Benzene removal tower and benzene washing tower

1、 Overview

The benzene removal tower is subordinate to the desorption tower and is an operating equipment that evaporates and releases benzene hydrocarbons dissolved in the washing oil solvent through direct contact heat transfer with a heat carrier.

Working principle: A general heat carrier superheated steam is used as the heating agent to make the equilibrium vapor pressure of crude benzene on the washing oil surface greater than the partial pressure of crude benzene in the heat carrier. The vapor-liquid two phases are in countercurrent contact for mass and heat transfer, gradually releasing crude benzene from the rich oil. A mixture of benzene vapor and steam is obtained at the top of the tower, and a relatively pure lean oil is obtained at the bottom of the tower.

This equipment is generally shared with other equipment such as ammonia washing tower, benzene washing tower, tube furnace, etc. The direct steam introduced during the distillation process is highly superheated steam. Therefore, the amount of direct steam used plays an extremely important role in the benzene distillation operation.

2、 Structural features and classification

The benzene removal tower is divided into plate tower and packed tower according to its internal structure. A plate tower consists of a tower body and internal components (trays). Commonly used plate trays include sieve tray, float valve tray, and bubble cover tray (Figure 1); The packed tower consists of a tower body, a packing layer, and a liquid distribution device (Figure 2).

A packed tower is a mass transfer equipment that uses the packing inside the tower as the contact component between gas and liquid phases. The packed tower belongs to continuous contact gas-liquid mass transfer equipment, and the two-phase composition changes continuously along the tower height. Under normal operating conditions, the gas phase is the continuous phase and the liquid phase is the dispersed phase.

The benzene removal towers produced by our company are mostly plate towers with circular bubble caps, CJST vertical bubble caps, or floating valve towers as internal components. According to the functions of each part inside the cylinder, they are divided into several parts: lean liquid tank section (some equipment does not have it, depending on requirements), crude distillation section, and refined distillation section. Generally, 17 layers of trays are set up in the crude distillation section, and 13 layers of trays are set up in the refined distillation section.

The plate type benzene removal tower consists of skirt, tower body, tower internals, tower kettle, suspension column, connecting pipe, etc. The tower internals are all stainless steel parts, mainly including steam distributor, tower plate, bubble cover or float valve. Outside the benzene removal tower

Figure 1 Figure 2

The shell is made of stainless steel material, divided into 2-3 sections and welded together on site.

Circular bubble cap trays and CJST vertical bubble cap trays each have their own characteristics, and can be used alone or interchangeably in the benzene removal tower.

1. The schematic diagram of gas-liquid contact of circular bubble cap tray is as follows:

characteristic:

The advantages are stable operation, good elasticity, high board efficiency, avoidance of dirt and blockage, and sufficient material inspection; The weaknesses are the complex structure, high cost, and large pressure drop of gas passing through each layer of the tray.

2. The schematic diagram of gas-liquid contact of CJST vertical bubble cap tray is as follows:

characteristic:

① High efficiency: The tower is equipped with multiple layers (more than 20 layers) of trays, and the ammonia vapor concentration at the top of the tower can reach more than 20%, with an ammonia distillation rate of 98% (traditional ammonia distillation towers are generally lower than 90%), anti clogging, and long operating cycle.

② High processing capacity: minimal entrainment of mist and foam, and the liquid flowing on the board is basically a clear liquid without gas, reducing

There is a possibility of overflow from the downcomer.

③ Good operational flexibility: Improved the mass transfer method for bubbling trays such as bubble caps and sieve plates, with gas-liquid being carried out in the form of jets

Contact, product quality is almost unaffected by fluctuations in gas-liquid load, and operation is stable.

④ Strong anti blocking ability: Due to the large holes in the tray plate and the absence of moving parts, it is generally not easily blocked by dirty or viscous materials. In addition, gas-liquid leaves the cap in a jet state, with a high gas velocity and strong self flushing ability for the cap hole itself, which will not block the tower.

⑤ Easy maintenance. The connection between the tower plate and the beam inside the tower is fixed with bolts, which is convenient for maintenance. Especially when replacing the tray, there is no need to dismantle the tower section, and it can be completed through the manhole, which can save maintenance costs.

3、 Design principles and selection criteria

1. Design principle: The working medium of the benzene removal tower is benzene vapor, benzene wash oil, and wash oil;

Design pressure: ≤ 0.1Mpa； Design temperature: 200℃

Material selection: Carbon steel materials are Q235C and Q245R, and stainless steel material is 304 (0Cr18Ni9).

Design Specification: JB/T4710-2005 "Steel Tower Containers".

2. Selection criteria:

The factors to consider when selecting a benzene removal tower include material properties and operating procedures

The performance of components and tower equipment, as well as the manufacturing, installation, and transportation of tower equipment

And maintenance, etc.

Reference standard for manufacturing and inspection of benzene removal tower: SH3098-93 Petroleum

Design Specification for Chemical Towers.

4、 Installation, use, and maintenance

1. Installation of benzene removal tower

① The selection of installation location must ensure ease of operation, maintenance, and disassembly.

② The foundation installation diagram should be made based on the outer diameter of the tower bottom and the position and size of the anchor bolts, paying attention to the detachment of the low skirt seat

The foundation of the benzene tower must be correspondingly raised (usually by 4/5 meters).

③ The production of this equipment generally consists of 2-3 sections of cylinder body, with clamps installed between each section. During on-site installation, select lifting equipment and tools according to the weight of the benzene removal tower, assemble the entire tower in sequence according to the requirements of the final assembly drawing, and ensure the levelness of the tower plate and the verticality of the tower body before welding.

④ When placing the tower base, use a spirit level to check the levelness of the plane on the base and ensure that the allowable deviation of levelness on the entire plane is 3mm. Then fix the base and tighten the nut.

⑤ During the installation process, its verticality must be verified at all times. The deviation between the actual axis of the erected tower and the design axis should not exceed 1/1500 of the tower height, and local bending should not exceed 1/750 of the measured length.

⑥ Verticality of tower body: During the installation of tower sections, the verticality of the tower body should be checked every 2 sections using the vertical method, and then checked again after the installation is completed.

The allowable inclination value is 1/1000 of the tower height, with a maximum of ≤ 15mm. Otherwise, it will cause unevenness and dead zones on the tray, thereby reducing the efficiency of the tray.

⑦ The tolerance of the overall and local orientations of the entire tower is measured based on the circumference of the tower bottom, and is ≤ 15mm.

⑧ For platforms with existing steel structures or concrete frames, it should be noted that the positions of the benzene removal tower's pipe openings should not be connected to the platform; The platform (or ladder) welded on site should be welded after the installation and adjustment of the entire tower, and should avoid each pipe opening.

⑨ When installing the entire tower, a lightning protection grounding device should be added.

2. Test run and operation of benzene removal tower

① Before testing, it is necessary to check the drawings for any special requirements and instructions, whether there are any special markings on the nameplate, and whether there are any special requirements for pressure testing and testing procedures.

② The test run of the tower body should be carried out by personnel familiar with the process flow.

③ Before testing, the entire system should be cleaned and a filter screen should be installed at the inlet connection.

④ If there is no bypass in the system, a temporary bypass should be added during testing.

⑤ Due to the fact that the equipment has undergone a water pressure test before leaving the factory, only the leakage test at the welding joint of the retaining ring can be conducted after on-site installation.

⑥ Trial operation: After the installation of the entire system is completed, water is pumped in and steam (a small amount) is fed into the inlet pipe at the bottom of the tower,

Check for any running, emitting, dripping, seeping, or leaking phenomena in various parts. If any are found, conduct maintenance until they are eliminated. At the same time, observe the working condition of the tray and check whether all instruments and safety facilities are complete and functional.

⑦ During the test drive, the temperature should be slowly increased and decreased to avoid excessive pressure difference and thermal shock.

⑧ The benzene stripping tower shall not operate at pressures exceeding those specified in the drawings.

⑨ After the test drive is completed, the entire tower is insulated as required (except for the mirrors and connecting pipes).

3. Use and maintenance of benzene removal tower

① When driving, first open the vent valve at the bottom of the tower, exhaust the air inside the tower, and then close it.

② When conditions permit, the remaining liquid in the tower kettle should be drained, the residual pressure should be relieved, and the tower should be washed with clean water to replace nitrogen or air. Only after passing the inspection can work be carried out; When there are no conditions, release the remaining liquid from the tower kettle and release the residual pressure, then steam the tower.

③ The oil inlet should be stable, the heating rate should be slow, and the valve adjustment should be frequent and slow.

④ When parking, first stop adding oil and drain the kettle liquid. Use a steam distillation tower for 15-20 minutes before emptying.

5、 Fault analysis and troubleshooting:

The possible faults that may occur during trial production and production process, the possible cause analysis and treatment methods are as follows, for reference only:

Yancheng Haiyun Environmental Engineering Technology Co., Ltd60—120Ten thousand tons/Engineering design, equipment supply, and installation services for the annual coke oven (cold drum section, desulfurization section, sulfur ammonia section, and benzene washing section).