Improvement of Feed Filter of Residue Hydrodesulfurization Unit

Two different types of feed filters, wedge wire filter and wire mesh filter, were used in the refinery in the wax oil hydrocracking unit and the residue hydrodesulfurization unit. The results show that both types of filters can be used to filter vacuum paraffin oil, and the backwash cycle of the filter element in the form of wire mesh is longer. 
When filtering vacuum residue, both types of filters could not function properly. The filter element in the form of a V wire tube with a wire mesh shell has better backwash performance and is more suitable for use in residual oil hydrogenation units.


The mechanical impurities in the feed oil of the hydroprocessing unit can cause the pressure drop of the reactor bed to increase and block the catalyst pores and cause the deactivation of the catalyst. Removal of mechanical impurities in the raw oil can extend the operating cycle of the device. Setting up a raw oil filter to ensure high filtration quality is an important measure for the long-term stable operation of the hydrogenation unit. Refinery Residue Hydrodesulfurization Units (VRDS) and Waxy Oil Hydrocracking Units (SSOT) typically use wire mesh and wedge wire (WW grade) filters. A modified filter in the form of a wedge wire tube with a wire mesh casing has been modified to filter VRDS feed and achieved better results. 

Filter operation before replacement

The raw oil filters of the VRDS device and the SSOT device were originally designed as filters in the form of wedge wire tubes, with a filtration accuracy of 25 μm.   
According to the difference of processing capacity and filtering raw materials, the VRDS device adopts four groups in parallel, and the SSOT device adopts two groups in parallel. 
The filter is controlled by a pneumatic actuator. When the pressure difference between the inlet and outlet of the filter reaches a high limit of 140 kPa, backwashing is performed. 
The mechanical impurities attached to the filter screen are backwashed by the filtered raw materials and sent to the dirty oil system, and the cleaned filter is put into operation. 
Since the residual oil is dirty, the filter is also equipped with a steam backflushing process. If the oil backflushing effect after filtering is poor, steam can be used to purge the impurities on the filter screen. 
The interval time of filter backwashing is determined by the impurity content in the raw oil. If the raw oil is dirty, the pressure drop will rise faster, and the switching interval will be small. 
Frequent washing of the filter can easily lead to damage to the filter screen or local deformation, and large particles of impurities enter the high-pressure feed pump and reactor, resulting in damage to the high-pressure pump equipment and reduction of catalyst activity. 
It is also easy to cause production accidents due to the interruption of reaction raw materials, which affects the safe and smooth operation of the device. Therefore, the filter should have good filtering ability and thorough backwashing ability. 
From the usage of SSOT device and VRDS device: 
The wax oil feed filter is operating normally. 
However, the residual oil filter does not operate normally due to the dirty raw materials. The filter is frequently washed, but the impurities attached to the filter screen are still difficult to wash off. It is often necessary to carry out steam backflushing and cleaning of the filter element, which is labor-intensive. When cleaning the filter element, it is often found that the filter element is deformed. even damaged and need to be replaced. 

Application of new filter in VRDS device

The VRDS unit was expanded from 0.84 Mt/a to 1.50 Mt/a, using an automatic filter with a built-in wire mesh filter element (R-grade). The system consists of 4 filters, and the process is shown in Figure 2. During normal operation, 3 units are kept online for filtration, and 1 unit is used for soaking. The filtered fluid is vacuum residue oil; temperature: 265 ~ 280 ℃. Pressure: 0.5 ~ 1.0 MPa. Flow rate: 250 t/h. Density: 843 kg/m3. Dynamic viscosity: 1.83 ~ 5.0 mPa·s.

Operation

Works fine with diesel feed.    

After the wax oil was introduced into the system, the pressure difference of the filter increased significantly faster, and the filter was flushed frequently. 
The pressure difference is between 150 and 180 kPa, and the backwash cannot be carried out. At this time, the auxiliary line valve cannot be fully closed. 
After draining the residual oil, the filter pressure difference is still as high as 250 kPa when the auxiliary line valve is opened for 4 to 5 buckles.
Afterwards, the filter element was chemically cleaned, and the R-grade filter element with higher filtration and flow capacity was used for 5 trials, which had a great impact on the production. The impurity brought in and the effect on the pressure drop of the bed are even more difficult to estimate. 

Proof of 1 to 6 trials: 
Neither M-class nor R-class elements of the filter can meet production needs. 
In the 5th trial use, replace the two groups B and D in the filter with the wedge wire filter element.  
In order to compare the operating effects of the R-level and wedge wire filter elements, the PLC program was modified so that two groups of R-level filter elements and two groups of wedge wire filter elements could be put into use respectively during the operation of the filter. 
Table 2 is the performance data of the filter element installed in this trial operation, and Table 3 is a typical set of pressure difference comparison data.  
The modified PLC program control is: (1) A and C are put into use at the same time, and B and D are ready for soaking. (2) When one of the following conditions is met, B and D are put into operation at the same time, and filters A and C are offline. 
Condition 1: The filtration time reaches 120 min. 
Condition 2: The pressure difference reaches 150 kPa and the filtration time reaches 60 min.  

Condition 3: A and C are ready for immersion and wait until the next cycle. 

Condition 4: If the filter pressure difference is greater than or equal to 300 kPa, the automatic control valve secondary line is opened. 

5th Installation Filter Performance Data

FilterABCD
TypeRWEDGE WIRERWEDGE WIRE
Quantity445316432316
Dia25.425.425.425.4
Length137111341371935
Total filter area/m248.832.845.726.8
Total opening area/m214.531.5714.201.28
Filtration accuracy75257525

Solution

The difference between two different filter element backflushing

The Wedge filter element has only one layer of winding tube, and the filter channel is a wedge-shaped structure with a gap between the teeth. There is no blocking force against impurities during backwashing, while the M-grade and R-grade filter elements use the perforations and cross-lamination of multi-layer wire mesh to retain impurities, and the inner layer of impurities is resisted by the wire mesh when backwashed and pushed out. It is not easy to be backwashed, and impurities will gradually accumulate, eventually causing backwashing to be ineffective.

Improvement measures

  • Filters A and C groups were also replaced with wedge wire filter elements, and 316 new filter elements were replaced, with an effective length of 1 134.4 mm. Filtration accuracy: 75% for 25-30 μm, 25% for 30-35 μm, 5% for 35-40 μm.
  • The direct thermal feed of the atmospheric and vacuum unit is adopted, and it is not stored in the tank area to prevent the reaction of oxidative radicals and the generation of organic impurities.
  • In order to increase the pulsating force of backwashing, part of the filter was modified, a pre-filter was connected in parallel for emergency backup, and the replacement oil replacement procedure was restored.

In conclusion

  • Since the vacuum wax oil contains less asphaltene and colloid, the filter adopts wedge wire filter element and metal mesh type R grade filter element, which can meet the production requirements. Both backwashing can clean the impurities on the surface of the filter screen, but the R-grade filter element has a higher flow capacity and a long interval between backwashing.
  • When filtering vacuum residue oil, the residue oil contains more colloidal asphaltenes, and the M-level and R-level filter elements with multi-layer wire mesh structure cannot effectively backwash the impurities in the filter screen.
    And as the operating time is prolonged, the pressure difference builds up, and finally a permanent filter cake is formed, making the filter unusable.
    The harm caused is that the high-pressure pump is easily damaged, the catalyst activity is reduced, and the operation period of the device is shortened.
  • The filter element in the form of a wedge wire tube is used to filter the vacuum residue oil, which can well backwash away impurities, which can meet the production needs.
  • The backwash of the wedge wire filter element uses the tooth gap to retain impurities, and the surface is a wedge-shaped structure, which has no blocking force against impurities.
    The multi-layer metal filter screen structure of the R-level filter element, because the inner layer of impurities is blocked by the multi-layer screen.
    Backwash resistance is high and leads to accumulation of impurities that eventually render the filter unusable.

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