used to demonstrate the heatwave analysis. The hydrocarbon liquid feed is pumped by the charge pumps to the reactor loop pressure, and the hydrocarbon liquid rate is regulated by flow control. Liquid hydrocarbon is combined with the recycle H2 from the recycle gas compressor. The combined feed stream goes through a series of feed/effluent exchangers, where the reactor feed absorbs heat from the reactor effluent before being heated to the reactor inlet temperature in the heater.

The reactor feed inlet temperature to the HDS reactor is controlled by adjusting the fuel gas flow to the heater burners. The reactor effluent is cooled in the feed/effluent exchangers and then mixed with wash water before entering the reactor effluent air cooler. The air cooler outlet stream enters the separator, which separates the sour water, liquid hydrocarbon and recycle gas. The liquid hydrocarbon and sour water are pressured out of the HDS reactor loop. The vapor stream is split, with some gas purged out of the HDS reactor loop to maintain H2 purity in the recycle gas. The remaining gas is routed through the recycle gas compressor and mixed with the hydrocarbon liquid feed.

The following example is for a heatwave resulting from a local power failure where the reactor feed is lost. It is assumed that normal control responses stop the H2 makeup and wash water flows. It is also assumed that fuel gas to the heater is shut off via safety interlock and that there is no heat input to the feed. The recycle gas compressor continues operating and initiates the heatwave. The dynamic model is run until peak temperatures on the feed/effluent exchangers are observed and begin to decay. Eight shells are used in series for the feed/effluent exchangers (Table 1).

For a new HDS unit, the peak temperatures can be incorporated into the selection of the design temperatures for both the shell and tube sides of the feed/effluent exchangers and the air cooler. A conservative approach is to set the design temperature of the effluent side of the feed/effluent exchangers to the same temperature as the HDS reactor. With the dynamic simulation, the peak temperatures can be used to reduce the design temperature, which may result in cost savings for the feed/effluent exchangers. Also, the dynamic simulation is useful in the design of the reactor effluent air cooler in terms of selecting fin tube type based on the design temperature.

For the examples shown in Table 1, if the peak temperature exceeds the existing equipment design conditions, it may be possible to rerate the impacted exchangers or consider reusing some of the existing exchangers in a different sequence. If rerating or reusing the impacted exchangers is not a viable option, then the exchangers will need to be replaced.

Calculating the relieving rate

Dynamic simulation can be used to calculate the required relieving rate of a PSV following the loss of recycle gas if the settle-out pressure is greater than the setpoint of the PSV. For a revamp case, the reactor throughput may be increased, resulting in a higher operating pressure in parts of the reactor loop. The reactor loop settle-out pressure may increase to a value higher than the PSV setpoint at the separator.

Fig. 2. Settle-out pressures and relieving rates.
Fig. 2. Settle-out pressures and relieving rates.

For the given example, the reactor loop PSV is located on the separator. The set pressure is 290 psig, and the normal operating pressure is 210 psig (Fig. 2). When the recycle compressor stops, the PSV opens and the relieving rate peak flow is approximately 56,000 lb/hr.

The pressure continues to increase after the PSV initially opens, but the dynamic model does not predict the PSV to be fully open since the separator pressure does not reach 10% overpressure (319 psig). The pressure peaks at around 302 psig. The results indicate that the PSV is adequately sized for this settle-out scenario.


Dynamic simulation is the best method to predict the transient responses of the temperatures and pressures in an HDS reactor loop from the loss of feed or recycle gas. For loss of feed, the transient temperature response predicts the magnitude of the heatwave needed to evaluate the design temperatures of the effluent side of the feed/effluent exchangers. Dynamic simulation allows the calculation of a realistic relieving rate from the products separator during a settle-out pressure caused by the loss of recycle gas. HP

The Authors

Related Articles

From the Archive



{{ error }}
{{ comment.comment.Name }} • {{ comment.timeAgo }}
{{ comment.comment.Text }}