The Naphtha Hydrotreating Unit is fairly simple to monitor from a calculation and data review standpoint. The chapter describes calculations that are typically done in a Naphtha Hydrotreating Unit. This does not include the Splitter section.
A. CALCULATIONS
Before any calculations are performed, the data should be reviewed to verify the unit was lined out during the period of time the calculations will cover. Usually, this is 24 hours. Good practice dictates that calculations be performed routinely, such as once a day, so that changes in performance can be quickly noted. Also, engineers find it very useful to have some data and calculations plotted in order to monitor trends and maintain a unit operating history.
Before the unit's performance can be properly monitored, the unit must first weight balance. Kilograms of liquid and gas in should equal kilograms out. A good balance is one where the percentage of kgs of products divided by the kgs of feeds equals 100 percent, plus or minus 2% maximum. If it is outside this range, the engineer will have to try to evaluate which indicator(s) is reading wrong and have it corrected. On the Naphtha Hydrotreating Unit there can be one to multiple naphtha feed streams and hydrogen make-up. The products typically consist of two streams: the stripper off-gas and stripper bottoms. If there is not a flow meter on the stripper bottoms, the product flow meter(s) from downstream vessels (splitter, intermediate tanks, Platforming Unit feed, etc.) should be used. Water injection and sour water product are not considered in the weight balance. Water and oil do not mix.
The first step of the calculations is to correct the feed and product flows to their actual mass flows at standard conditions. The averages on the logsheets are only approximations of the actual value. Liquid streams have to be corrected for changes in density as measured at a standard temperature due to variations in flowing temperatures. Gases have to be corrected for variations in specific gravity,
uop 117115 VIII-2
operating pressure and operating temperatures. The corrected flows are then used to check the unit weight balance and for other calculations as is noted below.
The following calculations are typically performed on the Naphtha Hydrotreating Unit daily:
1. Weight Balance
kg/hr products
kg/hr feed × 100
An acceptable weight balance is within 98 to 102 weight %.
2. Liquid Hourly Space Velocity (hr-1)
catalyst of volume hour per charge of volume LHSV =
3. Hydrogen to Hydrocarbon Ratio
Nm3/hr of hydrogen recycle gas
m3/hr of naphtha charge
4. Stripper Offgas
Nm3/hr of stripper offgas
m3/hr of naphtha charge
5. Stripper Reflux Ratio
m3/hr of reflux
uop 117115 VIII-3 6. Hydrogen Consumption charge naphtha of /hr m unit) the of out hydrogen /hr (Nm - makeup) H /hr (Nm 3 3 2 3 7. Cumulative Charge
Total m3 of charge to the unit. Usually, calculated from beginning of a run to a regeneration. If the unit has more than 1 feed source, the individual rates should be recorded as well.
8. Catalyst Life catalyst of kg m charge, cumulative 3
Catalyst life is measured from original startup to catalyst replacement.
9. Metals Contamination 2 - 10 catalyst of kg charge kg charge in metals wt% × ×
Usually, kg of charge is the total from the last time a metals analysis was performed on the feed to the latest one. The total metals contamination is then the summation of the incremental contaminations between analyses. See Section III, Part F for more information.
10. Water Injection
m3/hr of water
m3/hr of naphtha charge × 100
The typical continuous water injection target is 3 liquid volume percent of the charge rate. This is for when the water is injected just after the last combined- feed exchanger bundle. If the water is injected further upstream where the process temperature is higher, then the water rate must be increased. The
uop 117115 VIII-4
goal is to maintain at least 25% of the injected water in the liquid phase. The injection rate may require adjustments based on the separator water analysis. The separator water should be analyzed 3 times per day to insure it has the following qualities:
pH 6.0 + 0.5 (avoid 6.8 – 7.3 range) Iron 2 wt-ppm or less
Chloride Less than 500 wt-ppm
If the pH decreases below 5.5, then increase the water injection rate to bring the pH up to the desired range. Note that different water rates change the acceptable level of iron. It is the mass of iron being removed that is important to monitor. If increased water injection does not bring the pH into the proper range, then a “basic” water injection may be required. Contact UOP for further details.
11. Reactor pressure drop
Reactor inlet pressure – reactor outlet pressure
The maximum pressure drop of the reactor is typically set by the allowable pressure drop across the outlet basket. This is for guideline purposes only. For older units this is typically 60 psig (4.2 kg/cm2) and about 100 psig (7 kg/cm2) for newer designs. However, the pressure drop usually occurs at the top of the reactor bed. Thus, product quality and hydrogen flow are usually the limiting factor.
12. Reactor delta Temperature
Reactor outlet temperature – reactor Inlet temperature
For most straight run naphthas, there will be no temperature rise across the reactor, and may actually show a loss of temperature depending on heat loss. Naphthas that contain olefins, such as cracked naphthas, will exhibit a temperature rise. The magnitude will depend on the amount of olefins
present. As the olefin content increases so does the exotherm. As the outlet temperature approaches 343oC (650oF) then sulfur recombination can occur.
uop 117115 IX-1
IX. ANALYTICAL
Included in this section is the laboratory test method schedule for the Naphtha Hydrotreating Unit. This laboratory schedule is general in nature and is customized for each customer depending on the equipment included in the design. Please refer to the 934 specifications in the UOP Schedule A books for your unit.
uop 117115 IX-2