Trim heater

Fig. 38. Caustic purification system a, 50% caustic feed tank b, 50% caustic feed pumps c, caustic feed preheater d, amonia feed pumps e, ammonia feed preheater f, extractor g, trim heater h, ammonia subcooler i, stripper condenser j, anhydrous ammonia storage tank k, primary flash tank 1, evaporator reboiler m, evaporator n, caustic product transfer pumps o, purified caustic product cooler p, purified caustic storage tank q, ammonia stripper r, purified caustic transfer pumps t, overheads condenser u, evaporator v, evaporator vacuum pump w, aqueous storage ammonia tank x, ammonia scmbber y, scmbber condenser 2, ammonia recirculating pump aa, ammonia recycle pump. CW stands for chilled water.

We consider a plant designed to convert a feed stream rich in compound A (of molar concentration cao) into compound B in a high-temperature, mildly exothermic, first-order reaction carried out in an adiabatic reactor (Figure 6.8). For improved operability, the plant features a heater that is used at full capacity in startup mode and as a trim heater during operation, as well as a bypass stream that is used to regulate heat recovery in the FEHE. 

Figure 5.19 Adiabatic plug-flow reactor with feed-effluent heat exchanger and trim heater.

Step 8. Several control valves now remain unassigned. Steam flow to the trim heater controls reactor inlet temperature. Cooling water flow to the trim cooler is used to control the exit process temperature and provide the required condensation in the reactor effluent stream. Liquid recirculation in the absorber is flow-controlled to achieve product recovery, while the cooling water flow to the absorber cooler controls the recirculating liquid temperature. Acetic acid flow to the top of the absorber is flow-controlled to meet recovery specifications on the overhead gas stream. Cooling water flow to the cooler on this acetic acid feed to the absorber is regulated to control the stream temperature. Cooling water flow in the column condenser controls decanter temperature. 

Rather than maintain a high-temperature throughout the line, the refinery stores the material at flow temperature, 325°F. Separate inline heaters at each burner raise the fuel to atomization temperature. This trim heating often improves burner efficiency as w ell. Savings from reduced heat loss more than offset the cost of adding and operating the trim heaters. 

Modify the process in Problem 7.5 to use a counter-current heat exchanger (and trim heater) to heat the reactor feed and cool the reactor product. 

Figure 7 Schematic diagram of semipreperative-scale SFC chromatograph 1 carbon dioxide supply 1 a regulator 2 prechiller/heat exchanger 3 SD-1 Varian pump with (7) 200-mLpump head with special check valves 4 modifier reservoir 5 SD-1 modifier pump with (6) 200-mL standard pump heads 8 check valve 9 inlet pressure transducer 10 injection valve 11 check valve to prevent blow-back 12 mixer 13 fluid temperature preconditioner 14 column 15 column oven 16 uv detector 17 outlet pressure transducer 18 back-pressure regulator 1 9 evaporator 20 restrictor 21 trim heater 22 selection valve 23 peak detector 24 bank of collection vessels (or cassette) 25 individual collection tubes/bottles 26 pressure relief valves 27 waste container, waste vent. The manual cassette can be replaced with an automated cassette fed by a robot holding 128, 25 x 150-mm or 338, 16 x 150-mm test tubes, or with 7 large bottles.

Recycle. Self-regulation must be provided for components and energy in recycle loops. Recycling of material can recycle disturbances if the control is, for example, based on level control. Disturbances in the heat balance can be recycled if extensive heat integration is used. Add trim heaters and coolers to damp out thermal disturbances. 

A trim heater is to be designed to heat 116,000 Ib/hr of 57 t% ethane, 25 wt% propane, and 18 wt% n-butane from 80 to CT. The stream will enter the exchanger at 520 psia and must lot teach the bubble point in the exchanger. The stream will be leated with gasoline, which will enter at 240°F and 95 psia, with iflow rate of 34,000 Ib/hr. Standard practice of the company is... 

The chemical process shown in Figure 20.12 is based on an example by Stqphanopoulos (1984). It consists of a CSTR in which the species A reacts to form B in an exothermic reaction. The reactor effluent is fed to a flash vessel, where the heavier product B is concentrated in the liquid stream, and unreacted A is discarded in the vapor stream. A preheater recovers heat from the hot reactor effluent, with a so-called trim heater installed to ensure that the liquid reactor feed is at the desired temperature. To ensure that the reactor temperature remains on target, the CSTR is equipped with a jacket, fed with cooling water to attenuate the heat released. Seven flow-control valves are shown. An eighth valve could be placed on the liquid recycle to V-100, but is not. 

Further heating of the feed is done in a trim heater to help control the distillatiom The system shown in Figure 11-S may not be optimum, though, particularly if several columns are integrated. Nevertheless, the heat exchange ideas shown in Figure 11-5 are quite basic. 

The caustic from the bottom of the extractor is flashed to remove most of the dissolved ammonia. Like the ammonia stripper, the primary flasher operates at the intermediate pressure of about 1,450 kPa. The flashed vapor abstracts its latent heat from the remaining liquid. A trim heater on the caustic fed to the flasher supplies the necessary heat to offset the drop in temperature that would otherwise occur. The flashed ammonia, along with some water vapor, then can go to the stripper for separation into ammonia vapor and water. 

In operation it is envisaged that the vehicle will not normally be left for more thcin a few hours out of use without connecting to a battery charger. Trim heaters will be built into the battery box and these can be energised either from the mains, when connected, or else from the battery itself. 

In addition to the recovery of the latent heat of vapor streams, in many cases it is practical to recover part of the sensible heat in the column bottom product and steam condensate by exchange with column feed. Such schemes have been used in the chemical and petroleum industries for years. Since feed flow is typically set by level controllers or flow-ratio controllers, its flow rate will not be constant. The feed enthalpy or temperature, therefore, is apt to be variable. This may make column-composition control difficult unless one employs either feedforward compensation or a trim heater with control for constant temperature or enthalpy. (See Chapters 5 and 11.)... 

---