Vessel wall design

When synthesizing a fiowsheet, the designer should consider carefully the problems associated with operation under extreme conditions. Attenuation will result in a safer plant, providing the attenuation does not increase the inventory of hazardous materials. If the inventory does not increase, then attenuation not only will make the process safer but also will make it cheaper, since cheaper materials of construction and thinner vessel walls can be used and it is not necessary to add on so much protective equipment. 

A commercial design based on semicontinuous operation was developed for manufacture of silicate powders (27). A slurry, prepared containing the feed materials and water, is fed to the reactor tank and heated by circulating a heat-exchange fluid in channels located on the outside vessel wall. A six-bladed stirrer is operated at about 100 rpm in order to keep reagents well mixed. Once the slurry reaches the operating temperature, the vessel heat is maintained until reaction is complete. For most fine-particle products, this time is less than 1 hr. 

Spiral baffles, which are sometimes installed for hquid services to improve heat transfer and prevent channeling, can be designed to serve as reinforcements. A spiral-wound channel welded to the vessel wall is an alternative to the spiral baffle which is more predictable in performance, since cross-baffle leakage is eliminated, and is reportedly lower in cost [Feichtinger, Chem. Eng., 67, 197 (Sept. 5, I960)]. 

The Webre design as tested by Pollock and Work [14] showed (Figure 4-50C) that internal action in the separator was responsible for some of the entrainment, particularly liquid creep up the vessel walls. 

Unseen corrosion can be the most damaging type of attack. Items should be designed to permit periodic inspection. This involves the provision of sufficiently large manways, the installation of inspection pits, the placing of flat-bottomed vessels on beams instead of directly onto concrete bases and the facility for removal of thermal insulation from vessel walls. 

A typical plate is shown in Figure 11.22. The plate sections are supported on a ring welded round the vessel wall, and on beams. The beams and ring are about 50 mm wide, with the beams set at around 0.6 m spacing. The beams are usually angle or channel sections, constructed from folded sheet. Special fasteners are used so the sections can be assembled from one side only. One section is designed to be removable to act as a manway. This reduces the number of manways needed on the vessel, which reduces the vessel cost. 

The plates are not fixed to the vessel wall, as they are with sectional plates, so there is no positive liquid seal at the edge of the plate, and a small amount of leakage will occur. In some designs the plate edges are turned up round the circumference to make better contact at the wall. This can make it difficult to remove the plates for cleaning and maintenance, without damage. 

The strength of metals decreases with increasing temperature (see Chapter 7) so the maximum allowable design stress will depend on the material temperature. The design temperature at which the design stress is evaluated should be taken as the maximum working temperature of the material, with due allowance for any uncertainty involved in predicting vessel wall temperatures. 

The vessel wall thickness must be sufficient to ensure the maximum stress intensity does not exceed the design stress (nominal design strength) for the material of construction, at any point. 

The maximum compressive stress in a vessel wall should not exceed that given by equation 13.74 or the maximum allowable design stress for the material, whichever is the least. 

The value of e0 is only constant for a fixed volume V of solution inside the calorimetric vessel. The change of e0 with V is primarily due to an increase of the reaction vessel wall in contact with the liquid as the liquid volume increases [ 197,200]. This change, de0/dV, which is constant for well-designed calorimeters [197,200], can be determined by measuring e0 as a function of V. Because it has been found that as expected, e0 and d 0/dV are independent of the nature of the liquid used in the calorimeter, they are normally determined by performing electrical calibrations with the calorimeter filled with different volumes of water [200]. The energy equivalent of the calorimeter at any point during a titration can therefore be calculated from... 

Mechanical design of the adsorber then takes up the remainder of the engineering effort to produce a workable adsorption process design. Once a vessel is sized to provide the required inventory of adsorbenf we need to provide the mechanical details, which include flow distribution devices, bed supports and the required vessel wall thickness to withstand the working pressure and added stresses encountered during regeneration and repeated de-pressurization and re-pressurization. 

To optimize the applicability of the electrothermal vaporization technique, the most critical requirement is the design of the sample transport mechanism. The sample must be fully vaporized without any decomposition, after desolvation and matrix degradation, and transferred into the plasma. Condensation on the vessel walls or tubing must be avoided and the flow must be slow enough for elements to be atomized efficiently in the plasma itself. A commercial electrothermal vaporizer should provide flexibility and allow the necessary sample pretreatment to introduce a clean sample into the plasma. Several commercial systems are now available, primarily for the newer technique of inductively coupled plasma mass spectroscopy. These are often extremely expensive, so home built or cheaper systems may initially seem attractive. However, the cost of any software and hardware interfacing to couple to the existing instrument should not be underestimated. 

The second device shown is a cyclone inlet that uses centrifugal force, rather than mechanical agitation, to disengage oil and gas. This inlet can have a cyclonic chimney, as shown, or may use a tangential fluid race around walls. Designs arc proprietary, but most use an inlet nozzle sized to create a fluid velocity of about 20 fps around a chimney whose diameter is two-thirds the vessel diameter. 

Very high velocities tend to skim the liquid film off the vessel wall and off the liquid at the bottom. The liquid also tends to creep up the wall and down the exit pipe where it is picked up by the exit gas. The skirt shown on Figure 18.9 is designed to prevent reentrainment of the creeping liquid, and the horizontal plate in Figure 18.10 prevents vortexing of the accumulated liquid and pickup off its surface. 

---