Tips & Tricks: Direct Columns (DC) in Symmetry 2018
- Easy set up for Direct Columns
- Replacing Rigorous towers with Direct Columns
- Special cases - work around and suggestions
The Direct Column unit operation in Symmetry is a simplified separation unit that mimics the performance of a Distillation Column.
This article is geared towards providing some insights plus tips and tricks for a faster/easier set up of the unit operation in order to match the product TBP, flow rate and composition. This is accomplished by using pre-determined swing cut ranges based on PIONA structures in different boiling ranges, thus limiting the number of variables to be manipulated.
The DC has 4 possible split methods: end temperature, swing cuts, heuristic curve matching and linear programming. For the current case, we will only look into the use of the swing cuts methods. This option allows for a simple set up while giving the user the ability to adjust for product tailings and flow rates.
The suggested step-by-step procedure is as follows:
1. Set the number of feeds to the column
Set the number of feeds to the DC:
Then proceed to link to the appropriate streams in the flowsheet.
2. Set the number of products and name them
Set the number of product streams, the unit will automatically populate a Name and End Temperature for each one. We recommend you customize the products names to your preference.
At this point, you can select if the water will be decanted (checkbox). If so, an additional port will be created at the top for water separation, eliminating the need to account for the water stream in your number of product streams. If water is not decanted the unit op will split the water among the rest of the product streams.
3. Collect product distillation data
The required data will be only the 90% or 95% distillation point for each product. The distillation curve type can be selected inside the unit op, based on the available analysis; however, all products will have the same curve type. Additional data can be included as a reference in the unit op for quick comparison with the unit performance.
4. Select Split Method
Select the Split method (dropdown menu): Swing Cuts
Tip: If you want to include additional reference distillation curve data for one or multiple cuts you should select Heuristic curve method, select the number of points (example 5), then input the distillation percentage (%) and temperature for the selected products. After that, switch back to Swing cuts. The distillation data will be preserved for easy visual comparison, but it will not be used in any calculation.
5. Fix the number of swing cuts
Select the number of swing cuts to be used. It is recommended to use only 2 with the specific cut temperatures suggested (See Table 1), which have been found to be a good combination of simplicity and accuracy.
6. Set End Boiling Point (EBP) for each product
Based on the 90% or 95% distillation point of each product, go to the cut table selection (shown below, Table 1) and choose the EBP that is immediately lower in BP value. Then proceed to set that value as the EBP of the product (example: if the 90% point of your product is 250°C you should select EBP = 225°C, cut #9 in the table).
Table 1. Pre-determined cut ranges and swing cuts for the Full Refinery slate
P = Paraffin, I = Isoparaffin, O = Olefin, N = Naphthene, A = Aromatic, (AD) = Aromatic Dehydrogenated and (AH) = Aromatic Heteroatomic
The use of the 90% distillation point allows for the bulk of the product composition to be separated while the tailings of the products will be adjusted using the swing cuts.
This swing cut range selection guarantees that full PIONA components are part of the swing cuts. This prevents selective separation of one molecular type that would be an unrealistic separation (by boiling point distillation). This also guarantees that every product stream has at least one PIONA component in every range. These ranges are based on the use of the Full Refinery Slate available in Symmetry’s manual example and include the pure and pseudo components.
7. Use pre-defined delta Temp swing cuts
Based on the product EBP and the pre-determined swing cut ranges input the delta Temperature for each cut for both “under the IBP” and “above the EBP”.
Notice that the lightest cut will only require “above the EBP” and the heaviest only “under the IBP”.
Pay attention for the “under the IBP” the product IBP has to be used; the IBP on product n will be the EBP on product (n-1).
Example: say product n has an EBP =265°C and product (n-1) EBP = 175°C. So, product n is defined as IBP = 175°C and EBP = 265°C, meaning cuts # 9 and 10 (in Table 1) compose product n. The delta temperatures for product n swing cuts should be: Under IBP #2 = 50, Under IBP #1 = 25, Above EBP #1 = 50 and Above EBP #2 = 115 (See Table 1). Illustrated below product n is Kero6 and delta temperatures input in the DC.
8. Adjust Swing cut percentage
Once every product EBP and swing cut delta temperatures have been fixed, we proceed to adjust the Swing cuts percentage. This will be the truly manipulated variable to match the product data, which includes TBP, properties (e.g. density) and flow rate.
The desired cut percentages can be determined either manually or using one of Symmetry’s productivity tools, such as a controller (i.e. to guarantee a particular specification in one of the products) or a Regression Case.
Tip: The percentage of the swing cuts can be different for every swing cut, however, as a general rule of thumb the cut percentage of the smaller delta T should be greater than the cut percentage of the larger delta T.
9. Adjust Product stream conditions
The product stream's conditions will need to be adjusted outside of the Direct Column since the options for Temperature and Pressure calculations inside the DC are limited (refer to the advance dropdown menu in the Summary tab of the DC unit operation).
Use a heater or cooler unit operation to adjust every product stream’s condition. The recommended specifications for the Heaters/Coolers (Hx) are a pressure and a vapor fraction (0 for bubble point and 1 for dew point). If the product is subcooled or superheated, we recommend using a pressure spec and a “Deg Sub Cooling” or “Deg Super Heating”. These options are active once “Calc Sub Cooling” or “Calc Super Heating” checkbox are active in the “Advance” dropdown menu in the Summary tab of the Hx. Avoid as much as possible a temperature specification in the Heaters/Coolers as this will typically change with other shifting operating conditions.
10. Final checks
Some final checks might be appropriate after the DC is solved with satisfactory product TBPs and Flow rates:
- Make sure every product stream has full PIONA (go into the Refinery Tab of the stream to verify). Even the light gases should have soluble Aromatics (benzene) in small quantity that can be cross-checked with experimental data. This should guarantee if using the swing cut table provided above combined with the default refinery slate.
- Look for Phase changes, especially vaporization of heavy products that go into pumps for a recycle. This might have to do with the specifications of the Heater/Cooler that adjust the stream conditions or the delta temperature for the cuts selected, particularly if they are customized.
11. Special cases
A special case occurs when there is a small narrow cut in the middle of 2 overlapping cuts. This will give a continuous curve even if the small narrow cut was removed (see Figure 1).
Figure 1. Narrow cuts that are overlapped
In the green box “A”. The orange cut (light naphtha) is overlapped by the blue (lights) and red (heavy naphtha). If the cut was removed, and the mass distributed among the two remaining cuts, the distillation will still be continuous.
In the orange box “B”. The yellow cut (heavy diesel) is overlapped by the green (light Diesel) and the blue (cracked slurry oil).
You can identify this situation by looking at the overall distillation curve (Cuts Overall tab in the DC). It will also be noticed that the product flow rate cannot match no matter how large you make the Delta T or percentage swing since the DC doesn’t allow a swing cut to take mass from a product that is not immediately contiguous (this constraint will be resolved in future releases).
A workaround for this situation involves “splitting” the column to separate the products. As a general rule, the column will be split at the right (heavy side) of the narrow products (#1 = orange and #2 = yellow) this will allow good control over the separation, see Figure 2. The overall cuts resulting for each of the split towers is shown in Figure 3.
Figure 2. Column Splitting example
Figure 3. Overall distillation curves for each of the split DC
Victor M. Rodriguez, Ph.D., Technical Support Specialist
Please contact your local VMG office for more information.