Specific condensation heat

In addition to volume changes the effect of temperature is also important. Thus the specific latent heat of vaporization of a chemical is the quantity of heat, expressed as kJ/kg, required to change unit mass of liquid to vapour with no associated change in temperature. This heat is absorbed on vaporization so tliat residual liquid or tlie sunoundings cool. Alternatively an equivalent amount of heat must be removed to bring about condensation. Thus the temperature above a liquefied gas is reduced as tlie liquid evaporates and tlie bulk liquid cools. There may be consequences for heat transfer media and the strength of construction materials at low temperatures. 

When an agitated batch containing M of fluid with specific heat c and initial temperature t is heated using an isothermal condensing heating medium Tt, the batch temperature t2 at any time 0 can be derived by the differential heat balance. For an unsteady state operation as shown in Figure 7-27, the total number of heat transferred is q, and per unit time 0 is ... 

Subsequently, we used Aspen Dynamics for time-domain simulations. A basic control system was implemented with the sole purpose of stabilizing the (open-loop unstable) column dynamics. Specifically, the liquid levels in the reboiler and condenser are controlled using, respectively, the bottoms product flow rate and the distillate flow rate and two proportional controllers, while the total pressure in the column is controlled with the condenser heat duty and a PI controller (Figure 7.4). A controller for product purity was not implemented. 

In the case of methane combustion around 1.61 kg of water vapor is produced per m of methane (see Figure 34.4). The specific latent condensation heat of water is 2.418 MJ/kg at 25°C, so per m of methane 4.00 MJ of condensation heat is available. Compared to the HHV of methane of 39.8 M]/w , this is 10.04%. Compared to the LHV it adds an extra 11.05%. As natural gas consists mostly of methane, the same numbers apply roughly to natural gas. For other fuels, the stechiometric combustion equations are different and therefore the water vapor and the maximum amount of latent heat are different. For oil-fired boilers this is around 6% and for propane it is 8-9%. 

The design firm normally generates procurement specifications. These specifications will include definition of all major components such as materials of construction, agitator requirements, nozzles, etc. Major equipment for API processing is similar to fine chemical production and can include reactors, centrifuges, condensers, heat exchangers, distillation columns, extractors, absorption equipment, chromatography equipment, dryers, blenders, crystallizers, mills, etc. These components wiU normally require validation [installation... 

In Chapter 3, we studied how to design a distillation column, given the feed conditions, the desired product specifications, and the total number of stages. The calculated design parameters included the operating pressure of the column, the reboiler and condenser heat duties, and the length and diameter of the column vessel. 

Figure 5.32 gives the conceptual flowsheet. The specific system used as an example is methanol/water. Product specifications are 99.9 mol% methanol in the distillate streams (there is one from each column) and 99.9 mol% water in the two bottoms streams. The fresh feed is Ikmol/s with a composition of 60mol% methanol and 40mol% water. The feed is split between the two columns, so that the system operates neat, that is, the condenser heat removal in the high-pressure column is exactly equal to the reboiler heat input in the low-pressure column. Each column has 32 stages and is fed on the stage that minimizes reboiler heat input. 

The job of the column is to achieve the desired product purities in the face of disturbances. Recall that the design specifications are 1 mol% zC4 in the distillate and 0.5 mol% C3 in the bottoms. We are using single-end control with a temperature on a tray in the column controlled by manipulating reboiler heat input. The other degree of freedom is held constant and depends on the control structure. In CSl, it is the reflux-to-feed ratio, and in CS3, it is the RR. In control structure CS2, the other degree of freedom is a fixed condenser heat removal. 

Special attention should be paid in the definition of A. The specification of its variables should take into account the physical feasibility of the problem solution. Thus, vector A includes variables such as compositions of all incoming streams, compositions of all outgoing streams (side, distillate and bottom streams), temperatures of all incoming and outgoing streams, heat duties of reboiler and condenser, heat flows transferred to each compartment and the molar flows of all feed streams, side streams and bottom stream. 

Heat of vaporization, heat of condensation, heat of fusion, heat of sohdification, specific heat... 

The mean specific molar heat capacity of the dry gas mix Cpm.ilT l, can be easily calculated or extracted as a stream property from a process simulator, for example, Aspen Plus [2]. The moisture exergy ei,H,tni,H20 must include the complete condensation of any water vapor. Although the specific enthalpy ho = 1890.3 kJ/ kmol and entropy So = 6.616 kJ/(lcmol K) of the reference water are fixed, the enthalpy hi in kj/kmol and entropy s, in kJ/(kmol-K) are computed applying the IAPWS-IF97 formulation for the gas temperature and partial H2O pressure. 

Solid-Bed Dehydration. Sihca gel, bauxite, activated alurnina, or molecular sieves can be used for removing dissolved water to meet propane specifications. The soHd-bed dehydrators are used in a cycHc adsorption process. After an adsorption cycle has completed, the bed is heated with a purge gas or a vaporized Hquid-product stream for regeneration. If the latter is used, the Hquid product is condensed, separated from the free water, and returned to the process. After the beds are regenerated, they are cooled and returned to the adsorption cycle. 

The coolant for the HTGR is helium. The helium is not corrosive has good heat properties, having a specific heat that is much greater than that of CO2 does not condense and can operate at any temperature has a negligible neutron absorption cross section and can be used in a direct cycle, driving a gas turbine with high efficiency. 

Condenser and eboiler AT. The losses for AT are typically far greater than those for reflux beyond the minimum. The economic optimum for temperature differential is usually under 15°C, in contrast to the values of over 50°C often used in the past. This is probably the biggest opportunity for improvement in the practice of distillation. A specific example is the replacement of direct-fired reboilers with steam (qv) heat. 

---