TWO-STAGE PROCESS FOR PRODUCTION OF CALCIUM CHLORIDE AND ACETYLENE

Abstract: A two-staged process for producing acetylene and calcium chloride from calcium carbide (12) and water (12) has two successive reaction steps. In step Number 1, calcium carbide is charged into an entrained flow-type reactor (10) vessel containing water. The reaction which follows proceeds to about 60-90 % completion. The entrained reaction products and the unreacted feed material are carried over to a secondary reactor (16) to complete the reaction, and the acetylene gas is drawn off (26). Calcium hydroxide product is removed from the reactor (23) and reacted with hydrogen chloride to form calcium chloride. Heat generated by the exothermic reactions of water with calcium carbide and HCl with calcium hydroxide is used to dry the calcium chloride product and improve its value. In the presence of excess of water, the calcium carbide-water reaction is effectively a first order, irreversible reaction.

Description: DESCRIPTION TWO-STAGE PROCESS FOR PRODUCTION OF CALCIUM CHLORIDE AND ACETYLENE BACKGROUND OF THE INVENTION The invention comprises a process for simultaneous manufacture of acetylene and calcium chloride from calcium carbide and hydrogen chloride. In the currently practiced art, calcium carbide is charged to a large tank containing water and which is outfitted with slow moving stirring paddles. The carbide is dropped onto the surface of the water where it is allowed to react spontaneously. Acetylene gas is withdrawn from the top of the tank while calcium hydroxide and other impurities settle to the bottom of the tank. The reactor is cooled by internal coils containing cooling water or by spraying cold water on the exterior of the tank or both. The calcium hydroxide produced, along with its impurities, is discharged, stockpiled and disposed of as a low value neutralizer for industrial acid streams or for maintaining pH balance in sewage plants. Depending upon the application, acetylene is compressed and stored in high pressure vessels or, in cases where it is used as a chemical feedstock, is pipelined to a nearby chemicals processing plant. The current problems with the state-of-the-art technology are: A. Uncontrolled contact of the carbide with the water, resulting in lower reaction rate control and the possibility of incandescence. Incandescence results when calcium carbide particles are buoyed by the evolution of acetylene and rise to the surface of the water where they react violently but without the benefit of cooling that prevails when the particles are submerged. The lack of cooling and the high exothermicity of the reaction may result in a localized temperature rise which heats the particle to incandescent or glowing temperatures. If the pressure of the acetylene is higher than approximately 12,247.2 kilograms per square inch absolute, this condition may initiate detonation or deflagration reactions within the acetylene, resulting in a substantial pressure rise, rupture of the vessel/ and uncontrolled release of the contents. This condition presents a significant fire and safety hazard. Thus, poor reaction control results in a higher probability for adverse chemical reactions and necessitates the requirement that the primary reaction vessel be operated at a low operating pressure. B. Currently practiced technology utilizes a single reactor vessel of a stirred tank reactor design. Because of the inherent characteristics of a stirred tank reactor large vessels are required in relationship to the throughout of carbide to avoid discharge of unreacted carbide. In practice, slow mixing results in a reactor which is neither an ideal CSTR or ideal plug flow reactor and for which little is known of the local processes occurring. This condition results in poor rate control and unsteady operation. C. Currently practiced technology results in extremely low utilization of the available heat evolved in the exothermic reaction between calcium carbide and water. In currently practiced technology, little or none of the heat produced is used. This heat amounts to about 20% of the available chemical energy in the reactor and is wasted through the requirements of cooling. D. In currently practiced technology, little or no effort is made to improve the quality of the calcium hydroxide byproduct. Calcium hydroxide produced in acetylene manufacturing contains substantial impurities originating from coal, coke or limestone used in the manufacture of the calcium carbide. These impurities render the calcium hydroxide produced of extremely low or no market value. It is an objective of the present invention to overcome those problems in its prior art identified above. The process of the invention consists of a two-stage reactor system in which calcium carbide and water are reacted to form acetylene and calcium hydroxide. The calcium hydroxide is subsequently reacted with hydrogen chloride in a neutralizer to form calcium chloride. The calcium chloride may be marketed in an aqueous form or alternatively sent to a drying apparatus to produce marketable anhydrous calcium chloride. The invention embodies a unique, dual reactor configuration in which water and calcium carbide are mixed in an entrained flow-type reactor. The reaction is allowed to occur in a dilute aqueous phase with residence times such that the reaction proceeds to 60-90% completion while in the primary reactor. Entrained reaction products and unreacted feed material are carried overhead to the secondary reactor which consists of a dense phase, laminar plug-flow type reactor. Calcium hydroxide intermediate product is allowed to settle and is removed from the bottom of the reactor. Unreacted water is separated from the calcium hydroxide by use of an overflow wier and recycled to the primary reactor. The combination of the primary reactor and the secondary reactor are such that the majority of reaction occurs in a stirred-tank reactor type configuration which undergoes a transition in the secondary reactor to a plug-flow reactor configuration. The plug-flow reactor allows for the completion of the reaction of calcium carbide with water. In the presence of excess water, the reaction of calcium carbide with water is effectively a first order, irreversible reaction. THE DRAWING A preferred embodiment of the invention is shown in the accompanying drawings. FIG. 1 is a chart showing a reactor system in which calcium carbide hydrogen chloride and water are reacted to form acetylene and calcium chloride; FIG. 2, a chart showing the two-stage reactor configuration; FIG. 3, the details of the upper section of a vertical, tubular primary reactor; and FIG. 4, a detailed diagram of a secondary reaction vessel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 2 illustrates the complete dual reaction system showing the initial reactor 10 and the secondary reactor 16, described in further detail in Figs. 3 and 4. As shown in Fig. 3, the primary reactor 10 consists of an upper section 11 of a vertical, tubular design. The upper section 11 is outfitted with an annular downcomer tube 12 to which makeup water is charged. Calcium carbide is introduced to the center of the downcomer tube 12 and passes through the downcomer tube 12 in a co-current fashion with the water. Reaction is initiated at this point. The reactants proceed to the bottom of the upper section 11 where they comingle with recycled water passing through the velocity control restriction 13. The calcium carbide continues to react with the water in the bottom of the upper zone until the particle size reaches approximately 2-3 millimeters in diameter, at which time the particles are entrained by the upward flow of the fluid phase and carried via the side arm attachment to the secondary reactor. Residence times in the primary reactor 10 are on the order of 1-3 minutes. Unreacted impurities in the calcium carbide which are more dense than calcium carbide pass downward through the velocity control restrictor 13 and are accumulated in the lower section 14 of the primary reactor 10. Recycled water from the secondary reactor is introduced near the bottom of the lower Section 14 of the primary reactor 10. The rate at which this recycled water is introduced is determined by the required velocity in the velocity control restriction 13. The velocity in this section is controlled such that particles which have a density greater than calcium carbide pass downward through the zone, while calcium carbide particles are entrained upward due to the velocity which is in excess of the terminal velocity of the particle. Nonreacting and heavy impurities are withdrawn from the lower section 14 of the primary reactor 10 either continuously or periodically through an appropriate valve (not shown) . After the reaction is 60-90% complete, the unreacted calcium carbide and the intermediate reaction product, calcium hydroxide, along with acetylene and unreacted water pass through the side arm connector 15 to the secondary reaction vessel 16 shown in Fig. 4. The secondary reactor vessel 16 is a circular vessel of sufficient size so as to provide a calcium hydroxide settling time of 30 minutes to two hours. The overflow material from the primary reactor 10 is introduced into an inner annular zone 17 where it is allowed to flow in a downward fashion and where the calcium hydroxide is allowed to settle in a dense zone 18. Unreacted calcium carbide reacts to completion in the secondary reactor 16. Supernatant unreacted water is allowed to overflow an internal wier 19 of a concentric annular design, where it is pumped through a tube 20 to the lower zone of the primary reactor 10. Settled calcium hydroxide is raked toward the bottom of the reactor zone. The rake 21 is driven by a mechanical device 22 designed for an appropriate speed of revolution of approximately 1-6 revolutions per minute. Calcium hydroxide is withdrawn from the bottom of the secondary reactor 16 through a tube 23 and sent to a hydrogen chloride neutralizer (not shown) . The secondary reactor 16 is also outfitted with an over-pressure relief valve 24 to vent acetylene gasses in the event of failure of filters and back pressure relief devices (not shown) . The secondary reactor is also fitted with appropriate rupture disks 25 safely vented to areas of no human occupancy for the purpose of safely venting any deflagration of detonation which may occur as a result of the auto-reactions of acetylene. The acetylene product is generated and taken from the secondary reaction vessel 16 through a tube 26 through drying and filtering devices (not shown) as required to render the product marketable. As shown in Fig. 1, the calcium hydroxide is sent to a neutralizer section where it is mixed with hydrochloric acid. The neutralizer reaction consists of an open tubular reactor of sufficient volume to allow residence times between 1-5 seconds. The ensuing reaction generates heat and produces dissolved calcium chloride and water. Alternatively, anhydrous hydrogen chloride may be added to the calcium hydroxide stream from the secondary reactor to form the same products but without the additional water contained in the hydrochloric acid. The quantity of HC1 added is controlled through monitoring by pH meters or other appropriate monitors. Insoluble solids contained in the calcium chloride stream may be gravity separated through the use of cyclone separators or settling towers. Alternatively, the solution may be filtered to remove insoluble solids prior to drying. The aqueous calcium chloride may be marketed as a product or, alternatively, sent to a drying apparatus where the water is removed from the calcium chloride. Drying improves the market value of calcium chloride. Heat generated in the primary reactor and secondary reactor and the neutralizer are retained, insofar as is safely possible, as sensible heat of the calcium chloride solution. At this high temperature sensible heat contributes significantly to the energy efficiency of the drying apparatus and minimizes the amount of external heat which must be added to perform the drying operation. Excess heat generated in the reactors and the neutralizer (see Fig. 1) may be used to heat air used in the dryer. The dryer configuration may consist of any number of commercially available spray-type dryers designed for evaporation of liquid from crystalline or solid materials. A secondary dryer may be incorporated to further reduce the amount of water-of-hydration. The secondary dryer will require additional heat and may be of the configuration of commercially available fired-heater dryers. A product of the drying operation is an anhydrous calcium chloride which may be marketed. Obviously, numerous additional modifications and variations on the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims: CLAIMS We Claim: 1. A two-stage process for producing acetylene and calcium chloride, comprising the steps of: - reacting calcium carbide with an excess of water in a primary reaction vessel to form acetylene gas and calcium hydroxide; - permitting said reaction to proceed to 60% to 90% completion in the primary reaction vessel; transferring entrained reaction products acetylene gas and calcium hydroxide with unreacted water and calcium carbide to a secondary reaction vessel; - substantially completing the reaction in the secondary reaction vessel, and recovering the acetylene and calcium hydroxide from the secondary reaction vessel, 2. A process according to Claim 1, in which the calcium hydroxide produced by the reaction is reacted with hydrogen chloride in a neutralizer to form calcium chloride, which is recovered. 3. A process according to Claim 1, in which the first step of reacting calcium carbide with an excess of water is conducted in an entrained flow-type reactor; and the second step of completing the reaction is conducted in a dense phase, laminar plug-flow type reactor. 4. A process according to Claim 2, in which the calcium chloride is dried in dryer to produce anhydrous calcium chloride. 5. A process according to Claim 4, in which the heat required for drying is derived from the reactions occurring in the primary and secondary reactors and the neutralizer. 6. A process according to Claim 1, in which the residence time in the primary reaction vessel is about one to three minutes. 7. A process according to Claim 1, in which the residence time in the secondary reaction vessel is about 30 minutes to two hours.

Inventors: BUNGER JAMES W, DEVINENI PRASAD A V, RYU HOIL, BUNGER, JAMES, W., DEVINENI, PRASAD, A., V., RYU, HOIL
Applicants: UNIV UTAH, UNIVERSITY OF UTAH
Application year: 1990
Main classification: C01F11
IPC codes: C01F11, C10H3, C10H11, C10H15, C10H21