Vessels, process pressure

Under certain conditions of temperature and pressure, and in the presence of free water, hydrocarbon gases can form hydrates, which are a solid formed by the combination of water molecules and the methane, ethane, propane or butane. Hydrates look like compacted snow, and can form blockages in pipelines and other vessels. Process engineers use correlation techniques and process simulation to predict the possibility of hydrate formation, and prevent its formation by either drying the gas or adding a chemical (such as tri-ethylene glycol), or a combination of both. This is further discussed in SectionlO.1. 

Pressure Swing Adsorption. A number of processes based on Pressure Swing Adsorption (PSA) technology have been used in the production of carbon dioxide. In one version of the PSA process, CO2 is separated from CH using a multibed adsorption process (41). In this process both CH4 and CO2 are produced. The process requires the use of five adsorber vessels. Processes of this type can be used for producing CO2 from natural gas weUs, landfiU gas, or from oil weUs undergoing CO2 flooding for enhanced oil recovery (see Adsorption, gas separation). 

In the suspension process, which was the first method to be commercially developed, propylene is charged into the polymerisation vessel under pressure whilst the catalyst solution and the reaction diluent (usually naphtha) are metered in separately. In batch processes reaction is carried out at temperatures of about 60°C for approximately 1-4 hours. In a typical process an 80-85% conversion to polymer is obtained. Since the reaction is carried out well below the polymer melting point the process involves a form of suspension rather than solution polymerisation. The polymer molecular weight can be controlled in a variety of... 

Process flow diagrams are more complex and show all main flow streams including valves to enhance the understanding of the process as well as pressures and temperatures on all feed and product lines within all major vessels and in and out of headers and heat exchangers, and points of pressure and temperature control. Also, information on construction materials, pump capacities and pressure heads, compressor horsepower, and vessel design pressures and temperatures are shown when necessary for clarity. In addition, process flow diagrams usually show major components of control loops along with key utilities. 

At the instant a pressure vessel ruptures, pressure at the contact surface is given by Eq. (6.3.22). The further development of pressure at the contact surface can only be evaluated numerically. However, the actual p-V process can be adequately approximated by the dashed curve in Figure 6.12. In this process, the constant-pressure segment represents irreversible expansion against an equilibrium counterpressure P3 until the gas reaches a volume V3. This is followed by an isentropic expansion to the end-state pressure Pq. For this process, the point (p, V3) is not on the isentrope which emanates from point (p, V,), since the first phase of the expansion process is irreversible. Adamczyk calculates point (p, V3) from the conservation of energy law and finds... 

Some Data on the Reliability of Pressure Chemical Process Data derived from 1.4x10 vessel-year Process pressure vessels, pressure storage 44. 

Design Pressure of a Vessel the pressure established as a nominal maximum above the expected process maximum operating pressure. This design pressure can be established by reference to the chart in Chapter 1, which is based on experience/practice and suggests a percentage increase of the vessel design pressure above the expected... 

The pressure at which the valve is expected to open (set pressure) is usually selected as high as possible consistent with the effect of possible high pressure on die process as well as the containing vessel. Some reactions have a rapid increase in temperature when pressure increases, and this may fix the maximum allowable process pressure. In other situations the pressure rise above operating must be kept to some differential, and the safety valve must relieve at the peak value. A set pressure at the maximum value (whether maximum allowable working pressure of vessel, or other, but insuring protection to the weakest part of the system) requires the smallest valve. Consult manufacturers for set pressure compensation (valve related) for temperatures >200°F. 

The vessel is determined to be an uninsulated, horizontal, grade-level, cylindrical pressure vessel with a gas volume of 706 ft3 (20 m3). The vessel has a MAWP of 1,480 psig (102 bar) at 650°F (343°C) and a Minimum Design Metal Temperature (MDMT) of 40°F (4°C) since it is constructed of unnormalized steel material. Due to process conditions immediately prior to pressuring the vessel to 1,000 psig (69 bar), the vessel s metal temperature is approximately 30°F (-1°C). When the vessel is pressurized to 1,000 psig, it fails catastrophically. The distances to overpressure endpoints (1, 3, and 5 psig) are calculated as follows ... 

The relationship used for this purging process is identical to Equation 7-6, where nL is now the total moles at atmospheric pressure (low pressure) and nH is the total moles under pressure (high pressure). In this case, however, the initial concentration of oxidant in the vessel ( y0) is computed after the vessel is pressurized (the first pressurized state). The number of moles for this pressurized state is nH and the number of moles for the atmospheric case is nL. 

D espite many safety precautions within chemical plants, equipment failures or operator errors can cause increases in process pressures beyond safe levels. If pressures rise too high, they may exceed the maximum strength of pipelines and vessels. This can result in rupturing of process equipment, causing major releases of toxic or flammable chemicals. 

Arulanantham and Lees (1981) have studied pressure vessel failures in process plants such as olefins plants. They define failure as a condition in which a crack, leak or other defect has developed in the equipment to the extent that repair or replacement is required, a definition which includes some of the potentially dangerous as well as all catastrophic failures. In olefins plants fired heaters have failure rates of about 0.4 failures/year, while process pressure vessels have 0.0025 failures/year and heat exchangers 0.0015 failures/year. It is noticed that fired heaters are much unsafer than process pressure vessels, which are a little unsafer than heat exchangers. 

The inventory in the process is 100 tonnes when all seven vessels have been calculated together with one hour residence time. The maximum process temperature is 175°C in the reactor. The highest process pressure is 30 bar in the reaction section. The equipment safety is determined by the CO feed-gas... 

Example 2.7. To show what form the energy equation takes for a two-phase system, consider the CSTR process shown in Fig. 2.6. Both a liquid product stream f and a vapor product stream F (volumetric flow) are withdrawn from the vessel. The pressure in the reactor is P. Vapor and liquid volumes are and V. The density and temperature of the vapor phase are and L. The mole fraction of A in the vapor is y. If the phases are in thermal equilibrium, the vapor and liquid temperatures are equal (T = T ). If the phases are in phase equilihrium, the liquid and vapor compositions are related by Raoult s law, a relative volatility relationship or some other vapor-liquid equilibrium relationship (see Sec. 2.2.6). The enthalpy of the vapor phase H (Btu/lb or cal/g) is a function of composition y, temperature T , and pressure P. Neglecting kinetic-energy and potential-energy terms and the work term,... 

Pumps handling flammable materials represent a significant potential for spill and subsequent fire. This is due to damage to seals and failures of other potential leak points. The first consideration in fire protection for pumps is their location relative to other equipment, vessels, process structures and buildings housing personnel, and key control or utility systems. When locating a pump, consideration should be given to the size, properties of material handled, temperature, and pressure. 

A compressor is required to recover the vapour CO2 from the chamber at the end of the washing cycle, to minimize the loss in CO2 for each cycle (step 5). The vapour is condensed through a heat-exchanger and the liquid is recovered and returned to the storage vessel. The pressure-drop at the end of this step can be larger than 50 bar, with a temperature of the process-fluid higher than 80°C downstream the compressor. 

NH1CONH2 + H2O. The processing is complicated because of the severe corrosiveness of the reactants, usually requiring reaction vessels that are lined with lead, titanium, zirconium, silver, or stainless steel. The second step of the process requires a temperature of about 200 C to effect the dehydration of the ammonium carbamate. The processing pressure ranges from 160 to 250 atmospheres. Only about one-half of the ammonium carbamate is dehydrated in the first pass. Thus, the excess carbamate, after separation from the urea, must be recycled to the urea reactor or used for other products, such as the production of ammonium sulfate. 

In Figure 8.7, the curves determine the restriction downstream pressure at which hydrate blockages will form for a given upstream pressure and temperature. Gas A expands from 2000 psia and 110°F until it strikes the hydrate formation curve at 700 psia (and 54°F), so 700 psia represents the limit to hydrate-free expansion. Gas B expands from 1800 psia (120°F) to intersect the hydrate formation curve at a limiting pressure of 270 psia (39°F). In expansion processes while the upstream temperature and pressure are known, the discharge temperature is almost never known, but the discharge pressure is normally set by a downstream vessel or pressure drop. 

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