ApplicationSolution

Summary

Dynamic simulation can be

used to simulate a typical

distillation column or

fractionator relief scenario

to calculate the column

relief load more accurately

than using conventional

methods, while

maintaining an acceptable

margin of safety.

Business Value

• Eliminate the need for

additional relief valves

on unit revamp or

debottlenecking projects

• Eliminate or reduce the

cost for additional flare

system piping for refinery

expansion projects

• Increase throughput

through units that are

constrained by relief

capacity

INTRODUCTIONDynamic simulation can simulate a typical distillation column or fractionator relief scenario (such as cooling water failure, partial power failure, and total power failure). The objective of the analysis is to calculate the column relief load more accurately than using conventional methods while maintaining an acceptable margin of safety. The peak relief flow may then be used to size relief valves or used within an overall flare analysis to determine relief valve back pressures. In most cases, the use of dynamic simulation can reduce the need for additional relief valves or flare system expansion. This example discusses how dynamic simulation can reduce the calculated relief loads by 25% or more as compared with conventional calculations.

DYNAMIC SIMULATION BENEFITS SUMMARY• Eliminate the need for additional relief valves on unit revamp or debottlenecking

projects• Eliminate or reduce the cost for additional flare system piping for refinery

expansion projects• Increase throughput on units that are constrained by relief capacity

DYNAMIC SIMULATION APPROACHConventional methods for calculating column relief loads are very conservative and can lead to over-designed relief systems, unnecessary flare, or flare header replacements during unit revamp and debottlenecking projects.

Dynamic simulation provides an alternative to conventional calculation methods and provides substantially lower relief loads. For fractionation towers with wide boiling ranges, significant flare load reductions are possible. Dynamic simulation accounts for the limited inventory of light components and the sensible heat required to boil off the heavier components.

Dynamic simulation is consistent with the approach of API-521. API RP 521 Section 3.3 (1995) has two components:

1. “For mixtures, light components are relieved before heavier components.”2. “Change in vapor rate and molecular weights at various time intervals should be

investigated.“

Dynamic simulation provides a safe and documented alternative to conventional relief load calculations methods that result in substantially lower calculated relief loads.

Dynamic Simulation Example:Column Relief Load Analysis

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CONVENTIONAL APPROACHThe conventional approach used by many process designers is to use a heat balance method around the entire column. If condenser cooling is lost, the heat balance is recalculated accordingly and the net heat input is applied to a tray at or near the top of the column. The heat of vaporization is used to calculate relief flow based on the following equation:

Where:W - Relief flowf - Feed or product flowh - Feed or product enthalpyQ - Reboiler or condenser dutyλ - Heat of vaporization

DYNAMIC SIMULATION MODELDiagram 1 depicts a distillation column, which illustrates some of the principles involved in using dynamic simulation for column relief load analysis.

Diagram 1: Column Relief Load

The following assumptions were made in the development of this simulation model;

Tower• The DYNSIM® Tower Model includes both vapor and liquid holdup on each tray.• Tray liquid holdup assumes clear liquid height. This is a conservative assumption as it increases the

amount of volatile low boiling point components.• The Tower has the same number of theoretical stages as the steady state simulation. The inventory of

each theoretical stage is adjusted by the ratio of actual trays to theoretical stages.• A DYNSIM Separator simulates the column sump. The Separator includes an internal weir to simulate

the thermosyphon reboiler baffle. Accurately modeling the sump inventory is important as reboiler heat transfer rate reduces as the inventory is depleted.

Pumps and Valves• DYNSIM rigorously models pumps, valves, and controllers. However, there are simplified models

available for situations where not all the equipment data is available. For example, the DYNSIM StreamSet model simulates the overhead liquid product level control valve and reflux flow control valve.

• A DYNSIM Valve and PID simulate the feed flow control valve and flow controller. The rigorous valve model calculates the reduced feed flow to the column that may occur as the column pressure rises.

Condenser• The overhead condenser is modeled with an equation to reduce surface area when the overhead

accumulator floods.

Relief Valve• The DYNSIM Relief Valves are set to modulate rather than pop open. A modulating relief valve provides

the peak relief rate while holding the column pressure at a nearly constant rate as required to determine the relief load. This is the preferred approach when determining a peak relief flow to use in an overall flare system back pressure analysis or to size relief valves for a new column. Alternatively, the relief valve can be pop acting based on the available relief valve orifice area to determine if pressure rises above the allowable accumulation (10% for single valves and 16% for multiple valves).

Controls• Per API Recommended Practice 521, controllers fail in their last positions unless their automatic

action increases the relief load. Many controllers are not required to be modeled and the simulation does not need to match the control configuration used in the actual plant.

• Reflux cascade flow controller operates to keep a constant mass flow rate. The master controllers are assumed to not change the set point the slave controllers.

• The overhead pressure controller locks in position since its action to open on high pressure would reduce the relief valve load.

• Tray temperature controllers on reboiler steam are not required to be simulated since a temperature increase during relief would reduce reboiler heat transfer.

SCENARIO CASES AND RESULTSTable 1 summarizes the three relief scenarios and provides the results for the conventional calculation and peak DYNSIM relief flow. Table 2 shows the actual scenarios as implemented in the DYNSIM Scenario utility.

Table 1: Scenario Cases and Results

Table 2: DYNSIM Scenarios

DISCUSSION OF RESULTSFor all cases, the equipment failure event occurs at the five-minute timeframe.

Case 1 – Loss of Cooling Water

Figure 1 shows the relief rate for the loss of cooling water case. The peak relief rate is 42,000 kg/hr, 24% lower than the conventional calculation. Reflux continues to empty the contents of the overhead receiver into the column and cascades down through the trays to the reboiler where it is vaporized to increase pressure and create relief.

Figure 1: Case 1 - Loss of Cooling Water

Case 2 – Loss of Reflux/Overhead Pump

Figure 2 shows the relief rate for loss of the Reflux Overhead Product Pump. The peak relief rate is 120,000 kg/hr, 40% lower than the conventional calculation. The pressure initially decays as the reflux stops the supply of volatile material entering the column. However, the overhead receiver floods in an approximate four minutes, which eliminates condenser cooling, and the pressure begins to increase to create relief. There is a large spike at 23 minutes as the low boiling material reaches the reboiler.

Figure 2: Case 2 - Reflux Failure

Case 3 – Loss of Reflux / Overhead Pump and Feed

Figure 3 shows the relief rate for loss of the overhead pump. The peak relief rate is 15,000 kg/hr, 78% lower than the conventional calculation. Pressure first falls due to loss of reflux. However, since there is a loss of feed the reboiler is not supplied with low boiling material and the relief quickly subsides.

Figure 3: Case 2 - Reflux and Feed Failure

Additional Work

It is the process engineer’s obligation to ensure that the results of a simulation model are appropriate for the design. The following is a partial checklist to help validate a column relief study using dynamic simulation.

• Review electrical bus arrangements to see which electrical devices can trip at the same time. Loss of multiple devices due to a common electrical supply is a typical relief scenario.

• Check initial column tray holdup calculations to be sure that the inventory estimate is adequate. It is allowable if you overestimate the actual inventory using a clear liquid approach.

• Investigate pop-acting relief valve to determine maximum pressure using the actual relief system configuration.

• Be sure that the liquid levels are not low such that vapor is blowing through liquid control valves.

In many cases, conservative limiting case assumptions can be made to simplify simulation development. Perform sensitivity runs to determine the impact of these assumptions on the results to see if further simulation refinement is warranted. This approach is often far less time consuming than attempting to create a more rigorous simulation.

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