Isobaric operation

In isobaric operation the system pressure is maintained constant, by the exchange of gas or liquid with an external reservoir. The temperature is decreased until the formation of hydrate is indicated by significant addition of gas (or liquid) from a reservoir. After hydrate formation the temperature is slowly increased (maintaining constant pressure by fluid withdrawal) until the last crystal of hydrate disappears. This point, taken as the equilibrium temperature of hydrate formation at constant pressure, may be determined by visual observation of hydrate dissociation or at a constant temperature as simple hydrates dissociate with heat input. 

Mehta (34) has carried out a reactor network optimization study to find improved designs for the production of acrylonitrile in a collaboration between UMIST and one of its industrial partners. Most industrial installations employ fluidized-bed reactors (BP/Sohio process) with a well-mixed reaction zone. Previous process improvements have mainly resulted from better catalysts, which have produced an increase in yield from 58% to around 80%. The reaction model employed in the optimization study is taken from Ref. 81 and considers seven reactions and eight components. Air, pure oxygen, and propylene are available as raw material streams. The optimization study assumes negligible pressure drop along the reaction sections, isothermal and isobaric operation, and negligible mass gas-solid transfer effects. 

In Isothermal conditions a constant inlet gas pressure would automatically maintain a constant volumetric and linear flow rate. When the temperature of the column is programmed while the inlet gas pressure is kept constant, the flow rate will change considerably, becoming lower because of the modifications of the column geometry, expansion of the gas, and modifications of the gas viscosity, which increases with (T) . Besides isobaric operation, which is common, constant flow at programmed temperature capability is also available for certain instruments. 

The variation of the reactor volume with the reaction extent is given by Eq. 6.1.14. For isothermal-isobaric operations with single reactions, and when the initial state is selected as the reference, Eq. 6.1.14 reduces to... 

To express the temperature changes, we write the energy balance equation. For semibatch reactors, the expansion work is usually negligible, and assuming isobaric operation, the energy balance equation is... 

Sieverts hydrogenator modified from the standard configuration (Fig. 7.4) for isobaric operation by the addition of a needle valve (NV) and a second pressure transducer. The needle valve feeds gas to or from the sample under feedback control such that Pceii is constant. In addition to providing the feedback signal, the second pressure transducer allows the hydrogen content of the sample to be calculated at all times. 

Air at 294 K and 1 atm enters a fixed-bed adsorber at a flow rate of 0.146 m3/s with a benzene vapor concentration of 29 g/m3. The cylindrical adsorber is 0.61 m in inside diameter and is packed to a height of 1.83 m with 331 kg of silica gel particles having an effective diameter of 2.6 mm and an external porosity of 50%. The adsorption isotherm for benzene has been determined experimentally and found to be linear over the concentration range of interest, given by q = kc, where q is in kg benzene/kg gel, c is in kg benzene/m3 of gas, and k = 4.127 m3 of gas/kg of gel. It has been estimated that the overall volumetric mass-transfer coefficient for the conditions prevailing in the bed is Kc.a = 8.79 s-1. Assuming isothermal and isobaric operation, calculate ... 

We have three external constraints (1) no shaft work (Wg = 0), (2) isothermal operation (outlet temperatures are same as temperature in the unit), and (3) isobaric operation (outlet pressures are same as in the unit). So (12.3.11) gives... 

The coordinates of the point of maximal productivity w are analogous to the isobaric case (Kb,2. Ka.3). The point w has, in comparison to the isobaric operation at an average density, a larger distance from the diagonal, which is equivalent to a higher obtainable productivity. 

If the separation regimes for the gradient operation are compared to the one for the isobaric operation with respect to the purities of the products, the separation regions described for the isobaric case can be found. Notable are the new regions 7 to 9, in which one or both components accumulate in the apparatus. The assumptions of the triangle theory of linear isotherms and infinite solubility of the components lead to a nonsensical prediction of a permanent accumulation in the apparatus. In reality, the accumulating component will break through after a certain time. 

The heat requirement of a separation unit follows from the energy balance as shown in Chapter 1.3.1. For a simple distillation unit the heat flux may be derived, as shown in Fig. 2-9. An energy balance of the distillation unit under isobaric operation gives... 

The application of pressure is done by one or more hydraulic cylinders similar to static pillar presses. Depending on the application area, CCM machines can be designed according to an isochoric or an isobaric operating... 

Under isobaric operating conditions (hydrostatic head operating pressure) the gas volume flow at outlet for complete mixing of all phases (model <11>) is given by... 

Although a continuous and isobaric operated high-pressure column process seems to be beneficial, no absorption liquid with suitable distribution coefficients is often available. An incomplete purification of the supercritical fluid leads to strongly increased solvent to feed ratios. Thus, the extracted components are separated by pressure reduction or by adsorption like in supercritical processes for solid material. An example is given in Figure 8.18 for the deacidiflcation of vegetable oil [36]. 

The basic principle of SSITKA is the following. Under steady-state isothermal and isobaric operating conditions, an isotopic transient is introduced by an isotope switch or, in other words, a sudden replacement of one labeled compound with its isotope. Analysis is done by GC and MS. The amount of adsorbed intermediates Np converted in products is defined as... 

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