1 Feb, 2018
Optimum Feed Tray Location in an NGL Fractionation Column
A fractionator is a column equipped with trays or packing materials for separating a mixture of components into two or more products, at least one of which will have a controlled composition or vapor pressure. In crude oil or condensate systems, such a fractionator is often called a stabilizer and is an alternative to stage separation. The fractionator is essentially a constant pressure column that uses heat, absorption, and stripping to separate components based on the difference in their boiling points .
Fractionation or distillation columns are named based upon the products that they produce overhead, for example, a deethanizer will produce a distillate stream that primarily contains ethane and lighter components such as methane and nitrogen, with a bottoms product of propane and heavier components (C3+). Similarly, a depropanizer will produce a distillate stream that is primarily propane, and the bottoms stream is butane and heavier components (C4+). Chapter 16 of the Gas Conditioning and Processing presents an excellent overview of fractionation and absorption fundamentals .
Predicting the optimum feed tray location in the design phase is not easy, particularly if a short-cut calculation is used. Virtually all short-cut calculation methods of estimating feed tray location are based on the assumption of total reflux .
This tip of the month (TOTM) will demonstrate how to determine the optimum location of a feed tray in an NGL fractionation or distillation column by a short-cut method and the rigorous method using a process simulator. As an example, we will consider sizing a deethanizer by performing material and energy balances, distillation column short-cut calculations, and rigorous tray-by-tray calculations. Finally, the TOTM will determine the optimum feed tray location by the short-cut and rigorous methods.
Deethanizer Case Study:
Let’s consider a deethanizer column with the feed compositions, flow rate, temperature and pressure presented in Table 1. It is desired to size the deethanizer column:
A. To recover 90 mole percent of propane of the feed in the bottoms product and
B. Ethane to propane mole ratio equal to 2 % in the bottoms product
For understanding the concept, the TOTM will do the sizing in three steps:
1. Material and energy balances
2. Distillation column short-cut method
3. Distillation column rigorous tray-by-tray calculations
All of the above steps can be done by the available tools/operations in a process simulator. In this TOTM all calculations are performed using UniSim Design  with the Peng-Robinson  equation of state option. Figure 1 presents the process flow diagram (operations/tools) for the above steps .
Table 1. Feed composition and condition
Figure 1. Process flow diagram 
Material and Energy Balances:
Let’s choose ethane as the light key (LK) component and propane as the heavy key (HK) component because their separation requirements are specified. Assume that all of the components lighter than the LK component go to top and all of the components heavier than the HK component go to bottom.
Column condenser pressure is normally set based on the cooling media temperature. Typical operating pressure range for a deethanizer is 375–450 psia (2586–3103 kPa) . Since the feed pressure is 435 psia (3000 kPa), assume the column top pressure is 403 psia (2779 kPa) and bottom pressure is 410 psia (2828 kPa).
We can use the “component splitter” tool in the process simulator to perform initial material and energy balances. The “component splitter is shown in the lower part of Figure 1. The split for propane (HK) is specified (90 mole % goes to bottom and remaining 10 mole % to top). The ethane split is unknown but can be determined by trial and error manually or by using the “adjust” or “solver” tool of the process simulator which essentially varies the ethane spilt so the mole ratio of ethane to propane in the bottoms product becomes 2 %. The estimated ethane split of 97 mole % goes to top.
The estimated mole fractions of the LK and HK components in the top and bottoms and the specified values in the feed stream are presented in in Table 2. The “component splitter” also determines the estimates of top and bottoms flow rates, compositions, temperature and the energy requirement.
Table 2. Specified (feed) and estimates of key components compositions
Distillation column short-cut calculation method:
Using the top and bottom column pressures and the key components mole fractions (from Table 2), the short-cut distillation column operation in the process simulator can be used to determine the minimum number of equilibrium (theoretical) trays and the minimum reflux ratio (Reflux rate /Distillate rate), (L/D)min. The process flow diagram for the distillation column short-cut method is presented in the middle of Figure 1.
The estimated minimum number of trays using Fenske’s correlation[1,4] is 6.1 and the minimum reflux ratio using Underwood’s correlation [1,5] is (L/D)min = 0.618. The operating reflux ratio is typically in the range of 1.05–1.25 times (L/D)min . Assuming operating reflux ratio is 1.15 times (L/D)min then the operating reflux ratio is 0.711. For this operating reflux ratio, the program determines the number of equilibrium trays using Gilliland’s Correlation [1,6], the optimum feed tray using Kirkbride’s correlation [1,7], components compositions in the overhead and bottoms products, top and bottoms flow rates, temperatures, and condenser and reboiler duties. Table 3 presents the summary of the short-cut results.
Table 3. Summary of the specified and calculated values from column short-cut method
Predicting the optimum feed tray location in the design phase is not easy, particularly if a shortcut calculation is used. Virtually all the short-cut calculation methods of estimating feed tray location assume total reflux. A convenient empirical correlation by Kirkbride [1,7] is in Equation 1.
N + M = S (2)
Where: N = number of equilibrium trays above feed tray
M = number of equilibrium trays below feed tray
B = bottoms rate, moles
D = distillate rate, moles
xHKF = composition of heavy key in the feed
xLKF = composition of light key in the feed
xLKB = composition of light key in the bottoms
xHKD = composition of heavy key in the distillate
S = Number of equilibrium trays in column
Substituting the corresponding parameter values from Tables 2 and 3 in Equations 1 and 2 results in the values of N and M.
Since N + M = 16.9, N = 5.42 and M = 11.48, the estimated optimum feed tray location matches well with the value reported in Table 3. Approximately 5.42 equilibrium trays will be required above the feed tray and 11.48 equilibrium trays (including reboiler) below.
The actual number of trays in the column can be estimated by dividing equilibrium number of trays by the overall tray efficiency. Typical deethanizer overall tray efficiency is 50–70 % . Assuming an overall tray efficiency of 60%, the actual number of trays will be 16.9/0.6 = 28, which is in the range of typical deethanizer actual number of trays of 25–35 .
Distillation column rigorous tray-by-tray calculations:
By performing the short-cut calculations, we have good estimates of different variables for this deethanizer column. For the specified ethane and propane specifications, 17 equilibrium trays (including reboiler) plus a condenser, top and bottom pressure, estimated feed tray location, and an estimate of operating reflux ratio, rigorous computer simulation can be performed. Note that the number of equilibrium trays, the estimate of feed tray location, and the operating reflux rate were determined in the preceding sections.
Because the short-cut method estimated of feed tray location and other variables, we will use tray-by-tray calculations by computer simulation to improve deethanizer sizing and locate a better optimum feed tray location. The deethanizer column tray-by-tray process flow diagram is shown on the top of Figure 1.
The tray-by-tray rigorous simulation results for the conditions provided in this case study are presented in Table 4 and Figure 2. Several feed tray locations are simulated and the one yielding the lowest condenser duty (reboiler duty) is the optimum location. The optimum feed tray location is tray 3 from top (N=3 and M=14 including reboiler).
Table 4. Condenser and reboiler duty vs feed tray location
Figure 2. Condenser and reboiler duties as a function of feed tray location
The column temperature profiles as a function of feed tray location are shown in Figure 3. The optimum feed tray location should result in a smooth temperature profile. Improper feed tray location is usually manifested by a sharp discontinuity in the slope of the temperature profile. Multiple feed nozzles and or a feed preheater are typically used to provide flexibility to adjust to changing feed conditions.
Figure 3. column temperature profile vs feed tray location
Several key design parameters for feed tray location of 3 are presented in Table 5.
Table 5. Summary of key design parameters for feed tray location of 3
Alternatively, a column profile of molar ratio of LK/HK composition with tray number can be plotted. The optimum feed location is determined by matching the molar ratio of LK/HK in the feed to the column profile of LK/HK. This method results in minimizing the reboiler and condenser duties for the distillation column.
This TOTM demonstrated how a process simulator can be used to size a deethanizer and determine the optimum feed tray location by minimizing the reboiler and condenser duties. This procedure is equally applicable to other NGL fractionators.
Selection of the proper feed tray location is important in order to optimize the operation of the fractionator. Placing the feed tray too high in the tower can result in excessive condenser duty (reflux ratio) to meet distillate product specification. Too low a feed location may result in excessive reboiler heat to meet bottom product specification.
Because short-cut methods provide a rough estimate of feed tray location, a rigorous tray-by-tray simulation program should be used to determine the optimum location of the feed tray by minimizing the condenser/reboiler duties.
Multiple feed nozzles and or a feed preheater are typically used to provide flexibility to adjust to changing feed conditions.
To learn more about similar cases and how to minimize operational problems, we suggest attending our G4 (Gas Conditioning and Processing), G5 (Practical Computer Simulation Applications in Gas Processing), and G6 (Gas Treating and Sulfur Recovery) courses.
PetroSkills offers consulting expertise on this subject and many others. For more information about these services, visit our website at http://petroskills.com/consulting, or email us at consulting@PetroSkills.com.
By: Dr. Mahmood Moshfeghian
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